The measurement of interface mechanical properties between an adhesive layer and a substrate is significant for optimization of a high-quality interface. A common method for measuring these properties is the peel test. While there are many interesting applications of peel in such areas as cell and gecko adhesion, the focus here is to obtain a better understanding of the fundamental mechanics underlying the problem.
\r\n\r\nThe mechanics of the peel test is examined through experiments, finite element simulations, and theoretical analysis with the aim of developing governing relations to describe the role of fracture in the peel test for elastic adhesive tapes. An inverse formulation is developed to extract a cohesive zone law from a set of experimental peel tests using a theoretical framework based upon non-linear beam theory. Through extracting a cohesive zone law, the adhesion energy during a peel test is determined along with the force distribution in the process zone. This local method of determining the adhesion energy is compared to a global method used by Rivlin in the context of finite deformations, showing good agreement.
\r\n\r\nThe effect of rate-dependency in the peel test is also examined experimentally, with the results used to derive a rate-dependent power-law for the adhesion energy in a peel test as a function of the peel rate. The effects of varying different geometrical parameters during the peel test and how they affect the force distribution and adhesion energy are also presented. Finally, a study of the stability in the peel test, including the role of compliance through several newly developed force-controlled experimental configurations is discussed. The stiffness of the system is varied by altering the magnitude and direction of the applied load during a test. This change in stiffness can be tuned in order to trigger or delay the onset of instability. Theoretical stability criteria are also presented to in order to develop insights of the role of parameters investigated experimentally.
", "doi": "10.7907/W2KT-CY70", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:5216", "collection": "thesis", "collection_id": "5216", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05292008-163638", "primary_object_url": { "basename": "Christian_Franck_Thesis.pdf", "content": "final", "filesize": 19389049, "license": "other", "mime_type": "application/pdf", "url": "/5216/1/Christian_Franck_Thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Quantitative Characterization of 3D Deformations of Cell Interactions with Soft Biomaterials", "author": [ { "family_name": "Franck", "given_name": "Christian", "clpid": "Franck-Christian" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Tirrell", "given_name": "David A.", "clpid": "Tirrell-D-A" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "In recent years, the importance of mechanical forces in directing cellular function has been recognized as a significant factor in biological and physiological processes. In fact, these physical forces are now viewed equally as important as biochemical stimuli in controlling cellular response. Not only do these cellular forces, or cell tractions, play an important role in cell migration, they are also significant to many other physiological and pathological processes, both at the tissue and organ level, including wound healing, inflammation, angiogenesis, and embryogenesis. A complete quantification of cell tractions during cell-material interactions can lead to a deeper understanding of the fundamental role these forces play in cell biology. Thus, understanding the function and role of a cell from a mechanical framework can have important implications towards the development of new implant materials and drug treatments.
\r\n\r\nPrevious research has contributed significant descriptions of cell-tissue interactions by quantifying cell tractions in two-dimensional environments; however, most physiological processes are three-dimensional in nature. Recent studies have shown morphological differences in cells cultured on two-dimensional substrates versus three-dimensional matrices, and that the intrinsic extracellular matrix interactions and migration behavior are different in three dimensions versus two dimensions. Hence, measurement techniques are needed to investigate cellular behavior in all three dimensions.
\r\n\r\nThis thesis presents a full-field imaging technique capable of quantitatively measuring cell traction forces in all three spatial dimensions, and hence addresses the need of a three-dimensional quantitative imaging technique to gain insight into the fundamental role of physical forces in biological processes. The technique combines laser scanning confocal microscopy (LSCM) with digital volume correlation (DVC) to track the motion of fluorescent particles during cell-induced or externally applied deformations. This method is validated by comparing experimentally measured non-uniform deformation fields near hard and soft spherical inclusions under uniaxial compression with the corresponding analytical solution. Utilization of a newly developed computationally efficient stretch-correlation and deconvolution algorithm is shown to improve the overall measurement accuracy, in particular under large deformations.
\r\n\r\nUsing this technique, the full three-dimensional substrate displacement fields are experimentally determined during the migration of individual fibroblast cells on polyacrylamide gels. This is the first study to show the highly three-dimensional structure of cell-induced displacement and traction fields. These new findings suggest a three-dimensional push-pull cell motility, which differs from the traditional theories based on two-dimensional data. These results provide new insight into the dynamic cell-matrix force exchange or mechanotransduction of migrating cells, and will aid in the development of new three-dimensional cell motility and adhesion models.
\r\n\r\nAs this study reveals, the mechanical interactions of cells and their extracellular matrix appear to be highly three-dimensional. It also shows that the LSCM-DVC technique is well suited for investigating the mechanics of cell-matrix interactions while providing a platform to access detailed information of the intricate biomechanical coupling for many cellular responses. Thus, this method has the capability to provide direct quantitative experimental data showing how cells interact with their surroundings in three dimensions and might stimulate new avenues of scientific thought in understanding the fundamental role physical forces play in regulating cell behavior.
", "doi": "10.7907/VMN5-SP86", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:2322", "collection": "thesis", "collection_id": "2322", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05302008-161653", "primary_object_url": { "basename": "WinstonJacksonThesis.pdf", "content": "final", "filesize": 40556815, "license": "other", "mime_type": "application/pdf", "url": "/2322/1/WinstonJacksonThesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Characterization of Soft Polymers and Gels Using the Pressure-Bulge Technique", "author": [ { "family_name": "Jackson", "given_name": "Winston Paul", "clpid": "Jackson-Winston-Paul" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "A method to characterize the bulk hydrated properties of soft polymers and hydrogels, whose moduli are in the low MPa regime, using the pressure-bulge technique is presented. The pressure-bulge technique has been used extensively in the characterization of thin films, particularly for the case of metals. The extension of the plane-strain and circular bulge techniques to determine the Young's modulus and Poisson's ratio of bulk latex and silicone rubber sheets are shown here, in addition to the viscoelastic behavior of 5% agarose gels in the time domain using relaxation tests.
\r\n\r\nThe membranes are clamped between two stainless steel plates that are connected to a liquid pressure chamber. A syringe connected to a linear actuator causes changes in the pressure and displacement, and a pressure sensor and confocal displacement sensor are used to monitor these changes in real time. The theory presented converts the measured pressure and displacement data into stress and stretch data, using a geometrically nonlinear analysis, and the elastic/viscoelastic properties are then determined from this data.
\r\n\r\nThe results from the bulge tests are compared with data from uniaxial tension tests on hydrated specimens, and the data comparison with respect to each of the materials tested show good agreement between the two measurements. These results show promise regarding the use of pressure-displacement techniques to characterize other soft material systems, including biological polymers and tissues, as well as cell-to-matrix and cell-to-cell interactions under varying mechanical loading conditions of cell substrates.
\r\n", "doi": "10.7907/DMZ9-RE14", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:2067", "collection": "thesis", "collection_id": "2067", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05252007-000127", "primary_object_url": { "basename": "Dissertation_SamanthaDaly.pdf", "content": "final", "filesize": 2774907, "license": "other", "mime_type": "application/pdf", "url": "/2067/1/Dissertation_SamanthaDaly.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Deformation and Fracture of Thin Sheets of Nitinol", "author": [ { "family_name": "Daly", "given_name": "Samantha Hayes", "orcid": "0000-0002-7297-1696", "clpid": "Daly-Samantha-Hayes" } ], "thesis_advisor": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Molinari", "given_name": "Alain", "clpid": "Molinari-A" }, { "family_name": "Daraio", "given_name": "Chiara", "clpid": "Daraio-C" }, { "family_name": "Rittel", "given_name": "Daniel", "clpid": "Rittel-D" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Nickel-Titanium (Nitinol) is a Shape Memory Alloy (SMA) that exhibits superelasticity (pseudoelasticity) and shape memory by a solid-solid state diffusion-less phase transformation. Phase transformation and the resulting strain localization in Nitinol has long been a topic of study, both for its inherent scientific interest and also because of the large number of practical applications of this bimetallic alloy. Although Nitinol devices are extensively used in the medical industry, there is a fundamental gap in the amount of high-quality quantitative experimental data detailing strain localization. The numerous applications of shape memory alloys provide the motivation to understand the deformation and failure mechanisms of these materials, particularly their fatigue and fracture behavior. By using an in-situ optical technique called Digital Image Correlation (DIC), quantitative measures of strain localization in Nitinol are presented for the first time in both deformation and failure modes. In addition, a finite element small-scale transformation analysis near a crack tip in Nitinol subjected to mode-I loading under plane stress conditions is performed for the first time. The experimental results and finite element analysis provide new and detailed insights concerning the structure of phase transformation and crack tip fields in Nitinol.", "doi": "10.7907/RATX-WG46", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:834", "collection": "thesis", "collection_id": "834", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03022006-005723", "primary_object_url": { "basename": "02thesis.pdf", "content": "final", "filesize": 3203305, "license": "other", "mime_type": "application/pdf", "url": "/834/2/02thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "High Temperature Deformation of Vitreloy Bulk Metallic Glasses and Their Composite", "author": [ { "family_name": "Tao", "given_name": "Min", "clpid": "Tao-Min" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Huang", "given_name": "Yonggang", "clpid": "Huang-Yonggang" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "A complete understanding of the deformation mechanisms of BMGs and their composites requires investigation of the microstructural changes and their interplay with the mechanical behavior. In this dissertation, the deformation mechanisms of a series of Vitreloy glasses and their composites are experimentally investigated over a wide range of strain rates and temperatures, with focus on the supercooled liquid regime, by combining uniaxial mechanical testing with calorimetric and microscopic examinations. Various theories of deformation of metallic glasses and the composites are examined in light of the experimental data.
\r\n\r\nA comparative structural relaxation study was performed on two closely related Vitreloy alloys, Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit 1) and Zr46.7Ti8.3Cu7.5Ni10Be27.5 (Vit 4). Differential scanning calorimetric studies on the specimens deformed in compression at constant-strain-rate in supercooled liquid regime showed that mechanical loading accelerated the spinodal phase separation and nanocrystallization process in Vit 1, while the relaxation in Vit 4 featured local chemical composition fluctuation accompanied by annealing out of free volume. The effect of the structural relaxation on their mechanical behavior was further studied via single and multiple jump-in-strain-rate tests.
\r\n\r\nThe deformation and viscosity of a new Vitreloy alloy were characterized using uniaxial compression tests in its supercooled liquid regime. A new theoretical model named Cooperative Shear Model, which correlates the evolution of the macroscopic mechanical/thermal variables such as shear modulus and viscosity with the configurational energies of atom clusters in an amorphous alloy, was critically examined in this investigation. The model was successful in predicting the Newtonian and non-Newtonian viscosities of the material, as well as the shear moduli of the deformed specimens, in a self-consistent manner.
\r\n\r\nThe plastic flow of an in-situ metallic glass composite, beta-Vitreloy, was investigated under uniaxial compression in its supercooled liquid regime and at various strain rates. The composite, with ~0.25 volume fraction of crystalline beta-phase dendrites exhibited superplastic behavior similar to that of amorphous Vit 1. Significant strain hardening was observed when the material was deformed at high temperatures and low strain rates. A dual-phase composite model was employed in finite element simulations to understand the effect of the composite microstructure on its mechanical behavior.
", "doi": "10.7907/27SN-R187", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:1276", "collection": "thesis", "collection_id": "1276", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-04062004-165940", "primary_object_url": { "basename": "twc_thesis_electronic.pdf", "content": "final", "filesize": 9410907, "license": "other", "mime_type": "application/pdf", "url": "/1276/1/twc_thesis_electronic.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Gaseous Detonation-Driven Fracture of Tubes", "author": [ { "family_name": "Chao", "given_name": "Tong Wa", "clpid": "Chao-Tong-Wa" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "clpid": "Shepherd-J-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "Explosion Dynamics Laboratory" }, { "literal": "div_eng" } ], "abstract": "An experimental investigation of fracture response of aluminum 6061-T6 tubes under internal gaseous detonation loading has been carried out. The pressure load, with speeds exceeding 2 km/s, can be characterized as a pressure peak (ranging from 2 to 6 MPa) followed by an expansion wave. The unique combination of this particular traveling load and tube geometry produced fracture data not available before in the open literature. Experimental data of this type are useful for studying the fluid-structure-fracture interaction and various crack curving and branching phenomena, and also for validation for multi-physics and multi-scale modeling.
\r\n\r\nAxial surface flaws were introduced to control the crack initiation site. Fracture threshold models were developed by combining a static fracture model and an extensively studied dynamic amplification factor for tubes under internal traveling loads. Experiments were also performed on hydrostatically loaded preflawed aluminum 6061-T6 tubes for comparison. Significantly different fracture behavior was observed and the difference was explained by fluid dynamics and energy considerations. The experiments yielded comparison on crack speeds, strain, and pressure histories.
\r\n\r\nIn other experiments, the specimens were also pre-torqued to control the propagation direction of the cracks. Measurements were made on the detonation velocity, strain history, blast pressure from the crack opening, and crack speeds. The curved crack paths were digitized. The Chapman-Jouguet pressure, initial axial flaw length, and torsion level were varied to obtain different crack patterns. The incipient crack kinking angle was found to be consistent with fracture under mixed-mode loading. High-speed movies of the fracture events and blast wave were taken and these were used in interpreting the quantitative data.
\r\n\r\nNumerical simulations were performed using the commercial explicit finite-element software LS-Dyna. The detonation wave was modeled as a traveling boundary load. Both non-fracturing linear elastic simulations and elastoplastic simulations with fracture were conducted on three-dimensional models. The simulated fracture was compared directly with an experiment with the same conditions. The overall qualitative fracture behavior was captured by the simulation. The forward and backward cracks were observed to branch in both the experiment and simulation.
", "doi": "10.7907/TEZP-YC46", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:1409", "collection": "thesis", "collection_id": "1409", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-04192004-120604", "primary_object_url": { "basename": "Can_Aydiner_Thesis.pdf", "content": "final", "filesize": 2896990, "license": "other", "mime_type": "application/pdf", "url": "/1409/1/Can_Aydiner_Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Investigation of Thermal Tempering in Bulk Metallic Glasses", "author": [ { "family_name": "Aydiner", "given_name": "Cahit Can", "orcid": "0000-0001-8256-6742", "clpid": "Aydiner-Cahit-Can" } ], "thesis_advisor": [ { "family_name": "Ustundag", "given_name": "Ersan", "clpid": "Ustundag-E" } ], "thesis_committee": [ { "family_name": "Ustundag", "given_name": "Ersan", "clpid": "Ustundag-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Bulk metallic glasses are recent advanced materials which generate residual stresses due to rapid cooling from their surfaces during processing. These stresses arise from the thermal gradients that form within the sample at and above the glass transition region. A typical processing of BMGs involves feeding the alloy melt into a mold followed by severe quenching. The formation and nature of these stresses are analogous to the residual stresses due to the thermal tempering of silicate glasses. This analytical-experimental study investigates the thermal tempering phenomenon in BMGs for the first time.
\r\n\r\nOne of the best glass forming metallic alloys, Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vitreloy 1TM), is employed in this study. First, the best technique for the high-resolution measurement of residual stresses in BMGs is determined to be the crack compliance method. Second, the formation of the stresses is modeled with three different levels of viscoelastic phenomenology, namely, an instant freezing model, a viscoelastic model and a structural model. The first is a simplistic analytical model to estimate residual stresses whereas the structural model accounts for the temperature history dependence of the glassy structure. The constitutive laws for the viscoelastic and structural models are incorporated into the finite element method (ABAQUSTM software package) allowing the application of these models to complex geometries. To increase the accuracy of the analysis, the 'correct' temperature evolution in the sample during processing has to be input to these 'mechanical' models. Therefore, the heat transfer problem during the casting process of the BMG is analyzed in detail. Accuracy also requires a detailed knowledge of the thermal parameters of the material as a function of temperature; thus, some attention is also devoted to their measurement.
