**2.2 SONAR dome**

A special application of naval ships, submarines and fishing vessels is SONAR dome, which houses the arrays of acoustic transmitting and receiving transducers for detection of underwater objects. Conventionally titanium is used to make sonar domes due to its fair acoustic transparency, high strength-to-weight ratio, and good resistance to sea water corrosion and bio-fouling. However, acoustic impedance of titanium is not as close to that of sea water compared with glass-fiber based FRPs. Therefore, titanium domes are less efficient in underwater acoustic transmission and have underwater acoustic reflectivity more than FRP. For naval vessels such as submarines and battleships, FRP SONAR domes are being used in some countries, for example, the United Kingdom France, Sweden, Australia, Holland [1–3]. Such SONAR domes are very critical with respect to high drag force, compressive stresses at high depth in sea for submarines, and requirement of high acoustic transmission characteristics. The thickness of the dome is decided by the strength and modulus of the FRP, but higher thickness results in loss in acoustic transmission power. Therefore, the design and fabrication of an FRP dome are very critical and are done using Finite Element Method (FEM) so that the dimensional features, strain levels at different sections, and maximum stress can be somewhat accurately determined for both static and dynamic conditions. A prediction of acoustic transmission can also be done using general acoustic attenuation theories. Fabrication method can be very important, so that the dome would have nearly same theoretical strength and dimensional accuracy with acceptable tolerances. Among many possibilities, resin film infusion technique or vacuum-assisted resin infusion can be adopted to make the domes with precise dimensions, strength, and flawless integrity. The thickness of such domes can vary from 20 mm to 80 mm depending on the size. The vacuum process has two advantages, (1) the high fiber content resulting in high strength and (2) nearly zero air gap/flaw in the composite. The air gap is undesirable in sonar dome since any such air bubble would increase acoustic reflection, thus reducing the acoustic transmission across the dome thickness. The aspect of sea water diffusion and corresponding loss of strength, lowering of glass transition, and deterioration of acoustic transparency are main consideration of its long usability and depend on both material and fabrication process. Commonly used fabric is E-glass and S-glass while the resin can be a hybrid of vinyl ester and epoxy resin. For the purpose of enhancement of strength, carbon fibers are preferred over glass fiber, since a carbon fabric-epoxy FRP would have Young's modulus of nearly 70 GPa compared with GFRP of about 30 GPa.

Recently aligned carbon nanotube containing GFRP domes are being considered for mid-frequency acoustic application to reduce the thickness of the dome and to impart better structural vibration damping.

#### **2.3 Secondary marine components**

Of the superstructural components, carbon fiber-epoxy combination is best, provided there is no necessity of radar stealth features. However, carbon fiberepoxy composites are used in cabinets and covers of power electronics in ships and submarines for EMI shielding purpose.

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

Composite pipe can be made using a combination of prepreg lay-up on a mandrel followed by filament winding technique. This fabrication method can give sufficient Hoop Stress. A best possible fiber alignment in subsequent layers on the mandrel is determined by a stress-strain analysis by FEM method. Resin pick-up by the fiber strands in automatic winding method is minimized by two doctor's blades fixed on the fiber running line as one of the guide systems for the strands before winding onto the mandrel. Autoclave curing at high pressure and temperature can be adopted for such pipes.

Composite valves are made by dough molding compounds because of intricate dimensional requirement to make them leak-proof. Pipes and valves are special among all items because the fluid pressure (Pascal's pressure) in most commercial ships is designed for 12 bar, and for Naval standard, it should be 20 bar with continuous use and should withstand maximum 30 bar pressure for 24 h. This stringent requirement makes the fabrication method very critical, for example, the surface of the pipe must not "sweat" at high hydrostatic pressure and circularity and movement of the ball in a Ball valve must be very precise to avoid leakage of liquid, besides resistance to the "sweating." The processing and fabrication with dough molding compounds are best done by application of high pressure of 1–3 MPa to eliminate excess resin and to ensure compactness with precise dimensional tolerance and without layer gaps or air entrapment. Vacuum application is not beneficial since the dough, containing 20% short fiber, would have very poor flow property. Instead, kneader mixing can produce dough without air entrapped in the green dough. The molds are made with die steel for high-pressure molding.

The elements that are used on board such as ladder, stanchion, and guard rails are critical due to shape and require high-impact energy to resist crack or breakage on impact. Hence, a method of flexibilizing or nanoparticle reinforcement must be attempted to improve impact energy of common reins. As an example, a common epoxy thermoset Glass FRP has an impact energy of 750–850 J/m (Charpy impact), while a modified epoxy-Glass FRP would have 1300–1500 J/m, which may qualify the impact requirement. The strength must not be compromised too much. A maximum 10% reduction for the FRP could be accepted by a designer to prefer a flexibilized resin matrix. For such small and shaped components, hand lay-up of fabric and resin or prepreg lay-up in metallic mold can be adopted. High compression would be beneficial to eliminate any flaw, air gap, and better compactness. In these on-board components, carbon fabric prepregs cannot be used in naval vessels since carbon-based composites increase radar signature.
