Optical measurements of shock propagation in PMMA around arrays of inclusions

The optical refractive imaging techniques quantitative schlieren and coherent gradient sensing are applied here to study shock wave propagation in optically transparent polymethyl methacrylate (PMMA) samples. The approximately two-dimensional samples have holes and embedded metallic rods as inclusions with which the shock waves interact. The schlieren imaging yields quantitative measurement of the density gradients and thus density field as a function of time and space in the PMMA materials during the shock wave loading. Coherent gradient sensing provides quantitative stress field information behind the shock front. When combined, these techniques provide a characterization of the material state throughout the shock wave loading process. Shock loading is generated using gun-launched projectiles with impact velocities on the order of 1000 m/s. PMMA samples with varied geometrical arrays of inclusions demonstrate the ability to alter the shock propagation and locations of shock wave focusing due to complex shock wave reflections and diffractions. High-speed digital cameras are utilized to record the schlieren and coherent gradient sensing images at sub-microsecond timescales, allowing for detailed analysis of wave interactions.