扭转作用下有缺陷柱形壳的屈曲和后屈曲性能

分析扭转作用下有缺陷柱形壳的屈曲和后屈曲性能。基于Karman-Donnell-Type非线性微分方程建立计算公式,采用壳屈曲的边界层理论进行分析,以获得能严格满足边界条件的解决方案。采用奇摄动技术,以确定屈曲载荷和后屈曲平衡路径。数值结果显示,目前的理论能对柱形壳的后屈曲性能进行较好评估。同时分析了几何参数对柱形壳的屈曲和后屈曲性能的影响。证实了扭转作用下柱形壳的后屈曲平衡路径并不稳定,并且相对更短的壳体具有更高的后屈曲平衡能力。最后,指出初始缺陷对柱形壳屈曲和后屈曲性能的影响。对具有初始横向挠曲的有缺陷壳体的分析结果显示：即便是非常小的缺陷,也确实会减少屈曲承载力,并使得后屈曲稳定性变差。扭转作用下柱形壳的屈曲和后屈曲性能显示出明显的缺陷敏感性。此外,如果缺陷更大,那么带来的影响也随之会变得更大。     

扭矩和轴压共同作用下各向异性薄壁和中厚壁圆筒的屈曲性能

采用基于考虑了各向异性和横向剪切刚度影响的厚壳理论的分析模型,研究各向异性、横向剪切刚度、长度和它们的相互作用对纯扭或者扭矩与轴压共同作用下圆筒屈曲性能的影响。由于该模型仅轴压下的屈曲性能得到验证,故将轴压下分析结果与扭转作用下作对比。结果表明,该模型对扭矩作用下屈曲性能亦有较好的适用性。研究表明,各向异性、横向剪切刚度及壳体长度均影响柱壳的临界力大小。这说明空心壳理论(Donnell理论)并不准确,应该采用考虑了各向异性和横向剪切刚度影响的厚壳理论。     

轴压圆柱薄壳的屈曲分析

通过对大量的试验数据进行数理统计分析,提出了计算轴心受压圆柱壳屈曲应力的经验公式,该式表明在其他因素相同的情况下,屈曲应力与壳体厚度t1.5成正比,而经典理论则认为屈曲应力与厚度t成正比.通过圆柱壳在自重作用下的屈曲试验和有限元分析,验证了该经验公式的正确性.非线性有限元全过程分析显示,圆柱壳在其荷载位移全过程曲线上存在一个近似水平的后屈曲承载力"平台"(Plateau),与之相应的是在壳壁上形成了一个局部"凹陷"(Dimple),而试验得到的平均屈曲应力约等于平台对应的后屈曲应力.由此发现,试验或经验公式所得到的实际上是圆柱壳的后屈曲应力,而并非其初始屈曲应力.承受局部轴向压力的圆柱壳,在压力作用点附近亦形成一个与后屈曲承载力平台对应的局部凹陷.同上述凹陷一样,这类凹陷需要一个近似常量的压力来维持其不稳定的后屈曲平衡.给出了该作用力的计算公式,且它与t2.5成正比,从而亦间接证明了后屈曲应力或经验公式确定的应力与t1.5成正比的关系.对以往试验现象作出了合理解释,提出的两个计算屈曲应力和局部屈曲荷载的公式具有理论意义和实用价值.     

防屈曲技术在某网架加固工程中的应用

结合工程实践提出了一种基于防屈曲技术的网架杆件加固方案,并进行了理论研究和数值分析。推导了小挠度状态下加固杆件的挠曲微分方程,根据边界条件和节间变形协调条件,得到杆件失稳特征方程的解析表达式。分别采用梁单元和壳单元对加固杆件的欧拉临界力及弹塑性极限承载力进行有限元分析,欧拉临界力理论值与计算值吻合良好,验证了理论公式的正确性。壳单元模型能够更精确地计算杆件在非线性条件下的极限承载力,梁单元模型计算结果则需要进行修正。研究表明提出的加固方案能够有效提高杆件稳定承载力,并且具有施工简便、快速高效等优点。     

