前屈曲应力状态对组合加肋旋转壳失稳临界压力影响的探讨

推导了包含前屈曲弯矩和横剪力的旋转壳弹性稳定性基本方程.运用Riccati传递矩阵法对组合加肋旋转壳算例进行了稳定性分析,并与假设前屈曲状态为薄膜应力状态计算出的失稳临界压力进行了比较.结果表明,前屈曲弯矩和横剪力对组合加肋旋转壳失稳临界压力的影响较小,随着壳板厚度和肋骨尺寸的增大及肋骨间距的缩短,影响略有增大.因而,分析组合加肋旋转壳弹性稳定性时,前屈曲状态采用薄膜应力状态的假设是合理的.     

环壳屈曲的渐近解

本文提出分析圆环壳屈曲的一种渐近解析方法,由Sanders非线性平衡方程和壳中面变形协调方程推导出静水外压下环壳的稳定方程,求出了方程的渐近解,理论计算的临界压力值与Fishlowitz的实验结果符合良好,并研究了屈曲前非线性变形对临界载荷的影响。     

受压油管屈曲变形与内外层杆管接触分析

针对机械采油工程中杆管偏磨问题, 考虑直井内受压油管屈曲变形与套管、 抽油杆的双层接触, 以及油管和抽油杆的变截面结构和环空间隙, 采用有限元结构稳定性分 析方法对受压油管进行失稳计算, 将求得的多阶失稳波形叠加值作为油管柱的初始位移, 再 采用大位移和接触非线性分析方法模拟受压油管屈曲变形, 以及与套管和抽油杆间的接触摩 擦, 为套管内受压油管屈曲变形分析提供了一种计算方法.     

圆截面杆在曲线井中的屈曲问题

本文研究了线弹性的圆截面压杆在刚性井壁的曲线井中的屈曲问题。压杆两端为铰支，杆和井壁之间的摩擦力忽略不计，失稳前假设压杆位于曲线井的较低侧。分析了压杆屈曲时的几何变形条件，导出了压杆的曲率，变形能和各项外力势的表达式，用最小势能原理导出压杆失稳的微分方程和边界条件。引入梁单元，用有限单元法求得了压杆失稳时的临界压力和杆轴线状态。结果显示，临界压力随着压杆自重和曲线井半径的增加而增加。自重为零时，杆轴线为半个正弦曲线；自重不为零时，杆轴线为振荡的曲线，且振荡主要集中在压杆顶部。     

波形钢板-混凝土组合剪力墙的非线性有限元分析

以波形钢板-混凝土组合剪力墙的试验研究为基础,利用ANSYS建立了考虑黏结滑移的有限元模型;分析了混凝土墙体和波形钢板在试件加载过程中的受力性能,通过将有限元模型计算与试验所得的荷载-位移曲线进行对比分析,得出计算结果与试验结果吻合较好的结论。研究了栓钉对波形钢板和混凝土变形协调的影响,结果表明:墙体下部栓钉滑移量大于上部栓钉滑移量,可增大下部栓钉强度,以改善波形钢板和混凝土墙体的变形协调关系。分析了不同参数对波形钢板-混凝土组合剪力墙受力性能的影响,包括栓钉直径、栓钉间距、钢板厚度、墙体厚度、轴压比等。分析结果表明:增大栓钉间距会降低组合墙承载力和延性;栓钉直径、波形钢板厚度、配筋率对试件承载力影响不大;波形钢板厚度为2mm或轴压比为0.40时,试件延性明显降低;型钢厚度、墙体厚度、轴压比、剪跨比对试件承载力影响较大。     

考虑范德华力曲率效应的双壁碳纳米管外压屈曲

针对双壁碳纳米管外压屈曲问题,研究了层间范德华力的曲率效应对临界外压的影响。应用弹性双层圆柱壳模型,考虑层间范德华力不仅与层间距有关而且与挠度曲率的变化有关,导出了外压屈曲临界压力解析公式。计算得出在不同半径、不同长细比下,外压屈曲临界压力的数值结果,并与经典壳的结果和忽略范德华力曲率效应的结果做了比较。结果显示,对于小半径的双壁碳纳米管曲率效应对外压屈曲有效明显的影响。     

