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Environmental stress corrosion of hybrid fibre composites
Composites Science and Technology 45 (1992) 257-263
Environmental stress corrosion of hybrid fibre composites Mark A. French* & Geoffrey Pritchard School of Applied Chemistry, Kingston Polytechnic, Penrhyn Road, Kingston upon Thames, Surrey, UK, KT1 2EE (Received 23 June 1991; revised version received 30 September 1991; accepted 18 October 1991)
Unidirectional glass/epoxy, carbon/epoxy and glass/carbon/epoxy hybrid laminates (14 glass plies and 2 carbon) were immersed in water, humid air and dilute (0.5 M) sulphuric acid and subjected to various levels of constantly applied tensile stress until rupture occurred. When the liquids were allowed direct access to the cut edges of rectangular strip specimens, and hence to the glass plies, the position of the carbon plies made little difference to the failure time at a given stress. Protecting the cut edges and allowing access only to the major faces resulted in failure times which depended more strongly on the location of the carbon plies. Carbon plies used as external cladding greatly increased the failure time, and the variability in failure time, in acid at low strain levels, but did not completely inhibit failure. This supports the view that microcracks develop in the outer carbon plies. Failure modes are discussed.
Keywords: environmental, strain, corrosion, hybrid, epoxy, laminate, glass, carbon, diffusion, fracture
1 INTRODUCTION
It has been shown that under certain conditions the addition of a second reinforcing fibre to form a hybrid composite can enable the failure strain of the first (more brittle) plies to be increased above their normal value. Incorporation of a second phase into a glass composite subjected to water or acid attack might be expected to provide a means of restricting crack growth. Carbon fibre reinforced epoxy resins are themselves prone to brittle cracks, but not to the same extent as that demonstrated by glass/epoxy laminates under the special conditions of acid-induced stress corrosion. Carbon fibres are resistant to water and dilute acid attack and, provided that the energetic conditions are satisfied, there is a possibility that hybridisation might reduce the crack growth rate at a given strain. This investigation was concerned with the extent of the improvement in stress corrosion failure times obtainable when carbon plies are added to a unidirectional glass composite, and with the importance of the position of the carbon plies used. An account has already been given elsewhere of the behaviour of the same laminates
Unidirectional glass reinforced epoxy laminates normally fail in tension to give 'brush-like' failure surfaces, with some of the energy being absorbed by the process of pulling fibres out of one or both fractured pieces. However, the tensile strength of glass fibre reinforced composites is seriously reduced by hot, wet conditions or by acid. Unidirectional glass laminates held under a tensile stress in wet or acidic environments can fail catastrophically at stresses far below the short term tensile strength, 1,2 with failure times increasing as the applied stress is decreased. At low strains especially, the failure process is dominated by the growth of brittle cracks in a direction transverse to the applied load. These cracks are uninterrupted by the interfaces between fibre and matrix, and they therefore lead to a characteristically planar fracture surface. * Present address: British Aerospace, Stevenage, UK.
Composites Science and Technology 0266-3538/92/$05.00 © 1992 Elsevier Science Publishers Ltd. 257
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Mark A. French, Geoffrey Pritchard
Table 1. Reference codes for lay-ups Code
Stacking sequence
A1 C G-G-G-G-G-G-G-G-G-G-G-G-G-G C A2 G-G-G C G-G-G-G-G-G-G-G C G-G-G A3 G-G-G-G-G-G C G-G C G-G-G-G-G-G C = carbon ply; -G- -- glass ply.
immersed without edge protection in the same environments with no applied stress. 3
2 EXPERIMENTAL 2.1 Specimen fabrication Unidirectional all-glass, all-carbon and hybrid glass/carbon/epoxy laminates were manufactured at Westland Helicopters Ltd, Yeovil, Somerset, by stacking sheets of two kinds of prepreg (Fibredux ® 913C/XAS, containing 60% by volume of Grafil type II high tensile strength carbon fibres, and Fibredux 913G-E, containing 55% by volume of E glass fibres, both in an epoxy resin matrix, from Ciba Geigy) to form 16 ply (2 mm) sheets. The sheets were then cured for two hours at 90°C and one hour at 120°C. To make the hybrids, two carbon plies were interspersed with 14 glass ones and located symmetrically at various positions in the laminates (Table 1). The sheets were checked for void content by ultrasonic C-scanning and machined into parallelsided strip specimens approximately 2 0 0 m m l o n g × 1 5 m m wide, by means of a diamond wheel. The cut edges were sanded with carborundum paper and the strips were dried over silica gel at 50°C in a vacuum desiccator for 14 days. Aluminium alloy end-tabs, 2 m m thick, were bonded to the specimen ends with Redux 403, an aluminium filled epoxy adhesive (from Ciba Geigy). Static tensile characterization of each type of laminate was obtained by using a Zwick 1484 200 kN machine at a crosshead speed of 5 mm/min.
