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.

Delamination is one of the common failure types in the fiber composites. The trailing edge in the wind turbine blades, for example, is one of the structural components where the delamination is driven by mode III under predominantly flapwise loading. There has been an extensive effort in developing test procedures and test specimens for determination of the delamination toughness in mode I and mode II delamination while the mode III is neglected due to its complexity, on the one hand, and the lack of a reliable test procedure and numerical model for the mode III delamination toughness testing on the other hand. Therefore, the cur- rent project aims for developing an experimental test specimen capable of extracting cohesive laws using the J integral approach. To this end, the FEM is utilized for the analyses of various parameters.

Delamination can be modeled using Progressive Failure Analysis where the crack initia- tion and crack evolution are studied. The present thesis covers both aspects of the Progressive Failure Analysis in the unidirectional fiber composites. Before starting the mode III, mode I and II test specimens are analyzed to establish requirements for proper three-dimensional simulation. The reliable numerical model from this step is the fundamental model for the mode III delamination analysis. In the first part of the project, the Double Cantilever Beam cross-section is optimized to give the pure mode III. LEFM is invoked for the analyses of this section (Small Fracture Process Zone). The results have shown that the pure mode III cannot be accomplished and there always exists an induced coupling mode II under anti-plane load- ing. Furthermore, the results suggest higher mesh density is required at the free surface to further investigate the 1/√r-singularity at this location.
The second part of the thesis covers the Cohesive Zone Modeling (Large Fracture Process Zone) whereby the crack evolution is simulated by the cohesive elements. The implemented cohesive law is validated by the J integral. The bilinear cohesive law has been successful in simulating the crack extension for Large-Scale Bridging condition. The traction-separation law has been validated through the resistance curve generated by the FEM and derivation of the resistance w.r.t the tangential end-opening.

The commercial code, ABAQUS, is utilized in this study for the Finite Element Analyses.