Abstract:
The purpose of this study was to develop a design synthesis approach that can be used to reach an optimal design solution (in terms of the strength, stiffness and weight) of automotive body structures during the conceptual stages of the design process.
Two conceptual model variants; standard sedan and open-top unitary body structures that were made from the same platform were analysed for their maximum bending moment, stresses, deflections and their maximum load carrying capacity. Topology optimization was also undertaken in order to find a lightweight design of the body structures. The body structures were modelled using three different modelling techniques, namely; the simple beam model, the simple structural surface (SSS) method and the finite element (FE) method.
The simple beam model was used to determine the axle reaction forces and the maximum bending moment of a body structure that was subjected to static and dynamic loading conditions. Dynamic load factors and an extra safety factor were used to simulate the dynamic bending loads. The factors were varied from 1.0 to 4.5 with a step of 0.5. It was found that the maximum bending moment under dynamic loading is simply a multiple of the static maximum bending moment and they both occur at a position that is close to the rear part of the front seats.
The effects of different geometries on the strength, stiffness and the weight of body structures were studied using the finite element method. The two conceptual models were made into four different plane FE models with each concept having two different FE models. The panels of these models were constructed as simple structural surfaces and were based on the SSS analysis of the standard sedan. The models were subjected to bending and torsion load cases. Each load case was varied similarly for 19 different iterations until the yield point was reached for each FE model.
It was also found that the load-displacement graphs were linear for loading within the elastic range, even if there are subassemblies that are missing. However, it was found that this relationship ceases to apply once the body structures are subjected to the torsion loads that are above the yield load. It was also found that the qualitative response to torsion loads was similar for all four body structures. However, the quantitative response was quite observable. It was found that the stiffness can be reduced by at least 37% by omitting subassemblies for the same platform and almost the same mass of the body structure.
In addition, the effects of different materials on the strength, stiffness and the weight of body structures were also studied. It was found that lightweight designs can be achieved by using lightweight materials. However, both the bending and torsion stiffness were observed to be reduced or increased in proportion to the Young’s modulus or modulus of elasticity of the material that was used to construct the models. It was also noted that, the stiffness to weight ratio remained almost the same for the same models made from different materials.
Topology optimization was undertaken in order to determine alternative load paths of the body structures. The two conceptual models were made into four different solid FE models.
It was observed that the load paths remain similar for different volume fraction constraints for similar models under similar loading conditions. It was also noted that at least 20% in weight savings and at least 5% in torsion stiffness improvement can be achieved when topology optimization is used to determine the alternative load paths for a standard sedan model. Besides, the load carrying capacity was found to remain similar. However, the bending stiffness was noted to have reduced due to the reduction in the mass of the structure.
In contrast, it was found that for an open-top model, both the bending and torsion stiffnesses were reduced in proportion to the reduction in the mass of the body structure. In addition, it was observed that a further reduction in the mass of the open-top body structure can also significantly reduce its load carrying capacity. Although the stiffness of the optimized open-top model was noted to have reduced due to the reduction in the mass of the structure. The stiffness to weight ratio of the optimized body structure was higher than that of the non-optimal structure.
Description:
M. Tech. (Department of Mechanical Engineering, Faculty of Engineering and Technology), Vaal University of Technology.
