Optimization of composite stiffened panels under mechanical and hygrothermal loads using neural networks and genetic algorithms

The present work develops an optimization procedure for a geometric design of a composite material stiffened panel with conventional stacking sequence using static analysis and hygrothermal effects. The procedure is based on a global approach strategy, composed by two steps: first, the response of the panel is obtained by a neural network system using the results of finite element analyses and, in a second step, a multi-objective optimization problem is solved using a genetic algorithm. The neural network implemented in the first step uses a sub-problem approach which allows to consider different temperature ranges. The compression load and relative humidity of the air are assumed to be constants throughout the considered temperature range.     

Behaviour of structural fibre composite sandwich panels under point load and uniformly distributed load

An innovative fibre composite sandwich panel made of glass fibre reinforced polymer skins and a modified phenolic core material was developed for building and other structural applications. The behaviour of this new generation sandwich panel was studied with reference to the main fibre orientation in floor applications, so that the effect due to erroneous installation could be evaluated. The two- and four-edge supported sandwich panels with different fibre orientations and fixity systems between panel and joist were tested under point load and uniformly distributed load (UDL) to determine their strength and failure mechanisms. The results of this experimental investigation show that the panels behave similarly under both loading conditions. Moreover, the fixity does not have a major effect on its failure mode and deflection.     

Deformation behavior of a stiffened panel subjected to underwater shock loading using the non-linear finite element method

Given the superior strength-to-weight ratio, stiffened panels have been used extensively in the main structure of ships and underwater vehicles. The loads acting on a stiffened panel in a ship is in-plane compression or tension, resulting from the overall hull-girder bending moment or torsion, shear force resulting from the hull-girder shear force, and lateral pressure resulting from the external wave or shock loading. This work addresses the transient responses of a panel structure reinforced by ribs of different sizes to underwater shock loads using non-linear finite element code-ABAQUS. Verification of the reliability was made between the Ramajeyathilagam’s experiments results [Ramajeyathilagam K, Vendhan CP, Rao VB. Non-linear transient dynamic response of rectangular plates under shock loading. Int J Impact Eng 2000;24:999–1015, Ramajeyathilagam K, Vendhan CP. Deformation and rupture of thin rectangular plates subjected to underwater shock. Int J Impact Eng 2004;30:699–719] at several different locations on the plates. The shock factor is adopted to describe the shock severity. Additionally, the displacement–time histories under different shock loadings are presented which will be used in designing stiffened panels so as to enhance resistance to underwater shock damage.     

A hybrid genetic based optimization procedure for aircraft conceptual analysis

The problem to define a methodology for the analysis of aircraft performances, in the phase of conceptual design, is addressed. The proposed approach is based on a numerical optimization procedure where a scalar objective function, the take-off weight, is minimized. Deterministic and stochastic approaches as well as hybridizations between these two search techniques are considered. More precisely, we consider two-stage strategies where the optimum localization is performed by a genetic algorithm, while a gradient-based method is used to terminate the optimization process. Also, another type of hybridization strategy is investigated where a partially converged gradient-based method is incorporated in the genetic algorithm as a new operator. A detailed discussion is made and various different solutions are critically compared. The proposed methodology is consistent and capable of giving fundamental information to the designer for further investigating towards the directions identified by the procedure. A basic example is described, and the use of the methodology to establish the effects of different geometrical and technological parameters is discussed.     

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.     

一种混合优化策略在工步排序中的应用

结合创成式ＣＡＰＰ系统中工步优化问题，介绍了由遗传算法（ＧｅｎｅｔｉｃＡｌｇｏｒｉｔｈｍｓ，ＧＡ）和模拟退火算法（ＳｉｍｕｌａｔｅｄＡｎｎｅａｌｉｎｇＡｌｇｏｒｉｔｈｍｓ，ＳＡ）构成的混合寻优策略。最后以一箱体的工艺规划过程为例，将基于混合寻优策略的工步排序融入以加工中心为主要加工设备的ＣＡＰＰ工艺决策过程。     

Postbuckling behaviour of a pressurized stiffened composite panel – Part I: Experimental study

Results of an experimental study of the buckling and postbuckling behaviour until the collapse of a cylindrical stiffened composite panel are presented. The specimen is subjected to a uniform pressure on one of its faces using a combination of gas and liquid inside a hermetic box. The present analysis shows the postbuckling load carrying capacity of elements of this kind without developing failure mechanism. Due to the high sensitivity to geometric imperfections of these structures, a simple procedure to obtain their measurements once the specimen is placed in the experimental device is set out. The data registered in these tests will be used for the subsequent validation of the numerical model in order to develop more accurate solutions. This will produce a significant increment in the fidelity of those predictions, making possible a reduction in the number of tests to be performed in real applications.     

Failure analysis in postbuckled composite T-sections

Blade-stiffened skin designs made of composite materials have the potential to produce highly efficient structures, when the large strength reserves in the postbuckling range are utilised. This paper investigates the failure under postbuckling deformations of T-section specimens cut from a blade-stiffened panel, by comparing experimental results to finite element models. In the experimental work, T-section specimens with a particular lay-up and geometry were tested to failure in antisymmetric and symmetric loading rigs. These loading rigs simulate deformations on skin-stiffener interfaces during panel postbuckling. For the numerical analysis, two-dimensional models of the interface cross-section were used with a strength-based criterion that monitored failure within each ply. The use of a zero-thickness layer of cohesive elements has also been investigated in order to simulate the delamination behaviour. The numerical predictions are compared to the experimental results in terms of the failure load, specimen stiffness and specimen behaviour. The analysis approach is shown to be capable of predicting the critical damage locations and initiation loads for both antisymmetric and symmetric loadings. The successful prediction of failure in skin-stiffener interfaces can be linked to a global-local approach for efficient analysis of large, fuselage-representative composite structures.     

