【作者】 李威；

【导师】 那景新；

【作者基本信息】 吉林大学 ， 车辆工程， 2005， 硕士

【摘要】 本文对一步成形逆有限元法做了如下改进,首先将Cook 提出的基于弹性理论的四边形翘曲修正算法引入基于塑性的一步成形,并且提出了新的曲面展开的四边形单元翘曲修正方案,对单元内力和单元刚度矩阵进行了修正,从而提高了模拟的精度;然后提出了一种部分减缩积分处理方案,既解决了四边形全积分单元过刚和剪力自锁问题,又克服了单点减缩积分引起的沙漏模式造成的单元过柔问题,提高了应力计算的精度。在此基础上,初步研究了一步成形正有限元法,根据平面应力厚向应变不独立的特点,对一步成形几何关系作了进一步简化,并且研究了一步成形正有限元法的关键技术,其中包括:“Z 向投影法”、“面积坐标映射法”和“RS 映射法”三种初始场预示方法;节点法向的刚体运动约束机制、滑动界面约束方案两方面有效的节点滑动界面约束方案;最小失衡力准则作为Newton-Raphson 迭代的收敛判断标准。采用球冠件进行了实际模拟,并且与一步成形逆有限元法、KMAS 增量法模拟结果进行了对比和验证。更多还原

【Abstract】 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更多还原