Bearing and Delamination Failure Analysis of Pin Loaded Composite Laminates

Transcription

1 Volume 6, No. 2, February Bearing and Delamination Failure Analysis of Pin Loaded Composite Laminates V. Dinesh Babu, Professor, Nehru Institute of Engineering and Technology, Coimbatore T. Sivagangai, Assistant Professor, Nehru Institute of Engineering and Technology, Coimbatore ABSTRACT Because of high strength to weight ratio, composite materials find wide use in aircraft structures. Mechanical fastened joints are used to join the section of aircraft components such as fuselage, wings and empennages. Typical applications are spar to skin, frame to skin, Ribs to skin, Ribs to spar and so on. Mechanical fastening remains a critical aspect of designing composite aircraft joints. The holes drilled for composite joints are the primary source of structural failure in aircraft structures. Developing efficient design procedure is still a challenge for aircraft designers due to anisotropic nature of composite, complex failure response, geometric parameters and stacking sequence. In this paper, an attempt has been made to study bearing and delamination onset failure for a single pinned composite joint based on 3D finite element analysis using ANSYS code. For the analysis, the composite plate with four layers [/9⁰], and the metallic bolt made of steel are modeled using SOLID185 element available in ANSYS. A contact pair is developed between the plate and the bolt using CONTACT174 at the bolt hole and bonded contact is applied between the layers of the composite. The displacement at the bolt hole, contact stresses and delamination onset are studied at the bolt hole for three conditions namely; no friction between the metallic bolt and composite plate, friction with lubrication and without lubrication. A parametric study with different enforced displacement at the bolt is performed and the results are used for studying the delamination onset and growth. The bearing stress and bolt hole displacement for various loading conditions are compared for different composite material. Keywords Bearing, composite bolted joints, delamination, finite element analysis. 1. INTRODUCTION Composite materials like carbon fiber reinforced polymers, glass fiber reinforced polymer etc. find important structural applications in the aircraft industry due to better mechanical characteristics compared to their counterpart isotropic materials. Construction of aircraft structures requires the joining of various subparts to transfer loads in a proper manner. Typically, these connections are made by means of mechanically fastened joints and adhesive bonds. Mechanically fastened joints like pinned or bolted joints are widely used in the aerospace industry to fasten highly loaded fibre reinforced plastic (FRP) components to other FRP or metallic parts. Typical applications are skin-to-spar and tail-to-fuselage connection and bolted patch repairs. They are easy to assemble and can be disassembled when required for inspection and/or maintenance purposes without imparting damage.all connections or joints are potentially the weakest points in the structures so can determine its structural efficiency. As, in case of increasing efficiency of the structure, the operational load continues to grow, the load carried by each fastener increases accordingly. This increases probability of failure. Therefore, the assessment of the stresses around the fastener holes becomes critical for damage-tolerant design. Because of the presence of unknown contact stresses and contact region between the fastener and the laminate, the analysis of a pin-loaded hole becomes considerably more complex than that of a traction-free hole. The accurate prediction of the stress distribution along the hole edge is essential for reliable strength evaluation and failure prediction. In the past decades, various methods have been proposed to assess the strength of bonded structures. Atas et. al.[1] carried out a finite element analysis on a pin loaded composite plate with hole by creating a three dimensional model for CFRP composite plate and steel bolt in ANSYS The cohesive zone elements were introduced in between the unidirectional and 9 layers of fibers which were stacked together to give a total thickness of.5 mm and contact elements were used to produce contact between steel bolt and composite layers. Murat Pakdil[2] conducted an experimental study on composite single bolted joints. Specimens with different stacking sequences, variable joint geometry and applied bolt pretension were tested and corresponding variation in the bearing strength and the failure mode was studied. Aktas [3] conducted an experimental and numerical investigation on woven glass epoxy composite plates with one and two serial pinned joints. Specimens were prepared with variations in distance from the free edge of the plate to the diameter of the first hole ratio, and width of the specimen

