# Sewater Effect on Fatigue Behaviour of Notched Carbon/Epoxy Laminates
## 1 Introduction  
Composite materials are used in various engineering applications, such as marine, aerospace, civil, offshore, sports, and automobile industries, due to their high specific strength and stiffness, as well as their high fatigue and corrosion resistance. In particular, carbon (CFRP), glass (GFRP) and Kevlar (KFRP) fibre-reinforced polymer composites are being increasingly used in several industrial applications. Therefore, traditional materials such as steel and aluminium alloys have been replaced by fibre-reinforced composites, and this trend is likely to continue in the near future because of their outstanding mechanical properties [1]. Nevertheless, in several engineering applications, it is expected that these materials will be exposed to hostile environmental conditions, with consequent degradation of their load-bearing capacity, which can significantly affect their mechanical properties, reducing long-term durability. As is well-known, the oceans cover more than 70% of the Earth's surface and represent an important source of energy. During the last few years, the use of composite materials in the marine industry has increased significantly [2]. The marine environment, like seawater, involves adverse conditions that lead to a decrease in the structural integrity of composite components, with fatigue behaviour being one of the main concerns in the design of marine structures subjected to cyclic loads, with direct implications on maintenance costs [3]. Ocean energy devices made of composite materials operate in a very severe environment, and the selected materials must be thoroughly qualified to reduce the risk of failure in service [4]. Water uptake in this family of materials as well as its effect on their mechanical behaviour, treated as an uncoupled phenomenon, has been widely studied [5; 6; 7; 8; 9; 10; 11; 12]. According to [PERSON] et al. [13], the water is responsible forirreversible damage. The chemical changes induced in the matrix can promote serious degradation of mechanical properties, and debonding at the fibre/matrix interface and delamination. When delamination appears on the laminate, there is a reduction in the strength, stiffness, and fatigue life of the laminate, which can lead to catastrophic failure of the entire structure. [PERSON] et al. [14; 15; 16] investigated the hygothermal effect in the fatigue lifetime of CFRP composite laminates. They observed three absorption modes: bulk dissolution of water in the polymer network, moisture absorption onto the surface of vacuoles, and hydrogen bonding between polymer hydrophilic groups and water. Due to the chemical reactions between the water molecules and the matrix, the water acts as a plasticizer, and the composite becomes softer because the matrix tends to be more flexible [13]. As one of the most common reasons for material breaking down, fatigue failure needs to be investigated as a vital property in the design of composites subjected to cyclic loads [17]. Although there are some studies on the use of composites in adverse conditions, the durability of CFRP composites exposed to moisture or wet conditions needs further research to make them applicable to external applications with limited monitoring or inspection [18]. Due to their specific characteristics, composite materials are excellent candidates for developing reliable lightweight components and structures that are often subjected to cyclic loads. Under these peculiar service conditions, such materials are susceptible to progressive damage accumulation over the entire fatigue life. Due to the degradation of the mechanical properties of a laminate subjected to fatigue loading, a final failure can occur after a certain number of cycles [19]. [PERSON] et al. [20] studied the fatigue failure of materials immersed in seawater and concluded that the total failure takes place quickly after the appearance of the first crack. [PERSON] et al. [21] studied the effect of CFRP plates immersed on water and seawater at room temperature.