复合材料双曲率壳屈曲和后屈曲的非线性有限元研究

基于 ABAQUS软件分析平台 , 采用非线性有限元法研究了横向载荷作用下复合材料双曲率壳的屈曲和后屈曲行为。通过在有限元模型中引入 Tsai2Wu失效准则 , 预测了复合材料双曲率壳的初始失效及渐进破坏过程 , 数值结果和试验数据吻合较好 , 表明了该模型的合理有效性 , 并详细讨论了各种参数对屈曲和后屈曲行为的影响。经分析复合材料双曲率加筋壳在均布压载和剪力联合作用下的屈曲和后屈曲行为 , 得到了屈曲载荷的拟合曲线 , 研究表明顺剪力的存在有利于提高屈曲载荷。     

复合材料夹芯梁屈曲破坏模式及极限承载

为研究复合材料夹芯梁在轴压作用下的屈曲、后屈曲特性及承载能力,进行了试验研究与有限元仿真。首先,开展了系列复合材料夹芯梁屈曲特性试验,研究了铺层比例、梁长度、表层厚度及芯层厚度等因素对其屈曲、后屈曲破坏模式及极限承载的影响;然后,基于非线性屈曲理论,采用三维内聚力界面单元模拟面芯脱粘,并引入初始预变形及材料损伤准则对复合材料夹芯梁在轴压下的屈曲特性及极限承载进行仿真研究。结果显示:界面脱粘是屈曲破坏的重要模式;仿真计算的极限承载与试验结果相比,误差控制在10%以内。所得结论表明该方法可有效预报复合材料夹芯梁的后屈曲路径、破坏模式及极限承载。      

冲击后含损伤复合材料格栅加筋板的后屈曲

采用数值分析方法研究了冲击后含损伤的复合材料格栅加筋板的后屈曲特性。基于Mindlin 一阶剪切理论和Von Karman 大挠度理论, 建立了冲击后蒙皮内含分层损伤复合材料格栅加筋板后屈曲分析的有限元方法; 分析中同时考虑了蒙皮和肋骨中纤维断裂、基体开裂等损伤累积造成的刚度的退化和蒙皮分层子板间的闭合接触效应, 为含损伤复合材料格栅加筋板的后屈曲特性研究提供了一种有效的数值分析方法。分析结果表明, 蒙皮分层面积较大时, 格栅加筋板出现蒙皮分层上子板的局部屈曲后仍然具有较强的继续承载能力, 而在后屈曲分析中, 应考虑损伤累积对格栅加筋结构承载能力的影响; 采用非线性虚拟界面元可成功处理分层子板间的闭合接触效应。      

基于改进弧长法的层压复合壳后屈曲反应分析

研究了层压复合壳在横向均匀外压作用下的后屈曲反应。应用全拉格朗日公式描述和9结点退化三维壳单元。提出了一种渐进破坏的模式,并且引入改进的弧长法用于非线性有限元分析。重点研究了不同铺设顺序及方向的层压复合壳后屈曲变形形态和破坏过程。数值计算结果的精度和稳定性得以证实。     

Buckling of filament-wound composite cylinders subjected to hydrostatic pressure for underwater vehicle applications

The buckling and failure characteristics of moderately thick-walled filament-wound carbon–epoxy composite cylinders under external hydrostatic pressure were investigated through finite element analysis and testing for underwater vehicle applications. The winding angles were [±30/90]_FW, [±45/90]_FW and [±60/90]_FW. ACOS, an in-house finite element program, successfully predicted the buckling pressure of filament-wound composite cylinders with 2 ∼ 23% deviation from the test results. The analysis and test results showed that the cylinders do not recover the initial buckling pressure after buckling and that this leads directly to the collapse. Major failure modes in the test were dominated by the helical winding angles.     

复合材料加筋圆拱壳的失稳特性分析

本文应用增量形式的拉格朗日列式法对其有纵横加筋的迭层圆拱壳在均布载荷作用下的稳定性进行了非线性有限元分析。文中应用Sander 壳体理论及横向剪切的影响, 推导了矩形壳元及与该壳元变形相协调的直梁元和曲梁元的切线刚度矩阵。编制了FORTRAN 计算程序。计算并分析了加筋拱壳的局部及整体失稳过程。      

Circumferential stiffness tailoring of general cross section cylinders for maximum buckling load with strength constraints

Stiffness tailoring of laminated composite structures using steered fibre tows is a design method that maximally uses the directional properties of composite materials. Cylindrical structures usually have circular cross sections while some application, geometric or aerodynamic requirements can necessitate other cross sections, e.g. elliptical. Circumferential tailoring can increase the buckling load of thin cylinders by compensating for non-uniform sectional loading such as bending and/or varying radius of curvature in general cylinders. Here, strength constraints are considered in maximum buckling load design, to ensure that the failure load is greater than the buckling load. A two-step optimisation framework is used to separate the theoretical and manufacturing issues in design. A computationally cheap semi-analytical finite difference method is used to solve the linear static and buckling problems. Conservative failure envelopes based on Tsai-Wu failure criterion are used for strength evaluation. To avoid repetitive analyses, successive convex approximation method is used. For demonstration, circumferential tailoring framework is applied to a circular cylinder under bending and an elliptical cylinder under axial compression. The improvements in buckling capacity of variable over constant stiffness designs are shown and verified using nonlinear buckling analysis in the commercial FEM software Abaqus^TM, and the mechanisms of improvements are investigated.     

