Anomalous stress-strain response of fluid-filled polymer foams

Foams are used extensively in the field of impact mitigation, from athletic helmets to body armor. Being able to tune the toughness properties of foams would allow for the design of impact mitigation technologies that are comfortable for the user while also adapting to different types of impacts. Though the compressive stress-strain response of dry flexible foams is well studied, this behavior in fluid-filled flexible foams has not been fully explained. Under compression, fluid-filled foams are surprisingly less tough than their dry-foam counterparts. We investigate how the plastic resistance of the polymer struts, the viscous flow around the struts, and the impact speed (tested over several orders of magnitude) determines the toughness of these wet foams. This tunable toughness feature of wet flexible foams could enable innovations in smart impact mitigation materials, where the rigidity of the fluid-foam material could adapt according to the type of impact and the choice of filling fluid.