placed in wet atmospheres. All the jigs were fitted with load cells. Those jigs designed for placing in humidity chambers at 50°C/95% relative humidity were of stainless steel construction, and their load cells were protected with water-resistant putty (Fig. 1). The others were essentially small loading frames of mild steel, with glass jackets for surrounding the specimen with water or acid while it was under load. The loads were applied by tightening the nut on a threaded section attached to one of the grips. The creep rate of the unidirectional laminates was negligible. In one set of experiments box-shaped acrylic cells 6 0 m m by 13 m m by 11 mm, containing 0.5 M solutions of sulphuric acid, were attached to the specimen faces so that the liquid did not directly contact the cut edges. (This practice was adopted after simpler methods such as edge sealing with various sealants had proved unsuccessful.) Physical changes were observed and the specimens were photographed as failure approached. The times at which failure occurred could be correlated with reductions in the load sustained by using chart recorders connected to the load cells.
2.2 Immersion procedure Special jigs were used to apply a tensile stress while the samples were immersed in liquids or
Fig. 1. Stainless steel jig, with 16 ply A2 laminate after failure.
Environmental stress corrosion of hybrid fibre composites
259
3 RESULTS A N D DISCUSSION 3.1 Stress corrosion in water and humid air 1.5
The failure time was arbitrarily defined as the time to reach a 10% reduction in load, since once the failure process had initiated, the load fell progressively over a period of time. The process involved such events as longitudinal splitting at one or both edges, delamination, and some transverse cracking, particularly at lower loads. In all-glass specimens, transverse cracks were joined by longitudinal ones, so that the fracture surfaces appeared stepped, resembling those of unstressed, immersed specimens after tensile testing 3 but with far less longitudinal splitting. The stress rupture times were very variable in water and at high humidities, as shown in Fig. 2. Data points with arrows indicate that no failure occurred by the end of the experiment. The scatter in water and humid air was much greater than could be expected from variations in the strength or quality of the laminates, as reflected in the static tensile tests. It will be noted later that the scatter was also very much larger than it was in dilute sulphuric acid, and therefore the crucial factor must be environment-related. The cause must involve one of three parameters: (i) the number, distribution, and nature (size or shape) of existing flaws in the glass fibres; (ii) the rate of formation of new flaws; or (iii) the rate of 1011 90 nm.io,,
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Fig. 3. Loglo (failure time) as a function of applied tensile strmn for (A) 16 ply glass-epoxy laminates compared with
( i ) equivalent laminates having one carbon ply on each major surface, but with no edge protection in either case. Environment: 0.5 M sulphuric acid at 30°C.
flaw and crack growth. The number of pre-existing flaws is unlikely to be responsible since they must be statistically distributed throughout all the fibre surfaces. Flaw propagation is thought to involve the bond rupture of strained silicon-oxygen bonds by water molecules at the tips of flaws in the glass fibres, 4,5 and unless many flaws are too small for water to enter, this too is unlikely to be the dominant factor. In contrast, the formation of new flaws involves the exchange of acidic hydrogen ions, present in water or dilute acid (but much more numerous in dilute acid), with larger cations in the glass fibre surfaces, causing surface shrinkage. The surface shrinkage in turn creates tensile stresses which eventually lead to new surface flaws. 6.7 3.2 Stress corrosion in dilute sulphuric acid
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Fig. 2. Log10 (failure time) as a function of the applied tensile stress, expressed as a percentage of the mean tensile strength. 16 ply A1 laminates (symbol: Q) with one carbon ply on each outer surface, the remainder being glass, are compared with equivalent laminates (A2) having 14 glass plies and two carbon plies in positions 4 and 13 (symbol: II). Environment: 96% relative humidity at 50°C. The specimen edges were not protected. Arrows indicate no failure.
