In this study, mechanical properties of Al-Zn alloys were conducted, with various parameters including Zn contents, grain size, and tensile strain rate. Experimental samples were all manufactured with friction stir processing method. Samples of Al-Zn alloys with the grain size of 1.5£gm, 1£gm, or 0.5£gm and five Zn concentration were pulled in tension at strain rate of 10-3s-1,10-4s-1 and 10-5s-1 . The data set were then used to draw engineering and true tensile stress vs. strain curves , flowing stress vs. Zn contents curves, Hall-Petch equation curves, m vs. Zn contents curves and m vs. grain size curves. Quantitative analysis were conducted to discover that solid solute softening and inverse Hall-Petch relation were present in Al-Zn alloys, which were more prominent at slower tensile strain rate when grain size was less than 1£gm and the Zn contents was higher than 10wt%. Quantitative analysis of strain rate sensitivity (m) showed the trends of increasing value of m with higher Zn contents and smaller grain sizes when solid solute softening and inverse Hall-Petch relation were present. The high grain-boundary diffusion coefficient of Zn which accelerates the efficiency of dynamic recovery are considered the main reason. The effect gets more prominent with increasing Zn contents , smaller grain size , and slower tensile strain rate. For Zn concentration higher than 10wt%, dynamic recovery may drive inverse Hall-Petch relation to appear when grain size is about 1£gm large.

Ultra-fine grained (UFG) materials have attracted considerable interest due to the possibility of achieving simultaneous increase in strength and ductility. Effective use of these materials in engineering applications requires investigating the processing-microstructure-property inter-relations leading to a comprehensive understanding of the material behavior. Research efforts on producing UFG hexagonal close packed (hcp) materials have been limited in spite of their envisaged utilization in various technologies. The present study explores multiple UFG hcp materials to identify the general trends in their deformation behaviors, microstructural features, crystallographic texture evolutions and mechanical responses under strain path changes. UFG hcp materials, including commercial purity Ti, Ti-6Al-4V alloy and high purity Zr, were fabricated using equal channel angular extrusion (ECAE) as a severe plastic deformation (SPD) technique following various processing schedules. Several characterization methods and a polycrystal plasticity model were utilized in synergy to impart the relationships between the UFG microstructure, the texture and the post-ECAE flow behavior. Pure UFG hcp materials exhibited enhanced strength properties, making them potential substitutes for coarse-grained high strength expensive alloys. Incorporation of post-ECAE thermo-mechanical treatments was effective in further improvement of the strength and ductility levels. Strong anisotropy of the post-ECAE flow response was evident in all the materials studied. The underlying mechanisms for anisotropy were identified as texture and processing-induced microstructure. Depending on the ECAE route, the applied strain level and the specific material, the relative importance of these two mechanisms on plastic flow anisotropy varied. A viscoplastic self-consistent approach is presented as a reliable model for predicting the texture evolutions and flow behaviors of UFG hcp materials in cases where texture governs the plastic anisotropy. Regardless of the material, the initial billet texture and the extrusion conditions, ECAE of all hcp materials revealed similar texture evolutions. Accurate texture and flow behavior predictions showed that basal slip is the responsible mechanism for such texture evolution in all hcp materials independent of their axial ratio. High strength of the UFG microstructure was presented as a triggering mechanism for the activation of unexpected deformation systems, such as high temperature deformation twinning in Ti-6Al-4V and room temperature basal slip in pure Zr.

