Part I (Efficient Endochronic Finite Element Analysis: an Example of a Cyclically Loaded Boron/Aluminum Composite): A convenient and efficient algorithmic tangent matrix approach has been developed for 3-D finite element (FE) analysis using the endochronic theory without a yield surface. The underlying algorithm for integrating the endochronic constitutive equation was derived by piecewise linearization of the plastic strain path. The approach was employed to solve a micromechanics boundary value problem of a cyclically loaded unidirectional boron/6061 aluminum composite. All the FE results consistently demonstrate superior numerical stability and efficiency of the proposed method. Extensions of the method to endochronic plasticity with a yield surface and to the plane stress case are also presented. Part II (Simple and Unified Finite Element Formulation for Doubly Periodic Models: Applications to Boron/Aluminum Composites): A simple and unified weak formulation and its convenient FE implementation have been proposed. The weak formulation is valid for any doubly periodic model under uniform 3-D macro-stress, and serves as a common rational foundation of different FE approaches. The algorithmic tangent matrix approach for the endochronic theory has been incorporated into the FE formulation to model isothermal, rate-independent plastic macro-deformation of unidirectional fibrous composites with idealized two-phase micro-structure and backed-out inelastic matrix properties. Methods for determining inelastic material parameters of the matrix have been established. Numerical results for a B/6061 AI composite subjected to on-axis and off-axis monotonic tensile loadings are in good agreement with experimental data. The micromechanics model also shows the potential for quantitative characterization of complicated cyclic behavior. Finally, some effects of model geometry on overall plastic response of the B/6061 AI composite are discussed from the viewpoint of theoretical-experimental correlation. / Ph. D.

Presented is a model for predicting SMA-induced deformations in an unsymmetric cross ply laminate. A previously developed theory is used to predict the room-temperature shape of the cross-ply laminate by minimizing its total potential energy. Then, using the principle of virtual work, equations relating the shape of the laminate to a force applied on supports fastened to the laminate are derived. Induced strains and displacements are predicted as a function of the applied force. Experiments where the force is generated by known weights are conducted. Good correlations are established between the experimental results and the predictions. The developed theory is able to predict with good accuracy the shape, strains and, displacements of an unsymmetric cross-ply laminate to the force applied on the laminate. This theory is then used to develop a model relating the laminate response to forces produced by a SMA actuator, the actuator being a SMA wire. To describe the mechanics of the SMA actuator, constitutive equations derived by other researchers are used. These constitutive equations relate the temperature of the wire to forces generated in it. Experiments where a SMA wire is used as an actuator are conducted. These experiments consist of resistively heating a SMA wire attached to supports fastened to the laminate. During these experiments, laminate deformations are measured as a function of the applied voltage. Comparisons with the temperature-based constitutive model predictions are not made since the relation between the applied voltage and the SMA temperature is very difficult to establish. However, the experiments show that a SMA used in conjunction with cross-ply unsymmetric laminates can induce very large changes in the laminate shapes. Thus, the concept of using a SMA actuator to control the shape of cross-ply unsymmetric laminates is validated. / Master of Science

The stresses and deformations in cross-ply composite tubes subjected in circumferential temperature gradients are studied. The motivation behind the study is the anticipated use of composite tubes in space structures where the tube is exposed to the heat of the sun on one side and the cryogenic temperatures of space on the other. Experiments were performed to measure the functional form of the temperature gradient and the displacements. It was found that the form of the temperature gradient, T(Ɵ), can accurately be represented by T(Ɵ) = A + BcosƟ¸ and that the displacement of the tube is parabolic in the axial coordinate. Two types of analytical solutions were developed: an exact elasticity U solution and an approximate solution. The approximate solution includes a linear variation of the material properties with temperature and uses the principle of complementary virtual work in conjunction with a Ritz approximation on the stress field. The elasticity solution predicts that high tensile stresses could crack the matrix. The effect of including temperature-dependent material properties is to reduce the circumferential dependency of the stresses. / Master of Science

This study intends to develop a useful tool for the investigation of the behavior of three-dimensional elastic frame structures undergoing large deformations and large rotations, using a mini-computer with an attached array processor. An updated Lagrangian finite element formulation is established by employing conventional two node-twelve degree of freedom beam elements. In order to trace the pre- and post-buckling equilibrium path, an improved nonlinear solution procedure is proposed. The software is designed to make it possible to solve large-scale problems on a mini-computer by adopting a hypermatrix scheme and the segmentation into a number of processors which are independent programs. The software is simulated to estimate the performance of the software on a combined mini-computer/array processor system. By using the simulator time measurements are performed for three different cases of large-scale three-dimensional frame structure models, which verify the usefulness of the array processor in the solution of non-linear finite element structural problems. With the use of the hypermatrix scheme, an alternative solution algorithm for system of linear equations is proposed. The accuracy of the finite element formulation and the effectiveness of the solution algorithms implemented are demonstrated by carefully selected two- and three-dimensional frame examples. Finally, directions for further research are discussed.