网壳结构的屈曲分析研究(二):壳体屈曲和缺损的非线性分析

介绍了柯以特理论,在讨论了壳体临界荷载和敏感度、缺损的类型,并简单地回顾上个世纪60年代以来普遍采用的平衡路线跟踪方法的原理的基础上,提出基于广义稳定的概念、屈曲的定义及其数学描述的屈曲路线跟踪方法.通过分析,讨论了影响网壳结构屈曲类型及分枝荷载的因素,为空间格构结构中常用的网壳结构设计临界荷载值的确定提出了建议.     

GFRP管防屈曲加固槽钢构件轴压性能试验研究

基于传统防屈曲耗能支撑构件设计理念,通过对三种长细比、两种加固形式的槽钢单轴对称截面钢构件进行轴心受力性能试验研究,分别得到了9个试件的变形形态和失效模式。初步试验研究表明:相对于未加固的传统纯钢构件,GFRP管防屈曲加固槽钢构件明显改善了槽钢构件的延性和峰值变形能力,加固后试件抗压极限承载力提高了10%~62%。本文通过多种屈曲失稳理论公式对比,完善了FRP管防屈曲加固钢构件轴压稳定性能的计算公式。     

均匀外压下的锥壳减速机试验

有关壳结构屈曲性能的研究日益增多,尤其是锥壳。然而,有关细长柱端壳体减速机的足尺模型和实际模型的屈曲性能试验研究还较缺乏。试验研究和分析3个锥壳减速机在均匀周边压力下的屈曲性能。进行有限元模拟和理论分析,比较屈曲荷载和变形模式。在研究范围内,收集足尺锥壳减速机的有关数据。     

板件在单轴往复荷载作用下非线性屈曲分析的有限元法

本文根据板壳非线性有限元的基本理论，考虑几何和材料双重非线性，导出了分析板壳非线性问题的曲壳有限元法，并编制了计算程序。文中考虑了初始缺陷的影响，对板件在往复荷载作用下屈曲后性能和全过程性状进行了计算分析，并与试验结果进行了比较。     

防屈曲偏心支撑加固钢筋混凝土框架结构抗震性能试验研究

结合2榀1/2比例的3层单跨钢筋混凝土框架低周反复加载拟静力试验研究,对采用防屈曲偏心支撑加固的钢筋混凝土框架结构抗震性能开展研究。通过与未加固试件的对比,对采用防屈曲偏心支撑加固的钢筋混凝土框架结构的破坏形态、承载能力、变形能力、延性和耗能能力等抗震性能指标进行了深入分析。研究结果表明:经合理设计,防屈曲偏心支撑加固框架可有效实现预期的塑性铰机制,达到了防屈曲支撑先于框架结构屈服的设计目标,与未加固对比试件相比,采用防屈曲偏心支撑加固混凝土框架抗震性能显著提高,防屈曲偏心支撑加固混凝土框架具有良好承载能力、弹塑性变形能力和耗能能力。     

预应力FRP加固钢管的临界屈曲理论研究

预应力FRP加固技术应用于内压管线的修复、加固项目中,已经得到了人们的重视。本文针对预应力FRP加固钢管的临界屈曲进行了理论分析,重点分析了屈曲理论下钢管能承受的预应力极值。研究了钢管在预应力作用下其环向应力和径向应力的分布规律,获得了强度理论条件下的预应力极值P_(max1);基于壳体理论推导了预应力作用下钢管在径向方向的应变方程,得到了屈曲理论条件下的预应力极值P_(max2)。对比文献中的试验结果,通过本文分析得到的钢管径向应变的分布规律和文献给出的试验结果相一致。通过P_(max1)、P_(max2)获得了钢管在预应力作用下发生屈曲问题的临界条件,且P_(max1)、P_(max2)均随着径厚比的增加而递减,随材料屈服强度的增加而增加。     

