Composite Stiffened Panels Research Articles

Optimization of Blade Stiffened Composite Panel under Buckling and Strength Constraints

This paper deals with multiple constraints for dimension and stacking-sequence optimization of a blade-stiffened composite panel. In a previous study, a multiple objective genetic algorithm using a Kriging response surface with a buckling load constraint was the target. The present study focuses on dimension and stacking-sequence optimization with both a buckling load constraint and a fracture constraint. Multiple constraints complicate the process of selecting sampling analyses to improve the Kriging response surface. The proposed method resolves this problem using the most-critical-constraint approach. The new approach is applied to a blade stiffened composite panel and the approach is shown to be efficient.

Design "Feasilization" Using Knowledge-Based Engineering and Optimization Techniques

The multidisciplinary design optimization process can be supported by partial automation of analysis and optimization steps. Design-and-engineering engines are a useful concept to structure this automation. Within the design-and-engineering engines, a product is parametrically defined using knowledge-based engineering. This parametric product model needs to be initiated before global multidisciplinary optimization can be performed. The 'feasilization is done by the initiator component in the design-and-engineering engines that simulates the heuristic methods normally used by designers to estimate the first values for the parameters and variables describing their designs. The initiation of values for structural parameters and variables is elaborated for a sample composite-stiffened panel structure. It is shown that the initiator function of the design-and-engineering-engine concept can be implemented on the basis of optimization techniques using simplification of the design requirements, simplified representations of the design options, and the class of so-called schematic models to mimic the designer's job in the preliminary sizing phase of the design. An implementation of the initiator is used in a sample design-and-engineering engine for aircraft vertical tail design.

Prediction of flange debonding in composite stiffened panels using an analytical crack tip element-based methodology

Fracture mechanics-based predictive methodologies are commonly employed to estimate onset of interlaminar damage growth in composite structures. Full implementation of interlaminar fracture mechanics in design requires the continuing development of codes to calculate energy release rates and advancements in delamination growth criteria under mixed mode conditions. In this research, an analytical crack tip element (CTE) methodology was evaluated and applied to predict skin-to-stiffener separation, which is a typical failure mode in aerospace structures. The methodology was correlated against empirical data obtained from structural testing of a single blade stiffened panel with an embedded artificial debond. Failure initiation occurred at the location predicted by the CTE and the predicted load for initiation agreed with the average of a set of experimental results.

Damage Index Comparison for a Composite Stiffened Panel Using Lamb Wave

Various damage index (DI) algorithms of detecting changes such as a loosen bolt and a delamination development in a composite structure were examined using ultrasonic Lamb waves generated by embedded piezoelectric active sensors. The DI is a single value that is a function of response signal’s attenuation due to any damage or changes in a structure. Various DI algorithms such as active damage interrogation (ADI), time domain root men square (RMS), short time Fourier Transform (STFT) and time reversal (TR) were discussed. For experimental validation, a composite stiffened panel was used, and loosen bolt damage and low-velocity- impact damage were tested. In order to pitch and catch Lamb waves, surface mounted PZTs (lead zirconate titanate) were used. According to the DI algorithms, appropriate ultrasonic guided Lamb waves were selected for actuators. Each set of DI algorithm and drive signal showed different characteristics to detect the damage.

Optimization of composite stiffened panels subject to compression and lateral pressure using a bi-level approach

Composite stiffened panel optimization is typically a mixed discrete-continuous design problem constrained by buckling and material strength. Previous work applied a bi-level optimization strategy to the problem by decomposing the mixed problem to continuous and discrete levels to reduce the optimization search space and satisfy manufacturing constraints. A fast-running optimization package, VICONOPT, was used at the continuous optimization level where the buckling analysis was accurately and effectively performed. However, the discrete level was manually adjusted to satisfy laminate design rules. This paper develops the strategy to application on continuously long aircraft wing panels subjected to compression and lateral pressure loading. The beam-column approach used to account for lateral loading for analysis during optimization is reported. A genetic algorithm is newly developed and applied to the discrete level for automated selection of laminated designs. The results that are presented show at least 13% weight saving compared with an existing datum design.

