薄壁圆筒壳初始几何缺陷不确定性量化的极大熵方法

针对薄壁圆筒壳结构轴压屈曲载荷的缺陷敏感性以及真实几何缺陷的不确定性,提出一种基于实测缺陷数据和极大熵原理的初始缺陷建模与屈曲载荷预测方法。首先,将初始几何缺陷视为二维随机场,并利用实测缺陷数据和Karhunen-Loève展开法将初始缺陷的随机场建模转化为随机向量的建模;其次,利用极大熵方法确定随机向量的概率分布;最后,基于所构建的初始缺陷随机模型,利用MCMC抽样方法和确定性屈曲分析方法,进行随机屈曲分析并给出基于可靠度的屈曲载荷折减因子。数值算例表明,与直接假设随机场相关结构的方法相比,本文方法的结果是对薄壁圆筒壳屈曲载荷的一个更无偏估计。     

三维编织复合材料圆柱壳在外压作用下的屈曲分析

提出一种三维四向编织复合材料圆柱壳的细观力学模型,宏观上基于Reddy高阶剪切变形理论和广义Kármán型方程,采用奇异摄动法求解在固支边界条件下三维四向编织复合材料圆柱壳在外压作用下的屈曲和后屈曲.分析中同时考虑非线性前屈曲、大挠度和初始几何缺陷的影响.讨论了纤维体积含量、壳体几何参数等因素对圆柱壳屈曲行为的影响.     

易拉罐在轴-侧-扭-内压联合作用下的屈曲地貌

柱壳结构广泛应用于各个领域, 但由于其对初始缺陷较为敏感, 容易发生灾难性的屈曲失稳. 本文利用非线性有限元分析程序ABAQUS研究了柱壳屈曲问题, 并应用到了易拉罐的屈曲分析. 首先采用数值模拟的方法验证了Virot等学者的易拉罐屈曲试验结果, 然后为了获得屈曲的一些普适结果, 进一步考察了柱壳的屈曲表现. 对柱壳结构在不同载荷组合、不同几何参数作用下进行了细致分析. 为了讨论的直观, 本文绘制了柱壳结构在受到侧压-轴压载荷作用下外力-屈曲载荷-位移三维屈曲地貌图(称为landscape). 结果表明: 在侧压-轴压-扭转载荷作用下, 试件力-位移曲线出现了"cliff"(断崖)现象; 扭转载荷的施加不利于试件整体稳定性, 并造成了试件对初始缺陷的敏感性; 对于受到轴压-扭转载荷作用的试件, 本文定义承载力为零的平面为"sea level"(海平面)来区分试件破坏模式; 通过对不同边界条件的试件进行分析, 发现试件两端固定可以有效地增加结构的承载能力, 提高稳定性. 对柱壳结构内部充气可以大幅度提升结构的承载能力和稳定性, 减小对初始缺陷的敏感度.      

初始几何缺陷对网壳截面优化结果影响研究

单层网壳结构是缺陷敏感结构,初始几何缺陷不同对"网壳截面优化结果"有显著影响,该文研究了网壳杆件截面取值与缺陷分布、结构极限承载力之间的关系.结果表明:经截面优化设计的网壳结构最不利应力最早并且一直出现在初始几何缺陷较大处;优化寻优结果与初始几何缺陷分布有直接关系,增大缺陷较大处杆件截面能显著提高结构承载力;由于缺陷随机分布,经截面优化设计的网壳结构须校核其在不同缺陷下的稳定承载力以确保结构安全.     

内部填充泡沫铝的柱壳力学响应数值模拟

针对泡沫铝及其填充圆柱壳的变形模式复杂、理论分析困难的问题,在分析泡沫金属唯象本构模型的基础上,用Bilkhu/Dubois可压缩泡沫模型描述泡沫铝的力学行为,用随机几何缺陷描述结构的可能缺陷形式,采用有限元法对内部填充泡沫铝的圆柱壳结构在轴向载荷作用下的静、动态力学响应进行了数值模拟.数值模拟结果与实验结果较一致,表...     

矩形脉冲作用下复合材料层合扁球壳的冲击屈曲分析

研究了计及横向剪切的复合材料层合扁球壳在矩形脉冲载荷作用下的非线性动力屈曲问题;采用Galerkin方法得到以顶点挠度表达的动力响应方程,并用Runge-Kutta方法进行数值求解,应用Budiansky-Roth准则(简称B-R准则)确定冲击屈曲的临界荷载;讨论了壳体几何尺寸和物理参数对复合材料层合扁球壳冲击屈曲的影响;数值算例表明,该方法是可行的.     