Damage monitoring of CFRP stiffened panels under compressive load using FBG sensors

FBG sensors were embedded in each of two CFRP stiffened panels fabricated by VaRTM. Low-velocity impacts were applied to one of the panels in order to compare the methods of monitoring impact events using FBG sensors. The main impact damage was an interlaminar delamination inside the skin, which could be observed by an ultrasonic C-scan. A monitoring method using the full spectral signals was more effective in evaluating the impact damages in detail than that using the center wavelength. Following the impact tests, buckling behaviors were investigated under compressive loading using FBG sensors and surface-attached strain gauges. The FBG sensors could evaluate strain changes resulting from buckling behaviors under relatively low compressive loading. They could also evaluate damage growth until the final failure and difference of buckling behaviors between panels with and without impact damages.     

High-order finite elements reduced models for modal analysis of stiffened panels

A method is presented to build reduced (equivalent) models of stiffened panels made of thin-walled composite materials. The technique is developed to be used in the modal analysis of panels and wing boxes, allowing finite element modelling and analysis using a single-type, three-dimensional orthotropic p-element. The use of a single element guarantees speed and flexibility in the (re)modelling of the structure and reduces the modelling and analysis errors connected to finite element analysis in preliminary-design/multidisciplinary-optimization environments. The method is tested on two types of representative wing boxes. Different approaches for the equivalencing are tested and compared to each other. The results show that the equivalent models give results within few percent from those obtained running a full model, saving as much as one order of magnitude in the number of degrees of freedom employed.     

A two-step optimization approach for technology selection

An efficient optimization approach for the technology selection problem is described. Technology selection is a crucial step in the aircraft design process, especially when the performance and econo-mic requirements are not fulfilled for any combination of the configuration design variables. In such a case, the designer must search efficiently within a set of technology options for the optimal combination that achieves the required improvements. When the set of available technologies is large, as is usually the case, a difficult combinatorial optimization problem ensues, resulting in significant time and computational expense. The objective of the new approach is to reduce the computational cost of technology selection by decomposing the process into two smaller sub-problems. The new approach attempts to exploit the structure of the technology compatibility matrix to improve the efficiency of the technology selection process. Results from an application problem are presented and valuable insights and observations are discussed.     

Availability allocation to repairable systems with genetic algorithms: a multi-objective formulation

This paper describes a methodology based on genetic algorithms (GA) and experiments plan to optimize the availability and the cost of reparable parallel-series systems. It is a NP-hard problem of multi-objective combinatorial optimization, modeled with continuous and discrete variables. By using the weighting technique, the problem is transformed into a single-objective optimization problem whose constraints are then relaxed by the exterior penalty technique. We then propose a search of solution through GA, whose parameters are adjusted using experiments plan technique. A numerical example is used to assess the method.     

Optimization of Machining Characteristics for Al／SiCp Composites using ANN／GA

The present work is focused on optimization of machining characteristics of Al/SiCp composites.The machining characteristics such as specific energy,tool wear and surface roughness were studied.The parameters such as volume fraction of SiC,cutting speed and feed rate were considered.Artificial neural networks(NN) was used to train and simulate the experimental data.Genetic algorithms(GA) was interfaced with ANN to optimize the machining conditions for the desired machining characteristics .Validation of optimized results was also performed by confirmation experiments.     

Postbuckling behavior of a pressurized stiffened composite panel – Part II: Numerical analysis. Effect of the geometrical imperfections

Numerical simulations of the experimental tests performed with a pressurized composite stiffened panel are presented in this paper. As a consequence of the high slenderness of this structural typology, pressure caused the panel to enter the postbuckling regime. Previous experimental tests showed that the panel had a large safe postbuckling range, the experimental collapse pressure being over four times the first buckling load. Due to the relevant influence of the geometric imperfections on the global response, a procedure for taking into account actual imperfections in the development of the models is proposed. This procedure can be used as a tool to facilitate the modeling of the actual geometry of the panel, mainly the zone of the skin located between the stringers. A satisfactory agreement with experimental results has been reached using the proposed procedure.     

Integrated optimization of structure and control for piezoelectric intelligent trusses with uncertain placement of actuators and sensors

The finite element modeling of truss structures with piezoelectric members is presented. Based on the approach of independent modal space control, the controllability and observability indices of the system related to the positions of actuators/sensors are demonstrated. Consequently, the effective damping response time is evaluated. The object of the optimization model is to minimize a specified performance index of the intelligent truss subjected to constraints on the natural frequency and the amplitude of displacement response as well as the applied voltages under a given disturbance. Structural sizing variables, control parameters and actuator/sensor placements are treated as the independent design variables. Coding, the calculation of fitness and the optimization procedure of Genetic Algorithms are discussed so as to solve the integrated optimization with two different types of design variable space: discrete and continuous. Numerical examples are presented to show the effectiveness and usefulness of integrated optimization of structure and control for piezoelectric intelligent trusses.The authors would like to thank for the support by Natural Science Foundation of China under grant 10072050 and the Doctorate Creation Foundation of Northwestern Polytechnical University under grant 200236.