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Bus body lightweight analysis

Abstract: taking a vehicle model as an example, this paper studies the body lightweight method from the aspects of optimization process, optimization analysis model and analysis working conditions, and puts forward a set of practical technical route

key words: car body; Structure; Lightweight; Optimization technology

lightweight design is one of the themes in today's automobile design. It can improve the power of vehicles, reduce costs, reduce energy consumption and reduce pollution accordingly. However, lightweight structure is a double-edged sword, which will not only affect the strength and stiffness of the vehicle, but also affect the service life of the structure. These two aspects need to be dealt with in coordination. At present, most of the research on this aspect focuses on the use of light materials, but due to the problems of cost, processing technology and environmental protection [1,2], these materials are still difficult to be used in general vehicles, but the optimization from the perspective of structure is of more practical significance

structural lightweight is essentially an optimization problem. The existing optimization technology has been applied to the field of engineering design on a large scale, but the particularity of bus structure makes the existing optimization methods feasible only by improving. Among them, the optimization process, control conditions, constraints and optimization model are very key technical problems. There is little systematic research on this aspect in the existing references

1 optimization process determination

optimization analysis model is generally composed of objective function, constraint equation and optimization design variables. If these three aspects can be written in the form of analytical mathematical functions, they can be easily completed with the help of relevant software under the conditions of existing computer technology and optimization theory. However, due to many special conditions of lightweight design of bus structure, this method is not applicable

firstly, in the process of lightweight analysis, the objective function is generally selected as the minimum total body mass. For the existing analysis software, the definition of this objective function can be realized on the premise that the parametric geometric model is established in the analysis environment. However, most of the existing geometric models for analysis come from CAD software. They enter the analysis software through a certain format and lose some characteristic parameters, so they cannot be optimized. If the geometric modeling is carried out in the analysis software, it is acceptable for the beam element and unrealistic for the plate and shell element. Therefore, the beam element model based on approximate body modeling is the only model of this optimization method. Secondly, the constraint conditions mainly include reasonable body dynamic stiffness (mainly refers to that the natural frequency corresponding to the first several vibration modes is controlled within a certain range), the stress of body structural parts under various working conditions is not greater than the yield limit, and the deformation of each window under torsional working conditions is controlled within a certain range. These equations can be defined in the software. Finally, the optimal design variables can select the plate wall thickness, rod length and so on, and the optimal value is a continuous variable

from the above analysis of these factors, it can be seen that using the algorithm provided in the software can theoretically complete the bus structure optimization of the beam element model, but it can not be realized in the actual use process, mainly for the following reasons:

(1) the amount of optimization calculation is huge, and the existing hardware conditions are not enough to complete such optimization work. Even if we are full of confidence in the development speed of computer technology, it is still quite difficult to complete these optimization work in a short time

(2) topology optimization cannot be carried out, but the thickness of profile is changed on the basis of existing structure

(3) the optimization result generally has no practical significance. For example, the thickness of the profile is discrete and the specification is small, and the optimization result value is generally difficult to select in the actual section steel

(4) it is difficult to consider the process factors and make full use of the experience of engineers, which is the fatal weakness of the optimization method provided by the software

therefore, the existing optimization analysis process is improved for the lightweight analysis of bus body: the optimization process based on traditional experience is adopted, i.e. general layout optimization, topology optimization, geometric form optimization, section parameter optimization, etc. its objective function and constraint conditions are the same as the above software optimization methods. The expansion microspheres exhibited by the company have the advantages of low density, light weight The characteristic quantities such as good resilience are mainly based on the general profile library of bus manufacturing enterprises. The specific optimization process is as follows:

(1) general layout optimization. By changing the connection position of some parts, the possibility of concentrated double moment (bending moment and torque) and large local stress is reduced, and the load distribution is homogenized when the general layout can be realized

(2) optimize the topology and classify the functional parts and stiffeners. Functional parts mainly refer to the components used to meet the needs of structural layout and process; Stiffeners refer to additional members added to meet the strength needs. In the optimization process, the methods of addition design and subtraction design are adopted respectively. The so-called additive design is to add stiffeners on the basis of the structure of functional parts, while the subtractive design is to pay attention to the following 8 points in the inspection requirements of the original vehicle: reduce the components on the basis of the structure, and finally make the structure meet the constraints on the basis of less number of components and minimum mass, so as to select the better scheme. Theoretically, the results obtained by the two methods should be similar

(3) optimization of section geometry. In the better scheme, the deformation of each component is extracted and expressed in the form of list, the dangerous working conditions of each component are determined, and the bending type, torsion type or tension compression type are selected, so as to select the geometric form of the section

(4) optimization of section parameters. From the established section, I hope it can help you to select the corresponding profile for trial calculation

