A parametric finite element study of cracked plates and shells

The overall behavior of plates and shells as affected by the presence of a through crack in the elastic range has been studied. Due attention has been paid to finite element modeling aspects of the problem. Forty different cracked plate and shell FE models have been generated and analyzed by a special computer program developed for the purpose of this study. The significance of various parameters such as the order of mesh refining at the crack tip, the effect of boundary conditions, Poisson's ratio, crack length and shell curvature are studied. FE model consisting of isoparametric 4-noded shell elements moderately refined at the crack tip predicted the overall stress and displacement field with acceptable precision.     

空间网壳结构的理论研究

回顾了网壳结构的发展，介绍了网壳结构的计算模型及分析方法，分析了其稳定性，探讨了网壳结构的新型结构体系，指明了网壳结构在21世纪的发展动向及应用前景。     

自由曲面网壳合理壳面及其应力控制研究

通过选用较优的初应力分布来对最终应力分布进行控制的方式,提出了受力合理且满足建筑要求的自由曲面网壳目标壳面的生成方法,并对半球形网壳壳面算例进行了分析,结果表明:初应力均布时,所得最终应力的均匀性最佳;初应力布置时,对环向应力和边缘应力同时进行加强,最终径向应力与环向应力的协同性最佳。     

含面内转角自由度的四结点四边形单元

利用十二结点等参数单元的形函数，推导含转角自由度的四结点四边形膜元的单元刚度矩阵，该单元能直接与每个结点具有六个自由度的空间梁柱单元相联结，能有效地解决工程实际问题。     

Triangular higher-order element for better prediction of stress resultants and stresses in plated and shell structures

Although they furnish accurate displacements, conventional displacement-based lower order finite elements fail to predict accurate stress resultants and stresses in certain classes of plate and shell problems that involve free edges, steep stress gradients and singularities. In order to tackle such problems, a triangular higher-order shell element based on the nodal basis approach has been developed. The nodes of the element are located at optimal points and its more superior shape functions derived from orthogonal Proriol polynomials. To illustrate the improved performance of the higher-order element as compared to commonly used lower order shell elements in predicting the variations of stress resultants and stresses, three example problems involving a simply supported skew plate, a corner supported square plate, and a clamped cylindrical shell are solved. The stress resultants and the stresses furnished by the higher-order element for the problems considered are found to be accurate with the satisfaction of the natural boundary conditions and devoid of any oscillations. When compared to lower order elements, the higher-order element requires a simple mesh design and lesser degrees of freedom resulting in a considerable reduction in the computational effort, especially for large scale nonlinear analysis.     

Buckling of super ellipsoidal shells under uniform pressure

This paper is concerned with the elastic buckling of super ellipsoidal shells under external uniform pressure. The middle surface of a super ellipsoidal shell is defined by the following equation: (x/a)²ⁿ+(y/b)²ⁿ+(z/c)²ⁿ=1, where n is an integer varying from unity to infinity. It is clear from the equation that the range of shell shapes covered sphere (n=1, a=b=c) to cube () and ellipsoid (n=1) to cuboid (n=∞). By adopting a recently proposed solid shell element for the buckling analysis, the critical buckling pressures of thin to thick super ellipsoidal shells are obtained and tabulated for engineers. The shell element allows for the effect of transverse shear deformation which becomes significant in thick shells. Their buckling shapes are also examined. In addition, a simple approximate formula for predicting the critical buckling pressure of thick spherical shells is proposed.     

Buckling of super ellipsoidal shells under uniform pressure

This article is concerned with the elastic buckling of super ellipsoidal shells under external uniform pressure. The middle surface of a super ellipsoidal shell is defined by the following equation: (x/a)²ⁿ +(y/b)²ⁿ + (z/c)²ⁿ  = 1 where n is an integer varying from unity to infinity. It is clear from the equation that the range of shell shapes covered sphere (n = 1, a = b = c) to cube (n = ∞, a = b = c) and ellipsoid (n = 1) to cuboid (n = ∞). By adopting a recently proposed solid shell element for the buckling analysis, the critical buckling pressures of thin to thick super ellipsoidal shells are obtained and tabulated for engineers. The shell element allows for the effect of transverse shear deformation, which becomes significant in thick shells. Their buckling shapes are also examined. In addition, a simple approximate formula for predicting the critical buckling pressure of thick spherical shells is proposed.     