考虑周长约束的圆柱薄壳初始几何缺陷随机场建模方法

利用随机场对圆柱薄壳结构的初始几何缺陷进行建模，并据此建立了一种用于含初始几何缺陷轴压圆柱薄壳屈曲分析的随机分析方法。首先，指出已有将圆柱薄壳初始几何缺陷表征为二维高斯随机场的方法会导致与实际不相符的初始几何缺陷，如圆柱周长显著增大或缩小的几何缺陷。其次，提出一种考虑周长不变约束的随机场建模方法，以剔除与实际不相符的随机几何缺陷。最后，基于所建立的初始几何缺陷随机场模型，利用非干涉多项式混沌展开法进行圆柱薄壳的随机屈曲分析，给出临界屈曲载荷的概率分布。数值试验结果表明，基于随机场理论的初始几何缺陷建模方法可有效刻画几何缺陷对结构承载能力的影响，而提出的约束随机场建模方法又能有效减小结果的分散性。     

复合材料板的几何非线性行为综述

本文概述了利用分析方法了解弹性层合板静态大挠度,后屈曲和非线性动态响应方面的进展.在对称层合板情况下不存在弯曲-拉伸耦合.应用了正交异性板或各向异性板的经典非线性理论,还提供了有关这些板的参考文献.简要评述了许多类型边界条件下一般层合板的非线性剪切变形理论和一般解法.在本综述中讨论的使复合材料板几何非线性行为复杂化的一些影响因素是:横向剪应力和正应力,转动惯量和面内惯量,面内初始边界力,几何缺陷,切口,以及非经典边界条件.     

复合材料板的几何非线性行为综述

本文概述了利用分析方法了解弹性层合板静态大挠度,后屈曲和非线性动态响应方面的进展.在对称层合板情况下不存在弯曲-拉伸耦合.应用了正交异性板或各向异性板的经典非线性理论,还提供了有关这些板的参考文献.简要评述了许多类型边界条件下一般层合板的非线性剪切变形理论和一般解法.在本综述中讨论的使复合材料板几何非线性行为复杂化的一些影响因素是:横向剪应力和正应力,转动惯量和面内惯量,面内初始边界力,几何缺陷,切口,以及非经典边界条件.      

充满液体的封闭圆柱壳受轴压时屈曲过程的实验研究

报导了对充满水的金属圆柱薄壳在轴向压缩时屈曲性能的实验结果。实验观察表明，充满液体的金属圆柱壳在轴压作用下的屈曲模态呈轴对称型，液体的内压与外部轴压随轴向缩短的变化趋势大致相同。由于壳内液体的存在，同内空的圆柱壳相比只是临界载荷略有提高，屈曲后承载能力并无显著降低，且对初始几何缺陷表现得远不象内空柱壳那样敏感。     

Effects of imperfections on the buckling response of compression-loaded composite shells

The results of an experimental and analytical study of the effects of initial imperfections on the buckling and postbuckling response of three unstiffened thin-walled compression-loaded graphite-epoxy cylindrical shells with different orthotropic and quasi-isotropic shell-wall laminates are presented. The results identify the effects of traditional and non-traditional initial imperfections on the non-linear response and buckling loads of the shells. The traditional imperfections include the geometric shell-wall mid-surface imperfections that are commonly discussed in the literature on thin shell buckling. The non-traditional imperfections include shell-wall thickness variations, local shell-wall ply-gaps associated with the fabrication process, shell-end geometric imperfections, non-uniform applied end loads, and variations in the boundary conditions including the effects of elastic boundary conditions. A high-fidelity non-linear shell analysis procedure that accurately accounts for the effects of these traditional and non-traditional imperfections on the non-linear responses and buckling loads of the shells is described. The analysis procedure includes a non-linear static analysis that predicts stable response characteristics of the shells and a non-linear transient analysis that predicts unstable response characteristics.     

Repeated buckling and its influence on the geometrical imperfections of stiffened cylindrical shells under combined loading

The present experimental study aims at providing better inputs for improvement of the buckling load predictions of stiffened cylindrical shells subjected to combined loading. The work focuses on two main factors which considerably affect the combined buckling load of stiffened shells, namely geometric imperfections and boundary conditions. Six shells with nominal simple supports were tested under various combinations of axial compression and external pressure. The vibration correlation technique is employed to define the real boundary conditions. The geometric imperfections of the integrally stiffened shells are measured in the present experiments in situ and are used as inputs to a multimode analysis which yields the corresponding “knockdown” factor for various combinations of loading. Thus, when employing the repeated buckling procedure for obtaining interaction curves, each point on the curve is adjusted (using the multimode analysis) for the measured “new” surface of the shell and this results in more realistic interaction curves. The geometrical imperfections of the preloaded shells can also serve as an input to the International Imperfection Data Bank for future studies on the correlation between the manufacturing method of the shell and their geometric imperfections.     

