Internal reinforcement joints in grp under static and fatigue loading

Internal reinforcement joints in grp under static and fatigue loading M.J. OWEN and J. R. GRIFFI THS

Several forms of glass reinforcement for polyester resins are supplied as rolls of finite width and thus inevitably give rise to reinforcement joints in large laminates. Internal joints in flat and tubular specimens reinforced with chopped strand mat and a plain weave fabric were studied under static and fatigue loading. Under static loading a butt joint in chopped strand mat reinforcement had a small effect on the strength compared with a similar specimen without a joint. Under fatigue loading the presence of a joint produced serious reduction in strength. However with fabric reinforcement a lap joint produced a reduction in static strength and a negligible reduction in strength at long fatigue lives. Although the strength of bonded lap joints between laminated grp plates has been reported, 1' 2 the significance of internal joints in grp laminates has not been recognised. These joints occur because reinforcements are supplied in rolls of limited width and hence butt or lap jointing of the reinforcement may be unavoidable during industrial laminating. This paper describes the effect of reinlbrcement joints on the static and fatigue strength of two composites. The results arise from an investigation into the behaviour of grp under complex (biaxial) stresses. The uniaxial and biaxial stress properties of a chopped strand mat (csm) reinforced polyester resin were examined by testing flat laminates and thin-walled tubes under static and fatigue loading. 3 A thin-walled tube subjected to a tensile hoop stress only (R = 0 where R = ratio of axial hoop stress in the tube walls), and the flat laminate subjected to a uniaxial tensile stress are approximately equivalent. It has been reported that tubes may have marginally higher ultimate strengths than flat laminates because of the absence of edge effects. 4 However Owen and Found 3 have shown that although the static tensile strengths for both specimen types were similar, the tubes exhibited a much lower fatigue performance than the flat specimens. This is shown in Fig. 1 and the magnitude of the discrepancy increases with fatigue life. Several possible reasons for this anomaly were suggested - eg the effect of the pressurising medium (Shell Tellus 15 mineral oil), or size effects, or radial stress effects, but none were verified. Subsequently, one of the present authors, J.R. Griffiths, s took over the experimental work with an initial objective to investigate this anomaly. Static and fatigue tests were performed on tubes at R = 0 and flat laminates under uniaxial tension to substantiate the previous evidence and Dr Owen is a Reader in the Department of Mechanical Engineering, University of Nottingham, University Park, Nottingham NG7 2RD; Dr Griffiths is currently with the Mining Research & Development Establishment, Bretby, Nr Burton-on-Trent, England.

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Zero-tension stress fatigue results for csm-reinforced flat specimens compared with fatigue results for tubes at R = 0 (from Fig. 1

Owen and Found 3 and Owen et a16): I -- Flat laminate results (Reference 3); 2 -- Flat laminate results (Reference 5); 3 - Tube results (Reference 3 ) ; 4 -- Tube results (Reference 5)

to gain insight into failure modes. The additional results are also shown in Fig. i 6 where data points are included with Curve 4 to indicate the typical scatter. The discrepancy between the two sets of data was shown to be due to reinforcement batch differences, s Although csm is nominally isotropic, due to its random glass strand orientation, in practice it possesses slight anisotropy, usually being stronger along the length of the roll. To overcome this problem when making laminates in the laboratory alternate reinforcement layers are cut from across and along the reinforcement roll. This method was used for making tubes. The tubes were manufactured on a mandrel and each reinforcement layer was made from a separate piece of reinforcement wrapped to meet with a carefully made butt

0010-4361/79/020089-06 $02.00 © 1979 IPC Business Press COMPOSITES. APRIL 1979

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joint, a Hence a discontinuity, and thus a possible failure site, existed at the joint in each layer. The joints in the various layers were staggered at regtdar intervals. In addition to the csm work an extensive programme was tmdertaken to characterise the strength of a fabric-reinforced polyester resin under complex stress, s' 7 The reinforcement used was a balanced weave fabric having equal fibre counts in both warp and weft directions. Five layered tubes were made by continuously wrapping reinforcement soaked with resin around a mandrel. The end of the reinforcement on the outside of the tube overlapped the start of the reinforcement on the inside of the tube by approximately 40 mm to prevent peeling back of the reinforcement during test. The fabric tubes thus contained two reinforcement lap joints. During the course of the foregoing investigations the full significance of reinforcement joints became appreciated and the present paper concentrates on this aspect of the work. TEST PROGRAMME AND TECHNIQUES

To ascertain the cause of the anomaly illustrated in Fig. l the effects of an oil environment and of reinforcement joints were investigated using flat csm specimens. A similar technique was adopted to examine joint effects in fabric reinforced specimens. Static tests were performed on an 'E' type Tensometer at a crosshead speed of 1.27 mm/min using wedge type nonrotating grips. Fatigue tests were performed at a frequency of l O0 c/min in axial load fatigue machines previously described in detail by Owen. s Full details of the materials used are given in Table 1.