\r\n \r\nAt the end, calculated and measured stresses are compared and good agreement is achieved. BMGs are demonstrated to be capable of generating very high (around 400 MPa) compression on their surfaces. The study also yielded valuable physical insight into the thermal tempering process itself. It is seen that this process exhibits significant discrepancies in BMGs compared to its analogy in silicate glasses. For instance, the transient tensile stresses that develop in the latter are shown to be lacking in the BMGs. Another discrepancy between the two materials is that the density of BMGs is uniform across the sample cross section in contrast to that found in silicate glasses. Overall, this investigation developed sufficient understanding of the thermal tempering phenomenon in BMGs to establish it as a viable process to manipulate properties.
\r\n", "doi": "10.7907/ZC9Z-5Y06", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:3529", "collection": "thesis", "collection_id": "3529", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09142005-105805", "primary_object_url": { "basename": "Chasiotis_i_2002.pdf", "content": "final", "filesize": 18575077, "license": "other", "mime_type": "application/pdf", "url": "/3529/1/Chasiotis_i_2002.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "The Strength of Polycrystalline Silicon at the Micro- and Nano-Scales with Applications to MEMS", "author": [ { "family_name": "Chasiotis", "given_name": "Ioannis", "clpid": "Chasiotis-Ioannis" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Ustundag", "given_name": "Ersan", "orcid": "0000-0002-0812-7028", "clpid": "Ustundag-E" }, { "family_name": "Tai", "given_name": "Yu-Chong", "orcid": "0000-0001-8529-106X", "clpid": "Tai-Yu-Chong" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Three aspects concerning the reliability of MicroElectroMechanical Systems (MEMS) are discussed in this work. These aspects are: (1) the development of a new tensile testing technique for measuring the elastic modulus and rupture strengths of thin films, (2) an assessment of stress concentration and specimen size effects in failure of micron-sized specimens and (3) the consequences of Hydrofluoric Acid (HF) chemical treatment on the microstructural integrity and the tensile strength of polycrystalline silicon.
\r\n\r\nA new method for tensile testing of thin films by means of an improved apparatus has been developed to measure the elastic properties (Young's modulus, tensile strength) of surface micromachined polycrystalline silicon specimens. The newly designed tensile tester makes use of an Ultraviolet (UV) light curable adhesive to clamp micron-sized specimens. It permits for the first time the testing of thin film materials possessing high failure strength. The properties determination utilizes surface topologies of deforming specimens, acquired with an Atomic Force Microscope (AFM), for determining strain fields by means of Digital Image Correlation (DIC). This full-field, direct and local measurements technique provides the capability of testing any type of thin film materials with nanometer resolution. The gage section of the specimens tested in this study varied between 200 and 1000 \u00b5m in length, 6 and 50 \u00b5m in width, all for a nominal thickness of 2 \u00b5m.
\r\n\r\nThe dependence of fracture strength on micron and sub-micron sized geometries was studied by means of specimens containing various degrees of stress concentrations. A systematic study of small-scale size effects was thus performed by tensioning elliptically perforated specimens (minimum radius of curvature of 1 gm) so as to: (a) vary the stress concentration with constant radius of curvature, (b) increasing radius of curvature of micronotches relative to the grain size. The results demonstrate a strong influence of the size of the highly strained domain (decreasing notch radii) on the failure strength of MEMS scale specimens, while the effect of varying the stress concentration factor is rather insignificant. In addition, tests performed on unnotched tensile specimens of varying dimensions revealed a specimen size effect by which the values of strength scaled with the specimen length. The Young's modulus, however, is found to be rather insensitive to the specimen dimensions at the scale of microns.
\r\n\r\nIn an effort to assess the unexplained and puzzling large variation in properties reported for very small polysilicon specimens a study was conducted to search for a cause. Contrary to the common belief that 49% HF wet release represents a safe post-process for manufacturing polycrystalline silicon, this study has clearly identified the release process as a key item in determining thin film failure properties. It is found that surface roughness as characterized by groove formation at the grain boundaries depends distinctly on the HF release time. In addition, while the actual failure mechanism in polysilicon follows a transgranular fracture, moderate exposure in HF results in partial intergranular fracture at the film surface that is responsible for complete failure. Long exposures yield films of low mechanical strength that demonstrate clear intergranular failure.
", "doi": "10.7907/JPRZ-P277", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:6371", "collection": "thesis", "collection_id": "6371", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05032011-085654224", "primary_object_url": { "basename": "Linero_lg_2002.pdf", "content": "final", "filesize": 52715179, "license": "other", "mime_type": "application/pdf", "url": "/6371/1/Linero_lg_2002.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Global Fracture Analysis of Laminated Composite Materials for Aerospace Structures", "author": [ { "family_name": "Gonz\u00e1lez Li\u00f1ero", "given_name": "Luis", "clpid": "Gonz\u00e1lez-Li\u00f1ero-Luis" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Jennings", "given_name": "Paul C.", "clpid": "Jennings-P-C" }, { "family_name": "Phillips", "given_name": "Robert B.", "orcid": "0000-0003-3082-2809", "clpid": "Phillips-R" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The failure process of laminated composite materials originating from precut sharp cracks, as well as their propagation, is studied from a \"global\" perspective,\r\nappropriate for structural analysis. The size effect in the damage development is explored and the question of \"scaling\" of the results is addressed.\r\n\r\nTwo globally orthotropic sets of panels with the notches aligned along the axes of orthrotopy are studied. The internally evolving damage in the crack tip region is\r\nexamined through enhanced x-ray radiographic inspection and surface strain fields are measured by means of the Digital Image Correlation method (the applicability and limitations of which are analyzed and discussed). The results obtained from these two experimental techniques are joined to assess the feasibility of identifying internal damage solely from surface measurements.\r\n\r\nThe shape of the region of influence of the crack is described and its extension measured. A simplified model for damage progression analysis is proposed.\r\n\r\nThe process of initiation of the damage propagation is described in detail and the different responses for the two different layups are discussed. The maximum stress/strain and the Tsai-Hill failure criteria are compared with the experimental results on the laminates, and their reliability and limitations are addressed.\r\n\r\nThe effective properties of the two sets of laminates are measured at three different loading rates and compared to theory, and the relevance of the time dependence of the material is studied.\r\n", "doi": "10.7907/FCJ8-EW63", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:2515", "collection": "thesis", "collection_id": "2515", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06082005-151713", "primary_object_url": { "basename": "Lu_J_2002.pdf", "content": "final", "filesize": 14799070, "license": "other", "mime_type": "application/pdf", "url": "/2515/1/Lu_J_2002.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Mechanical Behavior of a Bulk Metallic Glass and Its Composite Over a Wide Range of Strain Rates and Temperatures", "author": [ { "family_name": "Lu", "given_name": "Jun", "clpid": "Lu-Jun" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Ustundag", "given_name": "Ersan", "orcid": "0000-0002-0812-7028", "clpid": "Ustundag-E" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThe development of bulk metallic glasses (BMG), which have exceptional mechanical properties such as high strength, high hardness and corrosion resistance, as well as good glass forming and shaping abilities, using relatively expensive materials and processing techniques, offers great opportunities to use this class of solids as structural amorphous materials (SAM). In this thesis, the mechanical behavior of a bulk metallic glass [...] (Vitreloy 1) and its composite [beta]-phase Vitreloy 1 composite, i.e., [...]) is investigated.\r\n\r\nThe stress-strain relations for Vitreloy 1 over a broad range of temperatures (from room temperature up to the crystallization temperature) and strain rates [...] were established in uniaxial compression using both quasi-static and dynamic Kolsky pressure bar loading systems. The effect of strain rate and temperature on steady state flow stress, viscosity and peak stress, as well as the effect of jump-in-strain-rate on the stress-strain behavior, were investigated. Based on the experimental results, boundaries between three main deformation modes are proposed, namely, Newtonian flow and nonlinear flow resulting in homogeneous deformation and shear-localized failure constituting inhomogeneous deformation. To characterize the constitutive behavior of the bulk metallic glass, a free volume based model as well as a fictive stress model are utilized to analyze the stress-strain behavior and a mechanism for shear band formation.\r\n\r\nA unique deformation characteristic of a bulk metallic glass is the shear localization of the material in response to external mechanical loading, which may lead to catastrophic shear failure immediately after yielding under uniaxial loading and at low temperatures. A dynamic indentation experimental setup was developed to evaluate the high-strain-rate inelastic post yield deformation behavior of Vitreloy 1 and its [beta]-phase composite. Time-resolved depth and load responses during the process of indentation on the materials were obtained. Both materials are found to be strain rate insensitive up to 2,000 [...]. Numerical simulations of the indentation experiments, using both pressure insensitive (J2 von Mises) and pressure dependent (Drucker-Prager) flow models, reveal that both materials are pressure (or normal stress) dependent. Intense multiple shear bands are observed in the indentation craters and are responsible for the observed overall inelastic deformation.\r\n\r\nTo further examine the inelastic deformation and as well as whether a pressure sensitive or normal stress is more appropriate for Vitreloy 1, multiaxial compression experiments using a confining sleeve technique were performed. In contrast to the catastrophic shear failure behavior in uniaxial compression, Vitreloy 1 exhibits large inelastic deformation of more than 10 percent under confinement, indicating the nature of ductile deformation under constrained conditions. It is found that the metallic glass follows a pressure dependent Tresca criterion, [...], and the coefficient of the pressure dependence, [beta], is 0.17. Multiple parallel shear bands are observed on the outer surfaces of the deformed specimens.\r\n\r\nMotivated by potential use of Vitreloy 1 in impact related applications, the shock compression characteristics of both Vitreloy 1 and [beta]-Vitreloy composite were studied using planar impact loading. A surprisingly low amplitude elastic precursor bulk wave, corresponding to the elastic response of the 'frozen structure' of the intact metallic glasses, was observed to precede the rate-dependent large deformation shock wave. A concave downward curvature after the initial increase of the [...] shock Hugoniots suggests that a phase-change-like transition occurred during shock compression. In addition, compression damage occurred due to the shear localization. The spalling inside Vitreloy 1 was induced by shear localization, while in [beta]-Vitreloy 1, it was due to debonding of the [beta]-phase boundary from the matrix. The spall strengths at strain rate of [...] were 2.35 GPa and 2.11 GPa for Vitreloy I and [beta]-Vitreloy 1, respectively.", "doi": "10.7907/3SZ8-Y947", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:6380", "collection": "thesis", "collection_id": "6380", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05102011-141326530", "primary_object_url": { "basename": "Zhuang_s_2002.pdf", "content": "final", "filesize": 20980970, "license": "other", "mime_type": "application/pdf", "url": "/6380/1/Zhuang_s_2002.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Shock Wave Propagation in Periodically Layered Composites", "author": [ { "family_name": "Zhuang", "given_name": "Shiming", "clpid": "Zhuang-Shiming" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Grady", "given_name": "Dennis E.", "clpid": "Grady-D-E" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Mathematically, a shock wave is treated as a discontinuity in a medium. In reality, however, a shock wave is always structured, i.e., its front takes a finite time to rise from an initial material state to the final shocked state. The structuring of a shock front is due to the competition between the nonlinearity of material behavior and the dissipation processes occurring during the wave propagation. There are many mechanisms which may be responsible for the dissipation and/or dispersion of shock wave energy. In homogeneous media, such as metals, one common interpretation for the structuring of a shock wave is that the viscoplasticity processes (dislocation, twinning, etc.) are responsible for the dissipation of energy. While in heterogeneous composites, besides the viscous dissipative processes existing in each of its constituents, due to the existence of internal interfaces, the scattering induced by the interface during shock compression could be another important mechanism.\r\n\r\nIn this study, the interface scattering effects on shock wave propagation in heterogeneous media were investigated by subjecting periodically layered composites to planar impact loading with a flyer plate. The flyer plate was accelerated to a desired velocity using a powder gun loading system. In order to measure shock particle velocity time history at an internal or the free surface of the specimen, the so-called VISAR (Velocity Interferometry System for Any Reflector) diagnostic system was constructed and used during shock compression experiments. Manganin stress gages were embedded inside the specimen at selected internal interfaces to measure shock stress time history. To study the scattering mechanisms of the interface to waves, two-component composite specimens with different interface mechanical properties and heterogeneity were prepared and tested. Different types of composites were prepared with differing mechanical impedance. Specimens with different heterogeneity were obtained by changing the geometrical configuration (length scale) of the layered stack. Two-dimensional numerical simulations were also carried out to understand the process of shock wave evolution in the layered composites.\r\n\r\nExperimental and numerical studies show that periodically layered composites support steady structured shock waves. The influence of internal interfaces on the shock wave propagation is through the scattering mechanism, i.e., multiple reflection of waves in the layers and their interaction with the shock wave. The interface scattering affects both the bulk and the deviatoric response of the composite to shock compression. The influence of scattering on the bulk behavior is to slow down the velocity of the shock wave in the composites, while its influence on the deviatoric response is to structure the shock wave profile. If all the dissipative and dispersive effects are collectively termed as viscosity, which causes the shock front structuring, i.e., the shock front rise-time increasing, then the effective shock viscosity increases with the increase of interface impedance mismatch and decreases with the increase of interface density (interface area per unit volume) and shock loading strength. The existing mixture model for constructing the constitutive relation for composites based on the known properties of its component materials can only, at best, reasonably predict the response of the composites under strong shock loading conditions. In order to fully describe the response of a heterogeneous composite to shock compression loading, accurate physics-based constitutive relations need to be formulated to take into account the scattering effects induced by the heterogeneous microstructure.\r\n", "doi": "10.7907/988X-1V27", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:6345", "collection": "thesis", "collection_id": "6345", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04212011-132230527", "primary_object_url": { "basename": "Zhu_w_2002.pdf", "content": "final", "filesize": 47725548, "license": "other", "mime_type": "application/pdf", "url": "/6345/1/Zhu_w_2002.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Nonlinearly Viscoelastic Response of Glassy Polymers", "author": [ { "family_name": "Zhu", "given_name": "Weidong", "clpid": "Zhu-Weidong" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" }, { "family_name": "Emri", "given_name": "Igor", "clpid": "Emri-I" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis consists of three chapters. After a brief introduction on the general aspects of polymer characterization and viscoelasticity in the first chapter, all major features of this research project are described in the following two chapters.\r\n\r\nThe second chapter deals exclusively with the nonlinearly thermo-mechanical creep behavior of (bisphenol A) polycarbonate under pure shear loading at different\r\ntemperatures (0 \u00b0C to 140 \u00b0C). The shear creep in the linearly viscoelastic range was measured with a torsiometer for reference purposes and a master curve, along with a shift factor curve, were deduced. While the master curve is well defined with no detectable deviation, the shift factor can be represented by two straight line segments interrupted at the \u03b2 transition temperature of polycarbonate. The shear creep tests in the nonlinearly\r\nviscoelastic range were conducted on an Arcan specimen geometry at different temperatures and under different stress levels, utilizing digital image correlation for the\r\nrecording of the creep strains. The difference between the nominal stress and the actual stress distribution in the Arcan specimen was explored via numerical simulations\r\n(ABAQUS) by assuming linear quasi-elastic and quasi-plastic analysis in place of the as yet uncertain material characterization. Isochronal plots were created from the creep data. Nonlinearly viscoelastic behavior starts to take effect near 1% strain at the temperatures considered. The applicability of the stress-clock representation for material characterization has been explored and is found to be dubious, at best, for this material. The \"yield-like\" behavior of polycarbonate has been examined in terms of the isochronal stress-strain response and a corresponding \"yield-like shear stress\" has been determined\r\nto be a monotonically decreasing function of the temperature, but again with an interruption at the \u03b2 transition temperature. Time-temperature trade-off as practiced for \"time-temperature shifting\" at small strains does not apply in the nonlinear domain. The results are generally in agreement with those found for Poly(Methyl Methacrylate), thus fostering the idea that the present results can be generalized -with additional work- to other amorphous polymers.