Skin-stringer Debonding Research Articles

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.

Impact area and debonding line detection assessment by cross-correlation analysis and distributed sensing

Debonding at two components interface (f.i., skin & stringer) is a common defect of composite stiffened structures. It may result from inefficient manufacturing or impacts from foreign objects. Discontinuities within an elastic solid medium under mechanical loads may cause local anomalies in the strain field. On this basis, the current work investigates disbonding at skin/stringer interface of an aeronautical plate, generated by low/medium energy impacts. Distributed fiber optics records longitudinal strain values; data are then elaborated by a methodology using different deformation-based features providing Local High Edge Observation (LHEO) in both time and space domains. Cross correlation is implemented to improve signal to noise ratio. Proposed LHEO algorithm proves its capability to detect impact area and occurred skin-stringer debonding, without reference to applied loads or a certain baseline. A dedicated Matlab routine elaborates the output further, with the aim of visualizing and estimating impact region extension.

Cross-Influence between Intra-Laminar Damages and Fibre Bridging at the Skin-Stringer Interface in Stiffened Composite Panels under Compression.

In this paper, the skin–stringer separation phenomenon that occurs in stiffened composite panels under compression is numerically studied. Since the mode I fracture toughness and, consequently, the skin–stringer separation can be influenced by the fibre bridging phenomenon at the skin–stringer interface, in this study, comparisons among three different material systems with different fibre bridging sensitivities have been carried out. Indeed, a reference material system has been compared, in terms of toughness performance, against two materials with different degrees of sensitivity to fibre bridging. A robust numerical procedure for the delamination assessment has been used to mimic the skin–stringer separation. When analysing the global compressive behaviour of the stiffened panel, intra-laminar damages have been considered in conjunction with skin–stringer debonding to evaluate the effect of the fibre and matrix breakage on the separation between the skin and the stringer for the three analysed material systems. The latter are characterised by different toughness characteristics and fibre bridging sensitivities, resulting in a different material toughness.

Progressive Failure Analysis Using Global-Local Coupling Including Intralaminar Failure and Debonding

Composite laminate stiffened panels are often used in aircraft fuselage design because of their favorable properties. To assess the failure load of these thin-walled structures and to exploit their reserves, a reliable simulation capability for their postbuckling behavior is often necessary. To perform a realistic failure analysis and to accurately detect final collapse, material degradation should be considered. Global-local approaches are computationally efficient techniques to perform a progressive failure analysis and to examine localized damaged areas in detail. In this paper, a two-way coupling global-local approach is presented, including a combination of different damage modes, such as matrix cracking, fiber damage, and skin-stringer debonding. An accurate exchange of information concerning the damage state between global and refined local models is performed. From the global to the local model, the displacements are transferred through a submodeling procedure. Afterward, the degraded material properties obtained from the local model analysis are returned to the global model with a special mapping technique that accounts for the different mesh sizes at the two levels. The two-way coupling procedure is applied to the progressive failure analysis of a one-stringer composite panel loaded in compression. Finally, the numerical results of the procedure are compared with experimental results.

Analysis of skin-stringer debonding in composite panels through a two-way global-local method

According to various experimental results, stiffened panels under compressive loading are prone to debonding between the skin and the flange of the stringer. In this paper, a novel two-way global-local coupling approach is presented that is able to model progressive separation of the skin and stringer in stiffened CFRP panels under compression. The main goal of this methodology is to examine skin-stringer debonding at two levels of accuracy, taking advantage of the fast calculations at the global level and assessing in detail the damage propagation at the local level. First, critical areas are defined in a global model with a standard mesh, and local models with a considerably finer mesh are created by means of a submodeling technique. Secondly, a local model analysis is conducted, in which cohesive elements are applied to simulate debonding. Particularly important is the appropriate information exchange in both directions between the different steps of the coupling analysis. Averaged degraded properties are defined at the local model level and transferred back to the global level. The applied compressive load is increased and induces a progression in skin-stringer separation. The global-local coupling loops are repeated until panel failure occurs. The approach is applied to a case of a representative one-stringer stiffened panel and to a stiffened panel for which test results are available. A good correspondence with reference results and test results demonstrates the effectiveness of the global-local approach presented.

