<p>The subject of this thesis is the mechanical response of inherently anisotropic granular materials. The study comprises both the experimental and numerical aspects and provides a rigorous methodology for the solution of geotechnical problems involving anisotropic frictional materials.</p> <p>The experimental investigation has been carried out at both the material and structural levels. The material tests involved a series of direct shear, triaxial and hollow cylinder experiments on crushed limestone sand, whose mechanical characteristics are strongly affected by the orientation of the sample. In addition, a scaled foundation setup was designed and a series of experimental tests was carried out to examine the effects of anisotropy on bearing capacity.</p> <p>The numerical part of this work was associated with development and implementation of a constitutive framework that describes the mechanical response of transversely isotropic frictional materials. The framework is based on elasto-plasticity and accounts for the effects of strain localization and inherent anisotropy of both the deformation and strength characteristics. An implicit scheme was proposed for identification of material parameters/functions, which incorporates predictions based on critical plane analysis. As a part of constitutive modeling, a suitable numerical algorithm was also developed to integrate the constitutive equations.</p> <p>The constitutive framework has been implemented in a commercial FE package (ABAQUS). A series of numerical simulations were carried out focused on the assessment of the bearing capacity of a shallow foundation in transversely isotropic granular medium. The results of numerical simulations have been compared with the experimental data. A parametric study was also carried out aimed at examining the influence of various simplifications in the mathematical framework on its predictive abilities.</p> / Thesis / Doctor of Philosophy (PhD)

This study investigated the role of Sic particles in the mechanical behaviour of a metal matrix composite (Al-SiC). Measurements of the development and magnitude of internal stresses were performed from Bauschinger experiments in the aluminum matrix A-356 reinforced with the Sic particles. The behavior of the Al matrix itself was also analyzed. The level of internal stresses in the particulate reinforced composite was found to saturate after 0.9% plastic strain and after 1.3% in the unreinforced matrix. The initial development of the unrelaxed internal stresses was analyzed using both microscopic and macroscopic models of the load-bearing role of the Sic particles. The Sic particles were found to have little influence on the plastic flow of the composite beyond the initial plastic deformation as the size and distribution of the Sic is very non-uniform. The effect of the Sic phase was compared to continuous fibers embedded in a metallic matrix. A model system of pure copper reinforced with continuous tungsten fibers was used for this purpose. The effect of the internal stresses on the dimensional stability of the particulate reinforced and the unreinforced matrix was also investigated. The generation of dislocations arising from the thermal cycling of those materials was also analyzed by reference to the increase in flow stress observed after thermal cycling and from a model based on dislocations production due to the difference in coefficient of thermal expansion of the phases. / Thesis / Master of Engineering (ME)

No description available.

Osteoporosis is a disease prevalent in older adults, characterized by increased bone porosity resulting in significant fracture risk. Orthopaedic implants are designed and validated against cadavers from the general ‘healthy’ population, but little is known about their response in osteoporotic bone. Orthopaedic implants can also be developed using synthetic bones, if they have been demonstrated to be representative of healthy bone, and offer a number of advantages. To date, no synthetic femur has been validated for the osteoporotic population. The purpose of this study was to assess novel synthetic femurs for representing this population. Custom jigs were manufactured to test two sets of ten synthetic femurs and five isolated cadaveric femurs in four-point bending, torsion, axial compression, axial failure, and screw pullout, using an Instron mechanical testing machine to record load-displacement data. Statistical significance was found in bending, torsion, and screw pullout between both synthetic sets and cadavers using one-way ANOVA with post-hoc Tukey analysis. In all instances, the synthetic femurs had lower coefficients of variation than natural specimens. Both synthetic and cadaveric femurs were CT scanned prior to testing. The data were used to measure key anatomical details and to develop a series of numerical models of the synthetic bones, using Materialize Mimics® and ABAQUS® software, evaluated using axial and bending data. The model was modified by reducing cortical thickness and modulus in an attempt to make the synthetic model better represent osteoporotic bone. Establishing synthetic femurs as suitable replacements for osteoporotic bone allows for improved orthopaedic implant development. The digital model constructed allows the synthetic to be further analyzed, improving expected response of the synthetic bones. These synthetic bones could provide a foundation for development of effective orthopaedics for this population. / Thesis / Master of Applied Science (MASc) / The considerations and parameters in the design of orthopaedic implants for osteoporotic bone are relatively unknown. Orthopaedic implants can be evaluated with synthetic bones, which offer a number of advantages to natural specimens, assuming they are sufficiently representative of natural bone. No physical synthetic model yet exists that represents an osteoporotic femur. In the present work, synthetic femurs were subjected to bending, torsion, axial compression, and screw pullout and compared to natural osteoporotic specimens. The synthetics were significantly different to natural specimens in bending, torsion, and screw pullout. A numerical model was created, evaluated, and tested in finite element software alongside modified models with reduced modulus and cortical thickness to assess stiffness. Recommendations were made to improve the accuracy of a future synthetic model. The synthetic femurs tested were not representative of osteoporotic femurs, but may be feasible alternatives with minor modifications and could be useful in future orthopaedics design.

