  Ever since the very first human-made knapped tools, the control of fracture
propagation in brittle materials has been a vector of technological
development. Nowadays, a broad range of applications relies on crack
propagation control, from the mitigation of damages, e.g., from impacts in
glass screens or windshields, to industrial processes harnessing fracture to
achieve clean cuts over large distances. Yet, studying the fracture in real
time is a challenging task, since cracks can propagate up to a few km/s in
materials that are often opaque. Here, we report on the in situ investigation
of cracks propagating at up to 2.5 km/s along a (001) plane of a silicon single
crystal, using X-ray diffraction megahertz imaging with intense and
time-structured synchrotron radiation. The studied system is based on the Smart
Cut process, where a buried layer in a material (typically Si) is weakened by
micro-cracks and then used to drive a macroscopic crack (0.1 m) in a plane
parallel to the surface with minimal deviation (1 nm). The results we report
here provide the first direct confirmation that the shape of the crack front is
not affected by the distribution of the micro-cracks, which had been a
postulate for previous studies based on post-fracture results. We further
measured instantaneous crack velocities over the centimeter-wide field-of-view,
which had only been previously inferred from sparse point measurements, and
evidence the effect of local heating by the X-ray beam. Finally, we also
observed the post-crack movements of the separated wafer parts, which can be
explained using pneumatics and elasticity. Thus, this study provides a
comprehensive view of controlled fracture propagation in a crystalline
material, paving the way for the in situ measurement of ultra-fast strain field
propagation.
