Experimental study and numerical simulation of the behaviour of viscoelastic materials to reduce the vibrations on board ships

Nowadays, noise and vibrations are the main factors for evaluating comfort levels on board ships. In the case of luxury vessels, such as cruise ships and mega-yachts, the required high levels of comfort are achieved through the mitigation of noise and vibrations generated by onboard sources. In this thesis the reduction of vibrations on board ships with the application of viscoelastic materials by means of the use of floating floors and the application of viscoelastic materials on the structures are studied. Concerning floating floors, several studies have been undertaken to demonstrate their effectiveness in isolating the receiving structures in ships, such as the passenger cabins, and also to evaluate their performance. However, there is a lack of design methods for controlling the capability of floating floors to diminish structure borne noise levels, and neither standard procedures to test their Transmission Loss. In the case of the application of viscoelastic materials, several studies have been done for their use in the aerospace and automotive fields, proving their effectiveness to reduce the structure borne noise and vibration levels. Despite the application of viscoelastic materials in the maritime industry for several years, their use is based mainly on the experience of shipyards and sub-contractors, since there is still a lack of a standard procedure for the design of these damping solutions, which is why, more research should be carried out in order to generate mathematical models and design procedures to allow ship designers to simulate more accurately the effects of this treatment on marine structures. Regarding the floating floors, a procedure has been developed for the design of new floating floors for marine applications, with the objective to improve their capabilities of isolating the receiving structures from structure borne noise. The procedure first uses Finite Element simulations to optimise the resilient material to be used for the decoupling of the receiving structure from the vibrating structure, as well as the structural beam supporting the resilient material. The optimal configurations are then built into prototypes for laboratory tests to validate the Finite Element models, to identify the floating floor resonances and to evaluate the Transmission Loss levels. The results of the research activity show the effectiveness of the proposed procedure and emphasise the importance of the experimental tests to validate the outcome of the numerical simulations. On the subject of the application of viscoelastic materials on board ships, a method is proposed to perform laboratory tests for the measurement of the Loss Factor of viscoelastic materials applied on rectangular plates, using the same theoretical background of the ASTM E756 standard, without recurring to highly precise and costly equipment. This study represents the starting point for the base of a long term joint research among the Memorial University of Newfoundland, the University of Trieste and C.S.N.I. scarl, that has as final objective the definition of a rational approach for an optimal application of viscoelastic materials in marine structures to better control the structure borne noise on board ships.