Failure characteristics of glass-reinforced plastic pipe and pipe assembly

Failure characteristics of glass-reinforced plastic pipe and pipe assembly

Composite Structures 18 ( 1991 ) 365-377 L,~ Failure Characteristics of Glass-Reinforced Plastic Pipe and Pipe Assembly R. Kitching, a R. Priester, ...

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Composite Structures 18 ( 1991 ) 365-377

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Failure Characteristics of Glass-Reinforced Plastic Pipe and Pipe Assembly R. Kitching, a R. Priester, b H. H a s h e m i z a d e h a & P. D. S o d e n a aApplied Mechanics Division,MechanicalEngineeringDepartment, bMaterials Science Centre, Universityof Manchester Institute of Scienceand Technology,PO Box 88, Manchester M60 1QD, UK A B S TRA CT In the examination of glass-reinforced plastic (GRP) pipes and components after failure the fracture is often so catastrophic that much of the evidence is destroyed or hidden. When a leak failure occurred during a hydraulic pressure test on a GRP pipe bend the opportunity was taken to examine the bend wall in detail at a number of localities. They included, not only the region where the leak was apparent but also at areas where strain and displacement measurements had indicated the possibility of failure arising. The 250 m m diameter pipe specimen tested consisted of a right-angled pipe bend with flanged, straight lengths each attached by a butt joint. The resin matrix was polyester and the reinforcement was E-glass in the form of chopped strand mat of four layers weighing 2"4 kg/m'-. Although the failure pressure was entirely satisfactory from a design point of view, three important areas of weakness were discovered. They were associated with procedures for butt jointing and for moulding of the bend unit. The resulting micrographs were of sufficient quality to act as comparators for examination of possible service faihtres.

1 INTRODUCTION A number of 90* #ass-reinforced plastic (GRP) pipe bends with straight pipe attachments of the same material have been tested at low load and to destruction under various types of load including internal pressure, inplane flexure, out-of-plane flexure, separately and in combination, t The fractures have often been so catastrophic that much of the evidence of the initial failure and its progression was destroyed. One of the specimens, however, failed under hydraulic internal pressure by leakage 2 365 Composite Structures 0263-8223/91/S03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain

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and micrographs were obtained to trace the origin of the leak. In addition, the opportunity was taken to examine other regions which had been shown to be areas of possible fracture from strain measurements taken during the test. 2 SPECIMEN The pipe bend specimen is shown in Fig. 1. It was of a construction typical of that used in process and chemical plant for applications requiring corrosion resistance. Manufacture was by hand lay-up, the resin

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being polyester and the E-glass reinforcement in the form of chopped strand mat (CSM). The basis of design was BS7159. 3 The pipe inside diameter was nominally 250 mm throughout and the wall thickness corresponded to 2-4 kg/m 2 of glass (four layers of CSM). The specimen consisted of one bend unit, two straight lengths and two stub flanges. The manufacturing procedure was arranged so that joints were not excessively thick and so that electrical resistance strain gauges could be attached to the inside, as well as the outside, surfaces especially in the bend region where maximum strains were expected to occur. The straight pipes and bend portion were delivered in skin form, i.e. with only two layers of E-glass reinforcement bonded by the resin over an inner resin rich laye', with a veil of C-glass as surface tissue. After checking the geometry and marking out, the internal gauges (Fig. lc) were attached and the leads soldered. A jig was then used to assemble the components so that joints could be made, after which the remaining layers of CSM were wrapped on to the assembly, together with a final resin-rich layer incorporating surface tissue. When in the jig a canvas bag was inserted and inflated to form a backing while each of the butt joints was made. This kept the components in position while a 2 mm gap (between components, e.g. straight pipe and bend) was filled with thixotropic resin paste. The manufacture of the delivered straight pipes was by spiral wrapping on a mandrel, while that of the bend was by wrapping on a mould. For purposes of removal from the sheet metal mould, the latter was made collapsible (see Fig. 2) with removal segments (¢t--90-112.5 °

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and ~ 247.5-270°). While the moulding operation was taking place adhesive strips covered the periphery at the joints. The result of this was that at these positions there were undulating irregularities in the otherwise circular profile of the inside surface of the bend unit after removal from the mould? =

