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: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:6354", "collection": "thesis", "collection_id": "6354", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04262011-100757709", "primary_object_url": { "basename": "Sane_s_2001.pdf", "content": "final", "filesize": 50485649, "license": "other", "mime_type": "application/pdf", "url": "/6354/1/Sane_s_2001.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Time-dependent compressibility of poly (methyl methacrylate) (PMMA) : an experimental and molecular dynamics investigation", "author": [ { "family_name": "Sane", "given_name": "Sandeep Bhalchandra", "clpid": "Sane-S-B" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "clpid": "Bhattacharya-K" }, { "family_name": "Ortiz", "given_name": "Michael", "clpid": "Ortiz-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis contains three chapters, which describe different aspects of an investigation of the bulk response of Poly(Methyl Methacrylate) (PMMA). The first chapter describes the physical measurements by means of a Belcher/McKinney-type apparatus. Used earlier for the measurement of the bulk response of Poly(Vinyl Acetate), it was now adapted for making measurements at higher temperatures commensurate with the glass transition\r\ntemperature of PMMA. The dynamic bulk compliance of PMMA was measured at atmospheric pressure over a wide range of temperatures and frequencies, from which the master curves for the bulk compliance were generated by means of the time-temperature superposition principle. It was found that the extent of the transition ranges for the bulk and shear response were comparable. Comparison of the shift factors for bulk and shear responses supports the idea that different molecular mechanisms contribute to shear and\r\nbulk deformations.\r\n\r\nThe second chapter delineates molecular dynamics computations for the bulk response for a range of pressures and temperatures. The model(s) consisted of 2256 atoms\r\nformed into three polymer chains with fifty monomer units per chain per unit cell. The time scales accessed were limited to tens of pico seconds. It was found that, in addition to the typical energy minimization and temperature annealing cycles for establishing equilibrium models, it is advantageous to subject the model samples to a cycle of\r\nrelatively large pressures (GPa-range) for improving the equilibrium state. On comparing the computations with the experimentally determined \"glassy\" behavior, one finds that,\r\nalthough the computations were limited to small samples in a physical sense, the primary limitation rests in the very short times (pico seconds). The molecular dynamics computations do not model the physically observed temperature sensitivity of PMMA, even if one employs a hypothetical time-temperature shift to account for the large\r\ndifference in time scales between experiment and computation. The values computed by the molecular dynamics method do agree with the values measured at the coldest\r\ntemperature and at the highest frequency of one kiloHertz.\r\n\r\nThe third chapter draws on measurements of uniaxial, shear and Poisson response conducted previously in our laboratory. With the availability of four time or frequency-dependent material functions for the same material, the process of interconversion between different material functions was investigated. Computed material functions were\r\nevaluated against the direct experimental measurements and the limitations imposed on successful interconversion due to the experimental errors in the underlying physical data\r\nwere explored. Differences were observed that are larger than the experimental errors would suggest.\r\n", "doi": "10.7907/saw5-7p32", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "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:6138", "collection": "thesis", "collection_id": "6138", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10122010-134248309", "primary_object_url": { "basename": "Samudrala_o_2001.pdf", "content": "final", "filesize": 158367031, "license": "other", "mime_type": "application/pdf", "url": "/6138/1/Samudrala_o_2001.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Subsonic and intersonic crack growth along weak planes and bimaterial interfaces", "author": [ { "family_name": "Samudrala", "given_name": "Omprakash", "clpid": "Samudrala-Omprakash" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "A combined experimental and analytical study has been conducted to investigate the phenomena of intersonic crack propagation along weak planes in homogeneous solids and dissimilar material interfaces. A single edge notch/crack oriented along a weak plane in a brittle polymer or along a polymer/metal interface was loaded in shear by impacting the specimen with a high velocity projectile fired from a gas gun. Homalite-100 or PMMA was chosen for the polymer and 6061 Aluminum or 4340 steel was chosen for the metal. The stress field information around the propagating crack tip was recorded in real time by two different optical techniques which provide complimentary information - photoelasticity and coherent gradient sensing (CGS), in conjunction with high speed photography. Along weak planes in Homalite-100, dynamic shear cracks were observed to initiate and propagate at speeds exceeding the shear wave speed (c_s) of the polymer. The isochromatic fringe patterns reveal two distinct lines of strong stress field discontinuity (Mach waves) emanating from the crack tip. Intersonic cracks were observed to initially accelerate up to the longitudinal wave speed (c_l) of Homalite and thereafter slow down to propagate at a near constant velocity slightly above \u221a2c_s . A series of short secondary opening cracks parallel to each other and at a steep angle to the weak plane (\u2248 80\u00b0) were also observed to initiate behind the main intersonic crack tip. Motivated by the experimental observations, an asymptotic analysis was performed to obtain the near tip fields for an intersonically propagating steady state mode II crack with a finite sized shear cohesive zone in front of it. The cohesive shear stress was chosen to be either a constant or to depend linearly on the magnitude of the local slip rate. Decohesion was chosen to occur when the relative slip between the two cohesive surfaces reaches a material/interface specific critical value. Unlike the case of a point sized dissipative region, it is shown that with a finite cohesive zone, the dynamic energy release rate is finite through out the intersonic regime. The influence of crack plane shear strength and of the rate parameter on the crack propagation behavior is investigated. Isochromatic fringe patterns were constructed using the cohesive crack tip fields, which compare favorably with the experimentally observed fringe patterns, and an attempt is made to extract the relevant analytical parameters. Unlike for a mode-I crack, a cohesive stress distribution that decreases with the local slip rate is found to match the experimental observations. The rate parameter was extracted by fitting the secondary crack angle observed in the experiments to that predicted by the analytical solution based on a maximum principal stress fracture criterion. Edge notches/cracks on polymer/metal interfaces were loaded under different impact configurations and the conditions governing the attainment of intersonic crack growth along a bimaterial interface were investigated. High resolution isochromatic fringe patterns were obtained to study the nature of the crack tip fields during subsonic/intersonic transition. Careful observations of the transition of an interface crack into the intersonic regime showed the formation of crack face contact at speeds beyond c_R of the