Figure 3 shows the effect of using one carbon ply on each outer face of a glass laminate, without protecting the edges. Rupture times of all-glass laminates, as a function of strain, are compared with those of hybrid lay-up type A1 in 0.5 M sulphuric acid at 30°C. The log~o rupture times were linearly related to the applied strain, and the variability was very much less than that previously noted in water. Since there was no edge protection, the acid could attack the glass fibres regardless of the carbon cladding. There was very little difference in stress rupture times for any of the samples containing glass fibres
6
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Mark A. French, Geoffrey Pritchard
when the edges were exposed. (Therefore, for clarity, A2 and A3 data are not shown in Fig. 3.) The only difference was observed in the all-carbon samples, which remained intact even after a month at the highest stresses applied, i.e. 95% of their ultimate tensile strength. Figure 4 compares the failure times of A1, A2 and A3 series laminates in 0.5 M sulphuric acid at 20°C. In this case the edges were protected against acid attack and the acid applied by means of small cells attached to the larger specimen surfaces, as mentioned earlier. Failure times of A1 laminates were generally now very much longer, as expected, because the glass was not directly in contact with the acid. But it was also noted that the failure times were also very much more variable than those of the A3 laminates, at a given strain. The variability between A2 specimens was intermediate between those of A1 and A3. So the time required for acid-induced rupture in A3 laminates, with several outer glass plies, was relatively reproducible, and depended only on the applied strain. In contrast, in the case of carbon-clad A1 laminates, the time required for penetration of the liquid through the outer carbon layer, and consequent rupture, varied a great deal from one specimen to another. Such variability can be explained most easily by assuming the formation of very small microcracks in the carbon plies. The time to failure of A1 laminates was nearly always longer than that of laminates without any carbon cladding, so it is
1.5
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Fig. 4. Log~o (failure time) as a function of applied tensile strain for 16 ply unidirectional glass/epoxy laminates containing 2 carbon plies as substitutes for 2 glass ones. ( , ) : A1 lay-up (carbon on the outside surfaces); (O): A2 lay-up (carbon in positions 4 and 13); (A) A3 lay-up: (carbon in positions 7 and 10). Environment: 0-5 M sulphuric acid at 20°C. All edges protected.
• O0 AA" • 0.5
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I
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i
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Fig. 5. Loglo (failure time) as a function of applied tensile strain for 16 ply unidirectional glass/epoxy laminates ( i ) with and (/k) without edge protection, compared with equivalent carbon-clad laminates (O). Environment: 0.5 M sulphuric acid, at 20°C.
unlikely that these cracks were present at the outset. It is also improbable that they formed by purely mechanical means, as the imposed strains were mostly far below the tensile failure strains of the dry laminates, which were, for example, about 1.6% in the case of A1 lay-ups. One possibility is that the microcracks could be caused by sorption, as a result of swelling strains in the resin rich zones between carbon fibre bundles, but the reason for such variable crack formation times is unclear. Microcracks have previously been detected and demonstrated in similar unidirectional epoxy laminates as a result of hygrothermat conditioning, both with all-glass s and all-carbon 9 reinforced materials. Interestingly, protection of the specimen edges achieved a small increase in failure time, even in the case of all-glass laminates (Fig. 5). We assume that acid access through the edges was more rapid than through the faces because the cut edges had no surface veneer of epoxy resin. Data for A1 carbon-clad laminates are reproduced here again alongside those for the two sets of glass experiments. They show that the benefits of carbon cladding vary from very little, up to about a one hundredfold increase in lifetime. No such benefit was seen with the specimens exposed to acid at their cut edges. 3.3 Fracture surfaces
The fracture surface morphology of all-glass specimens was dependent on the applied strain, as reported by Hogg & Hull. 1° High strains
Environmental stress corrosion of hybrid fibre composites (1"0-1.3%) resulted in sufficiently large stress concentrations at the tips of transverse cracks to induce fibre debonding, delamination and longitudinal cracking. Intermediate strains of 0.61.0% produced numerous crack nuclei, which coalesced by transverse and longitudinal cracking to result in irregular, 'stepped' fracture surfaces. At strains of around 0.3%, transverse crack growth dominated, leading to a planar fracture surface, with no evidence of fibre pull-out. The failure of glass plies during stress rupture experiments on hybrids at relatively low stresses did not necessarily lead to failure of the carbon plies, and in those cases where the carbon did not fail, the environmentally induced cracks travelled through the glass plies producing planar fracture surfaces. Glass plies on the outside of laminates sometimes failed separately, leaving the innermost glass plies, enclosed by carbon, apparently intact. In other cases, peeling the outer carbon plies off after failure revealed numerous planar fractures in the glass core (see Fig. 6). This suggests that the interface between the glass and carbon plies survived the first complete fracture of the glass plies, thus facilitating load transfer via the carbon from one part of the glass core to another. Three types of failure process were observed, regardless of the presence of edge protection:
Type /:Single transverse crack in the acidimmersed region. Type 2: Single transverse crack initiated at the acid-air interface. Type 3: Combination of transverse and longitudinal cracks. The Type 1 fracture surfaces of A3 laminates were often planar, in line with the model of
Fig. 6. The carbon-clad (A1) specimentshown here had not apparently failed, but when the outer carbon layer was removed, the glass core showed multiple transverse fractures.
