Research On Reduced Model Of Joints In Automotive Body Structure

A simplified model that describes the actual properties of body structure must be established in the concept design process of body structure. The stiffness of body structure is designed in order to control the performance by studying reduced models. Body joints are very important components in body structure. We define a body joint as any region of an automotive body structure containing the intersection of two or more loading-carrying beam-like members. In general there are seven joint pairs in body structures such as the A-pillar to roof rail, B-pillars to roof rail, C-pillar to roof rail, A-pillar to hinge pillar, shotgun and cowl, hinge pillar to rocker, B-pillar to rocker, C-pillar to rocker, and so on. Joints play an important role on the stiffness of body structures. They control the 60 percent of the whole stiffness. The importance of joints is reflected mainly in the following aspects:1) Strength requirementThe reduction of joint stiffness will bring about the increase of the stress and strain level and the modal response in the joint positions, as well as the decrease of fatigue life. These will lead to joints'first destroying in body structure.2) Stiffness requirementThe flexibility of joints will lead to the relative rigid rotation of adjacent components in body structure, which have enormous implication for the whole stiffness. Therefore the design of joint stiffness must be emphasized.3) Design requirementThe connection forms and the related components of joints are so complex that joints'performance couldn't be well guaranteed. So the design of detailed joints must be checked, furthermore the design criteria must be formulated.4) Modeling requirementIf joints were regarded as rigid connection, there should be much more error in simulation models of body structure. Since modeling a joint is a complex process, it's necessary to find an accurate but simple modeling method of joint. It's very important to study the mechanical properties of joints and establish accurate simplified joint models for the concept design process of body structure, where the research significance of this paper is.The purpose of this paper is to establish a simplified joint model that could accurately describe joints'mechanical properties. After a lot of documents were searched and read, the following researches were done: First, joint models are extracted from the detailed finite element model (FEM) of body structure using HYPERWORKS FEM software. Secondly, an accurate simplified joint model is put forward which is defined as"Δ-rotational spring and rigid beam model". Thirdly, a computer program that could calculate the parameters of simplified joint model and establish the model is written by re-developed HYPERWORKS by using TCL/TK language. Finally, in order to investigate joints effect on the whole body stiffness, 3 classes of simplified model of body structure under bending and torsional conditions are established.In the 1st chapter, the process of concept design of body structure is introduced. The significance and the status of research on the joint are showed by summing up the literatures. The research contents of this paper are presented here.In the 2nd chapter, the general principles of modeling detailed joints such as structural simplification, determining the element type and number, grid distribution and simulation of connections are presented. And after extracting joint models from body structure, we discuss the mechanical and the flexibility properties of joints.In the 3rd chapter, the flexibility properties are explained by using super-element theory. The terms"characteristic direction"and"characteristic system"are defined and used to deal with the coupling phenomenon between different rotation degrees. Based on the definitions, a new simplified joint model is presented and two methods of determining parameters of joints are put forward, which are called"super-element method"and"flexibility method". Finally, to take Joint C for example, the validity of the simplified model is proved and the"flexibility method"is recommended.In the 4th chapter, TCL/TK language in HYPERWORKS is introduced firstly. Then 3 classes of simplified model of body structure under bending and torsional conditions are established according to different processing methods of joint in body structure, which are simplified body model with detailed joints(Model 1), one with rigid joints(Model 2) and one withΔ-rotational spring and rigid beam joint(Model 3). Process automation for building simplified body models above-mentioned is realized in HYPERWORKS by using TCL/TK language. Finally, we compare results of the three simplified body models under bending and torsional conditions in order to discuss joints'effect on the whole body stiffness.