Anisotropy of Fracture Toughness of Rigid Low-Density Bio-Based Polyurethane Foams

Low-density closed-cell rigid polyurethane (PU) foams are used for thermal insulation in a wide range of applications including cryogenics. In the latter case, thermal stresses develop in foam insulation in service due to mismatch of thermal expansion of the materials forming thermal insulation envelope that may lead to appearance and propagation of cracks, therefore foam toughness characterization is required. The free rise PU foams are transversely isotropic, and previous work has mostly considered toughness in the principal directions of foams. Directional dependence of mode I fracture toughness in intermediate directions within a plane normal to the isotropy plane of the transversely isotropic foams was studied experimentally by compact tension tests. Approximately sinusoidal variation of toughness with crack orientation angle in this plane was revealed. The effect of the shape anisotropy of foam cells on directional dependence of fracture initiation toughness of foams was also modelled numerically by employing a lattice of elongated Kelvin cells. The predicted variation of toughness with crack orientation agrees with the experimental results for rigid low-density bio-based PU foams having moderate geometrical cell anisotropy.