Publication
Wind turbine rotor blades are usually fabricated using composite materials and epoxy adhesive to bond the elements together. Due to their large size, the thickness of the adhesive layer varies from 1 to up to 30 mm depending on the position. The failure of thick adhesive joints, such as the trailing edge joints within the wind turbine rotor blades, is frequently observed through post-mortem examinations and full-blade investigations, but limited research has been carried out so far on thick adhesive joint behavior and failure causes in static and fatigue modes. This thesis thus aims to understand the fracture and fatigue behavior of thick adhesive joints to predict and improve their performance, and to propose novel adhesive formulations that could limit damage and allow healing of the cracks in the structure. Due to the test configuration simplicity, this study focused on the cohesive failure of double cantilever beam joints with thick adhesive layers (> 10 mm) under Mode I static and cyclic loading. The quasi-static experiments addressed the qualitative effects of voids, joint geometry, and test conditions on the performance of thick joints made with a benchmark commercial epoxy. Side-grooved shape guided the crack propagation direction and assisted stable propagation, while lower cross-head displacement rates reduced the occurrence of unstable crack propagation and prevented crack deflection. Porosities, which are inevitable due to the high viscosity of the adhesive, led to unstable propagation and promoted crack path deviations. The fatigue performance of thick joints was further investigated to study the qualitative influence of displacement ratio (Rd-ratio) and void content on the crack path and fatigue crack growth curves under constant amplitude cyclic loading. The results showed that the fatigue crack propagation path was primarily determined by the void size and distributions. Secondary cracks were also observed, which could lead to the crack path switching on the lateral surface and influence the crack measurement accuracy. Data analysis using the total fatigue life model showed that the fatigue crack growth depends on the Rd-ratio and void content. To improve the performance of thick adhesive joints, one approach is to develop healable epoxy adhesives, in order to mend the cracks in situ. The commercial benchmark adhesive contains fillers, such as nanosilica to achieve a rheological yield stress behavior, preventing flow of the adhesive under its own weight during application on all composite surfaces. A filler replacement method was thus proposed to achieve both a sufficient yield stress and high healing efficiency: a portion of nanosilica was replaced by poly(epsilon-caprolactone) microparticles, which serves a dual purpose, acting as fillers to form a 3D network synchronizing with nanosilica and acting as healing agents to heal the crack once heated. The developed formulation fulfilled the majority of the requirements, including st