Finite deformations and the dynamic measurement of radial strains in compression Kolsky bar experiments

A novel technique has been developed for the direct non-contact measurement of the radial deformations of a specimen during a compression Kolsky bar (split-Hopkinson pressure bar) experiment. Application of the new technique makes possible an analysis of the compression Kolsky bar experiment in terms of finite deformations, since the technique provides a complete experimental determination of the deformation gradient tensor during dynamic loading. Using the new technique, we have determined the relative validity of the incompressibility and Bell constraints for finite deformation dynamic plasticity. The experimental results show that the plastic incompressibility constraint is more appropriate for the dynamic compression of 6061-T6 aluminum. It is also shown that the traditional measure of axial strain rate derived from Kolsky bar experiments should be replaced by the axial rate of deformation that is valid for finite deformations. Finally, the technique has been used to investigate the dynamic compression of porous pure iron. It is shown that the new technique extends the capabilities of the compression Kolsky bar technique to include the investigation of plastically compressible materials.