Microgels are a class of colloids that are mechanically soft, and while in many cases can behave similarly to their hard-sphere counterparts, their interaction potentials are quite different. The softness of the interaction between microgels makes them capable of deformation and compression into more concentrated assemblies. This concentrated regime is interesting because little, if any, experimental work has been done to see how the bulk properties of soft-sphere assemblies deviate from those of hard-spheres at the point where their interaction potentials begin to diverge. In this thesis the effects on assembly phase behavior and dynamics of both particle compression and softness of the interaction potential are addressed. Poly(N-isopropylacrylamide) (pNIPAm) microgels are an excellent model system in which to study these effects. The thermoresponsivity of the polymer provides the experimentalist with a dial to tune the volume fraction of an assembly, while maintaining a constant particle number density in the system. Optical microscopy, particle tracking analysis and rheology have been used to investigate the effects of packing and particle structure on equilibrium phase behavior and localized perturbations to the phase of the assembly of this soft-sphere system. It has been elucidated from these experiments and others involving deswelling of large microgel particles in the presence of high concentrations of smaller microgels, that the soft, repulsive interaction between microgels is caused by a longer-range repulsion than was previously believed. The particles are acting on each other from a distance through the osmotic pressure of the assembly, which causes each particle to deswell without coming into direct contact with a neighboring particle.

A Thesis Submitted Towards the Partial Fulfilment Degree of Master of Technology (M.Tech.) FACULTY OF ENGINEERING MECHANICAL ENGINEERING DEPARTMENT Cape Peninsula University of Technology 2011 / The split Hopkinson pressure bar (SHPB) test is the most commonly used method for determining material properties at high rates of strain. The theory governing the specifics of Hopkinson bar testing has been around for decades; however, it has only been for the last decade or so that significant data processing advancements have been made. It is the intent of this thesis to offer the insight of application of SHPB to determine the compressive dynamic behaviour for welded low carbon steel (mild steel). It also focuses on the tensile behaviour for unheat-treated and heat-treated welded carbon steel. The split Hopkinson Pressure bar apparatus consists of two long slender bars that sandwich a short cylindrical specimen between them. By striking the end of a bar, a compressive stress wave is generated that immediately begins to traverse towards the specimen. Upon arrival at the specimen, the wave partially reflects back towards the impact end. The remainder of the wave transmits through the specimen and into the second bar, causing irreversible plastic deformation in the specimen. It is shown that the reflected and transmitted waves are proportional to the specimen's strain rate and stress, respectively. Specimen strain can be determined by integrating the strain rate. By monitoring the strains in the two bars and the specimen's material, stress-strain properties can be calculated. Several factors influence the accuracy of the results, including the size and type of the data logger, impedance mismatch of the bars with the specimens, the utilization of the appropriate strain gauges and the strain amplifier properties, among others. A particular area of advancement is a new technique to determine the wave's velocity in the specimen with respect to change in medium and mechanical properties, and hence increasing the range of application of SHPB. It is shown that by choosing specimen dimensions based on their impedance, the transmitted stress signal-to-noise ratio can be improved. An in depth discussion of realistic expectations of strain gages is presented, along with closed form solutions validating any claims. The thesis concludes with an analysis of experimental and predicted results. Several recommendations and conclusions are made with regard to the results obtained and areas of improvement are suggested in order to achieve accurate and more meaningful results.

While the study of compression after impact of laminated composites has been under consideration for many years. the complexity of the damage initiated by low velocity impact has not lent itself to simple predictive models for compression strength. The damage modes due to non-penetrating. low velocity impact by large diameter objects can be simulated using quasistatic three-point bending. The resulting damage modes are less coupled and more easily characterized than actual impact damage modes. This study includes the compression testing of specimens with well documented initial damage states obtained from three-point bend testing. Compression strengths and failure modes were obtained for quasi-isotropic stacking sequences from 0.24 to 1.1 inches 'thick with both grouped and interspersed ply stacking. Initial damage prior to compression testing was divided into four classifications based on the type. extent, and location of the damage. These classifications are multiple through-thickness delaminations, isolated delaminations. damage near the surface. and matrix cracks. Specimens from each classification were compared to specimens tested without initial damage in order to determine the effects of the initial damage on the final compression strength and failure modes. A finite element analysis was used to aid in the understanding and explanation of the experimental results. It was found that specimens with multiple through-thickness delaminations experienced the greatest reduction in compression strength, from 50 to 75% below the strength of undamaged specimens. All the sublaminates formed by the delaminations failed at the same time. Individual sublaminate buckling was observed for isolated delaminations near 'the surface of the laminate. Delaminations far from the specimen surface had little effect on the final compression strength. Damage occurring in the outside 00 plies caused a 10 to 200/0 strength reduction according to both analytical and experimental results. The effects of increased interlaminar stresses near the specimen edges caused a reduction in undamaged strength of [05/455/-455/905]55 specimens, while having little effect on the [Osl60sl-605]75 specimens. / Master of Science