polymer. Subsequently, the contact zone is observed to expand in size, detach from the intersonic crack tip and finally vanish. The recorded isochromatic fringe patterns showed multiple Mach wave formation associated with such a scenario. Along PMMA/ Al and PMMA/steel bimaterial interfaces, dynamic cracks initiating from edge notches were observed to accelerate to speeds higher than c_l of PMMA (supersonic), almost reaching c_R of aluminum. The resulting crack growth was observed to be highly transient and the gradients of in-plane normal stress components were recorded using CGS interferometry. Motivated by the aforementioned experimental observations, an asymptotic analysis was performed to obtain the stress and deformation fields around a steadily propagating intersonic crack on an elastic-rigid interface with a finite zone of crack face frictional sliding contact located a finite distance behind the tip. A linear frictional contact model is adopted, wherein the shear stress is proportional to the normal stress through a constant, the coefficient of dynamic friction. Isochromatic fringe patterns predicted by the near-tip fields exhibit the essential features observed during the experiments. Frictional sliding contact is shown to be possible only for velocities between c_s and \u221a2c_s of the polymer. The relevant analytical parameters were predicted by comparing the model to the experimental isochromatic fringe patterns and comments are made on the merits of the model presented.", "doi": "10.7907/51b0-1c87", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:3721", "collection": "thesis", "collection_id": "3721", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09232004-112403", "primary_object_url": { "basename": "Patel_bb_1998.pdf", "content": "final", "filesize": 5629863, "license": "other", "mime_type": "application/pdf", "url": "/3721/1/Patel_bb_1998.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "An Experimental Investigation of Wave Propagation in Fractured Brittle Material", "author": [ { "family_name": "Patel", "given_name": "Bibhuti Bhusan", "clpid": "Patel-Bibhuti-Bhusan" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "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.\r\n\r\nAn experimental method for visualizing and analyzing the propagation of plate stress waves in a brittle plate is developed. A procedure has been developed to cast Break-Away glass (a low molecular weight polystyrene material) plate specimens in an open mold. The specimens are loaded with short duration (200 [...]s) stress pulses on one edge by an electromagnetic stress wave generator. The propagating stress waves generate out-of-plane deformations on the specimen surface, which are observed using Twyman-Green interferometry. The fringe patterns created by the propagating stress waves are captured using a high speed camera - pulsing laser combination at 4[...]s intervals.\r\n\r\nA generalized \"Fringe Analysis Procedure\" is developed to subtract the reference interferogram from the subsequent interferograms. The \"Fringe Analysis Procedure\" employs a fringe edge detection algorithm to obtain the sharp edge lines of the fringes in an interferogram. A digitizer is used to extract points on these edge lines and assign them fringe numbers. The \"griddata\" option in the commercial software \"Matlab\" is utilized to interpolate the deformation field on to the nodes of a uniform grid. The field values at these nodes in the reference image are then subtracted from corresponding values in the subsequent images to obtain the actual deformation patterns generated by the propagating stress waves. The \"Fringe Analysis Procedure\" has eliminated the subjective element introduced by human judgment in manual fringe tracing procedures.\r\n\r\nThe developed experimental method and the image analysis technique is used to investigate the propagation of stress waves in Break-Away glass plate specimens.", "doi": "10.7907/w1t3-1f55", "publication_date": "1998", "thesis_type": "engd", "thesis_year": "1998" }, { "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:4436", "collection": "thesis", "collection_id": "4436", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11072003-103136", "primary_object_url": { "basename": "Lu_h_1997.pdf", "content": "final", "filesize": 5463156, "license": "other", "mime_type": "application/pdf", "url": "/4436/1/Lu_h_1997.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Nonlinear thermo-mechanical behavior of polymers under multiaxial loading", "author": [ { "family_name": "Lu", "given_name": "Hongbing", "clpid": "Lu-H" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "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 order to better understand phenomena related to \"yield-like\" behavior of polymers, the nonlinear thermo-mechanical behavior of Polymethyl Methacrylate (PMMA) under combined axial (tension, compression) and shear stress states (torsion) is investigated on thin walled cylindrical specimens at temperatures between 22[degrees] and 100[degrees]C. A non-contact measurement technique based on digital image correlation method is developed to determine the surface deformations on curved cylindrical specimens at various temperatures, including temperatures close to the glass transition when the specimens become soft. The study indicates that in contrast to tile mutual independence of shear and dilatational response under conditions appropriate for linearized viscoelasticity, one observes an increasingly strong coupling between all deformation or stress invariants as assessed in creep experiments for strains in excess of' 0.4%. While shear stresses alone elicit nonlinear response in creep (rates) as \"intrinsically\" nonlinear shear response, the superposition of small positive dilatation accelerates shear deformations while negative dilatation retards it in quantitative agreement with free volume arguments. Passing below the glass transition still produces greater creep acceleration from positive dilatation than from a decrease in specific volume, but the dominance of the intrinsic shear nonlinearity vis-a-vis the dilatational ilifluence increases below [...] as the temperature drops. A constitutive model based oil free volume consideration is modified from the models proposed by Losi and Knauss (1992). Comparison of experimental results with model prediction indicates a good qualitative agreement, and reasonably good quantitative agreement.", "doi": "10.7907/SH42-W153", "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:67", "collection": "thesis", "collection_id": "67", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01082008-095104", "primary_object_url": { "basename": "Chea_l_1997.pdf", "content": "final", "filesize": 3638281, "license": "other", "mime_type": "application/pdf", "url": "/67/1/Chea_l_1997.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Finite Element Simulation and Analysis of Local Stress Concentration in Polymers with a Nonlinear Viscoelastic Constitutive Model", "author": [ { "family_name": "Chea", "given_name": "Limdara Ong", "clpid": "Chea-Limdara-Ong" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "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.\r\n\r\nGiven a nonlinear viscoelastic (NLVE) constitutive model for a polymer, this numerical study aims at simulating local stress concentrations in a boundary value problem with a corner stress singularity. A rectangular sample of Polyvinyl Acetate (PVAc)-like cross-linked polymer clamped by two metallic rigid grips and subjected to a compression and tension load is numerically simulated.