Influence of environmental factors on energy absorption degradation of polystyrene foam in protective helmets

Abstract

The environmental factors such as hot-wet and pre-compression may lead to degradation in energy absorption properties of polystyrene foam in protective helmets. In this paper, an experimental study on the mechanical properties of Expanded Polystyrene (EPS) foam was conducted using high precision micro-force compression tests. Tests were conducted with foam specimens from actual helmets at pre-compression strain ranging from 10% to 50% and hot-wet conditions at 60 °C with different relative humidity (RH) levels between 40% and 85%. The load–displacement behavior of EPS foam was carefully recorded for determining the compressive stress–strain curves. The test results indicated that when the pre-compression strain exceeded 10%, the elastic stiffness and plastic yielding region (plateau region) decreased and the plastic yielding point increased dramatically. When the pre-compression strain reached 50%, the plateau region vanished from the stress–strain curves. High humidity has a significant effect on lowering the plastic yield point, but has little effect on the plateau region range. The stress–strain curves from these tests were successfully implemented into a validated helmet finite element model to simulate guided helmet free drop experiments. Headform peak decelerations and head injury criterion (HIC) values were obtained from simulation and analyzed to estimate the energy absorption degradation in EPS foam. The finite element analysis predictions showed that pre-compression significantly degrades the energy absorption properties of EPS foam. The maximum permissible pre-compression strain should not exceed 10%. High humidity levels lead the helmet to transmit a lower HIC value but wider HIC duration to the head.