Automotive composite components design and test: a vibration reduction oriented approach

Vehicle manufacturers initially relied on horsepower and speed performances as “standout qualities” to sell their products, but as time progressed, it became increasingly clear that customers were very much concerned about their comfort and safety while driving vehicles. In the late seventies, requirements for driver and passenger comfort were increased significantly, since-then, a large amount of effort has been invested into the vehicle improvement of noise and vibration reduction nevertheless enhancing the safety. Nowadays NVH performance is considered as a critical factor in the purchase decisions of many buyers. This work focusses on the development of a composite automotive door but, the methodology and materials applied could be extended for other panels, as there are several kinds in a typical vehicle whatever class or purpose it has. In the last years, furthermore, lightweight design imposed different design methodologies and materials like fiber reinforced plastics are becoming more and more used in the construction of vehicles for structural and non-structural parts. Reinforced plastics could perform higher performance and higher tailorable possibilities compared to conventional metallic materials, more and more composites are targeted toward structural applications involving combined dynamic, mechanical, thermal and hygral loading. Composite materials are principally preferred in such applications because of their advanced elastic properties and tailoring capability to individual design requirements. They also have the potential for incorporating significant and tailorable passive damping into the candidate structure. At the beginning, tests on two different viscoelastic materials are performed in order to identify mechanical characteristics necessary to provide valid data for numerical analysis of structures. ‘Oberst Beam Method’ is the standard test method for measuring dynamic properties of materials (ASTM E756 – 05). Mechanical characteristics of damping materials have been determined for different temperatures (SAE J1637), frequencies and at different stage of aging (ISO 60068). Besides, low-velocity impact tests have been executed (ASTM 7136). After having post-processed the experimental data, the best material has been chosen. For each test, for each material and specimens topology a significant number of specimen has been tested for statistical purpose. ASTM 756 tests have been replicated virtually and to compare the results FRF Receptance, eigenfrequencies have been matched. Excellent correlation between simulation and experiment has been found. To evaluate the FEM virtual capabilities to reproduce and simulate the dynamic response of a real component and to validate correlation methodology, a first modal analysis has been carried out on a real automotive component. An innovative composite leaf spring is subjected to a free-free frequency response test, in which a series of accelerometers measures the response of the component to an excitation imposed by an electromechanical shaker. Moreover, LIPEZ methodology permitted to post-process data logged and to evaluate the characteristic vibrating modes of the structure. While the real component has been experimentally tested, FEM numerical simulation is reproduced. In the end, eigenfrequencies, eigenmodes (MAC) of experimental and virtual tests have been compared; excellent mode shape correlation has been found-out, errors on eigenfrequencies lower than 5%. Last part of this dissertation is focussed on the design, testing, and development of a damped door by means of interlaminar materials. The activity has been performed in two stages: undamped door testing and correlation with FEM and then manufacturing, testing and correlation of the damped solution. LIPEZ modal extraction algorithm has been used to extract modal parameters for the two artifacts, as well. The undamped door has been tested, real and virtual receptance, eigenfrequencies, eigenmodes computed and correlated. Damped material patches have been dimensioned and positioned considering a trade-off between added mass, costs and vibro-reduction capabilities. Modal analysis executed on the door have demonstrated that high damping capabilities have been achieved with limited drawbacks on weight increase (+5%) and inappreciable cost augmentation. In view of the scopes of the work, the results are deemed satisfactory.