裂纹对悬臂梁板振动频谱的影响

1.引言众所周知,弹性体(梁、板、壳)内产生裂纹后,由于刚度减小,使其固有力学特性,如固有频率、屈曲临界压力等随之降低,因此可以利用测频方法来监视裂纹发生与扩展.例如汽轮机的长叶片、压气机的叶片可分别简化为悬臂梁与悬臂板,依据理论分析和实测频率值,可以判断裂纹产生的部位和大小.以往,在分析梁的力学特性时,经常将裂纹对梁变形的影响用一弹性铰表示.Liu等用此模型得出屈曲临界压力与裂纹深度的关系.冈村弘之等也用此模型研究两端简支梁和具有集中质量悬臂梁(只考虑梁的弹性,不计梁的自重)的横向振动,计算结果与其实验值比较符合.对于裂纹板的分析,等采用经典的Ritz法得到悬臂板一至八阶自振频率和振型,并与实验结果作了比较.     

STUDY ON AMPLIFICATION FACTOR OF FLEXURAL BUCKLING CRITICAL LOAD OF AXIAL COMPRESSION BARS1)

为了建立一般条件下轴压构件屈曲临界载荷的计算理论,首先对轴心受压构件发生屈曲时的总势能方程进行了推导,然后采用Rayleigh-Ritz法并基于势能驻值原理得到了4种不同端部约束条件下轴压构件的屈曲临界载荷,对比欧拉临界载荷,给出了临界载荷放大系数 的计算式,全面考虑了构件长细比、压缩变形、剪切变形以及截面形状系数对临界载荷的影响,推导的计算式可用于较小长细比轴压构件发生屈曲时临界载荷的计算.圆截面和双轴对称工字形截面轴压构件屈曲临界载荷的分析表明构件长细比是影响放大系数的主导因素。     

轴压构件屈曲临界载荷放大系数的研究

为了建立一般条件下轴压构件屈曲临界载荷的计算理论,首先对轴心受压构件发生屈曲时的总势能方程进行了推导,然后采用Rayleigh--Ritz法并基于势能驻值原理得到了4种不同端部约束条件下轴压构件的屈曲临界载荷,对比欧拉临界载荷,给出了临界载荷放大系数的计算式,全面考虑了构件长细比、压缩变形、剪切变形以及截面形状系数对临界载荷的影响,推导的计算式可用于较小长细比轴压构件发生屈曲时临界载荷的计算.圆截面和双轴对称工字形截面轴压构件屈曲临界载荷的分析表明构件长细比是影响放大系数的主导因素。     

弯曲井眼中受压管柱的屈曲分析

本文通过微元体的变形和受力分析，导出了弯曲井眼中受压管柱的屈曲方程──一个合参数。的四阶非线性常微分方程。ε是一个综合考虑轴压（Ｆ０）、管柱截面抗弯刚度（ＥＪ）、井眼轴线轨迹曲车半径（Ｒ）以及共眼有效半径（ｒ）的无因次参数。本文利用线性化方法及小参数摄动法求得了屈曲方程的一次分叉点ε１及二次分叉点ε２。当ε＞ε１时，管柱处于稳定状态，对应于屈曲方程的零解；当ε２＜ε≤ε１时，管柱处于正弦屈曲状态，对应于屈曲方程的周期解；当ε≤ε２时，管柱处于螺旋屈曲状态，对应于屈曲方程的螺线解。本文给出的弯曲井眼中受压管柱的两个临界屈曲载荷（与两个分叉声、对应），发生螺旋屈曲时的螺距等理论结果与他人发表的实验结果吻合较好。     

Crack buckling in soft gels under compression

Recent interest in designing soft gels with high fracture toughness has called for simple and robust methods to test fracture behavior. The conventional method of applying tension to a gel sample suffers from a difficulty of sample gripping. In this paper, we study a possible fracture mechanism of soft gels under uni-axial compression. We show that the surfaces of a pre-existing crack, oriented parallel to the loading axis, can buckle at a critical compressive stress. This buckling instability can open the crack surfaces and create highly concentrated stress fields near the crack tip, which can lead to crack growth. We show that the onset of crack buckling can be deduced by a dimensional argument com- bined with an analysis to determine the critical compression needed to induce surface instabilities of an elastic half space. The critical compression for buckling was verified for a neo-Hookean material model using finite element simulations.     