Experimental investigation of the buckling of shallow spherical shells

In the present paper, the buckling behavior of clamped thin shallow spherical shells under external pressure is studied. Seventy-nine plastic shells formed by thermovacuum process were tested. The distributions of initial geometrical imperfections and vertical displacements were minutely measured with a differential transformer. It was possible to control the symmetrical initial geometrical imperfection of each specimen.Results indicate that the buckling phenomena of shallow spherical shells vary greatly with the symmetrical initial imperfection parameter η. In the case of the geometrical parameter λ larger than 5.5, the amplitude of the asymmetrical displacement component with the bifurcation buckling wave calculated by Huang becomes large immediately before buckling. The validity of Huang's theory for an initially perfect shell is experimentally demonstrated.     

The computation of nonlinear instability for multilayer composite cylindrical shells

In this paper,energy method and finite difference method are used to Compute theinstability behavior of multilayered fiber composite cylindrical shells under axialcompression,hydrostatic pressure and torsion.The influences of initial imperfections,geometrical nonlinearities of shells and physical nonlinearities of the materials to thebuckling and postbuckling behavior of the shells are considered.The effect of transverseshear is also discussed.The computational results of this paper are well agreed with theexperimental data.     

Effect of multiple localized geometric imperfections on stability of thin axisymmetric cylindrical shells under axial compression

Stability of imperfect elastic cylindrical shells which are subjected to uniform axial compression is analyzed by using the finite element method. Multiple interacting localized axisymmetric initial geometric imperfections, having either triangular or wavelet shapes, were considered. The effect of a single localized geometric imperfection was analyzed in order to assess the most adverse configuration in terms of shell aspect ratios. Then two or three geometric imperfections of a given shape and which were uniformly distributed along the shell length were introduced to quantify their global effect on the shell buckling strength. It was shown that with two or three interacting geometric imperfections further reduction of the buckling load is obtained. In the ranges of parameters that were investigated, the imperfection wavelength was found to be the major factor influencing shell stability; it is followed by the imperfection amplitude, then by the interval distance separating the localized imperfections. In a wide range of parameters this last factor was recognized to have almost no effect on buckling stresses.     

Influence of Initial Geometric Imperfections on the Vibrations and Dynamic Stability of Elastic Shells

The results from studies into the vibrations and dynamic stability of thin elastic shells with initial geometric imperfections are analyzed. The corresponding dynamic problems are solved in both linear and nonlinear formulations. The influence of initial axisymmetric and nonaxisymmetric deflections on natural, forced, parametrically excited, and self-excited vibrations (flutter) is studied. The dynamic buckling of imperfect shells under short-term impulsive loading is examined. Some aspects of experimental investigation into the vibrations of shells with small geometric imperfections (deviations from the design shape) are considered     

Two non-probabilistic set-theoretical models for dynamic response and buckling failure measures of bars with unknown-but-bounded initial imperfections

This paper is concerned with the problem of comparison of two non-probabilistic set-theoretical models for dynamic response and buckling failure measures of bars with unknown-but-bounded initial imperfections. Two kinds of non-probabilistic set-theoretical models are convex models and interval analysis models. In convex models and interval analysis models, the uncertain quantities are considered to be unknown except that they belong to given sets in an appropriate vector space. In this case, all information about the dynamic response and buckling failure measures of bars is provided by the set of dynamic responses and buckling failure measures consistent with the constraints on the uncertain quantities. The dynamic response estimate is actually a set in appropriate response space rather than a single vector. The set estimate is the smallest calculable set which contains the uncertain dynamic response, but it is usually impractical to calculate this set. Two set estimate methods are developed which can calculate the time varying box or hyperrectangle, i.e. interval vector in the response space that contains the true dynamic response. Comparison between convex models and interval analysis models in mathematical proofs and numerical calculations shows that, under the condition of the outer enclosed ellipsoid from a hyperrectangle or an interval vector, the set dynamic response predicted by interval analysis models is smaller than that yielded by convex models; under the condition of the outer enclosed hyperrectangle or an interval vector from an ellipsoid, the dynamic response set calculated by convex models is smaller than that obtained by interval analysis models.     

The Stability and Load-Carrying Capacity of Cylindrical Shells with Axisymmetric Dents

The classical method of stability analysis and the reduced-stiffness method are used to evaluate the critical loads and the lower bounds of sensitivity to imperfections in buckling of longitudinally loaded elastic cylindrical shells with local imperfections in the form of axisymmetric longitudinal dents.