A nonlinear double-superposition global–local theory for dynamic buckling of imperfect viscoelastic composite/sandwich plates: A hierarchical constitutive model

Nonlinear dynamic thermo-mechanical buckling and postbuckling analyses of imperfect viscoelastic composite laminated/sandwich plates are performed by a proposed theory that takes into account all the interlaminar kinematic and transverse stress continuity conditions, for the first time. Even the dynamic buckling analysis of the multi-layered/sandwich plates employing the hierarchical constitutive model has not been performed before. The proposed theory is a double-superposition high-order global–local theory that is calibrated based on the nonlinear strain–displacement expressions for the thermoelastic loadings taking into account the structural damping. The buckling loads are determined based on a criterion previously published by the author. Various complex sensitivity analyses evaluating effects of the relaxation parameters, rate of the loading, sudden heating, and pre-stress on thermo-mechanical buckling of the viscoelatic multi-layered/sandwich plates are performed. Results show that the viscoelastic behavior may decrease the buckling load. Sudden dynamic buckling loads are higher due to the reflected stress waves.     

Buckling and postbuckling of composite structures

The postbuckling behaviour of a flat, stiffened, carbon fibre composite compression panel has been studied, theoretically and experimentally. The panel had a collapse load in excess of three times the buckling load. An initial failure mechanism leading to eventual explosive collapse of the panel is identified and the damaging stress resultant is measured in the panel and predicted from a finite element analysis.     

Optimum design of cross-sectional profiles of pultruded box beams with high ultimate strength

Pultruded box beams under bending may be subjected to local buckling which causes premature failure of the beams. As such it is important to design pultruded box beams with high local buckling resistance to increase their ultimate strength. This paper presents an optimum design approach for cross-sectional profiles of pultruded box beams of (approximately) the same mass with emphasis on accomplishment of high local buckling resistance through finite element analysis. Five different sectional profiles have been designed by stiffening a simple box, and finite element analysis is used to study linear buckling and postbuckling of the beams. Results for the critical loads of linear buckling and local buckling judged by stress variation, stresses and deformations in postbuckling are presented. The computational results show one of the proposed sectional profiles does not develop local buckling and produces much smaller stresses and deformations within the load range of interest.     

A robust methodology for the simulation of postbuckling response of composite plates

The application of a robust numerical method, namely the differential quadrature method (DQM) for the analysis of buckling and postbuckling of laminated composite plates is introduced. The method is combined with an arc-length strategy to solve the resulting system of nonlinear equations. The treatment accounts for the effect of large deformation by including the von Karman strains. Imperfections in the form of global deflection, conforming to the preferred buckling modes are introduced throughout the plate. Case studies are used to evaluate the geometrically nonlinear response of composite plates under the set conditions, and the results are compared with those obtained by finite element solution and the results obtained from a published literature.     

Failure of composite cylinders under combined external pressure and axial loading

An experimental and theoretical investigation was carried out into the collapse behaviour of filament wound glass fibre/epoxy cylinders under combinations of external pressure and axial loading in the third quadrant of the stress plane. Samples were tested with length-to-diameter ratios from 2·5 to 20 and diameter-to-thickness ratios in the approximate range of 20 to 40. Four ratios of hoop to axial stress were employed: ∞, 2, 1 and 0·5. The theoretical study employed a special purpose finite element program to calculate first ply failure (FPF) and buckling loads for shells of revolution made from multi-layered orthotropic materials. In all cases the experimental collapse pressure was strongly influenced by the predicted buckling failure mode. For those samples predicted to fail by buckling, agreement between the model and the experimental results was excellent. With the samples predicted to undergo FPF prior to buckling it was found that the residual strength was often sufficient to permit the buckling load to be approached.     

Delamination buckling and postbuckling in composite cylindrical shells under combined axial compression and external pressure

A series of finite element analyses on the delaminated composite cylindrical shells subject to combined axial compression and pressure are carried out varying the delamination thickness and length, material properties and stacking sequence. Based on the FE results, the characteristics of the buckling and postbuckling behaviour of delaminated composite cylindrical shells are investigated. The combined double-layer and single-layer of shell elements are employed which in comparison with the three-dimensional finite elements requires less computing time and space for the same level of accuracy. The effect of contact in the buckling mode has been considered, by employing contact elements between the delaminated layers. The interactive buckling curves and postbuckling response of delaminated cylindrical shells have been obtained. In the analysis of post-buckled delaminations, a study using the virtual crack closure technique has been performed to find the distribution of the local strain energy release rate along the delamination front. The results are compared with the previous results obtained by the author on the buckling and postbuckling of delaminated composite cylindrical shells under the axial compression and external pressure, applied individually.     

