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<p>The main aim of this PhD dissertation is to investigate the inherent and induced anisotropy in granular materials. The study includes both the experimental and theoretical aspects and provides a methodology for characterizing the mechanical response of granular materials that display anisotropy.</p> <p>The content of this thesis is divided into two main parts. The first part is focused on investigating the mechanical properties of materials with inherent anisotropy. In particular, an experimental program designed to investigate the mechanical properties of Ottawa standard sand (C109), with inherent anisotropy that is generated by the initial densification process, is described. The program involves a series of direct shear as well as triaxial axial tests. Its primary objective is to demonstrate that anisotropy may occur in sands that have nearly spherical particles (i.e. are typically considered as isotropic) provided the distribution of pore space has a preferred orientation due to the initial densification process. Following the experimental part, the mathematical formulation based on the Critical Plane Approach (CPA) is presented for describing anisotropic mechanical behavior of the material. The procedure for identification of parameters embedded in the constitutive model is outlined and an extensive numerical analysis is conducted simulating the experimental tests.</p> <p>The second part of this thesis deals with induced anisotropy and its focus is on developing an evolution law for the fabric of particulate materials as a function of continuing deformation. The microstructure descriptors are based on lineal intercept measurements and include the areal porosity and the mean intercept length distribution. The methodology involves performing a series of Discrete Element simulations for a granular assembly under evolving directions of the principal stress/strain and defining a correlation with the evolution of material axes. It is demonstrated that granular materials with spherical particles may become anisotropic due to the initial compaction process and that the induced anisotropy is characterized by the coaxiality between the microstructure and the total strain tensors. The proposed evolution law is incorporated into the constitutive framework for anisotropic materials, as discussed in the first part, and some numerical simulations are conducted. It is demonstrated that the proposed approach can describe, at least in a qualitative manner, several manifestations of induced anisotropy in granular materials.</p> / Doctor of Philosophy (PhD)

The proton exchange membrane (PEM) is a key component in proton exchange membrane fuel cells (PEMFCs). During standard fuel cell operation, the PEM degrades due to cyclic hygrothermal loads, resulting in performance loss or total failure. Improvement of current PEM materials and development of cheaper, more durable materials is essential to the commercialization of PEMFC technology, which may provide an attractive alternative energy source for transportation. This dissertation investigates a new PEM material which is a blend of sulfonated perfluorocyclobutane (PFCB) and polyvinylidene fluoride (PVDF). Hereafter referred to as PFCB/PVDF, this polymer blend was developed by General Motors Company™ as a potential replacement for the current benchmark PEM, the DuPont™ product Nafion®. The PFCB/PVDF blend is less costly to manufacture than standard PEM materials and investigations into its long-term mechanical durability are ongoing. Specifically, this document discusses the experimental and analytical work performed in the material characterization, constitutive expression development, and implementation of that expression into uniaxial and biaxial finite element geometries. Extension of the model to time-varying temperature and moisture conditions is also explored. The uniaxial finite element model uses a non-linear viscoelastic viscoplastic (NLVE-VP) constitutive expression with parameters determined from uniaxial creep and recovery experiments at a single environmental condition. Validation tests show that this model accurately predicts results from uniaxial tension experiments, such as stress relaxation, force ramp, and multistep creep and recovery, to stresses of 8 MPa and strains approaching 15%, which is the maximum hygrothermal strain expected in an operating fuel cell. The biaxial finite element model combines the NLVE-VP constitutive expression with the geometry of a pressure-loaded blister experiment, which better approximates fuel cell membrane constraints. Results from the biaxial model are compared to experimental results. The model accurately predicts strain in the blister test but predicts stresses that differ from those estimated from blister curvature. Additionally, it is found that both the non-linear viscoelastic and viscoplastic parameters are functions of the operating environment. Future experimental work is needed to characterize that dependence before the constitutive model is used to simulate the response of the PFCB/PVDF blend to fuel cell operating conditions. / Ph. D.