\r\n\r\nThe third chapter focuses on the role of volumetric strain in nonlinearly viscoelastic behavior of polycarbonate. The creep responses of (bisphenol A) polycarbonate at 80 \u00b0C under combined two-dimensional shear and tensile/compressive\r\nstress states were measured on Arcan specimens in the nonlinearly viscoelastic regime. Of particular interest is the influence of the dilatational deformation component on the nonlinearly viscoelastic creep behavior. Because the nonlinear material response determines the stress distribution under fixed deformation or load, but is not known a priori, a re-estimation of the latter is essential to verify or adjust the stress state(s). This is\r\naccomplished by approximating isochronal stress-strain relations derived from shear creep behavior, encompassing the nonlinear domain, by a classical incremental elastoplastic\r\nmaterial description at appropriate times. Inasmuch as the two-dimensional character of the test configuration places limits on accessing three-dimensional information, a coherent representation of the results in terms of maximum shear and/or octahedral representation is examined. It is found that the creep behavior under shear and normal stress or deformation imposition differ significantly: when viewed as a response in terms of a maximum shear description, there are material responses under combined loading when either one or the other dominates. Once the response is formulated in terms of an octahedral description the representation becomes less sensitive to normal vs. shear\r\nbehavior. Within the precision underlying the measurements it is found that the shear and normal strain components accumulate under creep in nearly constant ratios. However,\r\nunder this scenario it is demonstrated quite clearly that the addition of negative dilatational stress (or deformation) to pure shear leads to distinctly lower creep rates. The converse is true, if positive dilatational stresses are added, though not monotonically so.\r\n", "doi": "10.7907/V7GW-0342", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:6220", "collection": "thesis", "collection_id": "6220", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12212010-112400412", "primary_object_url": { "basename": "Huang_y_2001.pdf", "content": "final", "filesize": 10325405, "license": "other", "mime_type": "application/pdf", "url": "/6220/1/Huang_y_2001.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Scanning Tunneling Microscopy and Digital Image Correlation in Nanomechanics Investigations", "author": [ { "family_name": "Huang", "given_name": "Ying", "clpid": "Huang-Ying" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Probe microscopy (scanning tunneling microscopy and atomic force microscopy) and digital image correlation together serve as a potentially powerful tool for experimentally\r\ninvestigating the mechanical behaviors of materials at the sub-micron and nanometer scales. Based on the tunneling effect in quantum physics, the scanning tunneling\r\nmicroscope (STM) records surface topography quantitatively and can achieve angstrom resolution. The digital image correlation (DIC) extracts the displacements and gradients\r\nfrom the undeformed and deformed topographical images.\r\nIn this work, a calibration has been performed on the existing STM built \"in-house\" and the coefficients used in the STM system were confirmed. Major improvements on several\r\ncomponents of the system have been made, including constructing a new actuator probe to decouple its in-plane and out-of-plane movements, designing and implementing a new\r\nfirst-stage amplifier to reduce the noise output by a factor of 10 and modeling of the controller in the STM feedback loop.\r\nFurther, systematic study of the digital image correlation has been conducted. In the simple case of one-dimensional correlation, key parameters involved are the subset size,\r\nvariables in the displacement representation, frequency content of the signal and noise. The one-dimensional study was then extended to two dimensions. In addition to those\r\nkey parameters identified in the one-dimensional study, the sampling rate poses substantial influence on the correlation accuracy. Low amplitude, high frequency noise\r\nstill increases the correlation error significantly.", "doi": "10.7907/exy7-5z58", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:279", "collection": "thesis", "collection_id": "279", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01222008-140642", "primary_object_url": { "basename": "Lee_s_1998.pdf", "content": "final", "filesize": 7347042, "license": "other", "mime_type": "application/pdf", "url": "/279/1/Lee_s_1998.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Failure of laminated composites at thickness discontinuities under complex loading and elevated temperatures", "author": [ { "family_name": "Lee", "given_name": "Sangwook", "clpid": "Lee-Sang" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Failure initiation of laminated composites with discontinuous thickness is examined in terms of typical structural load description (tension, shear force and bending moment) rather than in terms of micromechanics considerations. Because transverse shear produced relatively small effects in failure initiation, results are presented as tension-bending interactions. Two loading frames were designed to apply moments and tension simultaneously. Four types of specimens of different stacking sequence were examined to determine failure initiation, and analyzed subsequently via a finite element analysis (ABAQUS). Depending on the stacking sequence across the interface of the step, two different failure modes are identified: For uni-directional fiber orientation across the interface in the tension direction, failure occurs through cracking and delamination which is governed by a fracture mechanics criterion. While the initiation strength for this failure mode is higher than for the cross-ply configurations, the residual strength after initiation is only marginally higher, providing virtually no margin of safety (10%). For cases involving cross-plies on either side of the interface, failure initiation occurs by matrix cracking, with a critical strain across the fibers providing a universal failure criterion. In these cases the residual load bearing capability was 30 to 45% higher than the failure initiation loads. The interaction between moment and tension at failure initiation is linear, an observation that does not hold for the delamination failure driven by crack propagation. It is found that all failures can be described in terms of a common fracture principle; the stress or strain criteria are interchangeable with the fracture energy computations, provided one allows for a range of values of associated fracture energies. Assuming that time dependent aspects of the failure process are not dominant, elevated temperatures did not change the general results of how bending and tension loads interact, provided one accounts for residual thermal stresses; however, the stress magnitude at which the failure initiation occurs decreases with increasing temperature.\r\n", "doi": "10.7907/9p36-ht49", "publication_date": "1998", "thesis_type": "phd", "thesis_year": "1998" }, { "id": "thesis:71", "collection": "thesis", "collection_id": "71", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01082008-112920", "primary_object_url": { "basename": "Deng_th_1997.pdf", "content": "final", "filesize": 5170673, "license": "other", "mime_type": "application/pdf", "url": "/71/1/Deng_th_1997.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Measurement of the dynamic bulk compliance of polymers", "author": [ { "family_name": "Deng", "given_name": "Tony H.", "clpid": "Deng-T-H" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Sturtevant", "given_name": "Bradford", "clpid": "Sturtevant-B" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Caughey", "given_name": "Thomas Kirk", "clpid": "Caughey-T-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Measurements are described and analyzed for the determination of the dynamic bulk compliance for polyvinyl acetate) [PVAc] as a function of frequency and temperature. The real and imaginary parts of the dynamic bulk compliance over the frequency range from 10 Hz to 1,000 Hz have been measured at different temperatures by determining the compressibility of a specimen confined to an oil-filled cavity via pressurization by a piezoelectric driver and response of a piezoelectric sensor. The wavelength of the compressional wave generated by the piezoelectric transducer over the frequency range used is much larger than the size of the cavity so that the pressure can be considered uniform inside the cavity. The complex compliances of the specimen, confining liquid, and the cavity, are additive upon the pressure variations due to the piezoelectric transducer expansion and contraction. All deformations are considered to be purely dilitational.\n\nA master compliance curve over a total frequency range of about 12 decades is generated by the method of time-temperature superposition. Experimental problems addressing limitations in resolution capability are discussed. The results are compared with the classical measurements obtained by McKinney and Belcher over thirty years ago. Further comparison of the bulk with shear compliance data shows that the extent of the transition ranges for the shear and for the bulk functions are comparable, but the two transitions belong to different time scales: That of the bulk response falls mostly into the glassy domain of the shear behavior. One concludes thus that for linearly viscoelastic response the molecular mechanisms contributing to shear and bulk deformations have different conformational sources.\n", "doi": "10.7907/vxb1-4950", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:5096", "collection": "thesis", "collection_id": "5096", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12212004-164817", "primary_object_url": { "basename": "Gonzalez_j_1997.pdf", "content": "final", "filesize": 20079935, "license": "other", "mime_type": "application/pdf", "url": "/5096/1/Gonzalez_j_1997.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Full Field Study of Strain Distribution Near the Crack Tip in the Fracture of Solid Propellants Via Large Strain Digital Image Correlation and Optical Microscopy", "author": [ { "family_name": "Gonzalez", "given_name": "Javier Gonzalez", "clpid": "Gonzalez-Javier-Gonzalez" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "A full field method for visualizing deformation around the crack tip in a fracture process with large strains is developed. A digital image correlation program (DIC) is used to incrementally compute strains and displacements between two consecutive images of a deformation process. Values of strain and displacements for consecutive deformations are added, this way solving convergence problems in the DIC algorithm when large deformations are investigated. The method developed is used to investigate the strain distribution within 1 mm of the crack tip in a particulate composite solid (propellant) using microscopic visualization of the deformation process.", "doi": "10.7907/HRM1-RJ74", "publication_date": "1997", "thesis_type": "engd", "thesis_year": "1997" }, { "id": "thesis:4986", "collection": "thesis", "collection_id": "4986", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12132007-082556", "primary_object_url": { "basename": "Fey_ke_1996.pdf", "content": "final", "filesize": 4094956, "license": "other", "mime_type": "application/pdf", "url": "/4986/1/Fey_ke_1996.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Experimental and theoretical aspects of dynamic crack growth along bimaterial interfaces", "author": [ { "family_name": "Fey", "given_name": "Kate Elizabeth", "clpid": "Fey-K-E" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This work presents findings of an experimental and theoretical study of dynamic bimaterial crack growth. Bimaterial systems composed of constituents with large material mismatch were investigated under dynamic loading conditions. The materials used in this study consisted of Poly-Methylmethacrylate (PMMA) and AISI 4340 Steel, bonded together using a Methylmethacrylate monomer. One point bend loading was achieved using a drop weight tower. Dynamic crack growth, with velocities up to eighty percent of the Rayleigh wave speed of PMMA, was observed using the lateral shearing interferometric technique of Coherent Gradient Sensing (CGS) in conjunction with high speed photography. The results of these experiments are first discussed within the realm of the validity of the linear, elastodynamic asymptotic stress fields. The complex interdependency of stress intensity and mode mixity with crack tip speed is also discussed. The interpretation of |K[superscript d]| and [phi superscript d] in a dynamic bimaterial crack is clarified through the experimental observation of crack growth.\r\n\r\nComplications in analysis arising from this interdependency between the dynamic K[superscript d]-field and velocity are examined for experimentally obtained CGS fringe patterns. Improvements of existing analyzing procedures are made, resulting in increased confidence in data obtained utilizing the method of CGS in dynamic bimaterial fracture. Special attention is given to the interaction of loading and velocity in the behavior of these crack tip fields.\r\n\r\nPrevious methods of investigation have used an elastodynamic, asymptotic K[superscript d]-field to describe the deformations near a bimaterial crack tip. Attempts to develop a fracture criterion based on these results have suffered from the lack of natural length scale as the major criticism. Motivated by experimental observations, a cohesive zone model is presented in this thesis that allows an investigation of dynamic crack growth. The length of the cohesive zone is given by a combination of stress intensity and mixity, bimaterial behavior, and velocity, and emerges as a natural, time evolving length scale with which to examine the bimaterial crack problem. A fracture criterion based on critical cohesive displacements at the trailing edge of the cohesive zone is presented.\r\n\r\nThis cohesive zone model is subsequently used to examine data obtained from experiment. The model enhances our ability to extrapolate our experimental measurements to the near tip region, and to thus study the neighborhood close to the propagating crack tip. Within experimental error, predictions of the proposed fracture criterion are shown to correspond to the experimentally observed dependence of |K[superscript d]| and [phi superscript d] on the instantaneous crack tip velocity. The fracture criterion based on the cohesive model presented in this paper provides the natural next step in understanding dynamic bimaterial crack growth. It provides a criterion based on physically motivated parameters, introduces a natural length scale into the problem, and increases our understanding of dynamic bimaterial fracture mechanics.\r\n", "doi": "10.7907/ehea-x787", "publication_date": "1996", "thesis_type": "phd", "thesis_year": "1996" }, { "id": "thesis:58", "collection": "thesis", "collection_id": "58", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01072008-112449", "primary_object_url": { "basename": "Walter_me_1996.pdf", "content": "final", "filesize": 13790400, "license": "other", "mime_type": "application/pdf", "url": "/58/1/Walter_me_1996.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "The evolution of damage in ceramic matrix composites", "author": [ { "family_name": "Walter", "given_name": "Mark E.", "clpid": "Walter-M-E" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Hall", "given_name": "John F.", "clpid": "Hall-J-F" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In an effort to better understand the evolution of damage in brittle matrix composites, the mechanical behavior of a ceramic matrix composite, unidirectional SiC/CAS (SiC fibers reinforcing a calcium aluminosilicate matrix), was studied. The presented results are based on uniaxial tension experiments for specimens with the fibers aligned in the loading direction. Post-test optical and scanning electron microscopy was also used to identify the various micromechanisms of damage; axial and transverse strain gauges on all four gage section surfaces and in situ acoustic emission and ultrasonic wave speed measurements were used to monitor the evolution of damage. The experimental results demonstrate the existence of \"zones of deformation\" which are associated with the onset of different damage mechanisms. The energy dissipated in each of these zones was calculated. It is shown that the observed stress-strain behavior can be qualitatively explained in terms of the material properties of the matrix and the fiber, the material processing, and the postulated zones of deformation.\r\n\r\nThe experimental results for SiC/CAS were compared with an existing shear-lag model, and the shortcomings of the model are discussed. By approximating matrix cracks as penny shaped cracks, a micromechanical model was used to estimate the change in the axial modulus of the composite. These results also present another way to interpret the acoustic emission data.\r\n\r\nThe evolution of damage in the SiC/CAS experiments was found to be strain rate dependent even within the quasi-static strain rate regime. For higher rate experiments, the transition from elastic to matrix cracked occurred at a stress level that was nearly twice that of the same transition in the lower rate experiments. This phenomenon and the mechanisms which cause it was further investigated with a model material system (a brittle epoxy resin sandwiched between aluminum strips). In situ quantification of the stress during damage initiation and propagation was realized by the optical method of Coherent Gradient Sensing. Based on these results, the reasons for strain rate dependence of the composite are postulated.\r\n\r\nDetailed understanding of aspects of the evolution of in brittle matrix composites was achieved with finite element simulations. This modeling was based on an axisymmetric unit cell composed of a fiber and its surrounding matrix. The unit cell was discretized into linearly elastic elements for the fiber and the matrix and cohesive elements which allow cracking in the matrix, fiber-matrix interface, and fiber. The cohesive elements failed according to critical stress and critical energy release rate criteria (in shear and/or in tension). After failing, the cohesive elements could slide with Coulomb friction. The tension and shear aspects of failure were uncoupled. The cohesive elements were used to simulate a Dugdale penny shaped crack in a homogeneous cylinder; results compared well to the analytical solution. In order to solve the composite axisymmetric unit cell problem, inertia and viscous damping were added to the formulation. The resulting dynamic problem was solved implicitly using the Newmark Method. Results were compared to the experiment by assuming that only a given number of unit cells were active at any point during the simulation. The effects of changing material properties (e.g., interface strength and toughness and matrix toughness) and loading rate are discussed. Several aspects of the experimentally observed material response of SiC/CAS composite were reproduced by the numerical simulations.