Strain rate dependence of metals, metal alloys and amorphous metals
Principal Supervisor: Dr Dan Eakins
The Institute of Shock Physics has several unique experimental capabilities within the UK. The combination of onsite pulsed power facilities, a large bore gas gun, and testing apparatus such as drop weight and split Hopkinson bar enable materials to be characterised across a large range of strain rates, up to ~108 without the need for explosive drives. The results of such experiments are invaluable for comparison to theoretical and computational models of a material's EOS and strength characteristics.
The purpose of this PhD is to develop a robust method of measuring the stress-strain response of a set of mechanically distinct materials through five orders of magnitude in strain-rate. The materials to be investigated include pure metals, metal alloys and finally amorphous metal alloys of similar composition (where the absence of structure and grain boundaries dramatically influcences the yield process), which together will provide insight into different deformation and failure mechanisms.
In addition to standard dynamic testing techniques at intermediate strain-rates (i.e. split Hopkinson pressure bar), a portion of the PhD will involve isentropic or 'shockless' loading and unloading experiments, using wave profile analysis to interpret VISAR and Het-V measurements of material motion. The use of different material thicknesses in the same experiment and careful consideration of impedance matching to diagnostic windows will allow a complete stress-strain history of a target material to be obtained in an experiment, and supply a measure of a material's yield strength. Drives for isentropic loading and unloading will be produced either by the MACH pulsed power facility, or through the use of a graded-density impactor on the gas gun platform, which can be co-developed for use in a separate project.
The use of VISAR and Het-V signals to obtain information on material strength will be supplemented via the use of conductivity and reflectometry probes, and X-ray tomography of material samples recovered post experiment. This will enable determination of the modes of deformation in each material type as related to strain-rate, and possibly provide a link between material microstructure and the strength exhibited at the macroscopic scale.