Relationship Between the Creep of Solid and Foam Polyurethane Resulting From Combined Stresses

Abstract

A multiple integral representation was used to describe the constant stress creep of solid (full density) polyurethane under compression and combined tension and torsion. Also, the modified superposition principle was used to predict the material’s response to abrupt changes in stress with very satisfactory results. The functions found to describe the solid polyurethane were then used to predict the response of foam polyurethane with a specific gravity of 0.33 to similar stress histories. This was achieved by multiplying the stresses for the foam by 15.5, which was the ratio of the stiffness of the solid to that of the foam. This procedure satisfactorily predicted the creep behavior of the foam for constant stresses and abrupt changes in stress. Two models were considered for correlating the stiffness of a foam and its associated full-density material. One model, a close-packed array of spherical voids, was found to predict the correct density for the foam considered here. The observed stiffness of the foam fell within the bounds for the stiffness found for this model. The other model considered was a cubical space frame. This model was found to predict reasonably well the stiffness of the polyurethane foam and the stiffness of polystyrene foams of several densities. The space frame model also predicted several other observed features of the behavior of polyurethane foam.