A study of elastic precursor decay in FCC and HCP metals under shock loading

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The dynamic behaviour of crystalline solids subjected to shock compression is of relevance to a wide range of applications, influencing for example crashworthiness in the automobile industry, armour resistance in the defence industry, and resilience to foreign object damage in aerospace, etc. The decay of the elastic precursor wave formed under shock loading conditions contains rich information regarding the origins of dynamic yielding. Despite being a topic of study for over 40 years, the microstructural origins of its behaviour remain obscured and strongly material dependent. Efforts to analytically model the decay of the precursor wave based upon elementary dislocation theory has typically resulted in an over-prediction of the initial mobile dislocation density of at least 2 orders of magnitude. Previous studies of precursor decay have mostly concentrated on strain-rate and temperature dependence of various metals such as aluminium, iron and copper, typically in the idealised annealed or single-crystal conditions. The aim of this project is to address the lack of exploration into "real" engineering alloys, investigating the role of the initial microstructural state in dynamic material behaviour. Additionally, this work will supply much-needed characterisation of variations in initial defect density, crystal orientation distributions or material processing history, which is needed to fully appreciate defect behaviour under shock loading, and which has been sorely lacking in the literature.This project presents a series impact experiments on aluminium and magnesium using a single-stage gas gun, in which a range of initial microstructural states have been examined. In aluminium, both pure and alloyed samples with varying degrees of heavy prior cold working (swaging) of up to three passes are studied. In magnesium, the focus is on the effects of material texture and crystal orientation distributions. Both cases feature materials or processing routes which have yet to be studied under dynamic loading conditions. Electron microscopy capabilities (TEM and EBSD) are leveraged in order to provide in depth knowledge of the microstructural parameters influencing the dynamic response. As a whole, this study is able to provide a dataset which is unique in its range of materials under consideration, its use of modern velocimetry techniques and its delivery of detailed knowledge of the initial microstructural state. Together this will fill a crucial gap in the experimental data needed in order to advance our theoretical understanding of dynamic material response. A collaboration with Israel Institute of Technology researchers making use of this distinctive dataset in order to model the material behaviour is currently underway.This project falls within the EPSRC Physical Sciences research area and is partly funded by AWE plc through an iCASE studentship.