\r\n\r\nA modified version of the finite element code FEAP, that incorporated a NLVE model based on the free volume theory, was used. First, the program was validated by comparing numerical and analytical results. Two simple mechanical tests (a uniaxial and a simple shear test) were performed on a Standard Linear Solid material model, using a linear viscoelastic (LVE) constitutive model. The LVE model was \r\nobtained by setting the proportionality coefficient [...] to zero in the free volume theory equations. Second, the LVE model was used on the corner singularity boundary value problem for three material models with different bulk relaxation functions K(t). The time-dependent stress field distribution was investigated using two sets of plots: the stress distribution contour plots and the stress time curves. Third, using the NLVE constitutive model, compression and tension cases were compared using the stress results (normal stress [...] and shear stress [...]). These two cases assessed the effect of the creep retardation-creep acceleration phenomena.\r\n\r\nThe shift between the beginning of the relaxation moduli was shown to play an important role. This parameter affects strongly the fluctuation pattern of the stress curves. For two different shift values, in one case, the stress response presents a 'double peak' and 'stress inversion' characteristic whereas, in the other case, it presents a 'single peak' and no 'inversion'. Another important factor was the material's compressibility. In the case of a nearly-incompressible material, the LVE and NLVE models yielded identical results; thus, the simpler LVE model is preferable.\r\n\r\nHowever, in the case of sufficient volume dilatation (or contraction), the NLVE model predicted correct characteristic responses, whereas LVE results were erroneous. This proves the necessity of using the NLVE model over the LVE model.\r\n", "doi": "10.7907/13SD-9N16", "publication_date": "1997", "thesis_type": "engd", "thesis_year": "1997" }, { "id": "thesis:1198", "collection": "thesis", "collection_id": "1198", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03292007-093444", "primary_object_url": { "basename": "Breton_f_1995.pdf", "content": "final", "filesize": 3125728, "license": "other", "mime_type": "application/pdf", "url": "/1198/1/Breton_f_1995.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "A large deformation analysis of plates or membranes for the determination of Young's modulus and Poisson's ratio", "author": [ { "family_name": "Breton", "given_name": "Fabienne Anne", "clpid": "Breton-F-A" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "An analytical method for determining Young's modulus and the Poisson's ratio of thin films is considered; the method is based on the load-deflection behavior of a rectangular membrane of finite aspect ratio n = b/a subjected to uniform pressure (c.f. figure 0.1). Following numerical analysis, previously published model results are shown to be inaccurate, especially for aspect ratios greater than 1.5.\r\n\r\nAn improved model description decomposes the displacement field into two parts: following the Timoshenko formulation, the deflection is assumed to be sinusoidal near the edges, but for aspect ratios larger than unity the central portion parallel to the longer sides is assumed to bear two-dimensional character (the displacement field is independent of the coordinate parallel to the longer side).\r\n\r\nUsing energy methods and including the effects of the residual stress, the load-deflection relationship for a rectangular membrane of arbitrary aspect ratio n = b/a, under uniform pressure, is obtained.\r\n\r\nExperiments are simulated by using numerical analysis. By comparing the numerical data of load-versus-deflection behavior to that for the energy based approximation, Young's modulus can be determined to within 2% of the value specified for the numerical analysis, provided that Poisson's ratio, v, is known. If the latter is not the case, the error increases to 14%, if the full range of Poisson's ratio (0 <= v <= 0.5) is admitted. Narrowing of the uncertainty through bounding Poisson's ratio is demonstrated. This result constitutes a significant improvement over the previous models which were shown to elicit errors on the order of 45%.\r\n\r\nA method to evaluate Poisson's ratio is also proposed. Making use of the load-deflection relationship for a rectangular plate of any aspect ratio, Poisson's ratio may be evaluated through the comparison of the load-deflection behavior of membranes of different aspect ratios. This method was found to be valid for materials with Poisson's ratio in the range {0.25, 0.5}.\r\n\r\nSince it is generally difficult to obtain homogeneous films, this study was extended to considerations for bimaterial plates in terms of an effective thickness that is well defined. A layer of the material under examination is deposited onto a well-characterized substrate so that a sandwich film results. Young's modulus of the material can then be deduced from the load-deflection data of the bimaterial film.", "doi": "10.7907/0c9r-ve54", "publication_date": "1995", "thesis_type": "engd", "thesis_year": "1995" }, { "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:2623", "collection": "thesis", "collection_id": "2623", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06162005-104436", "primary_object_url": { "basename": "Vendroux_G_1994.pdf", "content": "final", "filesize": 7995444, "license": "other", "mime_type": "application/pdf", "url": "/2623/1/Vendroux_G_1994.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Scanning tunneling microscopy in micromechanics investigations", "author": [ { "family_name": "Vendroux", "given_name": "Guillaume", "clpid": "Vendroux-G" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "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\nA new experimental method is proposed for studying deformations of micromechanical material systems at the submicron scale. To that end, a Scanning Tunneling Microscope (STM) was designed and built to allow placement on a mechanically deforming specimen. Operating in constant current mode, this digitally controlled STM records detailed topographies of specimen surfaces with a resolution of 10.15 nm in-plane and [...] out-of-plane, over a [...] area.\n\nA pattern recognition type algorithm was written to extract the 3-D displacement field from topographies of a given specimen area but under different loading conditions. This Digital Image Correlation (DIC) scheme was found to have very robust convergence characteristics and a higher resolution than that of the images it compares. The accuracy of the DIC code on STM scans was assessed by measuring displacement fields resulting from a translation of the specimen under the microscope. Two major causes of noise were identified, namely drifting of the specimen during scan acquisition and hysteresis distortion of the scan grid. An experimental procedure was devised to limit the occurrence of such perturbations and under these guidelines the resolution of the DIC scheme was found to be 4.8 nm for in-plane displacement measurements and 1.5 nm for out-of plane's.\n\nA micromechanical study of the deformation mechanism of PolyVinylChloride (PVC) was undertaken. Analysis of STM scans revealed that, upon first loading the surface of PVC specimens is deformed irreversibly even at low strain levels. The size of the strain induced topographic changes suggests that, at the scale of [...], a continuum type constitutive modeling of PVC may not be appropriate. This investigation also uncovered the present limitations of the STM-DIC scheme in measuring displacement fields consistently at the nanometer scale.", "doi": "10.7907/KVSX-Q863", "publication_date": "1994", "thesis_type": "phd", "thesis_year": "1994" }, { "id": "thesis:6071", "collection": "thesis", "collection_id": "6071", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09282010-160254018", "primary_object_url": { "basename": "Sugawara_s_1994.pdf", "content": "final", "filesize": 1915397, "license": "other", "mime_type": "application/pdf", "url": "/6071/1/Sugawara_s_1994.