An experimental investigation using grids with a frequency of 125 lines/in. (5 lines/mm) was performed on inert propellant and pure binder at two different global head rates of 0.1 in./min (2.5 mm/min) and 1.0 in./min (25.4 mm/min). From the extracted data, displacements, strains, and dominant eigenvalue for displacement were calculated. An idealized model was used to explain the high strain zone ahead of inert propellant that caused severe blunting at the crack tip. Using the available algorithms and three dimensional photoelasticity, the dominant stress singularity order values were calculated in a four point single edged cracked bend specimen with both straight front and thumbnailed cracks. The free surface values are the same as for the inert propellant and in good agreement with analytical values. A boundary layer is observed in the singularity order which extends towards the mid-plane of the specimen. This region is about twenty percent of the distance from the free surface to mid-depth of the fractured body. The slow and fast head rates alter the global behavior of the specimen as well as the density of the displacement and strain contours. However, the near tip mechanisms are not altered. / Ph. D.

An epoxy resin (Epon 828) was chemically modified with two different elastomers, poly(dimethyl-co-diphenyl) siloxane (PSX) and carboxyl-terminated butadiene-acrylonitrile (CTBN), to enhance its fracture toughness. The friction and wear of specimens modified with different amounts of elastomer were investigated in a pin-on-disk wear machine. An attempt was made to correlate the fracture toughness of the epoxy material to its fatigue wear rate for experiments in which a steel ball was sliding on a modified epoxy disk. A different type of experiment, modified epoxy pin sliding on an abrasive disk, was performed to detect whether abrasive wear of modified epoxies responds differently than fatigue wear to the fracture toughness. Other experiments were performed in which the wear debris produced during sliding was blown out of the interface to study its influence on friction and wear behavior. The effect of surface morphology on friction and wear was also studied. The results indicated that a marked improvement in fracture toughness was achieved for samples with higher elastomer content. Regardless of the type of the experiment, epoxy pin-on-abrasive disk or steel ball-on-epoxy disk, wear rates correlated positively with inverse of fracture toughness. Both friction coefficient and wear rate were found to be influenced by the removal of the wear debris, especially for samples with higher elastomer content. The friction coefficient was reduced for samples with higher elastomer content and this was attributed to the low surface energy of the elastomer. CTBN-modified epoxies exhibited lower friction coefficients than epoxies modified with polysiloxane. It was found that sample morphology had a significant effect on both friction coefficient and wear rate; the sample with approximately the same domain size but the least number of elastomeric domains exhibited the highest friction coefficient and the highest wear rate. / Ph. D.

Compressive loads can cause local buckling in composite laminates that have a near-surface delamination. This buckling causes load redistribution and secondary loads, which in turn cause interlaminer stresses and delamination growth. The goal of this research effort was to enhance the understanding of this instability-related delamination growth in laminates containing either an embedded or an edge delamination. There were three primary tasks: 1) development of a geometrically nonlinear finite element analysis named NONLIN3D; 2) performance of a parametric analytical study to determine the effects of strain, delamination shape, and delamination size on the distribution of the strain energy release rate components along the delamination front; and 3) performance of a combined experimental and analytical study of instability-related delamination growth (IRDG). Two material systems (AS4/PEEK and IM7/8551-7) and two stacking sequences (0/90/90/0)₆ and (90/0/0/90)₆ were examined. The laminates were fabricated with Kapton inserts between the fourth and fifth plies from the top surface to give an initial delamination. The analysis predicted a large variation of G_I and G_II along the delamination front. The G_III component was always small. The location of maximum G_I and G_II depended on the delamination shape and applied strain. In general, the strain-energy release rates were small except in a small region. Hence, delamination growth was expected to occur over only a small portion of the delamination front. Experiments corroborated this prediction. The laminate stacking sequence had a large effect on the shape of the deformed region, the direction of delamination growth, and the strain at which delamination growth occurred. These effects were predicted by the analysis. The G_I component appeared to govern initial delamination growth in the IM7/8551-7 laminates. Matrix ply cracking generally accompanied delamination growth. In some cases fiber micro-buckling also occurred shortly after delamination growth occurred. / Ph. D.