Autonomous Impact Damage Monitoring in a Stiffened Composite Panel

This study is concerned with the detection and characterization of impact damage in a stiffened woven composite structure using high frequency Lamb waves and low frequency modal vibrations. The geometric and material complexities of the structure present practical difficulties in the direct analysis of both wave propagation and modal vibration data using theoretical constructs. Improved ultrasonic and vibration test setups consisting of distributed, high fidelity, and surface mounted sensor arrays are used here to determine changes in the dynamical properties of the composite structural components in the presence of damage. The sensors are assumed to provide both the low frequency global response (i.e., modal frequencies and mode shapes) of the structure to external loads and the (local) high frequency signals due to wave propagation effects in either passive or active mode of the ultrasonic array. A damage index, comparing the measured dynamical response of two successive states of the structure is introduced as a determinant of structural damage. The method relies on the fact that the dynamical properties of a structure change with the initiation or growth of damage. A diagnostic imaging tool is used for the interpretation and graphical representation of the indices to enable automated monitoring of the changes in the indices at a given instant of time. The value of the index at a given sensor increases with the proximity of the damage to the sensor. A sensitivity analysis is carried out in an effort to determine a threshold value of the index below which no reliable information about the state of health of the structure can be estimated. It is shown that the automated procedure is able to identify a defect right from its appearance, with some degree of confidence. The feasibility of developing a practical intelligent structural health monitoring system (ISHMS), based on the concept of `a structure requesting service when needed,' is discussed.

Bilevel Optimization and Postbuckling of Highly Strained Composite Stiffened Panels

The paper presents a bilevel strategy for the efficient optimum design of composite stiffened panels using VICONOPT, a fast-running optimization package based on linear eigenvalue buckling theory, and embracing practical composite design rules. Panel level optimization finds a minimum weight cross-sectional geometry based on a substitution of equivalent ortbotropic plates for laminated plates. Optimization at the laminate level finds stacking sequences satisfying laminate design rules. VICONOPT models are validated with ABAQUS finite element models, and with experimental compressive testing of two blade-stiffened panels. The buckling and postbuckling behavior of the two panels, with initial buckling in the stiffeners and skin, respectively, is investigated in a high load and high strain range. The bilevel strategy is evaluated by the design of a relatively short Z stiffened panel which has been manufactured and tested, and also by design of a long wing cover panel with combined loads. The weight saving from the wing cover panel is 13% compared with an existing datum design. This demonstrated that the strategy is efficient, reliable, and extendable into the long panel range.

Dynamic instability analysis of laminated composite stiffened shell panels subjected to in-plane harmonic edge loading

The dynamic instability characteristics of laminated composite stiffened shell panels subjected to in-plane harmonic edge loading are investigated in this paper. The eight-noded isoparametric degenerated shell element and a compatible three-noded curved beam element are used to model the shell panels and the stiffeners respectively. As the usual formulation of degenerated beam element is found to overestimate the torsional rigidity, an attempt has been made to reformulate it in an efficient manner. Moreover the new formulation for the beam element requires five degrees of freedom per node as that of shell element. The method of Hill's infinite determinant is applied to analyze the dynamic instability regions. Numerical results are presented to demonstrate the effects of various parameters like shell geometry, lamination scheme, stiffening scheme, static and dynamic load factors and boundary conditions, on the dynamic instability behaviour of laminated composite stiffened panels subjected to in-plane harmonic loads along the boundaries. The results of free vibration and buckling of the laminated composite stiffened curved panels are also presented.

Buckling of laminated composite stiffened panels subjected to in-plane shear: A parametric study

The presence of in-plane loading may cause buckling of stiffened panels. An accurate knowledge of critical buckling load and mode shapes are essential for reliable and lightweight structural design. This paper presents some parametric studies on simply supported laminated composite blade-stiffened panels subjected to in-plane shear loading. A total of 450 models were analyzed using ANSYS 7.1 and a database is prepared for different plate and stiffener combinations. Studies are carried out by changing the panel orthotropy ratio, stiffener depth, pitch length (number of stiffeners), smeared extensional stiffness ratio of stiffener to that of the plate and extensional stiffness to shear stiffness ratio of the plate. Based on the studies, few important parameters influencing the buckling behaviour are identified and guidelines for better stiffener proportioning are developed, which will be helpful for the designer.

