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.