Picking up the pieces: The importance of soft-recovery in spall fracture research

Spall fracture is one of the most studied topics in the shock physics field. Occurring when a shocked body is allowed to release, and multiple rarefactions interact, spall is a tensile damage process that can lead to complete material failure. While it has been demonstrated that the nucleation, growth, and coalescence of spall damage is extremely sensitive to intrinsic (material, microstructure, impurities) and extrinsic (strain-rate, stress, pulse duration) factors, spall is still generally characterized by a single parameter: the spall strength. This is measured with ex-situ diagnostics such as VISAR or PDV, and can mask the complex processes that are occurring inside the sample during failure. The simple velocity-pullback analysis contains little to no information about which microstructural features are acting as nucleation sites, how damage propagates through the material, and the resulting damage morphology – all required data to fully validate and exploit modern strength and damage models deployed in hydrocodes. Here, we present some key works demonstrating the importance of recovering the sample to further investigate dynamic material response, showing how traditional shock diagnostics are insensitive to this rich dataset. Soft-recovery techniques are covered, that ensure that any damage in the recovered samples is only from the spall event. Experiments covering strain-rate, pulse-duration, crystallographic texture, and the recompaction of spall damage are covered, along with some recent results using true in-situ diagnostics in the form of synchrotron light sources to probe spall failure in real time.