\r\n", "doi": "10.7907/w4b4-dx66", "publication_date": "1996", "thesis_type": "phd", "thesis_year": "1996" }, { "id": "thesis:4976", "collection": "thesis", "collection_id": "4976", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12122007-141145", "primary_object_url": { "basename": "Liu_c_1994.pdf", "content": "final", "filesize": 14303932, "license": "other", "mime_type": "application/pdf", "url": "/4976/1/Liu_c_1994.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Dynamic fracture problems involving highly transient crack growth histories : an investigation of dynamic failure in homgeneous and bimaterial systems", "author": [ { "family_name": "Liu", "given_name": "Cheng", "clpid": "Liu-C" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nHighly transient elastodynamic fracture processes in both homogeneous and bimaterial systems have been investigated. It is found that due to the wave character of the mechanical fields during transient and dynamic crack growth, the customarily assumptions of steady state and K[superscript d]-dominance may be violated. This may be particularly true during crack growth in laboratory size specimens where crack growth seldom reaches steady state conditions due to the persistence of the initiation transients and the influence of reflected stress waves from the specimen boundaries. By relaxing both restrictions of steady state and of K[superscript d]-dominance, and by permitting the crack-tip speed and the dynamic stress intensity factor to be arbitrary functions of time, the transient asymptotic elastodynamic field near the moving crack-tip was established in the form of higher order expansion for both homogeneous solids and bimaterial systems. In homogeneous solids, we considered cracks that propagated along arbitrary smooth paths, while in bimaterial systems, we only considered crack growth along a straight interface. The higher order coefficients of the asymptotic expansion were found to depend on the time derivative of crack-tip speed, the time derivative of the dynamic stress intensity factors, and for crack propagating along curved paths, on the instantaneous value of the local curvature of the crack path.\n\nThe issue of K[superscript d]-dominance during dynamic crack initiation and transient crack growth was further investigated by solving a particular transient initial/boundary value problem. This corresponds to a planar dilatational wave impinging on a semi-infinite crack in an unbounded elastic solid. The crack initiates under the influence of the wave, and then propagates dynamically. Through comparison of this full field solution and the equivalent K[superscript d]-dominant field or the field represented by the higher order transient terms, it is found that even for points which are relatively far away from the crack-tip, or for times very close to the crack initiation, the higher order transient representation provides a very good description of the actual stress field. \nThe K[superscript d]-dominant field, however, is incapable of approximating the complete stress field with any accuracy (lack of K[superscript d]-dominance).\n\nThe implications of the above observations (possible lack of K[superscript d]-dominance) on the interpretability of dynamic fracture experiments are also explored. The interpretation of experimental data in past laboratory investigations of dynamic fracture events is based on the assumption of K[superscript d]-dominance. However, as we have seen theoretically this assumption may often fail in laboratory situations. As a result, experimental measurements must be analyzed by techniques that allow for the possibility of the existence of transient higher order term effects. Several types of experiments are considered as examples. Plate impact experiments involving very high rates of loading are first analyzed by both a K[...]-dominant and a high order transient approach. The results clearly show the strong effects of transients on the interpretation of the data. As a second example, the optical method of caustics is reanalyzed. A new way of extracting the instantaneous value of the dynamic stress intensity factor K[...](t), which takes transients into account, is proposed and verified theoretically. For the bimaterial system, the issues are equivalent but much more complicated analytically. Here transient effects are found to be magnified by the material property mismatch between the constituent solids. It is shown however, that the higher order transient analysis can predict accurately the fringe patterns from actual experiment performed by means of the CGS (Coherent Gradient Sensing) technique and high speed photography.\n\nThe observations of this thesis suggest that a variety of conclusions made in the literature based on interpretations of experimental data on the basis of steady state or K[superscript d]-dominance may be suspect.", "doi": "10.7907/k32y-0450", "publication_date": "1994", "thesis_type": "phd", "thesis_year": "1994" }, { "id": "thesis:6070", "collection": "thesis", "collection_id": "6070", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09282010-150048718", "primary_object_url": { "basename": "Srinivas_mv_1994.pdf", "content": "final", "filesize": 2079384, "license": "other", "mime_type": "application/pdf", "url": "/6070/1/Srinivas_mv_1994.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Time dependent failure of thin films", "author": [ { "family_name": "Srinivas", "given_name": "Mullahalli V.", "clpid": "Srinivas-M-V" } ], "thesis_advisor": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": " The prediction of time dependent failure at the interface of a thin film bonded to a substrate is considered in terms of a thermorheologically simple viscoelastic material. An initial investigation of stress distribution at the interface is carried out to understand the possible location and the mechanism of failure. Subsequently, a detailed analysis is developed to obtain not only the longitudinal stress component but also the lateral and shear stress components at the interface. Viscoelastic material behavior is included in the model to account for the stress relaxation behavior of the film. The proposed model satisfies both the edge free boundary conditions as well as the displacement and traction continuity at the interface. Using the model, the magnitude of the stresses and their relaxation behavior has been studied for the case of a polyimide film bonded to a silicon substrate. The analysis is capable of incorporating the material property at different temperatures. Using this capability, the stress evolution in a polyimide film during a typical curing process is obtained. With a view towards reducing the residual stress developed in the film, the effect of cooling rate and cooling cycle on final built in residual stresses during the curing process is studied.\r\n\r\n Based on the stress analysis and also from experimental observations, the most likely type of failure mechanism appears to be edge decohesion. So, the problem of edge decohesion along the interface of a thin viscoelastic film bonded to an elastic substrate under tensile residual stresses is considered in the next chapter. An analytical model is developed to predict the crack growth along the interface and its velocity. The tensile residual stress in the film is replaced by a combination of edge loads and an explicit relation of strain energy with respect to time is obtained by simple beam analysis. The strain energy function is computed at small time intervals and the energy release rate is calculated using Griffith's energy balance approach; the discretized time step is assumed to be very small so that the dissipation effects over a given time step are neglected. Crack growth along the interface is computed based on a fracture criteria. The validity of the assumptions in the analytical results is checked by performing a finite element analysis.", "doi": "10.7907/n3rb-w197", "publication_date": "1994", "thesis_type": "engd", "thesis_year": "1994" }, { "id": "thesis:4162", "collection": "thesis", "collection_id": "4162", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10182005-153006", "primary_object_url": { "basename": "Duong_cn_1994.pdf", "content": "final", "filesize": 5501072, "license": "other", "mime_type": "application/pdf", "url": "/4162/1/Duong_cn_1994.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "A nonlinear thermoviscoelastic stress and fracture analysis of an adhesive bond", "author": [ { "family_name": "Duong", "given_name": "Cong N.", "clpid": "Duong-C-N" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Hall", "given_name": "John F.", "clpid": "Hall-J-F" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The evolution of residual stresses resulting from cooling an adhesive bond configuration on its lateral surfaces at a constant rate through the glass transition of the polymer are considered. A nonlinear, viscoelastic (free-volume) model serves for the thermoviscoelastic characterization of the polymer. The simultaneous solution to the heat diffusion and the transient thermoviscoelatic problems are addressed. Both an infinite (one-dimensional) and a finite (two-dimensional) domain are studied. A \"critical\" cooling time exists, in the present case on the order of a few seconds, which separates the control of the solidification process according to whether the relaxation or thermal diffusion time scale governs. The short time \"quenching process,\" i.e., when the time scale is governed by thermal diffusion, leads to essentially constant residual stresses. Slower cooling increasingly invokes the time and rate sensitive properties of the polymer and leads to monotically decreasing residual stresses with longer cooling times. To reduce residual stresses by a factor of two from their maximal values requires cooling times on the order of one or two days. These results are not drastically altered by changes in the thicknesses of the bond components. Apart from singular behavior of the stress components in the two-dimensionally finite domain \"quenching\" has the effect of producing significantly different stress distributions (including stress \"spikes\") than slow or thermoelastic analyses would suggest. This observation is attributed to the interaction of the bending response of the metal components early in the cooling history under the high thermal gradients, which deformations are then partially frozen in during the subsequent cooling of the polymer. Implications of these results for systems possessing geometric and material differences subjected to various thermal cooling ranges are also discussed. The results demonstrate the importance of knowing the bulk relaxation or creep spectrum for the polymer.\n\nIn the second part of the thesis the effect of the residual stresses on fracture behavior of an adhesive bond are addressed within the context of linear fracture mechanics for dissimilar materials. The crack faces are found to be in contact at the fractured end during the (residually stress) unloading process. A significantly error results if this contact zone is not taken into account. The combined effect of the mechanical loads and the residual stresses on the energy release rate is also studied. The total energy release rate from the combined effect is not necessarily higher or equal to the sum of the individual contribution from external loads and from residual stresses separately.\n", "doi": "10.7907/3HF2-K703", "publication_date": "1994", "thesis_type": "phd", "thesis_year": "1994" }, { "id": "thesis:4779", "collection": "thesis", "collection_id": "4779", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12042007-075432", "primary_object_url": { "basename": "Lambros_j_1994.pdf", "content": "final", "filesize": 10360499, "license": "other", "mime_type": "application/pdf", "url": "/4779/1/Lambros_j_1994.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Dynamic decohesion of bimaterial interfaces", "author": [ { "family_name": "Lambros", "given_name": "John", "clpid": "Lambros-J" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nIn the present work, findings of an experimental study of dynamic decohesion of bimaterial systems composed of constituents with a large material property mismatch are presented. PMMA/steel or PMMA/aluminum bimaterial fracture specimens are used. Dynamic one point bend loading is accomplished with a drop weight tower device (for low and intermediate loading rates) or a high speed gas gun (for high loading rates). High speed interferometric measurements are made using the lateral shearing interferometer of Coherent Gradient Sensing in conjunction with high speed photography. Very high crack propagation speeds (terminal crack tip speeds up to [...], where [...] is the shear wave speed of PMMA) and high accelerations ([...], where g is the acceleration of gravity) are observed and reported. Issues regarding data analysis of the high speed interferograms are discussed. The effects of near tip three dimensionality are also analyzed. In crack propagation regions governed by large crack tip accelerations it is found that for accurate analysis of the optical data use of a transient elastodynamic crack tip field is necessary. Otherwise use of a Kd-dominant analysis is sufficient. Using the dynamic complex stress factor histories obtained by fitting the experimental data, a dynamic crack growth criterion is proposed. In the subsonic regime of crack growth it is seen that the opening and shearing displacements behind the propagating crack tip remain constant, i.e., the crack retains a self-similar profile during crack growth at any speed. This forms the basis of the proposed dynamic interfacial fracture criterion. It is also found that the process of dynamic interfacial fracture is highly unstable. This is corroborated by both the very large measured values of crack tip speed and acceleration and by the observation that the energy release rate at the propagating crack tip decreases with increasing crack tip speed. A mechanism of energy transfer from the metal to the PMMA side of the specimen is believed to be responsible for the high transient and transonic effects. An analysis and discussion of this phenomenon is also presented in this work.\n", "doi": "10.7907/36hw-c185", "publication_date": "1994", "thesis_type": "phd", "thesis_year": "1994" }, { "id": "thesis:3305", "collection": "thesis", "collection_id": "3305", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-08312007-104217", "primary_object_url": { "basename": "Pulos_gc_1993.pdf", "content": "final", "filesize": 7107948, "license": "other", "mime_type": "application/pdf", "url": "/3305/1/Pulos_gc_1993.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Nonsteady crack propagation and craze behavior in PMMA", "author": [ { "family_name": "Pulos", "given_name": "Guillermo C.", "clpid": "Pulos-G-C" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This work is devoted to the study of nonsteady crack propagation under cyclic loading in polymers, specifically PMMA. The first part deals with the delineation of a precision loading facility allowing ultra-precise load or displacement control commensurate with the high resolution measurements of crack tip material response.\r\n\r\nA method of determining the advance of crack tip through combined microscope and computer-analyzed observation is presented. In particular, the experimental set up and software development is described by which these measurements are achieved. It is shown that automated crack tip location is possible with a precision of one to two microns, which is amply sufficient for present purposes to make definitive statements about the smoothness or discontinuity of crack propagation.\r\n\r\nThe craze and crack opening displacements are measured near the free surface of the specimen both under quasi-static step loading and cyclic loading. Eleven craze opening profiles for equal load increments are acquired during a single cycle under fatigue loading. A multi-linear craze stress model is used to match the opening displacements to the measurements. While the primordial thickness can be defined from the Lorentz-Lorenz equation and from the assumption of a constant index of refraction for the quasi-static loading, the effect of load history may prevent such determinations for cyclic loading. The damage accumulated through cyclic deformation reduces the strength of the fibrils in the middle of the craze and produces a drop in the middle of the stress distribution. The craze and crack opening displacement are monitored in connection with a jump-like crack/craze advance constituting 50% of the craze length. The newly-drawn craze fibrils after the jump show mechanical behavior that is different from their behavior before the jump and exhibit no deterioration in their stress-displacement relation.", "doi": "10.7907/4kqe-ec90", "publication_date": "1993", "thesis_type": "phd", "thesis_year": "1993" }, { "id": "thesis:3242", "collection": "thesis", "collection_id": "3242", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-08272007-104817", "primary_object_url": { "basename": "Geubelle_ph_1993.pdf", "content": "final", "filesize": 7565792, "license": "other", "mime_type": "application/pdf", "url": "/3242/1/Geubelle_ph_1993.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Nonlinear effects in interfacial fracture", "author": [ { "family_name": "Geubelle", "given_name": "Philippe H.", "clpid": "Geubelle-P-H" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The issue of the non-coplanar quasi-static propagation of a crack in homogeneous and bimaterial sheets is investigated. Through a preliminary linear analysis, it is shown that the interface crack kinking problem is confronted, in most practical cases, with difficulties which do not arise in the homogeneous situation: the crack path as predicted by the maximum energy release rate criterion cannot be determined uniquely and an additional length parameter, absent in the homogeneous case, needs to be specified to assure uniqueness. Following that development, the assumption of small deformations is relinquished and it is shown how the size of the nonlinear zone imparts possibly the physical significance of the additional length parameter. The analysis is performed numerically in the homogeneous and bimaterial cases within the framework of the nonlinearly elastic theory of plane stress and using a \"boundary-layer\" approach. Material and geometrical nonlinearities are combined through the use of the Generalized Neo-Hookean (GNH) model. As the length of the crack extension becomes comparable to the size of the nonlinear zone, a transition is observed between the value of the energetically most favorable kink angle predicted by the linear theory and a unique \"nonlinear\" value which is found to be independent of the crack extension length and the far-field loading conditions.\n\nThe results of the crack propagation analysis are related to those of a detailed asymptotic analysis of the structure of the near-tip stress and deformation fields for the GNU class of hyperelastic materials. The investigation addresses a) the symmetric (mode I) and non-symmetric (mixed-mode) homogeneous situations, b) the rigid substrate case and c) the general bimaterial problem which allows for an arbitrary choice, on both sides of the interface, of the three material parameters characterizing the GNH model. The asymptotic analysis allows to quantify the effect of the \"hardening\" characteristics on the blunting of the crack and the associated stress and strain singularities, and shows that the near-tip fields corresponding to a general nonsymmetric loading are, in the homogeneous situation, related to those of the symmetric (mode I) case through a rotation which depends on the material characteristics and the far-field loading conditions. A somewhat similar property is obtained in the bimaterial problem, where the existence of a non-oscillatory and \"contact-free\" solution is confirmed for all material combinations.", "doi": "10.7907/rkt0-7k90", "publication_date": "1993", "thesis_type": "phd", "thesis_year": "1993" }, { "id": "thesis:1658", "collection": "thesis", "collection_id": "1658", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05072007-105324", "primary_object_url": { "basename": "Kubr_tj_1990.pdf", "content": "final", "filesize": 2862604, "license": "other", "mime_type": "application/pdf", "url": "/1658/1/Kubr_tj_1990.