Frequency-shifted photonic Doppler velocimetry (PDV) for measuring deceleration of projectiles in soils

Velocity measurements of fast-moving objects are essential to a growing number of geotechnical applications. A photonic Doppler velocimeter (PDV) was configured to acquire the velocity–time history of a decelerating projectile from impact into soil through final burial. The PDV was setup using two independent lasers that were frequency shifted by approximately 0.2 GHz, which permitted recording low velocity data that are typically lost in homodyne configuration employing one laser only. Validation tests were carried out using a rotating target which verified the PDV measurements. The PDV was deployed in a vertical impact laboratory range to obtain deceleration records within carefully prepared soil targets. Two penetration tests into clayey sand targets that were performed by impacting conical projectiles are presented to demonstrate the repeatability of the experimental methodology. In this study, deceleration histories from impact at a velocity of 155 m/s were measured. Time-dependent depth of penetration and penetration resistance were calculated, with high fidelity, from the velocity–time records. The results were nearly identical for both tests for the velocity–time record, and it was possible to track the penetration velocity down to 20 m/s. This demonstrates that PDV measurements are well suited for measuring penetration resistance at ballistic velocities.