Effects of weathering,scale, and rate of loading on polycarbonate fracture toughness

The fracture toughness of polycarbonate specimens of 3–9 mm thickness obtained from an actual aircraft canopy, were studied under accelerated weathering conditions and different rates of loading. Although no significant effects of thickness and loading rate on the critical stress intensity factors were observed, two different failure modes, brittle fracture and ductile fracture triggered by “pop-in,” were observed. The mode of failure was a random event and the probability of ductile failure associated with pop-in increases with the weathering time. More insight to material characteristics are gained through analysis of the specific fracture energy (SFE). The average values of SFE decrease monotonically with accelerated weathering time. This effect is ascribed to physical aging of the PC in the weatherometer that was corroborated through increases in density. The values of SFE seem to correlate with the probability for ductile fracture. This information can be used to establish conservative critical stress values for design. © 1994 John Wiley & Sons, Inc.