Comparison of Damages Occurring on the Bonding Surface of Carbon and Glass Fiber Reinforced Polymer Composite Materials Used in Wind Turbine Blades and Marine Vessels in Terms of Three-Point Bending and Four-Point Bending Tests

Composite materials are preferred in many industrial fields due to their superior prop-erties such as high mechanical strength, light weight, corrosion resistance, and long service life. These materials play a critical role especially in the marine and renewable energy sectors. However, despite these advantages, composites are not completely im-mune to environmental conditions. Factors such as moisture, saltwater, UV radiation, and temperature variations can negatively affect the structural integrity of these mate-rials over time. In marine environments, composite materials are continuously exposed to high humid-ity, salt concentration, and ultraviolet radiation. These environmental factors lead to the diffusion of water molecules into the material, causing weakening at the fiber–matrix interface. As a result, the mechanical strength of the composite decreases, lead-ing to a reduction in its structural performance. This is a significant issue for long-term marine applications. Nevertheless, due to their advantages, composites are widely used in ship superstructures (decks, bulkheads, masts, rudders, pipes, valves) and in com-mercial vessels (lifeboats, ferries, fishing boats). They are also commonly used in recre-ational marine vehicles such as speedboats, racing yachts, and canoes. On the other hand, the depletion of fossil fuel resources and the increasing importance of environmental sustainability have accelerated interest in renewable energy sources. In this context, wind energy has become a prominent renewable source due to its environmentally friendly, cost-effective, and sustainable nature. Wind turbines con-sist of components such as the tower, rotor, shaft, generator, and especially blades. The design and material selection of the blades are of great importance for turbine efficien-cy. Therefore, composite materials are increasingly used in blade manufacturing. Alt-hough metals were initially employed, today glass fiber-reinforced polymers (GFRP) have become dominant in wind turbine blades. This preference is due to advantages such as the low cost, easy availability, light weight, and high strength of glass fibers. Furthermore, their impact and fatigue resistance, corrosion resistance, and ease of molding during production make GFRP ideal for large-scale turbine blades. Currently, approximately 95% of the composite material used in a turbine blade consists of glass fiber, while the remaining 5% is carbon fiber. In light of this information, both the marine and wind energy sectors greatly benefit from the technical and economic advantages provided by composite materials. In this study, the damage processes occurring in adhesive-bonded regions of fiber-reinforced polymer (FRP) composites under environmental conditions were experimentally inves-tigated. Bending tests were performed on GFRP and CFRP composite specimens com-monly used in marine vehicles and offshore wind turbine blades. For marine applica-tions, three-point bending (3PB) tests were conducted on GFRP and CFRP specimens conditioned in seawater. In contrast, four-point bending (4PB) tests were performed on specimens used in offshore wind turbine blade applications. The results provided a comprehensive evaluation of the onset of damage in adhesive joints, the effect of sea-water on mechanical properties, damage behavior, and load-carrying capacity. Within the scope of this study, adhesive-bonded joints of glass fiber-reinforced polymer (GFRP) and carbon fiber-reinforced polymer (CFRP) composite materials commonly used in marine and offshore wind turbine applications were experimentally investi-gated under environmental effects. The specimens were prepared with unidirectional twill weave at a 90° orientation; GFRP composites consisted of 7 layers, while CFRP composites consisted of 8 layers. The samples were cut into 24 pieces according to ASTM D5868-01 standard, and an epoxy–hardener mixture was manually applied to the prepreg surfaces, followed by a one-day curing period. After gelation of the resin, the final composite materials were produced using the hot-pressing method. The single-lap adhesive joints were immersed in seawater obtained from the Aegean Sea (22 °C temperature, 3.3–3.7% salinity) for 1, 2, and 3 months in separate containers to simulate marine environmental conditions. Three-point bending tests were per-formed on the specimens exposed to marine conditions in accordance with ASTM D790 standard, while four-point bending tests were conducted on specimens representing offshore wind turbine applications. The damages that occurred during testing were examined in detail using a ZEISS GEMINI SEM 560 scanning electron microscope (SEM). As a result of the three-point bending tests conducted under marine environmental conditions, the Young’s modulus of GFRP specimens decreased by 5.94%, 8.90%, and 12.98% after 1, 2, and 3 months, respectively, compared to the specimens tested under dry conditions.