- Detail

The application of NASTRAN in the thickness optimization of BIW

1 overview

the first-order torsional modal value of BIW is a very important technical index. The value of the first-order torsional modal frequency is too low. When it is close to the excitation frequency of the engine at idle speed, it is easy to cause the resonance of the whole vehicle, resulting in its low NVH performance. How to apply CAE in the design stage to significantly improve the first-order torsional modal value, reduce the weight of the car body, shorten the development cycle, save development costs, and avoid fatal quality problems when products are put on the market is an urgent problem to be solved. During the design of a van, it was found that the first torsional modal value of the BIW was lower than the target value, and the requirements could not be met through the modification of local parts. Simply increasing the strength of local parts can no longer fundamentally solve the problem

this paper discusses the method of improving the first-order torsional modal frequency of the body in white by globally optimizing the material thickness of the body in white, while reducing the weight of the body and ensuring the torsional stiffness

2 finite element analysis

the finite element model of BIW is established under the current design state. In the model, the average cell size is 10mm, and the solder joint is simulated by cweld cell. The whole BIW has 556906 nodes and 539762 elements, and the weight of the finite element model of the BIW is 272.4kg

first, the free mode analysis is carried out on the basis of this model, and it is concluded that the fourth mode is torsional mode. Table 1 lists the frequencies and modes of the first four modes. Table 1 frequency and mode shape of the first four modes

calculate the static torsional stiffness value of the model. The torsional stiffness refers to the test scheme to determine the boundary conditions, and MPC is used to limit the displacement of the central point of the mounting support of the two front shock absorbers. The condition is that the displacement in the Z direction: z1+z2=0, that is, the displacement of the two points in the Z direction is mutually exclusive. Method: when using these functions again, the software will call the tacit printer, etc., in the opposite direction. Constrain all displacement degrees of freedom of nodes on the installation plane of the rear shock absorber, that is, t1=t2=t3=0. Apply a torque around the central axis of the vehicle body on the center point of the left shock absorber. The displacement measurement points P1 and P2 are located at the center of the vertical plane passing through the center points of the two shock absorbers and the section line of the bottom surface of the front longitudinal beam, and their Z-direction displacements are D1 and D2 respectively; Two measuring points P3 and P4 are also arranged at the center of the vertical plane of the central point of the rear shock absorber and the section line of the bottom surface of the rear longitudinal beam. The displacement in the direction of Z with larger deviation is D3 and D4, the distance between P1 and P2 in the horizontal direction is L1, and the distance between P3 and P4 in the horizontal direction is L2

the calculation formula of torsion angle and torsion stiffness of BIW under torsion load is:

α= (D1-D2)/L1-(D3-D4)/L2 （1）

K=M/α (2)

where: m is the torsional moment, α It is the torsion angle

calculated torsional stiffness: 375759.4nm/rad

3 optimization analysis

msc NASTRAN's sol200 solver can carry out sensitivity and optimization analysis, which is mainly used in building materials, automobile industry, packaging and transportation of goods, decoration materials and ordinary household appliances. This paper uses this solver for optimization analysis. The first round takes the minimum body mass as the goal, the value of the first-order torsional mode (the fourth-order mode of the recent BIW) as the constraint condition, and the part thickness as the variable. The optimization analysis is carried out by using the optimization module of MSC Nastran. In the optimization calculation, the part thickness is set as a discrete variable. The optimization results show that the fourth mode of BIW meets the target value, but the mode shape has changed, and it is no longer a torsional mode

in the second round of optimization analysis, the static torsional stiffness is added as the constraint condition. D1 and D2 are deduced from the static torsional stiffness analysis results, and the fourth-order modal frequency value and D1 (or D2) are limited as the optimization analysis conditions. The results show that the fourth modal frequency increases to 34.23hz, which is still torsional mode; According to the calculation formula of torsional stiffness, the static torsional stiffness of the optimized model is 431317.8nm/rad, which is increased by 14.79%, meeting the design requirements. At the same time, the body mass decreased by 4.23kg at the same time

the following table shows the thickness comparison of some parts before and after optimization. Table 2 Thickness comparison of parts before and after optimization

4 conclusion

through the global optimization analysis of all parts of the BIW, the first-order torsional modal frequency and static torsional stiffness can be improved while reducing the weight of the body, providing a good reference and basis for design changes, and effectively saving the time and cost of design changes and tests. (end)

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