基于第二类四边形面积坐标的弹性力学纯弯解及MacNeal局限定理的破解

为了提高有限元的性能,弹性力学的解析解(齐次方程的通解)常常可用作有限元的试探函数。然而单元自由度数与完备的直角坐标解析解个数并不匹配,不完备的试函数会导致单元有方向依赖性。利用新型局部自然坐标——第二类四边形面积坐标QACM-II(S,T),给出了平面问题对应任意方向纯弯曲状态的应力函数解析解,即S3和T3的线性组合,并推导出了这两组应力函数对应的应力、应变和位移解析解。之后,利用QACM-II表示的解析解构造了非对称的平面4节点8自由度单元USQ4,该单元可以同时通过常应力/应变分片检验和纯弯测试,从而破解了MacNeal局限定理对平面低阶单元的限制。     

飞机撞击建(构)筑物研究进展

飞机撞击重要建(构)筑物会导致灾难性后果。本文从试验研究、理论分析、数值模拟等3个方面对飞机撞击建(构)筑物的国内外研究现状，相关研究难点、需要注意的问题和研究方向及趋势进行总结，包括:缩比试验的系统和验证，飞机撞击力模型，撞击所致局部破坏计算公式，飞机和建(构)筑物的精细化建模，撞击所致振动特性，撞击荷载和火荷载对结构的耦合毁伤效应，一般模型和精细化模型、解耦和耦合方法以及不同数值模拟程序计算结果的对比分析等方面，以期为后续研究提供参考。     

一个不闭锁和抗畸变的四边形厚板元

构造一个彻底消除剪切闭锁现象并且对网格畸变不敏感的四边形厚薄板通用单元RPAQ。在方法上有三个特点：第一，在厚板挠度和转角的试函数中，采用了合理匹配方案，从而在源头上彻底消除了剪切闭锁现象；第二，采用四边形面积坐标，以代替通常的等参坐标，从而使网格畸变时仍然保持高精度；第三，采用广义协调元做法，使协调条件的采用灵活多样，并保证单元的收敛性。进行了一系列数值例题测试，表明单元RPAQ能自动消除闭锁现象，在由薄板到厚板的不同情况下，在各种网格畸变的情况下，都能体现出良好的精度和数值稳定性。     

Characteristic equation solution strategy for deriving fundamental analytical solutions of 3D isotropic elasticity

A simple characteristic equation solution strategy for deriving the fundamental analytical solutions of 3D isotropic elasticity is proposed. By calculating the determinant of the differential operator matrix obtained from the governing equations of 3D elasticity, the characteristic equation which the characteristic general solution vectors must satisfy is established. Then, by substitution of the characteristic general solution vectors, which satisfy various reduced characteristic equations, into various reduced adjoint matrices of the differential operator matrix, the corresponding fundamental analytical solutions for isotropic 3D elasticity, including Boussinesq-Galerkin (B-G) solutions, modified Papkovich-Neuber solutions proposed by Min-zhong WANG (P-N-W), and quasi HU Hai-chang solutions, can be obtained. Furthermore, the independence characters of various fundamental solutions in polynomial form are also discussed in detail. These works provide a basis for constructing complete and independent analytical trial functions used in numerical methods.     

截面含切口圆柱体自由扭转的弹塑性分析

对于截面含切口圆柱体的弹塑性自由扭转问题的分析,可按受力特点分为三个阶段:全弹性阶段、全塑性阶段和弹塑性阶段.每一阶段对应的分析方法不同,其中,在全弹性阶段可以采用有限差分法分析;在全塑性阶段可以按沙堆比拟的方法采用等倾曲面模拟;弹塑性阶段可以结合上述两种方法的结果和思路进行分析.利用差分法可以求出自由扭转截面内各离散点应力函数φ的数值解.本文推导了自由扭转的应力函数φ与J积分之间的关系,得出了自由扭转的应力函数与Ⅲ型裂纹的J积分之间的关系式.数值计算结果验证了本文方法的有效性和精确性.     

一种求三维弹性力学基本解析解的特征方程解法

提出了一种简单的推导各向同性材料,三维弹性力学问题基本解析解的特征方程解法.应用三维问题控制微分方程的算子矩阵,通过计算其行列式可得到问题特征通解所需满足的特征方程.将满足各种不同简化特征方程的特征通解,代入到微分方程算子矩阵所对应的不同的缩减伴随矩阵,可推导得出相应的三维弹性力学问题的基本解析解,包括B-G解、修正的P-N(P-N-W)解和类胡海昌解.进一步对各类多项式形式的基本解析解的独立性进行了讨论.这些工作为构造数值方法中所需的完备独立的解析试函数奠定了基础.     

A hybrid-stress element based on Hamilton principle

A novel hybrid-stress finite element method is proposed for constructing simple 4-node quadrilateral plane elements, and the new element is denoted as HH4-3fl here. Firstly, the theoretical basis of the traditional hybrid-stress elements, i.e., the Hellinger-Reissner variational principle, is replaced by the Hamilton variational principle, in which the number of the stress variables is reduced from 3 to 2. Secondly, three stress parameters and corresponding trial functions are introduced into the system equations. Thirdly, the displacement fields of the conventional bilinear isoparametric element are employed in the new models. Finally, from the stationary condition, the stress parameters can be expressed in terms of the displacement parameters, and thus the new element stiffness matrices can be obtained. Since the required number of stress variables in the Hamilton variational principle is less than that in the Hellinger-Reissner variational principle, and no additional incompatible displacement modes are considered, the new hybrid-stress element is simpler than the traditional ones. Furthermore, in order to improve the accuracy of the stress solutions, two enhanced post-processing schemes are also proposed for element HH4-3β. Numerical examples show that the proposed model exhibits great improvements in both displacement and stress solutions, implying that the proposed technique is an effective way for developing simple finite element models with high performance.     

国际非均匀材料力学会议（ICHMM）简介

为了提高非线性有限元的教学和科研水平，经中国力学学会和北京市力学学会批准，清华大学工程力学系于2004年6月17日～19日在北京举办了全国非线性有限元讲习班，聘请了美国西北大学Ted Belytschko教授讲学3天，题目为非线性有限元与计算力学。