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "An experimental investigation of fracture at an interface between two epoxies", "author": [ { "family_name": "Sugawara", "given_name": "Satoshi", "clpid": "Sugawara-S" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": " The growth of a crack located at the interface between two linearly elastic solids is investigated experimentally. The requirements for the test pieces are strong bonding between the two materials, a well-defined, planar interface, and a pronounced difference in the stiffness between the two materials. To attain these requirements, castable liquid epoxy resins are used. It is demonstrated that the manufacturing process, which follows previously established procedures for the bi-material solid composed of Solithane, is also applicable for epoxy. To investigate the toughness of the interface crack, the measurement of crack speed and the estimation of stress intensity factors are carried out for several different temperatures. Master curves of crack speed for either of the two materials and for the bimaterial are presented. The experimentally obtained fracture toughness data are compared with the expected values by Knauss's model (1971). By varying the mode mixity at the crack tip, it is found that the crack might advance by kinking into the soft material or by propagating along the interface itself, depending on the applied loading conditions. Although the number of data points is small, the fracture data gathered from tests performed at two different temperatures indicate that rate effects significantly influence the kinking behavior. Crack tip speeds after kinking are also recorded.", "doi": "10.7907/850e-8c88", "publication_date": "1994", "thesis_type": "engd", "thesis_year": "1994" }, { "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: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:6332", "collection": "thesis", "collection_id": "6332", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04152011-095226678", "primary_object_url": { "basename": "Tsuyuki_rm_1993.pdf", "content": "final", "filesize": 11228323, "license": "other", "mime_type": "application/pdf", "url": "/6332/1/Tsuyuki_rm_1993.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Buckling of thermoviscoelastic structures under temporal and spatial temperature variation", "author": [ { "family_name": "Tsuyuki", "given_name": "Richard M.", "clpid": "Tsuyuki-R-M" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Ravichandran", "given_name": "Guruswami", "clpid": "Ravichandran-G" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The problem of lateral instability of a viscoelastic in-plane loaded structure is considered in terms of thermorheologically simple materials. As an example of a generally in-plane loaded structure, we examine the simple column under axial load: Both cyclic loading is considered (with constant or in-phase variable temperature excursions) as well as the case of constant load in the presence of thermal gradients through the thickness of the structure.\r\nThe latter case involves a continuous movement of the neutral axis from the center to the colder side and then back to the center.\r\n\r\nIn both cases, one finds that temperature has a very strong effect on the rate at which instabilities evolve, and under in-phase thermal cycling the critical loads are reduced compared to those at constant (elevated) temperatures. The primary effect of thermal gradients beyond that of thermally-induced rate accelerations is a rate increase occasioned by the generation of an \"initial imperfection\" or \"structural bowing.\" This latter effect, which is proportional to both the temperature gradient and the coefficient of thermal expansion (presumed homogeneous in this study), can in fact be dominant. Because the coefficient of thermal expansion tends to be large for many polymeric materials, it may be necessary to take special care in lay-up design of composite structures intended for use under compressive loads in high-temperature applications. Finally, the implications for the temperature\r\nsensitivities of composites to micro-instability (fiber crimping) are also apparent from the results delineated here.\r\n", "doi": "10.7907/T54W-0694", "publication_date": "1993", "thesis_type": "engd", "thesis_year": "1993" }, { "id": "thesis:2953", "collection": "thesis", "collection_id": "2953", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-07202007-110435", "primary_object_url": { "basename": "Gortsema_sc_1992.pdf", "content": "final", "filesize": 15694942, "license": "other", "mime_type": "application/pdf", "url": "/2953/1/Gortsema_sc_1992.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "An Experimental Investigation of the Failure of a Stepped Composite Plate", "author": [ { "family_name": "Gortsema", "given_name": "Steven Craig", "clpid": "Gortsema-Steven-Craig" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The progression of damage in a stepped composite plate has been investigated experimentally. This study is intended to provide insight into the failure behavior of co-cured, stringer-reinforced composite plates and shells. The specimens were constructed of four different lay-ups of carbon-fiber-reinforced epoxy and were subjected to various combinations of moment, tension, and shear static loadings.
\r\n\r\nThe primary means of damage observation was in situ microscopic inspection of the specimen edges. These observations were supplemented by penetrant-dye inspection, microscopic inspection of the plate face, and ultrasonic C-scanning of specimens which were unloaded prior to catastrophic failure. In addition to providing detailed descriptions of the damage development, this thesis presents hypotheses which attempt to explain the mechanisms involved in the failure process.
\r\n\r\nThe immediate goal of this work was to gather the experimental evidence needed to evaluate a recently proposed failure criterion. The criterion attempts to predict failure for an arbitrary lay-up by studying the behavior of the computed stresses within an extremely small vicinity of the singularity. The tests described in this thesis revealed that damage initiation does not always occur on the plate-stringer interface, as assumed in the failure criterion. Furthermore, it was discovered that in some lay-ups, damage initiates at a distance of over one lamina thickness away from the singularity. This fact makes it impossible to predict failure accurately with any criterion which is based solely on a computed stress singularity.
