Laser-Driven Flyers and Nanosecond-Resolved Velocimetry for Spall Studies in Thin Metal Foils

We describe a laser-launched micro-flyer apparatus designed for spall strength measurement. The launcher uses a single pulse from a pulsed laser that is stretched in time to nominally 20 nanoseconds using an optical ring cavity, while inexpensive multi-lens arrays are used to spatially homogenize the beam. The velocimetry technique that we developed for the experiment provides the required sub-nanosecond time resolution. We demonstrate the capability of the apparatus to interrogate the spall strength of AZ31B Mg alloy thin foils, a material system with potential applications as a lightweight protection material. Numerical simulations and fractography are very useful to determine the quality of the experimental data and help to interpret our results. The simulations and fractography analyses of the experiments suggest that the short shock pulse duration in the experiment causes incipient spallation. The short pulse also likely introduces stochasticity to the measured spall strength through limited activation of failure mechanisms within the samples. The shocked AZ31B Mg alloy has spall strengths that are greater than previously reported figures for fine grained Mg alloys, likely because the laser based system achieves higher strain rates than in prior work on this material.