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J. Mar. Sci. Eng. 2020, 8, 26 17 of 28 primarily to increase stiffness, represents a compromise between these competitive goals. Tests carried out on epoxy and vinyl-ester resin laminates with E-glass reinforcements showed that epoxy/E-Glass panels have 25% higher fatigue resistance than vinyl ester/E-Glass panels, in the case of in-plane fatigue loads on dry samples, without considering porosity, holes, and ply-drops [47]. Other studies showed that the static properties of glass-reinforced vinyl resins exhibit less degradation after aging in seawater than the one that epoxies [46,48]. A further concern about the use of FRPs is their poor recyclability. Thermosetting resins cannot be recycled and the composites containing these resins can be disposed of only by incinerating. Thermoplastic resins, on the other hand, offer a high hardness combined with a greater degree of recyclability. However, blade production requires high working temperatures and the bonding of finished pieces could be difficult [42]. Today, environmental concerns are pushing the development of highly recyclable materials such as natural cellulose fibers and bio-based polymers. Carbon fiber-reinforced composite proved to be the material that better withstood the marine environment: carbon fiber composite showed after 10 months the lowest percentage of dirt sedimented on the external surface of the components. In addition, tests showed that surface finishing of the materials affects the percentage of organisms stuck on the surfaces of the rotor: it is higher in case of rough surfaces or cracks promote the formation of fouling, conversely, it remains low in case of smooth surfaces. Results also indicated that the biological contamination is cumulative: when a surface is affected by a certain degree of dirt or corrosion, it becomes more susceptible. GFRPs are more susceptible to corrosion and biological contamination than the CFRPs, however, they are being still used due to cost reduction strategies. To solve this problem, post-manufacturing processes, such as the electrolytic deposition of Ni-P [48,49], are carried out. These increase the surface finish, reducing the porosity, the surface hardness and the corrosion resistance of the material, protecting it from chloride ions. First large-scale turbine blades in fiber-reinforced plastics were manufactured by hand layup of pre-preg. The main parts of the tidal turbine blades (i.e., upper and lower skins and box-section spars) were produced separately and then joined using adhesive at very high production costs. Resin infusion processes have been employed to minimize costs allowing for the reduction of the number of parts to be assembled and the increase of process automation [46,48]. Resin infusion has been already applied in the production of wind turbine blades, but the adhesive bonding of the upper and the lower skins of the blade is still necessary to achieve the final product. The “Integral Blade” process, patented by Siemens, represents an exception: it uses a closed external mold and a flexible and expandable inner bladder to produce at the same time the entire glass/epoxy blade. Airborne Composites company is employing a similar technique to manufacture tidal turbine blades. They produced four meters long blades, installed in the 500 kW CoRMAT turbine, developed by the European Marine Energy Centre (EMEC) since 2014, using a VARTM ‘One-shot’ process. The VARTM “one-shot” process ensures higher production efficiency, eliminating the presence of additional material around the interface of the connectors. The company has also developed a method to incorporate inserts at the root of the blade and to connect it to the hub before the resin infusion. As a consequence, drilling operations on the realized composite are removed and the thickness of the laminate is reduced [46]. Some issues related to the quality of the laminate and the presence of defects in the manufactured parts are still unsolved. For single laminates and sandwich structures, manufacturing defects include delamination, dry areas, and un-wetted fibers, porosity, wrinkles, defects in fiber reinforcement, fiber misalignment. Non-destructive testing represents the most effective technique to detect manufacturing defects for wind turbine parts, due to the large areas to be monitored, complex shapes, the difficulty of accessing inner parts and the variety of detectable defects [42,50]. Relevant advances have been made in non-destructive testing of FRP in recent years, however, major challenges are still present. Currently, visual and ultrasonic techniques are the most used methods, even do not provide a comprehensive and exhaustive inspection. Therefore, combinations of different control techniques are typically adopted. Besides the defects resulting from the manufacturing processes, it is of paramount importancePDF Image | Marine Application of Fiber Reinforced Composites
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