Investigation of the transverse compressive and buckling strength of aluminium grid reinforced hybrid GFRP composite

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ScienceDirect Materials Today: Proceedings 5 (2018) 25625–25631

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IConAMMA_2017

Investigation of the transverse compressive and buckling strength of aluminium grid reinforced hybrid GFRP composite S. Ilangovana, S. Senthil Kumaranb, Vasudevan. Ac, Mitisha Suranab a

Karpaga Vinayaga College of Engineering and Technology, Madurantakam - 603306, India b College of Engineering Guindy, Anna University, Chennai - 600025, India c CIPET, Chennai - 600032, India

Abstract This work is concerned with the effect of addition of aluminium grid in Glass Fibre Reinforced Polymer Matrix Composite (GFRP) on the transverse compressive and buckling (axial) compressive strength of the composite. The 5-layered cylindrical composite consisting of alternate layers of bidirectional E-glass fibres and aluminium grid in an epoxy matrix, were fabricated using hand lay-up technique and subjected to transverse compressive and axial compressive (buckling) loads. Non-linear stress – strain response was achieved in transverse and axial loading conditions due to the yielding of the aluminium fibres. Fractography studies revealed that the brittle failure of the glass fibres and the yielding of aluminium fibres resulted in fibre rotation and shear distortion that helped to dissipate the crack propagation energy thereby increasing the strain to failure. This method may be effectively used to arrest the crack propagation that might develop as leakage cracks in composite pipes, enabling the hybrid composite to be used for underground pipelines in chemical and offshore platform applications. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017]. Keywords: Aluminium grid; Hand lay-up; Transverse Compressive strength; Buckling Strength; Fractography

1. Introduction Owing to their high specific strength and ease of maintenance, polymer matrix composites (PMCs) are replacing the conventional steel pipes, in the offshore oil and gas pipelines. Glass Fibre Reinforced Polymer (GFRP) pipes composed of E-glass fibres and epoxy resin matrices find their application as underground pipelines, in the chemical and offshore oil industry where high compressive strength and corrosion resistance are needed [1,2]. The 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017].

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smooth finish of the internal walls and absence of welding joints offer excellent hydraulic characteristics, thereby saving cost and enhancing the service life of the pipelines. Despite these advantages, the GFRP pipes suffer from reliability and leakage integrity issues [3]. The main cause for the leakage in laminated pipes has been the development of transverse cracks in the matrix parallel to the fibre direction and joining of cracks in adjacent layers of the lamina to create a leakage opening, of which detection and repair has always been a challenge [4,5,6]. These cracks in turn reduce the compressive strength of the composite leading to catastrophic failures. Vedvik et. al. reported the use of metal liners on the inner surface of pipes but wearing out of the liners due to erosive action of the fluid has been a major issue [7]. This paper adopts an innovative method to integrate aluminium grid within the GFRP composite to arrest the transverse cracks of the matrix through the yielding of the aluminium fibres. In this paper, the effect of addition of aluminium fibre laminate on the transverse compressive strength and buckling strength of the short pipe configuration (slenderness ratio < 1) GFRP composite has been investigated and fractography studies were made to determine the mechanism of failure. 2. Experimental Methodology 2.1. Materials The materials used in this work are as follows: 1. 2. 3. 4.

Epoxy LY556 Hardener HY951 Bidirectional E- glass fibres (300 GSM) Aluminium grid (pore size – 2mm, fibre diameter – 760µm)

2.2. Fabrication of the cylindrical Al – hybrid GFRP Composite Epoxy resin LY556 and hardener Aradur HY951 supplied by HUNTSMAN were mixed in the ratio of 10:1 by weight respectively to make the curing agent. A cylindrical mandrel was made as the base upon which alternate layers of bidirectional E-glass fibres (represented as GF) and aluminium grid (Grade AA 1050 H 14 supplied by HINDALCO, represented as AL) were stacked as shown in Fig. 1. The curing agent was applied using hand lay-up technique. Curing was carried out for 4 hours at room temperature to obtain the cylindrical Al - hybrid GFRP composite of inner diameter 90mm and a wall thickness of 4mm. The density of the composite was found to be 1925kg/m3.

Fig. 1. Schematic representation of the Al – hybrid composite stacking sequence

2.3. Testing of the Al – hybrid GFRP cylindrical composite 2.3.1. Transverse compressive test Five cylindrical samples with 90mm inner diameter, 4mm wall thickness and 50mm length were subjected to transverse compressive testing in the Instron 5500 Universal testing machine with a capacity of 200kN as shown in Fig. 2(a). The experimental conditions followed were according to the ASTM D5449 standard.

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2.3.2. Buckling test Five cylindrical samples measuring 90mm internal diameter, 4mm wall thickness and 50mm length were subjected to buckling test (Axial Compression Test) in the Instron Universal testing machine with a capacity of 200kN as shown in Fig. 2(b). The experimental conditions followed were according to the ASTM D2924 standard.

Fig. 2. (a) Transverse Compressive Test of Al - hybrid GFRP; (b) Buckling Test of Al –hybrid GFRP composite.

2.4. Fractography Fractography studies were carried out to understand the mechanism of failure in the Al – hybrid composites using the Scanning Electron Microscope (HITACHI S – 3400N). Since the Al – hybrid GFRP composite consists of non-conductive epoxy and E – glass layers, the fractured samples were gold plated before mounting the sample in the SEM. Electron Gun voltage of 5kV was applied to study the fracture surface. 3. Results and Discussions 3.1. Transverse Compressive Strength of the Al – hybrid GFRP cylindrical composite The transverse compressive test on the aluminium grid reinforced hybrid GFRP yielded a true stress – strain curve as shown in Fig. 4(a) and the mechanical properties obtained are shown in Table 1. The top layers of the composite experienced compressive stress while the bottom layers experienced tension at the loading point as shown in Fig. 3.

