A theory is presented to examine the formation of wrinkles in plates when subjected to high rates of loading in the axial direction. The type of instability examined occurs in metals when the strains are well beyond the elastic range. For this reason the metals are assumed to be governed by the equations of a rigid plastic material. In particular, the von-Mises yield criterion is used in conjunction with the Levy-Mises flow rule. A parameter is introduced which measures the lateral restraint of the plate. By giving this parameter different values, all plate widths can be examined. The theory predicts wavelengths of the buckled plates which are compared with some experimental results obtained in 1968 by Goodier. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

An important consideration in the design of clay foundation for seismic or wave loading is the undrained response of clay during and after cyclic loading . Cyclic loading of clays causes, in general, a reduction in both stiffness and undrained strength on subsequent static loading. No systematic study has been carried out to assess this loss in stiffness, and there are conflicting conclusions as to the magnitude of strength reduction in studies reported in the literature. This thesis presents a study of the influence of cyclic loading on the post-cyclic undrained stress-strain characteristics of a marine clay. The influence of factors, such as, cyclic stress level, number of cycles, amplitude of maximum axial strain during cyclic loading, residual pore pressure and residual strain at the conclusion of cyclic loading is systematically investigated. In addition, the influence of initiating cyclic loading with the type of loading pulse (compression and extension) and the sense of residual strain in relation to the sense of strain during post-cyclic monotonic loading is studied. It is shown that the loss in undrained stiffness and undrained strength of the undisturbed clay as a consequence of cyclic loading are not uniquely related to the amplitude.of strain during cyclic loading, as commonly assumed. Nor can they be explained in terms of overconsolidation induced as a result of pore pressure generated due to cyclic loading. A rational explanation and correlation of both the changes in post-cyclic stress-strain and strength of clay is provided in terms of hysteretic work absorbed by the clay during cyclic loading. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

The research for this thesis was carried out to investigate the effects of anisotropy on stress and strain measurements made by the Leeman "doorstopper" technique. The stress field in rock is influenced by many different sources. These complicate the methods for obtaining a stress tensor which is a representive model of the in-situ stress condition. Research has been carried out to determine stress concentrations for the Leeman doorstopper technique in isotropic ground, but previously, no values were known to exist for anisotropic conditions. A three-dimensional finite-element computer model was used to investigate the effect of anisotropy on stress and strain concentration factors. Displacement data and strain concentration factors obtained from this analysis, although not quantitatively accurate, showed that the doorstopper technique can indicate erroneous stress levels in anisotropic ground unless the appropriate corrections can be made. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

D.Ing. / In mechanical design and analysis the mechanical properties of the material used are crucial to achieve effective design or analysis. In designing structures that are susceptible to dynamic loading different mechanical properties of the material may be needed than those used for quasi-static situations. Usually when one refers to the dynamic properties of a metal one refers to the notch toughness of the material. That is the resistance of the material to crack propagation under dynamic loading. Another less well known dynamic property of a metal is strain rate sensitivity. This implies that mechanical properties like yield strength, tensile strength and rupture strain varies according to strain rate. Typical applications where these properties are of use are in impact situations such as vehicle collisions and cold and hot working of metals in the manufacturing industry. The mechanical properties of certain metallic components or structures may change when the component or structure are subjected to dynamic loading that causes permanent deformation. The purpose of this investigation is to investigate the strain rate sensitive behaviour of certain stainless steels. The steels investigated are AISI Types 304, 316 and 430 stainless steels, 3CR12 corrosion resisting steel (a proprietary alloy also known as Type 1.4003) and mild steel which acts as a reference. The strain rate sensitivity of the above mentioned steels are investigated experimentally at room temperature for strain rates between 10' to approximately 100 s -1 . The steels are all tested in as delivered sheet form and testing is conducted in both rolling directions. The testing at the medium strain rates necessitated the design and construction of a dynamic tensile tester, the design of which, is also presented. The implementation of strain rate sensitive material properties into structural design and analysis are investigated and a constitutive model is proposed. The implementation of the proposed constitutive model into numerical methods analysis tools such as the finite element method is discussed and presented. The practical implementation of the proposed constitutive model is illustrated by numerically analysing the problem of a clamped beam struck transversely by a mass and comparing this with available experimental data. The validity of a typical constant velocity tensile test that is used to determine strain rate sensitive material properties is also investigated numerically to place the experimental results obtained into perspective. All the steels tested are found to be strain rate sensitive. Their behaviour is satisfactorily described by the constitutive model presented. No general trend regarding strain rate sensitivity is found when the results of the two rolling directions are compared. The importance of including strain rate sensitivity into structural design and analysis is illustrated by the analysis of the clamped beam struck transversely by a mass. The numerical results compare well with the available experimental data. It transpires from the numerical analysis of a typical constant velocity tensile test that it is difficult to obtain a constant strain rate throughout the gauge length of a typical test specimen. It also shows that there exists an optimum specimen geometry where the strain rate variation in the gauge length is at a minimum.

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