Prediction of the postbuckling response of composite airframe panels including ply failure

Future design scenarios aim to allow buckling in composite airframe panels. Reliable simulation procedures should be able to capture the postbuckling elastic as well as the inelastic response associated with damage. Damage in composite laminates in terms of ply failure may primarily occur as fiber fracture or matrix cracking. This paper presents a model which is able to capture both geometrical and material nonlinearity. It bases on the finite element formulation of a layered, iso-parametric, quadrilateral shell element which allows for an arbitrary reference surface as well as an arbitrary stacking sequence. Geometrical nonlinearity is accounted for by utilizing Green strains and second Piola–Kirchhoff stresses. Material nonlinearity is considered via a layerwise ideally brittle damage model. The model is applied to a buckling test of a stringer-stiffened composite airframe panel. The numerical results are compared with an experiment proving the applicability of the proposed concept.     

Post Buckling Progressive Failure Analysis of Composite Laminated Stiffened Panels

The present work deals with the numerical prediction of the post buckling progressive and final failure response of stiffened composite panels based on structural nonlinear finite element methods. For this purpose, a progressive failure model (PFM) is developed and applied to predict the behaviour of an experimentally tested blade-stiffened panel found in the literature. Failure initiation and propagation is calculated, owing to the accumulation of the intralaminar failure modes induced in fibre reinforced composite materials. Hashin failure criteria have been employed in order to address the fiber and matrix failure modes in compression and tension. On the other hand, the Tsai-Wu failure criterion has been utilized for addressing shear failure. Failure detection is followed with the introduction of corresponding material degradation rules depending on the individual failure mechanisms. Failure initiation and failure propagation as well as the post buckling ultimate attained load have been numerically evaluated. Final failure behaviour of the simulated stiffened panel is due to sudden global failure, as concluded from comparisons between numerical and experimental results being in good agreement.     

Collapse Analysis,Defect Sensitivity and Load Paths in Stiffened Shell Composite Structures

An experimental program for collapse of curved stiffened composite shell structures encountered a wide range of initial and deep buckling mode shapes. This paper presents work to determine the significance of the buckling deformations for determining the final collapse loads and to understand the source of the variation. A finite element analysis is applied to predict growth of damage that causes the disbonding of stiffeners and defines a load displacement curve to final collapse. The variability in material properties and geometry is then investigated to identify a range of buckling modes and development of deep postbuckling deformation encountered in the experimental program. Finally the load paths for the damaged panels are used to visualise the load transfer and enhance the physical understanding of the load displacement history.     

加筋复合材料圆柱壳的屈曲和初始后屈曲的研究

本文分析了筋和壳的儿何参数及壳的铺层对纵向或环向密加筋的复合材料层合圆柱壳在轴压和侧压下的稳定性和初始后屈曲性能的影响。初始后屈曲分析基于Koiter理论。对几种不同几何参数、壳体铺层和载荷情况的加筋壳的计算表明:在所有情况下,外加筋比内加筋更有效地提高了屈曲载荷;复合材料壳的加筋效率一般都高于各向同性材料加筋壳;壳体的铺层对屈曲和初始后屈曲性能有很大影响。     

Smooth postbuckling stresses by a modified finite element method

Exact postbuckling stresses usually vary fairly smoothly. Unfortunately, finite element postbuckling stresses tend to be much less well behaved. The result is that second order postbuckling constants determined by the finite element method may be highly inaccurate. The reason is that in finite element solutions transverse displacements associated with the buckling fields furnish too rapidly varying postbuckling strain contributions, while the postbuckling axial or membrane displacements contribute strain components that are sufficiently smooth, thus creating an internal postbuckling strain and stress mismatch. The present study suggests a modified finite element method that handles the problem, which is a special example of membrane locking, by introducing the postbuckling strains as independent variables. In general, the method provides rather complicated finite element expressions. However, by a suitable choice of interpolating functions, the resulting finite element equations themselves may be found to be the usual ones, and yet provide smooth postbuckling stresses and therefore good values of the postbuckling constants.     

Free vibration of buckled SMA reinforced composite laminates

The effect of shape memory alloys (SMA) on the free vibration behavior of buckled cross-ply and angle-ply laminates by varying the SMA fiber spacing was investigated using the finite element method. The formulation of the location-dependent linear, nonlinear stiffness and mass matrices due to non-homogeneous material properties and the temperature-dependent recovery stress stiffness matrix were derived. Numerical results show that the increase of SMA fiber volume fraction and prestrain may generate more recovery stress, and increase the stiffness of SMA reinforced composite laminates. Therefore, the postbuckling deflections of the plate will be decreased considerably and the natural frequencies of the plate may be modified significantly. The buckling mode and fundamental natural mode are dependent on the graphite fiber orientation of the SMA reinforced angle-ply laminates. The relationship between the buckling mode and fundamental natural mode is clearly displayed and studied in detail.