碟形封头压力容器在内压作用下的弹塑性屈曲及后屈曲行为研究

基于ABAQUS对受内压碟形封头压力容器的弹塑性屈曲及后屈曲行为进行分析.通过非对称网格剖分技术诱发结构的屈曲行为,采用Riks算法捕捉完整的屈曲及后屈曲路径,分析屈曲载荷和后屈曲形态.计算结果表明,在内压作用下碟形封头压力容器过渡区域出现波状的分叉屈曲形态,随着加载的继续,过渡段将逐渐生成肉眼可视的褶皱,进入后屈曲阶段.后屈曲路径可以分为第一阶段与第二阶段.对引入初始厚度缺陷的结果分析可知,初始缺陷厚度的布置将对后屈曲路径产生影响,并降低结构的分叉屈曲载荷.     

内压标准椭圆封头弹塑性屈曲分析及失效载荷研究

采用有限元弹塑性屈曲分析,考虑椭圆封头的最大形状偏差并在过渡区施加局部厚度减薄缺陷,针对不同材料和厚径比δ_e/D_i,较为系统地讨论了内压标准椭圆封头结构屈曲载荷P_(cr)与极限载荷P_L的变化规律。研究结果表明,厚径比小于某临界值时,屈曲载荷低于极限载荷,分析模型发生屈曲破坏;厚径比大于某临界值时,屈曲载荷高于极限载荷,分析模型发生强度破坏。不同材料分析模型厚径比临界值存在一定差异,对高强钢13MnNiMoR材料而言,GB 150—2011标准防止标准椭圆封头内压弹性屈曲失效的厚径比规定偏于冒进。本文结果可为内压薄壁标准椭圆封头设计准则提供指导。     

几何缺陷对复合材料加筋平板轴压屈曲影响研究

当前的设计准则要求复合材料结构在限制载荷下蒙皮不发生局部屈曲,因此对蒙皮屈曲载荷的准确预测至关重要。设计了三种构型复合材料加筋平板轴压试验件,研究蒙皮在轴压载荷下的屈曲和后屈曲;采用线性特征值有限元方法计算了蒙皮的轴压屈曲载荷;采用多阶屈曲模态线性组合的方式引入初始缺陷,应用非线性方法计算了蒙皮轴压屈曲载荷并进行了缺陷灵敏度分析。计算结果和试验结果对比表明:当前计算蒙皮屈曲的工程方法过于保守,有限元方法可以更准确的计算蒙皮的屈曲载荷;初始缺陷对复合材料平板屈曲载荷有一定影响,其影响大小与缺陷幅值息息相关,不考虑该缺陷,会得到非保守的计算结果。     

应变强化后容器的外压屈曲分析

在应变强化过程中,容器会发生塑性变形。为了解该变形对于容器外压稳定性的影响,确立了全面考虑强化影响的非线性屈曲分析方法,并选择不圆度作为初始几何缺陷,进行理想容器和含初始不圆度容器在强化前后的屈曲求解,通过比较屈曲载荷、屈曲模态以及载荷位移曲线的变化来探究应变强化过程对容器外压稳定性的影响规律。结果表明,建立的容器模型的屈曲载荷具有很强的不圆度敏感性;含初始不圆度的容器强化后的屈曲载荷提高,外压稳定性增强;应变强化过程可改善容器圆度,提高容器制造过程中壳体初始不圆度的最大允许值。     

电除尘器壳体的整体承载性能分析

研究电除尘器壳体结构的空间整体承载性能。用有限元方法进行壳体结构的特征值屈曲分析。根据壳体的施工和使用过程,考虑加载路径的影响,分三步施加荷载,进行完善壳体的承载能力非线性分析。将特征值屈曲模态和完善壳体结构的非线性极值点变形模态作为初始缺陷模态,进行带缺陷壳体的承载力非线性分析。特征值屈曲分析表明,结构的前60阶屈曲模态均为发生在立柱连接腹板、墙板和顶板的局部屈曲。完善壳体的非线性分析表明,结构的破坏主要由隔宽柱的弹塑性局部屈曲引起。带缺陷壳体的非线性分析表明,壳体对中间隔宽柱连接腹板上多处局部初始几何缺陷最为敏感,可使其承载力严重降低。电除尘器壳体的空间整体刚度较大,不会出现整体失稳破坏。壳体的失效是源于单个构件的破坏。单个构件失效后,壳体内不会发生内力重分布。工程中现行的平面分析设计方法是合理可行的。     