Electrical Muscle Stimulation (EMS) is used as a countermeasure in animal disuse studies that seek to determine which forms of exercise are most effective in mitigating the effects of disuse atrophy on bone and muscle. Although EMS has been used for many years in our lab and others, few researchers have been able to quantify the levels of strain on rat tibiae during EMS and far fewer have investigated the causal relationship between torque produced at the ankle and strain on the tibia. This thesis sought to investigate the relationship between ankle torque and tibial strain by using a combination of techniques, namely: (1) the addition of rosette strain gages, (2) improved synchronization between ankle torque and tibial strain recordings, and (3) spectral analysis between torque and strain waveforms. In previous work, few methods existed to align torque and strain recordings temporally, as those data were recorded on separate computers and synchronizing events were not captured. Attempting to create a torque-strain crossplot with unsynchronized data does not always yield valid results, so a method of reliably synchronizing those data is required. This thesis developed methods to capture simultaneous (synchronizing) events in both torque and strain recordings and then used those captured events to synchronize data between two computers. Following that synchronization, stiffness calculations were run on torque-strain crossplots to determine linear-model relationships between torque and strain for each method of synchronization. The results from those regressions were then used to determine if one or more synchronization techniques are superior to others, in terms of repeatability or precision. The results of these analyses have shown that using portions of the curves can dramatically increase computing speed while providing high levels of repeatability in synchronization measures. After synchronization techniques had been investigated, 3-element rosette data were used to calculate the principal strains on the surface of the tibiae, and the percentage of principal strains that are accounted for in the axial direction. Since the strain environment changes along the axis of the bone, the principal strain data were plotted versus the distance from proximal epiphysis to rosette gage, and statistical analysis was presented. After rosette data were analyzed, the torque and strain data pairs were fed into a signal processing suite for the purpose of transfer function calculation. Using the synchronization methods outlined above, two means of synchronization were compared in the transfer function program. Results for these analyses demonstrated that transfer functions are slightly dependent on synchronization methods, but that calculated gains do not differ between synchronization techniques. The specific shapes of the transfer functions highlight the relative attenuation/amplification of frequencies in torque and strain signals. Specifically, a range of frequencies, commonly called a band, between 24 and 32Hz is attenuated by the soft tissues and mechanical linkages in the lower leg of rats. This finding gives researchers looking to increase or decrease modeling stimulus to bone a new piece of information about the relative efficiency of EMS exercise. For example, EMS performed at 24-25Hz might produce less strain in the tibia than EMS at 22-23Hz, despite the 22-23Hz stimulation producing marginally less torque.

The goal of this research work is to further the understanding of crystal plasticity, particularly at reduced structural and material length scales. Fundamental understanding of plasticity is central to various challenges facing design and manufacturing of materials for structural and electronic device applications. The development of microstructurally tailored advanced metallic materials with enhanced mechanical properties that can withstand extremes in stress, strain, and temperature, will aid in increasing the efficiency of power generating systems by allowing them to work at higher temperatures and pressures. High specific strength materials can lead to low fuel consumption in transport vehicles. Experiments have shown that enhanced mechanical properties can be obtained in materials by constraining their size, microstructure (e.g. grain size), or both for various applications. For the successful design of these materials, it is necessary to have a thorough understanding of the influence of different length scales and evolving microstructure on the overall behavior. In this study, distinction is made between the effect of structural and material length scale on the mechanical behavior of materials. A length scale associated with an underlying physical mechanism influencing the mechanical behavior can overlap with either structural length scales or material length scales. If it overlaps with structural length scales, then the material is said to be dimensionally constrained. On the other hand, if it overlaps with material length scales, for example grain size, then the material is said to be microstructurally constrained. The objectives of this research work are: (1) to investigate scale and size effects due to dimensional constraints; (2) to investigate size effects due to microstructural constraints; and (3) to develop a size dependent hardening model through coarse graining of dislocation dynamics. A discrete dislocation dynamics (DDD) framework where the scale of analysis is intermediate between a fully discretized (e.g. atomistic) and fully continuum is used for this study. This mesoscale tool allows to address all the stated objectives of this study within a single framework. Within this framework, the effect of structural and the material length scales are naturally accounted for in the simulations and need not be specified in an ad hoc manner, as in some continuum models. It holds the promise of connecting the evolution of the defect microstructure to the effective response of the crystal. Further, it provides useful information to develop physically motivated continuum models to model size effects in materials. The contributions of this study are: (a) provides a new interpretation of mechanical size effect due to only dimensional constraint using DDD; (b) a development of an experimentally validated DDD simulation methodology to model Cu micropillars; (c) a coarse graining technique using DDD to develop a phenomenological model to capture size effect on strain hardening; and (d) a development of a DDD framework for polycrystals to investigate grain size effect on yield strength and strain hardening.