2 Volume 6, No. 2, February to the diameter of the holes ratio and tested. ANSYS was used to perform the numerical analyses with the same variations. Faruk Sen et. al. [4] carried out an experimental study to determine the effect of variations in preload moments on the failure behavior of bolted composite joints. In, addition variable stacking sequences of composite plates and joint geometric parameters were used and bearing strength was calculated in all the cases. Alderliesten[5] proposed a generic damage tolerance approach based on the SERR concept which is validated on metal-to metal and metal-to-composite bonded structures.buketokutan[6] has carried out numerical and experimental study to determine the effects of joint geometry, ply orientation and material non-linearity on the stresses, failure strength and failure mode of composite laminate with pin loaded hole. The numerical analysis was performed with ANSYS and the results were verified by experimenting around 16 specimens with different variations. McCarthy et. al. [7] used commercial code MSC NASTRAN to develop two models to determine effect of friction in the composite bolted joints. Two available models within the code are examined for their ability to model load transfer by friction in torqued joints and the stress distribution at the bolt hole interface in a pinned joint. Turon et. al. [8] proposed a thermodynamic model for progressive delamination in composite materials under variable mode ratio. The model is implemented in a finite element formulation, and the numerical predictions are compared with experimental results obtained in both composite test specimens and structural components. Ivana Ilić[9] has conducted an experimental and numerical study on the failure analysis of mechanically fastened joints of layered composites. The Tsai- Wu failure criterion is used to predict the failure load and failure mode. The finite element model, which simulates the contact between the lug and the pin was used for analysis and the FEA results were compared with experimental results. This study is the extension of the previous work done by Atas et. al. [1], in which the authors created three dimensional finite element model with cohesive zone elements to simulate a mechanically fastened [/9]s pinloaded joint in a composite laminate. The same approach has been carried forward to model the composite plate and metallic pin, but the cohesive zone elements were not used. A contact pair is established between the fastener and the plate hole boundary. The effects of various pin displacements, in the three cases of (i) no friction, (ii) friction with lubrication and (iii) without lubrication between bolt and hole is analysed for different composite materials. 2. DESCRIPTION OF THE PROBLEM In this study, a mechanically fastened pin-loaded cross-ply [⁰/9⁰] s CarbonFibre Reinforced Plastic (CFRP) laminate is chosen which is clamped at one edge of the panel and a quasi-static displacement is applied to the pin in the direction of the free edge.1 The four layered composite plate with length is l and width w is modelled by placing a bolt hole of diameter d at a horizontal distance of e from the free edge of the plate. This study emphasizes on fundamental failure mechanisms. So, a pinloaded laminate is selected rather than the more representative bolted/riveted joint configuration which involves complexities due to washer and nuts. In, general, three basic failure modes are observed in pin loaded composites as shown in Fig. 1: These are net tension, shear-out and bearing. The mixed modes are also observed. The three modes of failure depend on the geometric parameters of the joint i.e. e/d ratio and w/d ratio. The Net tension occurs catastrophically with least strength whereas bearing failure mode presents the highest strength. So, designers mainly focus on the optimum values for e/d and w/d to obtain bearing mode of failure in pinned joint uses. Fig.1 Modes of failure of joints in composite laminates The behaviour of the joint is influenced by four groups of parameters 3 : (i) Material Parameters: types of fibres, types of resins, fibre orientation and stacking sequence of laminates. (ii) Geometry parameters: specimen width, specimen thickness, ratio of edge distance to hole diameter (e/d), specimen width to hole diameter(w/d), edge distance(e) (iii) Fastener parameters: fastener type, clamping area, hole size and fastener diameter; and (iv)design parameters: loading type, loading direction and failure criteria. So, there are so many factors influence the practical joints, therefore, joint geometry including specimen width, edge distance, e/d ratio and w/d ratio highly affects the complete behaviour of the joints and the failure mode. The geometry of the [⁰/9⁰] s pin loaded laminate (w/d=6, e/d=3) is chosen so that bearing is the