Multiobjective optimization of orthogonally stiffened cylindrical shells for minimum weight and maximum axial buckling load

In the present research, the weight and axial buckling optimization of orthogonally stiffened cylindrical shells is carried out by the Genetic Algorithm. Constraints include two nondimensional functions of weight and buckling load in such a way that the stiffened shell has no increase in the weight and no decrease in the buckling load with respect to the initial unstiffened shell. In analytical solution, the Rayleigh–Ritz energy procedure is applied and the stiffeners are treated as discrete members. The optimization is implemented for shells with simply supported end conditions stiffened by four shapes of stiffeners including rectangular-, cee-, I-, and hat-shaped ones. The results show that the I-section and rectangular-section stiffeners are, respectively, the most and the least efficient in designing stiffened cylindrical shells for minimum weight and maximum critical axial buckling load.     

A semi-analytical model for local post-buckling analysis of stringer- and frame-stiffened cylindrical panels

A fast semi-analytical model for the post-buckling analysis of stiffened cylindrical panels is presented. The panel is comprised of a skin (shell) and stiffeners in both longitudinal (stringers) and circumferential direction (frames). Local buckling modes are considered where the skin may buckle within a bay and may induce rotation of the stiffeners. Stringers and frames are considered as structural elements and are thus not ‘smeared’ onto the skin. Large out-of-plane deflections and thus non-linear strain–displacement relations of skin and stiffeners are taken into account. The displacements of skin and stiffeners are approximated by trigonometric functions (Fourier series). First, a linear buckling eigenvalue analysis is carried out and some combination of buckling eigenmodes is chosen as imperfection. Then the load history is started and the Fourier coefficients are determined by minimizing the stiffened panel's energy at each load level. A curve-tracing algorithm, the Riks method, is used to solve the equations. The present model can be used to assess the post-buckling behavior of stiffened panels, for example, aircraft fuselage sections.     

Elastic buckling analysis of ring-stiffened cylindrical shells under general pressure loading via the Ritz method

This paper presents the Ritz method for the elastic buckling analysis of shells with ring-stiffeners under general pressure loading. The stiffeners may be of any cross-sectional shape and arbitrarily distributed along the shell length. Using polynomial functions multiplied by boundary equations raised to appropriate powers as the Ritz functions, the method can accommodate any combination of end conditions. As far as it is known, the Ritz method has not been automated in this way for the buckling of ring-stiffened shells. By formulating in a nondimensional form, generic buckling solutions for shells with various end conditions, stiffener distributions and under various pressure distributions, were presented. These new buckling solutions should serve as useful reference sources for checking the validity and accuracy of other numerical methods and software for buckling of cylindrical shells. This paper also shows that the appropriate distribution of ring stiffeners can lead to a significant increase in the buckling capacity over that of a stiffened shell with evenly spaced and identical ring stiffeners.     

An experimental study on externally pressurized stiffened and thickened cylindrical shells

It has long been identified that stiffening of steel shells is one of the most effective ways of enhancing the capacity of these structures. Stiffeners largely in the form of welded elements have been employed to strengthen shell structures in which the stiffeners generally cover the whole length of the structure. In this research the effect of partial and full length stiffening of shells was studied in which the stiffeners were attached without welding to avoid the adverse effect of the residual stresses. Furthermore, local thickening of the shells by the same stiffening strips was investigated and the results were evaluated against the plain specimen. The effect of strengthening provided by local thickening was slightly less but comparable to that provided by the stiffeners.     

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.     