Estrogens are a group of endocrine disrupting compounds capable of causing abnormalities in the reproductive systems of the wildlife. Wastewater is a major source of environmental estrogens, in part due to incomplete removal of estrogens in biological wastewater treatment processes. This dissertation investigated factors affecting estrogen biodegradation in bioreactors. Specifically, research efforts were placed on characterization of several bacterial estrogen degraders (model strains: Aminobacter strains KC6 and KC7, and a Sphingomonas strain KC8) and examination of the effects of operating parameters on estrogen removal and estrogen-degrading microbial community structure. Sphingomonas strain KC8 can use 17beta-estradiol as a sole carbon source, suggesting that estrogen degradation by KC8 is a growth-linked, metabolic reaction; however, estrogen degradation by strains KC6 and KC7 might be a non-growth linked, cometabolic reaction. One important finding was that strain KC8 can also degrade and further utilize testosterone as a growth substrate. Strain KC8 was characterized in terms of its utilization kinetics toward estrogens and testosterone with the results that showed relatively smaller kinetic parameters than the typical values for heterotrophs in activated sludge. Strain KC8 can also grow on other organic constituents (glucose, succinate, and acetate). Strain KC8 retained its ability to degrade both 17beta-estradiol and estrone (after 15 d of growth on a complex nutrient medium without 17beta-estradiol). Effective removals (>98.7 %) of 17beta-estradiol with no significant differences were observed in sequencing batch reactors (SBRs) under three solid retention times (SRTs of 5, 10, 20 d). The population ratios of known estrogen degraders (strains KC8 and ammonia-oxidizing bacteria (AOB)) and amoA gene (associated with ammonia oxidation) to total bacteria decreased as SRT increased in SBRs. These observations correspond to the decreasing percentages of 17 beta-estradiol biodegraded in SBR when SRT increased from 5 to 20 d, when the sorption of 17 beta-estradiol onto biomass was considered. Real-time terminal restriction fragment length polymorphism showed that more ribotypes were observed in SBR-20d than SBR-5d. The species evenness (E) in microbial community structures in SBRs was not affected by SRT. However, diversity indices (Shannon-Weaver diversity index (H) and the reciprocal of Simpson?s index (1/D)) suggest that longer SRTs might lead to a more diverse microbial community structure.

Eshelby-type inclusion problems of an infinite or a finite homogeneous isotropic elastic body containing an arbitrary-shape inclusion prescribed with an eigenstrain and an eigenstrain gradient are analytically solved. The solutions are based on a simplified strain gradient elasticity theory (SSGET) that includes one material length scale parameter in addition to two classical elastic constants. For the infinite-domain inclusion problem, the Eshelby tensor is derived in a general form by using the Green’s function in the SSGET. This Eshelby tensor captures the inclusion size effect and recovers the classical Eshelby tensor when the strain gradient effect is ignored. By applying the general form, the explicit expressions of the Eshelby tensor for the special cases of a spherical inclusion, a cylindrical inclusion of infinite length and an ellipsoidal inclusion are obtained. Also, the volume average of the new Eshelby tensor over the inclusion in each case is analytically derived. It is quantitatively shown that the new Eshelby tensor and its average can explain the inclusion size effect, unlike its counterpart based on classical elasticity. To solve the finite-domain inclusion problem, an extended Betti’s reciprocal theorem and an extended Somigliana’s identity based on the SSGET are proposed and utilized. The solution for the disturbed displacement field incorporates the boundary effect and recovers that for the infinite-domain inclusion problem. The problem of a spherical inclusion embedded concentrically in a finite spherical body is analytically solved by applying the general solution, with the Eshelby tensor and its volume average obtained in closed forms. It is demonstrated through numerical results that the newly obtained Eshelby tensor can capture the inclusion size and boundary effects, unlike existing ones. Finally, a homogenization method is developed to predict the effective elastic properties of a heterogeneous material using the SSGET. An effective elastic stiffness tensor is analytically derived for the heterogeneous material by applying the Mori-Tanaka and Eshelby’s equivalent inclusion methods. This tensor depends on the inhomogeneity size, unlike what is predicted by existing homogenization methods based on classical elasticity. Numerical results for a two-phase composite reveal that the composite becomes stiffer when the inhomogeneities get smaller.

In this study, the behavior of pressure in the Rapid Distortion Limit, along with its concomitant modeling, are addressed. In the first part of the work, the role of pressure in the initiation, propagation and suppression of flow instabilities for quadratic flows is analyzed. The paradigm of analysis considers the Reynolds stress transport equations to govern the evolution of a dynamical system, in a state space composed of the Reynolds stress tensor components. This dynamical system is scrutinized via the identification of the invariant sets and the bifurcation analysis. The changing role of pressure in quadratic flows, viz. hyperbolic, shear and elliptic, is established mathematically and the underlying physics is explained. Along the maxim of "understanding before prediction", this allows for a deeper insight into the behavior of pressure, thus aiding in its modeling. The second part of this work deals with Rapid Pressure Strain Correlation modeling in earnest. Based on the comprehension developed in the preceding section, the classical pressure strain correlation modeling approaches are revisited. Their shortcomings, along with their successes, are articulated and explained, mathematically and from the viewpoint of the governing physics. Some of the salient issues addressed include, but are not limited to, the requisite nature of the model, viz. a linear or a nonlinear structure, the success of the extant models for hyperbolic flows, their inability to capture elliptic flows and the use of RDT simulations to validate models. Through this analysis, the schism between mathematical and physical guidelines and the engineering approach, at present, is substantiated. Subsequently, a model is developed that adheres to the classical modeling framework and shows excellent agreement with the RDT simulations. The performance of this model is compared to that of other nominations prevalent in engineering simulations. The work concludes with a summary, pertinent observations and recommendations for future research in the germane field.