3 ELASTIC TEST The specimen was subjected to water pressure after sealing the ends by means of the flanges, flat steel end plates, half collars and bolts. For the low load tests the pressure was cycled between zero and 0.437 MPa in small increments. The output from the strain gauges (Fig. lc) was read, logged and found to be sensibly linear. Distribution of strain around the pipe circumference for the mid-bend position is shown in Fig. 3 corresponding to the pressure of 0.437 MPa. A comprehensive strain analysis indicates that distribution is very close to the membrane condition which

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is shown for comparison in Fig. 3. The features of the experimental strains are as follows: (1) There were appreciable and variable shell bending strains in addition to the direct strains. These were greatest in the hoop direction. (2) The maximum strains, which were 0-21% at ~ -- 90* (point B) and 0.20% at 4 = 240 ° (point D) were significantly higher than the maximum membrane strain 0.156% at ~=180*). The highest bending strain existed at position B close to that corresponding to the join of the removable portions of the mould, where the inner surface of the bend was undulating. (3) Significant bending strains of opposite sign existed at A and C on either side of position B. This implies a high bending strain gradient, hence high transverse shear force and thus high interlaminar shear stress. A similar situation occurs near point D.

4 FRACTURE TEST The specimen was later loaded in increments of 0-05 MPa until the pressure reached 3-19 MPa when excessive leakage occurred at the joint between the bend portion and one of the straight pipes. It compares with an allowable design pressure 3 of 0"4 MPa. The maximum recorded strain at mid-bend, ~t-- 90 °, was 1.28% on the outside surface.

5 PRELIMINARY EXAMINATION The specimen was inspected near the leakage position and, although there was clear whitening, indicating delamination (Fig. 4), no obvious resin cracks were visible. Thickness measurements near the joint where failure took place did show that this was the thinnest region around that section (7.8 mm compared with an average thickness of 8"8 mm). No cracks were visible in the vicinity of maximum measured strains. A detailed investigation mainly by optical microscope, was then undertaken. The specimens were taken from four regions, namely the leakage area, (see Fig. 4), that near B (Fig. 3) where the maximum strain was recorded and either side of it where transverse shear was maximum, the similar region D and the position of maximum membrane strain (4 = 180*). Some electron micrographs were also studied.

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6 PREPARATION Care was taken to ensure that the initial diamond sawing operation caused minimum delamination at the edges. For optical microscopy specimens were mounted in cold setting resin (Metset FT) before being ground on water lubricated abrasive papers of 220, 320, 500 and 1000 grades in turn. They were held in such a way that each successive grinding operation was made at right ,angles to the previous cut. Before transferring to the next grade it was ensured that the mechanical damage of the previous stage had been removed and the less intense damage replacing it was as uniform as possible. The specimen was then washed with water to prevent residual debris remaining. To remove the resulting ridged surface layer and obtain a mirror finish, polishing was carried out with two cloths impregnated with 6 !Lm and l ~ m diamond paste. Times were 45 min and 10 min respectively, with washing in detergent and rinsing in water between the stages. To improve the contrast between fibre and resin, the specimens were polished for approximately 10 min using Op-S suspension. Polishing on finer diamond wheels was not desirable as it tended to lead to a reduction in contrast between fibre and resin. Use of the 1 /~m wheel gave polishing of fibre ends without breakage and the surface had negligible scratches.

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All grinding and polishing was carried out using a machine which was able to maintain constant pressure throughout a stage and gave rise to high-quality finishes. For scanning electron microscopy (SEM), the fracture surfaces of specimens were coated with a thin layer of gold by vapour deposition by means of a sputter-coating unit. 7 RESULTS 7.1 Leakage area

Figure 5 is a diagram showing the locations of nine specimens which were cut and prepared. Optical microscopy resulted in the photograph in Fig. 6a which shows a crack initiated at the butt joint, propagating to the outside surface (Fig. 6b). Delamination occurred in the process, and this gave rise to leakage. Interlaminar failure took place along the taper where the overlay at the joint was bonded to the straight pipe. The defect in the butt joint where initiation took place was undoubtedly caused