2 Establishment of optimization model

when establishing the finite element model of body beam element, it is mainly necessary to study the simulation of composite beam, variable section beam and welding riveting. In this model, as in general references, the influence of skin is omitted

since the stress and deformation of the beam element in the calculation depend on the interaction of the section shape, moment of inertia and external boundary conditions, the section shape and moment of inertia of the simplified section of the composite beam should be the same. In this paper, several closely adjacent beams in the body frame structure are combined and replaced by one beam. Figure 1 shows two rectangular tubes welded to each other and with different cross-sectional dimensions; For the variable cross-section beam, it can be simulated by multi segment constant cross-section beam; For welding simulation, the master-slave node method is mainly used in this paper, which can avoid the model geometric error (variable length) caused by node coincidence, but the stiffness equivalence and strength equivalence should be considered. Based on the above simplification principles, the finite element model of body beam element is established based on the CAD model of a luxury car. The simplified model is shown in Figure 2

Figure 1 section definition of composite beam

Figure 2 finite element model of beam element

weight and mass of engine and compressor are allocated to each loaded node by master-slave node method; The weight/mass of batteries, televisions, etc., which have little impact on the stiffness of the whole vehicle, are evenly distributed to the nodes connected to it in the body structure

in this paper, the driving conditions and test conditions are taken as the analysis conditions. The driving conditions include straight-line driving, driving, braking and steering; The test conditions are torsion condition and bending test condition of single wheel suspension

the main constraint condition is that the strength of each component under driving conditions does not exceed the yield limit; The natural frequency of the first two-stage body twist and bending modes does not change by more than 10% compared with the original model (this constraint is only considered from the perspective of technical feasibility and does not necessarily have practical significance); Under the test condition, the deformation of the window shall not exceed 5 mm. The design variables are based on the general profile library of bus manufacturing enterprises

3 optimization results

before the optimization work is carried out, the analysis model must be confirmed first. Since this model is a newly developed model and there is not enough data accumulation, the analogy method is adopted in this paper to compare with the results of the same model with the published data and the models of different element forms (plate shell element and beam element). The results show that the simulation results of this model are reliable and will not be described in detail here

according to the above optimization process, the vehicle structure is optimized. The specific optimization results are as follows:

(1) considering the process factors in the side wall, the top beam of the window frame is not modified, and the bottom beam of the window frame is 50 × fifty × 115 section steel replaces the original 50 × fifty × 215 section steel, and the original section steel is replaced by bending parts for window edge beams

(2) the longitudinal beam in the top cover is 30 × thirty × 1. 5 section steel replaces the original structure

(3) remove the diagonal brace between the middle cross beam and the vertical support of the longitudinal beam and some vertical supports on the rear steps from the underframe, and add a vertical support between the longitudinal beams, a support beam connecting the rear underframe and the longitudinal beam, and a 40 at the rear seat support × forty × 1. 75 beam

(4) remove the longitudinal beam, the connecting plate between the rear wall and the side wall from the rear wall, and add four connecting beams with the same characteristics as the connecting beams at the bottom of the side wall and the rear wall

see Table 1 for the comparison of the maximum stress of the body before and after the improvement. It can be seen from the table that after the lightweight improvement of the car body, the maximum structural stress is 168m PA, which is within the allowable range of the selected materials; In addition, the stress of some parts with large stress in the original structure decreases more after improvement, and the stress of some parts with small stress increases significantly after improvement, indicating that the application level of the structure has been greatly improved. Due to the limited model of profile, that is, the requirements of process, many structures that can be lightweight in theory have not been improved, such as the stress level of underframe, side wall and other structures is very low

see Table 2 for the comparison of modal characteristics of vehicle body before and after improvement. From the above analysis, it can be seen that the frequency change corresponding to the main vibration modes of the improved structure is small and meets the design requirements. Under standard test conditions, the diagonal length change of door frame and front and rear windshield frame shall not exceed 5 mm, which meets the design requirements of passenger cars. Through calculation, the mass of the whole vehicle is reduced by 150 kg

4 Conclusion

(1) the beam element model is practical and feasible for passenger car lightweight analysis, and the analysis of plate and shell element model is expensive, so it is not suitable for lightweight design

(2) based on the optimization process of general layout optimization, topology optimization, geometric form optimization and section parameter optimization, it is effective for bus lightweight and can achieve good optimization effect

(3) in this paper, the driving condition and test condition are used as the evaluation condition, and the analysis results are used as the constraint conditions, so the calculation cost of this method is large. Setting the strength control condition is the key to the next work research

References:

[1] lightweight and stamping forming of Liu Shucheng 1 automobile [J] 1 light vehicle technology, 1997 (1)

[2] Li Hongyu 1 new vehicle and new material [J] 1 light vehicle technology, 1997 (2) (end)

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