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Stresses near a change of thickness in a continuous-fiber-composite plate", "author": [ { "family_name": "Kubr", "given_name": "Thomas J.", "clpid": "Kubr-T-J" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis presents the results of a numerical investigation of the stresses near an abrupt change in thickness of a composite plate. The plate is a laminate of unidirectional, continuous fiber plies. The study is aimed at gaining insight into the failure behavior of co-cured stringer reinforced composite plates and shells.\n\nThe analysis is performed in a plane normal to the stringer axis. The problem formulation is similar to plane strain. Because orthotropic materials are involved, a solution method allowing three-dimensional displacements as functions of two spatial variables is applied. The method is called plane-coupled strain. Failure is assumed to initiate at the sharp 90\u00b0 reentrant corner of the step-down geometry due to a rise in stresses. The resulting stress singularity is characterized for different combinations of stacking orders in the stepped plate. Stresses in the structure are determined by means of the finite element method.\n\nThe results are presented in two parts: The first describes the differences obtained with plane-coupled strain and classical plane strain; the second characterizing the stress singularity. The more computer intensive plane-coupled strain solution produces significantly different results for the stresses near the singularity. The character of the stress singularity is found to be highly dependent on the fiber direction of the material at the sharp corner.\n\nThe results for the stress singularity are used in an example failure criterion. Stacking sequences are examined with respect to their susceptibility to failure under different combinations of in-plane loading.", "doi": "10.7907/rra5-sv66", "publication_date": "1991", "thesis_type": "engd", "thesis_year": "1991" }, { "id": "thesis:2848", "collection": "thesis", "collection_id": "2848", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-07102007-104514", "primary_object_url": { "basename": "Pfaff_rd_1991.pdf", "content": "final", "filesize": 7181768, "license": "other", "mime_type": "application/pdf", "url": "/2848/1/Pfaff_rd_1991.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Three-dimensional effects in nonlinear fracture explored with interferometry", "author": [ { "family_name": "Pfaff", "given_name": "Richard D.", "clpid": "Pfaff-R-D" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The prospects for understanding fracture mechanics in terms of a general material constitutive description are explored. The effort consists of three distinct components.\n\nFirst, optical interferometry, in its various forms (Twyman-Green, diffraction moire, etc.), can potentially be used under a wide range of conditions to very accurately measure the displacement and strain fields associated with the deformation surrounding a cracktip. To broaden the range of fracture problems to which interferometry may be applied, certain of the necessary experimental improvements have been developed:\n\n1. High speed camera designs capable of extremely high (> 10(9) frames/second) framing rates with large array sizes, (> 4000 x 4000 pixels per frame) so that the application of optical techniques to solid mechanics may be considered without limitation on the rate of deformation.\n\n2. An accurate and adaptable device for dynamic loading of fracture specimens to high load levels utilizing electromagnetic (Lorentz force) loading with ultrahigh (> 2,000,000 Amp/cm(2)) current flux densities.\n\n3. Implementation of high sensitivity (2 nm), large range (2 nm x 3,200,000) interferometry achieved with wide field array sizes of 50,000 x 50,000 and 8 bit gray scale (error restricted to 1 bit) for surface deformation measurements on fracture specimens.\n\nSecond, functional descriptions for certain aspects of the displacement fields associated with fracture specimens are developed. It is found that the fully three-dimensional crack tip field surrounding a through-thickness crack in a plate of elastic-plastic material shows a hierarchical structure of organization and that the primary aspects of the deformation field would seem to have a relatively simple form of expression if the deformation is viewed in a properly normalized form.\n\nThird, a comparison is made between interferometrically measured surface displacements for a notched 3-point-bend speciemn of a ductile heat treatment of 4340 steel and a numerical simulation of the specimen based on a material constitutive description determined from uniaxial tests performed on the same material. The small but finite notch tip radius (0.15 mm) fabricated by a wire-cutting electrical discharge machine allows one to explore the limits of applicability of standard continuum plasticity theories without involving a process zone model for the very near tip region extent in a cracked specimen geometry.\n", "doi": "10.7907/6jnw-kh62", "publication_date": "1991", "thesis_type": "phd", "thesis_year": "1991" }, { "id": "thesis:4297", "collection": "thesis", "collection_id": "4297", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10292003-112909", "type": "thesis", "title": "Time-Temperature Response of Multi-Phase Viscoelastic Solids Through Numerical Analysis", "author": [ { "family_name": "Brinson", "given_name": "Lynda Catherine", "clpid": "Brinson-Lynda-Catherine" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Hall", "given_name": "John F.", "orcid": "0000-0002-7863-5060", "clpid": "Hall-J-F" }, { "family_name": "Tschoegl", "given_name": "Nicholas W.", "clpid": "Tschoegl-N-W" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "A numerical model has been constructed for the study of the properties of multi-phase viscoelastic composites. The model utilizes the dynamic correspondence principle of viscoelasticity in a finite element program to solve boundary value problems simulating uniaxial tension or simple shear and obtains the global complex Young's or shear moduli of the composite.
\r\n\r\nEach phase of the composite is considered to be thermorheologically simple. The resulting modulus properties of the composite however, are thermorheologically complex and this investigation examines the nature of time-temperature behavior of multi-phase composite materials. The specific composite considered throughout this study contains viscoelastic inclusions embedded in a different viscoelastic matrix material. The deviation of the composite moduli from thermorheologically simple behavior of the matrix material is shown to occur at frequencies and temperatures where the glass-to-rubber transition of the included phases are reached.
\r\n\r\nProperties of polystyrene and polybutadiene are used to investigate the thermorheological complexity (non-shiftability) of a Styrene-Butadiene-Styrene (SBS) block copolymer. To achieve congruence of the results with experimental data, it is necessary to consider a transition phase of properties \"intermediate\" to those of styrene and butadiene. Using accurate physical information on the individual phase properties and on the interphase region, it is possible to utilize the numerical model to predict long term properties of multi-phase composites from short term laboratory data. Lacking detailed information on the properties of a particular phase (e.g., the interphase), but knowing the time dependent properties for the composite material at a broad range of temperatures, it is also possible to use the numerical tool to solve an inverse problem and determine the unknown properties of the phase in question.
", "doi": "10.7907/SYAS-2A50", "publication_date": "1990", "thesis_type": "phd", "thesis_year": "1990" }, { "id": "thesis:4242", "collection": "thesis", "collection_id": "4242", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10252002-160528", "type": "thesis", "title": "Robust Control of Flexible Structures: Theory and Experiments", "author": [ { "family_name": "Balas", "given_name": "Gary John", "clpid": "Balas-Gary-John" } ], "thesis_advisor": [ { "family_name": "Doyle", "given_name": "John Comstock", "orcid": "0000-0002-1828-2486", "clpid": "Doyle-J-C" } ], "thesis_committee": [ { "family_name": "Doyle", "given_name": "John Comstock", "orcid": "0000-0002-1828-2486", "clpid": "Doyle-J-C" }, { "family_name": "Caughey", "given_name": "Thomas Kirk", "clpid": "Caughey-T-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Morari", "given_name": "Manfred", "clpid": "Morari-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Stringent requirements envisioned for the pointing and shape accuracy of future space missions necessitate advances in the control of large flexible structures. These structures will be extremely flexible, with little natural damping and modes densely packed throughout the frequency domain. Due to their size and complexity, testing of these structures will lead to system models that are inaccurate for control purposes. Therefore, control design methods must be developed to account for model inaccuracies or uncertainties. Such methods should optimize the robustness and performance characteristics of control laws based on the accuracy of the design model.
\r\n\r\nThis thesis focuses on incorporating knowledge of the mismatch between the physical system and its mathematical models into the control design process. Control design models are developed to fit into the structured singular value (\u00b5) framework that is used in the analysis and synthesis of control laws. To validate and verify theoretical developments, a flexible structure experiment is developed to investigate large flexible control problems in a laboratory environment. The Caltech experiment has a number of their attributes: closely spaced, lightly damped modes, collocated and noncollocated sensors and actuators combined with numerous modes in the controller crossover (roll off) region.
\r\n\r\nThe experimental structure is used to investigate several important issues related to control of flexible structures: tradeoffs between robustness and performance associated with uncertainty modeling for flexible structures, robust control of flexible modes in the controller crossover region and benefits and limitations of collocated versus noncollocated control design. A consistent trend in the results indicates that an accurate description of the flexible structure and model errors is required to synthesize high performance, robust control laws for flexible structures.
", "doi": "10.7907/T7NG-T636", "publication_date": "1990", "thesis_type": "phd", "thesis_year": "1990" }, { "id": "thesis:4417", "collection": "thesis", "collection_id": "4417", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11062003-112730", "type": "thesis", "title": "Dynamic Crack Propagation in Elastic-Plastic Solids", "author": [ { "family_name": "Deng", "given_name": "Xiaomin", "clpid": "Deng-Xiaomin" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Hall", "given_name": "John F.", "orcid": "0000-0002-7863-5060", "clpid": "Hall-J-F" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Vreeland", "given_name": "Thad", "clpid": "Vreeland-T" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The present finite element study addresses several issues of interest pertaining to the phenomenon of dynamic crack propagation in elastic-plastic solids. Three classes of materials, namely elastic-perfectly plastic materials, linear hardening materials and power-law hardening materials, are considered. The materials are assumed to obey the von Mises yield criterion and the associated flow rule.
\r\n\r\nUnder conditions of Mode I, plane stress, steady state and small scale yielding, we investigated the structures of the near-tip stress and deformation fields. A preliminary asymptotic analysis for crack-tip stress and velocity fields in elastic-perfectly plastic solids was provided to reveal and explain some special features of the crack tip fields observable only in the case of rapid crack propagation. We studied the theoretical basis of a fracture criterion based on the dynamic stress intensity factor for crack growth in materials which fail in a locally ductile manner. We explored the behavior of crack tip fields under non-K-dominance conditions and its effects on the dynamic fracture toughness vs. crack propagation speed relationship.
\r\n\r\nAn Eulerian finite element scheme is employed. Finite element meshes with extremely small elements near the crack tip are carefully designed. The ratio of the crack tip plastic zone size to that of the element nearest to the crack tip is of the order of 1.6 x 10\u2074. In order to overcome numerical difficulties associated with crack-tip strain singularities and the use of small near-tip elements, an efficient stress integration algorithm is devised. The existing stress state determination procedure is modified to prevent the occurrence of negative plastic flow and to avoid mistakenly treating elastic unloading as plastic flow. The above measures are proven to be essential for the convergence of the numerical solution.
", "doi": "10.7907/WHJV-C644", "publication_date": "1990", "thesis_type": "phd", "thesis_year": "1990" }, { "id": "thesis:4299", "collection": "thesis", "collection_id": "4299", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10292003-134326", "type": "thesis", "title": "On the Domain of Dominance of the Asymptotic Elastodynamic Crack-Tip Fields", "author": [ { "family_name": "Krishnaswamy", "given_name": "Sridhar", "clpid": "Krishnaswamy-Sridhar" } ], "thesis_advisor": [ { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Wood", "given_name": "David Shotwell", "clpid": "Wood-D-S" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "A substantial part of the experimental data in dynamic fracture mechanics has been obtained under the assumption that the two-dimensional asymptotic elastodynamic stress-intensity factor field (the KdI-field) is dominant over at least the region around the crack-tip over which the experimental measurements are made. The validity of this assumption is investigated in this thesis both experimentally and through finite-element simulations of the experiments.
\r\n\r\nThe experiments reported in this work were on 4340 steel, three-point bend specimens loaded dynamically using a drop-weight tower. The two cases of dynamically loaded stationary cracks and dynamically propagating cracks were considered. An optical configuration is proposed that leads to a bifocal high-speed camera capable of focusing on two different planes simultaneously. This was used in conjunction with the method of caustics to measure the apparent stress-intensity factor simultaneously from two different regions (initial-curves) around the crack-tip. If the initial-curves lie within the domain of dominance of the asymptotic field, the measured values of the dynamic stress-intensity factor must agree to within experimental error. By suitably adjusting the optical set-up, a range of initial-curves was scanned in an attempt to map the domain of dominance of the KdI-field.
\r\n\r\nThe impact hammer and supports of the drop-weight loading device were instrumented in order to monitor the time dependent loads acting on the specimen. These loads were subsequently used as boundary tractions in dynamic two- and three-dimensional finite-element simulations of the experiments. The simulations were carried only up to the point of crack initiation. Comparison of the numerical simulations with the experimental results help in identifying the role of three-dimensionality and transient conditions on the measured stress-intensity factor values.
\r\n\r\nOn the basis of both the experimental results as well as the numerical simulations, no sizeable annulus of dominance for the asymptotic elastodynamic field was found for the laboratory situation studied. It appears that the assumption of an underlying KdI-dominant (or two-dimensional) field might not hold to a level of accuracy that would warrant many of the conclusions made in the literature regarding the crack-initiation toughness values as well as the uniqueness of the dynamic fracture toughness - crack velocity relation or its specimen and acceleration dependence.
", "doi": "10.7907/cfz9-nk80", "publication_date": "1989", "thesis_type": "phd", "thesis_year": "1989" }, { "id": "thesis:4539", "collection": "thesis", "collection_id": "4539", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11132007-092151", "type": "thesis", "title": "Considerations for the Design of Gas-Lubricated Slider Bearings", "author": [ { "family_name": "Smith", "given_name": "Paul Wesley, Jr.", "clpid": "Smith-Paul-Wesley-Jr" } ], "thesis_advisor": [ { "family_name": "Iwan", "given_name": "Wilfred D.", "clpid": "Iwan-W-D" } ], "thesis_committee": [ { "family_name": "Iwan", "given_name": "Wilfred D.", "clpid": "Iwan-W-D" }, { "family_name": "Caughey", "given_name": "Thomas Kirk", "clpid": "Caughey-T-K" }, { "family_name": "Hall", "given_name": "John F.", "clpid": "Hall-J-F" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "An approach is developed that simplifies calculation of the dynamic characteristics of a self-acting, gas-lubricated slider bearing system. This technique avoids a lengthy simultaneous solution of the equations of motion of the slider and the time-dependent Reynolds' equation, while providing additional design information that is otherwise unobtainable.
\r\n\r\nThe equilibrium pressure distribution in the gas film is obtained using the Bunov-Galerkin formulation of the finite element method. By considering small perturbations of the slider bearing system about equilibrium, two coupled, second-order partial differential equations are derived, which define the in-phase and out-of-phase perturbation pressures in the gas film. These perturbation pressures are integrated to obtain the frequency dependent, non-symmetrical stiffness and damping matrices for the slider bearing.
\r\n\r\nUsing the stiffness and damping properties of the gas bearing and slider support, the equations of motion for the entire slider bearing system are derived. The frequency dependence of the stiffness and damping matrices renders the eigenvalue problem nonlinear, and the eigensolutions are obtained iteratively using Brent's method.
\r\n\r\nBecause of the non-symmetrical stiffness and damping matrices, a similarity transformation based on the left and right modal matrices is used to decouple the equations of motion. This decoupling is approximate because of the frequency dependence of the stiffness and damping matrices, but the resulting damped natural frequencies are shown to be in excellent agreement with published experimental data. Fractions of critical damping obtained for several slider geometries also successfully predict observed instabilities.
\r\n\r\nThe mode shapes of slider oscillation, unobtainable with other methods, permit calculation of the center of rotation for the coupled, pitch-heave modes; this information can be used to determine the optimum location for the magnetic transducer. Closed-form solutions are obtained for the response to disk surface displacement, and for the response to a random force applied to the slider body. These forced-response solutions are useful in identifying the critical parameters of slider design.
