Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 167 (2016) 103 – 108 Comitato Organizzatore del Convegno Internazionale DRaF 2016, c/o Dipartimento di Ing Chimica, dei Materiali e della Prod.ne Ind.le Influence of stitching on skin stringer debonding in stiffened composite panels A Riccioa,*, P Lindeb, A Raimondoa, A Buompanea, A Sellittoa a Department of Industrial and Informatics Engineering, Second University of Naples, via Roma 29, 81031 Aversa (CE), Italy b Airbus, Kreetslag 10, 21129 Hamburg, Germany Abstract 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 © 2016 2016The TheAuthors Authors Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd This (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility ofthe Organizing Committee of DRaF2016 Peer-review under responsibility of the Organizing Committee of DRaF2016 Keywords: Laminates; Debonding; Damage Mechanics;Finite Element Analysis (FEA) Introduction Beyond clear advantages of using composite materials for aerospace structural applications, such as the high specific strength and stiffness, there are also several disadvantages that have to be considered Indeed, fibrereinforced plastic (FRP) materials show excellent mechanical properties in the fibre direction, while exhibit poor * Corresponding author Tel.: +39 081 5010 407; fax: +39 081 5010 204 E-mail address:aniello.riccio@unina2.it 1877-7058 © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the Organizing Committee of DRaF2016 doi:10.1016/j.proeng.2016.11.675 104 A Riccio et al / Procedia Engineering 167 (2016) 103 – 108 mechanical properties in the third direction, namely the z-direction, basically due to the poor mechanical properties of the polymeric matrix Therefore, the composite laminate is likely to be highly vulnerable to delaminations In order to increase the poor out-of-plane inter-laminar strength, mechanical fasteners could be added to the structure, however, this would lead to higher manufacturing costs and weight An alternative more effective solution is the introduction of translaminar reinforcements (TLR) or through-the-thickness reinforcements (TTR) into the composite in order to create mechanical links between adjacent plies of the laminate leading to an overall improvement of its out-of-plane mechanical properties and damage tolerance behaviour [1] The TTR can be either continuous, such as weaving, braiding, threads, yarns and tows or discontinuous, such as short fibres, whiskers and pins Stitching is one the most common TTR and involves the insertion along the thickness axis, of one or two hightensile threads by means of a needle that might be interlocked or not The material used for the thread is usually glass or aramid fibres but carbon fibres threads could be adopted as well A recent research has suggested that stitching in only selected areas may have limited but positive effects, potentially without involving the severe reduction of in-plane stiffness and strength related to full aerial stitching [2] The idea behind the selective stitching is to adopt this technique only in the “weak zones” which can be, potentially, more affected by the delamination phenomenon As a matter of facts, selective stitching aims to alter or arrest damages by using stitching seams at only the necessary locations The selective stitching is expected to provide not only a better damage tolerance but also weight saving and simplification in the manufacturing procedures with a consequent reduced cost The Selective Stitching can be successfully applied to reduce the delaminations occurring between skin and stringers in aircraft fuselages stiffened composite panels which can be considered as a network of stringers connected to thin skins For these structures, for analytical, experimental and numerical studies at coupon level, a realistic representation of their mechanical behaviour is given by a stringer foot-skin system stitched along the edges [3-9] The scope of this work is the development of a finite element model able to correctly simulate the mechanical behaviour of stiffened composite panels with selective stitching This model should also be able to assisting 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 In order to gain in-house experience of the structural behaviour of selectively stitched stiffeners, Airbus R&T decided to perform some mechanical tests and analytical analyses on the potential effect of selective stitching on the debonding of stringers and on the effect of different parameters related to stitching (stitching techniques, pitch, stitching yarn) These tests were performed by Airbus R&T in collaboration with EADS IW in Ottobrunn and KSL Keilmann Sondermaschinenbau GmbH A limited part of these mechanical tests have been reported in this paper for a first validation of the proposed Finite Element Model In section 2, the Numerical model is introduced In section 3, all the results in terms of load versus applied displacement are introduced, compared to the experimental data and assessed Numerical Model Definition In order to characterize the flexural behaviour of