外套有/无约束条件下波纹管扭转特性的实验研究

波纹管是一种具有极高扭转刚度，但易于失稳的弹性薄壁壳体。由于波纹管通常在很小的扭转角度下就会发生失稳，限制了其在传递扭矩方面的用途。通过实验研究发现，波纹管扭转屈曲时其材料远未到达强度极限，使得材料的利用率较低。波纹管失稳以后依然可以在一定范围内承受增大的扭矩，其在弹性范围内的扭转角度远大于屈曲发生时的扭转角度。为提高波纹管在后屈曲阶段的承载能力，采用具有合适内径的刚性外套来约束其横向弯曲变形。通过实验研究与有限元分析相结合的方法，比较了后屈曲阶段外套在提高波纹管承载能力方面的作用，两者得到了一致的结论，外套约束对波纹管后屈曲阶段的承载增加作用明显。     

轴向应力波作用下圆柱壳塑性轴对称动力屈曲

运用有限元特征值分析方法对应力波作用下圆柱壳塑性轴对称动力失稳问题进行了研究。基于应力波理论和相邻平衡准则导出了圆柱壳轴对称动力失稳时的特征方程,在分析中同时考虑了应力波效应及横向惯性效应,把圆柱壳塑性动力失稳问题归结为特征值问题。通过引入圆柱壳动力失稳时的波前约束条件实现了此类问题的有限元特征值解法。计算结果揭示了圆柱壳塑性轴对称动力屈曲变形发展的机理,以及轴向应力波和屈曲变形的相互作用规律。     

含分层损伤复合材料层合板的压缩强度研究

给出了基于一阶剪切变形理论的含分层损伤层合板有限元分析模型,将含分层损伤层合板在压缩载荷作用下的强度破坏分析和屈曲破坏分析统一起来。先区分其破坏形式,然后再进行具体破坏分析,在屈曲特性分析中考虑了铺层强度破坏引起的刚度折减的影响,数值结果表明,该文给出的方法和结论对含分层损伤复合材料层合板的设计更具参考价值。     

Length dependence of critical measures in single-walled carbon nanotubes

This paper presents an investigation on the buckling behaviour of single-walled carbon nanotubes under various loading conditions (compression, bending and torsion) and unveils several aspects concerning the dependence of critical measures (axial strain, bending curvature and twisting angle) on the nanotube length. The buckling results are obtained by means of an atomistic-scale generalized beam theory (GBT) that incorporates local deformation of the nanotube cross-section by means of independent and orthogonal deformation modes. Moreover, some estimates are also obtained by means of non-linear shell finite element analyses using Abaqus code. After classifying the buckling modes of thin-walled tubes (global, local and distortional), the paper addresses the importance of the two-wave distortional mode (flattening or ovalization mode) in their structural behaviour. Then, the well known expression to determine the critical strain of compressed nanotubes, which is based on Donnell theory for shallow shells, is shown to be inadequate for moderately long tubes due to warping displacements appearing in the distortional buckling modes. After that, an in-depth study on the buckling behaviour of nanotubes under compression, bending and torsion is presented. The variation of the critical kinematic measures (axial strain, bending curvature and twisting angle) with the tube length is thoroughly investigated. Concerning this dependence, some uncertainties that exist in the specific literature are meticulously explained, a few useful expressions to determine critical measures of nanotubes are proposed and the results are compared with available data collected from several published works (most of them, obtained from molecular dynamics simulations).     

Sandwich modeling with an application to the residual strength analysis of a damaged compression panel

Finite element models of sandwich structures are often based on classical sandwich theory which reduces the essentially three-dimensional composite — a combination of two high strength faces separated by a light weight core — to a two-dimensional, deformable reference surface to which certain appropriate stiffness properties against stretch, bending and transverse shear are attached.The simplification introduced in this way is well established, but it suffers from a number of drawbacks that restricts its range of applicability rather severely. The drawbacks concern among others, the kinematic and dynamic boundary conditions that prohibit a realistic application of the loading along the edges. They also concern the inability of this model to study local effects such as buckling of the faces.In the present paper, we demonstrate how, with a relatively simple means, a sandwich model can be introduced that does not suffer from the deficiencies mentioned above. Moreover, this improved model provides the possibility to study the detrimental effect of delamination of the faces, and/or, decohesion zones between the core and faces, on the buckling strength of sandwich compression panels.The modification proposed here makes use of existing shell finite elements that are standard in many finite element codes. These shell elements are used to model the deformation of the faces of the sandwich, while the coupling between the two shell components is carried out by applying an appropriate penalty function that represents the deformation of the core.In this paper, we describe this model in some detail and solve an example problem involving the buckling of a sandwich plate with a partially debonded face plate.     

