Propagation and crack-arrest fracture toughness of injection-moulded short-fibre reinforced thermoplastics at low temperatures

Injection-moulded (IM) short-fibre reinforced thermoplastics (SFRPs) are lightweight material suitable for high-volume application; however, the research about fracture behaviours considering their microstructure under low service temperatures still remains limited. This study investigates the fracture of IM-SFRPs for different fibre orientation under low temperature conditions, focusing on the initiation and propagation fracture toughness (full R-curves) and their associated damage mechanisms. Full R-curves were obtained through Compact tension (CT) testing using an experimental J-integral method based on 3D-DIC measurements. The results revealed a pronounced dependence of both initiation and propagation fracture toughness on fibre orientation and testing temperature. A substantial reduction in fracture toughness was observed under −30 °C condition, particularly for fibre orientations aligned along the crack face. In addition, the load-displacement curves exhibited a reproducible crack jump; the corresponding crack-arrest fracture toughness was characterised for the first time in IM-SFRPs. Fractographic analysis identified a brittle-ductile transition and revealed preferential crack propagation in the core region under −30 °C condition, which was attributed to local embrittlement and reduced fracture toughness in this core region. Interestingly, Finite Element simulations successfully reproduced the data points corresponding to propagation and crack-arrest toughness used as inputs for Cohesive Zone Modelling. This study provides the first comprehensive characterisation of full R-curves of IM-SFRPs under low-temperature conditions, including crack-arrest toughness. The findings provide valuable insights into the design of efficient energy-absorbing automotive components operating in cold environments.