Ductile behaviour of ethylene-propylene block copolymers

Abstract

Flow behaviour during the tensile loading of ethylene-propylene diblock copolymers (EP copolymers) has been investigated at various strain rates. The strain rate sensitivity index indicated that the short chain (EP-TC) copolymer is more sensitive to strain rate than is the long chain (EP-M) copolymer. Scanning electron microscopy of ethylene-propylene diblock copolymers deformed at various strain rates revealed two types of fracture morphology, fibrillation and ductile ploughing. The area fraction of fibrillation decreased with an increase in strain rate, with a consequent increase in ductile ploughing. The surface of tensile deformed specimens of both block copolymers was characterised by wedge formation that increased with strain and strain rate of the tensile test. The density of wedges was greater in the centre of the longitudinal direction of the tensile specimen and less away from the centre. The higher plastic flow associated with wedges led to fibrillation type failure in the centre of the fracture, surrounded by a less ductile mode of failure, referred to as ductile ploughing. Regarding the difference in ductile behaviour of the two copolymers, long chain EP-M diblock copolymer exhibited greater fibrillation compared with short chain EP-TC diblock copolymer at a constant strain rate.