# Processing–Microstructure–Performance Relations in Thermoformed Auxetic Hyperelastic Foams with Enhanced Energy Absorption Capacity

## TL;DR

This paper introduces a thermoforming method to create auxetic foams with improved energy absorption and mechanical properties.

## Contribution

A scalable thermoforming process is developed to induce auxeticity in hyperelastic polyurea foams.

## Key findings

- Auxetic foams achieved negative Poisson’s ratios approaching −0.6.
- Energy absorption capacity was several times greater than the original foam.
- The auxetic transformation occurs near the foam’s nominal densification strain.

## Abstract

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.