The wind energy sector has achieved significant milestones driven by rapid industrialization and the adoption of larger wind turbine rotor blades. These blades, manufactured with thick composite shells and spars, rely on adhesive bonding technology for assembly. However, the trend towards producing larger blades using conventional manufacturing processes poses challenges in adhesive bonding, as evidenced by a notable increase in defects with blade size. Enhancing the toughness of the adhesive bondline through various methods, such as toughening and incorporating crack-arresting features (CAFs), holds promise in realizing the damage tolerance philosophy. This thesis aims to develop crack-arresting strategies for bulk adhesives and thick bonded interfaces while addressing several factors, including manufacturing defects, toughening techniques, and specimen geometry effects. A comprehensive assessment of the effects of manufacturing processes, toughening methods, and standard specimen geometries on the adhesive properties of wind turbine blades is conducted through instrumented experiments. Additionally, a data-leakage-free, generalized machine learning (ML) framework is proposed to estimate the fatigue life of adhesive materials based on the characteristics of failure surface voids. Thin PEI and PVDF thermoplastic interlayers are incorporated within bulk adhesives as crack-arresting features and their Mode I fracture, and fatigue response are investigated using single edge notched beam (SENB) specimens. In thick glass-fiber reinforced polymeric (GFRP) composite joints, additively manufactured, architected crack-arresting features are incorporated, and their Mode I fracture and fatigue behavior are investigated using double cantilever beam (DCB) specimens.
Manually mixed and manufactured adhesive specimens exhibit higher void content compared to machine-mixed adhesives. Hybrid adhesives are developed by combining non-toughened and toughened adhesives. Toughening decreases the modulus and strength and makes the adhesives sensitive to strain rate. Toughening do not affect the S-N curve slope, toughened adhesives exhibited lower fatigue life compared to non-toughened adhesives. Considering the specimen geometry effect, standard geometry does not always lead to consistent fatigue life characterization in epoxy adhesives. ASTM D638-22 Type I specimens exhibited an increased S-N curve slope and decreased fatigue life compared to Type II and Type IV specimens. Considering the crack-arresting capability of the thin PEI and PVDF interlayers in bulk epoxy adhesives under Mode I quasi-static fracture loading, the interlayer materials can effectively arrest the initial crack at the thermoplastic-epoxy interface. Crack bifurcation and the formation of epoxy-thermoplastic interface cracks are critical for fatigue crack-tip shielding. Architected crack-arresting feature materials based on tough thermoplastics and soft elastomers can be used to engineer joint stiffness, crack initiation energy, maximum strain energy release rate (SERR), and overall fracture resistance curve behavior. Maximum SERR can be attained with tough materials, whereas soft materials provide very stable fracture behavior. Under fatigue, tough CAF materials provide better FCG resistance compared to pristine and soft CAF materials.