Composite Stiffened Panels Research Articles

DAMAGE CHARACTERIZATION OF COMPOSITE STIFFENED PANELS UNDER IMPACT LOADING IN AIRCRAFT APPLICATION

Composite Materials have becomesa popular due to itshigh specificstrength and high stiffness to weight ratio. They found extensive applications in automobile, aerospace, defence equipment’s and other critical components. Composite plates are predominantly used as alternative materials to regular materials. In order to provide even better strength and resistance to deformation, stiffeners are attached to the composite plates thereby increasing the bending stiffness to a large extent. These stiffened panels have found principal application in aircraft wings, ship hulls and bridge decks. In this project, low velocity impact on composite plate and composite stiffened panel has been studied. Numerical models of composite plates and stiffened panels are impacted with different energies (4J, 9J, 16J) and are analysed by finite element software ABAQUS explicit and four different oblique impact angles 90◦, 60◦, 45◦and 30◦ are analysed. Different parameters such as displacement, contact force, energy absorbed were compared for both composite plates and stiffened panels. It was noted that stiffened panels offer more resistance to deformation and absorb more energy due to high stiffness. Keywords:Stiffened Panels, low-velocity impact, FE Model, ABAQUS Explicit.

An innovative modeling strategy for flexural response of fiber-reinforced stiffened composite structures

The use of composite stiffened panels is common in a large variety of engineering structures. An advanced computational model is developed to accurately predict the complex behavior of laminated composite stiffened panels having thin-walled open or closed section stiffeners with in-filled materials. In the approach proposed, the plate and stiffeners are discretely modeled using 2D plate elements for the skin or plate, and 1D beam elements for the stiffeners, where the deformations of the stiffeners are completely expressed in terms of deformations of the plate to minimize the number of unknowns. The key novelty in this study is the model formulation of the stiffeners, which is based on a rigorous cross-sectional analysis to include all effects including out-of-plane and in-plane warping along with their couplings. To validate the proposed model, numerical examples of stiffened panels with different stiffener cross-sectional geometries and composite layups have been analyzed and benchmarked against results from the literature. Moreover, the predictions of the new model have also been successfully compared against detailed finite element model results to study the behavior of stiffened panel configurations, which are not available in the open literature. The results predicted by the proposed model confirmed a very good performance in terms of accuracy and range of applicability.

Cohesive Zone Model Modification Techniques According to the Mesh Size in Finite Element Models of Stiffened Panels with Debonding

When catastrophic failure phenomena in aircraft structures, such as debonding, are numerically analyzed during their design process in the frame of “Damage Tolerance” philosophy, extreme requirements in terms of time and computational resources arise. Here, a decrease in these requirements is achieved by developing a numerical model that efficiently treats the debonding phenomena that occur due to the buckling behavior of composite stiffened panels under compressive loads. The Finite Element (FE) models developed in the ANSYS© software (Canonsburg, PA, USA) are calibrated and validated by using published experimental and numerical results of single-stringer compression specimens (SSCS). Different model features, such as the type of the element used (solid and solid shell) and Cohesive Zone Modeling (CZM) parameters are examined for their impact on the efficiency of the model regarding the accuracy versus computational cost. It is proved that a significant reduction in computational time is achieved, and the accuracy is not compromised when the proposed FE model is adopted. The outcome of the present work leads to guidelines for the development of FE models of stiffened panels, accurately predicting the buckling and post-buckling behavior leading to debonding phenomena, with minimized computational and time cost. The methodology is proved to be a tool for the generation of a universal parametric numerical model for the analysis of debonding phenomena of any stiffened panel configuration by modifying the corresponding geometric, material and damage properties.