基于初始缺陷敏感性的轴压薄壁圆柱壳屈曲分析研究进展

屈曲是轴压薄壁圆柱壳结构最主要的失效模式之一。围绕轴压下薄壁圆柱壳结构的承载能力对初始缺陷具有高度敏感性这一问题,系统回顾半个多世纪以来国内外学者在考虑初始缺陷对轴压薄壁圆柱壳屈曲临界载荷影响方面所做的理论、试验以及数值模拟等工作。研究表明,目前已形成了两类等效考虑初始缺陷影响的方法,即：基于数值模拟的确定性分析方法和基于概率统计的不确定性分析方法。其中,前者主要针对初始几何缺陷,而后者则还包括非传统初始缺陷,如材料、厚度及加载缺陷等。两类方法在开展轴压薄壁圆柱壳屈曲载荷预测和设计方面各具特点和优势,相较于试验结果,多点扰动载荷法和单边界扰动法的预测精度最高,两者均能达到90%以上的预测精度且同时具有一定的安全裕度。最后展望未来基于初始缺陷敏感性的轴压薄壁圆柱壳屈曲研究方向和亟待开展的研究工作,为相关研究的开展提供了一定的参考和指导。     

含初始几何缺陷薄壁圆柱壳屈曲载荷的数值模拟方法研究

对3个薄壁圆柱壳试件进行了轴压屈曲试验,得到了屈曲临界载荷值和屈曲变形。基于试验结果,比较了弧长法、非线性稳定算法和显式动力算法3种有限元数值模拟方法的差异,其中非线性稳定算法的模拟结果与试验结果吻合最好,该方法可以比较准确地计算轴压屈曲临界载荷,误差在6.0%以内,且屈曲变形与试验吻合得较好。试验表明,初始几何缺陷的存在大幅降低了薄壁圆柱壳的屈曲临界载荷,试验值仅为理论值的42%左右。     

薄壁压力容器在内压作用下的弹塑性屈曲行为研究

基于ABAQUS对受内压薄壁压力容器的屈曲行为进行计算时,采用非对称的网格剖分是打破对称性、诱发分叉屈曲的有效方法。与在结构中引入初始缺陷的传统做法相比,该法具有更加客观真实,且不破坏原始结构的优势。通过一系列的数值试验,揭示薄壁压力容器在内压作用下的弹塑性屈曲规律。计算结果表明,在内压作用下薄壁压力容器过渡区域出现波状的分叉屈曲形态,随着加载的继续,过渡段将逐渐生成肉眼可视的褶皱。通过与试验结果以及传统方法对比,非对称网格剖分方式可以更加简单而有效地诱发结构的分叉屈曲行为,且与试验结果吻合较好。     

碟形封头在外压作用下的屈曲分析

采用有限元法分析了碟形封头在外压作用下的屈曲问题。通过静力学分析观察碟形封头内部的应力分布情况以判断封头发生屈曲的位置,并使用Newton—Raphson法和弧长法对封头进行非线性屈曲分析,追踪碟形封头发生屈曲失稳前后过程,获取其典型的结构特征。最后将稳定安全系数引入到有限元分析结果的处理中,从而得到可以在工程中应用的许用外压载荷。整个分析方法和结论对碟形封头的设计和校核具有实际的工程意义。     

含腐蚀缺陷弯管的极限载荷分析

应用有限元分析软件ANSYS对含腐蚀缺陷弯管在内压载荷作用下进行有限元分析。分析中考虑了材料非线性和几何非线性的影响,并根据Binfu失效准则,计算分析出弯管不同缺陷尺寸对弯管极限载荷的影响、弯管缺陷处的应力分布状态以及缺陷处随内压改变而变化的应力应变图。根据腐蚀区的应力应变图,对模型的塑性极限载荷压力进行预测,得出了缺陷尺寸对塑性极限载荷的影响及变化规律。     

Buckling limit evaluations for reactor vessel of ABTR

We evaluated the buckling limit of a conceptually designed reactor vessel of the Advanced Burner Test Reactor (ABTR) for a horizontal safe shutdown earthquake (SSE) seismic load. In this evaluation, both seismic isolation and non-isolation designs were considered for thin reactor vessels subjected to elevated temperature services. For calculating the buckling load, two kinds of methods, a numerical simulation method using finite element analysis and an evaluation formula driven from a theoretical basis, were used. To consider the material aging effect caused by a 60-year design lifetime and 510°C normal operating temperature, an isochronous stress-strain curve corresponding to these conditions was used for the nonlinear elastic-plastic buckling analysis method. From the evaluation results of the buckling load, it was found that plasticity behavior significantly affects the buckling strength, but that the initial geometrical imperfections have little effect. Also, the non-seismic isolation design does not satisfy the buckling limit rules of the ASME BPV Section III, Subsection NH, but the seismic isolation design does satisfy it with sufficient margins.     

