Fracture and creep properties of flow-formed tubes

Abstract

The effects of varying the percentage thickness reduction on the fracture toughness and creep properties of flow-formed polypropylene tubes at different angles of orientation with respect to the direction of flow-forming have been investigated. Thickness reductions of up to 76% were employed. The results show that both the fracture toughness and creep modulus vary with the angle of orientation and reach a peak value at an angle close to the helix angle of flow-forming, indicating that anisotropy is induced into the flow-formed tubes. While the un-flow-formed material was isotropic, anisotropic effects after flow-forming appeared prominent at thickness reductions above 50%. Significant improvement in fracture toughness could be achieved also at this thickness reduction. Although the fracture toughness values were lowest at an orientation angle of 90°, they were however not lower than those for the un-flow-formed material. Nevertheless, this lower fracture toughness value is manifested in that final failure of the flow-formed tubes under pressure occurred often by splitting along the helix angle. The rate of feed investigated in the flow-forming process, within the range of 0.3–1.0 mm/rev, did not appear to alter significantly the fracture toughness properties of the polypropylene. Although the variation in creep modulus with the angle of orientation in general followed that for fracture toughness, only the modulus at 76% thickness reduction was higher than that for the unflow-formed material. This implies that flow-formed tubes creep more readily initially than the original un-flow-formed tubes.