Strain-rate dependent brittle deformation of rocks during impact cratering: Linking mechanical data and microstructure

在撞击事件中，矿物和岩石的变形是高度动态的，应变速率达到每秒100万立方米。传播冲击波的半球形衰减导致应变率和压力随距离减小。很大一部分岩石在高于每秒10到100次的应变速率下变形，在这种速率下，脆性材料的行为可能变得对速率敏感。该项目的目的是了解和量化岩石在准静态到高动态条件下在撞击过程中所经历的典型载荷下应变率敏感脆性变形的作用，并将其与变形岩石内相应的微观结构联系起来。为了实现这一目标，我们将系统地确定石灰岩、粘土、花岗岩和玄武岩在压缩和拉伸载荷下的力学数据（强度、弹性模量、泊松比），应变速率范围从准静态（~0.1秒）到高动态（1000000秒）。数据收集采用三轴加载框架（0.1-1 /秒），Split-Hopkinson-Pressure-Bar（1-1000 /秒）和飞板设备（>>1000 /秒）。我们将推导出给定应变率下拉伸和抗压强度的动态增加因子（DIF）。力学分析将与实验变形样品的综合微观结构研究相结合，特别是关于断裂形貌、裂纹分支特征、断裂密度和断裂/碎片尺寸-频率分布。这将有助于理解脆性材料在不同应变速率条件下的破坏机制、能量耗散和应力抗力。我们期望识别出一种与应变率相关的岩石结构指纹，可以应用于自然变形的岩石。这是缩小自然环境中变形的物理边界条件的关键。我们将对巴林杰陨石坑和希克苏鲁伯陨石坑的样本进行测试和应用。推断的应变率相关的动态增加因子（DIF）将被实现到现有的数值模型中（例如，iSALE冲击物理代码）。我们将特别评估应变率相关变形对撞击效率的作用。这项研究的结果也可能与其他地质灾害有很大的相关性，这些地质灾害涉及到中至高应变速率下的岩石变形，特别是发震断裂。

During an impact event, the deformation of minerals and rocks is highly dynamic and reaches strain rates of >1000000 per second. The hemispherical attenuation of propagating shock waves causes strain rates and pressures to decrease with distance. A significant volume of the rock -is deformed at elevated strain rates larger than 10 to 100 per second at which the behavior of brittle materials likely becomes rate-sensitive. The aim of this project is to understand and quantify the role of strain rate sensitive brittle deformation of rocks for typical loads experienced during impact cratering in the range from quasi-static to highly dynamic conditions and to associate it with corresponding microstructures within the deformed rock.To accomplish this, we will systematically determine mechanical data (strength, elastic moduli, Poisson ratio) for limestone, claystone, granite, and basalt at strain rates ranging from quasi-static (~0.1 per second) to highly dynamic (1000000 per second) conditions under both compressive and tensile loading. Data are gathered with a triaxial loading frame (0.1-1 per second), a Split-Hopkinson-Pressure-Bar (1-1000 per second), and a Flyer-plate facility (>>1000 per second). We will derive the dynamic increase factors (DIF) of tensile and compressive strength for a given strain rate. The mechanical analyses will be combined with a comprehensive microstructural investigation of the experimentally deformed samples, in particular with respect to fracture topography, crack branching characteristics, fracture density, and the fracture/fragment size-frequency distribution. This will lead to an under-standing of failure mechanisms, energy dissipation and stress resistance of brittle materials at diverse strain rates conditions. We expect to identify a strain-rate dependent structural fingerprint of rocks that can be applied to naturally deformed rocks. This is key to narrow down the physical boundary conditions of deformation in natural settings. We will test and apply our results to samples from Barringer and Chicxulub impact craters.The inferred strain rate dependent dynamic increase factor (DIF) will be implemented into existing numerical models (e.g., the iSALE shock physics code). We will particularly evaluate the role of strain rate dependent deformation for the efficiency of impact cratering. The results of this study can potentially have a great relevance also for other geo-hazards involving rock deformation at medium to high strain rates, in particular seismogenic faulting.