Numerical investigation into the stress wave transmitting characteristics of threads in the split Hopkinson tensile bar test

Abstract

The Split Hopkinson tensile Bar (SHTB) is one of the most widely used methods to study various material behaviors under tensile loading and high nominal strain rate. Since the specimens and grips with thread may disturb the wave propagation between bars and specimen, the stress-strain relation of the specimen, which is calculated from the strains of the incident and transmitter bars, may not match with the stress-strain relation measured directly from the specimen. In order to ensure a high accuracy of SHTB system, specimen calibration is usually carried out. To ensure the no wave distortion in the thread region, the stress wave should be entirely transmitted from the incident bar to the transmitter bar in tests using the calibration specimen (not the real specimen) of the same material and the same diameter with the transmitter and incident bars. Therefore, the strain signals at the incident and transmitter bars need to be the same. This study investigated the wave transmit characteristics through threads in SHTB test. To investigate the effect of the thread on the wave transmission, a specimen with the same diameter of the incident and the transmitter bars is used in the SHTB test. The effects of the thread inner diameter, yield stress, thread type, and strike velocity on the wave propagation characteristics are investigated. Based on the analysis results for various conditions, a proper design guide for a thread shape to secure the measurement accuracy in the SHTB test is proposed.