Research On Topological Optimization Light-weight Design Method Of Truck Frame

It is well known that the excessive levels of vibration in heavy trucks negatively affect driver comfortability, cargo safety and road condition. The automotive designers pay great attention to optimize the vehicle structure, that is, to increase stiffness and strength and to reduce weight. Weight optimizing structural components for heavy trucks is becoming more and more important.Therefore the structural optimization has been the subject of numerous studies and widely used to develop the truck. The optimal methods used in the automotive component design can be divided into three types, size, shape and topology optimization.In the size optimization, the design variables are the used material parameters such as the elastic module(E) and density(ρ), and the size parameters such as the cross sectional area of a beam, bending moment of inertia, the thickness of a plate, an elastic support with spring stiffness and an connector of stiffness between two components, etc. In the shape optimization, the design variables are the coordinates of the boundary node points. In order to account for mesh changes, the basis vector approach and the perturbation vector approach, are used.The topology optimization is used to affirm an optimized shape and material distribution for a given structure. The topology algorithm calculates the material properties for each element,and alters the material distribution to optimize the defined objective function under given constrains. It has been recognized that topology optimization can more greatly improve the structure design than fixed-topology optimization. Up to date significant progress has been made with a variety of different approaches. For example, homogenization or a density method has been widely used to solve the topological optimization problem of trucks. For the density method, each element has one variable representing its material density. For the homogenization method which is limited to 2-D and 3-D elements, there are two size variables for each shell element and three void size variables for each solid element, and an additional variable for each element is the void orientation angel.In recent years, all three optimal methods described above have been widely applied to the component design of the truck and significant successes have been achieved. However, for the optimal problems of the large scale finite element model with a large number of degree of freedom and design variable, the difficulties arise and three methods described above are difficult to deal with. Up to date, most of the optimizations are limited to deal with the individual component of the truck. A few studies can be found for the optimization under full truck assembly.It should be pointed out that even though the loads on the full truck are usually known, the loads on each component, such as cab, body and frame, are rarely known. Since there exist the elastic and dynamic coupling relations between components under the assembly environment of the truck, it is difficult to obtain the boundary conditions of each component. In this case, the optimization for the individual component may not only give inaccurate result but also may be meaningless, if exact loads and boundary conditions can not be given. As can be seen that even through the optimal design for the individual component is obtained, the design of full assembly truck may not be optimal, In other words, the optimization of the individual is not equal to the optimization of full assembly truck. Therefore, it is necessary to discuss the optimization of the component under full truck assembly environment in order to obtain the optimal performances of the full truck. To this end, this paper presents the optimization of the frame under full truck assembly environment including the simplified modeling and more effective topological optimal method for solving the large scale optimal problems of components under full truck assembly environment. The main contents of this paper are studied as following:A review on the applications of structural optimal method to the component design of the truck is given. The state-of-the-arts of the size, shape and topological optimizations for the individual component of trucks is presented. The difficulties resulting from the large scale optimal problems of the structure and problems to be discussed in this paper are also given.The effect of frame stiffness on the ride comfort and the cargo ride safety is discussed. As can be seen that excessive levels of vibration in commercial vehicles, due to excitation from the road irregularities, can lead to cargo damage and safety problems. Therefore a finite element model of full vehicle was established. The computation algorithm for acceleration power spectral density (PSD) and root mean square (RMS) are also given. In order to show the effect of frame stiffness on the ride comfort and cargo ride safety, two frames with different stiffness were given in the computations of PSD and RMS of the drivers and body vertical accelerations for four excitation cases. The computed results showed that when the stiffness of the frame is increased, the values of RMS the driver and body are decreased significantly at the frequency band 14～26Hz, which can effectively improve the ride comfort and cargo ride safety. This conclusion can be used to improve the design of frame as a valuable guide.The topological optimization and topological sensitivity analysis of the truck structure are presented in this paper. Firstly, the sensitivity analysis methods for the static displacements and modal frequencies, and the topological optimal methods which including, the homogenization and the density methods, are reviewed, And then, focusing on difficulties of resulting from the optimal design problems in large scale finite element model of the complex structure, a new conceptions and new methods for the topology optimization design of the truck component are developed. The strain energy resulting from the loads applied to the structure is defined as the inverse measurement of the stiffness of the structure. The new conception of the static sensitivity for topological modification based on the element and superelement is presented and the corresponding static topological optimal method is developed for structures. The eigenvaluesλr can be defined as the modal stiffness. The modal stiffness sensitivities based on the element and super element(substructure) are presented for the topological modification, and the corresponding dynamic topological optimal method are developed. By using the present methods for sensitivity computation the static topological modification sensitivities of the torsional and bending stiffness of the truck frame, and the modal stiffness sensitivities of the torsional and bending modes are obtained. The topological modification sensitivities can be to improve the frame design as a valuable guide.In the discussion of the optimal design of the truck frame under full vechiche assembly environement, the response analysis methods including multi-body system simulation, component mode synthesis and structural reduction are discussed. It is pointed out that it is difficult to use these methods to deal with the optimal design of the truck component under full vehicle assembly environment. By considering, difficulty resulting form the optimal problem with large scale degrees of freedom and design variables, the new methods for the optimization of the frame under full vehicle assembly environment are developed. Using the normal mode reduction and the static reduction, the dynamic substructural reduction and the dynamic topological optimization method are developed for the frame design under full vehicle assembly environment. The present method is applied to the design of the truck frame, and the obtained results indicate that the proposed method is valid and effective. The sensitivities of the modal stiffness of each component and the modal stiffness of the frame to the full vehicle are obtained from the computations.Finally, the conclusions are drawn and problems to be further investigated are also presented for the truck.