\r\n", "doi": "10.7907/FYJ3-X454", "publication_date": "1988", "thesis_type": "phd", "thesis_year": "1988" }, { "id": "thesis:4628", "collection": "thesis", "collection_id": "4628", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11212007-083506", "type": "thesis", "title": "Plastic Buckling of Cylinders Under Biaxial Loading", "author": [ { "family_name": "Giezen", "given_name": "Jurgen Johannes", "clpid": "Giezen-Jurgen-Johannes" } ], "thesis_advisor": [ { "family_name": "Babcock", "given_name": "Charles D.", "clpid": "Babcock-C-D" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Babcock", "given_name": "Charles D.", "clpid": "Babcock-C-D" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Singer", "given_name": "Josef", "clpid": "Singer-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "An experimental investigation is carried out to study the effects of nonproportional loading in the plastic range on a buckling load. The discrepancy between experimental and theoretical results points to some principal shortcoming in the analysis. The problem has been simplified by applying axial tensile load and external press to simple cylindrical shell specimen and observing the buckling load for various nonproportional load-paths. Results are compared to numerical predictions (BOSOR5) using classical type plasticity models such as J\u2082 deformation and J\u2082 incremental theory. Significant discrepancy was found an attributed to inadequate modeling of the nonlinear material behavior. The effects of geometrical imperfections and large deflections were found to insignificant, thereby leading to an idea how much of the discrepancy between test and theory is due to a use of inadequate plastic model. The introduction of the Southwell plot into elastic shell buckling problem reduced the already minor effects of geometric imperfections.
\r\n\r\nThe Christoffersen-Hutchinson corner theory model was introduced into BOSAR5 in its simplest form as presented by Poh-Sang Lam. Results obtained with this model, which allows corners to form on an initially smooth yield surface, displayed better agreement with experimental data. However, increased computational time and problems related to abrupt changes in load-path at the corner are a major concern at this present time.
\r\n", "doi": "10.7907/ycv3-kp17", "publication_date": "1988", "thesis_type": "phd", "thesis_year": "1988" }, { "id": "thesis:4415", "collection": "thesis", "collection_id": "4415", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11062003-092741", "primary_object_url": { "basename": "Waas_am_1988.pdf", "content": "final", "filesize": 26765568, "license": "other", "mime_type": "application/pdf", "url": "/4415/1/Waas_am_1988.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Compression Failure of Fibrous Laminated Composites in the Presence of Stress Gradients: Experiment and Analysis", "author": [ { "family_name": "Waas", "given_name": "Anthony Marius", "orcid": "0000-0002-5258-2749", "clpid": "Waas-Anthony-Marius" } ], "thesis_advisor": [ { "family_name": "Babcock", "given_name": "Charles D.", "clpid": "Babcock-C-D" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Babcock", "given_name": "Charles D.", "clpid": "Babcock-C-D" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Hall", "given_name": "John F.", "clpid": "Hall-J-F" }, { "family_name": "Lorenz", "given_name": "Jens", "clpid": "Lorenz-Jens" }, { "family_name": "Singer", "given_name": "Josef", "clpid": "Singer-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "A series of experiments were performed to determine the mechanism of failure in compressively loaded laminated plates in the presence of stress gradients generated by a circular cutout. Real time holographic interferometry and in-situ photomicrography of the hole surface, were used to observe the progression of failure.
\r\n\r\nThe test specimens are multi-layered composite flat plates, which are loaded in compression. The plates are made of two material systems, T300/BP907 and IM7/8551-7. Two different lay-ups of T300/BP907 and four different lay-ups of IM7/8551-7 are investigated.
\r\n\r\nThe load on the specimen is slowly increased and a series of interferograms are produced during the load cycle. These interferograms are video-recorded. The results obtained from the interferograms and photo-micrographs are substantiated by sectioning studies and ultrasonic C-scanning of some specimens which are unloaded prior to catastrophic failure, but beyond failure initiation. This is made possible by the servo-controlled loading mechanism that regulates the load application and offers the flexibility of unloading a specimen at any given instance in the loadtime history.
\r\n\r\nAn underlying objective of the present investigation is the identification of the physics of the failure initiation process. This required testing specimens with different stacking sequences, for a fixed hole diameter, so that consistent trends in the failure process could be identified.
\r\n\r\nIt is revealed that the failure is initiated as a localized instability in the 0\u00b0 plies at the hole surface, approximately at right angles to the loading direction. This instability emanating at the hole edge and propagating into the interior of the specimen within the 0\u00b0 plies is found to be fiber microbuckling. The microbuckling is found to occur at a local strain level of \u2243 8600 \u00b5strain at the hole edge for the IM material system. This initial failure renders a narrow zone of fibers within the 0\u00b0 plies to loose structural integrity. Subsequent to the 0\u00b0-ply failure, extensive delamination cracking is observed with increasing load. The through thickness location of these delaminations is found to depend on the position of the 0\u00b0 plies.
\r\n\r\nThe delaminated portions spread to the undamaged areas of the laminate by a combination of delamination buckling and growth, the buckling further enhancing the growth. When the delaminated area reaches a critical size, about 75-100% of the hole radius in extent, an accelerated growth rate of the delaminated portions is observed. The culmination of this last event is the complete loss of flexural stiffness of each of the delaminated portions leading to catastrophic failure of the plate. The levels of applied load and the rate at which these events occur depend on the plate stacking sequence.
\r\n\r\nA simple mechanical model is presented for the microbuckling problem. This model addresses the buckling instability of a semi-infinte layered half-plane alternatingly stacked with fibers and matrix, loaded parallel to the surface of the half-plane. The fibers are modelled using Bernoulli-Navier beam theory, and the matrix is assumed to be a linearly elastic foundation. The predicted buckling strains are found to overestimate the experimental result. However, the dependence of the buckling strain on parameters such as the fiber volume fraction, ratio of Youngs moduli of the constituents and Poisson's ratio of the matrix are obtained from the analysis. It is seen that a high fiber volume fraction, increased matrix stiffness, and perfect bonding between fiber and matrix are desirable properties for increasing the compressive strength.
", "doi": "10.7907/PP86-PE50", "publication_date": "1988", "thesis_type": "phd", "thesis_year": "1988" }, { "id": "thesis:4421", "collection": "thesis", "collection_id": "4421", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11062007-080516", "type": "thesis", "title": "On the Collapse of Long Thick-Walled Circular Tubes under Biaxial Loading", "author": [ { "family_name": "Madhavan", "given_name": "Raghu", "clpid": "Madhavan-Raghu" } ], "thesis_advisor": [ { "family_name": "Babcock", "given_name": "Charles D.", "clpid": "Babcock-C-D" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Hall", "given_name": "John F.", "clpid": "Hall-J-F" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" }, { "family_name": "Singer", "given_name": "Josef", "clpid": "Singer-J" }, { "family_name": "Babcock", "given_name": "Charles D.", "clpid": "Babcock-C-D" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The collapse phenomenon of long, thick-walled tubes subjected to axial tension and external pressure is investigated. A combined experimental and analytic approach is adopted. The diameter to thickness ratio (DA) of the tubes studied is in the range 10-40.
\r\n\r\nA series of collapse tests are conducted using thick-walled, small diameter tubes of two different materials. Careful measurements of geometrical and material parameters are carried out before each collapse test. Tension-Pressure collapse envelopes are obtained for tubes of different D/t and loading paths. Collapse tests involving initially ovalized tubes are also carried out. The results show that collapse strength is strongly influenced by initial ovality.
\r\n\r\nA two-dimensional model is used for predicting the collapse strength. The limit point behavior of a long tube with initial geometric imperfections has been modeled. The tube is assumed to be under generalized plane strain conditions and the possible variations of material and geometric parameters along the length are not considered. Hill's anisotropic plasticity theory involving a quadratic yield function is used to model the anisotropies in yield shown by drawn tubes. A power law creep model is employed to assess the effect of primary creep on collapse strength.
\r\n\r\nThe interaction between collapse pressure and tension is found to depend on both material and geometric parameters. The yield behavior of the tube material strongly affects the collapse phenomenon. Initial ovality of the tube is shown to be a very important geometric parameter that influences collapse strength. The effect of primary creep on collapse is shown to be not very significant, for the type of materials used (304 stainless steel and 6061-O aluminum).
\r\n", "doi": "10.7907/VPAC-QV51", "publication_date": "1988", "thesis_type": "phd", "thesis_year": "1988" }, { "id": "thesis:900", "collection": "thesis", "collection_id": "900", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03082008-083510", "primary_object_url": { "basename": "Silling_sa_1986.pdf", "content": "final", "filesize": 5434228, "license": "other", "mime_type": "application/pdf", "url": "/900/1/Silling_sa_1986.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Singularities and Phase Transitions in Elastic Solids: Numerical Studies and Stability Analysis", "author": [ { "family_name": "Silling", "given_name": "Stewart Andrew", "orcid": "0000-0002-4201-2910", "clpid": "Silling-Stewart-Andrew" } ], "thesis_advisor": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" } ], "thesis_committee": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Sternberg", "given_name": "Eli", "clpid": "Sternberg-E" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Numerical studies of the deformation near the tip of a crack are presented for a family of incompressible solids in the context of the theory of finite anti-plane shear of an elastic material. The numerical model computes the near-field and far-field solutions simultaneously, enabling observations of both small-scale and large-scale nonlinearity. The computed near-field solution is compared with a lowest-order asymptotic solution. An approximation for the J-integral under conditions of very large loads is discussed and compared with numerical results. The size of the region over which the lowest-order solution applies is observed.
\r\n\r\nNumerical solutions are presented for the same crack problem with materials for which the equilibrium equation changes in type from elliptic to hyperbolic as a result of deformation. These results show the emergence of surfaces of discontinuity in the displacement field in some cases. In other cases they show a chaotic mixture of elliptic phases near the crack tip.
\r\n\r\nAnalysis of the stability of such coexistent phases is carried out for a specific material, the trilinear material. It is shown that the Maxwell relation, and therefore local stability, cannot in general be satisfied exactly for an arbitrary boundary value problem with this material. However, in those cases where it cannot be satisfied exactly, it may be satisfied in the sense of a limit of a certain sequence of deformations. This sequence produces a progressively chaotic pattern of two coexistent elliptic phases, as was observed numerically. The phases mix over a definite region in a given boundary value problem. This region may be computed using a constitutive relation which characterizes the mixture in the limit of the sequence.
", "doi": "10.7907/0ytn-e775", "publication_date": "1986", "thesis_type": "phd", "thesis_year": "1986" }, { "id": "thesis:728", "collection": "thesis", "collection_id": "728", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02242004-152909", "primary_object_url": { "basename": "Ungsuwarungsri_t_1986.pdf", "content": "final", "filesize": 5055837, "license": "other", "mime_type": "application/pdf", "url": "/728/1/Ungsuwarungsri_t_1986.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "The Effect of Strain-Softening Cohesive Material on Crack Stability", "author": [ { "family_name": "Ungsuwarungsri", "given_name": "Tawach", "clpid": "Ungsuwarungsri-Tawach" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Iwan", "given_name": "Wilfred D.", "clpid": "Iwan-W-D" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Sabersky", "given_name": "Rolf H.", "clpid": "Sabersky-R-H" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Part I
\r\n\r\nFailure mechanisms of materials under very high strains experienced at and ahead of the crack tip (such as the formation, growth and interaction of microvoids in ductile materials, microcracks in brittle solids or crazes in polymers and adhesives) are represented by one-dimensional, nonlinear stress-strain relations possessing different post-yield softening (unloading) behaviors. These reflect different ways by which the material loses capacity to carry load up to fracture. A DCB type specimen is considered in this study. The nonlinear material is confined to a thin strip between the two elastic beams loaded by a wedge. The problem is first treated as a beam on a nonlinear foundation for which the pertinent equation is solved numerically as a two-point boundary value problem for both the stationary and the quasi-statically propagating crack. A finite element model is then used to model the problem in more detail to assess the adequacy of the beam model for reduction of the experimental data.
\r\n\r\nIt is found that the energy release rate G = 2(\u03b3b) = {3P2\u03b4)2/EI}1/3 derived by assuming the built-in conditions at the crack tip could be used to calculate the fracture (surface) energy more accurately and conveniently than the conventional scheme even in cases where the built-in assumption is invalid. Results for the deformations of the beam prior to or during crack growth suggest ways to approximately characterize the complete material stress-strain behavior, including loading and strain-softening characteristics.
\r\n\r\nPart II
\r\n\r\nThis study investigates the effects of nonlinear fibril behavior on the mechanics of craze and crack growth. We developed a numerical method for determining the equilibrium shape of a craze in an infinite elastic plane whose fibrils exhibit very general nonlinear force-displacement (P-V) behavior, including strain softening characteristics.
\r\n\r\nThe problem formulation is based on the superposition of the relevant elasticity Green's function. The solution is effected by using Picard's successive approximation iterative scheme. Both field equilibrium and the Barenblatt condition for vanishing stress and strain singularities (KI = 0) are satisfied simultaneously, rendering the craze tip profile cusp-like as observed experimentally. The formulation allows the stress distribution profile and the corresponding P-V relation to be computed from experimentally measured craze/crack contours with certain advantages over the methods proposed to date.
\r\n\r\nFurther numerical investigations indicate that only certain classes of the fibril P-V relations are consistent with realistic craze profiles, i.e., profiles with nonnegative displacements at all points. In addition, it is found that for a given P-V relation, nontrivial solutions -- the 'trivial solution' refers to the solution corresponding to a fully closed craze, i.e., zero displacements throughout or, simply: no craze exists -- exist only for certain ranges of craze lengths depending on the P-V characteristics under consideration.
\r\n\r\nQuasi-static growth of a craze with a central crack is analyzed for different nonlinear P-V relations for the craze fibrils. A 'critical crack tip opening displacement' (CTOD) or more precisely, 'critical fibril extension' is employed as the criterion for fracture. The P-V relation is further assumed to be invariant with respect to the craze and crack lengths. For comparison purposes, the results are compared and contrasted with the Dugdale model. The craze zone size and the energy dissipation rate are shown to approach asymptotic values in the limit of long cracks.
\r\n\r\nThe problem of craze growth from a precut crack under increasing far-field loading is then studied. Instability is shown to occur in the case where the P-V relation is monotonically softening: The crack could start to grow unstably before the crack tip opening displacement reaches its critical value.
", "doi": "10.7907/sqjv-pf95", "publication_date": "1986", "thesis_type": "phd", "thesis_year": "1986" }, { "id": "thesis:11441", "collection": "thesis", "collection_id": "11441", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04052019-121036771", "type": "thesis", "title": "Cumulative Damage of Structures Subjected to Response Spectrum Consistent Random Processes", "author": [ { "family_name": "Jeong", "given_name": "Garrett Duane", "clpid": "Jeong-Garrett-Duane" } ], "thesis_advisor": [ { "family_name": "Iwan", "given_name": "Wilfred D.", "clpid": "Iwan-W-D" } ], "thesis_committee": [ { "family_name": "Iwan", "given_name": "Wilfred D.", "clpid": "Iwan-W-D" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Hall", "given_name": "John F.", "orcid": "0000-0002-7863-5060", "clpid": "Hall-J-F" }, { "family_name": "Caughey", "given_name": "Thomas Kirk", "clpid": "Caughey-T-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "Earthquake Engineering Research Laboratory" }, { "literal": "div_eng" } ], "abstract": "A theoretical analysis of the effect of duration on the damage of structures subjected to earthquakes is presented. Earthquake excitation is modeled as a nonstationary random process. Estimates of the first-passage probability of a simple oscillator are employed to choose modulated Gaussian random processes consistent with a prescribed response spectrum. The response spectrum is assumed to be specified independent of the duration. Expressions for the mean damage of a structure are derived using an approach similar to the Miner-Palmgren rule for failure caused by cyclic loads. The expected damage expressions are then evaluated for a structure subjected to modulated Gaussian random processes of varying duration.
