FATIGUE-FRACTURE MECHANISM OF SLOWLY NOTCHED POLY(ETHYLENE-TEREPHTHALATE) POLYMERS

Fatigue fracture behavior of slowly notched polyethylene terephathalat e (PET) polymers were investigated at temperatures close to their beta transition temperatures up to well above their glass transition tempe ratures. Detailed characterization on the morphology of the notched ro ots showed that the crack tip during crack propagation became more dul l with increasing testing temperature. The failure cycle (N(f)) of the se samples increased with increasing temperatures until it reached the alpha transition temperatures of PET polymers, and most of the increa se in N(f) is due to the increased time consumed in the initiation per iod. On the other hand, the initial crack growth rate increased signif icantly and N(f) of these samples decreased dramatically as the temper ature increased well above the glass transition temperature. This inte resting temperature dependence of fatigue behavior is explained due to the change of molecular motion of PET polymers at this temperature ra nge.