The Effects of Nanostructure upon the Dynamic Ductile Fracture of High Purity Copper

Dynamic fracture is a fundamentally important physical process. However, the effect of material nanostructure, including both lattice and grain structure, and their respective defects, on such events is not well understood. Ductile fracture is widely accepted to proceed through the nucleation, growth and coalescence of voids to form a failure plane. The formation of voids at grain boundaries is established, and impurities and secondary phase particles are often found at the centre of these voids suggesting their involvement in the nucleation process. In a pure metal, where impurities are absent, the void nucleation process is unclear, although theories suggest the importance of dislocations and their substructures. Dislocation parameters are underpinned by the plasticity behaviour of the relevant material, which is often highly strain rate dependent, and in the case of copper, is also history dependent.Here, we describe experiments that study the ring fragmentation of OFHC copper, at strain rates around 104 s-1. Diagnostics include high speed photography, laser velocimetry, and soft capture of fragments. The number of fragments and strain at fracture was observed to increase with the applied strain rate, suggesting an increase in ductility with rate. The cross-section of the fragments changed significantly during the applied loading, due to interaction of the ring with the driver cylinder, suggesting a strongly tri-axial stress state, where the strain rate is likely to be higher than that which was estimated, based on circumferential expansion from laser velocimetry data. The fracture surfaces displayed macroscale surface topology, and showed evidence of several different fracture mechanisms across the failure plane. The length scales of the fracture surfaces were to the order of tens of microns, and below, suggesting that structures below the grain size of the material are dominating the observed behaviour.