Shallow shell finite element for the large deflection geometrically nonlinear analysis of shells and plates

An investigation into the large deflection, geometrically nonlinear behaviour of shells is carried out in the present paper. The finite element method is used in conjunction with linearised incrementation and the Newton-Raphson iterative technique.The finite element used is based on independent strain assumptions insofar as it is allowed by the compatibility equations. Strain-displacement relationships based on shallow shell formulation are used and applied to an element having three principal curvatures. The resulting element has the only essential external degrees of freedom, satisfies the exact requirement of strain free rigid body modes of displacements and can be used for the representation of cylindrical, spherical and hyperbolic paraboloid shells.Complex load-deflection curves are obtained for cylindrical and spherical shells by incrementing loads as well as deflections. The relative behaviour of cylindrical and spherical panels having the same overall dimensions are also discussed in terms of stiffness, instability and snap-through phenomena.     

A comparative study of concrete properties using coconut shell and palm kernel shell as coarse aggregates

The high cost of conventional building materials is a major factor affecting housing delivery in Nigeria. This has necessitated research into alternative materials of construction. This paper presents the results of an investigation carried out on the comparative cost analysis and strength characteristics of concrete produced using crushed, granular coconut and palm kernel shells as substitutes for conventional coarse aggregate in gradation of 0%, 25%, 50%, 75% and 100%. Two mix ratios (1:1:2 and 1:2:4) were used. A total of 320 cubes of size 100×100×100 mm were cast, tested and their physical and mechanical properties determined. The results of the tests showed that the compressive strength of the concrete decreased as the percentage of the shells increased in the two mix ratios. However, concrete obtained from coconut shells exhibited a higher compressive strength than palm kernel shell concrete in the two mix proportions. The results also indicated cost reduction of 30% and 42% for concrete produced from coconut shells and palm kernel shells, respectively. Considering the strength/economy ratio, it was concluded that coconut shells were more suitable than palm kernel shells when used as substitute for conventional aggregates in concrete production.     

基于离散单元法的单层网壳结构冲击破坏#br# 数值模拟研究

基于传统离散单元法的接触模型,通过能量守恒定律推导适用于杆系结构的离散元模型接触刚度。将分层梁理论引入到离散单元法中,给出接触的塑性判断准则以及推导接触进入塑性后的刚度系数,从而可以应用离散元方法解决杆系结构的弹塑性问题。通过定义离散单元法基于应变的断裂准则,将其应用于解决杆件的断裂问题。选取适用于碰撞问题的赫兹模型,给出赫兹模型刚度的设置方法。最后应用改进后的离散单元法对单层网壳冲击试验进行了考虑弹塑性、断裂、碰撞的全过程模拟,验证了该方法的正确性。并对Kiewitt单层球面网壳单点冲击和多点冲击进行数值模拟研究,分析了单点和多点冲击下单层网壳的破坏模式、冲击力、节点位移和支座反力的异同点。     

网壳结构基于性能的抗震设计方法研究

基于性能的抗震设计是结构抗震设计的发展趋势,该文在考虑材料损伤及损伤累积的基础上,建立网壳结构的数值模型,通过基于IDA的全过程分析方法,考察网壳结构强震下的抗震性能;建立了分别基于多项特征响应和变形能量的结构损伤模型,对比两种损伤模型的精度,实现结构不同损伤状态的量化,并利用此模型量化结构的分级性能水准;通过多组地震动的IDA分析,实现结构分级性能水准易损性分析和综合易损性分析;确定结构性能目标,初步实现结构的优化设计,从而为建立网壳结构完整的基于性能的抗震设计方法奠定了基础。     

Experimental studying of buckling of stringer cylindrical shells under axial compression

Results of tests on axial compression of small-sized quality steel cylinder shells strengthened by 24 and 36 longitudinal thin-walled stiffeners are presented. The shell length was varied. Shells both with inside and outside stiffening were tested at simply supported and clamped edges. The shell carrying capacity that was governed in the tests by overall buckling in the elastic range was compared with the estimated critical loads based on structural-orthotropic theory. The satisfactory quantitative correlation has been received only for the long simply supported shells with 36 inner stiffeners, which demonstrated insignificant effect of local undulation that preceded overall deflections. The experimental and the theoretical results differed significantly (twice as much) when the actual mechanism of lateral deflection caused by the intensive local undulation differed from the adopted model.     

