Deformation transitions of ethylene-hexene copolymers under creep loading

Abstract Creep tests were conducted in the temperature range from 296 to 358 K, on three commercial ethylene-hexene copolymers (PE) with mass density of 0.938, 0.941 and 0.952 g/cc. PE with the highest density shows the well-known ductile-brittle (DB) transition in the relationship between applied stress and failure time, and PE with the medium density a different transition which was detected through the change in the dependence of failure time on the applied stress, but the failure mode remained ductile. Therefore, this transition is named ductile-ductile (DD). The work shows feasibility of establishing creep master curves to include the two transitions, and discusses exclusiveness between DD and DB transitions so that detection of the DD transition can be used to determine the likelihood of the DB transition to occur in the long-term service. The work also examines applicability of the Monkman-Grant relationship to PEs that involve DD or DB transition.