The effect of cell shape anisotropy on fracture toughness of low-density brittle foams

Abstract

Low-density brittle foams, although not expected to carry mechanical loads, have to retain structural integrity during service in order to deliver the expected functional characteristics. The fracture toughness of low-density foams exhibits anisotropy when foam cells acquire anisometric shape due to, e.g., the presence of a preferential direction for free-rise foams, which have cells elongated in the foam rise direction. In the current study, the effect of foam morphology on anisotropy in mode I and mode II initiation fracture toughness is evaluated numerically. A lattice model of foams with elongated and generalised Kelvin cells is employed that provides a more realistic representation of foam morphology than rectangular grids studied previously. The results obtained are compared with an analytical scaling model based on parallelepiped unit cell. The predicted toughness anisotropy is found to agree with the experimental results for low-density rigid polyurethane foams with moderate cell shape anisotropy.