The “Open” Hopkinson Pressure Bar: Towards Addressing Force Equilibrium in Specimens with Non-uniform Deformation

Abstract

Conventional Split Hopkinson Pressure Bar (SHPB) testing requires the verification of force equilibrium in a specimen for the test data to be considered valid. For very low impedance materials the large impedance mismatch between input bar and specimens leads to significant uncertainty in the force measurement at the input face. This makes it difficult to fulfil the equilibrium requirement for conventional SHPB testing of very low impedance materials. Cellular materials further complicate matters, as non-uniform densification can lead to different stress states on either side of a densification front. A novel configuration, termed the Open Hopkinson Pressure Bar (OHPB), is proposed to address the difficulties in measuring small differences in forces on either side of the specimen. The specimen is placed on a HPB, and impacted directly by an instrumented striker (effectively another HPB). This arrangement only requires the processing of one wave in each bar, as opposed to the three waves required in a conventional SHPB. This technique allows significant improvements to be made in the resolution of the force measurements.