261
Fig. 7. Type 1 failure of an A3 laminate in acid at 0-5% strain, by means of a single planar fracture, with small steps induced by limited delamination.
confined acid attack of Hogg & Hull.~° The acid was confined to the crack region, with a limited amount of delamination which resulted in small steps. Figures 7 and 8 show A3 specimens which were both subjected to 0-5% strain; they failed by Type 1 and Type 2 methods, respectively. Type 2 failures began with numerous small transverse cracks on both sides of the specimens. Complete failure of the glass core in A1 lay-ups, and of the glass plies in A2 and A3 lay-ups, occurred at the acid-air interface. Specimens of A3 type showed delamination of the outer glass plies from the still intact carbon plies on both sides of the specimens. In Fig. 8, the level of acid has been raised slightly after the failure, to allow the crack to be seen more clearly. Jones et al.ll has explained Type 2 failures in terms of stress concentrations, arising from the formation of products such as calcium sulphate as a result of acid leaching of the glass. The salts then crystallize inside the composite; planar fracture results. This explanation suggests that such failures would not occur if acids with soluble salts were to be used. Figure 9 shows an A1 lay-up with Type 2 failure. The existence of salt crystals on fibres is not conclusive proof that they
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Mark A. French, Geoffrey Pritchard
Fig. 8. Type 2 failure of an A3 laminate in acid at 0.5% strain. The transverse crack occurred at the acid-air interface, but the level of liquid was subsequently raised slightly, to enable the fracture region to be seen.
acted as stress raisers and precipitated failure. There is no evidence as to whether the crystals formed before failure or afterwards. A later paper will show some of the damage features developing after the failure of the glass fibres. Type 3 failures consistent with the Hogg and Hull mechanism occurred typically as follows. An A2 lay-up specimen held at 0-9% strain showed two widely separated nucleation sites for horizontal cracks (i.e. at 90 ° to the fibres) after 35 minutes. Twenty minutes later, one crack had grown from 3 to 7 m m long and the other (near the acid-air interface) had grown by the coalescence of several small cracks, with shear cracking along the fibres at the crack tips, producing a slightly stepped appearance. Other cracks were also beginning. Three minutes later, limited delamination had taken place around the stepped crack, and the inner carbon ply became visible as a dark region. Almost immediately afterwards, at 63 minutes, the first two growing cracks had joined by means of a longitudinal crack extending several centimetres. The carbon core remained intact holding the specimen together after failure of the outer glass plies. Colour changes which had previously been observed at this stage in failed, unstressed samples were not noticeable until about an hour after failure had occurred. 3.4 The stress corrosion mechanism
Fig. 9. Type 2 failure of an A1 laminate in acid. The carbon cladding has been removed to show the planar fracture in the glass core. The fracture occurred at the air-acid interface.
The stress rupture times and the appearances of the fracture surfaces were quite different in acid from those in water and humid air. Dilute acid produced failure at much lower stresses than water, and resulted in a different fracture morphology. The factors producing a different mechanism may be" (i) the greater availability in dilute sulphuric acid of hydrogen ions for initiating flaws in the glass surface; (ii) differences between the rate of diffusion into the resin matrix of ions from the sulphuric acid, whether anions or cations, and ions from water. Previous work suggests that dilute sulphuric acid cannot diffuse into highly cross-linked thermosetting resins ~2a3 probably because of the large size of the sulphate anion. Regester ~2 has shown that sulphate ions do not diffuse through polyester resins, although chloride ions can diffuse to some extent. Our measurements of the diffusion of 36S labelled sulphuric acid into epoxy
Environmental stress corrosion of hybrid fibre composites
resins confirmed that sulphate ion migration was negligible. Therefore for reasons of electrical neutrality there would be little migration of the hydrogen ions from the acid unless microcracks developed in the matrix. Acid would contact the glass fibres and because of the high hydrogen ion concentration, would rapidly induce new and substantial cracks in the fibres. If the induced cracks did not lead to immediate fibre failure, the cracks produced would still be extended and sharpened by hydrolysis. This process is consistent with relatively short stress rupture times. Long times to failure can be expected in the absence of matrix microcracks in the outer plies. The degradation processes could also be slowed down by a protective surface layer such as a gelcoat, but water would still diffuse through all except completely impermeable outer layers, and would eventually induce hydrolysis and glass fibre failure by r a n d o m flaw growth, Matrix cracks would then develop, allowing acid ingress, with further fibre attack and eventual specimen failure.