Table 1.

Details of materials used

Material

Details

Chopped strand mat

'SuprEmat', Fibreglass Ltd 600 g/m 2 'E' glass mat

Fabric

'Tyglas' Y449, T5 finish 375 g/m = plain weave fabric with equal counts in both fibre directions, supplied by Fothergill & Harvey Ltd

Resin

Polyester resin L2615 supplied by BIP Chemicals Ltd Resin components: Maleic anhydride 2 mol Phthalic anhydride 1 mol Propylene glycol 3 mol Alkyd/styrene ratio 65/35 Hydroquinone 0.008% on blended resin

Catalyst

1% Methyl ethyl ketone peroxide (MEKP)

Accelerator

1~% BIP Chemicals Accelerator 'B' (cobalt type)

Cure schedule

18 h at room temperature followed by 3 h at 80°C

90

Csm-reinforced material

To simulate tube construction two- and three-layered flat laminates containing a carefully made butt joint in known positions in one reinforcement layer were manufactured. The technique is described elsewhere in more detail by Griffiths. s The butt joint was usually located within the first reinforcement layer, ie the first layer during laminate manufacture (Fig. 2). In addition one three-layered laminate was made with the joint located in the second reinforcement layer. For comparison purposes, plain non-jointed two- and three-layered laminates were manufactured. Static and fatigue specimens of types A and B shown in Fig. 3 were cut so that the joints were either parallel to or perpendicular to the specimen axis at the neck. To examine the effect of oil in contact with the surface both plain and jointed fatigue specimens of type B (Fig. 3) were tested by enclosing the specimen neck within a polyethylene jacket containing Shell Tellus 15 mineral oil. Fabric-reinforced material

To examine lap joint effects on fabric reinforced tubes, live-layered laminates were manufactured to simulate the tube construction. These, shown in diagrammatic form in Fig. 4, had a lap joint on the first and final layers with the joint in the final layer overlapping that on the inner layer by 40 ram. It must be made clear that these lap joints were not joints that connected two pieces of reinforcement together within the same laminate layer. They were effectively two extra half-lengths of reinforcement bonded to the inside and outside of the laminate. Static and fatigue specimens of types a and c (Fig. 3) respectively were cut so that the inner layer lap joint ran along the transverse axis of the specimen and both joints ran across the full specimen width. Fatigue specimens if type c were used in preference to those of type b so that similar applied stress conditions existed in both the joint and overlap regions. Non-jointed specimens of type c were also tested for comparison purposes. To examine oil effects a similar procedure to that described for the csm material was employed but using, in this case,

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b Fig. 2 Simulation of tube construction by csm laminates: (a) section through a 3-layered csm tube showing the inner reinforcement laver joint; and (b) section through a 3-layered csm laminate containing an inner reinforcement laver joint

COMPOSITES. APRIL 1979

plain non-jointed specimens of type b (Fig. 3). Specimens of type b were also tested in a normal environment for comparison purposes. IDENTIFICA TION OF JOINTS Unless a joint is identified in some way at the laminating stage it is remarkably difficult to locate subsequently, especially for csm type materials. I f transverse sections are

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cut and polished by the usual metallographic techniques to show resin containing cut fibres it is quite impossible to identify the joints. They can however be revealed in the plane of the reinforcement by the following procedure: surface grind down to the plane of interest (by internal grinding in the case of cylinders); etch the ground surface in hydrofluoric acid thus dissolving the glass from the resin surface; fill the resultant surface with Indian ink, dry and wipe clean to leave the glass filaments replaced by ink. Fig. 5a shows a three-layered csm plate, containing a joint in its second reinforcement layer, that has been surface ground down by only 0.5 mm and prepared as above. No joint is visible and the laminate appears to be perfectly normal. Fig. 5b shows the same plate ground down by 1.7 mm (ie into the second layer) and clearly shows the resin rich joint region which is characterised by the lack of glass strands crossing the joint zone. Figs. 6a and 6b show respectively a fabric-reinforced laminate, containing a lap joint, that has been surface ground, and a fabric-reinforced cylinder that has been internally ground and then prepared as above. The lap joints are clearly visible and comparison of the two confirms that the simulation of cylinder joints was successful.

