Assessment of bending waves in Torsion Hopkinson Bar experiments using Photon Doppler Velocimetry

A Photon Doppler Velocimetry system that measures the propagation of elastic shear waves in a Torsion Hopkinson Bar (THB) system is presented. The method uses multiple fibre optic probes located symmetrically on opposing sides of the apparatus bars, and provides data with high spatial (a laser irradiated spot size of $35\mu m$) and temporal resolution that is ultimately limited by the data acquisition system and used electronic components. A series of validation experiments simulating the movement of the bar subjected to bending and misalignments demonstrated that this approach is effective in detecting and accounting for the bending waves. The THB experiment under non-ideal conditions, where a combination of shear and bending waves propagates in the system, conclusively confirmed that the disturbance in the acquired signals can be properly addressed with the proposed arrangement of the PDV probes. It was reflected in similar measurements of the component of tangential velocity to the strain gauges. This approach shown to be complementary to the conventional strain gauge technique, but can provide better precision and be more robust under loading and/or temperature conditions that may affect the reliability of strain gauge measurements.