Effect of peak stress on spall in copper and a copper-lead alloy

Spall fracture is often seen in shock loaded materials. It has been shown extensively that spall is a weak-link driven event, with microstructural features such as grain boundaries, impurities, porosity etc. acting as nucleation sites for voids and cracks. Here, we present results studying spall fracture in a two-phase material, copper-lead. The alloy contains approximately 1% lead, which is contained as precipitates primarily at the copper grain boundaries. The effect of the peak compressive stress prior to spall is investigated, with a pure copper target also included as a control. It is shown that this small amount of lead drastically reduces the spall strength. Further, the spall strength of the copper-lead alloy begins to decrease at a far lower peak stress than the pure copper. Modeling efforts, at both continuum and molecular dynamics length scales, are used to elucidate the reasons behind these results. Initial conclusions are based on the slow wave velocities in the lead compared to the copper leading to a ‘shock-focusing’ in the lead precipitates, causing thermal softening and even melt, producing weak points and lowering the spall strength.