**1. Introduction**

Use of Fiber Reinforced Plastics (FRPs) is rapidly expanding in all fields such as medical equipment, engineering plants, packaging, transportation, aviation, space technology, building construction, heavy vehicles, and defense forces. Application of FRPs in marine construction industry is also not new and ever increasing with rapid advancements in exotic fibers, nanoparticles, and special polymers. For engineering application, principal requirements are inherent strength and a defined temperature limit up to which the strength is sustained to the desired level. The secondary requirements are high toughness, resistance to cyclic fatigue, low creep, low relaxation, environmental stability, and ease of joining and maintainability. The third most important factor is investment cost and processing cost. So far as mechanical strength is concerned, the Elastic Modulus in all modes and ultimate strengths are important. However, too stiff composites lack toughness, which often cause premature brittle failure. It has to be a tread-off between ultimate property and elastic modulus for restricting strain on the one hand and sustain low/high cycle fatigue on the other hand. The toughness imparted by flexible long-chain resin matrix results in high creep and relaxation, which are undesirable for engineering

structures but improve the fatigue life. Inclusion of rubbery moieties in a stiff matrix may result in phase separation and stress concentration at the interface and may cause premature failure. Toughening by nanoparticles, such as functionalized carbon nanotubes and reduced graphene oxide and derivatives, are being actively researched at present with apparently encouraging results. Detailed study of creep and stress relaxation of CNT-polymer or graphene-polymer composites is not done yet in a comprehensive manner, but with a general understanding, it is expected to be even better than the pristine polymer.

Thermal properties are more extensive, since the thermal agitation of polymers undergoes very drastic rise beyond a characteristic temperature called glass transition, where the stiff polymer transforms into a rubbery soft material. For polymers with partial crystallinity, flow takes place at further enhancement of temperature, and finally, a polymer starts to decompose at even higher temperature. A design of structural element then has to depend on the limit of temperature at which the modulus starts decreasing. Ideally it should be glass transition temperature. However, in practice, dynamic mechanical analysis shows that the modulus decreases even about 10–15°C below the glass transition. The extreme hazards of heat for an organic polymer (and FRP) are the fire propagation and evolution of toxic gases. The fire-retardant additives both as physical addition and chemical modification of resins are widely used and are also currently being researched in the light of possible benefits of nanoparticle reinforcements.

Marine application both for static off-shore structures and sea-going vessels needs robust and durable FRP composites, which can compete well with metals in terms of specific strength, durability, and cost-effectiveness. The replacement of a metal requires some special properties in FRPs apart from strength and degradation. One of the most difficult solutions is joining the Thermoset FRP elements since the joint should be almost similar in mechanical strength and toughness. Identical thermoset as the FRP element is best preferred, with a short fiber dough molding system that must be cured at ambient, yet provide acceptable joint strength. There can be special drilling technique for joining through riveting using the dough as rivets. Thermoplastics can be "welded" by melt joining as metals, most suitable for particulate reinforced composites and short fiber composites. The second and very important property of an FRP to qualify marine standard is effect of sea water aging considering all the chemical and biological adversaries of the sea. This single factor mostly decides the design and service life of a marine-grade FRP structure.

Although marine corrosion of FRP is not so severe as for steel, the FRP structures and underwater hulls need to be protected from bio-fouling. With the advancement of anti-fouling coatings, it is possible to protect a hull for minimum 3 years without any maintenance painting. Modern low surface energy foul release coatings based on silicones and fluoro-silicones are environment-friendly as they do not release toxins in the sea. These are non-depleting coatings and hence can have higher service life. However, These types of coatings are more effective for high-speed boats.

### **2. FRP components in marine vessels**

The different elements of a ship can be defined as primary, such as superstructure, hull, SONAR Dome, bulkhead, decks, propeller shafts, masts, doors, hatches, machinery foundations, support frames, etc. Secondary items are rudder, pipes, valves, ladder, stanchions, guard rails, etc.

In naval vessels, three important advantages of using FRP composite are (1) ability to damp vibration, thereby reducing the radiated noise in the sea. In addition, FRPs are acoustically transparent, hence reduce the acoustic reflection (2)

### *FRP for Marine Application DOI: http://dx.doi.org/10.5772/intechopen.101332*

FRPs without carbon or conducting material inclusions are radar transparent. These two features enhance the stealthy character of a battle ship and submarine and importantly (3) most common reason is no corrosion of FRP in sea water and saline atmosphere.

The most used application areas for FRP in ships are superstructure and bulkhead, where thick FRP panels are used with flap joint overlapping at the corner to flush the sides. Riveting with composite rivets can be done along with interface adhesion using a hand layup of fabric with resin so that the joint is sufficiently strong.

Vibration and fatigue are other important aspects. Machinery and propeller movement cause vibration of the hull and hull-mounted SONAR dome, which adversely affect SONAR performance and also results in fatigue. Normally, the fundamental frequency of machinery and propeller is up to 34 Hz, and prominent modes are up to about 200 Hz. Also, slow cycle fatigue results from sea waves, which is approximately 0.8 Hz. It is well known that slow cycle fatigue is quite important to decide the service life for steel hulls and is expected to have similar effect on FRP hull. Till now, there is no such detailed study on fatigue at various frequency envelops for marine FRPs, which are actually exposed in sea water with vibrations.
