Static and dynamic mode I fracture toughness of rigid PUR foams under room and cryogenic temperatures

Abstract

The research presented in this paper is an effort to better understand the fracture toughness of closed-cell rigid polyurethane (PUR) foams under different loading and temperature conditions. The effect of density (100, 145 and 300 kg/m3) and anisotropy (in-plane and out-of-plane loading directions) on both quasi-static and dynamic fracture behavior was also experimentally investigated. The three-point bending (3PB) tests were performed on Single Edge Notched Bend (SENB) samples, at room (25 °C) and cryogenic (−196 °C) temperatures, and the mode I fracture toughness (KIC) was calculated from their load-displacement curves. It was observed that all PUR foam samples, regardless of foam density and loading direction, showed a significant increase in KIC at the cryogenic temperature. The out-of-plane obtained samples showed a slight improvement in fracture toughness (highlighting an anisotropic behavior), both under quasi-static and dynamic 3PB loads. The dynamic KIC values were found higher than quasi-static ones, and irrespective of foam density and test condition, a brittle deformation mechanism without plastic deformation was observed for all samples. Finally, empirical formulations for cryogenic and dynamic KIC based on room temperature mode I fracture toughness were proposed.