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

受到冲击压缩的结晶固体的动态行为与广泛的应用相关，例如影响汽车工业的耐撞性，国防工业的抗装甲性以及航空航天中对外来物体损伤的弹性等。在冲击载荷条件下形成的弹性前驱波的衰减包含了关于动力屈服起源的丰富信息。尽管作为一个研究主题已有40多年，但其行为的微观结构起源仍然模糊不清，并且强烈依赖于材料。基于基本位错理论对前驱波衰减进行分析建模的努力通常会导致对初始可移动位错密度的过度预测至少达到2个数量级。先前对前驱体衰变的研究主要集中在铝、铁和铜等各种金属的应变速率和温度依赖性上，通常是在理想退火或单晶条件下进行的。该项目的目的是解决对“真正的”工程合金缺乏探索的问题，研究初始微观结构状态在动态材料行为中的作用。此外，这项工作将提供急需的初始缺陷密度、晶体取向分布或材料加工历史的变化特征，这是充分了解冲击载荷下缺陷行为所需要的，而这在文献中是非常缺乏的。本项目采用单级气枪对铝和镁进行了一系列冲击实验，并对一系列初始微观结构状态进行了研究。在铝，纯和合金样品的不同程度的重预先冷加工（锻压）多达三次进行了研究。在镁中，重点研究了材料织构和晶体取向分布的影响。这两种情况的特点是材料或加工路线尚未在动态加载条件下进行研究。利用电子显微镜功能（TEM和EBSD），以提供影响动态响应的微观结构参数的深入知识。总的来说，这项研究能够提供一个数据集，该数据集在考虑的材料范围内是独一无二的，它使用了现代测速技术，并提供了初始微观结构状态的详细知识。总之，这将填补实验数据的关键空白，以推进我们对动态材料响应的理论理解。目前正在与以色列理工学院的研究人员合作，利用这一独特的数据集来模拟材料的行为。该项目属于EPSRC物理科学研究领域，部分由AWE plc通过iCASE学生资助。

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.