\r\n\r\nTwo types of structures are examined: a steel structure and a reinforced concrete structure. Results are presented for systems with constant linear stiffness and a particular form of softening behavior. The nonlinearity of the softening system is accounted for by statistical linearization. The level of expected damage is found to be a strong function of both the duration of the excitation and the ductility of the response.
", "doi": "10.7907/97b3-2427", "publication_date": "1985", "thesis_type": "phd", "thesis_year": "1985" }, { "id": "thesis:7555", "collection": "thesis", "collection_id": "7555", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03262013-085556300", "primary_object_url": { "basename": "Stanley_ea_1985.pdf", "content": "final", "filesize": 17795444, "license": "other", "mime_type": "application/pdf", "url": "/7555/1/Stanley_ea_1985.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Diffusion in Glassy Polymers", "author": [ { "family_name": "Stanley", "given_name": "Elizabeth Ann", "clpid": "Stanley-Elizabeth-Ann" } ], "thesis_advisor": [ { "family_name": "Cohen", "given_name": "Donald S.", "clpid": "Cohen-D-S" } ], "thesis_committee": [ { "family_name": "Kreiss", "given_name": "Heinz-Otto", "clpid": "Kreiss-H-O" }, { "family_name": "Saffman", "given_name": "Philip G.", "clpid": "Saffman-P-G" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Cohen", "given_name": "Donald S.", "clpid": "Cohen-D-S" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Fluid diffusion in glassy polymers proceeds in ways that are not explained by the standard diffusion model. Although the reasons for the anomalous effects are not known, much of the observed behavior is attributed to the long times that polymers below their glass transition temperature take to adjust to changes in their condition. The slow internal relaxations of the polymer chains ensure that the material properties are history-dependent, and also allow both local inhomogeneities and differential swelling to occur. Two models are developed in this thesis with the intent of accounting for these effects in the diffusion process.
\r\n\r\nIn Part I, a model is developed to account for both the history dependence of the glassy polymer, and the dual sorption which occurs when gas molecules are immobilized by the local heterogeneities. A preliminary study of a special case of this model is conducted, showing the existence of travelling wave solutions and using perturbation techniques to investigate the effect of generalized diffusion mechanisms on their form. An integral averaging method is used to estimate the penetrant front position.
\r\n\r\nIn Part II, a model is developed for particle diffusion along with displacements in isotropic viscoelastic materials. The nonlinear dependence of the materials on the fluid concentration is taken into account, while pure displacements are assumed to remain in the range of linear viscoelasticity. A fairly general model is obtained for three-dimensional irrotational movements, with the development of the model being based on the assumptions of irreversible thermodynamics. With the help of some dimensional analysis, this model is simplified to a version which is proposed to be studied for Case II behavior.
", "doi": "10.7907/pjzx-hb67", "publication_date": "1985", "thesis_type": "phd", "thesis_year": "1985" }, { "id": "thesis:543", "collection": "thesis", "collection_id": "543", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02082005-114440", "primary_object_url": { "basename": "Vahala_kj_1985.pdf", "content": "final", "filesize": 4779219, "license": "other", "mime_type": "application/pdf", "url": "/543/1/Vahala_kj_1985.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Dynamic and Spectral Features of Semiconductor Lasers", "author": [ { "family_name": "Vahala", "given_name": "Kerry John", "orcid": "0000-0003-1783-1380", "clpid": "Vahala-Kerry-John" } ], "thesis_advisor": [ { "family_name": "Yariv", "given_name": "Amnon", "clpid": "Yariv-A" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" } ], "thesis_committee": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Sternberg", "given_name": "Eli", "clpid": "Sternberg-E" }, { "family_name": "Wu", "given_name": "Theodore Yao-tsu", "clpid": "Wu-T-Y-T" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" }, { "family_name": "Yariv", "given_name": "Amnon", "clpid": "Yariv-A" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis is divided into two main subject areas: the fluctuation properties of state of the art semiconductor lasers and the improvement of modulation and fluctuation properties in these devices through a technique called detuned loading.
\r\n\r\nThe discussion of fluctuations in lasers is a topic as old as the device itself, and much of the pioneering work in this field was done in the sixties. Surprisingly, however, several new chapters in this field are being written, because of certain pecularities only recently observed in semiconductor lasers. Chapters 2 and 3 of this thesis will consider these pecularities, which, as it turns out, are quite important in many potential system applications of these devices.
\r\n\r\nOne of the driving forces behind the development of semiconductor lasers has been their application as sources and local oscillators in optical communication systems. In general, such applications require lasers which have low phase and intensity noise, and which can be modulated at high data rates. As is often the case, these requirements are to a certain extent mutually exclusive. Chapter 4 introduces a technique which is an exception to this rule. It relies upon the semiconductor laser physics which produces the fluctuation abnormalities discussed in Chapters 2 and 3. The technique can be used to improve modulation speed while simultaneously reducing noise as compared to the conventional device.
", "doi": "10.7907/H7JA-K512", "publication_date": "1985", "thesis_type": "phd", "thesis_year": "1985" }, { "id": "thesis:1050", "collection": "thesis", "collection_id": "1050", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03212008-094413", "primary_object_url": { "basename": "Wakugawa_jm_1985.pdf", "content": "final", "filesize": 2552580, "license": "other", "mime_type": "application/pdf", "url": "/1050/1/Wakugawa_jm_1985.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "On the Existence and Uniqueness of the Solution to the Small-Scale Nonlinear Anti-Plane Shear Crack Problem in Finite Elastostatics", "author": [ { "family_name": "Wakugawa", "given_name": "Jason Masao", "clpid": "Wakugawa-Jason-Masao" } ], "thesis_advisor": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" } ], "thesis_committee": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Sternberg", "given_name": "Eli", "clpid": "Sternberg-E" }, { "family_name": "Wu", "given_name": "Theodore Yao-tsu", "clpid": "Wu-T-Y-T" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "orcid": "0000-0003-0559-0794", "clpid": "Rosakis-A-J" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis addresses the issue of existence and uniqueness of the solution to the small-scale nonlinear anti-plane shear crack problem in finite elastostatics. The hodograph transformation, commonly used in the theory of compressible fluid flows, plays an essential role. Existence is established by exhibiting an exact closed form solution, constructed via the hodograph transformation. Uniqueness is established by first proving the uniqueness of the solution to a related boundary-value problem, which is linear by virtue of the hodograph transformation, and then examining the implications of this result on the original problem. The possibility of making some of the conditions imposed on the solution to the small-scale nonlinear crack problem less restrictive is then investigated. This leads to several further results, including estimates of the nonvanishing shear stress component of the stress tensor along the crack faces.
", "doi": "10.7907/dfcw-j110", "publication_date": "1985", "thesis_type": "phd", "thesis_year": "1985" }, { "id": "thesis:2601", "collection": "thesis", "collection_id": "2601", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06152004-145433", "primary_object_url": { "basename": "Moonan_wk_1984.pdf", "content": "final", "filesize": 3844779, "license": "other", "mime_type": "application/pdf", "url": "/2601/1/Moonan_wk_1984.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Superposition of the Effects of Time, Temperature, and Pressure in Polymetric Materials", "author": [ { "family_name": "Moonan", "given_name": "William Kevin", "clpid": "Moonan-William-Kevin" } ], "thesis_advisor": [ { "family_name": "Tschoegl", "given_name": "Nicholas W.", "clpid": "Tschoegl-N-W" } ], "thesis_committee": [ { "family_name": "Tschoegl", "given_name": "Nicholas W.", "clpid": "Tschoegl-N-W" }, { "family_name": "Shair", "given_name": "Fredrick H.", "clpid": "Shair-F-H" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Chang", "given_name": "W. V.", "clpid": "Chang-W-V" } ], "local_group": [ { "literal": "div_chem" } ], "abstract": "Stress relaxation was studied in torsion under superposed hydrostatic pressure using a newly constructed device. Combination of these measurements with the determination of expansivities and compressibilities and with earlier measurements of stress relaxation in tension leads to verification of the description of the combined effects of temperature and pressure on shear relaxation by an extension of the classical free volume theory. The predictions of an extension of the Adam-Gibbs theory to the pressure domain are also compared with the data. This theory is not as successful in predicting superposition. The data are also examined in light of an adaptation of the Simha-Somcynsky equation of state to the free volume theory. Prediction of the effect of pressure by the theory requires a single adjustable parameter. It was possible to calculate the free volume parameters of several polymers without use of information from high pressure experiments.
", "doi": "10.7907/e970-mg79", "publication_date": "1984", "thesis_type": "phd", "thesis_year": "1984" }, { "id": "thesis:11308", "collection": "thesis", "collection_id": "11308", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12122018-092221333", "primary_object_url": { "basename": "Huang_M_1984.pdf", "content": "final", "filesize": 79571343, "license": "other", "mime_type": "application/pdf", "url": "/11308/1/Huang_M_1984.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Investigation of Local Geology Effects on Strong Earthquake Ground Motions", "author": [ { "family_name": "Huang", "given_name": "Moh-jiann", "clpid": "Huang-Moh-jiann" } ], "thesis_advisor": [ { "family_name": "Housner", "given_name": "George W.", "clpid": "Housner-G-W" } ], "thesis_committee": [ { "family_name": "Housner", "given_name": "George W.", "clpid": "Housner-G-W" }, { "family_name": "Jennings", "given_name": "Paul C.", "clpid": "Jennings-P-C" }, { "family_name": "Scott", "given_name": "Ronald F.", "clpid": "Scott-R-F" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Kanamori", "given_name": "Hiroo", "orcid": "0000-0001-8219-9428", "clpid": "Kanamori-H" } ], "local_group": [ { "literal": "Earthquake Engineering Research Laboratory" }, { "literal": "div_eng" } ], "abstract": "Accelerograms recorded at four stations in the Pasadena area during the 1971 San Fernando, the 1970 Lytle Creek, the 1968 Borrego Mountain and the 1952 Kern County earthquakes are analyzed to investigate local geology effects on strong earthquake ground motions. Spatial variations of the ground motions at two nearby stations are also investigated. It is found that the ground motions in this area caused by the local geology effects depend on the 3- dimensional configuration of the local geology and the direction of arriving seismic waves. Local geology effects are less evident on the leading portions of the accelerograms than the trailing portions, indicating the effects of local geology on surface wave propagations. Comparison of the Fourier amplitudes of the motions recorded at the same station during different earthquakes shows that there are no significant spectral peaks can be identified as site periods. Hence, it is not appropriate to characterize such local sites by a site period.
\r\n\r\nA two-dimensional model considering inclined propagating P and S waves in a horizontal-layered structure, which is more realistic and closer to the actual seismic environment within a local geology than the one-dimensional model of vertically propagating waves, is studied. The transfer functions between the free surface and the half-space outcrop for a single incident P, SV or SH wave from the half-space at an incident angle are defined and derived by a matrix method. Two numerical examples are given to demonstrate the effects of incident angle and material damping on the transfer function. It is found that the transfer function between the free surf ace motions and the bedrock outcrop motions to multiple incident waves having different amplitudes, angles and arrival times is quite different from that for a single incident wave. Completely satisfactory results cannot be expected when using the analytical model for evaluating the local geology effects on the motions during a nearby shallow-focus earthquake for which seismic waves emitted from different parts of the fault will approach the bedrock from different directions and at different angles.
\r\n\r\nThe analytical model for assessing local geology effects is evaluated in the light of the data recorded at stations in the Pasadena area during the 1971 San Fernando earthquake. The observed site transfer functions between the alluvial and the rock sites are obtained and compared with the computed results from a two-dimensional model with 7-layers overlying a half-space. Values of model parameters are optimally adjusted to give a best least-squares fit between computed and observed amplitude ratios. It is concluded that the analytical model oversimplifies the local geological structure in the Pasadena area and the actual seismic environment in the area during the 1971 San Fernando earthquake.
\r\n\r\nIt is concluded from these studies that the effects of the source mechanism and the seismic wave travel paths upon the site ground motion can be comparable to the effect of the local geology. The characteristics of the source mechanism, such as type of faulting, direction of fault-slip propagation, nature of stress drop across the fault surface, orientation of fault, depth beneath ground surface, etc., can appreciably influence the ground motion at the site. Also, the travel path can have a significant effect through influencing the types of waves that reach the site, and the directions of approach of the waves. The results of this study indicate that a better understanding of the spatial variation of ground motions, of the role played by different types of waves and their contributions to an accelerogram, and of the propagation directions of the waves is needed for assessing local geology effects on earthquake ground motions. A local array is desirable to provide data for giving a reasonably complete picture of the nature of ground motions in a local area.
", "doi": "10.7907/pm3k-w086", "publication_date": "1984", "thesis_type": "phd", "thesis_year": "1984" }, { "id": "thesis:1821", "collection": "thesis", "collection_id": "1821", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05152007-111322", "primary_object_url": { "basename": "Heymans_lj_1983.pdf", "content": "final", "filesize": 17479717, "license": "other", "mime_type": "application/pdf", "url": "/1821/1/Heymans_lj_1983.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "An Engineering Analysis of Polymer Film Adhesion to Rigid Substrates", "author": [ { "family_name": "Heymans", "given_name": "Luc J.", "clpid": "Heymans-Luc-J" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Babcock", "given_name": "Charles D.", "clpid": "Babcock-C-D" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Tschoegl", "given_name": "Nicholas W.", "clpid": "Tschoegl-N-W" }, { "family_name": "Fornberg", "given_name": "Bengt", "clpid": "Fornberg-Bengt" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "An important source of interface fracture contributing to adhesive failure in a bimaterial sandwich, consisting of a rigid substrate and a viscoelastic encapsulant material, arises from residual stresses. The encapsulant is often deposited on the substrate above its glass transition temperature region but used below this temperature range. In order to determine the magnitude of the residual stresses a viscoelastic stress analysis of a bimaterial sandwich is carried out, taking into account the time-dependent material properties of the polymeric layer and the environmental \"loading\" conditions. The theoretical analysis is paralleled by an experimental examination of the time-dependent out-of-plane deformation of thin, circular sandwiches.
\r\n\r\nPolyvinyl acetate was chosen as a model material exhibiting significant viscoelastic effects under room test conditions. Therefore the pertinent physical and mechanical properties of PYAc are determined; these include the thermal coefficient of expansion, the shear creep compliance and the relaxation modulus. In the experimental work BK-7 glass is taken as the \"rigid\" substrate. The measurements connected to the stress analysis are monitored with laser interferometry (Newton's rings). A comparison between theory and experiment completes the viscoelastic stress analysis.
\r\n\r\nIn the second part of this study time dependent adhesive failure of rubbery materials is investigated. Polymeric materials are being used increasingly for a wide variety of applications. Some of these materials are applied as protective layers to isolate their substrates from a hostile environment. Others achieve remarkable structural bond strengths thereby displacing the traditional mechanical fasteners like bolts and rivets. If one wants to investigate the long time integrity of a layer assembly the time dependence of the material properties of the adhesives needs to be carefully analyzed. This time dependence is also reflected in the energy required to create new surfaces as interfacial debonding proceeds the adhesive fracture energy is one of the dominant parameters in time dependent adhesive failure. In our investigation it is characterized through peel testing.
\r\n\r\nWith the knowledge of the pertinent material properties as well as of the adhesive fracture energy, we then proceed to formulate a criterion for continuing interfacial crack propagation. The analysis is carried out for elastic solids, with the effect of viscoelastic behavior incorporated later on. Debond tests provide a way to check how well the theoretical predictions correspond to experimental debond results.
