Evaluating fracture toughness in composite structures is challenging when material properties are unknown and residual stresses are present, such as aged or repurposed wind turbine blades. This thesis validates stiffness- and curvature-based analytical J-integral formulations for double cantilever beam (DCB) specimens under these conditions. The analytical methods compute the J-integral along the external boundaries, and their accuracy was assessed by comparison with local J-integral values evaluated around the crack tip in finite element models. A surface-based bilinear cohesive zone was implemented in three finite element models: a single-material baseline, a multilayer configuration, and a multilayer model incorporating residual stresses.

The analytical methods showed strong agreement with local J-integral results across a range of load cases, with errors typically below 2% of the fracture toughness. Increased stiffness in the multilayer model lengthened the cohesive zone and improved accuracy by aligning more closely with the analytical assumptions. In the residual stress model, the auxiliary problem, free of residual stresses, accurately reproduced the real problem’s crack plane opening profiles for variations in temperature, stiffness, thermal expansion coefficient, and mechanical loading. Validating the analytical expressions for the auxiliary case--combined with opening profile consistency and the assumption of a potential-function-based J-integral--validated their applicability to the real problem.

These results confirm that the analytical J-integral formulations can accurately estimate the energy release rate when material properties are unknown and residual stresses are present, enabling reliable structural integrity assessments of composite structures.

This thesis explores finite element modelling of fibre-matrix debonding and frictional sliding in
Abaqus, with particular attention given to their influence on neighbouring fibres. It aims to improve model accuracy through advanced simulations and validation methods. A literature review underscores the need for robust finite element models in predicting composite material behaviour in fatigue. Theoretical equations for validating the finite element model are developed.
Single-fibre and multi-fibre models are utilised, with the former being used primarily for validation and the latter being used to simulate various realistic cases. Results demonstrate successful validation and provide insights into the effects of friction along the debond interface on stress concentrations in neighbouring fibres. Key findings indicate that reducing interfacial friction increases crack tip energy release rates, leading to stress fields that could potentially cause fractures in neighbouring fibres near the debond crack tip.