The yield behaviour of oriented polyethylene terephthalate

Abstract

Oriented sheets of polyethylene terephthalate have been deformed in tension and simple shear, with the primary variable as the angle between the tensile stress or the shear stress and the initial draw direction. In tension, deformation bands were observed in all cases, as has been reported previously (Brown and Ward 1968 a), and a clear yiold point was observed in all cases. The stress–strain curves for simple shear on the other hand showed very appreciable changes with the direction of the shear stress, and deformation bands were observed only in certain cases. These qualitative features are consistent with the principles proposed in the previous paper, on the basis of the tensile behaviour. An attempt is made to obtain a yield criterion which is consistent with both the tensile and shear data. It was found that neither a critical resolved shear stress criterion nor this modified by a normal stress term was adequate to explain the tensile yield data. However, a modification of the von Mises yield criterion proposed by Hill for ideally plastic materials of orthorhombic symmetry did provide good fit to the tensile data, and gives a reasonable approximate fit to the shear data. The direction of the deformation band is also found to be consistent with Hill's theory, when the yield surface is equated directly with the plastic potential. The yield stress results for simple shear show unequal maxima as the yield varies with the direction of the applied shear stress. This can be explained by introducing a Bauschinger term into Hill's equation. It is then possible to obtain a yield criterion which gives a good fit to the tensile data on yield stresses and band angles, and predicts the yield stress behaviour in shear to a good approximation.