Abstract
High density polyethylene with a single crystal texture has been used to examine the variation of yield stress with orientation in both tension and compression. The Coulomb yield criterion, which has previously been applied to yield data in oriented polyethylene, must be rejected since the experimental data can only be fitted over a limited range of orientation in tension only. The von Mises criterion, as modified by Hill ^12^ for anisotropic materials, appears to give a reasonably good fit to the tensile data. However, further consideration of the theory with respect to the compressive data shows that the criterion is not applicable. The pressure dependent yield criterion proposed by Caddell et al^13^ also appears unable to predict the experimentally observed tensile and compressive behaviour. A threeβpart criterion previously applied to fibreβcomposites ^11^ gives a good fit to the tensile data; this model also predicts the changeover from ductile to brittle behaviour observed in tensile specimens with ΞΈ > 70dΜ (ΞΈ is the angle between the tensile axis and the molecular axis). None of the yield criteria examined have been able to explain satisfactorily the lack of any marked degree of anisotropy in compression.