The objective of this study was to observe and characterize the out-of-plane fracture of a 2-D carbon-carbon composite and to gain an understanding of the factors influencing the stress distribution in such a laminate. The experimental portion of this study consisted of performing an out-of-plane tensile test in a scanning electron microscope and determining the modes of failure. Failure was found to be interlaminar, with cracks propagating along the fiber-matrix interface. Finite element analyses of a two-ply carbon-carbon composite under in-plane, out-of-plane, and thermal loading were performed. Stress distributions were studied as a function of stacking sequence, undulation aspect ratio, and undulation offset ratio. The results indicated that under out-of-plane loading σ_x and τ_xz were strongly dependent on the geometric parameters studied, but σ_z and σ_y were relatively independent of geometry. Under in-plane loading all components of stress were strong functions of the geometry, and large interlaminar stresses were predicted in regions of undulation. The thermal analysis predicted the presence of large in-plane normal stresses throughout the laminate and large interlaminar stresses in regions of undulation. An elasticity solution was utilized to analyze an orthotropic fiber in an isotropic matrix under uniform thermal load. The analysis reveals that the stress distributions in the fiber are singular when the radial stiffness C_rr is greater than the hoop stiffness C₀₀. Conversely, if C_rr < C₀₀ the maximum stress in the composite is finite and occurs at the fiber-matrix interface. In both cases the stress distributions are radically different than those predicted assuming the fiber to be transversely isotropic (C_rr = C₀₀). / Ph. D.

In this study, a test technique, referred to as the probe test, has been developed as a quantitative tool for measuring the adhesion in thin adhesive films and coatings. The technique was initially developed as a qualitative test by the Hewlett-Packard Company for measuring adhesion of thin film microelectronic coatings. In the probe test method, an inclined needle-like probe with a conical tip is advanced underneath the free edge of a thin polymeric coating bonded to a substrate, causing the edge to lift-up from the surface of the substrate. A debond is thus initiated at the loading point and propagates as a semi-circular crack at the interface as the probe slides under the coating. A standard test procedure has been developed for testing thin adhesive coating/substrate systems. The sample system used is a thin film epoxy polymer coated silicon system. The interfacial fracture energy (Gc) (or critical strain energy release rate) has been used as a quantitative measure of adhesion for the given adhesive coating/substrate system. The probe test experiments were conducted using an optical microscope and a WYKO optical profiler. Using the optical microscope, the debond radius was measured for different debond sizes. Using the WYKO optical profiler, the three-dimensional surface topography of the debonded coating around the crack front was measured for different debond sizes. Using the experimental data from the probe test, analytical and numerical (finite element-based) techniques have been developed to determine the interfacial fracture energy (Gc) for the given adhesive coating/substrate system. The analytical techniques were developed based on different plate theory formulations (thin/thick plate - small/large deflection) of the probe test geometry and local curvature measurement at the crack tip. The finite element based techniques were developed using a hybrid numerical-experimental approach and surface-based contact interaction analysis in ABAQUS. The results obtained using thick plate-large deflection formulation correlated with finite element contact interaction analysis results. The probe test can be used with transparent or opaque coatings and thus offers a promising alternative to indentation and other tests methods for characterizing thin film and coating adhesion. / Master of Science