Paradoxical buckling behaviour of a thin cylindrical shell under axial compression

The widely accepted theory of buckling of thin cylindrical shells under axial compressive loading emphasises the sensitivity of the buckling load to the presence of initial imperfections. These imperfections are conventionally taken to be minor geometric perturbations of a shell which is initially stress-free. The original aim of the present study was to investigate the effect on the buckling load of imperfections in the form of local initial stress, which are probably more typical of practice than purely geometric ones. Experiments were performed on a vertical “melinex” cylinder of diameter 0.9 m and height 0.7 m, with radius/thickness ratio 1800. The upper and lower edges of the cylinder were clamped to end discs by means of circumferential belts — an arrangement that allowed states of self-stress to be introduced to the shell readily by means of local “uplift” at the base. The upper disc was made sufficiently heavy to buckle the shell, and it was supported by a vertical central rod under screw control. Many buckling tests were performed. Surprisingly, the buckling loads were generally at the upper end of the range of fractions of the classical buckling load that have been found in many previous experimental studies. Even when the local uplift at the base caused a local “dimple” to be formed before the shell was loaded, the buckling load was relatively high. A surface-scanning apparatus allowed the geometric form of the shell to be monitored, and the progress of such a dimple to be followed; and it was found that a dimple generally grew in size and migrated in a stable fashion up the shell as the load increased, until a point was reached when unstable buckling occurred. These unexpected and paradoxical features of the behaviour of the experimental shell may be attributed to the particular boundary conditions of the shell, which provide in effect statically determinate support conditions. This study raises some new issues in the field of shell buckling, both for the understanding of buckling phenomena and for the rational design of shells by engineers against buckling.     

The dynamics of symmetric post-buckling

This work examines the relationship between natural frequency and compressive load for a conservative elastic mechanical system characterised by symmetric buckling. Application of some general theory shows that for a perfect system, exhibiting a stable-symmetric point of bifurcation, the initial post-buckling curve of compressive load against the square of the natural frequency is linear and has a slope of that of the pre-buckling curve. Simple link models and discretised continuous systems are analysed to illustrate the results of the general theory. These models are also used to show that the initial post-buckling curve is independent of axial inertia. The influence of initial imperfections is then considered and compared with experimental evidence.     

Numerical analysis of static performance comparison of friction stir welded versus riveted 2024-T3 aluminum alloy stiffened panels

Most researches on the static performance of stiffened panel joined by friction stir welding（FSW） mainly focus on the compression stability rather than shear stability. To evaluate the potential of FSW as a replacement for traditional rivet fastening for stiffened panel assembly in aviation application, finite element method（FEM） is applied to compare compression and shear stability performances of FSW stiffened panels with stability performances of riveted stiffened panels. FEMs of 2024-T3 aluminum alloy FSW and riveted stiffened panels are developed and nonlinear static analysis method is applied to obtain buckling pattern, buckling load and load carrying capability of each panel model. The accuracy of each FEM of FSW stiffened panel is evaluated by stability experiment of FSW stiffened panel specimens with identical geometry and boundary condition and the accuracy of each FEM of riveted stiffened panel is evaluated by semi-empirical calculation formulas. It is found that FEMs without considering weld-induced initial imperfections notably overestimate the static strengths of FSW stiffened panels. FEM results show that, buckling patterns of both FSW and riveted compression stiffened panels represent local buckling of plate between stiffeners. The initial buckling waves of FSW stiffened panel emerge uniformly in each plate between stiffeners while those of riveted panel mainly emerge in the mid-plate. Buckling patterns of both FSW and riveted shear stiffened panels represent local buckling of plate close to the loading corner. FEM results indicate that, shear buckling of FSW stiffened panel is less sensitive to the initial imperfections than compression buckling. Load carrying capability of FSW stiffened panel is less sensitive to the initial imperfections than initial buckling. It can be concluded that buckling loads of FSW panels are a bit lower than those of riveted panels whereas carrying capabilities of FSW panels are almost equivalent to those of riveted panels with identical geometries. Finite elem     

