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This paper summarizes a recently developed continuum theory for the elastic-viscoplastic deformation of amorphous solids such as polymeric and metallic glasses. Introducing an internal-state variable that represents the local free-volume associated with certain metastable states, we are able to capture the highly non-linear stress-strain behavior that precedes the yield-peak and gives rise to post-yield strain-softening. Our theory explicitly accounts for the dependence of the Helmholtz free energy on the plastic deformation in a thermodynamically consistent manner. This dependence leads directly to a backstress in the underlying flow rule, and allows us to model the rapid strain-hardening response after the initial yield-drop in monotonic deformations, as well as the Bauschinger-type reverse-yielding phenomena typically observed in amorphous polymeric solids upon unloading after large plastic deformations. We have implemented a special set of constitutive equations resulting from the general theory in a finite-element computer program. Using this finite-element program, we apply the specialized equations to model the large-deformation response of the amorphous polymeric solid polycarbonate, at ambient temperature and pressure. We show numerical results to some representative problems, and compare them against corresponding results from physical experiments. / Singapore-MIT Alliance (SMA)

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This research investigates the compositional homogeneity of potash-limesilica glasses from the 12th-17th centuries in Northern Europe, and the significance of this with respect to compositional studies of archaeological glasses. The variables in the glass making process that influence the formation of a homogeneous glass are discussed, and investigated using laboratory replication of beech and bracken ash glasses. The experimental results are compared to archaeological material from glass production sites at Blunden's Wood, Knightons, Sidney Wood, and Little Birches in England, and Hils in Germany. Backscattered scanning electron microscope (SEM) imaging is used to qualify the extent of inhomogeneity in both the experimental and archaeological samples. It is confirmed that visually homogeneous glasses can contain inhomogeneities that are only visible under backscattered SEM imaging. It is seen that the size and orientation of inhomogeneities is varied, and specific glass artefact types (such as crucible and waste glass) are more prone to inhomogeneity than fully formed glass (such as window and vessel glass). Electron microprobe analysis (EPMA) is used to quantify the extent of elemental variations present in the inhomogeneous archaeological glasses. The results show that a number of elements are significantly influenced by inhomogeneity, including those (such as calcium, magnesium and sodium) which are commonly used to form compositional groupings of medieval glass. It is concluded that although a number of variables in the glass making process influence the formation of a homogeneous glass, specific variables, such as increased furnace temperature and a high alkali concentration in the ash, appear to be the dominating factors. The presence of large elemental variations in a number of the archaeological glasses analysed confirms that inhomogeneity is a vital consideration in compositional studies of this material, and that particular care must be exercised when using analytical techniques that require only a small sample size.

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The liquid impact properties of a range of infra-red and optical transmitting materials have been investigated using the liquid jet impact technique. In particular the effects of temperature and pre-existing stress fields on the liquid impact performance of certain materials have been examined. Consideration of these aspects of liquid impact has been supplemented by measurements of related fracture properties. The correlation between liquid jet impact and liquid drop impact has been re-examined for normal impact by comparing impacts on perspex. This correlation has been extended to consider angled impact. Components that are subjected to liquid impact may simultaneously be subjected to elevated temperatures. A system has been developed to allow experimental investigation of the liquid impact performance of materials at elevated temperatures. Preliminary results were obtained on a polymer (PEEK). The amount of damage resulting from liquid impact increased with temperature. Subsequently two brittle materials (glass and zinc sulphide) were investigated. Thermal shock has been shown to be the dominant failure mechanism for these materials at temperatures of 300<SUP>o</SUP>C and greater. Pre-existing stresses in a material can affect not only its static but also its dynamic loading response. The liquid impact performance and some associated fracture properties of toughened glass systems have been investigated. Chemically and thermally toughened systems were compared. There is a small increase in the threshold velocity for the toughened glasses considered compared to soda-lime glass. The toughening stresses distributions led to differences in the behaviour under single and multiple impact. A novel computer simulation of liquid impact onto pre-stressed substrates has been developed. It is based on the interaction of a model Rayleigh surface wave with a flaw distribution. Good agreement with experiment was obtained for a thermal toughening stress distribution.

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No description available.