ИСТИНА

Presentation will summarise results of recent and earlier investigations of the behaviour of silicate glasses under uniaxial shock compression. 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. Glasses have high Hugoniot elastic limit (HEL) but its value is usually difficult to evaluate because of smoothed transition from elastic to elastic-plastic compression. Because of that, indirect data are attracted. 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 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. Beginning of plastic deformation is accompanied with irreversible densification of glass. 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. Experiments with pre-stressed glass samples have shown the glass densification make essential contribution into the mechanism of the failure wave processes. This conclusion was supported by determining the stress range in which the failure wave phenomena can be observed. It has been shown there are lower and upper stress limits for the failure waves. The lower stress limit is certainly much below the HEL whereas upper stress limit slightly exceeds the HEL value.