Buckling and post-buckling behaviour of elastic seven-layered cylindrical shells – FEM study

The paper is devoted to buckling and post-buckling problems of an elastic seven-layered cylindrical shell under uniformly distributed pressure. The shell is an untypical sandwich structure composed of main corrugated core and two three-layer faces. Numerical FEM model for the shell has been elaborated. The calculations have been performed with the use of the ANSYS code for elastic shells of different dimensions. The linear and non-linear analyses of the shells have been performed with the use of the finite elements method. Critical pressure and equilibrium paths for the family of seven-layered shells subjected to uniformly distributed external pressure are calculated. The influence of corrugation pitch of main core and the length of the shell on the critical pressure has been determined. The results of these investigations are presented on the graphs.     

钢-混凝土组合薄壳屋盖的研究进展--施工阶段钢底壳的模型试验研究

钢-混凝土组合薄壳屋盖(Comshell屋盖体系)由兼作永久模板并替代钢筋的薄壁加劲钢底壳和现浇混凝土两部份组成。薄壁钢底壳由模块化单元件通过螺栓连接而成,单元件呈无盖扁盒状,由底板及周边板组成。其周边板在钢壳上构成两个方向的薄壁加劲板。这一新体系保留了混凝土薄壳屋盖的所有优点。又不需要使用临时模板．并大幅减少临时支撑。本文对这种新型结构体系及其各种可能的破坏模式进行了简单介绍,并给出了针对该结构体系施工阶段稳定性所进行模型试验的主要结果。     

Free vibration of non-uniformly ring stiffened cylindrical shells using analytical, experimental and numerical methods

In this research, the free vibration analysis of cylindrical shells with circumferential stiffeners, i.e. rings with non-uniform stiffeners eccentricity and unequal stiffeners spacing is investigated using analytical, experimental and finite elements (FE) methods. Ritz method is applied in analytical solution while stiffeners treated as discrete elements. The polynomial functions are used for Ritz functions and natural frequency results for simply supported stiffened cylindrical shell with equal rings spacing and constant eccentricity is compared with other's analytical and experimental results, which showed good agreement. Also, a stiffened shell with unequal rings spacing and non-uniform eccentricity with free–free boundary condition is considered using analytical, experimental and FE methods. In experimental method, modal testing is performed to obtain modal parameters, including natural frequencies, mode shapes and damping in each mode. In FE method, two types of modeling, including shell and beam elements and solid element are used, applying ANSYS software. The analytical and the FE results are compared with the experimental one, showing good agreements. Because of insufficient experimental modal data for non-uniformly stiffeners distribution, the results of modal testing obtained in this study could be as useful reference for validating the accuracy of other analytical and numerical methods for free vibration analysis.     

Non-standard stabilization of the post-buckling path for elastic–plastic cylindrical shells under combined state of loadings

The post-buckling behavior of elastic–plastic cylindrical shells under external pressure and twisting moment is unstable. The effect of stabilization of the post-buckling behavior is in most cases obtained by changing geometry of the structure. In this paper an alternative concept of modification of the post-buckling behavior is applied, namely modification of post-buckling path is achieved by application of additional loadings acting on the structure without changing the shape or sizes of the optimized element. Calculations are performed using ANSYS code for elastic–plastic shells of different length and thickness for the whole range of load from a pure external pressure to a pure twisting moment. It turned out that the active stabilizing force can always stabilize the post-buckling paths whereas the passive and ‘mixed’ variant of stabilizing loads are able to stabilize the post-buckling paths only for thicker elastic–plastic shells and for limited states of loading.     

Experimental Investigation of Damaged Stiffened Cylindrical Shells

Thin walled cylindrical shells, stiffened with rings and longitudinal stiffeners, are components in marine and aeronautical structures. The shells may become damaged in use and require some form of repair to restore some of the strength which may have been lost due to the damage. This paper presents the results of an experimental investigation into the effects of damage, and the efficacy of one method of repair, on an orthogonally stiffened shell. The shells were tested with combinations of external pressure and axial compressive loading. It is shown that local damage reduces the strength of the shell and that the addition of the chosen form of strengthening does not restore the intact strength. However, the efficacy of the repair is greater for a combination of pressure and axial loading.