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Fig. 3 Flat laminate specimens -- joints are located at X-X or Y-Y when required: (a) static tensile; (b) fatigue tensile; and (c) fatigue tensile (all dimensions in mm)

Fig. 5a Fig. 5b joint

Csm plate showing random fibre orientation The plate shown in Fig. 5a ground down to reveal a butt

a Resin

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Lap joint

Fig. 4 Simulation of tube construction by fabric-reinfroced laminates: (a) section through a 5-layered fabric tube; and (b) section through a 5-layered fabric laminate having a joint and overlap

COMPOSITES. APRIL 1979

Fig. 6a Fig. 6b

Jointed fabric laminate Internally ground fabric-reinforced tube

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RESULTS AND DISCUSS~ON

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A detailed analysis of the static csm results showed that

after making due allowance for glass content variation the effect of joints on the static tensile properties, including initial and secondary tangent moduli, were only small (2 10%). s (After debonding, csm shows a permanent stiffness loss and the secondary modulus is the tangent modulus after the onset of this damage stage.) Static strength results uncorrected for glass content are presented in Table 2. However, as shown in Fig. 7 and Table 2, a reinforcement joint has a considerable effect on the tensile fatigue properties of a csm-reinforced polyester resin. Comparisons between curves 1 and 4 of Fig. 7 show that the strength reduction due to the presence of a butt joint perpendicular to the axis in the first laminate layer is 38%and 58%at 103 and 106 cycles respectively. The magnitude of the effect increases with fatigue life because the failure mechanism is crack growth through the resin rich joint zone. A jointed layer, as shown in Fig. 5b is characterised by the lack of fibres crossing the joint zone and hence no barrier to crack growth exists. (It should be noted that great care went into making these joints and rollers were used to try to knit the joint together.) As a results of the crack growth mechanism the specimens used to generate curve 4 experienced bending stresses due to a shift of the neutral axis with respect to the loading points. Curve 3 of Fig. 7 is derived from specimens containing a middle layer reinforcement joint. Here no significant bending occurs because of symmetry and although the joint effect at short fatigue lives is less marked than curve 4 the trend indicates that at longer fatigue lives only small differences would exist between curves 3 and 4. An unexpected result is shown by curve 2. Here a joint effect is shown by a three-layered laminate containing a joint parallel to the loading axis in the first laminate layer.

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CSM-REINFORCED LAMINA TES 3-layer, no joint 103.8 3-layer, joint parallel to axis 91.1 3-layer, joint perpendicular to axis 90.3 3-layer, joint in middle layer perpendicular to axis 104.3 2 layer, no joint 96.1 2-layer, joint parallel to axis 2-layer, joint perpendicular to axis 88.0 3-layer cylinder (R -- 0) 97.1 FABRIC-REINFORCED LAMINA TES 5-layer, no joint 209.2 5-layer, joint and overlap 184.3 5-layer cylinder, (R = 0) 262.2

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Static strength (MPa)

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Fig. 7 Effect of joints on the zero-tension fatigue properties of csm-reinforced flat specimens: I - 3-layer laminate, no j o i n t ; 2 - 3-layer laminate, j o i n t parallel to axis; 3 - 3-layer laminate, j o i n t in m i d d l e layer p e r p e n d i c u l a r to axis; 4 -- 3-layer laminate, j o i n t in o u t e r layer p e r p e n d i c u l a r to axis; 5 -- t u b e (R = 0); 6 -- 2-layer laminate, no j o i n t ; 7 --2-layer laminate, j o i n t parallel t o axis; 8 -- 2-1aver laminate, j o i n t p e r p e n d i c u l a r to axis

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Similar results to those discussed above were shown by the two-layered laminates. The difference existing between curves 1 and 6 at 10 a cycles is due to glass content variation. Laminate strengths

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By considering forces instead of stresses Fig. 8 shows a three-layered laminate containing a joint in the first laminate layer parallel to the loading axis requires a lower failure force at fatigue lives above 10 a cycles than a nonjointed two-layered laminate. Hence the jointed layer cannot simply be considered as non-load bearing. The reason for this is a combination of the pre-determined failure initiation site, the crack growth failure mechanism and the associated superimposed bending stresses in a jointed laminate. Fig. 9 shows through the thickness failure in such a laminate and illustrates the failure mechanism. Cracking occurs initially at the resin rich joint zone (point A in Fig. 9).

COMPOSITES . APRI L 1979

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Fig. 11 Effect o f oil on the zero-tension fatigue properties o f 3-layered csm-reinforced flat laminate specimens: I - in air, no j o i n t ; 2 -- in oil, no j o i n t ; 3 -- in air, j o i n t p e r p e n d i c u l a r to axis; 4 -- in oil, j o i n t p e r p e n d i c u l a r to axis

In Fig. 10 individual results are plotted for transverse butt-jointed specimens and tubes (R = 0). The conclusion is inescapable that it is the reinforcement joints which dominate the fatigue behaviour and give rise to the anomaly shown in Fig. 1.

