Composite Stiffened Panels Research Articles

Experimental study on delamination growth of stiffened composite panels in compression after impact

Abstract The purpose of this paper was to investigate the failure behaviors of composite stiffened panels with skin low-velocity impact (LVI) damage under axial compression . Barely visible impact damage (BVID) was introduced into the L-shaped and T-shaped stiffened panels within the skin between stiffeners . Compression after impact (CAI) tests were performed and delamination growth of specimens was observed directly. The results show that there are two types of delamination growth processes, which are unstable and stable respectively. Due to the restriction effect of the stiffener, the delamination growth is easier to initiate in the outer sublaminates on the smooth side of the skin compared to the ones on the stiffener side. In addition, according to the thermal-deply result, adjacent impact delaminations can be connected by matrix cracks , which results in a larger area of the sublaminate . It is revealed that buckling of the sublaminates and subsequent transverse delamination propagation in large area are the triggers leading to the final failure of the stiffened panel.

Concurrent optimization of stacking sequence and stiffener layout of a composite stiffened panel

ABSTRACTThis article is to optimally design laminated composite stiffened panels by optimizing both stacking sequences of the panel skin and stiffeners as well as the layout of stiffeners. Starting from initial designs of stiffener layout and stacking sequences for each stiffener and the panel skin, the problem is formulated with discrete and continuous variables, where discrete 0/1 variables represent the absence/presence of each layer in initial stacking sequences, and continuous variables represent layer thicknesses. A first-level approximate problem is established to make the problem explicit. Genetic algorithm is used to determine the existence of each layer in the laminates. When the number of retained layers in stiffener becomes zero, that stiffener can be seen as unnecessary and removed. For individual fitness calculation, a second-level approximate problem is constructed to optimize continuous ply thicknesses of retained layers. Correspondingly, laminated stacking sequences and stiffener layout are concurrently optimized.

A methodology to simulate low velocity impact and compression after impact in large composite stiffened panels

Low velocity impact events significantly reduce the mechanical performance of composite structures even though the damage might be barely visible. Numerical simulations can be used to understand and improve the damage resistance and tolerance of composite structures. However, numerical simulations are usually computationally intensive and their application in large composite structures is limited. Furthermore, the numerical models require many parameters that affect their efficiency, accuracy, objectivity and robustness. The present work describes a methodology to simulate low velocity impact and compression after impact which is applied to a composite stiffened panel undergoing visible impact damage. The key definitions are discussed and special attention is devoted to the computational efficiency. The numerical results are compared with experimental data, and the suitability of the proposed methodology is discussed.

A general multi-scale two-level optimisation strategy for designing composite stiffened panels

This work deals with the problem of the least-weight design of a composite stiffened panel subject to constraints of different nature (mechanical, geometrical and manufacturability requirements). To face this problem, a multi-scale two-level (MS2L) design methodology is proposed. This approach aims at optimising simultaneously both geometrical and mechanical parameters for skin and stiffeners at each characteristic scale (mesoscopic and macroscopic ones). In this background, at the first level (macroscopic scale) the goal is to find the optimum value of geometric and mechanical design variables of the panel minimising its mass and meeting the set of imposed constraints. The second-level problem focuses on the laminate mesoscopic scale and aims at finding at least one stacking sequence (for each laminate composing the panel) meeting the geometrical and mechanical parameters provided by the first-level problem. The MS2L optimisation approach is based on the polar formalism to describe the macroscopic behaviour of the composites and on a special genetic algorithm to perform optimisation calculations. The quality of the optimum configurations is investigated, a posteriori, through a refined finite element model of the stiffened panel making use of elements with different kinematics and accuracy in the framework of the Carrera’s Unified Formulation (CUF).

