Prediction of Fracture Toughness for Open Cell Polyurethane Foams By Finite-element Micromechanical Analysis

The fracture toughness was determined for cellular polymers by micromechanicalmodelling using finite element analysis. In this study, mode I and mode II offracture toughness were evaluated with a 2D-solid model using fracture analysiscode FRANC2D/L. Simulation was performed for open cell polyurethane foams ofdifferent densities. Two cases were considered: constant cell length, l, and variable cellwall thickness; the former for constant cell wall thickness, t, and the latter for variablecell length. For estimation of fracture toughness the applied loads were progressivelyincreased to the point reaching the fracture strength of the solid material (130 MPa) inan un-cracked strut in front of the crack. The estimated fracture toughness wasindependent on crack length, indicating that the obtained values could be consideredas material property. The values of the fracture toughness of polyurethane foams arein the range of 10-3-10-1 MPa.m0.5. Lower values were obtained for mode II fracturetoughness. A strong dependency of the fracture toughness on the density of thecellular material was featured by present study. The obtained results for mode Ifracture toughness were compared with Gibson-Ashby micromechanical model, bywhich a good agreement was obtained. While, mode II fracture toughness wascompared with Choi and Sankar micromechanical models. The predicted fracturetoughness was finally validated with some experimental data. Another advantage of thismodel was to obtain a fully described the stress field in the solid struts. The stressdistribution in the first un-cracked strut showed a combined stress (bending andtension) for mode I loading, while for mode II loading a pure bending appeared