Toughness of Confined Auxetic Foams

Auxetic (negative Poisson's ratio) materials offer benefits such as impact mitigation, thermal insulation, vibration damping, and reduced deviatoric/shear strain—a key measure of material failure risk. However, the fracture mechanics of auxetic materials remain largely unexplored. This study investigates damage initiation and propagation in confined re‐entrant foam structures exhibiting auxetic behavior. These structures are fabricated by thermo‐mechanical transformation of pristine polyurethane foams. The confined foam is especially relevant for mechanical joints and bonding, illustrating practical advantages. Experimental mechanical characterization, combined with Ogden's hyperelastic formulation, underpins the analysis of the confined foam within a fracture mechanics framework, further supported by a traction‐separation law. A one‐dimensional semi‐analytical model, integrating beam theory and experimental material properties, predicts fracture processes under a double cantilever beam configuration. The model shows a very good agreement with experimental results, with confidence intervals ranging from 67% to 83%. The fracture toughness of the auxetic foams is reliably quantified, revealing the influence of the microstructural conversion process and a 50% improvement over conventional foams. This work transforms conventional foams by leveraging auxetic behavior for superior mechanical performance and provides a comprehensive investigation into their fracture mechanisms, offering critical insights for designing next‐generation mechanical joints and bonding technology.