On localized thermal track buckling

The thermal buckling of a railroad track in the lateral plane is analyzed with the track modelled as an elastic beam resting on an elastic-plastic foundation representing the ballast. The nonlinearity of the resistance forces exerted by the ballast on the track is accounted for, both in the lateral and axial directions. For a perfectly straight track the critical bifurcation mode is a periodic one and the effect of periodic imperfections on the instability temperature is analyzed numerically. The transition to the localized buckling pattern observed in practice takes place by a bifurcation from the periodic deflection pattern. The transition to this localized mode can occur with only little growth of the periodic deflections. The instability temperatures for some tracks with various nonperiodic initial imperfections are also determined. It is shown that the instability temperature depends on both the magnitude and the form of the initial imperfections.     

Linear and nonlinear buckling analysis of a locally stretched plate

Uniformly stretched thin plates do not buckle unless they are in special boundary conditions. However, buckling commonly occurs around discontinuities, such as cracks, cuts, narrow slits, holes, and different openings, of such plates. This study aims to show that buckling can also occur in thin plates that contain no defect or singularity when the stretching is local. This specific stability problem is analyzed with the finite element method. A brief literature review on stretched plates is presented. Linear and nonlinear buckling stress analyses are conducted for a partially stretched rectangular plate, and various load cases are considered to investigate the influence of the partial loading expanse on the critical tensile buckling load. Results are summarized in iso-stress areas, tables and graphs. Local stretching on one end of the plate induces buckling in the thin plate even without geometrical imperfection.     

Buckling analysis of discretely stringer-stiffened cylindrical shells

Engineering approach for computation of stringer stiffened cylindrical shells is realized mainly using the structurally orthotropic theory with momentless pre-buckling state. On the other hand, experimental results suggest that in many cases the mentioned theory provides excessive values of buckling load. The influence of imperfections for stringer stiffened shells seems to be less important than in an isotropic case. Considering axially symmetric momentous components of pre-buckling state cannot essentially improve theoretical results. Specific experiments showed a significant influence of stringer discreteness on the buckling loads of reinforced shells. The mentioned influence can be divided into two parts: excitation of essentially non-axially symmetric pre-buckling and buckling states. Usually, only the latter phenomenon is taken into account. In this paper we show that the first factor dominates. We propose simple analytical expressions governing non-axially symmetric pre-buckling state components. We also propose an asymptotic simplification of the buckling boundary value problem. Results obtained are compared numerically with the known theoretical and experimental data.     

Interaction between local and overall buckling

Numerical solutions of the equations governing the interaction of local and overall modes on box columns are presented. In confirmation of previous results these solutions show that elastic buckling is highly sensitive to changes in the magnitude of both local and overall small imperfections, and that the strong optimum indicated for coincident mode design is effectively eliminated when this imperfection sensitivity as well as effects of plasticity are included. It is also clearly demonstrated that the reduced modulus approach provides a conservative estimate of the elastic load-carrying capacity.The inclusion of plasticity into the analysis shows that the effects of elastic interaction may also be highly significant in controlling the plastic collapse load, and for most practical cases this plastic interactive buckling will control design. An extension of the Perry-Robertson procedure is shown to provide a convenient means of presenting the design information. Finally, the usefulness of the van der Neut model as the starting point of the analysis is verified.     

Postbuckling analysis of imperfect stiffened laminated cylindrical shells under combined external pressure and thermal loading

A postbuckling analysis is presented for a stiffened laminated cylindrical shell of finite length subjected to combined loading of external pressure and a uniform temperature rise. The formulation is based on a boundary layer theory of shell buckling which includes the effects of nonlinear prebuckling deformations, nonlinear large deflections in the postbuckling range and initial geometrical imperfections of the shell. The “smeared stiffener” approach is adopted for the stiffeners. The analysis uses a singular perturbation technique to determine the interactive buckling loads and the postbuckling equilibrium paths. Numerical examples are presented that relate to the performance of perfect and imperfect, stiffened and unstiffened cross-ply laminated cylindrical shells. Typical results are presented in dimensionless graphical form for different parameters and loading conditions.