Improvement Of One Step Forming Inverse Approach And Research On One Step Forming Direct Approach

Due to increasing environmental, energy sources demand, the weight of a conventional car body will most likely increase in the future. At the same time, legal safety demands will become stronger and weight reduction will play an important part in fulfilling them. These demands are contradicting and body designer are puzzled in this subject. Modern body, especially frames and panels of saloon car body, are manufactured using stamping process, and thickness and mechanics performance variations will happen in the sheet forming process with unavoidable residual stress and strain, which can affect strength, stiffness, durability and security. Before CAE structure analysis, thickness, residual stress and strain variations are predicted and then introduced into structure analysis, more precise and valid results will be gotten and more data for further lightweight and optimization of the body structure design will be provided. However, there is not a very effective method using for pre-process. Therefore one step direct approach is put forward based on improved one step inverse approach in this paper. Primary research contents of this paper are as followed. In first chapter, general situations and developments of considering stamping process influences in body structure CAE analysis at present were expatiated. And developments of the introducing sheet metal forming into structure analysis technologies, especially pre-process technologies in them, were analyzed. Incremental method and one step inverse approach using to simulate sheet metal forming were compared each other and from merits and limitations of them a result was gotten that a more effective pre-process method was needed to be studied. In the second chapter, elementary theories and ideas of one step forming inverse approach were described and warped modification method, implemented by Cook, was introduced into one step forming simulation and based on idea of surface spreading; new quadrilateral element modification arithmetic was put forward. By spreading warped quadrilateral onto plane in local coordinate system, a plane quadrilateral element whose shape and area were equal to original warped quadrilateral element was gotten and internal force vectors of four nodes and stiffness matrix of elements were improved to increase precision of simulations on this plane element. In order to overcome the influences of single point integral, especially stress solution would be impacted by sandglass mode, a partial reduced integral was adopted. Concretely, only shearing stresses in internal force vectors adopted single point integral and other internal force vectors used four points integral; in order to prove the validation of this approach, some typical parts and certain automobile front fender were used to simulate sheet metal forming process. By comparison of the simulation result through modified one step forming inverse approach with that by increment method and that by experiment, it was found that they agreed very well, which proved the validity of the approach proposed in this paper. Based on one step forming inverse approach, one step direct approach was put forward in third chapter to overcome some of limitations of one step inverse approach and increase precision to be introduced into body structure analysis described in first chapter. Firstly, a new primary theories system was set up by set forth kinematics relationship, physics constitutive relationship, boundary conditions, and setting up of finite element equation. Secondly, some pivotal technologies of one step direct approach were studied, including initial guess methods such as z-direction projection method, area coordinate mapping method and RS mapping method; nodes normal direction constraint mechanisms including rigid body constraint and sliding interface constraint schemes; minimum unbalance force convergence criteria. Initial results of area coordinate mapping method and RS mapping method was better than those of Z direction mapping method and adaptive to be used in one step direct forming solution. However, RS mapping method needed to solve some quadratic equations, which made efficiency lower, and in RS mapping method an assumption of plane quadrilateral element was made and low precision would appear in warped quadrilateral element. Area coordinate mapping method was selected because of better solution effect, which a quadrilateral element was divided into two triangle and warped instances were taken into account, and higher efficiency. Because of three-dimensional local coordinate systems were used in one step direct approach and in order to making balance of forces and constraint of rigid body distance, node normal constraints must be taken into account. Because nodes must slide on the die face in stamping process, criteria of pullback constraint must be adopted to the penetrating and departure nodes besides node normal constraints. Concrete constraint criteria were pullback along blank node normal, that is, intersecting point of node normal and die face needed to solve. Through tests to the typical parts, numerical values of unbalance force criterion were different and even it was difficult to find a fixed value as convergence precision of unbalance force. At the same time, by control process of convergence, numerical values of unbalance force criterion display a trend from increasing to decreasing. To this character, nodes displacement vector arrays would be saved, when numerical values of unbalance force criterion reached minimum, as displacement vector assays of stress solution. Then more reasonable results were gotten. Lastly one step direct approach was used in a typical sphere hat stamping simulation from analytic viewpoint while compared with one step forming inverse approach and...