The presence of residual stresses in engineering components may significantly affect damage evolution and progression towards failure. Correct evaluation of residual stress is of crucial importance for assessing mechanical components, predicting response and ensuring reliability. For example, when failure occurs due to cyclic loading, the underlying damage begins at the nano-, and then micro-scale. It is clear that improving engineering reliability at the micro-scale requires the ability to evaluate residual stress and mechanical properties at the appropriate scale. The key objective of the thesis is to advance the understanding and practice of residual stress evaluation at the micro-scale, and to examine the implications and applications that follow. Significant effort was devoted to the evaluation of two aspects of the relatively novel FIB-DIC micro-ring-core experimental technique: assessing the effects of Ga-ion damage and the quantification of uncertainty in stress evaluation due to unknown crystal orientation. FIB-DIC micro-ring-core milling was then used alongside with synchrotron XRD to study residual stress effects on fatigue crack growth propagation rate following the occurrence of overload or underload. The effects of the two principal mechanisms of crack retardation following an overload, residual stress and crack closure, were separated by testing samples at different loading ratios. Whilst, the acceleration after an underload was studied using validated non-linear FEM analyses. Conceptual focus was placed on the macro-micro-nano residual stress decomposition into Type I, II &amp; III according to scale and, detailed examination was conducted experimentally and numerically. In the context of shot-peening surface treatment, residual stresses were modelled using a novel eigenstrain-based modelling procedure for arbitrarily shaped components. Furthermore, a fine scale characterisation was performed of the recast layer produced by EDM, with particular attention paid to the residual stress. The investigations presented in this thesis open new perspectives for the assessment of material reliability. Improved failure prediction models will be elaborated based on the insights obtained in the present study.

Detta arbete genomförs på uppdrag av Skara Sommarland. Arbetet är till stor del baserat på att utföra en jämförande studie mellan två hjulaxlar till en åkattraktion. Den befintliga axeln består av tre delar, två axelpar och ett centerfäste, medan den nya designen av axeln enbart består av en del. Den befintliga axelns ändar som kopplas in i centerinfästningen utsätts för onödigt slitage. Arbetet belyser även orsaken som leder till nötningen samt möjligheter att minska nötningen. Den nydesignade axelns högre pris medför höga kostnader för företaget.I arbetet används Haigh-diagram för att undersöka om arbetspunkten för den nya axeln ligger under Haigh-diagram kurvan, som teoretiskt sett leder till oändlig livslängd. I arbetet utförs en FEM analys för att simulera båda axelkonstruktionerna och för att beräkna spänningarna i axeln under belastning.Resultatet av genomförda analyser visar att den befintliga axeln plasticerar medan den nya axeln inte når sträckgränsen för materialet. Varken den befintliga eller den nya axeln når brottgränsen. Eftersom brottgränsen inte nås finns ingen risk för sprickbildning enligt finite fracture mechanics. För den befintliga axeln visar resultatet att det uppstår en rörelse mellan axel och infäste som är tillräckligt stor för att klassificeras som en orsak för nötning.För att undvika eller minimera nötningen kan möjliga åtgärder genomföras vilka beskrivs i arbetet. Exempel på förbättringsförslag som skyddar mot korrosion och nötning ges som till exempel rengöring före ytbehandling och ökad korrosionshärdighet. / This thesis is performed on behalf of the amusement park Skara Sommarland. The work is based on a comparative study of two different shafts for a rollercoaster. An already existing shaft in three parts, two shafts and one hub, while the new shaft design consists of one homogenous part machined from a solid bloc of material. The ends of the shafts for the current shaft design that are mounted in the hub are exposed to unnecessary wear. The thesis partly focuses on the causes of the fretting wear and possible actions to prevent or minimalize the damaged caused by fretting. The new shaft design leads to increased costs, which leads to monetary losses for the client compared to before.Haigh-diagrams are used to evaluate if the critical point of the design is in the safe area of the diagram, theoretical leading to an infinite lifespan of the shaft. Finite element analyses were performed in 3D CAD to simulate the shaft design and calculate the stresses in the design when under the maximum operational load.The result of the analyses shows that the current shaft suffers from plastic deformation while the new shaft design does not reach the yield strength of the material. Neither the current nor the new shaft design reach the ultimate tensile strength. Because the ultimate tensile strength is not reached there is no risk of crack initiation through finite fracture mechanics. For the current shaft design the results show that there occur movement between shaft and hub with high enough magnitude to be classified as one of the causes of the fretting.To avoid or minimize fretting several actions can be performed. Examples of pre-emptive actions are cleaning of surfaces before surface treatment for corrosion resistance.

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Thesis: M.S., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 1978 / Includes bibliographical references. / M.S. / M.S. Massachusetts Institute of Technology, Department of Materials Science and Engineering