Biaxial yield failure of polyurethane foams

V. PETTARÍN; L. FASCE; P. FRONTINI

Kerkrade, The Netherlands

Conferencia; 14th International Conference on Deformation, Yield, and Fracture of Polymers (DYFP2009); 2009

Plastic cellular foams are frequently used as core materials. The properties of foams depend on the structure of the cell, the density and the material of which they are made. A detailed characterization of their mechanical behavior is essential for their efficient use in structural applications. In this work, a deep investigation of the mechanical behavior of polyurethane (PUR) cellular foam under uniaxial and biaxial stress conditions was undertaken. A PUR foam with density ñ=150Kg/m3 was studied. Mechanical testing under uniaxial loading, i.e. uniaxial tensile and uniaxial compression configurations, was performed. Also a series of biaxial tests were conducted, including: simple shear, constrained strip specimens in tension and constrained strip specimens in compression. Experiments were conducted at room temperature at two different loading rates. True strains were measured with the aid of a video-extensometer. True stress-strain curves along the in-plane and through-the thickness directions were then obtained. PUR foam exhibited a nearly isotropic behavior. However, it showed quite different stress-strain behavior in tension and compression. The strain rate sensitivity of the PUR foam was found to be small. The suitability of different failure criterions in describing the mechanical behavior was also analyzed. The yield behavior of porous materials such as polymeric foams is affected by the mean normal stress. Therefore, applied yield criteria should include its effect. It was found that the deformation under tensile loading was governed by cell wall bending. In contrast, under compressive loadings, the deformation of this foam was governed by the elastic buckling of the cell walls. This switch in deformation mechanism resulted in asymmetry of the failure surface with respect to the sign of the stress tensor. Thus, failure surface was in accordance with a quadratic criterion with an added buckling cap. This buckling cap was approximated closely by a maximum compressive first stress invariant in principal stresses criterion.