Influence of stiffener configuration on post-buckled response of composite panels with impact damages

In this paper, the effect of T, I and J stiffer configurations on post-buckling response of composite stiffened panels with impact damage is investigated through experiments and numerical simulations. Two identical panels in all three configurations were designed and manufactured. Each panel has four stringers of the same type. All panels were designed to have a skin buckling load of 92 kN ± 5 kN and weight of 1.45 kg ± 0.05 kg to separate the effect of impact damage irrespective of stiffener type. One panel from each configuration is impacted with 50 J energy above stiffener flange from skin side to create a Barely Visible Impact Damage (BVID) and followed by a compression test till collapse. A comparative study between pristine and impacted panels is presented. The effect of impact damage on buckling load, post-buckling response, collapse load and end-shortening were investigated. Moreover, finite element numerical models were developed for all panels which include intra-laminar and inter-laminar damage initiation and growth models. Impact damage area measured from ultrasonic C-scan was modelled in commercial finite element software Abaqus®. Failure modes in pristine and impacted panels such as disbond, tearing of stiffener web and locations from experiments are discussed and validated by numerical simulations.

Multi-objective optimization of a composite stiffened panel for hybrid design of stiffener layout and laminate stacking sequence

This paper presents a two-level approximation method for multi-objective optimization of a composite stiffened panel. The purpose is to seek the minimum structural mass and maximum fundamental frequency subject to given displacement constraints and manufacturing limitations. The design variables are the stiffener layout, and laminate stacking sequences for stiffeners and the panel skin. By introducing the concept of ground structure in both stiffener layout and laminate stacking sequence, the design problem is formulated with mixed discrete and continuous variables. Two types of discrete variables represent the existence of each stiffener and the existence of each ply in the laminate, respectively, with continuous ones for ply thicknesses. Considering the objectives are of different dimensions, a weighted min-max objective function is defined and minimized. The problem is firstly made explicit with branched multipoint approximate functions. Genetic algorithm (GA) is then adopted to optimize two types of discrete variables, determining which stiffeners/layers are deleted or retained. For fitness calculation in GA, a second-level approximation is built to optimize continuous ply thicknesses of the necessary layers that are retained. By giving different initial designs of stiffener layout and laminate stacking sequences, reasonable optimization results, which are tradeoffs between the considered two objectives, are obtained as design options. From the number of required structural analysis, it shows that the proposed method has a good efficiency in seeking rational solutions, which are tradeoffs between conflicting objectives and also feasible designs satisfying all considered constraints.

Effects of plies orientations and initial geometric imperfections on buckling strength of a composite stiffened panel

The use of composite stiffened panels is common in several activities such as aerospace, marine and civil engineering. The biggest advantage of the composite materials is their high specific strength and stiffness ratios, coupled with weight reduction compared to conventional materials. However, any structural system may reach its limit and buckle under extreme circumstances by a progressive local failure of components. Moreover, stiffened panels are usually assembled from elementary parts. This affects the geometric as well as the material properties resulting in a considerable sensitivity to buckling phenomenon. In this work, the buckling behavior of a composite stiffened panel made from carbon Epoxy Prepregs is studied by using the finite element analysis under Abaqus software package. Different plies orientations sets were considered. The initial distributed geometric imperfections were modeled by means of the first Euler buckling mode. The nonlinear Riks method of analysis provided by Abaqus was applied. This method enables to predict more consistently unstable geometrically nonlinear induced collapse of a structure by detecting potential limit points during the loading history. It was found that plies orientations of the composite and the presence of geometric imperfections have huge influence on the strength resistance.

A Rayleigh-Ritz approach for postbuckling analysis of variable angle tow composite stiffened panels

A Rayleigh-Ritz solution approach for generally restrained multilayered variable angle tow stiffened plates in postbuckling regime is presented. The plate model is based on the first order shear deformation theory and accounts for geometrical nonlinearity through the von Kármán’s assumptions. Stiffened plates are modelled as assembly of plate-like elements and penalty techniques are used to join the elements in the assembled structure and to apply the kinematical boundary conditions. General symmetric and unsymmetric stacking sequences are considered and Legendre orthogonal polynomials are employed to build the trial functions. A computer code was developed to implement the proposed approach and to establish its applicability and its features for investigating variable angle tow structures. The proposed solution is validated by comparison with literature and finite elements results. Original results are presented for postbuckling of variable angle tow stiffened plates showing the potentialities of the method.