Aluminum alloys are increasingly being used in a broad spectrum of applications such as lightweight structures, light rail, bridge decks, marine crafts, and off-shore platforms. The post-fire (residual) integrity of aluminum structures is of particular concern as a severe degradation in mechanical properties may occur without catastrophic failure, even for short duration, low intensity fires. The lack of research characterizing residual mechanical behavior results in an unquantified mechanical state of the structure, potentially requiring excessively conservative repair. This research aims to develop an in-depth understanding of the mechanisms governing the residual aluminum alloys so as to establish a knowledge-base to assist intelligent structural repair. In this work, the residual mechanical behavior after fire exposure of marine-grade aluminum alloys AA5083-H116 and AA6061-T651 is characterized by extensive mechanical testing. Metallography was performed to identify the as-received and post-fire microstructural state. This extensive characterization was utilized to develop constitutive models for the residual elasto-plastic mechanical behavior of the alloys. The constitutive models were developed as a series of sub-models to predict (i) microstructural evolution, (ii) residual yield strength, and (iii) strain hardening after fire exposure. The AA5083-H116 constitutive model was developed considering the microstructural processes of recovery and recrystallization. The residual yield strength was calculated considering solid solution, subgrain, and grain strengthening. A recovery model was used to predict subgrain growth and a recrystallization model was used to predict grain nucleation and growth, as well as subgrain annihilation. Strain hardening was predicted using the Kocks-Mecking-Estrin law modified to account for the additional dislocation storage and dynamic recovery of subgrains. The AA6061-T651 constitutive model was developed considering precipitate nucleation, growth, and dissolution. A Kampmann-Wagner numerical model was used to predict precipitate size distribution evolution during elevated temperature exposure. The residual yield strength was calculated using solid solution and precipitate strengthening, considering both shearable and non-shearable precipitates. A modified KME law was used to predict residual strain hardening considering the additional effects of the precipitate-dislocation interactions, focusing on the efficient of dislocation (Orowan) loop storage and recovery about the precipitates. In both cases, the constitutive models were bench-marked against experimental data. / Ph. D.

Ligaments and tendons are rope-like structures in our body that possess time- and history-dependent material properties. Despite the many advances made in experimental and theoretical biomechanics, the material properties of these biological structures are still not fully characterized. This dissertation represents a step forward in the development of combined theoretical and experimental tools that capture the time- and history-dependent material properties of ligaments and tendons. The mechanical behavior of bundles of collagen fibers which form ligaments and tendons was investigated. Axial stress-stretch data and stress relaxation data at different axial stretches were collected by testing rat tail tendon fascicles. The experimental results demonstrated, for the first time, that the shape of the normalized axial stress relaxation curve depends on the axial stretch level thus suggesting that the fascicles are nonlinear viscoelastic. A constitutive model was then formulated within the nonlinear integral representation frame- work proposed by Pipkin and Rogers (1968). Unlike the well-known quasi-linear viscoelastic model, the proposed constitutive law was able to capture the observed nonlinearities in the stress relaxation response of rat tail tendon fascicles. By extending the constitutive model for collagen fiber bundles, a new nonlinear three- dimensional model for the stress relaxation of skeletal ligaments was formulated. The model accounts for the contribution of the collagen fibers and the group substance in which they are embedded. Published uniaxial experimental data on the stress relaxation of human medial collateral ligaments were used to determine the model parameters. The model predictions for simple shear in the fiber direction, simple shear transverse to the fiber direction, and equibiaxial extension were then examined and, for the case of simple shear in the fiber direction, such predictions were found to be in good agreement with published experimental data. The relationship between the mechanical response and structure of suspensory ligaments was examined by performing state-of-the-art small angle x-ray diffraction experiments in tandem with incremental stress relaxation tests. Specifically, small angle x-ray diffraction was used to measure changes in strain and orientation of collagen fibrils during the stress relaxation tests. Throughout the tests the collagen fibrils were found to gradually orient towards the loading direction. However, the collagen fibril strain did not change significantly suggesting that collagen fibers do not play a significant role in dissipating load during stress relaxation. / Ph. D.

MScEng / Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009. / In this project the shape and attachment method of tissue leaflets for a percutaneous aortic valve is designed and tested as a first prototype. Bovine and kangaroo pericardium was tested and compared with natural human valve tissue by using the Fung elastic constitutive model for skin. Biaxial tests were conducted to determine the material parameters for each material. The constitutive model was implemented using finite element analysis (FEA) by applying a user-specified subroutine. The FEA implementation was validated by simulating the biaxial tests and comparing it with the experimental data. Concepts for different valve geometries were developed by incorporating valve design and performance parameters, along with stent constraints. Attachment techniques and tools were developed for valve manufacturing. FEA was used to evaluate two concepts. The influence of effects such as different leaflet material, material orientation and abnormal valve dilation on the valve function was investigated. The stress distribution across the valve leaflet was examined to determine the appropriate fibre direction for the leaflet. The simulated attachment forces were compared with suture tearing tests performed on the pericardium to evaluate suture density. In vitro tests were conducted to evaluate the valve function. Satisfactory testing results for the prototype valves were found which indicates the possibility for further development and refinement.