A robust cumulative damage approach for the simulation of delamination under cyclic loading conditions

Interlaminar damage evolution is one of the topics of main interest for composite materials research. An excellent level of knowledge has been achieved in the frame of the studies on delaminations propagation under static loading conditions (as a consequence of low velocity impacts). On the contrary, delaminations evolution under cyclic loading conditions is still an open challenge for the scientific community. Since expensive experimental campaigns are needed to assess the fatigue behaviour of Carbon Fibre Reinforced Polymers (CFRPs), the development and implementation of robust and efficient computational finite elements methodologies has become relevant. In this framework, a robust numerical finite element procedure able to simulate the fatigue driven delamination growth is proposed in this paper. A Paris-law based fatigue tool has been implemented in the commercial Finite Element Code ANSYS MECHANICAL via the Ansys Parametric Design Language (APDL) together with a static delamination procedure (named SMXB) previously developed by the authors. The proposed numerical procedure, based on the Virtual Crack Closure Technique (VCCT), is characterised by load step and element size insensitivity within non-linear incremental analyses. The proposed fatigue procedure, named FT-SMXB, has been preliminary validated at coupon level, by comparing the numerical results to literature experimental data on a unidirectional graphite/epoxy Double Cantilever Beam (DCB) specimen. Actually, the excellent agreement between the achieved numerical results and the literature experimental benchmark data proves the accuracy and the potential of the proposed methodology. Subsequently, a numerical application on a composite stiffened panel with skin stringer debonding under compressive-compressive fatigue loading conditions, has been performed. The Excellent agreement found between the obtained numerical results and available experimental literature data, for this application, in terms of number of cycles to failure and debonding propagation as a function of number of cycles, proves that the use of the proposed numerical tool can be extended, with profit, to geometrically complex and more realistic fatigue test cases.

Support Vector Machine Applied to the Optimal Design of Composite Wing Panels

One of the core technologies in lightweight structures is the optimal design of laminated composite stiffened panels. The increasing tailoring potential of new materials added to the simultaneous optimization of various design regions, leading to design spaces that are vast and non-convex. In order to find an optimal design using limited information, this paper proposes a workflow consisting of design of experiments, metamodeling and optimization phases. A machine learning strategy based on support vector machine (SVM) is used for data classification and interpolation. The combination of mass minimization and buckling evaluation under combined load is handled by a multi-objective formulation. The choice of a deterministic algorithm for the optimization cycle accelerates the convergence towards an optimal design. The analysis of the Pareto frontier illustrates the compromise between conflicting objectives. As a result, a balance is found between the exploration of new design regions and the optimal design refinement. Numerical experiments evaluating the design of a representative upper skin wing panel are used to show the viability of the proposed methodology.

Moisture absorption behavior of epoxy resin matrix composite stiffened panel

Moisture absorption behavior of epoxy resin matrix composite stiffened panel

Modified engineering method for ultimate load prediction of composite hat-stiffened panels

In this paper, the ultimate load of a typical hat type composite stiffened panel under axial compression is simulated by finite element method. Based on the ultimate load value and failure mode obtained by numerical simulation, the thickness of the lower flange of the stiffener in the engineering method is modified by using the stiffness equivalent method. The modified engineering method is in good agreement with the finite element results, which has a certain reference significance for the rapid calculation and analysis of stiffened plate in the early design stage.

Vibration-based detection of skin-stiffener debonding on composite stiffened panels using surrogate-assisted algorithms

Stiffened panel is a common application of fiber reinforced polymer (FRP) laminates in engineering industries, particularly those in mechanical and aerospace domains. However, laminated structures such as stiffened FRP panels are sensitive to impact and frequently suffer from invisible inter-laminar defects, for instance, in the bonding interface between the stiffener and the skin plate, which results in a considerable reduction of structural load capability and service lifetime. In this paper, a vibration-based approach is applied to assess skin-stiffener debonding on carbon fiber reinforced polymer (CFRP) stiffened panel using changes in natural frequencies. Three computational intelligence approaches, namely artificial neural network (ANN), genetic algorithm (GA) and Jaya algorithm, are employed as inverse algorithms for predicting the type, location and size of debonding damage. Validations of the proposed inverse algorithms are firstly conducted through numerical test cases. Thereafter, experimental modal testing is conducted using scanning laser Doppler vibrometer on CFRP stiffened panel specimens, and the measured frequency changes are then fed into inverse algorithms for the experimental validation. The performances of three inverse algorithms in prediction accuracy, running speed and reliability are compared. Lastly, a noise sensitivity analysis is conducted for evaluating the robustness of algorithms’ performance for this application.