The purpose of this study was to assess the validity, as well as to test novel approaches to improving the reliability, of mechanical response tissue analysis (MRTA). Twenty composite tibiae underwent MRTA testing on three separate days to determine intra- and inter-day reliability of bending stiffness. The bones were then subjected to three-point bending tests to directly determine elastic modulus. Within- and between-day reliability of tibial bending stiffness with MRTA was moderate (CV = 24%) and poor (CV = 74%), respectively. No relationship was observed between the two testing methods due to the wide variation in tibial bending stiffness values with MRTA. The second part of the study sought to determine within- and between-day reliability of MRTA in young women with the current testing protocol and compare the results with those from newly-designed protocols. Twelve women (23 ± 2 yr, 162 ± 7 cm, 57 ± 7kg, 19 ± 4 % fat) were tested for tibial bending stiffness with MRTA over 5 days. The current protocol was compared to protocols where day-to-day subject positioning was quantified, subjects were tested in a supine position, and various bending stiffness prediction models were used. Within- (CV = 20%) and between-day (CV = 19%) reliability of tibial bending stiffness with MRTA was moderate using the original methodology. Modifications to this protocol either resulted in similar or worse reliability. / Ph. D.

Mechanical response tissue analysis (MRTA) provides a noninvasive means of estimating the cross-sectional bending stiffness (EI) of long bones, and thus can serve as a predictor of bone strength. Estimates of bone bending stiffness are derived from the point impedance response of a long bone to low frequency (70-500Hz) stimulation according to beam vibration theory. MRTA has demonstrated the ability to reliably estimate human ulnar bending stiffness with between-test coefficients of variation of 5%, and in vivo measurements of monkey tibiae have been validated with ex vivo 3-point mechanical bending tests. Human tibial MRTA measurement has achieved between-trial coefficients of variation of only 12%, so a new physical MRTA configuration and improved computer algorithms have been developed in an attempt to improve upon this level of reliability. The new configuration removes the rigid proximal and distal tibial restraints and models the tissue behavior with a 12-parameter algorithm that accounts for free vibration at the ankle and knee joints. Initial testing with only the hardware changes and application of the 7-parameter model of tissue behavior used in earlier systems yielded unacceptable variation. Subsequent reliability testing with application of 6-, 9-, and 12-parameter models demonstrated modest improvements, prompting the development of the more robust 12-parameter model used in the present study. Evaluation of 110 college-age females (age 20.2±1.8 yr, height 163.3±5.9 cm, weight 60.7±9.3 kg, BMI 22.8±3.1 kg·m⁻²) with the current MRTA system has demonstrated an improvement in within-trial reliability for unsupported tibial EI measurement with a coefficient of variation of 11.2%. These results demonstrate the ability of the system to measure tibial response characteristics when both proximal and distal ends are free of rigid support. Long-term measurement reliability is still problematic with a coefficient of variation of 36.5% for a set of 4 measurements spanning 21 months. / Ph. D.

