Fracture studies on polypropylene

Measurements of fracture surface energy have been made on polypropylene in the undrawn state and at different states of orientation over the temperature range ?60 to 60° C. Tear tests were employed and it was found that the fracture surface energy of unoriented material was of the order of 10⁴ to 10⁵ J m^?2. As orientation (represented by birefringence) increased, the fracture surface energy decreased by a factor of approximately 100 at room temperature but this factor was found to decrease with decreasing temperature. For all degrees of orientation, the fracture surface energy increased with increasing temperature in the range ?60 to 60°C, Scanning electron microscope studies showed a direct relation between the crack tip diameter and the fracture surface energy of unoriented specimens. From comparable studies on the tearing of rubber, Thomas has interpreted such a relationship as implying that the high values of fracture surface energy arise from the energy required to deform the material in the crack tip up to the breaking point. On this basis the reduction in fracture surface energy with increase in orientation may be regarded as being due to the associated diminution of the crack tip diameter. This interpretation is substantiated by direct measurements of crack tip diameter for specimens of intermediate and high orientation. Further microscopic studies of fracture surfaces indicate three modes of fracture which have been correlated with the appearance of the crack tip and tend to occur in certain ranges of birefringence.