Measurement of the evolution of the axial strain distribution in Hopkinson specimens

Abstract
To investigate the dynamic properties of materials very often a split Hopkinson bar setup (or Kolsky apparatus) is used. During an experiment a specimen is subjected to a high strain rate, uni-axial compressive or tensile stress. Classical measurements provide the history of the strain, the strain rate and the stress in the specimen material during an experiment. However, at each moment, only one, mean, value for the stress, the strain and the strain rate are obtained. In this contribution a combined optical-numerical technique is presented which allows to obtain the evolution of the strain along the full length of the specimen during an experiment. Results are presented. They clearly show that the developed technique is a powerful tool to monitor the non-homogeneous deformations in a Hopkinson specimen. Attention will be paid to the influence of the specimen geometry on the strain distribution in Hopkinson specimens. Some comments will be made on classical assumptions with regard to strain homogeneity and extraction of the strain history.