Mg alloys have spurred a renewed academic and industrial interest because of their ultra-light-weight and high specific strength properties. Hexagonal close packed Mg has low deformability and a high plastic anisotropy between basal and non-basal slip systems at room temperature. Alloying with Li and other elements is believed to counter this deficiency by activating non-basal slip by reducing their nucleation stress. In this work I study how Li addition affects deformation mechanisms in Mg using atomistic simulations. In the first part, I create a reliable and transferable concentration dependent embedded atom method (CD-EAM) potential for my molecular dynamics study of deformation. This potential describes the Mg-Li phase diagram, which accurately describes the phase stability as a function of Li concentration and temperature. Also, it reproduces the heat of mixing, lattice parameters, and bulk moduli of the alloy as a function of Li concentration. Most importantly, our CD-EAM potential reproduces the variation of stacking fault energy for basal, prismatic, and pyramidal slip systems that influences the deformation mechanisms as a function of Li concentration. This success of CD-EAM Mg-Li potential in reproducing different properties, as compared to literature data, shows its reliability and transferability. Next, I use this newly created potential to study the effect of Li addition on deformation mechanisms in Mg-Li nanocrystalline (NC) alloys. Mg-Li NC alloys show basal slip, pyramidal type-I slip, tension twinning, and two-compression twinning deformation modes. Li addition reduces the plastic anisotropy between basal and non-basal slip systems by modifying the energetics of Mg-Li alloys. This causes the solid solution softening. The inverse relationship between strength and ductility therefore suggests a concomitant increase in alloy ductility. A comparison of the NC results with single crystal deformation results helps to understand the qualitative and quantitative effect of Li addition in Mg on nucleation stress and fault energies of each deformation mode. The nucleation stress and fault energies of basal dislocations and compression twins in single crystal Mg-Li alloy increase while those for pyramidal dislocations and tension twinning decrease. This variation in respective values explains the reduction in plastic anisotropy and increase in ductility for Mg-Li alloys.

by Cheung, Shing Kwong. / Thesis (Ph.D.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references (p. 137-143). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

The structure and interactions of the defects in material on an atomistic scale ulti- mately determine the macroscopic behavior of that material. A fundamental understanding of how defects behave is essential for predicting materials failure; this is especially true in an irradiated environment, where defects are created at higher than average rates. In this work, we present two different atomistic scale computational studies of defects in body centered cubic (bcc) iron. First, the interaction energies between screw dislocations (line defects) and various kinds of point defects will be calculated, using anisotropic linear elastic theory and atomistic simulation, and compared. Second, the energetics and behavior of hydrogen and hydrogen-helium gas bubbles will be investigated.

Shape memory alloys are a class of alloys that display the unique ability to undergo large plastic deformations and return to their original shape either through the application of heat (shape memory effect) or by relieving the stress causing the deformation (superelastic effect). This research takes advantage of the unique characteristics of shape memory alloys in order to provide a moment resisting connection with recentering capabilities. In this study, superelastic Nitinol, a nickel-titanium form of shape memory alloy that exhibits a flag-shaped stress versus strain curve, is used as the moment transfer elements within a partially restrained steel beam-column connection. Experimental testing consists of a one-half scale interior connection where the loading is applied at the column tip. A pseudo-static cyclic loading history is used which is intended to simulate earthquake loadings. The energy dissipation characteristics, moment-rotation characteristics, and deformation capacity of the connection are quantified. Results are then compared to tests where A36 steel tendons are used as the moment transfer elements. The superelastic Nitinol tendon connection showed superior performance to the A36 steel tendon connection, including the ability to recenter without residual deformation.

Transformation Induced Plasticity (TRIP) steels undergo a phase transformation when subjected to high levels of mechanical strain. This transformation from a paramagnetic austenitic parent phase to a ferromagnetic martensitic phase is irreversible and the resultant magnetic properties may therefore be used as a measure of strain history. The transformation behaviour of TRIP steels has been recognised as a potential smart characteristic and various proposals have appeared aimed at producing a structure that performs its primary structural function as well a strain sensing function simultaneously. However the strain induced nature of the transformation implies that transformation will occur in areas of high stress concentration and therefore engineered stress concentration features will be required to provide a consistent measure of the changes in the magnetic properties of the material as a function of applied load. In order to predict the performance of smart TRIP steel sensors, an analysis method capable of quantifying the effectiveness of a component in its dual role as structure and sensor is needed. The thesis addresses the development of a methodology for correlating the changing magnetic permeability of TRIP steel sensors and structures with martensitic transformation behaviour. The prediction of the deformation behaviour including transformation is implemented by considering a mechanical analysis based on the finite element method and a constitutive model incorporating strain-induced martensitic transformation kinetics. .Extensions to the model which allow for a wide range of deformation rates and temperatures are also discussed. In order to demonstrate the application of the methodology, an analysis of a simple tensile element used in strain measurement applications is presented. The analysis also includes the effect of temperature on the performance of the sensor. An analysis of a design proposal for a smart aircraft bolt is also included to investigate the effects of geometry, particularly engineered stress concentrations, and sensor placement. / Thesis (Ph.D.)-University of Natal, Durban, 2003.

A new philosophy in structural health monitoring was explored, with the view to the creation of a smart mining bolt: one which would bear the normal load of any bolt used in South African gold mining tunnels, but at the same time be capable of monitoring its own level of damage. To this end, a survey of various smart materials currently used in structural health monitoring applications, was conducted, and a group known as strain memory alloys isolated as holding the most promise in this regard. Strain memory alloys give an indication of peak strain based on an irreversible transformation from paramagnetic austenite to ferromagnetic martensite, which occurs in direct proportion to the amount of strain experienced by the material. A measurement of magnetic permeability can therefore be correlated to peak strain. An extensive study of the alloying chemistry, material processing and transformation characteristics was therefore carried out, including an analytical model for the quantification of the energy associated with martensitic nucleation, at a dislocation-disclination level. The conditions within typical South African gold mining tunnels were evaluated, and a smart mining bolt design produced, based on the loading and environmental conditions present. Several material formulations were then proposed, melted, tested and evaluated against the relevant strength, corrosion and transformation criteria. A suitable material was selected and further tested. A working prototype bolt has been produced, and in situ tests of complete bolts, are scheduled to take place shortly. / Thesis (Ph.D.)-University of Natal, Durban, 2003.