机械结构分析中的新型低阶高精度单元理论研究
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摘要
在计算金属体积成型或车身覆盖件冲压和碰撞等牵涉到动态大变形的实际机械类工程问题时,有限元方法存在精度较低或计算能力不足的问题;无网格方法能很好地用于大变形问题,却存在计算复杂、计算效率过低的问题。针对这些现有数值算法的不足,本文旨在形成一套新型的单元构造体系,提出几种有效的低阶高精度板壳单元,使其能够有效地用于工程计算,为工程实际问题的求解提供有效的数值手段和理论基础。基于这一目的,本文从弱-弱形式和广义梯度光滑技术出发,提出几种有效的低阶高精度数值计算方法和板壳单元,并对其精度、收敛特性等进行了分析,建立了相应的单元算法构造体系。具体工作为:
     1.基于弱-弱形式的新型数值算法理论
     提出子区域应变光滑的概念和区域光滑径向点插值法,将传统无网格所用的高斯积分转化为可任意控制积分点数目的子区域光滑积分,并给出基于背景网格的支撑点选点方案,将无网格插值方式和有网格选点进行有机结合,既保证了新算法处理网格畸变的能力,又提高了计算效率和精度。
     给出了边光滑弹塑性有限单元法,并从理论上证明边光滑技术能够降低系统的模型刚度,有效地解决有限元存在的模型过刚问题。通过求得边光滑域所对应的等效弹性模量和等效泊松比求解弹塑性问题,有效获得材料发生弹塑性变形而产生硬化后的材料参数。
     2.建立几种新型Mindlin板壳单元
     给出基于区域光滑径向点插值法的Mindlin-Reissner板构造过程,提出消除剪切自锁的方案,获得了具有高应力精度的中厚板公式。
     构造了基于光滑有限单元法的四边形板壳单元,并提出平均形函数的概念,有效消除了剪切自锁问题。直接在笛卡尔坐标系下构造形函数,不需要进行等参变换,并将传统的面积积分转化为线积分,既能够提高计算效率,又能够有效地处理网格畸变问题,是一种在工程上较为实用的优质四边形板壳元。
     提出了边光滑三角形板壳单元构造理论和边局部坐标系的概念,给出了非同面内的梯度光滑实现方案。在现有低精度三角形板壳单元基础上只需做简单改动便能够获得高精度,受网格畸变影响较小并能有效地用于解决工程问题的三角形板壳元,是一种简单高效实用的三角形板壳单元构造方式。
     3.建立基于线性插值的薄梁薄壳元构造理论
     提出采用线性插值求解四阶微分方程思想,首次构建相关理论框架,并通过构造线性插值薄梁单元,证明采用线性插值求解四阶微分方程问题的可行性。
     提出一种连续性二次松弛技术,进一步形成了全新的薄板单元构造新理论。通过散度定理将域积分转换为边界积分对积分域内的曲率进行平均处理,能够有效地降低试函数的连续性要求,对于2后阶偏微分方程,k-1阶连续的试函数便能够有效地离散数值模型。对于需要试函数具有C1连续性的四阶偏微分方程的薄板问题能够很容易地采用C0连续的试函数进行离散求解,并能够有效地进行求解。
     基于径向点插值法和三角形背景网格,给出了不同的积分域形成方案,并构造了相应的连续性二次松弛薄板单元,讨论不同的积分域形成方案对计算结果的影响,证实了采用连续性二次松弛技术构造薄板单元的有效性与精确性。
     提出了曲率重构理论,基于线性插值的挠度场构造兴趣点处的转角,并利用构造的转角重构积分域内的曲率场,从而获得相应的离散方程。给出了几种曲率场重构方案,构造了薄板公式,讨论了不同方案的计算精度,并选择了合适的方案用于构造薄壳单元。
     4.构建基于新型单元理论的动态大变形显式求解格式
     将基于弱-弱形式的数值计算方法用于求解包含接触、几何和材料三大非线性的金属成形问题,给出了相关的接触算法,并提出了光滑变形梯度和光滑应力应变张量的概念,为金属成形分析模拟提供了新而有效的数值工具。
     将前面提出的两种表现较好的三角形壳单元用于求解显式动态问题,给出了所提出的三角形壳单元显式格式,并在光滑迦辽金弱形式下推导光滑应变率形式的节点内力、节点外力以及节点惯性力。
The finite element method meets low accuracy or can not work for some engineering practice problems, such as metal forming、vehicle body panel forming and crashing, containing dynamic large deformation analysis. Meshless methods can work perfectly for large deformation analysis, but they encounter the major technical barrier-low computation efficiency. To solve these shortages of existing numerical method, this thesis propose a novel numerical theory system, which are very effective for engineering analysis. To pursue this goal, this thesis present several effective numerical method and plate and shell elements based on weakend weak form and generalized gradient smoothing technique. Some important properties, such as accuracy and convergence, are proved, and a numerical theory system is constructed. Main work includes the following aspects:
     1. Novel numerical methods based on weakend weak form
     The subdomain gradient smoothing technique and a cell-based smoothing radial point interpolation method are proposed in this thesis. Conventional gauss integration becomes line integration along smoothing subdomain edges. The supporting node selection for shape function construction is based on the background cells, which incorporating the meshless interpotation can deal with the mesh distortion effectively, and also has a high accuracy and efficiency.