A cysteine-containing elastin-like polypeptide (ELP2cys) was successfully synthesized and purified, and was shown to behave in a similar fashion to other well-characterized ELPs. Incorporating the ELP2cys as a crosslinking agent into a solution of sulphated hyaluronan (CMHA-S) not only decreased the gelation time of the solution but also increased the crosslinking density of the resultant hydrogel, in turn increasing both the resiliency and stiffness of the construct. Preliminary in vitro work involved culture of human disc cells, followed by their encapsulation within the hydrogel. Unfortunately the results were inconclusive, although it appeared as though the addition of ELP2cys to the matrix did not negatively affect the viability of the cells, as compared to hydrogels with CMHA-S only. This study showed that ELP2cys is a valuable addition to the family of recombinant elastin-like polypeptides, and shows promise as a crosslinking agent in the formation of hyaluronan hydrogels.

Il est bien établi que les propriétés mécaniques et rhéologiques d'une large classe de matériaux vitreux amorphes met en jeu - contrairement aux dislocations dans les cristaux - des rearrangements structuraux localisés formant par un processus de cascade des bandes de cisaillements. Cette localisation de la déformation est observée dans divers systèmes vitreux ainsi que dans des simulations numériques. Cette réponse mécanique complexe reste mal comprise à une échelle microscopique et il n'est pas clair si l'écoulement plastique peut être associé à une origine structurale locale ou à des processus purement dynamiques.Dans cette thèse nous envisageons ces problématiques à l'aide de simulations atomiques athermales sur un système Lennard-Jones modèle. Nous calculons le tenseur élastique moyenné localement sur une échelle nanométrique. A cette échelle, le verre est assimilable à un matériau composite comprenant un échafaudage rigide et des zones fragiles. L'étude détaillée de la déformation plastique à différents taux de cisaillement met en évidence divers régimes d'écoulement. En dessous d'un taux de cisaillement critique dépendant de la taille du système, la réponse mécanique atteind une limite quasistatique (effets de taille fini, cascades d'événements plastiques, contrainte seuil) alors que pour des taux de cisaillement plus importants les propriétés rhéologiques sont fixées par le taux de cisaillement imposé. Dans ce régime nous mettons en évidence la croissance d'une longueur de coopérativité dynamique et discutons de sa dépendance avec le taux de cisaillements.

The ability of current clinical tools to predict bone fractures is poor, likely because these tools focus on bone mass and mineral content and neglect bone quality and the collagen phase. The Mechanical Response Tissue Analyzer (MRTA) is an instrument that provides a non-invasive mechanical measurement of the whole bone. It has traditionally been used to obtain a bone stiffness constant (Kb), but can provide a bone damping constant (Bb) that has not previously been considered. The goal of this research is to determine whether the MRTA can detect three damage modes that do not alter bone mass or mineral density: γ-irradiation, collagen over-crosslinking, and fatigue. The MRTA detected a reduction in Bb due to over-crosslinking. Fatigue was found to increase Bb and decrease Kb, and these changes were confirmed through dynamic bending tests. The MRTA shows potential to diagnose increased fracture risk in scenarios where damage is currently undetectable.

The ability of current clinical tools to predict bone fractures is poor, likely because these tools focus on bone mass and mineral content and neglect bone quality and the collagen phase. The Mechanical Response Tissue Analyzer (MRTA) is an instrument that provides a non-invasive mechanical measurement of the whole bone. It has traditionally been used to obtain a bone stiffness constant (Kb), but can provide a bone damping constant (Bb) that has not previously been considered. The goal of this research is to determine whether the MRTA can detect three damage modes that do not alter bone mass or mineral density: γ-irradiation, collagen over-crosslinking, and fatigue. The MRTA detected a reduction in Bb due to over-crosslinking. Fatigue was found to increase Bb and decrease Kb, and these changes were confirmed through dynamic bending tests. The MRTA shows potential to diagnose increased fracture risk in scenarios where damage is currently undetectable.