Experimental investigation of the deformation of Hopkinson bar specimens

Split Hopkinson bar (SHB) experiments are often used to study the strain rate dependent mechanical properties of materials. During a SHB experiment a small sample of the material under study is subjected to a high strain rate, uni-axial, tensile, compressive or torsion load. From the classical measurements the time history of the mean stress, strain rate and strain in the specimen can be derived. For some applications, more detailed information concerning the variation of the deformation in the specimen is necessary. In this contribution a technique is presented which makes it possible to obtain the deformation along the length of the specimen. The deformation of a line grid attached to the specimen is recorded during an experiment by means of a streak camera. An advanced and innovative numerical technique, based on a combination of geometric moiré and phase shifting, is developed to extract the time history of the deformation along the axis of the specimen from the picture of the deforming grid automatically. Large specimen deformations are allowed, and the technique proved to give highly accurate results. In this contribution results are presented of a SHB experiment on a steel sheet specimen. Some remarks are formulated concerning the generally assumed homogeneity of the deformation in the specimen, and the deformation obtained with the classical measurement techniques.