Imaging of Barely Visible Impact Damage on a Complex Composite Stiffened Panel Using a Nonlinear Ultrasound Stimulated Thermography Approach

Thermosonics, also known as ultrasonic stimulated thermography, is a rapid non-destructive evaluation technique that uses an infrared camera to visualise material defects by detecting the frictional heating at crack surfaces when a part under inspection is vibrated. These vibrations are usually produced by an ultrasonic horn being pressed against the surface of the test sample, which result in uncontrolled generations of frequency components and excitation amplitude. This makes thermosonics highly non-reproducible and unreliable. This paper presents a novel thermographic method, here named as nonlinear ultrasound stimulated thermography, for the detection and imaging of real material defects such as impact damage on a complex composite stiffener panel. This technique combines nonlinear ultrasonic techniques with thermography. A nonlinear ultrasonic approach was used as signature for a reliable frequency-selective excitation of material defects, while an infrared camera was employed to reveal the damage location and severity. A nonlinear narrow sweep excitation method was employed to efficiently excite the local resonance frequencies of the damaged region in order to give rise to the highest nonlinear harmonic response in the material leading to a high heat generation at the crack surface. The experimental tests were carried out with a laser vibrometer in order to better understand the interaction of elastic waves with nonlinear scattering. An ad-hoc nonlinear thermal-structural finite element and crack model was developed to study the heat generation caused by the movement of the crack surfaces when elastic waves with a particular frequency impinges on the crack interphase with good agreement with the experimental results. The proposed new method allows to detect single and multiple barely visible impact damage in a quick, reliable and reproducible manner and overcomes the main limitations of classical thermosonics.

Optimisation of stiffeners for maximum fundamental frequency of cross-ply laminated cylindrical panels using social group optimisation and smeared stiffener method

A review of the literature reveals a dearth of research on the maximisation of natural frequencies of laminated composite stiffened panels. The present study combines an analytical solution based on the smeared stiffener method with a novel social group optimisation (SGO) algorithm to develop a procedure for the solution of such an optimisation problem. For this purpose, cross-ply laminated circular-cylindrical panels with a simply supported condition, stiffened by stringers and rings, are considered. The quantities and sizes of the stiffener are optimised to maximise the fundamental frequency of the panels under practical constraints. Example optimisations of various panel geometries are performed. The optimisation results show that the fundamental frequency can be increased significantly by using tailored rings and that for the considered numerical examples, stringers do not make any beneficial contribution to the maximisation of the fundamental frequency. It is also demonstrated that the SGO algorithm is an efficient tool for this kind of mixed-integer constrained optimisation problem.

Fatigue life and damage tolerance of postbuckled composite stiffened structures with indentation damage.

The fatigue life and damage tolerance of composite stiffened panels with indentation damage are investigated experimentally using single-stringer compression specimens. The indentation damage was induced to one of the two flanges of the stringer of every panel. The advantages of indentation compared to impact are the simplicity of application, less dependence on boundary conditions, better controllability, and repeatability of the imparted damage. The tests were conducted using advanced instrumentation, including digital image correlation, passive thermography, and in situ ultrasonic scanning. Specimens with initial indentation damage ranging between 32 and 56 mm in length were tested quasi-statically and in fatigue, and the effects of cyclic load amplitude and damage size were studied. A means of comparison of the damage propagation rates and collapse loads based on a stress intensity measure and the Paris law is proposed. The stress intensity measure provides the means to compare the collapse loads of specimens with different damage types and damage sizes, while the Paris law is used to compare the damage propagation rates in specimens subjected to different cyclic loads. This approach enables a comparison of different tests and the potential identification of the effects that influence the fatigue lives and damage tolerance of postbuckled structures with defects.

Effect of Z-pinning on fatigue crack propagation in composite skin/stiffener structures

The skin/stiffener interface debonding has been a longstanding problem for composite stiffened panels. Proper crack-arresting reinforcements become a necessity for the wide application on large-scale framed structures. Z-pinning was employed to strengthen composite skin/stiffener bond in this study. Herein, static and fatigue tensile tests were conducted on a generic configuration to characterize the improvement of Z-pinning on skin/stiffener debonding resistance to skin stretching. Results show that the improvements on ultimate debonding strength and fatigue life are significant, even though the effect on crack onset is marginal under either monotonic or cyclic loading. Z-pinning changes the unstable continuous crack growth into a propagation-suspension-propagation evolution pattern. The crack growth rate is decreased by up to three orders of magnitude due to Z-pinning. Effects of pin distribution were experimentally studied. A locally densified distribution is found to be more effective than the traditional uniform distribution.

