TEMPERATURE AND STRAIN-RATE DEPENDENCE OF YIELD STRAIN AND DEFORMATION-BEHAVIOR IN POLYETHYLENE

The deformation behavior of a range of polyethylene materials which di ffer with respect to both their short-chain branch content and molecul ar weight has been studied. Mechanical measurements carried out over a wide range of temperatures have shown that there is a sudden transiti on in the measured tensile yield strain at a temperature which is depe ndent on both the grade of material and the applied strain rate. Above the transition temperature all of the materials behave in a nonlinear viscoelastic manner and the wide-angle X-ray scattering patterns obta ined have shown that at low applied strains reorientation of the lamel lae is observed before necking. Below the transition temperature the m aterials all behave in an elastic-plastic manner and there is no evide nce of lamellar reorientation before necking. This transition in yield mechanism is not apparent when considering the yield stress data alon e. (C) 1997 John Wiley & Sons, Inc.