64Recently, the dynamic strength for aluminum, the AMg6M aluminum-magnesium alloy, and polyme thyl methacrylate (PMMA) were studied experimen tally by the method of laser generation of shock waves [1][2][3]. It was established that the mechanical strengths of these materials attained the limiting values at strain rates of 10 6 -10 7 s -1 . Further, it seems to be important to reveal the intrinsic features of similar processes occurring at strain rates exceeding 10 7 s -1 . This range is available under the condition of either increasing the amplitude of the shock action or substantially reduc ing its duration.In this paper, we present the results of studies of the dynamic mechanical strength for aluminum, the AMg6M alloy, and PMMA under the action of pulsed laser radiation of 70 ps duration. In the preceding experiments [1-3], the duration of the laser pulse attained 2.5 ns. The use of shorter pulses made it pos sible to realize in our experiment strain rates exceeding 10 7 s -1 .In these experiments, we employed a neodymium glass laser of the Kamerton T setup constructed at the Prokhorov Institute of General Physics, Russian Academy of Sciences. The basic radiation was trans formed to the second harmonic of 0.527 μm wave length. The laser beam energy attained 1.5 J. Laser radiation was formed at a target to spots of 0.2 to 0.8 mm in diameter. In this case, the maximum energy density of the laser radiation flux in the focal region and ablation pressure were 6.2 × 10 13 W cm -2 and 13.5 Mbar, respectively. Targets made of aluminum, the AMg6M alloy, and PMMA were applied in the form of plates 50 to 220 μm thick.The spall phenomenon was used to extract infor mation on the dynamic mechanical strength of the materials under investigation [4]. This phenomenon arises on the target rear (free) side as a result of the reflection of a pressure (compression) pulse caused by laser radiation, which is generated on the target front surface. By virtue of the reflection, the target free sur face is pushed into motion, which initiates the propa gation of a tension wave propagating counter to the compression wave. At a certain distance from the rear surface, pressure in the target can become negative. In this case, the tensile strength in the material can exceed its ultimate tensile strength. This leads to the formation of the spallation layer that leaves the origi nal sample.In our study, we have employed a novel approach that has allowed us to determine the spall strength and the strain rate of the material. This approach is based on both the determination of the spallation groove depth upon the pulsed laser beam action on the target and the subsequent mathematical simulation of the shock wave process in the substance under study [5].In the calculations, we employed a numerical code similar to that of the Courant-Isaacson-Riesz scheme, which was based on the hydrodynamic equa tions [6]. The wide range semiempirical equations of state for the substances under study, which was taken from [7,8], were used. We also supposed that the...
