The effect of temperature on the fracture of two partially crystalline polymers; polypropylene and nylon

Brittle fractures in polypropylene and nylon were obtained in the temperature range -180 to -10° C and -160 to 20° C, respectively. The fracture toughness results of surface notches and single edge notches showed a remarkable thickness dependence. This effect was explained in terms of a plane strain (K_C1) and a plane stress (K_C2) toughness value using a bi-modal fracture analysis. While K_C1 in both materials was generally insensitive to temperature, K_C1 was temperature dependent showing substantial viscoelastic effects. In polypropylene, the linear relationship between K_C2 and temperature (7") was associated with the β and γ processes over the same temperature range. In nylon, there was a one-to-one correspondence between the K_C2 changes and the tan δ peaks due to the β and γ relaxation processes. Using a modified crack tip opening displacement (u) equation, i.e. u = ((K_C2 - K_C1 )/σ_y)²e_y, where σ_y and e_y are the yield stress and yield strain respectively, a constant u criterion was found to describe the fracture behaviour of both materials in the temperature ranges of the β and γ processes.