Failure modes of closed-cell polyurethane foams

Abstract

The failure modes of closed-cell polyurethane foams were studied by applying the elliptic paraboloid failure surface criterion. A series of three polyurethane rigid foams (PUR-foams) were examined presenting different amounts of porosity from a highly porous material having a low density of 64 kg/m³ to a compact one with a density of 192 kg/m³. All these PUR-foams were of the same batch of material presenting a cell-wall density ϱ₅=1200 kg/m³. Samples were tested in simple tension and compression along the three principal axes of anisotropy of the materials. It was shown that all three types of foams may be closely represented by transversely isotropic materials.

The elliptic paraboloid failure surfaces (EPFS) for these three materials were defined from the six values of principal failure stresses in tension and compression. It was shown that the theoretically plotted paraboloid surfaces along all their principal-plane intersections were in good agreement with experiments.

Since cellular materials collapse, either under elastic buckling in the compressive octant of the principal stress space, or under fast brittle fracture in the tensile octant, it was shown that the elliptic paraboloid failure surface is truncated by the intersection of the EPFS and an ellipsoid whose position and dimensions are interrelated with those of the EPFS. Again, experimental evidence with elastic buckling of foams corroborated the results of this theory.

An important feature for the failure behavior of the foams was derived by this study according to which the foamed materials change mode of failure from a compression strong to a tension strong mode as their porosity is increased. In between they pass through a quasi-isotropic state.