", "doi": "10.7907/4R72-Y162", "publication_date": "1992", "thesis_type": "engd", "thesis_year": "1992" }, { "id": "thesis:3053", "collection": "thesis", "collection_id": "3053", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-08082007-100442", "primary_object_url": { "basename": "Minahen_tm_1992.pdf", "content": "final", "filesize": 5437488, "license": "other", "mime_type": "application/pdf", "url": "/3053/1/Minahen_tm_1992.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Structural instabilities involving time dependent materials : theory and experiment", "author": [ { "family_name": "Minahen", "given_name": "Timothy M.", "clpid": "Minahen-T-M" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-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": "Christman", "given_name": "Tom", "clpid": "Christman-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The creep buckling of viscoelastic structures is studied analytically and experimentally to investigate structural stability in the presence of time dependent materials. The theory of linear viscoelasticity is used to model polymeric column specimens subjected to constant compressive end loads. A strength of materials approach (Euler-Bernoulli beam theory) is employed to model the moment-curvature relation for the column. The growth of initial imperfections is calculated using the hereditary integral formulation. Solution techniques are developed for small displacements and then generalized to include the effects of large displacements and rotations. A failure criterion based on maximum deformation allows the column life to be estimated directly from the material relaxation modulus. A discussion generalizing the results to include plates and shells is presented.\n\nRectangular cross-section polymethylmethacrylate (PMMA) specimens with hinged boundary conditions are used to study viscoelastic buckling experimentally. Constant compressive end loads are applied using a servo-controlled load frame while the specimens are kept in a temperature cabinet at elevated temperatures (accelerating the creep behavior). Specimen shortening and out-of-plane deflections are monitored during the tests. The relaxation modulus of PMMA is approximated by a Prony-Dirichlet series and the model is used to simulate the laboratory experiments. Model and experimental results show good agreement during the \"glassy\" and slow growth phases of the column response. As the growth rate increases some deviations between theory and experiment are seen. It is shown that the deviations are not a result of geometric nonlinearities, but may, in part, be explained by material nonlinearities not accounted for in the model.\n", "doi": "10.7907/7h70-2p51", "publication_date": "1992", "thesis_type": "phd", "thesis_year": "1992" }, { "id": "thesis:2951", "collection": "thesis", "collection_id": "2951", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-07202007-092222", "primary_object_url": { "basename": "Bowen_jm_1992.pdf", "content": "final", "filesize": 6180318, "license": "other", "mime_type": "application/pdf", "url": "/2951/1/Bowen_jm_1992.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "An experimental investigation of fracture at a bimaterial interface", "author": [ { "family_name": "Bowen", "given_name": "John Murray", "clpid": "Bowen-J-M" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The growth of a crack located at the interface between two linearly (visco)elastic solids is investigated experimentally. It is found that the crack may advance by kinking into either of the adherends or by propagating along the interface itself, depending on the applied loading. For the separation problem, in which crack advance occurs along the bimaterial interface, it is demonstrated that the time-dependent unbonding of the two joined viscoelastic solids follows a rate-dependent fracture process that can be described to a large extent by the viscoelastic properties of the two adherends. Moreover, the strength of the interfacial bond can be characterized in terms of an equilibrium interface-intrinsic fracture energy, the magnitude of which represents the bond strength quantitatively. In particular, interface strength on the same order as those of the adherends is achieved. In contrast to the time-dependent approach developed for the separation problem, the propensity of the interface crack to kink out of the interface upon loading is evaluated in the context of (time-independent) linearly elastic fracture mechanics. It is demonstrated that crack propagation along the interface occurs for a finite range of load mixity, a phenomenon predicted by linear analysis of the bimaterial joint but absent from the corresponding homogeneous development. Agreement between observed kinking behavior and analytical results is seen to improve by suitable manipulation of a presumably material characteristic length, the origin of which is rooted in the linearized analysis. The influence of material rate effects on the crack kinking behavior \nis also investigated.\n", "doi": "10.7907/esfz-eb60", "publication_date": "1992", "thesis_type": "engd", "thesis_year": "1992" }, { "id": "thesis:2862", "collection": "thesis", "collection_id": "2862", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-07122007-132245", "primary_object_url": { "basename": "Schultheisz_cr_1991.pdf", "content": "final", "filesize": 20817271, "license": "other", "mime_type": "application/pdf", "url": "/2862/1/Schultheisz_cr_1991.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Comparison of experimental and computational crack-tip deformations using Moire interferometry and finite elements", "author": [ { "family_name": "Schultheisz", "given_name": "Carl R.", "clpid": "Schultheisz-C-R" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The large plastic deformations at the tip of a crack in a ductile heat treatment of 4340 steel are studied experimentally and numerically to investigate the details of the deformation in a tough material. The specimen is loaded in a three-point-bend arrangement. The finite-element model of the experiment uses a small-strain, incremental plasticity law, with a power-law hardening behavior. Both the in-plane and out-of-plane deformations were measured on the same specimen at the same time.\n\nThe experimental technique of moire interferometry is used to measure the in-plane displacements. This technique is described in detail, including an analysis of the effect of out-of-plane rotations on the use of the technique. A four-beam interferometer for measuring orthogonal displacement components is described, and its performance analyzed.\n\nThe three-dimensional, finite-element model has 11913 degrees of freedom, and provides data for comparison with the experiment between 4000 N (linear behavior) up to 73.5 kN (continuous fracture of the steel specimen). The model material properties are determined from a uniaxial test on specimens taken from the same bar as the fracture specimens and with identical heat treatment. This model characterizes the crack as a rounded notch to match the notch in the steel fracture specimen. The effects of tunneling of the crack are introduced through the release of nodes along the crack plane corresponding to measured crack profiles.\n\nResults indicate that the numerical model matches the experiment quite well up to a load of 52.3 kN; mismatch at higher loads may be caused by a lack of finite-strain formulation in the code. The finite notch tip negates the singularity in either the stress or strain fields; the HRR field seems to have no region of dominance. However, the function of the J-integral appropriate to the HRR field does normalize the stresses and strains well, indicating that the J-integral is still a good fracture criterion. The effects of the added tunnel indicate that failure of the material depends on both the plastic strain and the hydrostatic stress.", "doi": "10.7907/etqv-z784", "publication_date": "1991", "thesis_type": "phd", "thesis_year": "1991" }, { "id": "thesis:3699", "collection": "thesis", "collection_id": "3699", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09222006-155328", "primary_object_url": { "basename": "Shimabukuro_sr_1991.pdf", "content": "final", "filesize": 14349234, "license": "other", "mime_type": "application/pdf", "url": "/3699/1/Shimabukuro_sr_1991.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Stress Assisted Diffusion in Polymers", "author": [ { "family_name": "Shimabukuro", "given_name": "Sy Ross", "clpid": "Shimabukuro-Sy-Ross" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "A model for diluent diffusion into a polymer which includes mass flux contributions from both a concentration gradient and pressure gradient is proposed. The pressure is a result of the diluent-induced swelling of the polymer and a constitutive law is incorporated that is analogous to that used in nonlinear thermo-viscoelasticity. The concept of free volume is used as the common basis to relate the relaxation processes for a viscoelastic material to the diffusion of the diluent molecules. The resultant strains (swelling) and stresses must satisfy a strain-displacement relationship and equilibrium equations, respectively.
