Crack propagation in biaxially oriented polypropylene at low temperature

Abstract

AbstractThe crack growth behavior of polypropylene biaxially oriented by cross‐rolling was studied at low temperature. Single edge notch testing produced a stable tearing type of crack growth in both 50% and 80% biaxially oriented polypropylene at −40°C, in contrast to the brittle fracture of unoriented polypropylene. The crack growth in the two oriented materials began slowly and accelerated to a constant rate that was higher in the 80% oriented material than in the 50% oriented material. The main difference between the crack growth behavior of the two was the longer period of initial slow growth in the case of 80% orientation. This period of slow growth corresponded to crack growth through the notch tip damage zone. Residual strength diagrams were used to present the crack growth data obtained when the stress state was intermediate between plane stress and plane strain. Fractography revealed large differences among the fracture surfaces of the three materials with the unoriented polypropylene showing a grainy appearance from the brittle fracture. The two oriented materials showed considerable ductility. The 50% oriented material showed many voids in the fracture surface, indicating that voiding during the fracture process contributed significantly to the toughness improvement. The 80% oriented polypropylene showed delamination crazing on the fracture surface with layered material and fibrils bridging the crazes.