Experimental failure investigation of pull-off tests of single T-stiffened composite specimens

This paper investigates the initiation and progression of interlaminar failure complying with characteristic postbuckling deformation patterns using single T-joint specimens with different geometric definitions and laminate disposals. The analysis herein conducted addresses the role of these aspects regarding the characteristic failure behavior in terms of: (i) damage initiation location, and (ii) progression path within the specimen. A thorough qualitative discussion concerning the damage initiation and subsequent progression path at the skin-stringer joint is supported by the development of a simple analytical model based on the Classical Laminate Theory denominated as 3-spot-joint model. These results provide a valuable insight into the principal mechanisms that govern such interlaminar damage events, which usually provoke the structural collapse in actual carbon fiber reinforced polymer composite stiffened panels.

Molding process influence on pull-off performance of composite stiffened panels

Two types of molding process of composite stiffened panels which are co-curing and secondary-bonding are investigated, and pull-off experiment was designed to obtain the load-carrying capability and failure modes. The results show that the main failure mode of the secondary-bonded composite stiffened panels is adhesive debonding between the skin and stiffener. The failure mode of co-cured stiffened panels is complex, which includes skin-stiffener debonding, fiber damage, delamination and so on. The performances of the co-cured stiffened panels are better than that of secondary-bonded stiffened panels. The debonding initiation and propagation were simulated by using the nonlinear finite element method. Using user subroutine and cohesive interface, Hashin and B-K failure criterion were applied to simulate co-cured stiffened panels and secondary-bonded stiffened panels respectively, and the simulation results agreed well with experimental results.

Experimental Study on Fibre Reinforced Resin-Based Composite Stiffened Panel with Low Velocity Impact Damage under Shearing Load

Drop hammer impact tests on the Fibre Reinforced Resin-Based composite stiffened panels are used to make low velocity impact damage. And then shearing experiments on impacted specimen and intact ones are conducted to obtain the buckling load and failure load. Test results of impacted specimens and intact ones are compared to analyse the effect of damage on the carrying capacity of the structure. The results show that there is no obvious buckling during the tests for two kinds of panels. But the low velocity impact damage on the panel can reduce the failure load of the structure a lot. The failure modes mainly contain damage of four corners, cleaving of skin and deformation of specimen and so on. The experimental results offer some valuable references to engineering application of the structure.

Detection, Localization and Quantification of Impact Events on a Stiffened Composite Panel with Embedded Fiber Bragg Grating Sensor Networks.

Nowadays, it is possible to manufacture smart composite materials with embedded fiber optic sensors. These sensors can be exploited during the composites’ operating life to identify occurring damages such as delaminations. For composite materials adopted in the aviation and wind energy sector, delaminations are most often caused by impacts with external objects. The detection, localization and quantification of such impacts are therefore crucial for the prevention of catastrophic events. In this paper, we demonstrate the feasibility to perform impact identification in smart composite structures with embedded fiber optic sensors. For our analyses, we manufactured a carbon fiber reinforced plate in which we embedded a distributed network of fiber Bragg grating (FBG) sensors. We impacted the plate with a modal hammer and we identified the impacts by processing the FBG data with an improved fast phase correlation (FPC) algorithm in combination with a variable selective least squares (VS-LS) inverse solver approach. A total of 164 impacts distributed on 41 possible impact locations were analyzed. We compared our methodology with the traditional P-Inv based approach. In terms of impact localization, our methodology performed better in 70.7% of the cases. An improvement on the impact time domain reconstruction was achieved in of the cases.

Modelling the geometry of the unit cell of woven fabrics with integrated stiffener sections

Composite-stiffened panels of naval and aerospace applications are under the risk of delamination due to the lack of reinforcement at the joints of panel and stiffener section. One of the sustainable remedy to overcome such problems is to use 3D woven performs as a reinforcement. The benefit of 3D weaving is its intricate weave architecture and its ability to create near net shaped preforms for structural reinforcements. The technique provides endless permutations and combinations for weave design, however, repeat unit cell is the common factor that represents the complete weave architecture of the fabric. The behavior of the unit cell represents the behavior of the composite as a whole. The work presented in this paper details on the geometrical representation of an unit cell of woven stiffener fabrics and formulation of a modeling approach to determine its total weight and Fiber Volume Fraction. In this work, woven fabrics with integrated stiffener sections of four different configurations were produced using modified face-to-face (terry) weaving principle and their geometrical parameters were formulated through the modeling approach. The computed results from the models show close approximation with experimental values thus justifying the prediction of performance through them.