\r\n\r\nThe governing equations are solved using the finite element technique. An iterative scheme is developed to solve the stress-coupled diffusion process as two separate problems: one for the diffusion process and one for the mechanical response. The two problems are solved individually where the solution from one problem is used as input to the other.
\r\n\r\nNumerical simulations are performed for both one-dimensional and two-dimensional axisymmetric problems in which the diluent is assumed to diffuse along the axis of the cylinder. The results suggest that certain anomalous experimental diffusion results can be explained through the relaxation behavior of the polymer. There are indications that Case II behavior is included in the model by an appropriate choice of material parameters. The results also indicate that typical diffusion experiments may be inadequate to uniquely define the physical model for the diffusion process when more than one type of diffusion driving force exists.
\r\n", "doi": "10.7907/2g5v-v304", "publication_date": "1991", "thesis_type": "phd", "thesis_year": "1991" }, { "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:4481", "collection": "thesis", "collection_id": "4481", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11092007-101356", "primary_object_url": { "basename": "Washabaugh_pd_1990.pdf", "content": "final", "filesize": 13356988, "license": "other", "mime_type": "application/pdf", "url": "/4481/1/Washabaugh_pd_1990.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "An experimental investigation of mode-I crack tip deformation", "author": [ { "family_name": "Washabaugh", "given_name": "Peter D.", "clpid": "Washabaugh-P-D" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Rosakis", "given_name": "Ares J.", "clpid": "Rosakis-A-J" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The out-of-plane displacement of amorphous polymethylmethacrylate plates rupturing at slow (0.1 mm/s), and fast (0.5 to 0.9 mm/\u00b5s) rates are measured using a Twymann-Green interferometer. The measured surface shapes within one plate thickness of the crack-tip do not compare well with the two-dimensional planar asymptotic approximation, but compare favorably with the published slopes for three-dimensional finite element solutions when normalized with the static material properties. Discrepancies, on the order of ten percent, between the magnitude of the three-dimensional finite element solutions suggest that the stress intensity factor does not fully characterize the near tip deformations.\n\nA dynamically propagating crack is found to move in a non-steady, periodic, submicrosecond fashion. This result is supported both by the surface measurements and the fracture morphology. The material toughening, as measured by the surface roughness, correlates well with the stress intensity factor and not with the crack velocity. The details of the sub-microsecond propagation and toughening was not resolvable with the microsecond temporal resolution of the experiment.\n\nInhibiting the material toughening at the crack tip by artificially introducing a weak material plane augments the crack motion to velocities close to the material's shear wave speed. The crack propagates more steadily along the weak plane than through a virgin solid, while maintaining the character of the out-of-plane displacement of a crack propagating in an unsullied material.\n", "doi": "10.7907/wv0e-g535", "publication_date": "1990", "thesis_type": "phd", "thesis_year": "1990" }, { "id": "thesis:4381", "collection": "thesis", "collection_id": "4381", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11032003-103818", "primary_object_url": { "basename": "Losi_gu_1990.pdf", "content": "final", "filesize": 6324450, "license": "other", "mime_type": "application/pdf", "url": "/4381/1/Losi_gu_1990.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Nonlinear thermoviscoelastic behavior of polymers", "author": [ { "family_name": "Losi", "given_name": "Giancarlo Umberto Maria", "clpid": "Losi-G-U-M" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The rheological behavior of polymers in the neighborhood of the glass transition has been investigated in the framework of the free volume theory of nonlinear viscoelastic behavior. Free volume theory as normally applied above the glass transition was modified to account for the effect of the residual volume of vacancies below the glass transition; this modification was accomplished by modelling the changes in the state of the polymer as the sum of viscoelastic changes and a random disturbance deriving from the thermal collisions between molecules. The changes in mechanical properties going across the glass transition follow from the freezing-in of relaxation mechanisms and of free volume. The pressure dependence of the glass transition was found to be in qualitative agreement with measurements on PVAc, while the ratio of the glassy and rubbery heat capacities was found to coincide with the ratio of the equilibrium bulk compliances in the glassy and rubbery domains. The predictions of the model for the problem of transient and residual thermal stresses were compared with those of two simpler models.\n\nThe second part of the thesis studies the consequences of the nonlinear viscoelastic behavior on the decohesion zone in front of a crack propagating through an adhesive layer. The softening of the material response in the cohesive zone is taken to be effected by free volume induced change in relaxation times of the cohesive material and by void growth; the latter is assumed to depend on a critical value of strain at the beginning of the cohesive zone. The stress intensity factor for steady crack propagation is obtained by imposing the finiteness of strains at the crack tip. For the case where the properties of the adherends are the same as the linearized properties of the adhesive, the predictions show three regimes of crack propagation: a low speed regime where the adherends behave elastically with the rubbery properties, an intermediate range where their response becomes increasingly stiffer, and a high speed regime characterized by glassy behavior of the adherends and control of the crack growth process exclusively by the nonlinearly viscoelastic behavior of the failing material.", "doi": "10.7907/2dk7-z575", "publication_date": "1990", "thesis_type": "phd", "thesis_year": "1990" }, { "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: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:2542", "collection": "thesis", "collection_id": "2542", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06102005-155309", "primary_object_url": { "basename": "Chang_sky_1983.pdf", "content": "final", "filesize": 4643223, "license": "other", "mime_type": "application/pdf", "url": "/2542/1/Chang_sky_1983.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Crack Propagation in Viscoelastic Materials under Transient Loading with Application to Adhesively Bonded Structures", "author": [ { "family_name": "Chang", "given_name": "Samuel Kwang Yeh", "clpid": "Chang-Samuel-Kwang-Yeh" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Linear viscoelasticity theory is applied to the fatigue problem of solids sensitive to the deformation rate. A series of experiments have been performed to investigate the range of applicability in whch the theory is valid and to examine the accuracy of the theory. The experimental results are compared with an existing theory.", "doi": "10.7907/ZHV3-TK46", "publication_date": "1984", "thesis_type": "engd", "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: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:4301", "collection": "thesis", "collection_id": "4301", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10292003-135400", "primary_object_url": { "basename": "Liechti_km_1980.pdf", "content": "final", "filesize": 6430522, "license": "other", "mime_type": "application/pdf", "url": "/4301/1/Liechti_km_1980.