Processing-Microstructure-Performance Relations in Thermoformed Auxetic Hyperelastic Foams with Enhanced Energy Absorption Capacity.

Auxetic (negative Poisson's ratio) foams with reentrant cell structures exhibit enhanced mechanical properties such as superior strength, energy absorption, and fracture resistance, compared to their nonauxetic counterparts. A well-established method for inducing auxeticity in cellular solids involves permanently changing the cell ribs that are buckled under compressive loads. This permanent change can be achieved by heating a deformed foam for a specific duration. In this study, a thermoforming process is developed to convert closed-cell hyperelastic polyurea foams into auxetic structures. The approach relies on rationally identifying critical compression ratios by assessing key mechanical performance attributes of the pristine foam. Auxetic transformation is achieved by applying compressive strains exceeding a defined threshold, with lateral confinement provided by a custom-designed thermoforming die. Microstructural observations and mechanical testing, including stress-strain and Poisson's ratio measurements, confirm the successful auxetic transformation in the foam. Notably, the transition occurs at compression ratios near the nominal densification strain of the original foam. The resulting auxetic foams demonstrate negative Poisson's ratios approaching -0.6 and exhibit energy absorption capacities several times greater than those of the pristine foam. The simplicity and scalability of the proposed thermoforming method underscore its potential for broader application in the development of the next-generation energy-absorbing structures.