复合材料加筋板屈曲/后屈曲分析的应用

研究了复合材料加筋板翼面结构稳定性问题,分析了加筋板在压缩和剪切等载荷作用下的稳定性安全裕度。利用计算复合材料加筋板屈曲及后屈曲承载能力的方法,验证复杂受载情况下结构的稳定性。验证对象是一个优化后的满足强度、刚度和工艺制造要求的复合材料机翼。该机翼在各种载荷工况下的内力分布情况由MSC.NASTRAN分析得到,通过本文提出的方法得到每块蒙皮的稳定性承载能力。然后给出复合材料层合板在复杂载荷下的屈曲及后屈曲安全裕度的计算准则,验证优化后的机翼加筋板是否满足稳定性设计要求。该方法可作为约束集成到结构优化系统平台中。     

Computational strategy for multiobjective optimization of composite stiffened panels

This paper presents a multiobjective optimization methodology for composite stiffened panels. The purpose is to improve the performances of an existing design of stiffened composite panels in terms of both its first buckling load and ultimate collapse or failure loads. The design variables are the stacking sequences of the skin and of the stiffeners of the panel. The optimization is performed using a multiobjective evolutionary algorithm specifically developed for the design of laminated parts. The algorithm takes into account the industrial design guidelines for stacking sequence design. An original method is proposed for the initialization of the optimization that significantly accelerates the search for the Pareto front. In order to reduce the calculation time, Radial Basis Functions under Tension are used to approximate the objective functions. Special attention is paid to generalization errors around the optimum. The multiobjective optimization results in a wide set of trade-offs, offering important improvements for both considered objectives, among which the designer can make a choice.     

Semi-analytic probabilistic analysis of axially compressed stiffened composite panels

The influence of scattering input parameters on the response of axially compressed stiffened composite panels is investigated. In order to estimate the stochastic distributions and the correlations between the first buckling load (or local buckling load), the global buckling load and the collapse load, a semi-analytic probabilistic analysis is performed. A procedure is given for evaluating the probability of failure of stiffened panels from the determined stochastic distributions, and probabilistically justified safety factors are derived.     

Optimum design for buckling of plain and stiffened composite axisymmetric shell panels/shells

The buckling of plain and discretely stiffened composite axisymmetric shell panels/shells made of repeated sublaminate construction is studied using the finite element method. In repeated sublaminate construction, a full laminate is obtained by repeating a basic sublaminate, which has a smaller number of plies. The optimum design for buckling is obtained by determining the layup sequence of the plies in the sublaminate by ranking, so as to achieve maximum buckling load for a specified thickness. For this purpose, a four-noded 48-dof quadrilateral composite thin shell element, together with fully compatible two-noded 16-dof composite meridional and parallel circle stiffener elements are used.     

Post-buckling behaviour of flat stiffened composite panels: Experiments vs. analysis

The paper is focused on the development of a validated procedure for modelling, by means of Finite Element tools, the post-buckling behaviour of stiffened composite flat panels subjected to compression loads. The experimental data for model validation were collected during a test campaign on two sets of CFRP flat stiffened panels.     

Formulation of an improved smeared stiffener theory for buckling analysis of grid-stiffened composite panels

An improved smeared stiffener theory for stiffened panels is presented that includes skin-stiffener interaction effects. The neutral surface profile of the skin-stiffener combination is developed analytically using the minimum potential energy principle and statics conditions. The skin-stiffener interaction is accounted for by computing the bending and coupling stiffness due to the stiffener and the skin in the skin-stiffener region about a shift in the neutral axis at the stiffener. Buckling load results for axially stiffened, orthogrid, and general grid-stiffened panels are obtained using the smeared stiffness combined with a Rayleigh-Ritz method and are compared with results from detailed finite element analyses.     

基于蚁群算法的光学膜系初始结构优化设计

本文在分析和研究蚁群算法原理、算法实现过程的基础上,针对多膜料设计,以1200～1600nm波段内带通滤波器、1200～1600nm波段内截止滤波器、600～1000nm波段内截止滤波器和400～800nm波段内截止滤波器等4个滤波器实例为优化对象,以理论反射率和实际反射率之差最小为目标函数,采用蚁群算法和MATLAB语言编制了膜系初始结构优化设计的仿真计算程序,计算机仿真结果准确,证明了蚁群算法应用于膜系设计的可行性.     

Ant Colony Optimization for dispersed laminated composite panels under biaxial loading

The current study aims to show the benefits of dispersed laminates (laminates in which the orientation angles are not limited to the conventional, 0°, ±45° and 90°, orientations) over conventional ones, in terms of stiffness, buckling resistance and strength, in structural applications. The Ant Colony Optimization algorithm is used with strength constraints to find the best candidate to achieve this goal. A study is conducted to select the most suitable failure criterion among three common ones.The methodology is used for two loading cases: biaxial compression and biaxial tension. In the case of biaxial compression, the problem is formulated to maximize the critical buckling load whereas with the biaxial tension, the formulation is to minimize the failure index. For both loading cases, the methodology succeeds in improving the response of dispersed laminates with respect to the conventional ones. These results support the movement of the composite industry toward using dispersed laminates.     

