The Effect of Layering Interfaces on the Mechanical Behavior of Polyurea Elastomeric Foams

The performance of density-graded elastomeric foams has been a cynosure of the pursuit of superior impact mitigation materials and structures. Elastomeric foams exhibit a remarkable mechanical response, including resilience, toughness, and recoverability. However, recent research has only focused on the performance of uniform-density foam paddings in response to various strain rates. Concurrently, the body of research on the potential of density gradation has been burgeoning, suggesting an untapped potential to achieve higher levels of protection than those offered by their ungraded counterpart. This research aims to elucidate the layering interfaces effect on the performance of density-graded elastomeric foams in response to quasi-static and impact loading. The approach is to manufacture foam laminates consisting of bi- or tri-layered polyurea elastomeric foams using two different layering techniques. In one set of samples, the foam was natively adhered by casting subsequent layers with different densities by adjusting the mixing and pouring ratios. In the second set of samples, separately cast polyurea sheets were adhered using ultrathin polyurea adhesive to mimic the configuration of the first set. All foam samples were submitted to quasi-static loading up to densification and impact loading at 7 J. The static and dynamic stress-strain curves were accompanied by full-field digital image correlation analysis, revealing the contributions of the density gradation and layering interfaces to the overall deformation. While the primary outcomes include insights into the mechanistic processes responsible for the mechanical behavior, the natively bonded density-graded polyurea foams provide an exciting platform to explore additional mechanics of elastomeric foams.