The design of damage-tolerant aeronautical composite structures often involves thin-walled components that are susceptible to in-plane mixed-mode fracture. Unlike with metals, this process is complicated by the composites anisotropy and the lack of standardized procedures for predicting failure in notched, holed or cracked composites under mixed-mode loading. This study introduces a novel Modified Arcan Fixture (MAF) for testing Compact Tension Shear (CTS) specimens of carbon fibre woven reinforced polymer composite. Digital Image Correlation (DIC) was used to capture strain fields and calculate Stress Intensity Factors (SIFs), which were then compared to analytical predictions for different mode combinations and notch lengths. R-curves were generated for specimens exhibiting self-similar crack propagation. The results revealed that failure modes were dominated by tensile cracking in Mode I and compressive cracking in Mode II, indicating that a single-parameter fracture criterion inadequate for the failure description. A theoretical model that incorporates both tensile and compressive cracking is proposed, which can accurately predict the complete mixed-mode fracture envelope. Furthermore, Scanning Electron Microscopy (SEM) and X-ray micro-tomography were used to elucidate the mechanisms of surface failure and the morphology of internal damage.

Adhesive bonding technologies for thermoset polymer composites have been used in marine, automotive, construction and aerospace industries due to their superior mechanical behaviour (high strength-to-weight ratio, damage tolerance and fatigue resistance) compared to conventional joining methods. The main disadvantage of this joining technology is its susceptibility to delamination due to disbonding during use. Loading conditions, adhesive type, ageing effects and lack of inspection procedures are just some of the elements that affect the overall structural performance of the composite joint during the manufacturing process. A deeper understanding of how these elements affect joint behaviour is required to improve joint performance and design. This work provides a comparative fractographic analysis for two different joint types: co-bonded (CB) and secondary bonded (SB) joints, under Mode I delamination at elevated temperature and high humidity conditions. Fractographic analysis was used to compare the two joint technologies and explain the differences in toughness values and fracture behaviour, revealing crack propagation mechanisms in composite joints. While the CB and SB joints have comparable fracture toughness (G_IC) values, different fracture characteristics and bonding methods can discern these two bonding technologies, indicating that SB joints are more susceptible to environmental conditioning.