Initiation and propagation fracture toughness of injection-moulded short fibre composites under different environmental conditions

Injection-moulded short-fibre composites combine lightweight and manufacturability; however, their fracture behaviour and how it is affected by the microstructure and environmental conditions are yet to be fully characterised. The initiation and propagation fracture toughnesses of injection-moulded short glass-fibre reinforced polyamide 6.6 composites were characterised through compact tension testing under the combined effect of fibre orientation, moisture level and temperature. Full R-curves were calculated using either an FE-based compliance calibration method, or the J-integral method based on full-field measurements from Digital Image Correlation; both data reduction methods provided consistent propagation values for the fracture toughness, although only the J-integral method can characterise the initiation toughness and the shape of R-curves reliably. This work revealed that the material became tougher with increasing fibre orientation along the loading direction, increasing moisture content, and/or increasing temperature; the corresponding increase in toughness was related to changes in failure and toughening mechanisms, identified through fractography. FE simulations of the compact tension tests have demonstrated the need to consider both initiation and propagation values of fracture toughness to accurately predict the response of notched specimens. The thorough characterisation of fracture toughness presented in this paper can contribute to design safer and more efficient damage-tolerant IM-SFRP components.