Fig. 9 testing

T h r o u g h - t h i c k n e s s failure in a j o i n t e d laminate after fatigue

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Crack growth then takes place along BC and CD in turn. These regions represent the portion of the joint where some degree of fibre overlap across the joint has been achieved by the use of rollers. Even so, crack growth appears to proceed easily because knitting of the joint throigh its thickness is virtually impossible to achieve and thus there must exist a thin layer of resin where cracking has no barrier. Between D and E the superimposed bending stresses have caused delamination and final separation occurs by tensile overload at point E.

C O M P O S I T E S . A P R I L 1979

Fig. 11 presents the oil environment test results on csmreinforced laminates. Surface effects of oil would be greatest on jointed specimens since an easy oil penetration path is present after the onset of joint cracking. However, both the plain and jointed specimen results indicate a slight strengthening effect in oil at long lives. A similar result was observed for the fabric-reinforced laminates as shown in Fig. 12. The enhancement of fatigue strength is possibly due to a combination of (i) moisture exclusion; and (ii) lubricating effects at (a) the glass/resin interface, and (b) the glass roving to roving contact points thus reducing the number of micro-flaws initiated. For the fabric reinforcement, the lap-jointed laminates exhibited a 10 % lower static ultimate tensile strength than the plain laminates (see Table 2). This was because just prior to failure the 'overlap' layer delaminated and the subsequent load redistribution caused failure of the remaining layers: No effect was observed on the static tensile modulus. Fig. 13 shows the effect of lap joints on the zero-tension fatigue behaviour of fabric-reinforced laminates. Results are shown in Table 2. An adverse effect is in evidence at short fatigue lives reducing to zero as the life increases. Thus, the difference in behaviour between lap-jointed fabric-reinforced laminates and butt-jointed csm-reinforced laminates is quite distinct. In industrial laminates fabricated from mats, fabrics, or woven rovings, reinforcement joints are inevitable because the reinforcement rolls are suppfied in finite widths (usually 1 yard or 1 m). It is a common practice to vary the total reinforcement thickness locally by providing additional layers of reinforcement. Mixed types of reinforcement are used in some cases, eg chopped strand mat pressure vessels with additional filament wound hoop reinforcements are used in the chemical industry. The present paper shows that reinforcement joints or overlaps may be weak in

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A CKNOWL EDGEMENTS This paper is based on results included in a PhD thesis by J.R. Griffiths. s The work was supported by a Science Research Council grant to M.J. Owen and by a research studentship awarded to J.R. Griffiths. The authors are indebted to Mr G. Budd for assistance in the preparation of specimens.

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Jointed reinforcement layers have a marked adverse effect on the zero-tension fatigue strength of csm-reinforced polyester resins when the number of reinforcement layers is small, although only a small effect is observed on the static tensile properties. Joints are inevitable in structures and their positioning requires careful consideration if premature catastrophic failure is to be avoided. Lap-joints of the type reported in this paper in continuous balanced weave fabric-reinforced polyester resins have a significant effect on the static strength which, however, becomes negligible at long fatigue lives. It has been found that the effect of Shell Tellus 15 mineral oil on the fatigue strength of the two composites considered is negligible.

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Kendall, IL 'Crack propagation in lap shear joints', J Phys (D) App Phys 8 (1975) pp 512-522 Renton, J.W. and Vinson, J.R. 'On the behaviour of bonded joints in composite material structures', Engng Fract Mech 7 (l 975) pp 41-60 Owen, M.J. and Found, M.S. 'Static and fatigue failure of glass fibre reinforced polyester resin under complex stress conditions', Faraday Special Discussion o f the Chemical Society 2 (1972) pp 77-88 Pagano, N.J. and Whi~ey, J.M. 'Geometric design of composite cylindrical characterisation specimens', J Composite Mater 4 (1970) pp 360-378 Gtiffiths, J. IL 'Fatigue of glass reinforced plastics under complex stresses', Ph D thesis (University of Nottingham, 1974) Owen, M.J. Griffiths, J.R. and Found, M.S. 'Biaxial stress fatigue testing of thin-walled grp cylinders', Proc 1st Int

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Conf on Composite Materials, Met Soc of the AIME 2

fatigue irrespective of their direction. These features are not normally regarded as defects. The fact that a crack at a resin-rich butt joint in chopped strand mat can propagate readily into the adjacent continuous layers is also significant for the case of additional hoop winding which is prone to

(1976) pp 917-941 Owen, M.J. and Griffiths, J.R. 'Evaluation of biaxial stress failure surfaces for a glass fibre reinforced polyester resin under static and fatigue loading', JMater Sci 13 (1978) pp 1521-1537 Owen, M.J. 'A new fatigue machine for reinforced plastics', Trans J Plastics Inst 35 (1967) pp 353-357

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