", "doi": "10.7907/3GQW-YP87", "publication_date": "1983", "thesis_type": "phd", "thesis_year": "1983" }, { "id": "thesis:3617", "collection": "thesis", "collection_id": "3617", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09182006-085128", "type": "thesis", "title": "Two Problems in Plane Finite Elastostatics", "author": [ { "family_name": "Connolly", "given_name": "Jeremiah Finbarr", "clpid": "Connolly-Jeremiah-Finbarr" } ], "thesis_advisor": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" } ], "thesis_committee": [ { "family_name": "Sternberg", "given_name": "Eli", "clpid": "Sternberg-E" }, { "family_name": "Cohen", "given_name": "Donald S.", "clpid": "Cohen-D-S" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Miklowitz", "given_name": "Julius", "clpid": "Miklowitz-J" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In this paper the fully nonlinear equilibrium theory of homogeneous and isotropic incompressible elastic solids is used to study the elastostatic fields in plane strain near the point of application of a concentrated force on a deformed half plane and near the vertex of a circular sector whose plane deformed faces are subjected to prescribed tractions.
\r\n\r\nIn the concentrated force problem, restricting only the form of the elastic potential at large extensional deformations, it is shown that, for materials which \"harden\" in simple shear, the displacement is bounded at the point of application of the load. This is not the case for materials which \"soften\" in shear. Estimates of the true stress tensor near the singular point are given.
\r\n\r\nIn the sector problem, for a class of the materials mentioned, the deformation and stress field near the vertex of the deformed cross-section are derived and discussed.
\r\n", "doi": "10.7907/yhjy-mb48", "publication_date": "1983", "thesis_type": "phd", "thesis_year": "1983" }, { "id": "thesis:3336", "collection": "thesis", "collection_id": "3336", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09052006-154543", "primary_object_url": { "basename": "Pence_tj_1983.pdf", "content": "final", "filesize": 4399827, "license": "other", "mime_type": "application/pdf", "url": "/3336/1/Pence_tj_1983.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "The Emergence and Propagation of a Phase Boundary in an Elastic Bar", "author": [ { "family_name": "Pence", "given_name": "Thomas James", "orcid": "0000-0002-5136-3384", "clpid": "Pence-Thomas-James" } ], "thesis_advisor": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" } ], "thesis_committee": [ { "family_name": "Sternberg", "given_name": "Eli", "clpid": "Sternberg-E" }, { "family_name": "Cohen", "given_name": "Donald S.", "clpid": "Cohen-D-S" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This dissertation is concerned with the dynamical analysis of an elastic bar whose stress-strain relation is not monotonic. Sufficiently large applied loads then require the strain to jump from one ascending branch of the stress-strain curve to another such branch. For a special class of these materials, a nonlinear initial-boundary value problem in one-dimensional elasticity is considered for a semi-infinite bar whose end is subjected to either a monotonically increasing prescribed traction or a monotonically increasing prescribed displacement. If the stress at the end of the bar exceeds the value of the stress at any local maximum of the stress-strain curve a strain discontinuity or \"phase boundary\" emerges at the end of the bar and subsequently propagates into the interior. For classically smooth solutions away from the phase boundary, the problem is reducible to a pair of differential-delay equations for two unknown functions of a single variable. The first of these two functions gives the location of the phase boundary, while the second characterizes the dynamical fields in the high-strain phase of the material. In these equations the former function occurs in the argument of the latter, so that the delays in the functional equations are unknown. A short-time analysis of this system provides an asymptotic description of the emergence and initial propagation of the phase boundary. For large-times, a different analysis indicates that the phase boundary velocity approaches a constant which depends on material properties and on the ultimate level reached by the applied load as well. Higher order corrections depend on the detailed way in which the load is applied. Estimates for the various dynamical field quantities are given and a priori conditions are deduced which determine whether the phase boundary eventually becomes the leading disturbance.
", "doi": "10.7907/85D8-5A19", "publication_date": "1983", "thesis_type": "phd", "thesis_year": "1983" }, { "id": "thesis:4316", "collection": "thesis", "collection_id": "4316", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10302003-104624", "type": "thesis", "title": "An Experimental Investigation into the Mechanics of Dynamic Fracture", "author": [ { "family_name": "Ravi-Chandar", "given_name": "K.", "clpid": "Ravi-Chandar-K" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Babcock", "given_name": "Charles D.", "clpid": "Babcock-C-D" }, { "family_name": "Housner", "given_name": "George W.", "clpid": "Housner-G-W" }, { "family_name": "Kanamori", "given_name": "Hiroo", "orcid": "0000-0001-8219-9428", "clpid": "Kanamori-H" }, { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Current theories of dynamic fracture are based on elastodynamic analyses of mathematically sharp plane cracks and as such do not explain the observed terminal velocities or the phenomenon of crack branching satisfactorily. The present investigation addresses the above problems by using both microscopic and macroscopic interpretations. The experimental scheme that is used in this investigation is the configuration of a pressure loaded semi-infinite crack in an infinite medium. The loading is achieved through an electromagnetic device which provides highly repeatable loading. The method of caustics is used in conjunction with a high speed camera to obtain the time histories of the crack tip stress intensity factor and the crack position.
\r\n\r\nThe problems of crack initiation and crack arrest are explored. The stress intensity factor at initiation is found to be independent of the rate of applied loading when the latter is below about 104MPA/sec, but the initiation stress intensity factor increases considerably when the loading rate is increased further. Crack arrest is obtained in large specimen by using very low energy loading pulses. It was found that the stress intensity factor at crack arrest was constant and also that, within the time resolution of the high speed camera (5 \u03bcsec), the crack comes to a stop abruptly.
\r\n\r\nThe crack propagation and branching aspects were investigated first using post-mortem analysis of the fracture surfaces and high speed photomicrography to get an idea of the microscopic processes that occur in the fracure process. From this investigation, it was found that crack propagation involving high stress intensity factor and high velocity situations takes place by the growth and interaction of microcracks, due to the voids present in the material. A surprising result of this investigation was that cracks propagated at a constant velocity, although the stress intensity factor varied. Current theories of dynamic fracture cannot explain such behaviour. The crack branching process was found to be a continuous process arising out of propagation along a straight line. High speed photomicrographs of the branching process indicated the presence of a number of part-through attempted branches that interact with one another and finally the successful emergence of a few full fledged branches.
\r\n\r\nThe microscopic observations on the crack propagation and branching process leads to a new interpretation of dynamic fracture that attempts to qualitatively explain the constancy of the velocity of propagation, the terminal velocity and crack branching. The crack branching mechanism is a logical continuation of the mechanism for crack propagation.
\r\n", "doi": "10.7907/ZSX5-Q128", "publication_date": "1982", "thesis_type": "phd", "thesis_year": "1982" }, { "id": "thesis:1366", "collection": "thesis", "collection_id": "1366", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-04132004-084629", "type": "thesis", "title": "The Growth of Impact Damage in Compressively Loaded Laminates", "author": [ { "family_name": "Chai", "given_name": "Herzl", "clpid": "Chai-Herzl" } ], "thesis_advisor": [ { "family_name": "Babcock", "given_name": "Charles D.", "clpid": "Babcock-C-D" } ], "thesis_committee": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Miklowitz", "given_name": "Julius", "clpid": "Miklowitz-J" }, { "family_name": "Housner", "given_name": "George W.", "clpid": "Housner-G-W" }, { "family_name": "Corcoran", "given_name": "William Harrison", "clpid": "Corcoran-W-H" }, { "family_name": "Babcock", "given_name": "Charles D.", "clpid": "Babcock-C-D" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The thesis is divided into two chapters. The first chapter describes an experimental program carried out to determine the phenomenological aspects of composite panel failure (graphite/epoxy laminate) under simultaneous in-plane compression and low velocity transverse impact (0-250 ft/sec). High-speed photography and the shadow moire technique were used to record a full-field surface deformation history of the impacted plate. The information gained from these records, supplemented by plate sectioning and observation for interior damage, has shown that the predominant failure mechanism is the coupled effect of delamination and buckling.
\r\n\r\nIn chapter 2, this process of failure is modelled by delamination buckling wherein the local delamination stability, growth or arrest are governed by an energy release rate criterion. Various degrees of problem simplifications are employed, starting with a one-dimensional, delaminated-beam model. In the most sophisticated treatment, it is assumed that the damaged area has an elliptical shape. It was found that this model is capable of describing the growth conditions and the growth behavior of the damaged area. It was also found that the predominant parameter controlling delamination growth or arrest is the fracture energy associated with delamination.
\r\n\r\nIn the appendix at the end of this work, the fracture energy of a composite laminate is considered. A modified cleavage analysis is developed, and is applied to evaluate this quantity for two composite materials of current interest. The test results are then examined with reference to impact damage tolerance data available for these materials. A viscoelastic characterization of the two resins used to fabricate the two composites mentioned above is also provided. The results of these measurements are then examined with reference to long-term applicability of the matrix material.
", "doi": "10.7907/58D3-ZZ41", "publication_date": "1982", "thesis_type": "phd", "thesis_year": "1982" }, { "id": "thesis:3228", "collection": "thesis", "collection_id": "3228", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-08262005-154143", "type": "thesis", "title": "The Coherent Structure of Turbulent Mixing Layers. I. Similarity of the Primary Vortex Structure. II. Secondary Streamwise Vortex Structure", "author": [ { "family_name": "Bernal", "given_name": "Luis Paulino", "clpid": "Bernal-Luis-Paulino" } ], "thesis_advisor": [ { "family_name": "Roshko", "given_name": "Anatol", "clpid": "Roshko-A" } ], "thesis_committee": [ { "family_name": "Roshko", "given_name": "Anatol", "clpid": "Roshko-A" }, { "family_name": "Coles", "given_name": "Donald Earl", "clpid": "Coles-D-E" }, { "family_name": "Dimotakis", "given_name": "Paul E.", "clpid": "Dimotakis-P-E" }, { "family_name": "Ingersoll", "given_name": "Andrew P.", "orcid": "0000-0002-2035-9198", "clpid": "Ingersoll-A-P" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Saffman", "given_name": "Philip G.", "clpid": "Saffman-P-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The primary spanwise organized vortex structure and the secondary streamwise vortex structure of turbulent mixing layers have been investigated. Flow visualization motion pictures of a constant density mixing layer were used to measure the properties of the large scale vortices. It was found that after an initial transition region mean properties of the large scale vortices reach the expected linear growth with downstream distance required by similarity. In the self-similar region, the vortex core area and visual thickness increase continuously during its life-span.
\r\n\r\nA theoretical model of probability distribution function for the large-scale vortex circulation was developed. This distribution is found to be lognormal and to have a standard deviation, normalized with the mean of 0.28. From this model the mean life-span of the vortices could also be obtained and was found to be 0.67 times the mean life-span position.
\r\n\r\nThe streamwise streak pattern observed by Konrad (1976) and Breidenthal (1978) in plan-view pictures of the mixing layer was investigated, using flow visualization and spanwise concentration measurements. It was confirmed that this pattern is the result of a secondary vortex structure dominated by streamwise, counterrotating vortices. A detailed description of its spatial relation to the primary, spanwise vortex structure is presented. From time average flow pictures, the onset position and initial scale of the secondary structures were determined. From concentration measurements, spanwise variations in mean properties, resulting from the secondary structure, were found. This also showed an increase of the spanwise scale with downstream distance and the existence of the streamwise vortices in the fully developed turbulent region. In this region the mean spacing is found approximately equal to the vorticity thickness.
", "doi": "10.7907/Z52D-SA34", "publication_date": "1981", "thesis_type": "phd", "thesis_year": "1981" }, { "id": "thesis:3993", "collection": "thesis", "collection_id": "3993", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10092006-080016", "type": "thesis", "title": "An Experimental Study of a Turbulent Vortex Ring", "author": [ { "family_name": "Glezer", "given_name": "Ari", "clpid": "Glezer-Ari" } ], "thesis_advisor": [ { "family_name": "Coles", "given_name": "Donald Earl", "clpid": "Coles-D-E" } ], "thesis_committee": [ { "family_name": "Coles", "given_name": "Donald Earl", "clpid": "Coles-D-E" }, { "family_name": "Roshko", "given_name": "Anatol", "clpid": "Roshko-A" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Kubota", "given_name": "Toshi", "clpid": "Kubota-T" }, { "family_name": "Saffman", "given_name": "Philip G.", "clpid": "Saffman-P-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "An apparatus has been constructed to study the formation and dynamics of vortex rings. The rings are formed in water by a momentary jet discharge downward into a glass tank from an orifice in a submerged plate. The necessary impulse is provided by a pressurized reservoir and is controlled by a fast programmable solenoid valve.
\r\n\r\nA particular turbulent vortex ring with a relatively thin core was chosen for detailed study. The velocity field of this ring was measured at numerous locations in the tank, using a two-channel laser-Doppler velocimeter with digital tracking electronics. Beamsplitting and frequency-shifting were accomplished by two partially-overlapping radial phase gratings driven by hysteresis-synchronous motors.
\r\n\r\nThe main aim of the research was to verify the similarity properties of the turbulent vortex ring and to determine mean particle paths and mean vorticity contours in the appropriate nonsteady similarity coordinates x/t\u00bc, r/t\u00bc. Some novel pattern-recognition techniques were developed to overcome the problem of dispersion in the trajectories of individual vortices.
", "doi": "10.7907/RC4P-DA56", "publication_date": "1981", "thesis_type": "phd", "thesis_year": "1981" }, { "id": "thesis:10745", "collection": "thesis", "collection_id": "10745", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03012018-170438409", "primary_object_url": { "basename": "Abeyaratne_RC_1979.pdf", "content": "final", "filesize": 42465129, "license": "other", "mime_type": "application/pdf", "url": "/10745/1/Abeyaratne_RC_1979.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Discontinuous Deformation Gradients in Plane Finite Elastostatics of Incompressible Materials. (I) General Considerations. (II) An Example", "author": [ { "family_name": "Abeyaratne", "given_name": "Rohan Chandra", "clpid": "Abeyaratne-Rohan-Chandra" } ], "thesis_advisor": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" } ], "thesis_committee": [ { "family_name": "Knowles", "given_name": "James K.", "clpid": "Knowles-J-K" }, { "family_name": "Hughes", "given_name": "Thomas J. R.", "clpid": "Hughes-T-J-R" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Sternberg", "given_name": "Eli", "clpid": "Sternberg-E" }, { "family_name": "Wu", "given_name": "Theodore Yao-tsu", "clpid": "Wu-T-Y-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This investigation is concerned with the possibility of the change of type of the differential equations governing finite plane elastostatics for incompressible elastic materials, and the related is sue of the existence of equilibrium fields with discontinuous deformation gradients. Explicit necessary and sufficient conditions on the deformation invariants and the material for the ellipticity of the plane displacement equations of equilibrium are established. The issue of the existence, locally, of \"elastostatic shocks\" -- elastostatic fields with continuous displacements and discontinuous deformation gradients -- is then investigated. It is shown that an elastostatic shock exists only if the governing field equations suffer a loss of ellipticity at some deformation. Conversely, if the governing field equations have lost ellipticity at a given deformation at some point, an elastostatic shock can exist, locally, at that point. The results obtained are valid for an arbitrary homogeneous, isotropic, incompressible, elastic material. In order to illustrate the occurrence of elastostatic shocks in a physical problem, a specific displacement boundary value problem is studied. Here, a particular class of isotropic, incompressible, elastic materials which allow for a loss of ellipticity is considered. It is shown that no solution which is smooth in the classical sense exists to this problem for certain ranges of the applied loading. Next, we admit solutions involving elastostatic shocks into the discussion and find that the problem may then be solved completely. When this is done, however, there results a lack of uniqueness of solutions to the boundary value problem. In order to resolve this non-uniqueness, dissipativity and stability are investigated.
", "doi": "10.7907/seym-cd95", "publication_date": "1979", "thesis_type": "phd", "thesis_year": "1979" } ]