The study on stability of composite panel structure under axial compression load

The axial compressive load test and finite element simulation are made to study the mechanical properties of aircraft stiffened composite panel. Firstly, the numerical simulation model of composite stiffened panel structure is constructed, the accuracy and feasibility of the simulation model are verified by calculating the value of MAC (Modal Assurance Criterion) between the computational free modal and experimental free modal. Then, the nonlinear post-buckling numerical simulation is carried out to extract the buckling load, the post-buckling load path and the failure load. Then, the axial compression test of the panel structure is carried out by the testing machine, and the load-displacement curve and the load-strain curve of the loading position are obtained. Finally, the correctness of the analysis method is proved by comparing the numerical simulation value with the experimental analysis value. The experimental research method of the mechanical properties of the composite stiffened panel under axial load is explored, which can be used as a reference for the study of the bearing capacity of the stiffened panel.

Damage detection in large composite stiffened panels based on a novel SHM building block philosophy

This paper proposes a structural health monitoring (SHM) building block (BB) approach for guided wave based structural health monitoring (GWSHM) of large composite stiffened panel subjected to varying temperature conditions. The proposed approach follows a similar philosophy to industrial substantiation of composite structures through BB by conducting SHM analysis and associated tests at different levels of structural complexity, beginning with small coupon specimens and progressing through structural elements and details (mono-stringer) and finally sub-components (large stiffened composited panel). Each level builds on the knowledge gained at previous, less complex levels. The detection of multiple barely visible impact damage (BVID) in the large panels is achieved by outlier analysis using a reference pristine database gathered from simple coupons and mono-stringer panels under a wide range of temperature variation. This approach enables the data utilisation from lower-level structure in the proposed BB test scheme to higher-level structures and hence reduce the number of tests required for large and complex structures. The detected damage is then localised by a proposed two-step multi-damage localisation method, which first confirms the presence of the damage and narrows down the damaged section then provides an enhanced damage imaging for precise location estimation. Damage detection with a true positive rate of 0.909 and a false positive rate of 0.111 as well as accurate multiple damage localisation are achieved in the flat fuselage panels for a temperature range of 25 °C–50 °C.

Nonlinear ultrasonic imaging of damage in composite materials using a higher harmonic and modulated multi-path reciprocal method

Structural health monitoring has become an important factor in the assessment of defects/damage in material components. Ultrasonic methods generally incorporate a sparse array of sensors/transducers as they provide a low number of piezoelectric sensors per area, thus providing savings with regard to system costs and weight. Many structural health monitoring techniques rely on linear ultrasonic effects such as reflections, amplitude changes, time of arrival and wave scattering effects which rely on precise baseline measurements. In this work, a nonlinear ultrasonic method based on a sparse array of surface-bonded ultrasonic transducers was used to evaluate the second harmonic and modulated elastic responses from a damaged medium. A complex composite stiffened panel with barely visible impact damage was evaluated. The points closest to damage are found on the paths between transmitter–receiver pairs through a reciprocal relationship of nonlinear elastic parameters and a statistical approach was used to select a cloud of points so that a two-dimensional image of the damaged region is created. Experimental results revealed that the second-order nonlinear parameter provided accurate damage localisation and imaging and the use of modulation bands further improved imaging accuracy and damage localisation. The maximum error between the calculated and the real damage area centres was only 1.3 mm. The proposed nonlinear elastic multi-path imaging technique based on higher harmonic generation and modulation coupled with a statistical approach provides damage detection without a priori knowledge of the material characteristics, which is in direct contrast to conventional linear ultrasonic methods which rely on precise measurement of elastic wave effects both before and after the initiation of damage.

Effects of hygrothermal environment on the shear behavior of composite stiffened panels

Hygrothermal effects on shear behaviors of composite stiffened panels are investigated experimentally and analytically. Firstly, a moisture absorption test of the stiffened panel is performed, and a new moisture absorption model of the stiffened panel is established based on the non-Fickian model. Compared with the existing model, the proposed model can better describe the moisture absorption behavior of the stiffened panel. Shear tests are then carried out on the virgin and hygrothermally aged stiffened panels. Observed effects on hygrothermally aged specimens include change in buckling mode, increase in skin–stiffener separation, and aggravation of local skin warpage. Compared with virgin specimens, the average buckling load and failure loads of aged specimens are reduced by 15.9% and 9.7%, respectively. In addition, a simplified, semi-analytical method is proposed to predict the critical buckling load of stiffened panels subjected to shear load in the hygrothermal environment. In this method, a micro-mechanical model is built considering the hygrothermal degradation of material properties and the hygrothermal strain of the material. The buckling load calculated by this method is in good agreement with both the FEM results and experimental results.

