Speckle measurements of sample deformation in the split Hopkinsons pressure bar

Abstract
In conventional Hopkinson bar measurements the sample is required to be in stress equilibrium and homogeneous. By using an optical technique to measure the strains in the sample these problems can be overcome. This has already been donc in Tensile Hopkinson bars [1], but not in compression. In this paper, a Polybutaleneteraphthalate (PBT) sample is tested using a compression Hopkinson bar combined with speckle metrology. Speckle measurements allow high accuracy displacement fields, and hence strains, on the material's surface to be measured. The speckles are produced either by laser illumination, or by using the natural relief and features of the surface. By capturing the speckled images using highspeed photography, the progression of strains on the sample surface can be followed throughout the experiment. The combination of Hopkinson bar data with the optical technique will also provide modellers with both macro and meso-scale data from the same experiment.