轴向应力波作用下圆柱壳弹性轴对称动力失稳有限元特征值分析

本文运用有限元特征值分析方法对应力波作用下圆柱壳弹性轴对称动力失稳问题进行了研究。基于应力波理论和相邻平衡准则导出了圆柱壳轴对称动力失稳时的有限元特征方程,在此方程中考虑了应力波效应及横向惯性效应,把圆柱壳弹性动力失稳问题归结为特征值问题。通过引入圆柱壳动力失稳时的波前约束条件实现了此类问题的有限元特征值解法。计算结果揭示了圆柱壳弹性轴对称动力屈曲变形发展的机理,以及轴向应力波和屈曲变形的相互作用规律。     

Torsional,flexural, and torsional-flexural buckling modes of a cylindrical shell under combined loading

Stability problems for cylindrical shells under various loading modes were considered in numerous papers. A detailed analysis of such problems can be found, e.g., in the monograph [1]. We refer to the solutions presented in this monograph as classical.For long cylindrical shells in axial compression, one of the buckling modes is the purely beam flexural mode similar to the classical buckling mode of a straight rod. It is well known that it can be studied by using the nonlinear or linearized equations of the membrane theory of shells. In [2], it was shown that, on the basis of such equations constructed starting from the noncontradictory version of geometrically nonlinear elasticity relations in the quadratic approximation [3], under the separate action of the axial compression, external pressure, and torsion, there are also previously unknown nonclassical buckling modes, most of which are shear ones.In the present paper, we show that the use of the above equations for cylindrical shells under compression and external pressure with simultaneous pure torsion or bending permits revealing the earlier unknown torsional, beam flexural, and beam torsional-flexural buckling modes, which are nonclassical, just as those found in [2]. The second of these buckling modes is realized when axially compressing forces are formed in the shell with simultaneous torsion, and the third of them is realized under compression combined with pure bending.It was found that, earlier than the classical buckling modes, the torsional buckling modes can be realized for relatively short shells with small shear rigidity in the tangent plane, while the second and third buckling modes can be realized for relatively long shells.     

Postbuckling of long unsymmetrically laminated composite plates under axial compression

An approximate solution is given for the postbuckling of infinitely long and unsymmetrically laminated composite plates. This solution is obtained by superposing a polynomial transverse displacement given by bending due to unsymmetric laminate configurations and a simple functional representation for the buckling mode in conjunction with the Galerkin method. Nondimensional parameters are used to express the approximate solution in a very simple and clear formulation. The results given by this solution for axial compression in the longitudinal direction are compared with the results given by the nonlinear finite element method (FEM) for finite length rectangular long plates. The influence of the boundary conditions on postbuckling response is also studied. For the FEM analysis, two different simply supported boundary conditions on the long edges of the plate are considered. It is found that these two sets of boundary conditions give different results for the buckling and postbuckling finite element analysis. In most cases the FEM analysis overestimate and, respectively, underestimate the approximate closed form solution, depending on the type of simply supported boundary condition considered. Thus, the approximate solution appears useful for design purposes as an averaged quantity between the two FEM analyses. Also, it is found that the reduced bending stiffness method can be successfully used for determining the approximate solution.     

The influence of mechanical deformation on the electrical properties of single wall carbon nanotubes

Recent experimental studies and atomistic simulations have shown that carbon nanotubes (CNTs) display strong interplay between the mechanical deformation and electrical properties. We have developed a simple and accurate method to determine atom positions in a uniformly deformed CNT via a continuum analysis based on the interatomic potential. A shift vector is introduced to ensure the equilibrium of atoms. Such an approach, involving only three variables for the entire CNT, agrees very well with the molecular mechanics calculations. We then study the effect of mechanical deformation on the band gap change of single wall CNTs under tension, torsion, and combined tension/torsion via the k-space tight-binding method. Prior studies without this shift vector lead to significant overestimation of the band gap change. It is established that the conducting CNTs may easily become semi-conducting ones subject to mechanical deformation, but the semi-conducting CNTs never become conducting ones upon deformation.