Failure Modes of Foams: The Influence of Orientation of Voids

Abstract

An experimental investigation is undertaken in this paper for the assessment of the failure behavior of a series of polyurethane foams of variable specific density. All foamy materials examined here were acknowledged as transversely isotropic materials, whose axis of symmetry is, however, the weak axis of the medium. Simple uniaxial tension and compression tests were executed, and the experimental data were introduced into the elliptic paraboloid failure surface criterion. This condition was utilized for the comparison of the failure loci, associated with primary failure, of the polyurethane foams of different porosities. As a result, it is shown that, when the specific density of the foamy solids is raised, the shape, position and alignment of their failure loci intersections by the principal stress planes, as well as by the principal diagonal planes, is adjusted significantly. In addition to this fact, their degree of anisotropy is augmented; however, it is noted that the totality of the foamy materials tested exhibit solely compression-strong failure characteristics. Next, the influence of the voids’orientation on the failure modes of foams is investigated, by evaluating the variation of the essential parameters, required for the precise definition of the failure loci of the weak-axis polyurethanes, with respect to their specific density. And, further, by contrasting this variation with the respective one for the strong-axis foams encountered in previous studies of the senior author (Theocaris, 1992a,b; 1998). Hence, a comprehensive comparison of the failure behavior of foams, which display a weak and strong-axis of symmetry, is succeeded, revealing their similarities, but most importantly, their peculiarities and idiosyncrasies. Finally, a numerical analysis is disclosed as a means of determining the failure characteristics of mixed foams, that consist of both strong and weak-axis polyurethane materials of different specific densities. Accordingly, the effect of variation of the relative proportions and the specific densities of the constituent foamy phases on the characteristic parameters, designating the failure loci of mixed foams, is studied, offering interesting results concerning the modes of failure of such foams.