Analysis of skin-stringer debonding in composite panels through a two-way global-local method

Analysis of skin-stringer debonding in composite panels through a two-way global-local method

Skin–stringer debonding detection using distributed dispersion index features

Statistical approaches have been extensively used to detect structural damage. This article proposes a preliminary investigation on the use of a non-model-based damage identification method, implementing “dispersion feature” indicators, extracted from the differential strain signal. In detail, the method takes advantage of the elaboration of the full-spectrum acquisition, irrespective of the healthy state of the structure under test. Large variations of some selected statistical features, representative of the acquired signals, are taken as indicators of the presence of anomalous structural condition. At this stage, these features have been arbitrarily selected. In synthesis, a cumulative dispersion index is defined as the combination of the selected feature variations, contemporarily overcoming prescribed thresholds, providing a scalar output that can be used as an index to identify and possibly quantify the extension of a damage location. Experimental results confirm the envisaged potentiality of the proposed technique. In this study, damage in the form of a stringer debonding is referred to, obtained as a consequence of point impacts on a wing box stiffened panel. A distributed fiber optic strain sensor network is used to detect the input data.

On the use of selective stitching in stiffened composite panels to prevent skin-stringer debonding

Abstract Composite laminates are generally characterized by poor mechanical properties along the thickness direction, therefore they are highly susceptible to delaminations. A potential increase in structural efficiency may be obtained by using 3D reinforcements technologies. In this paper, an experimental/numerical study is presented focusing on the effects of a single reinforcement seam of stitches along the edge of a stringer foot, in a stiffened composite panel. Three-point bending tests have been performed on a skin-stringer configuration by considering variations of the skin thickness, the stitching technique, the pitch and the yarn diameter. Then, a Finite Element Model has been developed capable to simulate the mechanical behaviour of stiffened composite panels with selective stitching and to assist the design of more complex geometrical configurations by estimating the damage behaviour, as well as the onset and the propagation of delamination taking into account the effect of the selective stitching. The numerical results, in terms of load vs applied displacements, have been found in good agreement with the experimental data proving the effectiveness of the introduced numerical model. The numerical results have confirmed the potential beneficial effects of stitching in terms of delay of the crack initiation and growth, of smoother delamination profile, of increase in the stiffness of the structure with a reduction of the delamination area.

Development and application of a numerical procedure for the simulation of the “Fibre Bridging” phenomenon in composite structures

In this paper, a novel numerical procedure for delamination growth simulation able to evaluate the Fibre Bridging effect has been used to numerically investigate the mechanical behaviour of stiffened composite panels. The proposed methodology, implemented in ANSYS® FEM code by Ansys Parametric Design Language (APDL), allows to take into account the real variation of the Critical Energy Release Rate associated to the Fibre Bridging phenomenon. A first numerical application has been performed for preliminary validation purposes, simulating the flexural behaviour of representative section of a stiffened composite plate. The considerable agreement between the obtained numerical results and the literature experimental data confirms the effectiveness and the accuracy of the proposed model. Finally, a parametric analysis has been carried out in order to study the influence of the geometrical parameters of single stiffened composite panels in terms of skin-stringer debonding including Fibre Bridging.

Inter-laminar and intra-laminar damage evolution in composite panels with skin-stringer debonding under compression

Abstract In this paper, a numerical investigation on the inter-laminar and intra-laminar damage mechanisms, characterising the skin-stringer debonding growth in stiffened composite panels subjected to compressive load, has been carried out. A novel numerical methodology for inter-laminar damage growth simulation, based on VCCT technique and able to overcome mesh and time step sensitivity issues, has been adopted in conjunction with a User Material Subroutine (USERMAT), based on Instantaneous Degradation Models (IDM), to investigate the influence of the intra-laminar damages, in terms of fibers and matrix failures, on skin-stringer debonding evolution. The proposed numerical model has been applied to two configurations of single stiffener composite panels with different geometry and debonding size. The numerical results, in terms of compressive load as a function of applied displacements and debonding evolution up to failure, have been compared to experimental data to assess the effectiveness of the implemented methodology. A further comparison with a previously developed numerical model, taking into account only the inter-laminar damage growth, has allowed assessing the influence of the intra-laminar damage on the skin-stringer debonding propagation and the benefit of the proposed integrated inter-laminar intra-laminar numerical model.

A Numerical Assessment of Fibre Bridging Influence on Composite Panels Skin-stringer Debonding

Numerical simulations on stiffened composite panels have been performed in order to investigate the influence of fibre bridging phenomenon on the onset and on the evolution of skin-stringer debonding. A novel numerical procedure for delamination growth simulation, implemented in ANSYS® FEM code by Ansys Parametric Design Language and enhanced to take into account GIc variations related to the fibre bridging phenomenon, has been used to numerically simulate the skin-stringer separation. Indeed, fibre-reinforced composites can develop high fibre bridging under mode I (peeling) delamination growth due to the additional resistance to crack opening offered by delaminated fibres, leading to a significant increase of mode I critical strain energy release rate. Within the present paper, a limited sensitivity analysis is presented to assess the influence of geometrical parameters of stiffened composite panels on the fibre bridging relevance in skin-stringer debonding in terms of debonded area.