Soil properties (shear modulus and damping) are normally determined from laboratory tests. These tests provide both values of the shear modulus in the linear elastic range for very small levels of strain, and its variation with the level of strain. It has become more common to measure the maximum shear modulus at low levels of strain directly in the field, using geophysical techniques. The values obtained in situ can differ significantly in some cases from those determined in the laboratory, and a number of reasons and correction factors have been proposed in the literature to account for this variation. As a result, when in situ properties are available, it is normal to use these values for very low levels of strain, but still assume that the variation of the ratio G/Gmax (normalized shear modulus) with shear strain is the same as determined in the laboratory. Recently, tests have been performed using large vibrators (the Thumper and Tyrannosaurus Rex of the University of Texas at Austin) to determine soil properties in situ for larger strains, and the variation of G/Gmax obtained from these tests has been compared to that reported in the literature from lab tests. Observation indicates some generally good agreement, but also some minor variations. One must take into account, however, that in the determination of the shear modulus versus strain in the field from vibration records, a number of approximations are introduced. The objective of this work is to evaluate the accuracy of some the procedures used and to assess the validity of the simplifying assumptions which are made. For this purpose, a shear cone that would reproduce correctly the horizontal stiffness of a circular mat foundation on the surface of an elastic, homogeneous half space, was considered. The cone was discretized using both a system of lumped masses and springs and a finite difference, using second-order central difference formulation, verifying that in the linear elastic range the results were accurate. A number of studies were conducted next, increasing the level of the applied force and using nonlinear springs that would reproduce a specified G/Gmax vs. γ curve. Using a similar procedure to that used in the field tests, the shear wave velocity between hypothetical receivers and the levels of strain were determined. The resulting values of G/Gmax vs. γ were then compared with the assumed curve to assess the accuracy of the estimated values.

Racial inequality within the juvenile justice system has been cited by numerous studies. This racial inequality is generally referred to as disproportionate minority contact (DMC), and the causes have been debated in the literature for decades. Using a relatively unique methodology for DMC literature, this study incorporated in-depth interview data from thirty male juveniles residing in a private correctional facility to elucidate possible causes of DMC. By analyzing and comparing the experiences of incarcerated juveniles, support for theories of systemic racism, Donald Black’s self-help or the community justice theory, and Agnew’s general strain theory was found. Themes that emerged from the qualitative data include differences in neighborhood and family contexts for minorities compared to whites, variations in motivations for engagement in criminal activity, and differences in the interactions with police officers and perceptions of the police based on race. Specifically, major findings show minority participants were more likely to describe anger and revenge as the most common reason for committing crimes compared to whites, who frequently cited boredom as their primary reason for engaging in criminal activity. Furthermore, black, Latino, and Native American participants were more likely to report growing up in dangerous neighborhoods than whites. Police interactions also showed a racial discrepancy, with whites receiving more chances from the police, and minorities being repeatedly arrested by the same officer slightly more frequently than whites. Overall, findings suggest that disproportionate minority contact is a result of disproportionate levels of strain and injustice experienced by minorities compared to whites.

In this thesis, the relation between shear load and displacement for the lead-free solder (Sn3.0Ag0.5Cu) and the tin-lead solder (63Sn37Pb) are investigated. Except that, a new shear strength of the solder balls is suggested with considering the plastic strain energy of the solder balls. Three diameters of the Sn/Ag/Cu and Sn/Pb solder balls are studied. The variation of the plastic strain energies for the balls undergone different number of thermal cycles is compared. The effect of high temperature aging on the shear strength is also discussed. The difference between the failure fractures of the Sn/Ag/Cu and Sn/Pb solder ball are executed by using SEM. The experimental results show that the failure mechanism for the Sn/Ag/Cu is quite different from the Sn/Pb solder ball. Generally, the lead-free Sn/Ag/Cu solder is much ductile than the Sn/Pb solder ball in the shear test. Also the better fatigue performances are observed for the Sn/Ag/Cu solder balls.