Free vibration analysis of joined conical shells: Analytical and experimental study

Natural frequencies and mode shapes of two joined isotropic conical shells are presented in this study. The joined conical shells can be considered as the general case for joined cylindrical–conical shells, joined cylinder–plates or cone–plates, conical and cylindrical shells with stepped thicknesses and also annular plates. Governing equations are obtained using thin-walled shallow shell theory of Donnell and Hamilton׳s principle. The continuity conditions at the joining section of the cones are appropriate expressions among stress resultants and deformations. The equations are solved assuming trigonometric response in circumferential and series solution in meridional directions and all combinations of boundary conditions can be assumed in this method. The results are compared and validated with the available results in other investigations and also modal testing. The effects of semi-vertex angles and meridional lengths on the natural frequency and circumferential wave number of joined shells are investigated.     

不同应力路径下旋转硬化方程的统一表示及验证

在收集到的大量试验数据的基础上,结合旋转硬化表达式,提出一个适用于不同应力路径下的旋转硬化表达式,该公式中共有两个参数,通过理论分析得到这两个参数的确定方法。将该公式导入修正剑桥模型来模拟土体在不同加载路径下的应力应变行为,将模拟结果与试验结果进行对比,结果显示,采用该旋转硬化表达式的各向异性本构模型得到的结果与试验结果均吻合较好,从而验证了该旋转硬化表达式的合理性。     

Analysis of multiple stiffened barrel shell structures by strain-based finite elements

In this paper strain-based rectangular cylindrical shell and curved arch finite elements are coupled to straight beam finite elements to analyse multiple stiffened barrel shell structures. The first two finite elements are based on assumed strains rather than displacement fields. The results obtained with this analysis are successfully compared with values derived from several commercial computer codes.     

索拉穹顶结构动力特性分析

索拉穹顶是一种新型大跨空间结构,比较了索拉穹顶和普通穹顶的自振频率及其对应的振型且较详细地讨论了这种新型结构的动力特性。然后,计算了各种参数(长跨比、矢跨比、索截面、索初始预应力、钢杆件截面)对索拉穹顶动力特性的影响。计算结果显示:索拉穹顶的动力特性优于普通穹顶;索拉穹顶的自振频率随矢跨比的增大,先增大后减小,随钢杆件截面、索截面的增大而增大;索的初始预应力对索拉穹顶的动力特性影响很大。     

圆柱形网壳的塑性极限分析

本探讨了圆柱形网壳结构的塑性极限分析，在分析过程中，采用拟壳法将圆柱形网壳结构等效为连续的夹层壳，然后对等效夹层壳进行塑性极限分析，由于夹层壳结构的极限条件是线性的，因而在求解时问题得到了大的简化，鉴于有矩理论的复杂性，本在作极限分析时，采用了无矩理论和半无矩理论的平衡方程分别作为中短壳和长壳的平衡方程。     

On the buckling of cylindrical shells with through cracks under axial load

Presence of cracks or similar imperfections can considerably reduce the buckling load of a shell structure. In this paper, the buckling of cylindrical shells with through cracks has been studied. A general finite element model has been proposed, verified and applied to some novel cracked shell buckling problems for which documented results are not available. A special purpose program has been developed for generating finite elements models of cylindrical shells with cracks of varying length and orientation. The buckling behavior of cracked cylinders in tension and compression has been studied. The results of the analysis are presented in parametric form when it seems to be appropriate. Sensitivity of the buckling load to the crack length and orientation has also been investigated.     

Stability analysis of cross-ply laminated shells of revolution using a curved axisymmetric shell finite element

A curved axisymmetric shell finite element based on a consistent first-order shear deformable shell theory is developed for the linear stability analysis of cross-ply laminated shells of revolution under compressive loads. Finite element analysis results are presented for isotropic, orthotropic and cross-ply laminated shells of revolution in comparison with the analytical and numerical results found in the literature. These comparisons demonstrate the applicability and the high performance of the element in stability analysis of thin and moderately thick cross-ply laminated composite shells of revolution under compressive loads.     

A triangular conforming element for laminated shells

A 30 degree of freedom (DOF) conforming shell element is developed for laminated composite materials. This element is 10-noded and has a triangular shape. Sander's thin shell theory is used. The element is used to perform both static and dynamic analysis on a composite cylindrical shell. Results obtained by the present element are compared with those available in the literature (exact, experimental, and numerical) for simple support and cantilever boundary conditions. These comparisons show that one can get reasonably accurate results with the present element and good convergence characteristics.