Procedure setting to determine the Dynamic transfer stiffness of a resilient mounting element in a low frequency range

Comfort and security of passengers on board ships, such as cruise ship or mega yacht, is one of the most focused problematics in the marine engineering sector. Among the several possible sources of discomfort, a particular attention was paid on the perceived vibrations and air born noises. In fact, the vibrations and noises generate high discomfort especially in low excitation frequencies where body resonance mechanism may occur. It was emphasised in several publications that the principal source of disturbance are the ship engines. Actually, the engines and propellers in function are at the origin of acoustic noise and structural vibrations along the ship structure, therefore disturb the comfort of passengers in all ship areas, especially cabins and public accommodations ones. Although it is very difficult and may be impossible to remove completely such noises and vibrations, it is possible to reduce them below the passengers’ detection level. To decrease the vibration level created by diesel engines, these latter’s are generally suspended by resilient and damping systems such as passive isolators in terms of mechanical vibrations. From a practical point of view, theses isolators are placed between the feet of the engines and the ship structure. They are called resilient mounting element (RME). The first function of the RME is to reduce the level of vibrations transmitted by the engine to its base frame through the use of damping constitutive material. The RME is commonly made up of by three parts. The upper one is tightly connected on the structure of the engine foot and is constituted by a quite rigid metallic material like steel. It lies on the intermediate part, which is constituted by a damping material like rubber. This part, rests on the lower part, which is connected to the structure of the ship foundation. It is also constituted by commonly by steel. The principal deterministic characteristic to predict the RME behaviour in function of the environment conditions is the dynamic transfer stiffness (DTS). This DTS depends on several parameters such as the excitation frequency spectrum, the excitation amplitude. Generally, DTS is obtained by experimental tests usually performed on real scale RME in laboratory. Therefore it requires substantial means and lasts between two days and two weeks depending on the result exhaustiveness. The main purpose of this thesis is to develop an alternative methodology to characterize the RME by obtaining the same dynamic transfer stiffness using finite element (FE) simulations. It includes quasi static simulations in addition to non-linear dynamic ones. It also contains rubberlike material characterization and, for the purpose of benchmarking, a classical experimental campaign. The start point of the thesis and commonly provided information about the resilient mount element is the static deflection curve of the element. This curve represents the static displacement of the top of the tested element under a compression load. The first step in the static simulation is to simulate this experiment. However, on board ship, the element is under only one working load, the compression force due to the engine weight. The working load of the element considered in this thesis is around 73 KN. Using reverse engineering and knowing the matching displacement it is possible to acquire the young modulus or the coefficient in the hyperelastic law. The static simulation allows the obtaining the elastomer strain field, which further serves as input data for the dynamic characterization of the damping material. The results are used as material law coefficients in the dynamic simulation model. The comparison between the results obtained experimentally and numerically is discussed at the end of the thesis. A good correlation could warrant the endorsement of the scientific approach to assist or replace the resilient mounting element behaviour experimental prediction.