Fig. 3. Nature of stress in laminates at the loading point during the transverse loading of Al – hybrid GFRP composite

Xia et al. developed a model to predict the stress at individual layers of a laminated composite shell and anticipated that the stress was maximum in the fibres which are at 90° to the loading axis [4]. As the stress at the

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loading point increased, the total strain was limited by the tensile strain experienced by the bottom aluminium and glass fibres [8, 12]. The initial non-linearity of the curve was due to the onset of yielding of the bottom aluminium layer. The curve followed a linear trend due to the predominant elastic strain of the glass fibres. The bottom glass fibres experienced the maximum tension and the composite reached the UTS of 3.369 MPa as the bottom glass layers started breaking. The aluminium grid helped to sustain the load as it yielded under tension that gave a total strain percentage of 4%, ultimately leading to the failure of the composite at the tension side. 3.2. Buckling Strength of the Al – hybrid GFRP cylindrical composite Fig. 4(b) shows the true stress – strain curve for the axial compressive loading of the aluminium grid reinforced GFRP that provided the buckling strength of the composite. Very little deformation was observed and the curve remained linear at the start. Due to the shearing nature of the load along the thickness [9], lateral displacement in the central part of the cylinder was observed. On reaching the buckling stress, non-linearity was observed with large lateral strains, because of the yielding of the aluminium layers within the composite. The deformation of the cylindrical pipe resembled a half sine wave shape that denoted instability, resulting in the global failure of the composite. Table 3.1 shows the buckling stress and strain of the aluminium grid reinforced hybrid composite.

Fig. 4. (a) True stress vs true strain curve obtained from Transverse compressive testing; (b) True stress vs true strain curve obtained from buckling test. Table 1. showing the mechanical properties of the Al – hybrid GFRP composite under transverse and axial compressive loading. Total strain of the composite (% gauge length)

Non-linear strain percentage (% of total strain)

0.59

4.2

1.67

3.36

5.4 (lateral strain %)

2.7

Nature of compressive load

Maximum compressive stress (MPa)

Transverse Axial

3.3. Fractography of the Al – hybrid GFRP composite 3.3.1. Transverse Compressive Loading The transverse loading of the composite resulted in a tension - compression zones at the bottom and top layers of the composite respectively. Fig. 5(a) shows the delamination of the aluminium layer on the tension side. The aluminium grid has increased the ductility of the composite through the yielding of the aluminium fibres and the blunting of the crack at the matrix – aluminium interface as shown in Fig. 5(b). The fractured aluminum fibre, as shown in Fig. 5(c), reveals the bend and distorted cross section of the aluminium fibre. Energy is dissipated through the fibre yielding and fibre rotation thereby delaying the crack propagation through thickness eventually delaying

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the delamination and ultimate failure. The glass fibres have undergone brittle fracture with minimum fibre pull out suggesting good interface strength and served as crack initiation sites as can be seen from Fig. 5(d), substantiating that the aluminium fibres at the tension side are responsible for the delay in crack propagation.

Fig. 5. SEM images of fracture surface under transverse compressive loading showing (a) Delaminated aluminium grid layer; (b) Crack tip blunted by the yielding of the Al layer; (c) Al grid breakage; (d) Fracture and fibre pull out of glass fibres.

3.3.2. Axial Compressive Loading Fig. 6(a) shows the fracture surface of the aluminium grid reinforced GFRP failed under axial compressive loading (buckling). Delamination of the aluminium layer can be observed owing to the differential strains experienced by the glass and aluminium fibres for the same stress. The fracture has happened at a shear angle of 45° suggesting that shear force was the dominant force. The shear strain and distortion of the aluminium fibre can be observed in Fig. 6(b). The deflection of the crack along the Al – matrix interface suggested that the presence of aluminium has delayed the crack propagation. Fig. 6(c) shows a clearer image of the Al – matrix delaminated interface. Brittle glass fibre fracture was observed in Fig. 6(d) at 45° angle to the loading axis. The non-linear strain response of the composite during axial compression was substantiated by the distortion of the aluminium fibres proving that non-linear yielding was due to presence of aluminium fibres.

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Fig. 6. SEM images of the fracture surface under axial compressive loading showing (a) Delamination of the aluminium layer; (b) Yielding of aluminium; (c) Aluminium epoxy interface; (d) shows the shear fracture of glass fibres and fibre pull out.

4. Conclusion The addition of aluminium grid to the GFRP composite has induced non-linear strain response during the transverse and buckling (axial) compression test. Fractography revealed that the aluminium fibres dissipate the crack propagation energy through fibre yielding and fibre rotation while the glass fibres failed in brittle manner. The crack arresting capability of aluminium fibres can be used prevent the formation of leakage cracks and increase the strain to failure of the GFRP composite. References [1] Myler. P, Kitching. R, Extended Time Tests on GRP Pipe Bends, Composite Structures Vol.7 (1987), p. 255-266. [2] Tarakcioglu. N, Samanci. A, Arikan. H, Akdemir. A, The fatigue behavior of (±55°)3 filament wound GRP pipes with a surface crack under internal pressure, Composite Structures Vol.80 (2007), p. 207–211. [3] Martens. M, Ellyin. F, Biaxial monotonic behavior of a multidirectional glass fiber epoxy pipe, Composites: Part AVol.31 (2000), p. 1001– 101. [4] Xia. M, Takayanagi. H, Kemmochi. K, Analysis of transverse loading of laminated cylindrical pipes, Composite structure Vol. 53 (2001), p. 279 – 285.

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