Experimental and theoretical investigations on stiffened and unstiffened composite panels under uniform transverse loading

An experimental investigation on fibre-reinforced stiffened and unstiffened panels under transverse uniform pressure has been carried out. The deflections and strains measured inside the laminate are compared with a finite element analysis. The effect of one or two stiffeners within the panel is small since the stiffener fail at small loads. In contrary to this, the effect of a different lay-up is big, because the angle-ply panels [+45°/−45°]₂ carried higher loads than the cross-ply panels [0°/90°]₂. The failure of the panels near the centre of the long edge at the clamping is correctly predicted by the FEA. In addition, calculated and predicted stresses are close to each other.     

Buckling behavior of stiffened composite panels with variable thickness skin under compression

In this paper, the effect of the skin configuration on the buckling behavior of stiffened composite panels subjected to uniaxial compression was numerically and experimentally investigated. P-version finite element models containing all the structural details were used to predict the buckling load and buckling mode of stiffened composite panels. The upper and lower ends of the panel were fixed by potting a tin bismuth alloy with melting temperature around 70?°C to get an uniformly loading condition in the test. The alloy could be easily recycled by heating and reutilized later for potting the other test specimens.     

Multi-objective optimal design of sandwich panels using a genetic algorithm

In this study, an optimization problem concerning sandwich panels is investigated by simultaneously considering the two objectives of minimizing the panel mass and maximizing the sound insulation performance. First of all, the acoustic model of sandwich panels is discussed, which provides a foundation to model the acoustic objective function. Then the optimization problem is formulated as a bi-objective programming model, and a solution algorithm based on the non-dominated sorting genetic algorithm II (NSGA-II) is provided to solve the proposed model. Finally, taking an example of a sandwich panel that is expected to be used as an automotive roof panel, numerical experiments are carried out to verify the effectiveness of the proposed model and solution algorithm. Numerical results demonstrate in detail how the core material, geometric constraints and mechanical constraints impact the optimal designs of sandwich panels.     

Global buckling of composite structural insulated wall panels

This paper presents a new type of composites structural insulated panels (CSIPs) for structural wall applications. The proposed composite panel is made of low-cost thermoplastic orthotropic glass/poly-propylene (glass–PP) laminate as a facesheet and expanded polystyrene foam (EPS) as a core with very high facesheet/core moduli ratio. The proposed CSIP walls are intended to overcome problems of traditional Structural Insulated Panels (SIPs) such as termite attack, mold buildups and poor penetration resistance against wind borne debris. This paper investigates the behavior of CSIPs under concentric and eccentric loading. CSIPs specimens failed by global buckling mode in which no debonding was observed. The eccentric specimens failed at load 35% lower than that of the concentric ones. Global buckling formulas for concentric and eccentric loading were presented and validated using the experimental results and were in a good agreement. An equivalent stiffness formula was also developed for sandwich wall under in-plane loading considering the shear deformations effect of the core. Design study for CSIP walls is also presented to help in designing this new type of composite panels.     

Similitude of sandwich panels with a ‘soft’ core in buckling

The paper presents the similarity analysis of a sandwich unidirectional panel with a transversely flexible core under buckling loads. The governing equations are those used in the high-order analysis of sandwich panels with a ‘soft’ core. The study derives the similitude conditions in the case of external in-plane compressive loads that yield buckling of the panel with and without imperfections. In the first part, the buckling analysis is presented and it is based on the linearized version of the governing equations of the non-linear geometrical bending equations. The presentation includes an analytical proof of the applicability of similarity for the buckling of a sandwich panel with identical faces and a numerical demonstration of the response when full similarity and partial similarity exist. The effects of full and partial similarity are presented for a panel with imperfections.     

Influence of post-buckling behaviour of composite stiffened panels on the damage criticality

In stiffened panels with defects, such as skin delaminations or stringer debonding, buckling may occur prior to the designed critical buckling load. Depending on the damage parameters, such defects may also affect the post-buckling behaviour and consequently the structural performance. An automated finite element (FE) modelling tool has been developed to predict the post-buckling behaviour of panels. It was coupled with a linear elastic fracture mechanics approach to determine damage criticality, based on the “no-growth” principle. The structural behaviour in the post-buckling range and its interaction with the damage parameters were analysed. Local buckling occurred as a result of localised stiffness reduction in the damage region. Global buckling occurred when sufficient in-plane strain was reached. The onset of local buckling was an important factor on stringer debonding criticality as the local buckling mode had an effect on the corresponding global buckling. In comparison, the onset of local buckling for the skin delamination was lower due to the thin sub-laminate separation. However, it was less influential on the damage criticality because the local buckling slowly dissipated in the far post-buckling range. It was found that the initiation of local buckling, and the interaction between the local and global buckling mode, would determine the damage criticality.     

