​Dynamic strength of sapphire crystal (2)

Dynamic strength of sapphire crystal (2)

With the short-wave loading (10-100 MPa), the deformation state of the crystal changes from elastic to plastic deformation, eg: the crystal "flows". Because the short-wave behavior is short-lived (on the order of nanoseconds), the crystal lattice arrangement does not change. will be disturbed by the flow of the material. However, under such conditions, the lattice undergoes isotropic compression, eg in the direction of wave propagation and perpendicular to the plane. But if the material already has the properties of a liquid, then Pascal's law comes into play. In fact, X-ray diffraction experiments often reveal uniform compression of the lattice in all directions.

Tests of short-wave-induced lattice lattice deformations in silicon and copper have shown that these materials have fundamentally different response mechanisms [76]. In particular, under loads of 10–100 MPa, the elastically deformed state in copper crystals transforms into plastic deformation, While the crystal lattice remains the same. Its isotropic compression occurs only in the wave propagation and vertical directions. However, the silicon crystal lattice is compressed by 11% in the wave propagation direction, while there is no change in the vertical direction. Although the stress exceeds the static yield strength and deforms strongly in the longitudinal direction, the silicon does not enter a state of plastic deformation; its response remains fully elastic.

We know that dislocations in a crystal start to move under increasing loads, interact and create new dislocations. Macroscopically, dislocations are merely plasticity of the material. However, it takes considerable time to transform into a plastic state, which depends on the The initial concentration of dislocations and their mobility. Like other covalent crystals, silicon is also characterized by low dislocation mobility. Therefore, no transition into a plastically deformed state is observed during short-time force application . For copper crystals, the time to enter the plastic state is short (10-100 ps), however, under an equivalent shock loading behavior, copper acquires a hydrodynamic state. For ionic crystals, dislocation density and mobility are higher than covalent Bonds are orders of magnitude higher in crystals, which provide the necessary conditions for the crystal to transform into a plastic state when it interacts with a force. In general, it is assumed that the arrangement of the structure of a solid under vibrational compression persists for 10-9-10- 7 or so.

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