**8. Conclusion**

Application of fiber reinforced composites based on thermosets is increasing as the cost and availability of fuel increase with time. This is simply because a lighter marine vessel has the fuel efficiency much higher than metallic vessels. For static RCC construction too, a simple outer jacket of a composite can protect a concrete pillar of off-shore structure for quite a longer period than an exposed RCC. The undersea pipelines are the other areas of potential use of carbon-vinyl ester composites. Nano-carbons can effectively improve the toughness of such items.

A large number of studies are already done to examine the efficacy of using FRPs for use in marine environment considering the chemistry of the matrix resins and relevant properties. Similarly, various fibers were also investigated by many researchers. Recent advancement is focused at incorporation of nanofillers of different chemistry and forms such as nanotubes, fibers, rods, spherical, platelets, etc.

The composites studied so far are widely varying in the resin-fiber ratios, forming sequence, and processing methods. Therefore, the results of each study cannot be fully generalized, but a broad conclusion on quantitative figures of merit can be made for each polymer-fiber combination.

A very general conclusion on durability in marine environment is that vinyl ester resin with carbon fiber is the best choice for applications in static structures, high strength ship components, and commercial speed boats where durability and weight reduction are important. However, for naval ships and submarines, for use in superstructures, the CFRP composites have a problem of radar reflections similar to metallic structures. Modern-day stealth ships exclusively use GFRP since it is radar transparent material (RTM). CFRP is only used in radar absorbing structures (RAS). Internal areas of the vessel can be made with CFRP for better strength and hence reduced weight. However, GFRP has slightly more damping capability than CFRP, which is stiffer. The SONAR Dome can be made with glass fiber or carbon fiber, also with hybrid fiber system. CFRPs are electrically more conductive than GFRP, and hence it is better to use GFRP as inside layers to avoid electrical problems for securing the transducer arrays.

Most studies on durability of FRPs reported so far are either using moisture, or distilled water or artificial sea water, but very rarely natural sea water has been

used. Artificial sea water does not simulate the natural sea water. The variations in types of microbes across the world are so much that a result of durability study in sea water at Mumbai coast in India is not applicable in a coast of the United States. There are no comprehensive reports on effect of microbial activity and effect of sea water constituents both considered together to decide a service life of an FRP.

To determine service life of an FRP in marine water, it is required to use the panels immersed in actual sea water using a raft and periodically observing the change in water uptake, chemical groups, mechanical strength, dynamic mechanical properties, surface restructuring, glass transition, etc. Mathematical models commonly used may not be directly applicable for considering the influence of all unforeseen parameters of the sea, but a functional property such as bending strength/modulus can be monitored with time. The data can be superimposed with the similar value of the property with that from a simultaneous laboratory experiment at different temperature as is normally done. The time-temperature superposition will be better used in such cases with graphical shift method to avoid any assumptions. Although the microbe activities are not mapped in temperature scale in such method, it is fairly accurate since the microbe activity is constant due to approximately constant sea water temperature and salinity. The study must be done for at least two cycles of breeding of microbes. This means that the experiment may be only for at least 12 months. Vinyl-ester-based GFRP and CFRP are therefore required to be studied to observe the effects of sew water chemistry and microbiological activity to decide the service life.
