The effects of microstructure and nanostructure upon dynamic ductile fracture
Dynamic fracture is an important process in the behaviour of materials. However, the effects of material microstructure on dynamic fracture processes are not well understood. This thesis investigates the role of material microstructure and nanostructure in dynamic ductile fracture, such that the results can ultimately be used to improve fracture models. C110 OFHC copper was selected as a suitable material through which to investigate the role of microstructure and nanostructure in dynamic ductile fracture. The material was procured in a cold worked state. In order to control its microstructure, the copper was annealed to reduce the dislocation density, without altering its grain size. A novel method was developed to produce rings of pre-shocked material. The microstructures and quasi-static mechanical behaviour of all three states of copper (annealed, cold worked and pre-shocked) were characterised. An explosively loaded ring fragmentation experiment was developed, based upon an existing apparatus for brittle materials. The apparatus consisted of a mild steel driver cylinder containing an explosive charge. The copper ring was fixed to the outside of the cylinder over the centre of the charge. Diagnostics used to monitor the experiment included soft capture, photon Doppler velocimetry (PDV) and high-speed photography. A non-repeating pattern was marked on the rings and driver cylinders, enabling them to be reassembled and the precise location of the PDV measurement points known. Thus, information was obtained about velocity variation during the fragmentation process. A series of ring fragmentation experiments were performed on cold worked copper at strain rates of 10³ to 10⁴ s¯¹, which were controlled by varying the mass of the explosive charge. The experiments suggested a very slight decrease in the ductility of the copper at higher strain rates. The characteristic nature of the fracture surfaces varied with strain rate and showed different failure mechanisms on a single surface, suggesting that multiple failure mechanisms were competing during deformation. The conditions that allow one particular mechanism to dominate will be very sensitive to the local loading and microstructure. Patterns of similarly sized features were observed on some fracture surfaces and the dominant length scales in these patterns correlated well to the grain size and dislocation cell size of the cold worked copper. In order to investigate the role of microstructure further, particularly dislocation cell structure, additional ring fragmentation experiments were performed on annealed and pre-shocked copper. The differences between the three materials (when tested under quasi-static conditions) were clearly defined, yet the fragmentation behaviour was very similar. The detonation shock wave pressures applied to the rings suggested that the dislocation density would increase in experiments conducted on annealed copper with the largest explosive charges. However, for the other states of copper and for annealed copper with smaller explosive charges, the dislocation densities generated were suggested to be small in comparison to the existing dislocation densities. Therefore, the similarity in the fragmentation behaviour of the three states of copper was unexpected. All three states of copper showed reduced failure strains for an increase in strain rate during the ring fragmentation experiments, the trend being strongest for the annealed copper. It is hypothesised that the apparent decrease in ductility is a result of the ring fragmentation geometry, where necks may not have sufficient time to form at the highest strain rates. To explore the effect of the detonation shock wave, a series of tensile split Hopkinson pressure bar experiments were performed on the annealed and cold worked states of copper. The difference in the fracture behaviour of the two states of copper was much more clearly defined than for the ring fragmentation experiments. The failure strain of the annealed copper was significantly lower in the ring fragmentation experiments than the tensile bar experiments, despite being conducted at similar strain rates. The failure strain of the cold worked copper was similar in both the tensile bar experiments and the ring fragmentation experiments. The results suggest that annealed copper is much more susceptible to modification by the detonation shock wave than the cold worked copper, even at relatively low shock pressures where existing models suggest that the dislocation density would be unaffected.
Ring fragmentation, Dynamic fracture, Tensile SHPB, Copper, Ductile fracture, Microstructure, Nanostructure
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