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Acoustic waves are used in SONAR systems for the detection and classification of underwater objects. Acoustically transparent materials are required for encapsulation of sensors for protection from sea water ingress. Sound absorbing and insulating linings find applications in underwater calibration facilities and as submarine stealth linings. Polyurethanes are a class of materials formed by the reaction between diisocyanates and diols. They can be tailored in different ways by selection of different raw materials, addition of fillers, copolymerisations, IPN formation, incorporation of geometric inclusions etc for the desired acoustic properties. Polyurethane based matrix materials were modified in different ways for the development of acoustic absorbers as well as window materials for underwater applications. The absorber was developed by a judicious choice of fillers and additives with the help of FEM modelling and experimental validation was done in the pulse tube as well as anechoic tank. The same material explored for application in the mulitilayer composite baffle for attenuating vibrations from hull structures for mounting conformal array sensors on them. The hydrophones used for underwater applications are to be protected from water permeation by encapsulating with a suitable material. The material should have suitable mechanical, electrical and physical properties compatible with the devices. It is essential to get encapsulation with no air entrapment with moulding temperature below 70o C to avoid damage to the transducer components. Polyurethane based encapsulants were extensively studied and used for these applications. In the same manner, an acoustic window material formulated was used for embedding sensors in a solid linear sensor array alleviating the problems of an oil filled array. The technology is already inducted into systems now. The expected functional life of polymers and polymer-based composites subjected to a set of environments and stress conditions is often determined by issues of aging, dimensional stability and durability. Therefore it is important to know the service life of a material by the principle of accelerated aging. The polyurethanes developed were evaluated for their service life using this technique. Among a variety of sonars used for underwater surveillance, towed array sonars forms a major class due to their low frequency operation capability offering long range detection. Towed array consists of various type of array modules housed in a polyurethane tubing filled with a suitable acoustic coupling fluid. The properties of the tube including its stability (chemical as well as physical), resistance to weathering and compatibility with the surroundings is of immense importance. As a case study, aging and life prediction of thermoplastic polyurethane array housing of towed array sonars was carried out to predict its service life in simulated underwater conditions. The results of these studies will be presented.
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