     An edge-based smoothed finite element method (ES-FEM) for elastic-plastic analysis is formulated. Theorem-proof of the results demonstrate that the edge-based smoothing operation reduces the stiffness of the discretized system, and compensates nicely the "over-stiffness" of the FEM model. The effective elastic modulus and Possion's ratio in each smoothing domain are obtained for computing the elastic-plastic problems, and the effective material parameters are the real material parameters of the hardening material.
     2. Proposed several novel Mindin plate and shell elements
     A formulation of Mindlin-Reissner plates is proposed using the cell-based smoothed radial point interpolation method with sub-domain smoothing operations. Effective treatment for shear-locking are given and a plate formulation with high accuracy of stress is obtained.
     A novel quadrilateral plate and shell element is presented based on the smoothed finite element method. An average shape function is proposed, and shear locking phenomenon is avoided by using only one smoothing cell. Area integration over each smoothing cells is recast into line integration along its edges, and the shape functions are obtained directly in the Cartesian coordinate, hence no mapping is needed and extremely distorted elements can be used. This quadrilateral plate and shell element provides very stable and accurate results with the low computational effort compared, which is very effective for engineering analysis.
     A novel edge-based smoothed triangular plate and shell element theory is proposed, and an edge local coordinate system is introduced for performing strain smoothing operations for the elements consisting the smoothing domain not in a plane. The present method can be easily implemented with little changes to the esxisting triangular shell elements. The new element is very high accuracy and not sensitive to mesh distortion, which is very useful to engineering analysis.
     3. Novel thin plate and shell element theory using linear interpolation
     This thesis presents a way to solve the4th order boundary value problems using simple linear point interpolation method, and a rotation-free Euler-Bernoulli beam element is proposed. A novel numerical theory is formed, and the feasibility of this theory is proved.
     A novel thin plate element theory is proposed based on a continuity re-relaxed technique to overcome the high continuity requirement for thin plate formulation which need the high computation cost. The curvatures over integration domains are restructured through the divergence theorem, and the continuity requirement of the trial deflection function for thin plate problems can be re-relaxed. Using the proposed continuity re-relaxed technique, the weak form of a2k order PDE can be modeled using shape functions of k-1consistence. The C1continuity thin plate problem can be easily formulated and stable results can be obtained only using Co interpolating functions.
     Based on the radial point interpolation method and triangular background cells, several types of smoothing domains are constructed, and different thin plate formulation using continuity re-relaxed technique. The properties of the different smoothing domains constructing scheme are discussed, the efficiency and accuracy of the novel thin plate element theory are confirmed.
     A curvature-constructed method (CCM) for thin plate problems is proposed using three-node triangular cells and assumed piecewisely linear deflection field. The slopes at special points are obtained using the gradient smoothing techniques (GST) over different smoothing domains. The curvature in each cell can be constructed using these slopes, and then used to create the discretized system equations. Some schemes are devised using the present CCM and the numerical results are presented, to test the efficiency and accuracy of these schemes.Outstanding scheme is choosed for constructing shell formulation.
     4. Dynamic explicit formulation of proposed numerical method and shell element for large deformation analysis
     An edge-based gradient smoothing based on weakened weak form is proposed for the simulation of metal forming processes, which involves geometric, material, and contact non-linear. A simple but general contact searching algorithm is used to treat the contact interface and an algorithm for the contact force is presented. The smoothed deformation gradient and the smoothed stress tensor are derived at the reference (or initial) configuration. The integration domains are different from the interpolation domain, and the method can work for extremely distorted meshes. Solutions to upsetting, extrusion of metals with large material distortions are given to show the effectiveness and efficiency of the proposed method.
     The explicit dynamic formulations of two outstanding triangular shell elements are extended for large deformation analysis. The nodel internal force, external force and inertial force are obtained using the weakened-weak form. Various numerical tests demonstrate the accuracy and efficiency of the proposed triangular shell elements.
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