Study On The Key CAE Techniques Of The Structural Dynamics And Acoustics Of The Bus Body

Supported by The Jiangsu 10th Five-Year scientific tackling key project and The National Natural Science Fund project, and focused on the development of the IVECO A40 new bus, this dissertation systematically studied the key CAE techniques of the structural design dynamics and acoustics of the bus body, which included the modelling method of the finite element (FE) model of the structure of the whole bus body, modification of the FE model based on the modal test, the analyses and optimization of the dynamic and static characteristics of the body structure, the analyses of the structural-acoustic coupling of the bus body structure with its inter-noise, FE modelling method of the skin shell of a bus body with surface-damping-treatment and estimation method of acoustic radiation, etc. The contents and innovative points of this dissertation include:1. Based on the study on the spot welding, sewing welding and rivet joint of bus body structures, a FE model, which was composed of more than 100000 elements, about 600000 degrees of freedom and 885 sets of real constant, was set up. The model could reflect the dynamic properties of the structure exactly and the scale of the FE model was controlled to a level which could be accepted by the present computer and the modelling efficiency was also taken into account during the modelling process. The modelling accuracy was guaranteed by modifying the properties of the interface of the parts, which was based on the modal tests of typical spot-welding structures, parts and sub-assemblies of the bus body.2. The modal test of the body-in-white was designed and carried through successfully, and the dynamic properties of the body of the IVECO A40 new bus were obtained. Based on the re-analyzing formula of the linear eigenvalue on broad sense owing to matrix-perturbation, the most sensitive positions to modify the model were located, and the FE model was modified according to the modal test results. The error percentage of the first-three-frequencies between the FE analysis results and the test results were within 3% and their mode shapes fit quite well. Thus the FE model could be guaranteed to reflect the dynamic properties of the physical bus body.3. Based on the results of the modal test and FE analyses, the weak points of the bus body structure were found out and corresponding methods to improve them were proposed. The modal tests were carried out after the structures were improved. The test results showed that the first several frequencies were elevated, which meant the dynamic properties of the body-in-white were improved.4. After other additional parts such as passengers, chairs, luggage, oil box, air-conditioner systems, glass, diesel engine etc. were added to the above body model, simulations under static-bending and bending-torsion conditions were carried out on the FE model, and corresponding stress and distortion distribution were obtained. The transient response analysis in time domain while the bus passing through sinuous obstacle was simulated, and stress and distortion time histories of any point during the process were obtained.Dynamic stress-test was carried out on the partial stress-concentrating areas, and the test results talliedbasically with the FE analyses results on the whole, which provided theoretical bases for improving structures of the bus body and overcoming its weak points further.5. The sub-structure methods for both dynamic and static sensitivity analyses were put forward. According to the sensitivity analyses results, those variables which influenced the objective sensitively, were selected as design variables of the optimization. Based on the ANSYS optimal function and its parametric design language, the dynamic and static optimization were carried out on the whole body-in-white of the bus, which had more than 100000 elements, about 600000 degrees of freedom, 885 sets of real constant, and 309 design variables. With this method, the analyzing efficiency was increased and calculation time was decreased by about 50 percent. This provided a practicable analyzing method with high efficiency for dynamic and static sensitivity analyses and optimization of large-scale structures.6. The detailed FE model of the acoustic cavity and the structural-acoustic coupling FE model of the light bus were set up, and modal and harmonic analyses were carried out, so the acoustic properties of the acoustic cavity of the light bus were portrayed; The coupled results of the body structure with its inner acoustic field were found out. The low-frequency noise level inside the bus was analyzed under the excitation of the engine while the bus kept motionless. The results of the analyses coincided well with the results of measurement, which showed that the model was accurate and the analyzing method was feasible.This could be of beneficial exploration to forecast the low-frequency noise level inside the light bus effectively.The measures to reduce the noise and vibration level were proposed according to the results of measurement and analyses, and the noise level inside the bus was reduced effectively, which resulted in the noise level inside the bus ranking on top among the similar buses in our country.7. The scheme of surface-damping-treatment on the skin shells of the bus body to reduce the noise and vibration level was proposed. The FE models for shells and beam-stiffened shells, both with free-damping-treatment and with constraining-damping-treatment, were established using integral-divided element method. The mass matrices and the stiffness matrices of the elements were developed and modal loss factors were predicted by means of the modal strain energy method.The infinitesimal algorithm was proposed to analyze the acoustic radiation of the surface-damping-treated shells or plates. The example given in the dissertation showed that the method is feasible to analyze the acoustic radiation of surface-damping-treatment structures. This provided an effective method for estimating the acoustic radiation of the surface-damping-treated shells or plates.Based on the structural-acoustic coupling theory, the noise level inside the bus under certain excitation was analyzed for both with and without partial surface-damping-treatment on the inner skin shell of the bus body. The effect of the damping-treatment of the structure on reducing noise and vibration level was evaluated quantitatively, and this provided theoretical support for the design of surface-damping-treatment on the skin shells of the body bus.The study in the dissertation greatly supported the development of the body of IVECO A40 new bus. The project was appraised by the Scientific and Technology Office of Jiangsu province in Feb. 2005 and the achievement was appraised to attain at advanced level of international standard. After the design was finalized, the buses of this style have been released into the market in scale and have won broad appraise. The research of this dissertation could be of universal significance to the development of other bus bodies.