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Low-speed fracture instabilities in a brittle crystal

机译：脆性晶体中的低速断裂不稳定性

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When a brittle material is loaded to the limit of its strength, it fails by the nucleation and propagation of a crack. The conditions for crack propagation are created by stress concentration in the region of the crack tip and depend on macroscopic parameters such as the geometry and dimensions of the specimen. The way the crack propagates, however, is entirely determined by atomic-scale phenomena, because brittle crack tips are atomically sharp and propagate by breaking the variously oriented interatomic bonds, one at a time, at each point of the moving crack front. The physical interplay of multiple length scales makes brittle fracture a complex 'multi-scale' phenomenon. Several intermediate scales may arise in more complex situations, for example in the presence of micro-defects or grain boundaries. The occurrence of various instabilities in crack propagation at very high speeds is well known, and significant advances have been made recently in understanding their origin. Here we investigate low-speed propagation instabilities in silicon using quantum-mechanical hybrid, multi-scale modelling and single-crystal fracture experiments. Our simulations predict a crack-tip reconstruction that makes low-speed crack propagation unstable on the (111) cleavage plane, which is conventionally thought of as the most stable cleavage plane. We perform experiments in which this instability is observed at a range of low speeds, using an experimental technique designed for the investigation of fracture under low tensile loads. Further simulations explain why, conversely, at moderately high speeds crack propagation on the (110) cleavage plane becomes unstable and deflects onto (111) planes, as previously observed experimentally.
机译：当脆性材料加载到其强度极限时，它会由于裂纹的形核和扩展而失效。裂纹扩展的条件是由裂纹尖端区域内的应力集中产生的，并且取决于宏观参数，例如试样的几何形状和尺寸。但是，裂纹的扩展方式完全由原子尺度的现象决定，因为脆性裂纹尖端在原子上很尖锐，并且通过在移动的裂纹前沿的每个点上一次破坏一个不同方向的原子间键来扩展。多个长度尺度的物理相互作用使脆性断裂成为复杂的“多尺度”现象。在更复杂的情况下，例如在存在微缺陷或晶界的情况下，可能会出现几个中间尺度。众所周知，在高速裂纹扩展中会出现各种不稳定性，并且最近在理解其起源方面取得了重大进展。在这里，我们使用量子力学混合，多尺度建模和单晶断裂实验研究了硅中的低速传播不稳定性。我们的模拟预测了裂纹尖端的重构，该重构将使低速裂纹在（111）分裂平面上的传播不稳定，而传统上认为该分裂平面是最稳定的分裂平面。我们使用为研究低拉伸载荷下的断裂而设计的实验技术，进行了在低速范围内观察到这种不稳定性的实验。相反，进一步的模拟解释了为什么相反地，如先前实验所观察到的，为什么在（110）分裂平面上裂纹以中等速度高速传播时变得不稳定并偏向（111）平面。

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