3 Volume 6, No. 2, February dominant mode of failure allowing more progressive damage to occur 1. coefficient values i.e.,.1,.14 to determine the effect of friction. Friction between the bolt and the composite plate plays an important role in the bearing and delamination and it can significantly alter the stress distribution at a pin-hole interface. Thus, in this study, analysis is performed on the pin-loaded composite plate with the various friction Table1 Properties of fibers The geometrical parameters on mechanical joints such as specimen width (w) and distance of the hole centre from the free edge (e) on the failure mode are given in table.1 and fig.1 for a given pin diameter(d). The properties for the fibers chosen for the analysis are given in the table 1. Fibers E 1 (Gpa) E 2 (Gpa) E 3 (Gpa) G 12 (Gpa) G 13 (Gpa) G 23 (Gpa) V 12 V 32 V 31 Carbon Glass (E) Graphite Kevlar Table 2 Geometrical Parameters of the composite plate with hole l (mm) w (mm) e(mm) d(mm) Fig. 2 Geometry of laminate 3. FINITE ELEMENT MODEL OF PIN- LOADED COMPOSITE JOINTS were connected using bonded contact. The finite element model consists of 4 elements and 2412 nodes. Linear elastic material behaviour was assumed for the composite laminate and steel pin with orthotropic material model for the composite and isotropic material model for the pin. The related orthotropic materials properties of fibers are given in Table.1 and the steel properties were E = 21 GPa and µ=.3. Fully clamped end has been simulated by applying all the rotations and translations as zero. A quasi-static displacement was applied to the pin in the x direction against the clamped position. As large contact compressive stress gradients and damage were expected to occur at the boundary of the hole, the mesh was refined in this area with an aspect ratio of Due to the presence of high strain gradients in the vicinity of the hole in composite plate, a relatively high mesh density was used in this region and was reduced further from the hole to reduce solution runtimes as this area was less critical in the analysis. The surface-to-surface contact was established between the pin and the composite hole boundary by using the TARGE17 and CONTA174 elements as shown in Fig. 4. A three dimensional finite element model of a cross-ply (⁰/9⁰) composite plate having four layers of thickness.125 mm stacked together to give a total laminate thickness as.5mm and steel pin has been modelled for the study using ANSYSv14.5.Fully-integrated 8-noded brick elements (SOLID185) were used to model both composite plate and steel pin. Fig.3 shows the finite element model of the pin and composite plate where same mesh size is used in meshing the composite plate and solid steel pin. Considering the structure and loading symmetry, to reduce computational cost, a half symmetry model is modelled. Each layer was modelled with unidirectional fibers of ⁰ and 9⁰ independently and then all the layers

4 Volume 6, No. 2, February both Mode II and Mode III delamination due to the complex stress field around the hole boundary. Fig. 3 Finite element model of the half symmetry pin and the plate Fig.4 Finite element model of the contact pair 4. RESULTS AND DISCUSSIONS Various pin displacements of.1,.15,.2,.25,.3 were applied to the loading axis to determine the delamination onset at the (⁰/9⁰) interface of the [⁰/9⁰] s laminate. The pin displacement is expressed as ΔL/d; where ΔL is the applied pin displacement and d is the pin diameter. The various applied pin displacements have resulted in the onset of delamination which is shown in Fig.5 (a) to Fig.5(f). The delaminated area develops an elliptical shape which grows in a non-self-similar manner with increasing pin displacement. In the contact based delamination modelling approach, the interface elements (contact elements) are attached to the top surface of the hole boundary and the top adjacent surface layer of the pin. Therefore, same nodes are shared between the elements of both the adjacent surfaces. This has resulted in the development of shear stresses at the interface elements due to development of in-plane shear stresses at the laminate layers. These shear stresses cause Fig.5 (a) un-deformed model Fig. 5 (b) deformed model with pin displacement of.1 Fig 5 (c) deformed model with pin displacement of.15

