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CFRP offers high specific stiffness and strength in comparison with materials such as steel and aluminium. CFRP parts are currently used in aero-structures and assembled by mechanical fasteners. The first applications of hybrid (composite/metal) joints are met in the

Chapter 2. Literature review

18 aerospace structures, with the use of composite patch repairs of aluminium structures [57]. This type of design implies weight penalties emanating from the need to deal with the stress concentrations developed around the bolts. Moreover, bolts and rivets damage the continuous reinforcing fibres and, consequently, can greatly affect the overall load-carrying capacity of the structure [58].

Adhesive bonding of aerospace components is a fabrication technique which has increased markedly in popularity during the last two decades. While adhesives have been used in a number of repair and maintenance operations they have yet to make a great impact.

Military applications initiated the use of adhesively bonded advanced composites, and aircraft such as the F-18 and the F-22, which employed significant amounts of bonded polymer matrix composite laminates at wing skins and control surfaces [59]. Similar applications may be found on many types of commercial aircraft, whose economic operation benefits considerably from the reduced weight offered by the bonded composite assemblies (e.g. AIRBUS A380) Figure 2.7, where around 42% of the joints are based on adhesive bonding structure [60]. Another example of the extensive use of CFRP is in Figure 2.8, which shows a fuselage section of the A350 with skin panels, doublers, joints and stringers entirely made of carbon fibre composites.

Chapter 2. Literature review

19 Figure 2.8: A fuselage section of the A350

Composite and adhesive bonding has been adopted by the marine industry since the 1980s. The construction of composite superstructures has been increasing in the use of composite materials for naval superstructures [61]. The French navy have implemented such a superstructure on their La Fayette class frigate in the form of a helicopter hangar [62, 63]. It has been a field of research by several scientists since then, with the design of adhesive joints in marine structures (deck-to-hull joints) gaining the greatest focus. Specific structural parts of a ship, such as superstructures, bulkheads, masts or even the entire deck, may be replaced by composite materials [61, 64, 65, 66], properly designed and adhesively bonded either to composite or metal parts. The study of composite hybrid joints has been extended to applications in the marine industries [62, 63, 61, 67]. Figure 2.9 presents a modern large- scale application where a composite deck is adhesively bonded to a steel hull.

Additionally, modern adhesive bonding technologies offer many techniques for repairing defected structural elements. By using an adhesively bonded patch, the repair is much easier and quicker to carry out. The design of each repair is a very promising technique in marine applications, where carbon, glass or combined fabric patches are either directly laminated, or cured composite patches are adhesively bonded on the cracked or corroded area of metallic parts, for example, the fuel tank, water tank, or hull of an oil tank.

This technology is advantageous in the marine industry [67, 68] since the hot work of welding is avoided, thus reducing the risk of a fire. Also, if a composite material is used for

Chapter 2. Literature review

20 the patching of steel, then the materials used are easy to transport and handle, with no need of heavy lifting machinery.

Figure 2.9: Composite deck adhesively bonded to steel hull [69]

Adhesives are used in the construction industry in repair and strengthening of existing structures and new built structures. During the 1950’s and 1960’s an enormous amount of new constructions were built and, as these structures age, many faults have become evident. The repair substrate may be ordinary concrete or polymer concrete, with or without reinforcement. If increased structural capacity is needed then external plate bonding can be an alternative.

Figure 2.10: CFRP used to strengthen bridges [70]

During the 1970s and up to the end of the 1990s, steel plate bonding was not unusual; however in the last decade, the use of advanced composites for external strengthening has

Chapter 2. Literature review

21 become quite common ( a photo of a bridge strengthened this way is shown in Figure 2.10 [71]). Also a great number of models for debonding have been presented [72, 73].

Figure 2.11: World’s largest carbon composite bridge in Paris (left) and Composite bridge on UK motorway (right) [70]

At the 2007 JEC-fair in Paris (the largest European exhibition on composite materials), the world's largest carbon composite bridge was shown. The carbon composite bridge is 24.5 meters long and 5 meters wide and weighs only 12 metric tonnes. This makes the bridge about 30 times lighter than a comparable concrete bridge (Figure 2.11).

Gradually, bridges are also being introduced on highways, where load-carrying capacity is much larger. A fibre-reinforced polymer bridge was constructed over the M6 motorway in the UK, carrying 40 tonnes which is the standard for road bridges on the highway network. The bridge uses a glass fibre composite deck on steel girders, placed on reinforced concrete substructures (Figure 2.11).

Chapter 2. Literature review

22 Finally, composites are an attractive solution for repair of damaged concrete structures. Carbon fibre-reinforced strips are glued on the exterior surface of the structure and help in taking up the loads of the structure (Figure 2.12).