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Ever since the very first human-made knapped tools, the control of fracturepropagation in brittle materials has been a vector of technologicaldevelopment. Nowadays, a broad range of applications relies on crackpropagation control, from the mitigation of damages, e.g., from impacts inglass screens or windshields, to industrial processes harnessing fracture toachieve clean cuts over large distances. Yet, studying the fracture in realtime is a challenging task, since cracks can propagate up to a few km/s inmaterials that are often opaque. Here, we report on the in situ investigationof cracks propagating at up to 2.5 km/s along a (001) plane of a silicon singlecrystal, using X-ray diffraction megahertz imaging with intense andtime-structured synchrotron radiation. The studied system is based on the SmartCut process, where a buried layer in a material (typically Si) is weakened bymicro-cracks and then used to drive a macroscopic crack (0.1 m) in a planeparallel to the surface with minimal deviation (1 nm). The results we reporthere provide the first direct confirmation that the shape of the crack front isnot affected by the distribution of the micro-cracks, which had been apostulate for previous studies based on post-fracture results. We furthermeasured instantaneous crack velocities over the centimeter-wide field-of-view,which had only been previously inferred from sparse point measurements, andevidence the effect of local heating by the X-ray beam. Finally, we alsoobserved the post-crack movements of the separated wafer parts, which can beexplained using pneumatics and elasticity. Thus, this study provides acomprehensive view of controlled fracture propagation in a crystallinematerial, paving the way for the in situ measurement of ultra-fast strain fieldpropagation.

Brittle fracture studied by ultra-high speed synchrotron X-ray diffraction imaging