5 Volume 6, No. 2, February Fig 5 (d) deformed model with pin displacement of.2 Fig 5 (f) deformed model with pin displacement of.3 The different values of friction coefficients were applied between the pin and the composite plate according to three conditions i.e. no friction, friction with lubrication and friction without lubrication. The maximum stress vs. enforced pin displacement with various friction coefficients (in three conditions: no friction, with lubrication and without lubrication) of the composite plates with different materials is shown in Fig.6 to Fig.9. In, all the cases, the maximum stresses are least in the condition of no friction and highest in the condition of friction without lubrication. The effect of friction in all the three cases: no friction, friction with lubrication and friction without lubrication on the bearing stress in pin loaded composite plates of different materials with various pin displacements is shown in Fig. Fig 5 (e) deformed model with pin displacement of Stress(N/mm 2 ) Fig. 6 Stress vs. Enforced Displacement of glass fibered composite plate

6 Stress(N/mm 2 ) Stress(N/mm 2 ) Stress(N/mm 2 ) International Journal of IT, Engineering and Applied Sciences Research (IJIEASR) ISSN: Volume 6, No. 2, February With librication Fig. 7 Stress vs. Enforced Displacement of kevlar fibered composite plate Fig. 8 Stress vs. Enforced Displacement of graphite fibered composite plate Fig. 9 Stress vs. Enforced Displacement of carbon fibered composite plate

7 Volume 6, No. 2, February P/dt(N/mm 2 ) Fig. 1 Bearing Stress vs. Enforced Displacement of carbon fibered composite plate 35 3 Stress((N/mm 2 ) Without friction With friction Fig. 11 Bearing Stress vs. Enforced Displacement of glass fibered composite plate Stress(N/mm 2 ) Fig. 12 Bearing Stress vs. Enforced displacement of kevlar fibered composite plate

8 Stress(N/mm 2 ) International Journal of IT, Engineering and Applied Sciences Research (IJIEASR) ISSN: Volume 6, No. 2, February Fig. 13 Bearing Stress vs. Enforced displacement of graphite fibered composite plate 5. CONCLUSION A 3D FE model is presented which predicts the onset and growth of delamination damage in a pin loaded cross-ply [/9]s composite laminate and the contact bearing stresses. The bearing stress is found to increase with increase in friction and increase in pin displacement. The contact bearing stress is found to be more for Graphite fibre compared to other fibre composites REFERENCES [1] A.Atas, G.F. Mahamed, C.Soutis, Modeling delamination onset and growth in pin loaded composite laminate by, Composite Science and Technology 72 (212), [2] Murat Pakdil, Failure analysis of composite single bolted-joints subjected to bolt pretension, Indian Journal of Engineering and Materials Sciences, Vol.16, April 29, pp [3] AlaatinAktas, Failure analysis of serial pinned joints in composite materials, Indian Journal of Engineering & Materials Sciences, Vol. 18, April 211,pp [4] Faruk Sen, OnurSayman, ResatOzcan & RamazanSiyahkoc, Failure response of single bolted composite joints under various preload, Indian Journal of Engineering & Materials Sciences, Vol. 17, February 21, pp [5] R.C. Alderliesten, Damage tolerance of bonded structures, International Journal of Fatigue 31(29), [6] BuketOkutan, Stress and failure analysis of laminated composite pinned joints, A PhD thesis Submitted to the Graduate School of Natural and Applied Sciences of DokuzEylul University, December 21. [7] C. T. McCarthy, M. A. McCarthy,W. F. Stanley And V. P. Lawlor, Experiences with Modeling Friction in Composite Bolted Joints, Journal of COMPOSITE MATERIALS, Vol. 39, No.21/25. [8] A. Turon, P.P. Camanho, J. Costa, C.G. Da vila, A damage model for the simulation of delamination in advanced composites under variable-mode loading, mechanics of Materials 38 (26) [9] Ivana Ilić, Numerical Simulation of Composite Structure Failure in the Areas of Geometric Discontinuities, Scientific Technical Review, Vol.LVIII, No. 2, 28.