Impact Response of Polyurea Elastomeric Foams

Providing effective protection from impacts in contact sports has been a foundation for the research of elastomeric cellular solids. Due to their superior energy absorption properties, polyurea elastomeric foams have emerged as a novel material candidate for impact mitigation in biomechanical and sports applications. This research utilized a small-scale shock tube to extend the application domain for polyurea foams to encompass higher strain rates. The experimental approach consisted of submitting foam plugs to a shock-propelled aluminum projectile with a velocity of ~ 24.5 m/s while capturing the impact event using high-speed imaging. The latter was analyzed using digital image correlation to extract the distribution of in-plane strain components in mono-density and density-graded foams. The density-graded samples were assembled using two interfacing strategies, namely adherent-free and thin layers of bulk polyurea adhesive. Experimentally measured strain-time histories revealed the effect of gradation and interfacing strategies at strain rates up to 4000 s^{− 1}. The results affirmed that adherent-free, density-graded polyurea foams exhibited higher deformations than the adhered counterparts, even with relatively thin adhesive layers. In addition, polyurea foams developed a noticeable strain lag between the lateral and axial strains, exemplifying their hyper-viscoelastic behavior and improving energy absorption by broadening the strain-time peak. Most notably, polyurea foams, irrespective of the configuration, underwent reversible and momentary pseudo-liquefaction upon densification, reaching strains greater than 90%. This unique behavior indicates a new deformation mechanism for polyurea foams in sports applications where the foam rapidly conforms to the geometry of the impacted body, thereby shielding larger areas from violent impacts. All polyurea foam samples exhibited significant recovery within minutes, a promising attribute for greater impact efficacy in repeated loading scenarios.