An analysis of stress uniformity in split Hopkinson bar test specimens

The assumption of uniform stress in a test specimen is fundamental to the split Hopkinson bar (SHB) test technique. Based on the method of characteristics for one-dimensional waves, the degree of stress uniformity in specimens tested dynamically by the SHB method is examined. The time duration t b (or number of transits n an elastic wave makes from one end to the other of a specimen) required to attain stress equilibrium is established and related to the relative mechanical impedance β of the specimen. The effects of the rising edge in the incident pulse on this time duration are also studied and the results appear to indicate that the shortest time to achieve uniformity of stress in a specimen requires the incident pulse to have a specific profile. The results of this study provide guidelines to assess the validity of experimental data.