Buckling and postbuckling of stringer stiffened fibre composite curved panels – Tests and computations

The European Commission was funding within its 5th Framework Programme the project POSICOSS, which coped with the demand to design fibre composite fuselage structures for postbuckling under ultimate load. The main objective of the work was the development of improved, fast and reliable procedures for postbuckling analysis and design of fibre composite stiffened panels of future fuselage structures. Another substantial objective was the creation of comprehensive experimental data bases for the purpose of validation. This paper deals with the buckling and postbuckling experiments on axially compressed, stiffened CFRP (carbon fiber reinforced plastics) curved panels, performed by DLR within the POSICOSS project for validation purposes, and it compares experimental results with computations.

POSICOSS—improved postbuckling simulation for design of fibre composite stiffened fuselage structures

The European Commission was funding within its fifth Framework Programme the project POSICOSS, which coped with the demand to design fibre composite fuselage structures for postbuckling under ultimate load. The main objective of the work was the development of improved, fast and reliable procedures for postbuckling analysis and design of fibre composite stiffened panels of future fuselage structures. They were essential, because so far postbuckling calculations were extremely time consuming and as such not applicable for design. Another substantial objective was the creation of comprehensive experimental data bases for the purpose of validation. The paper presents an introduction to the POSICOSS project; it derives the main objective of the project from the main general objectives for the design of the next generation of aircraft structures, and it summarizes the work done. The results achieved are only touched, because details of them can be found by consulting the references given.

Post-buckling optimization of composite stiffened panels: Computations and experiments

A multi-objective optimization procedure for the design of composite stiffened panels capable to operate in post-buckling is presented. The procedure is based on Genetic Algorithms and three different methods of global optimization: Neural Networks, Radial Basis Functions and Kriging approximation. Response surfaces are used to approximate the post-buckling behaviour of the panels using a limited number of sample points. Optimization results underline the significance of non-dominated solutions, as the best panel configurations inside the domain of interest. Finally, one of the non-dominated solutions is selected, manufactured and tested up to collapse. The analyses, performed a priori without considering any kind of imperfection, are closed and in good agreement with the tests in terms of pre-buckling and post-buckling stiffness, as well as in terms of collapse loads. The introduction of imperfections reduced the percentage errors between computations and tests within 5% in so far as buckling and collapse loads are concerned. The obtained results prove the influence of the initial imperfections not only on the first-buckling load but also in the post-buckling range up to collapse.

Compressive property degradation of composite stiffened panel due to debonding and delaminations

The effect of localized damage due to impact on compressive buckling as well as postbuckling behaviors of blade stiffened composite plates was numerically studied. A partial debonding between a skin panel and a flange and multiple delaminations in the skin panel were chosen as the localized damage. The three-dimensional composite elements used to analyze the compressive behavior of the stiffened panel with multiple delaminations was found to be suitable for this kind of complex composite structure. The contact problem between the skin and flange panels was approximated by a spring element with no restraint of the positive relative displacement and a strong restraint of the negative displacement. At the delaminations in the skin panel, which tended to close during compression, the normal relative displacement was constrained to prevent the delaminated portions from overlapping; this constraint made the convergence of the solution quite smooth compared to the case where the contact problem was exactly considered. When a debonded area only was located at the edge of the flange, no notable reduction of compressive buckling load was found until the size of the debonding reached a half wavelength of the buckling mode. The compressive buckling load dropped significantly when multiple delaminations, which were small in comparison to the half wavelength of the buckling mode, accompanied the skin-flange debonding. The energy release rate distributions at the damage edges, calculated using the virtual crack closure technique, increased quite rapidly, particularly in the transverse direction after buckling became sufficient to increase the delamination.

Influence of loading conditions on the impact damage resistance of composite panels

In this paper the onset of impact induced delaminations in stiffened composite panels is analysed by using an approach based on a threshold critical impact force. This approach is adopted in combination with geometrically non-linear finite element analyses in order to investigate the influence of the in-service loading conditions on the impact damage resistance of the composite panels. An application to a stiffened composite panel under compression is presented in order to point out the sensitivity of the impact damage resistance to the level of load. Finally, indications on a novel approach, able to take into account the non-linearity of the composite stiffened panels’ response, are given.

