The fracture behaviour of oriented polyethylene at high pressures

Abstract

The fracture behaviour of die-drawn biaxially oriented polyethylene sheet has been studied under superimposed hydrostatic pressure in a new testing facility. Average fracture toughness values of K_Ic = 13 MN m^-3/2 and 4.4 MN m^-3/2 were obtained for the tensile stress applied parallel and perpendicular to the draw direction, respectively. The corresponding value for the polymer in its isotropic state was K_Ic = 1.5 MN m^-3/2, showing that the introduction of molecular orientation has increased considerably the resistance to brittle failure. When the notch in the tensile test was normal to the draw direction and the stress parallel to it, the cracks followed paths that were at non-zero angles to the notches from which they initiated. The ratio of the K_lc value obtained in this test to that when the notch was parallel to the draw direction and the stress normal to it is discussed first in terms of a crack opening critical stress and secondly in terms of a minimum energy fracture criterion. Some comparative tests are also reported for the fracture toughness relating to cracks propagated in the plane of the sheet, and for die-drawn uniaxially oriented polyethylene rod of comparable draw ratio.