High strain rate deformation of metals
The evolution of the physical sciences and engineering has involved a detailed and quantified understanding of the properties of metals. In particular, it is necessary to know how metals deform and the stresses that are involved which, in turn, are affected by the rate at which strain is applied. The nature and layout of this work is outlined. The history of the pressure bar transducer is summarised. The original concept of Hopkinson in 1914 was to use a long metal bar to study the propagation of wave pulses. During the Second World War, Davies refined the instrumentation and studied the shape of such pulses as modified by dispersion. Kolsky in 1949 adapted the technique to investigate the dynamic plasticity of specimens wedged between two instrumented pressure bars. Subsequent workers have used variants of this apparatus to make measurements at strain rates up to 105 s-1, whilst others have considered the effects of friction and inertia on the specimen. After an explanation of the particular design requirements, a description is given of the high strain rate apparatus that forms the basis for the research reported in this dissertation. The components that make up the system are described separately and the experimental procedures are outlined. The accuracy of components critical to the experimental technique is investigated. The effects of friction at the specimen interfaces, inertia during deformation and wave dispersion in the pressure bar are discussed. Bar calibration is described. Experiments have been carried out on copper in five different microstructural states at average strain rates of 6 x 104 s-1 and 5 x 10-2 s-1 and their behaviour compared. The metal has been specially worked to induce anisotropy in the form of texture. Special techniques have been developed to prepare specimens of known orientation from the bulk of the raw material. The results show correlations between the texture severity and the anisotropy of stress-strain properties. A dynamic work hardening effect is observed. There is evidence that the Petch relationship holds at high strain rates. The high strain rate deformation of uranium alloyed with titanium or molybdenum is investigated. Specimens often display evidence of macroscopic localised shear bands whose adiabatic formation is accompanied by a sharp fall in the materials' dynamic strength. Metallographic sections reveal the morphology of these bands and the relative motion of microstructural features during deformation. Results are presented on a eutectoid zinc-22% aluminium alloy in a lamellar and superplastic microstructural state and a gun steel. The high strain rate deformation of titanium-6% aluminium-4% vanadium alloy is compared with uranium-0.75% titanium alloy regarding their tendency to form macroscopic shear bands. The dynamic behaviour of copper is contrasted with that of uranium alloy. In conclusion, the current work is viewed in the context of the historical development of the miniaturised Hopkinson pressure bar. Some comments are made about the application of the technique, and the scope for further research.