A numerical and experimental investigation on composite stiffened panels into post-buckling

The structural behaviour of composite stiffened flat panels under axial compression is here investigated up to collapse. The panel configuration is designed to buckle once the limit load is reached and to work in post-buckling until the ultimate load. The design phase is based on the use of four different kinds of finite element analyses: eigenvalue, non-linear static with modified Riks’ method and both implicit and explicit dynamic analyses. Once the final configuration is identified, two specimens are manufactured. The initial geometrical imperfections are measured and analyzed, then axial compression tests are performed until collapse. As foreseen by the numerical analyses, experimental results prove the ability of the panels designed to work in the post-buckling field until collapse which takes place due to the failure of the stiffener blades. Finally, the measured initial imperfections are included in the model significantly increasing the numerical–experimental correlation.

Minimum-weight design of compressively loaded composite plates and stiffened panels for postbuckling strength by Genetic Algorithm

The minimum-weight design of compressively loaded composite plates and composite stiffened panels under constrained postbuckling strength is addressed herein. A nonlinear finite element code, COSAP (COmposite Structural Analysis Program) was applied to analyze the buckling and postbuckling behavior. As an optimization technique, a modified Genetic Algorithm was used to find the optimum points. The parallel computing scheme was implemented with MPI (Message Passing Interface) library and realized in a parallel-computing supercomputer and a pc cluster. The optimal design was performed with two chosen examples: a composite plate and a composite stiffened panel. The design variables were the number of plies and the ply angle in each ply. In case of the stiffened panels, the size and the location of the stiffeners were also considered as the design variables. The objective function was defined as the product of the nondimensional weight and the strength. The optimization results showed better performances than conventional designs.

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.

Reliability analysis of a ship hull in composite material

A simple and effective analytical method for ultimate longitudinal strength calculation and reliability analysis of a ship’s hull in composite materials is presented. The ultimate strength formula of composite stiffened panel is derived from composite column theory. Deck and ship bottom structures are modelled as assemblies of stiffened panels. Then, progressive collapse analysis is applied to calculate the longitudinal ultimate strength of ship hull in composite materials. The response-surface method and the first order reliability method are combined to calculate the safety index and failure probability. Moreover, the sensitivity of each variable that has effect on the reliability is discussed.

Assessment of Delamination Fracture Load of Stringer Stiffened Composite Panel

Conclusions The concept and design freedom introduced by adopting spatial Ž lters to second-orderpartial differential equationsystems have been successfully veriŽ ed by an one-dimensional clamped-free shaft-based rotational acceleration rate sensor. It was shown that spatial Ž lters can introduce a no-phase delay low-pass Ž lter by offering a weighting function with the prechosen surface electrode, and the introduction of this odd function brings a differentiation to the transfer function. The method of imaging was introduced to extend the spatial Ž lter application area from a Ž nite structure to an inŽ nite domain, that is, spatial Ž lters can now be placed near or even at the boundaries.BeneŽ ts associatedwith adoptingspatial Ž lters to point sensors successfully demonstrate theoretically as well as experimentally the advantagesof the newly developed rotational acceleration rate sensor. In summary, expanding the bandwidth of rotational accelerationrate sensors by using newly developed Ž nite domain spatial Ž lters has been successfully veriŽ ed both theoretically and experimentally.

Linear static analysis of composite hat-stiffened laminated shells using finite elements

Analysis of composite stiffened panels used in aerospace, ship and other engineering structures by the method of finite elements has been presented. The formulation presented is based on the concept of equal displacements at the shell–stiffener interface. An eight-noded isoparametric shell element is used in association with three-noded curved beam element for the formulation of the stiffened panel element. First-order shear deformation theory is used in the present study. The stiffness matrix of the stiffener is computed first irrespective of its position within the plate/shell element. The stiffness matrix so computed for the stiffener is then transferred to that of the plate/shell element nodes depending on its position and orientation within the element before assembling the element stiffness matrix. A generalised formulation for the hat-shaped stiffener is presented. The investigation is restricted to linear static analysis of composite stiffened panels and parametric studies have been made to study the various aspects of laminated composite shell with open and closed shaped stiffeners.