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The high strain rate dependence of the flow stress of metals and alloys is described from a dislocation mechanics viewpoint over a range beginning from conventional tension/compression testing through split Hopkinson pressure bar (SHPB) measurements to Charpy pendulum and Taylor solid cylinder impact tests and shock loading or isentropic compression experiment (ICE) results. Single crystal and polycrystal measurements are referenced in relation to influences of the crystal lattice structures and nanopolycrystal material behaviours. For body centred cubic (bcc) metals, the strain rate sensitivity (SRS) is in the yield stress dependence as compared with the face centred cubic (fcc) case of being in the strain hardening property. An important consequence is that an opposite ductility influence occurs for the tensile maximum load point strain that decreases with strain rate for the bcc case and increases with strain rate for the fcc case. Different hexagonal close packed (hcp) metals are shown to follow either the bcc or fcc case. A higher SRS for certain fcc and hcp nanopolycrystals is explained by extrapolation from conventional grain sizes of an inverse square root of grain size dependence of the reciprocal activation volume determined on a thermal activation strain rate analysis (TASRA) basis. At the highest strain rates, additional deformation features enter, such as deformation twinning, adiabatic shear banding and very importantly, for shock induced plasticity, transition from plastic flow that is controlled by the mobility of the resident dislocation density to plasticity that is controlled by dislocation or twin generations at the shock front. The shock description is compared with the very different high rate shockless ICE type loading that occurs over nanoseconds and leads to higher compressive strength levels because of dislocation drag resistance coming into play for the originally resident mobile dislocation density. Among the high strain rate property, concerns are the evaluation of ductile to brittle transition behaviours for bcc and related metals and also, projectile/target performances in ballistic impact tests, including punching. Very complete metallographic and electron microscope observations have been reported in a number of the high rate deformation investigations.

We review some of the factors that influence the hardness of polycrystalline materials with grain sizes less than 1 lm. The fundamental physical mechanisms that govern the hardness of nanocrystalline materials are discussed. The recently proposed dislocation curvature model for grain size-dependent strengthening and the 60-year-old Hall-Petch relationship are compared. For grains less than 30 nm in size, there is evidence for a transition from dislocationbased plasticity to grain boundary sliding, rotation, or diffusion as the main mechanism responsible for hardness. The evidence surrounding the inverse Hall-Petch phenomenon is found to be inconclusive due to processing artefacts, grain growth effects, and errors associated with the conversion of hardness to yield strength in nanocrystalline materials.

Organic and inorganic explosives were first developed and put into service in the 19th century, before there was much understanding of how the energy release mechanisms differed from those of the long established gunpowder. Theoretical advances in the understanding of shock waves combined with improvements in photographic and electronic techniques led to the hypothesis that a detonation is a shock wave maintained by the rapid release of chemical energy. Studies of accidental ignitions/initiations showed that explosive events can occur even when the energy input is much less than that required to heat the bulk explosive to the deflagration temperature. Hence, the highly fruitful idea of the localised hot spot was conceived. Apart from electrical stimuli, the main hot spot mechanisms are currently accepted as being adiabatic asymmetric collapse of gas spaces (producing gas heating, jetting, viscoplastic work) and the rubbing together of surfaces as in friction or adiabatic shear. Initiation mechanisms are also connected with the anisotropy of plasticity and fracture in explosive crystals. Decomposition of molecules can take place as they are forced past each other in a deforming crystal. There is, however, still much to discover about reaction pathways. Novel optical and electron microscopy techniques have given a great deal of new and precise information about displacements and failure mechanisms when explosive crystals are bonded together using polymers. The deflagration-detonation transition (DDT) has been extensively studied in model one-and two-dimensional systems.