Experimental Investigation of the Effect of Seawater on Glass and Carbon Fiber Composites via Mechanical Characterization
Authors: Yalçınkaya Senai , Mertgenç Yoldaş Dudu , Yoldaş Mehmet Fatih 
Published: February 2025

Since composite materials are light and corrosion-resistant, they have replaced many traditional materials in the aviation and marine industries. Composite materials have the advantages of a much higher strength–weight ratio, lower maintenance requirements, and the ability to form complex shapes, such as bodies, compared to carbon steel. In this study, the mechanical properties of glass fiber reinforced (GFRP) and carbon fiber reinforced (CFRP) composite materials were investigated in marine applications in which composite materials had been used. In this study, 0/90 oriented twill weave eight-ply GFRP and eight-ply CFRP composite materials were used, incorporating the hand lay-up method and hot-pressing method. Seawater was taken from the Aegean Sea, Izmir Province (Balçova/İnciraltı), and had an average temperature of 22.43 °C. This seawater was kept in different containers for 30 days and 60 days (a total of 1440 h of keeping in seawater) with the intent to test the GFRP and CFRP composite samples separately. The produced CFRP and GFRP sheets were then cut with a wet (circular) saw in accordance with the standard procedure in the Composite Research and Testing Laboratory of the Dokuz Eylul University Department of Mechanical Engineering. Moisture retention percentages and Charpy impact tests were carried out. Then, three-point bending tests were carried out according to TS EN ISO 14125. The damage in the material was examined using a ZEISS Stereo Discovery.V12 imaging microscope (Oberkochen, Germany). The mechanical properties of CFRP- and GFRP-reinforced composite samples before and after aging were investigated using the Charpy impact test and three-point bending test. Then, the effects of the seawater environment on the mechanical properties of the CFRP and GFRP composite materials were evaluated by comparing the results. The aim was to better understand what kind of damage would occur in GFRP and CFRP composite materials given the effects of seawater and at what stages changes would occur in the mechanical properties of these materials. Moisture retention rates (%) in the tested samples after the Charpy impact test were 2.56% in GFRP and 0.47% in CFRP after 30 days. In the tested samples after the three-point bending test, these values were 1.41% in GFRP and 0.31% in CFRP after 30 days. Subsequent to the Charpy impact tests, the fracture toughness values of the CFRP samples tested at the 30 J impact energy level before aging in seawater conditions for 30 days or 60 days were found to be increased by 15.79% and 21.08%, respectively. The fracture toughness values of the GFRP tested at the 30 J impact energy level in dry conditions and kept in seawater for 30 days or 60 days were found to be 27.69% and 29.23%, respectively. The energy absorbed during the impact tests by the GFRP samples was higher than in the CFRP samples. This showed that the GFRP samples were more brittle. Subsequent to the three-point bending tests, the CFRP composite samples kept in seawater for periods of 30 days and 60 days showed changes in the modulus of elasticity of 7.48% and 7.46%, respectively, compared to the dry samples. The GFRP composite samples kept in seawater for periods of 30 days and 60 days showed changes in the modulus of elasticity of 7.015% and 11.53%, respectively, compared to the dry samples. The change in the modulus of elasticity was less in the CFRP samples than in GFRP. All of these results showed that the mechanical properties of CFRP were better than those of GFRP.