ИСТИНА

Presentation will summarise results of recent and earlier investigations of the behavior of several silicate glasses under plate impact loading. Silicate glasses exhibit high yield strength and low fracture toughness as a result of their high homogeneity. The fracture of glasses under compression occurs by axial splitting. At high pressures, brittle glasses become ductile. Ductility of glass is caused by a loose microstructure with a large concentration of molecular-size voids. It is known that glasses show gradual structural changes resulting in increased density. Since densification occurs under Vickers indentation, it is supposed that the irreversible densification of the silicate structure is responsible for the plastic flow properties of glasses under high pressure. The compression waves in silicate glasses usually do not exhibit a distinct transition from the elastic to plastic response. Most of silicate glasses have anomalous longitudinal compressibility within the region of elastic compression where the longitudinal sound speed decreases as the compressive stress increases that, in turn, causes broadening of the elastic compression wave with its propagation. It is not clear yet whether or not the bulk compressibility of glasses is anomalous also. As a result of an anomalous decrease of longitudinal sound speed with increasing stress, a rarefaction shock is formed in glass at unloading from a shock-compressed state below the Hugoniot elastic limit (HEL). Since the reversibility of stress–strain processes is a main attribute of elastic deformations, observation of the rarefaction shock may be considered as evidence of an elastic regime of deformation. However, even in this stress range the response is not purely elastic: the waveforms distinctly demonstrate signatures of internal friction. Above the HEL the unloading wave speed becomes greater than the compression wave speed. Silicate glasses have high dynamic tensile strength (spall strength) within the elastic deformation region and maintain it or large part of it when the HEL is exceeded. The impact loading of a glass can result in the appearance of a failure wave. The failure wave is a network of cracks that are nucleated on the surface and propagate into the elastically stressed body. It is a mode of catastrophic fracture in an elastically stressed media whose relevance is not limited to impact events. It has been shown that the failure wave is really a wave process which is characterized by small increase of the longitudinal stress and corresponding increments of the particle velocity and the density. The propagation velocity of the failure wave is less than the sound speed; it is not directly related to the compressibility but is determined by the crack growth speed. The failure wave is steady if the stress state ahead of it is supported unchanging and it stops at unloading. The kinematics of the failure waves differ from those of elastic–plastic waves. In particular, since the propagation velocity of a failure wave and the final stress are fixed, the stress in the leading elastic wave ahead of the failure wave is governed by these values and should not necessarily be equal to the failure threshold. The glass surface plays an important role in the failure-wave process because the surface is a source of cracks. In some sense the process is similar to a subsonic combustion wave. Computer simulations based on the phenomenological combustion-like model reproduces well all kinematical aspects of the phenomenon. It has been shown recently the glass densification make essential contribution into the mechanism of the failure wave processes.