the selective stitched and unstitched specimens, three-points bending tests have been simulated The mixed-mode stress configuration of this test gives a stress combination close to the real life situation for a T-section stiffener From the results of a three-points bending test it is possible to rapidly estimate the effect of the selective stitching and of the geometry parameters on delamination onset and evolution In the next sub-sections the geometrical characteristics of the specimens and the numerical model are introduced 2.1 Geometrical description of the tested specimen The analysed selective stitched test specimens consist of a skin and a stringer foot (see Figure 1) with throughthe-thickness stitching across both parts along the edge of the stringer foot The geometrical description of the specimens is introduced in Figure 105 A Riccio et al / Procedia Engineering 167 (2016) 103 – 108 Figure 1: Selective Stitched coupon geometry (stitches are represented by red lines) 100 mm R10 mm 150 mm 200 mm 120 mm 70 mm 60 mm Figure 2: Geometrical description of the three-point bending test specimens The material, for both the parts (skin and stringet-foot), is a NCF SAERTEX quadri-axial 505 sgm with 12k Fiber Tenax HTS, recommended by EADS Innovation Work The denomination is RTM6/Tenax HTS 12k The specimen has been stitched with glass yarns from Culimeta: references EC-9 3x34 The properties of the adopted Material System are shown in Table Table 1: Material properties (RTM6/Tenax HTS 12k) E1 E2 G12 ν12 135 GPa 8.5 GPa 4.2 GPa 0.35 The stitching pitch parameter has been varied in order to assess the optimization of the stitching process The analysed specimen, stitched with tufting stitching technique and EC-9 3×34 stitching yarns are reported in Table The unstitched configuration has been used as a reference Table 2: Specimens Configuration Conf ID Stitching Technique Stitching pitch Stitching yarn TUF-34-P5 Tufting mm EC-9 3×34 TUF-34-P10 Tufting 10 mm EC-9 3×34 106 A Riccio et al / Procedia Engineering 167 (2016) 103 – 108 For the analysed configurations, the thickness and the stacking sequences of the two laminates, namely the skin and the stringer foot, are described in Table Table 3: Coupons layups Thickness Stacking Sequence Number of plies mm (-45/0/+45/90)2S 16 2.2 Numerical Implementation The aim of the numerical activities, presented is this paper, is the development of a finite element model able to correctly simulate the mechanical behaviour of stiffened composite panels with selective stitching For this purpose, the FEM software ABAQUS/Standard 6.12-3 [10] has been adopted The structure under investigation has two planes of symmetry, so it would be theoretically possible to simplify the problem by modelling only a quarter of the specimen Nevertheless, due to the presence of initial defects and misalignments, during the experimental tests, the crack has been found to on-set and grow only on one side of the specimen Therefore, no symmetry has been considered in the numerical model and the specimens have been fully modelled In order to replicate the growth on only one side of the specimen, observed during the experiment test, the debonding between skin and stringer foot has been driven along one edge of the modelled specimen by slightly increasing the cohesive elements coefficients on the other edge The geometrical model adopted for the analyses is presented in Figure Figure 3: Three-point-bending test numerical model The two laminates (skin and stringer foot) have been modelled by using Continuum Shell elements (SC8R), while the supports have been modelled as a discrete rigid surfaces Cohesive Zone Model (CZM) elements have been adopted to numerically simulate the onset and the propagation of the inter-laminar damage Indeed, a layer of cohesive elements has been placed between the skin and the stringer foot The adopted Cohesive constitutive response is characterized by two different phases, a failure initiation phase and a damage evolution phase which cover the inter-laminar damage progression up to complete failure of the element Numerical Results In this Section, the numerical results are presented both for the Unstitched configuration (taken as a reference configuration) and the stitched configurations (introduced in Table 2) 107 A Riccio et al / Procedia Engineering 167 (2016) 103 – 108 3.1 Unstitched Reference Configuration Figure shows the obtained numerical results compared with experimental data in terms of load-displacement curve for the unstitched specimen Load vs Applied Displacement Unstitched Specimen 2500 NUMERICAL EXPERIMENTAL Force [N] 2000 1500 1000 500 0 10 12 Displacement [mm] Figure 4: Load-displacement curve – Unstitched Specimen An important observation can be made regarding the comparison between numerical and experimental data shown in Figure As we can see, the numerical simulation allows to predict just the onset of the delamination Indeed, the introduced numerical model predicts an unstable skin-stringer debonding probably because it is not able to simulate the fiber-bridging