# A 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 via Three-Point Bending and Four-Point Bending Tests

## TL;DR

This study compares how seawater exposure affects the strength of carbon and glass fiber composites used in wind turbines and ships, showing carbon fiber is more durable.

## Contribution

The study provides new empirical data on the degradation of GFRP and CFRP adhesive joints under simulated marine conditions using standardized bending tests.

## Key findings

- GFRP specimens showed greater Young’s modulus reduction (up to 12.98%) compared to CFRP (up to 3.74%) after 3 months of seawater exposure in 3PB tests.
- CFRP joints demonstrated more stable mechanical behavior in both 3PB and 4PB tests under simulated marine and wind turbine conditions.
- The results emphasize the importance of material selection for long-term durability in offshore composite structures.

## Abstract

The aim of this study is to experimentally evaluate the damage mechanisms occurring in the adhesive-bonded regions of glass fiber-reinforced polymer (GFRP) and carbon fiber-reinforced polymer (CFRP) composites, which are widely used in marine and offshore wind turbine applications, under environmental conditions. In particular, this study focuses on the degradation caused by long-term seawater exposure and its effects on the bending behavior and load-carrying capacity of adhesive joints. For this purpose, the specimens were prepared in accordance with ASTM D5868-01, using 7-layer GFRP and 8-layer CFRP laminates. Single-lap adhesive joints were fabricated. To simulate marine environmental conditions, the single-lap adhesive joints were immersed in natural seawater obtained from the Aegean Sea (22 °C temperature and 3.3–3.7% salinity) for 1, 2, and 3 months in separate containers. Three-point bending (3PB) tests were performed on specimens representing marine applications, while four-point bending (4PB) tests were conducted on specimens representing offshore wind turbine blade structures. The results quantitatively revealed the influence of seawater on adhesive-bonded composite joints. In 3PB tests, the reductions in the Young’s modulus of GFRP specimens after 1, 2, and 3 months of exposure were measured as 5.94%, 8.90%, and 12.98%, respectively. For CFRP specimens, degradation was more limited, with corresponding reductions of 1.28%, 3.39%, and 3.74%. A similar trend was observed in 4PB tests representing offshore wind turbine applications, where GFRP joints exhibited modulus reductions of 3.15%, 6.42%, and 9.45%, while CFRP joints showed reductions of 1.29%, 2.62%, and 3.48% for the same exposure durations. Overall, the findings demonstrate that CFRP composites exhibit more stable mechanical behavior under environmental exposure, whereas GFRP structures undergo more pronounced performance losses, particularly in moisture- and salt-rich environments. These results highlight the critical importance of material selection for long-term durability in offshore composite structures. The outcomes of this study contribute to a better understanding of the damage processes occurring in composite adhesive joints under environmental conditions and provide a scientific basis for developing more reliable design and material selection strategies in both the marine and wind energy sectors.