UNIFICATION OF DUCTILE FAILURE AND SLOW CRACK-GROWTH IN AN ETHYLENE-OCTENE COPOLYMER

The time dependence of ductile and brittle failure of an ethylene-octene gas pipe resin was measured as a function of stress, notch depth and temperature. The ductile failure mode depends on the ligament stress and all the ductile data could be unified by normalizing the ligament stress with respect to the short-time yield point at each temperature. The brittle data were unified by correlating the stress and notch depth in terms of the stress intensity and by introducing a time-temperature shift. At each temperature there is a critical stress intensity above which the notch becomes blunted so that ductile failure ensues. The unified curve for the ductile failure is about the same for all polyethylenes. The unified curve for the brittle regime varies greatly with different polyethylenes where the primary difference is associated with the minimum time required to produce brittle behaviour for a common temperature and stress intensity.