Structural energetic materials (SEM) are a class of multicomponent materials which may react under various conditions to release energy. Fragmentation and impact induced reaction are not well characterized phenomena in SEMs. The structural energetic systems under consideration here combine aluminum with one or more of the following: nickel, tantalum, tungsten, and/or zirconium. These metal+Al systems were formulated with powders and consolidated using explosive compaction or the gas dynamic cold spray process. Fragment size distributions of the indicated metal+Al systems were explored; mean fragment sizes were found to be smaller than those from homogeneous ductile metals at comparable strain rates, posing a reduced risk to innocent bystanders if used in munitions. Extensive interface failure was observed which suggested that the interface density of these systems was an important parameter in their fragmentation. Existing fragmentation models for ductile materials did not adequately capture the fragmentation behavior of the structural energetic materials in question. A correction was suggested to modify an existing fragmentation model to expand its applicability to structural energetic materials. Fragment data demonstrated that the structural energetic materials in question provided a significant mass of combustible fragments. The potential combustion enthalpy of these fragments was shown to be significant. Impact experiments were utilized to study impact induced reaction in the indicated metal+Al SEM systems. Mesoscale parametric simulations of these experiments indicated that the topology of the microstructure constituents, particularly the stronger phase(s), played a significant role in regulating impact induced reactions. Materials in which the hard phase was topologically connected were more likely to react at a lower impact velocity due to plastic deformation induced temperature increases. When a compliant matrix surrounded stronger, simply connected particles, the compliant matrix accommodated nearly all of the deformation, which limited plastic deformation induced temperature increases in the stronger particles and reduced reactivity. Decreased difference between the strength of the constituents in the material also increased reactivity. The results presented here demonstrate that the fragmentation and reaction of metal+Al structural energetic materials are influenced by composition, microstructure topology, interface density, and constituent mechanical properties.

The failure behavior of composite laminates is modeled numerically using the Generalized Layerwise Plate Theory (GLPT) of Reddy and a progressive failure algorithm. The Layerwise Theory of Reddy assumes a piecewise continuous displacement field through the thickness of the laminate and therefore has the ability to capture the interlaminar stress fields near the free edges and cut outs more accurately. The progressive failure algorithm is based on the assumption that the material behaves like a stable progressively fracturing solid. A three-dimensional stiffness reduction scheme is developed and implemented to study progressive failures in composite laminates. The effect of various parameters such as out-of-plane material properties, boundary conditions, and stiffness reduction methods on the failure stresses and strains of a quasi-isotropic composite laminate with free edges subjected to tensile loading is studied. The ultimate stresses and strains predicted by the Generalized Layerwise Plate Theory (GLPT) and the more widely used First Order Shear Deformation Theory (FSDT) are compared with experimental results. The predictions of the GLPT are found to be in good agreement with the experimental results both qualitatively and quantitatively, while the predictions of FSDT are found to be different from experimental results both qualitatively and quantitatively. The predictive ability of various phenomenological failure criteria is evaluated with reference to the experimental results available in the literature. The effect of geometry of the test specimen and the displacement boundary conditions at the grips on the ultimate stresses and strains of a composite laminate under compressive loading is studied. The ultimate stresses and strains are found to be quite sensitive to the geometry of the test specimen and the displacement boundary conditions at the grips. The degree of sensitivity is observed to depend strongly on the lamination sequence. The predictions of the progressive failure algorithm are in agreement with the experimental trends. Finally, the effect of geometric nonlinearity on the first-ply and ultimate failure loads of a composite laminate subjected to bending load is studied. The geometric nonlinearity is taken in to account in the von Kármán sense. It is demonstrated that the nonlinear failure loads are quite different from the linear failure loads, depending on the lamination sequence, boundary conditions, and span-to-depth ratio of the test specimen. Further, it is shown that the First order Shear Deformation Theory (FSDT) and the Generalized Layerwise Plate Theory (GLPT) predict qualitatively different results. / Ph. D.