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when the joint was made and was probably due to a wrinkle in the inflated canvas bag providing an imperfect backing surface. 7.2 M a x i m u m strain region (region D)

Figure 7 is a composite photograph showing a radial crack starting from the resin-rich layer at the inside surface of the pipe at ¢ = 270 °, propagat-

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ing outwards through the thickness. The crack grew until it was temporarily arrested by a bundle of fibres, (see enlarged view of area a of Fig. 7 in Fig. 8a). It then continued radially but with branching along the glass-resin interface as seen in Fig. 8a, propagation being around and not across fibres. The crack then continued to move radially until again deflected by a region of high fibre volume, when there was branching and propagation along another interface (see Fig. 8b which is an enlargement of area b in Fig. 7). It was finally arrested further along the interface. The crack extended in the longitudinal direction for approximately 15 ° of the bend (Fig. 8c). This area was thus one of incipient failure which would be initiated near the irregularity in the inner surface profile corresponding to the removable portions of the mould. Similar cracks could not be found in the vicinity of the equivalent position B where strain measurements were highest, but, of course, strain gauging, though extensive, could not be

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guaranteed to pick up the highest strain position. It may be significant that in addition to the high bending strain in region D, the direct strain was higher than the level of direct (as opposed to bending) strain in region B.

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7.3 Maximum bending strain gradient region (AB) Figure 9 shows a crack of 1-5 m m longitudinal length at a distance 3-5 mm from the inside surface, where the total thickness was 8-3 ram. Its position and its interlaminar nature imply that it occurred due to high transverse shear force. T h e photograph shows that it was possibly initiated at a void before travelling along the interface between fibre layers, and extending over a small area. An SEM photograph confirmed the presence of the void and its position as the likely initiation point of the crack. 7.4 Maximum membrane strain region No evidence of failure could be observed in this region.

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8 CONCLUSIONS (1) Failure was by leakage at a butt joint and was probably due to a manufacturing defect in making the butt joint. (2) Radial cracks on the inside surface were already in evidence near a position of high inside surface strains. This would clearly have caused failure at a slightly higher pressure than the maximum pressure attained. The position of high strain was caused by an uneven inner surface in the bend unit, caused in turn by the design of the collapsible mould, which has since been changed. (3) lnterlaminar shear cracks occurred in a region of high bending strain gradient. This was again caused by an uneven surface which had the same origin as that described in conclusion (2). (4) The grinding and polishing procedures developed for this composite give rise to high quality microscopy which can be used to characterise different types of failure in practical GRP structures.

9 ACKNOWLEDGEMENTS The investigation was carried out jointly in the laboratories of the Mechanical Engineering Department (Applied Mechanics Division) and the Material Science Centre at the University of Manchester Institute of Science and Technology. The authors express their thanks to all colleagues who were involved in the project and the preparation of this paper, in particular Dr E Myler (now of Lancashire Polytechnic) who carried out the test to failure. The authors are also grateful to the Science and Engineering Research Council, in association with the Polymer Engineering Directorate and later the Polymer Engineering Group, for their financial support.

REFERENCES 1. Myler, E & Kitching IL, Fracture conditions for GRP pipe bend under simple and combined loads. In Proc. 2nd Conf. Pipework Engng Operation. I. Mech. E, Mechanical Engineering Publications, Ltd, London, 1989, pp. 187-95.

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2. Kitching, 1L & Myler, P., Final report on SERC grants GR/B67964 (Strength and deformation characteristics of GRP pipes with smooth and discontinuous bends) and GR/C53305 (GRP smooth pipe bed behaviour under a variety of loads). Dept. of Mechanical Engineering, UMIST, Manchester, 1985. 3. BSI, Code of practice for design and construction of glass reinforced plastic (GRP) piping systems for individual plants or sites. BS7159, British Standards Institution, London, 1989. 4. Kitching, R., Tan, A. L. & Myler, P., Tests to failure of GRP pipe bends under in-plane flexural loading. In Proc. 1st Conf. Pipework Design Operation. I. Mech. E., Mechanical Engineering Publications, Ltd, London, 1985, pp. 21-34.