Impact analysis of embedded delamination growth in stiffener of hybrid laminated composite stiffened panel

Modern aerostructures are predominantly of curved construction characterized by a skin and stiffeners. The latest generation of large passenger aircraft also mostly uses composite materials in their primary structure and there is trend toward the utilization of bonding of subcomponents. A stiffener is a simple beam attached to the skin to support the applied load. The stiffeners are provided to improve the bending stiffness of the panel which then improves the structural efficiency. Metals, unlike composites, offer plasticity effects to evade high stress concentrations during postbuckling. Under compressive load, composite structures show a wide range of damage mechanisms. A pre-damaged configuration is loaded to study the effect of delamination location and mode for delamination initiation and propagation. A parametric study is conducted to investigate the effect of the location of the delamination propagation when delamination is embedded inbetween plies of the stiffener, with the cases (i) delamination embedded at front and inbetween plies of the stiffener, (ii) delamination embedded at middle and inbetween plies of the stiffener, (iii) delamination embedded at the end and inbetween plies of the stiffener. Further, the influence of the location of the delamination in between plies of stiffener on load carrying capacity of the panel is investigated. The effect of delamination location on crack growth and collapse behavior is analyzed using analysis tool. The effect of delamination location on crack growth and collapse behavior is analyzed with numerical method of Virtual Crack Closure Technique.

Potentiality of SHM and PFA for a new design approach of low weight aircraft structures

The object of this work is to show the potentiality of the application of SHM systems and PFA methodology, from the beginning of the design phase of wing box composite stiffened panels, for obtaining lighter structures respect to the current ones. The common industrial approach to satisfy the certification requirements (EASA AMC 20-29) for composite structures, based mainly on the application of high conservative knockdown factors to the material strength properties and/or performing extensive test campaigns, can lead to oversized structures and to an increase in costs and timing. The use of information coming from SHM systems and of the PFA methodology could exploit the full potential of damaged composite materials, in favour of a greater weight reduction. By detecting the damages thanks to SHM systems, the structure could be designed with higher design allowables (more reliable detection of BVID) improving the static strength for a reduced damage size detection.Two aircraft wing box composite stiffened panels have been preliminary designed under static compressive load, one panel at the wing root and the other one at the wing tip, according to the traditional industrial design approach. Then, they have been re-designed releasing some of the current conservative criteria, because they were considered resolved by SHM systems: no BVID knockdown factor, no notch material design allowables (only bonded joints and bonded repair are considered) have been applied. The new design has shown the potential weight reduction achievable, the design parameters and panel subparts to which the panel weight is more sensitive. The results of these analyses provide fundamental requirements for the SHM system definition in terms of “where to monitor and why”.Successively, in order to exploit the actual residual strength of impact damaged panels, PFA has been performed also on the stiffened panels considering a discrete damage model against the traditional design approach; the latter, based on the first ply failure design criteria and assuming the structure uniformly damaged by using reduced design allowable obtained at coupon level. The results show that a significant weight reduction is potentially achievable by using PFA, and the potentiality of this methodology as a valid design tool to investigate a new design philosophy based on the evaluation of the residual strength of panels with discrete damages.

A Bayesian Approach for Sensor Optimisation in Impact Identification.

This paper presents a Bayesian approach for optimizing the position of sensors aimed at impact identification in composite structures under operational conditions. The uncertainty in the sensor data has been represented by statistical distributions of the recorded signals. An optimisation strategy based on the genetic algorithm is proposed to find the best sensor combination aimed at locating impacts on composite structures. A Bayesian-based objective function is adopted in the optimisation procedure as an indicator of the performance of meta-models developed for different sensor combinations to locate various impact events. To represent a real structure under operational load and to increase the reliability of the Structural Health Monitoring (SHM) system, the probability of malfunctioning sensors is included in the optimisation. The reliability and the robustness of the procedure is tested with experimental and numerical examples. Finally, the proposed optimisation algorithm is applied to a composite stiffened panel for both the uniform and non-uniform probability of impact occurrence.