The plastic deformation of oriented polypropylene: tensile and compressive yield criteria

Abstract

Polypropylene, oriented by hot-drawing, has been deformed in tension and compression, at various angles to the initial draw direction. The behaviour in tension is described well by a three-part criterion of the type applied in previous work to fibre-composites [8, 9]. The Hill-von Mises treatment of the plasticity of an anisotropic metal, which has been previously extended successfully to polymers [1], can be applied to describe the tensile yield stresses but is unsatisfactory in its prediction of the form of the strain in polypropylene. In compression, the variation of the yield stress with the angle of the stress axis is much less than in tension, and the modes of deformation are different from those operating in tension at the same angle. In particular, compressing at right angles to the molecules produces shear in a direction normal to the molecules (transverse shear). The critical stress for transverse shear in compression is approximately the same as the critical stress for shear parallel to the molecules in tension.