​Dynamic strength of sapphire crystal (6)

Dynamic strength of sapphire crystal (6)

To understand the mechanism of shearing [75], one should imagine a compressive stress pulse (Fig. 2.64) through the test sample in the impact result. The compression wave (Fig. 2.64b) reaches the free surface of the sample and is emitted as a tensile wave. The reflected tension The extensional wave then interacts with the incident compressional wave (Fig. 2.64c). At d from the back surface of the sample, the ultimate tensile stress becomes equal to the critical normal failure stress, the material cracks and a fragment of it breaks away from the surface. So, the cut out is entirely generated by the wave effect.

The described failure process is also observed in rectangular disks. In general, the angular regions are destroyed first.

The plastic rheology of sapphire under shock loading is revealed. On the sapphire surface, a sudden relief of deformation is formed at the point of shock (Fig. 2.65).

Earlier, similar patterns of deformation relief with plastic rheology were found only in sapphire diffusion welding at T > 2,200 K and high pressure. The magnitude and character of the rheological relief was related to the orientation of the sample (Fig. 2.66). (0001 ) oriented samples have an impact symmetry trace on the surface and their deformation relief is sharp. The energy of the impact is mainly dissipated in the formation of this relief, e.g. in the plastic rheology of the material. Therefore, crystals with (0001) orientation Often better withstand shock (Fig. 2.66a). In R-directed crystals, plastic rheology is rather weak (Fig. 2.66b). In many cases it is not detected at all, and the applied shock energy causes crystal failure . The behavior of guide A is between guides C and R. The major part of the impact energy is dissipated by plastic rheology at the surface of the impact point, and such crystals may be subjected to impact loads.

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