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Driveline vibrations of a truck are a cause of strong discomfort for drivers, and have to be investigated in early design stages. In order to develop analytical and numerical tools for the prediction of noise and vibration of such a heterogeneous and composite structure, a deep knowledge of the physical phenomena involved is imperative.
Few experimental studies have been performed on truck vibrations, and they mostly concerned single components of a vehicle (chassis, cabin, powertrain). Therefore an Experimental Modal Analysis (EMA) of a complete truck has been performed in order to observe vibratory phenomena and determine influencing parameters involved in the vibration transmission. This study brings new insights into truck vibrations knowledge. All the phases of EMA have been performed on a real Medium Duty physical prototype. Modal testing allowed especially to identify how the behaviour of the structure changes with frequency: low frequency and high frequency ranges where located, and the so called medium frequency range was determined. In the latter, interesting transition and interaction phenomena take place, which are thought to have a first order influence on vibration transmission over trucks.
The results of the test campaign have firstly been used to validate a pre-design finite element model of the complete vehicle. The target of this validation was the achievement of a representative model for phenomena and trends. Although measurements and numerical models involve the entire structure, the analysis has been focused on the chassis behaviour, this latter being the main transfer path for driveline vibrations to the rest of the structure and in particular to the driver's seat (vibratory response) and driver's hear (acoustic response). Once the model validated, it was profitably used to investigate and better explain the phenomena observed in the experimental phase. Specific investigations involved the dynamic coupling of the chassis with the engine, cabin, and several accessories (fuel tank, battery box ...); the interaction of these stiff and flexible components is considered to be one of the key features of the medium frequency domain. Finally, a study has been performed on the effect of adding mass to the structure and changing stiffness conditions at the interfaces.
This work constitutes the preliminary part for an on-going project aiming at the development of reduced numerical models for the prediction of sound and vibration in truck cabins. Reduced models are best suited to the early design stages, when there is a need to perform fast evaluations of response to driveline excitations and sensitivity analyses. Reduced models thus represent an alternative and an evolution of the current Finite Element model.
In the second phase, the reduced models are developed in the framework of the Wave Finite Element Method (WFEM). The WFEM has been chosen above all to properly simulate the dynamic coupling phenomena occurring between components of the composite structure.