phenomenon, which significantly slows down the skin-stringer debonding growth rate, observed during the experimental test However, for the purposes of this paper, which is focused on the selective stitching effects mostly affecting the delamination onset phenomenon, the correct simulation of the stability of delamination growth, influenced by the fiber-bridging phenomenon, has not been considered of main concern 3.2 Stitched Configurations Figure introduces the obtained numerical results, in terms of load-displacement curves obtained for the analysed configurations with the two different stitching pitch (5-10 mm) The numerical results are also compared to experimental data Load vs Applied Displacement TUF-34-P5 Load vs Applied Displacement TUF-34-P10 3000 3000 NUMERICAL 2500 NUMERICAL 2500 EXPERIMENTAL EXPERIMENTAL 2000 Force [N] Force [N] 2000 1500 1500 1000 1000 500 500 0 Displacement [mm] 10 12 Displacement [mm] Figure 5: Load-displacement curve – Numerical-experimental comparisons for the stitched specimens 10 12 108 A Riccio et al / Procedia Engineering 167 (2016) 103 – 108 From comparison between Figures and 5, it is clear that the numerical model is able to correctly take into account the effects of the presence of selective stitching on the delamination onset However, as expected (see previous subsection), the proposed numerical model is only partially able to simulate the delamination propagation Indeed the model is able to correctly predict the Inter-laminar failure growth only up to the complete failure of the stitches then, being not able to simulate the fiber-bridging phenomenon, it shows again a premature unstable growth of the debonding leading to an underestimation of the final failure load of the specimens It should be remarked that, for the analysed configurations, the yarn’s failure (last drop in load before finale failure) has been very well predicted Conclusions In this paper, the results of a combined numerical/experimental study on selective stitched composite laminates are presented A numerical model developed in ABAQUS, adopting assembled fasteners to simulate the stitches, has been introduced and preliminary validated by comparisons with experimental data from three-point bending tests on skin-stringer foot interfaces Configurations with different stitching pitch sizes have been experimentally and numerically analysed The introduced numerical tool has been found able to predict the effect of selective stitching on skin-stringer debonding resistance Only a qualitative description concerning the effects of selective stitching on the skin-stringer debonding growth retardation has been provided here due to the incapability of the introduced numerical model to simulate of the fibre-bridging phenomenon (strongly influencing the stability of delamination propagation) Finally, both the experimental and numerical investigation shows that selectively stitched specimens are characterized by a superior mechanical behaviour if compared with the unstitched ones with increased load carrying capability and ductility related to the stitch density References [1] G Dell’Anno, Effect of tufting on the mechanical behaviour of carbon fabric/epoxy composites, Ph.D thesis, Cranfield University, 2007 [2] P Baisch, Nähtechnische Montage und Modifizie-rung eines faserverbundverstärkten Rumpf-schalen-Segments im Kontext einer hoch automatisierten Prozesskette M.Sc thesis CTC, Stade, July, 2004 [3] A Velicki, Blended Wing Body Structural Development, Proc Royal Aero-nautical Society First Structural Design Conf., London, 2008 [4] A Velicki, Damage Arrest Design Approach using Stitched Composites, Proc Royal Aeronautical Society 2nd Structural Design Conf., London, 2010 [5] D Jegley, Experimental Behavior of Fatigued Single Stiffener PRSEUS Specimens, NASA/TM-2009-215955, 2011 [6] A Bergan, J G Bakuckas, Jr., A Lovejoy, D Jegley, K Linton, Full-Scale Test and Analysis of a PRSEUS Fuselage Panel to Assess Damage-Containment Features, FAA techn paper, 2011 [7] Y Tan, G Wu, S.S Suh, J.M Yang, H.T Hahn, Damage tolerance and durability of selectively stitched stiffened composite structures Int J Fatigue, 30(3), (2008) 483-492 [8] K Dransfield, C Baillie, Y.-W Mai, Improving the delamination resistance of CFRP by stitching-a review Compos Sci Technol., 50(3), (1994) 305-317 [9] D.C Jegley, Improving strength of postbuckled panels through stitching Compos Struct., 80(2), (2007) 298-306 [10] ABAQUS Analysis user’s manual 6.12 (2012) ... effect of selective stitching on skin- stringer debonding resistance Only a qualitative description concerning the effects of selective stitching on the skin- stringer debonding growth retardation... analyses on the potential effect of selective stitching on the debonding of stringers and on the effect of different parameters related to stitching (stitching techniques, pitch, stitching yarn)... phenomenon, the correct simulation of the stability of delamination growth, influenced by the fiber-bridging phenomenon, has not been considered of main concern 3.2 Stitched Configurations Figure introduces