Pressure sensing in clay: A new metric for characterizing the ballistic backface deformation response of personnel protection equipment

Abstract

Clay indentation depth is traditionally used to quantify the performance of personnel protection equipment (PPE) under non-penetrating ballistic impact events. However, plastic deformation of the clay ‘witness’ media or backing following an impact event does not have a direct correlation to risk of injury. Alternative methods for assessing PPE have been developed including instrumented test dummies or gel-based surrogates, but results from these methods are not backward compatible with decades of existing clay witness data. Adding clay pressure measurements to the current clay-indent methodology will provide time-resolved data related to impact load transmission through the PPE that may better quantify the risks for biological damage. Four different commercial pressure sensors (piezoelectric and piezoresistive types) were selected and tested in air and clay using a shock tube and a step-less pressure generator with various conditioning and data acquisition methods. For both sensor types, sensitivity changed by only a few percent in clay compared to air, and both were able measure the amplitude, rise time, and shape of pressure waves in the clay media. The piezoresistive sensors appear to be better suited to this application due to their smaller size (necessary for helmet testing), greater bandwidth and resonant frequency, and superior mechanical robustness with lower sensitivity to radial loading and mounting torque variations.