Damage and failure analysis of composite stiffened panels under low-velocity impact and compression after impact

Impact resistance and damage tolerance after impact are important performance reference indices in the structural design of composite stiffened panels. The damage evolution and failure mechanism of composite panels with foam-filled hat-stiffeners under low-velocity impact and compression after impact (CAI) at different positions and different energies were studied by experimental and numerical simulation methods. The delamination damage caused by low-velocity impact was captured by ultrasonic phased array C-scan, and a fringe projection profile (FPP) measurement system was employed to measure the full-field buckling deformation and mode evolution of the stiffened composite panel during CAI. A damage model based on the Hashin failure criterion was established to study the complex damage and failure mechanisms, and the interface cohesion behavior embedded in ABAQUS/ Explicit was used to define the interlaminar damage model for characterizing delamination. The interlaminar delamination damage and intralaminar damage behavior in the process of impact and CAI were considered. The results obtained from the numerical simulation and the experimental results were combined to analyze the damage modes of the composite stiffened panel under different impact positions and impact energies and their effects on the damage development and failure during compression.

Influence of adhesive interface properties on the post-buckling response of composite I-stiffened panels with lateral support under axial compression

The post-buckling carrying capacity is a major concern in the engineering design of composite stiffened panels, and is obviously influenced by the properties of the skin-stiffener adhesive interface. This work investigated the influence of adhesive interface properties on the post-buckling response of composite I-stiffened panels with lateral rib support under axial compression. After the compression tests, the buckling instability, post-buckling carrying capacity and failure mode of the specimens were obtained. The post-buckling process and failure mechanism of the stiffened panels were then analyzed in detail through finite-element (FE) method. With the validated FE model, the post-buckling response of composite stiffened panels with different interface adhesive materials combined with different bonding methods were simulated and compared. A further parametric analysis was implemented in analyzing the influence of skin-stiffener interface strength and critical strain energy release rate on the post-buckling responses of composite stiffened panels. The results indicate that the property of interface strength affects the post-buckling carrying capacity within a certain limit, and the critical interface strength is achieved. The properties of both interface strength and interface critical strain energy release rate have effects on the structural failure process and failure mode.

Effects of debonding defects on the postbuckling and failure behaviors of composite stiffened panel under uniaxial compression

Effects of stiffener debonding defects on postbuckling responses and failure behaviors of composite stiffened panels which have extensive applications in aerospace structures attract significantly increasing attention recently. Aiming to this problem, a series of nonlinear analyses comprehensively accounting for multiple failure modes including intra- and inter-laminar damage in composite laminates, adhesive layer damage between stiffeners and skin, as well as buckling and postbuckling responses were carried out via a numerical model. This numerical model is combined with a progressive damage model and a cohesive zone model, and has been validated to be effective for predicting the postbuckling and failure responses of composite stiffened panels. Seventeen computational cases with different parameters including debonding sizes and positions which can affect buckling and postbuckling responses were conducted. It follows that debonding defects can change postbuckling deformation modes of stiffened panels and their development paths, which induce different degrees of inflection and distortion in structures, and change failure behaviors as well as the failure process of composite laminates, and further weaken the ultimate failure strength of the whole composite stiffened panels.

Ananalytical and Experimental Study of T - Shaped Composite Stiffened Panels: Effect of 90° Plies in Stringers on Curing and Buckling Performance

The effect on curing deformation and strength of 90° plies in stringers for composite T-shaped stiffened panel were presented by numerical analysis and experimental measurement. In this work, the finite element model (FEM) for curing deformation based on cure kinetics formulation was utilized. The model for buckling and failure based on Tsai-Wu failure criteria and incremental/ Newton -Raphson mixed iteration equation was established. The curing, buckling and post-buckling analysis were performed on composite T-shaped stiffened panels to obtain the deformation, the critical load and mode of failure, with different 90° plies-ratios in stringers. Furthermore, the experimental investigation has been carried out for specimens with different plies-ratios of 90° plies in stringers. The good agreement between the numerical results obtained by finite element method and the experimental ones demonstrated that this kind of numerical analysis could estimate appropriately the behavior of the structure. In addition, it was found that the 90° plies in stringers play an important role in reducing the curing deformation, but it has little effect on the axial compression loading capacity for composite T-shaped stiffened panel, which is useful for application in civil aviation aircraft.