Influence of Stitching on Skin Stringer Debonding in Stiffened Composite Panels

A potential increase of the out-of plane structural efficiency of laminated composites may be obtained by using 3D reinforcements technologies. In this paper, a numerical study is presented focusing on the effects of a single reinforcement seam of stitches (selective stitching) along the edge of a stringer foot, in a stiffened composite panel. A Finite Element Model has been developed in order to simulate the mechanical behaviour of the stiffened composite panels with selective stitching. The numerical model has been preliminary validated by comparison with experimental data on three-point bending tests on a skin-stringer configuration. The numerical results, in terms of load vs applied displacements, have been found in good agreement with the experimental data proving the effectiveness of the introduced numerical model. The numerical results have confirmed the potential beneficial effects of stitching in terms of delay of the crack initiation and growth.

Analytical Tool for the Preliminary Design of an Adhesively Bonded T Joint

Several different methodology for the study of adhesively bonded joints can be found in literature. However, most of those methods are joint-specific. One of the most interesting joint case is the bonding between the skin and the stringer of an aeronautical panel. Unfortunately, this case is one of the less studied, due to the complexity of the load conditions. In this paper, the problem of the skin stringer debonding under a T-pull load condition is analyzed. The analysis is performed by means of the methodology developed by Bigwood and Crocombe, that differs from other bonding technique because it is not joint-specific. Indeed, this method allows to solve any type of joints for which it is possible to specify the end loading values on every adherends’ sides. The results, in terms of peel and shear stress, are compared with the ones obtained via finite element methods, in order to validate the results for the specific load case considered.

A robust numerical approach for the simulation of skin–stringer debonding growth in stiffened composite panels under compression

In this paper, a numerical study on skin–stringer debonding growth in stiffened composite panels has been carried out. A novel numerical methodology is proposed here to investigate the compressive behaviour of a stiffened composite panel in the presence of skin–stringer partial separation. The novel numerical methodology, able to overcome the mesh size and time increment dependency of the standard Virtual Crack Closure Technique (VCCT), is an evolution of a previously developed and tested numerical approach for the circular delaminations growth. The enhancements, with respect to the previously developed approach, rely mainly in the capability to deal with the different defect shapes characterising a skin–stringer debonding. The proposed novel methodology has been implemented in a commercial finite element platform and tested over single stiffener composite panels. The effectiveness of the suggested numerical methodology, in predicting the compressive behaviour of stiffened panels with skin stringer debondings, has been preliminary confirmed by comparisons, in terms of load versus applied displacement and debonding size at failure, with literature experimental data and numerical results obtained with the standard VCCT approach.

Skin Stringer Debonding Evolution in Stiffened Composite Panels under Compressive Load: A Novel Numerical Approach

In this paper, a numerical study, on the compressive behaviour of stiffened composite panels with skin-stringer debonding has been carried out. The analysis has been performed by adopting a novel robust (mesh and time step independent) finite elements based numerical model on a single stiffener panel with an artificial debonding. In order to prove the effectiveness of the proposed numerical tool, the results in terms of debonded area growth and compressive load versus applied displacement, have been compared with experimental data available in literature.

Damage Characterisation of Carbon Fibre Reinforced Composite Plate Using Acoustic Emission

Acoustic Emission (AE) is a sensitive technique which can be used to characterise damage in high strength composite plate. This paper describes an extension to an earlier piece of research work carried out by the ERC which resulted in the successful development of a novel source location methodology for the said material. The previous work concentrated on the source location in plate-like composite structures using acoustic emission. The work presented in this paper focuses on establishing the correlation between the different damage types suffered in the material namely de-lamination, matrix cracking, fibre rupture and stringer to skin debonding with key signal features of the AE activities. Controlled bending tests were initially carried out on laterally grooved slender composite specimens to progressively propagate damage in the weakened region of these specimens. The composite laminate plate itself is made from 16 plies of carbon fibre twill weaved in an epoxy matrix with bidirectional fibre alignments in the 0° and 90° directions with 60/40 fibre-matrix volume composition. These prepared samples were fully instrumented with broad band (100 kHz to 1MHz) Physical Acoustic AE sensors linked to the necessary signal conditioning hardware. The AE events were recorded using a high speed DAQ card accessed by customised software written in LabVIEWTM. Gathered raw data were analysed off-line for key signal features including energy and frequency contents and subsequently correlated to actual damage types. It can be concluded from the empirical evidence that feature vectors are distinct to the type of damage. Results gathered from additional test on the progressive skin-stringer debonding of the same material to failure confirmed the uniqueness of the AE feature trends. An integrated system which is capable of both in-situ location of compromised sites and the diagnostic of flaw types in composite plate can potentially find engineering applications including the structural health monitoring of composite aircraft parts.