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "The Application of Optical Interferometry to Time Dependent Unbonding: Parts 1, 2, and 3", "author": [ { "family_name": "Liechti", "given_name": "Kenneth Macdougall", "clpid": "Liechti-Kenneth-Macdougall" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Improved methods of adhesive joining for constructing structural elements have led to an increase in the use of bonding. Bonding is already widely used in the aerospace industry. It allows potential for greater weight and manufacturing cost savings when compared to mechanical fastening as well as providing better fatigue resistant capabilities. More accurate design or life prediction based on a better understanding of adhesive joint failure will allow far more extensive and safer use of adhesively bonded structures. The possibility of time dependent failure is introduced in bonding,by the fact that many adhesives are polymers and measurably viscoelastic under many circumstances. Failure can occur by unbonding at the adherend-adhesive interface or totally within the adhesive. Here the time dependent failure by unbonding at the interface is addressed as one initial investigation of the whole problem.\r\n\r\nIn many engineering problems the modelling of failure has been based on linear fracture mechanics. The extent to which the use of the linear theory can be justified is often questionable in view of the nonlinearity of the local crack front deformation field. One aspect of the current investigation is therefore the measurement of the local crack front deformation to examine the limits of the validity for the linear theory and to provide a basis for future modelling, Within this framework, particular attention is paid to the aspects of local failure mode interaction and the time dependence of the unbonding process.\r\n\r\nThe small displacements in the crack front region require a resolution on the order of a wavelength of light. The experimental techniques used are therefore based on optical interferometry. The displacements applied to the adherends are controlled to the same order by a novel, thermally actuated servoloading device.\r\n\r\nThe thesis is divided into three distinct parts. The first part deals with the development of the experimental techniques used to measure the unbound profiles as well as to measure and control the displacements applied to the adherends. For stationary cracks, the second part describes experiments which examine the extent of the nonlinearity in the crack front deformation field while fracture criteria are developed and compared for unbonding rates in steadily propagating cracks. Finally, proposals for improvement of the experimental techniques and further experiments are discussed in the third part.", "doi": "10.7907/9yk1-v212", "publication_date": "1980", "thesis_type": "phd", "thesis_year": "1980" }, { "id": "thesis:4775", "collection": "thesis", "collection_id": "4775", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12042006-082245", "primary_object_url": { "basename": "Smith_gc_1975.pdf", "content": "final", "filesize": 5820847, "license": "other", "mime_type": "application/pdf", "url": "/4775/1/Smith_gc_1975.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "An Experimental Investigation of the Dynamic Fracture of a Brittle Material", "author": [ { "family_name": "Smith", "given_name": "Gordon Carl", "clpid": "Smith-Gordon-Carl" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "A new method of experimentally investigating the behavior of a crack in a stress wave environment has been developed and used to study the initial stages of dynamic crack propagation in Homalite 100, a polyester. The experimental method, which employs an electromagnetic loading device, permitted the application of pressure pulses to the surfaces of an 18 inch crack. The amplitude (51 psi to 1020 psi) and duration (~ 200 \u00b5sec) of the pulses were highly repeatable. The experimental configuration simulates, in the vicinity of the crack tip, a tension wave impacting a stationary semi-infinite crack in an infinite two-dimensional body where the wave front is parallel to the crack. A high speed framing camera, synchronized with the loading device, was used to record the time required for the crack to begin to propagate and its subsequent extension and velocity. The experimental results were analyzed within the bounds of linearly elastic fracture mechanics and a correlation was made between the dynamic stress intensity factor and the time at which the cracks began to propagate. It was found that the critical stress intensity factor increased dramatically with increasing loading rates at very high rates, which contrasts with quasi-static loading where the critical stress intensity factor decreases with an increasing loading rate. A simple model suggests that temperature effects at the crack tip may account for the observed increase in the critical stress intensity factor at high loading rates. Crack branching of running cracks was observed but unlike previous reports of branching, the cracks in this study branched mostly into three separate fast-running cracks.", "doi": "10.7907/35AK-XJ82", "publication_date": "1975", "thesis_type": "phd", "thesis_year": "1975" }, { "id": "thesis:6000", "collection": "thesis", "collection_id": "6000", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08192010-160559026", "primary_object_url": { "basename": "Mahale_nk_1973.pdf", "content": "final", "filesize": 44111624, "license": "other", "mime_type": "application/pdf", "url": "/6000/1/Mahale_nk_1973.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Failure of Precracked Fiber Reinforced Composite Plate", "author": [ { "family_name": "Mahale", "given_name": "Narayan Krishna", "clpid": "Mahale-Narayan-Krishna" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" }, { "family_name": "Babcock", "given_name": "Charles D.", "clpid": "Babcock-C-D" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "In the following pages a theoretical and experimental investigation of the fracture of a precracked carbon fiber reinforced composite is presented. The work deals mainly with a unidirectional composite having the crack aligned parallel to the fibers. We put forth some ideas on how to extend the failure criterion of the above restricted case to a more general one of multidirectional composites.\r\nThe failure criterion proposed is based on an energy argument similar to that proposed by Griffith for isotropic solids. For the purpose of stress analysis the material under consideration is assumed to be homogeneous and orthotropic. Lekhnitskii's complex variable method\r\nof stress analysis of the two dimensional elasticity problems for anisotropic bodies is used in the theoretical investigation.\r\n", "doi": "10.7907/DPK2-YA94", "publication_date": "1973", "thesis_type": "engd", "thesis_year": "1973" }, { "id": "thesis:6065", "collection": "thesis", "collection_id": "6065", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09282010-083924490", "primary_object_url": { "basename": "Pucik_ta_1972.pdf", "content": "final", "filesize": 3202534, "license": "other", "mime_type": "application/pdf", "url": "/6065/1/Pucik_ta_1972.