Response of iron to two symmetric laser shocks

Laser-driven shocks provide a means of studying the dynamic behaviour of solid materials at very high strain rates. Here, we investigate the effects of a new pulsed load, generated by the crossing of two symmetric laser shocks. Both surfaces of thin iron foils are irradiated simultaneously by two high-power laser beams, producing two compressive pulses of duration about 3 ns and amplitude about 10 to 60 GPa. When they cross each other in the central region of the sample, considerable increases of the pressure and the temperature are induced, leading to twin formation and phase transition. Then, the interactions of all the incident and reflected release waves which propagate inside the sample result in various types of spall damage, depending strongly on the sample thickness and on the shock pressure. All those effects have been observed in the recovered targets, and explained by a phenomenological analysis of wave propagation. The influences of various experimental parameters have been investigated. Finally, one-dimensional computations have been performed to test the ability of a simple constitutive model, including twin formation, phase transformation and spallation, to predict the observed results. A rough agreement between computations and experiments, better at lower shock pressure, has been obtained.