STEPWISE FATIGUE-CRACK PROPAGATION IN POLYETHYLENE RESINS OF DIFFERENT MOLECULAR-STRUCTURE

Stepwise fatigue crack propagation in a range of polyethylene resins, some of which are candidates for use in pipes for natural gas distribu tion, was studied. Examination of the effect of molding conditions on fatigue crack propagation in a pipe resin indicated that fast cooling under pressure produced specimens with the same crack resistance as sp ecimens taken from a pipe extruded from this resin. The mechanism of s tepwise crack propagation in fatigue was the same as reported previous ly for creep loading. Observations of the region ahead of the arrested crack revealed a complex damage zone that consisted of a thick membra ne at the crack tip followed by a main craze with subsidiary shear cra zes that emerged from;he crack tip at an angle to the main craze. The effects of molecular parameters, such as molecular weight, comonomer c ontent, and branch distribution, on the kinetics of fatigue crack prop agation were examined. Correlation of creep and fatigue crack resistan ce made it possible to relate fatigue fracture toughness to molecular parameters by invoking concepts of craze fibril stability developed fo r creep. (C) 1998 John Wiley & Sons, Inc.