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Elastostatic Interaction of Cracks in the Infinite Plane", "author": [ { "family_name": "Pu\u010dik", "given_name": "Thomas Antone", "clpid": "Pu\u010dik-Thomas-Antone" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": " The stress boundary value problem of an infinite, planar region with embedded rectilinear cracks is investigated from the viewpoint of two-dimensional, static, linear elasticity theory (plane strain or generalized stress). Any finite number of cracks may be considered. Their orientation may be arbitrary, so long as they do not intersect. Boundary loadings may take the form of quite general in-plane tractions along the crack surfaces, together with a bounded in-plane stress field at infinity.\r\n\r\n Using Muskhelishvili\u2019s solution for colinear cracks, the problem is reduced to a set of one-dimensional Fredholm integral equations. A simple numerical technique is presented for the approximate solution of these equations. The method is established to possess an extremely high rate of convergence.\r\n\r\n Results are presented for a number of two-crack interaction problems. As expected, the interaction of the cracks generally tends to reduce the fracture strength of a material, relative to the strength that would exist with either crack acting independently. However, for certain orientations, it is found that the interaction phenomenon can actually increase the resistance to fracture.", "doi": "10.7907/BVJ3-BV94", "publication_date": "1972", "thesis_type": "phd", "thesis_year": "1972" }, { "id": "thesis:6007", "collection": "thesis", "collection_id": "6007", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08252010-104555143", "primary_object_url": { "basename": "Palaniswamy_k_1972.pdf", "content": "final", "filesize": 48268085, "license": "other", "mime_type": "application/pdf", "url": "/6007/1/Palaniswamy_k_1972.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Crack Propagation under General In-Plane Loading", "author": [ { "family_name": "Palaniswamy", "given_name": "Karuppagounder", "clpid": "Palaniswamy-Karuppagounder" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The problem of crack extension in brittle materials under\r\ngeneral loading conditions is investigated. Methods of solution of the related two-dimensional elasto-static boundary value problem are discussed. Using Kolosov-Muskhelishvili stress functions, an approximate solution is obtained. The effect of the approximation on the results is estimated by solving two related problems exactly. Then using two postulates the critical loads and crack extension direction are determined under loading conditions\r\nunsymmetrical to the crack axis. Results are compared with those obtained using a different set of postulates.\r\n", "doi": "10.7907/M7WW-G964", "publication_date": "1972", "thesis_type": "phd", "thesis_year": "1972" }, { "id": "thesis:4574", "collection": "thesis", "collection_id": "4574", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11152005-142505", "primary_object_url": { "basename": "Mueller_hca_1968.pdf", "content": "final", "filesize": 5138326, "license": "other", "mime_type": "application/pdf", "url": "/4574/1/Mueller_hca_1968.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Stable Crack Propagation in a Viscoelastic Strip", "author": [ { "family_name": "Mueller", "given_name": "Hans-Karl Christian Alfred", "clpid": "Mueller-Hans-Karl-Christian-Alfred" } ], "thesis_advisor": [ { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "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.\r\n\r\nA crack of length 2a which propagates with small, constant speed through a viscoelastic strip of width 2b is considered. The strip is strained by displacing its shearfree edges. Linear theory is applied. The stress on the line of crack advancement and the shape of the crack surface are calculated for a state of plane stress.\r\n\r\nThe stress intensity factor which is independent of material properties is given as a function of a/b. It exhibits a maximum at [...]. For a/b > 1.5 the stress intensity factor becomes essentially independent of crack length. The crack surface deflection is obtained in the form of a superposition integral and is a function of material properties and crack speed.\r\n\r\nThe energy which is released when the crack extends a small distancee is calculated. This crack energy depends on the crack speed and involves the creep function of the material. A characteristic length enters in the course of its derivation. This length does not appear in the case of an elastic material and is considered as an additional material property necessary to describe viscoelastic crack propagation.\r\n\r\nThe energy conservation equation is established by considering a small control volume surrounding the crack tip. A relationship emerges from this equation which implicitly gives a stable crack speed as a function of applied strain, temperature, and material properties. The creep function is the controlling factor in this equation.\r\n\r\nThe relevant material properties are discussed and presented for a Polyurethane rubber (Solithane 113 - 50/50). The lower bound of the surface energy is determined from fracture tests on the swollen material. The results of the material characterization are used to calculate the crack speed as a function of applied strain and temperature. Good agreement is found to exist between theory and experiment.", "doi": "10.7907/MGSD-R362", "publication_date": "1968", "thesis_type": "phd", "thesis_year": "1968" }, { "id": "thesis:4643", "collection": "thesis", "collection_id": "4643", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11232005-113650", "primary_object_url": { "basename": "Lindsey_gh_1966.pdf", "content": "final", "filesize": 5171370, "license": "other", "mime_type": "application/pdf", "url": "/4643/1/Lindsey_gh_1966.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Hydrostatic Tensile Fracture of a Polyurethane Elastomer", "author": [ { "family_name": "Lindsey", "given_name": "Gerald Herbert", "clpid": "Lindsey-Gerald-Herbert" } ], "thesis_advisor": [ { "family_name": "Williams", "given_name": "Max L.", "clpid": "Williams-M-L" }, { "family_name": "Knauss", "given_name": "Wolfgang Gustav", "clpid": "Knauss-W-G" } ], "thesis_committee": [ { "family_name": "Unknown", "given_name": "Unknown" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The investigation of fracture of polymeric materials in hydrostatic tensile fields constitutes an avenue of approach to the study of fracture in more general three-dimensional environments. The advantages created by the symmetry of the stress field are considerable and, in one of the cases studied, facilitates a theoretical treatment that includes large deformations, which are characteristic of this class of materials.\r\n\r\nThe analysis is developed through the concept of fracture originating from a flaw, which in this instance is taken to be a spherical cavity. Through the application of energy principles, a theoretical prediction of ultimate strength is made for hydrostatic tensile fields.\r\n\r\nExperiments were conducted to demonstrate the existence of such flaws and to evaluate the theory. Results of the tests on specimens containing both residual flaws and artificially inserted ones indicate a fundamental difference in behavior as contrasted with cracks.\r\n\r\nAn explanation is given linking experimental results and theoretical predictions. It is based on the concept that a flaw \"grows\" in the material under load using the cavity as a nucleating point. Upon this hypothesis is built a theory of rupture in which planar cracks grow radially from the center of the cavity in the form of Saturn-ring cracks.", "doi": "10.7907/K12J-X907", "publication_date": "1966", "thesis_type": "phd", "thesis_year": "1966" } ]