An Engineering Correction Method for Buckling Load of Dense Stiffened Panels

It is convenient for designers to get the buckling loads of sparse stiffened panels quickly by using engineering calculation method to analyze the stability of composite stiffened panels, but it is still unable to meet the accuracy requirements of analysis of dense stiffened panels. The buckling loads of stiffened panels are closely related to the buckling modes. Based on capturing and analyzing the Compressive Buckling waveforms of T-shaped densely stiffened panels, this paper presents a formula for calculating the buckling loads according to the geometric coefficients. The results are very similar to those of finite element simulation, and can be used to calculate the buckling loads of sparse and dense stiffened panels with different stiffeners.

Experimental Study on Compression Behavior of Fiber-Reinforced Resin-Based Composite Stiffened Panels in Hygrothermal Environment

Fiber-reinforced resin-based composite stiffened panels are put in the hygrothermal box at 70°C and 85% RH until moisture absorption reaches balance. Then, the compression experiments on the common stiffened panels and the stiffened panels after moisture absorption are conducted to get buckling and failure loads. Test results are compared to analyze the effect of damage on the carrying capacity of the structure. The buckling load does not significantly change, but the failure load of the panels after moisture absorption is reduced by 10.2% as compared to the common panels. The failure modes are mainly composed of debonding and fracture of the stiffener, the skin cleavage and deformation of the specimen. The experimental results will offer some valuable guidelines to engineering applications of the structure.

Probabilistic optimisation of mono-stringer composite stiffened panels in post-buckling regime

In this paper, a multi-objective probabilistic design optimisation approach is presented for reliability and robustness analysis of composite structures and demonstrated on a mono-omega-stringer stiffened panel. The proposed approach utilises a global surrogate model of the composite structure while accounting for uncertainties in material properties as well as geometry. Unlike the multi-level optimisation approach which freezes some parameters at each level, the proposed approach allows for all parameters to change at the same time and hence ensures global optimum solutions in the given parameter design space (for both probabilistic and deterministic optimisations) within a certain degree of accuracy. The proposed approach is used in this study to conduct extensive multi-objective probabilistic and deterministic optimisations (without considering safety factors) on a mono-stringer stiffened panel. In particular, a global surrogate model is developed utilising the computational power of a high-performance computing facility. The inputs of the surrogate model are the omega-stringer geometry and the mechanical properties of the composite material, while the outputs are the fundamental linear buckling load (LBL) and the nonlinear post-buckling strength (NPS). LBL and NPS are obtained via detailed parametric finite element models of the mono-stringer stiffened panel; in the nonlinear model, the interface between the skin and the omega-stringer is modelled via cohesive elements to allow for debonding in the post-buckled regime. Extensive multi-objective optimisations are conducted on the surrogate model using deterministic and probabilistic approaches to examine the omega-stringer geometric parameters mostly affecting the system robustness and reliability. The differences between deterministic and probabilistic designs are highlighted as well.

A Metamodel Based on Non-Uniform Rational Basis Spline Hyper-Surfaces for Optimisation of Composite Structures

This study presents a new strategy to generate a surrogate model used for design purposes. The metamodel is based on Non-Uniform Rational Basis Spline (NURBS) hyper-surfaces and is able to fit non-convex sets of target points (TPs). The proposed method aims at determining all the parameters involved in the definition of the NURBS hyper-surface, i.e. control points (CPs) coordinates, weights, degrees, CPs number and knot-vector components. To this purpose, the problem of finding a suitable metamodel is formulated as a constrained non-linear programming problem (CNLPP) wherein the above variables are optimised in order to fit a set of TPs. Nevertheless, when the number of CPs and the degrees of the basis functions are included among the design variables, the resulting problem is defined over a space having a variable dimension. This problem is solved by means of a special genetic algorithm able to determine simultaneously the optimum value of both the design space size (related to the integer variables of the NURBS hyper-surface) and the NURBS hyper-surface continuous parameters. The NURBS-based metamodel is then used to emulate the first buckling load of a composite stiffened panel and it is used in the framework of a meaningful design problem.