The use of a genetic algorithm to improve the postbuckling strength of stiffened composite panels susceptible to secondary instabilities

The use of genetic algorithms (GAs) for structural optimisation is well established but little work has been reported on the inclusion of damage variables within an optimisation framework. This approach is particularly useful in the optimisation of composite structures which are prone to delamination damage. In this paper a challenging design problem is presented where the objective was to delay the catastrophic failure of a postbuckling secondary-bonded stiffened composite panel susceptible to secondary instabilities. It has been conjectured for some time that the sudden energy release associated with secondary instabilities may initiate structural failure, but this has proved difficult to observe experimentally. The optimisation methodology confirmed this indirectly by evolving a panel displaying a delayed secondary instability whilst meeting all other design requirements. This has important implication in the design of thin-skinned lightweight aerostructures which may exhibit this phenomenon.     

A genetic algorithm with memory for optimal design of laminated sandwich composite panels

This paper is concerned with augmenting genetic algorithms (GAs) to include memory for continuous variables, and applying this to stacking sequence design of laminated sandwich composite panels that involves both discrete variables and a continuous design variable. The term “memory” implies preserving data from previously analyzed designs. A balanced binary tree with nodes corresponding to discrete designs renders efficient access to the memory. For those discrete designs that occur frequently, an evolving database of continuous variable values is used to construct a spline approximation to the fitness as a function of the single continuous variable. The approximation is then used to decide when to retrieve the fitness function value from the spline and when to do an exact analysis to add a new data point for the spline. With the spline approximation in place, it is also possible to use the best solution of the approximation as a local improvement during the optimization process. The demonstration problem chosen is the stacking sequence optimization of a sandwich plate with composite face sheets for weight minimization subject to strength and buckling constraints. Comparisons are made between the cases with and without the binary tree and spline interpolation added to a standard GA. Reduced computational cost and increased performance index of a GA with these changes are demonstrated.     

Residual strength prediction of multiple cracked stiffened panels

Prediction of the coalescence of adjacent cracks is critical for residual strength estimation of structures under multiple site damage conditions. A methodology successfully developed for the case of crack link‐up prediction of un‐stiffened plates, is extended for the case of typical cracked stiffened aircraft panels. The proposed link‐up criterion is based on the change in the magnitudes of elastic and plastic strain energies of the stiffened panel, before and after the cracks coalesce. The strain energy magnitudes of interest are calculated using non‐linear elastic–plastic finite‐element analysis. For the application and verification of the method, experimental results from the open literature are used. Residual strength values calculated by the proposed methodology are in good agreement with the experimental results. The present criterion provides superior results when compared to the existing and commonly applied link‐up criteria.     

Determination of robustness for a stiffened composite structure using stochastic analysis

It is a common practice to only consider the nominal means as input variables for both classical solid mechanics and finite element (FE) analysis problems. A single solution based on the mean values is then used in design. In reality all input variables are stochastic, existing within a range of possible values. Different combinations of these stochastic input variables will lead to differing output responses and the introduction of variability will cause each structure to have a response that deviates from the original specification, sometimes with catastrophic consequences. In this paper two variables, influence and sensitivity, have been identified as parameters affecting structural robustness. Variability and uncertainty in loads, geometry and lamina stiffness are introduced via a stochastic finite element analysis (SFEA) procedure. The procedure is applied to the design of composite yacht hulls comparing the robustness of designs aimed at satisfying a range of performance and cost requirements. It is shown that influence and sensitivity are useful in identifying designs that lead to imperfection tolerant structures.     

Inverse identification of elastic properties of composite materials using hybrid GA-ACO-PSO algorithm

The main emphasis of this paper is placed on the effectiveness of the proposed optimization method in material identification. The primary motivation of integrating GA, ACO and PSO is to minimize each other’s weaknesses and to promote respective strengths. In the proposed algorithm, the effect of random initialization of GA is subdued by passing the products of GA through the ACO and PSO operators to well organize the exploitative and exploratory search coverage. In return, GA improves the convergence rate and alleviates the strong dependency on the pheromone array in ACO as well as resolves the conflict arisen in identifying the trade-off parameter and further refine the exploitative search of PSO with the introduction of two-point standard mutation and one-point refined mutation. The proposed algorithm has been verified and applied in composite material identification with absolute percentage errors between measured and evaluated natural frequencies not more than 2%.