Carbon nanotube (CNT) forests are an important class of nanomaterials with many potential applications due to their unique properties such as mechanical compliance, thermal and electrical conductance, etc. Their deformation and failure in compression loading is critical in any application involving contact because the deformation changes the nature of the contact and thus impacts the transfer of load, heat, and charge carriers across the interface. The micro- and nano-structure of the CNT forest can vary along their height and from sample to sample due to different growth parameters. The morphology of CNTs and their interaction contribute to their mechanical behavior with change of load distribution in the CNT forest. However, the relationship is complicated due to involvement of many factors such as density, orientation, and entanglement of CNTs. None of these effects, however, are well understood. This dissertation aims to advance the knowledge of the structure-property relation in CNT forests and find methodologies for tuning their mechanical behavior. The mechanical behavior of CNT forests grown with different methodologies is studied. Furthermore, the effects of coating and wetting of CNT forests are investigated as methods to tailor the degree of interaction between CNTs. In situ micro-indentation of uncoated CNT forests with distinct growth-induced structures are performed to elucidate the effects of change of morphology along the height of CNT forests on their deformation mechanism. CNT aerial density and tortuosity are found to dictate the location of incipient deformation along height of CNT forests. Macro-compression testing of uncoated CNT forests reveals mechanical failure of CNT forests by delamination at the CNT-growth substrate. Tensile loading of CNT roots due to post-buckling bending of CNTs is proposed to be the cause of this failure and simple bending theory is shown to estimate the failure load to be on the same order of magnitude as experimental measurements. Furthermore, delamination is observed to occur in the in situ micro-indentation of CNT forests coated with aluminum on the top surface, which demonstrates the role of the mechanical constraints within the CNT forest in the occurrence of delamination at the CNT-substrate interface. In addition, this dissertation explores the mechanical behavior of CNT forests coated conformally (from top to bottom) with alumina by atomic layer deposition. In situ micro-indentation testing demonstrates that the deformation mechanism of CNT forests does not change with a thin coating (2 nm) but does change with a sufficiently thick coating (10 nm) that causes fracturing of the hybrid nanotubes. Ex situ flat punch and Berkovich indentations reveal an increase in stiffness of the CNT forests that are in range with those predicted by compression and bending theories. An increase in the recoverability of the CNTs is also detected. Finally, solvent infiltration is proposed as a method of decreasing stiffness of CNT forests and changing the deformation mechanism from local to global deformations (i.e., buckling in the entire height). Presence of solvents between CNTs decreases the van der Waals forces between them and produces CNT forests with lower stiffness. The results demonstrate the effect of interaction between CNTs on the mechanical behavior. This dissertation reveals important information on the mechanical behavior of CNT forests as it relates to CNT morphology and tube-to-tube interactions. In addition, it provides a framework for future systematic experimental and theoretical investigations of the structure-property relationship in CNT forests, as well as a framework for tuning the properties of CNT forests for diverse applications.

A new class of materials, known as multi-functional energetic structural materials (MESMs), has been developed. These materials possess both strength and energetic functionalities, serving as candidates for many exciting applications. One of such applications is ballistic missiles, where these materials serve as part of structural casing as well as explosive payload. In this study, the dynamic compressive behavior of two types of MESMs in the intermediate strain rate regime is investigated. The first type is a thermite mixture of Al and Fe₂O₃ particles suspended in an epoxy matrix. The second type is a shock compacted mixture of Ni and Al powders. Compression experiments on a split-Hopkinson pressure bar (SHPB) apparatus are carried out at strain rates on the order of 103 s-1. In addition, a novel method for investigating the dynamic hardness of the Al + Fe₂O₃ + Epoxy materials is developed. In this method, high-speed digital photography is used to obtain time-resolved measurements of the indentation diameter throughout the indentation process. Experiments show that the shock compacted Ni-Al material exhibits a rather ductile behavior and the deformation of the Al + Fe₂O₃ + Epoxy mixtures is dominated by the polymer phase and significantly modulated by the powder phases. The pure epoxy is ductile with elastic-plastic hardening, softening, and perfectly plastic stages of deformation. The Al and Fe₂O₃ particles in Al + Fe₂O₃ + Epoxy mixtures act as reinforcements for the polymer matrix, impeding the deformation of the polymer chains, alleviating the strain softening of the glassy polymer matrix at lower levels of powder contents (21.6 - 29.2% by volume), and imparting the attributes of strain hardening to the mixtures at higher levels of powder contents (21.6 - 49.1% by volume). Both the dynamic and quasi-static hardness values of the Al + Fe₂O₃ + Epoxy mixtures increase with powder content, consistent with the trend seen in the stress-strain curves. To quantify the constitutive behavior of the 100% epoxy and the Al + Fe₂O₃ + Epoxy materials, the experimentally obtained stress-strain curves are fitted to the Hasan-Boyce model. This model uses a distribution of activation energies to characterize the energy barrier for the initiation of localized shear transformations of long chain polymeric molecules. The results show that an increase in powder content increases the activation energy, decreases the number of transformation sites, causes redistribution of applied strain energy, and enhances the storage of inelastic work. These effects lead to enhanced strength and strain hardening rate at higher levels of powder content.