The decline in fossil fuels and environmental pollution have made renewable energy sources essential. In this context, solar and wind energy have come to the forefront to re-duce carbon emissions. While wind energy was previously used for agriculture and water pumping, today its largest contribution is electricity generation. It is rapidly gaining pop-ularity due to its environmental friendliness and affordability. Wind turbines consist of a tower, rotor, shaft, generator, and blades. Blades are critical for system efficiency and en-vironmental protection. While metal materials were initially used, composite materials have now been adopted. Composites offer properties such as light weight, durability, cor-rosion resistance, and long life. Approximately 95% of the composite material used in tur-bine blades is glass fiber and 5% is carbon fiber. The reasons for the preference of glass fi-ber-reinforced composites (GFRP) are low cost, light weight, easy raw material supply, and high mechanical strength. It also offers advantages such as impact resistance, fatigue life, corrosion resistance, and easy shaping during production. Thanks to these properties, glass fiber offers both an economical and structurally safe solution for large-scale blades. The aim of this study is to experimentally investigate the damage processes occurring in the adhesively bonded areas of fiber-reinforced polymer composite materials commonly used in offshore wind turbine blades. Carbon fiber-reinforced composites (CFRP) offer significantly higher strength and stiffness than glass fiber, enabling them to carry the same loads with thinner and lighter structures. However, due to its higher cost and more complex manufacturing process, the use of car-bon fiber is limited. It is generally used in specialized turbines or blade tips requiring high performance. While glass fiber offers an ideal solution for general use, carbon fiber is pre-ferred to a lesser extent as a support or complement in critical areas. GFRP and CFRP specimens were prepared as 7-layer and 8-layer laminates, respectively, using unidirec-tional twill weave with a 90° orientation. A total of 24 specimens were cut according to ASTM D5868-01. Epoxy and hardener were applied to the prepregs by hand lay-up, fol-lowed by a one-day curing process. After resin gelation, the composites were produced using a hot press method. To examine the marine performance of adhesively bonded joints, commonly used in offshore wind turbine blades, single-layer adhesively bonded specimens were conditioned in separate containers for 1, 2, and 3 months in natural sea-water obtained from the Aegean Sea at 21.0°C and with a salinity of 3.3–3.7%. During this period, the moisture absorption of the joints was weighed periodically using high-precision scales. The obtained data was compared with dry reference samples to characterize the moisture absorption behavior in terms of weight change (%). To evaluate the mechanical strength of the specimens under the influence of humidity, four-point bending tests were conducted in accordance with the ASTM D790 standard. The damage types and microstructural deteriorations observed during the tests were examined in detail using a ZEISS GEMINI SEM 560 scanning electron microscope (SEM), which pro-vides high-resolution imaging. These analyses provided comprehensive experimental findings regarding the physical and mechanical effects on adhesively bonded composite joints. In the experimental studies, maximum moisture absorption in GFRP specimens was: After 1, 2, and 3 months of seawater exposure, the moisture absorption rates were measured as 1.02%, 2.97%, and 3.78%, respectively. In CFRP samples tested under the same conditions, the moisture absorption rates were determined as 0.49%, 0.76%, and 0.91%, respectively. According to the results of the four-point bending tests, Youngs modulus in GFRP sing-le-lap adhesively bonded joints decreased by 3.15%, 6.42%, and 9.45% for the respective durations compared to specimens tested in a dry environment. Similarly, Youngs modu-lus in CFRP single-lap joints decreased by 1.29%, 2.62%, and 3.48% depending on the du-ration of moisture exposure. These results demonstrate that environmental conditions, particularly those of GFRP materials, have a more pronounced impact on their mechanical performance.
Yalçınkaya et al. (Tue,) studied this question.