Determining Strength of Materials Under Dynamic Loading Conditions Using Hydrodynamic Instabilities

Abstract

Hydrodynamic instability experiments allow access to material properties at extreme conditions where the pressure exceeds 100 GPa and the strain rate exceeds 106 1/s. Laser ablation dynamically loads a sample, causing a manufactured initial perturbation to grow due to hydrodynamic instability. The instability growth rate depends on the strength of the sample. Material strength can then be inferred from a measurement of the instability growth. Past experiments relied on in-flight diagnostics to measure the amplitude growth, which are not available at all facilities. Recovery instability experiments, where the initial and final amplitude of the instability are measured before and after the sample is dynamically loaded, obviate the need for in-flight diagnostics. Recovery targets containing copper and tantalum samples coined with 2D (hill and valley) and 3D (eggcrate) initial perturbations were dynamically loaded using the Janus laser at the Jupiter Laser Facility, Lawrence Livermore National Laboratory. The energy of the laser pulse was varied to cover a range of conditions in the dynamically compressed sample with pressures in the range 10 GPa to 150 GPa and strain rates in the range 105 1/s to 108 1/s. The coupling of laser energy into a loading wave was studied with a combination of laser-matter interaction simulations (Hyades) and velocity interferometry data (VISAR). Laser ablation of the recovery targets generated a blast wave, loading the coined initial perturbations with a shock wave followed by a release wave. Different ablator materials and variations in the amount of laser energy deposited in the ablator lead to variations in the loading wave and consequently variations in instability growth. The growth of the initial perturbation amplitude from initial to final conditions was studied with hydrocode simulations (CTH). During dynamic loading of the sample, the shock wave caused amplitude growth due to hydrodynamic instability. The release wave accelerated the perturbed interface and slowed amplitude growth, in some cases reversing growth. The sensitivity of the instability growth to coarse changes in the strength model was demonstrated. However, uncertainty in modeling the laser ablation loading prevented a definitive comparison between simulation and experiment.