ИСТИНА

The shock-wave techniques provide unique capabilities to study mechanical behavior of materials at extremely high strain rates. Under these conditions, response of solids in some cases is unexpected and exotic. Temperature effects on the flow stress at high strain rate may differ even in sign from that we observe at low and moderate strain rates. Strengthened metals and alloys may demonstrate even lower HEL value than normally less hard ones. At highest strain rates, so-called ideal (ultimate) shear and tensile strength is reached. In the presentation, recent experimental data on the elastic precursor decay and rise times of plastic shock waves in several metals and alloys in various structural states at normal and elevated temperatures are discussed and systematized. The data on precursor decay include measurements at micron and submicron distances where realized shear stresses are comparable with their ideal values. It has been found the precursor decay may occur in several regimes which are characterized by different decay rates. Anomalous growth of the Hugoniot elastic limit with heating correlates with a fast decay regime and is not observed when the decay is relatively slow. Results of measurements have been transformed into dependences of initial plastic strain rate on the shear stress. The strong non-linearity of these dependences is treated as an evidence of nucleation of dislocations under applied stresses. An analysis of the rise times of plastic shock waves shows by order of magnitude faster plastic strain rates at corresponding shear stresses than that at the HEL that is treated as an evidence of intense multiplication of dislocations. Results of measurements of the resistance to high-rate fracture – spall strength show gradual increase of the later with increasing rate of tension and approaching the ideal strength in a picosecond time range. The spall strength not necessary correlates with dynamic yield stress. Although grain boundaries, in general, reduce the resistance to fracture as compared to single crystals, the spall strength of ultra-fine-grained metals usually slightly exceeds that of coarse-grain samples. The spall strength usually decreases with heating although in less degree than the strength at low strain rates does. The temperature dependences of the spall strength do not correlate with dependences of the yield stress that points on larger contribution of the fracture nucleation processes as compared to the void growth.