263
ACKNOWLEDGEMENTS The authors acknowledge the financial assistance of the Science and Engineering Research Council by the provision of a C A S E studentship to M.A.F. The National Physical Laboratory (NPL), Teddington, Middlesex, was the collaborating C A S E organisation. John M. Sillwood and John Aveston of NPL took part in several useful discussions. Westland Helicopters Ltd provided fabrication facilities.
REFERENCES 1. Hayashi, T., On the improvement of mechanical properties of composites by hybrid composition. Proc. 8th
International Reinforced Plastics Conference,
Brighton, UK. Paper 22, 1972, p. 149. 2. Bunsell, A. R., & Harris, B., Hybrid carbon and glass fibre composites. Composites, 5 (1974) 157. 3. French, M. A. & Pritchard, G., Proc. Symposium on Durability of Polymer Based Composite Systems for Structural Applications, Free University of Brussels,
4 CONCLUSIONS (1) Stress rupture times in water and humid air were longer and more variable than in acid. (2) Failure times in acid became shorter as the applied stresses increased. (3) Low strains favoured planar fracture surfaces. (4) Hybrid laminates A1 and A2 showed longer failure times than all-glass laminates, provided that the specimen edges were protected. (5) Edge protection and carbon cladding together did not completely prevent rupture in acid. (6) While the results do not constitute direct evidence of the mechanism of environmental attack on the glass fibres, they are nevertheless consistent with mechanistic theories recently reported in the literature. (7) It is likely that microcracks provide access to the glass surfaces in edge-protected, carbon-clad hybrids.
4. 5. 6. 7.
August 1990, ed. A. H. Cardon & G. Verchery. Elsevier Science Publishers, Barking, Essex, 1991, p. 345. Michalske, T. A. & Freiman, S. W., Nature, 295 (1982) 511. Michalske, T. A. & Bunker, B. C., J. Amer. Ceram. Soc., 70 (1987) 780. Metcalfe, A. G. & Schmitz, G. K., Glass Technology, 13 (1972) 5. Barker, H. A., Baird-Smith, I. G. & Jones, F., Symposium on Reinforced Plastics: Anti-corrosion Applications, National Engineering Laboratory, East
Kilbride, UK, 1979, Paper 12. 8. Kasturiarachchi, K. A. & Pritchard, G., J. Mater. Sci. Letters, 3 (1984) 283. 9. Stansfield, K. E., The effects of stress and thermal spiking on the hygrothermal response of carbon fibre reinforced plastics. PhD thesis, Kingston Polytechnic, UK, 1989. 10. Hogg, P. J. & Hull, D., Metal Science, 14 (1980) 120. 11. Jones, F. R., Rock, J. W. & Bailey, J. E., J. Mater. Sci., 18 (1983) 1059. 12. Regester, R. F., Behaviour of fibre reinforced plastic materials in chemical service. J. Comp. Mater., 10 (1976) 2. 13. Caddock, B. D., Evans, K. E. & Hull, D., Proc. 2nd Conference on Fibre Reinforced Composites, Liverpool University, 1986. Institute of Mechanical Engineers, Mech. Eng. Publications Ltd, Paper C25-86, 55-62.
Environmental stress corrosion of hybrid fibre composites Mark A. French* & Geoffrey Pritchard School of Applied Chemistry, Kingston Polytechnic, Penrhyn Road, Kingston upon Thames, Surrey, UK, KT1 2EE (Received 23 June 1991; revised version received 30 September 1991; accepted 18 October 1991)
Unidirectional glass/epoxy, carbon/epoxy and glass/carbon/epoxy hybrid laminates (14 glass plies and 2 carbon) were immersed in water, humid air and dilute (0.5 M) sulphuric acid and subjected to various levels of constantly applied tensile stress until rupture occurred. When the liquids were allowed direct access to the cut edges of rectangular strip specimens, and hence to the glass plies, the position of the carbon plies made little difference to the failure time at a given stress. Protecting the cut edges and allowing access only to the major faces resulted in failure times which depended more strongly on the location of the carbon plies. Carbon plies used as external cladding greatly increased the failure time, and the variability in failure time, in acid at low strain levels, but did not completely inhibit failure. This supports the view that microcracks develop in the outer carbon plies. Failure modes are discussed.
Keywords: environmental, strain, corrosion, hybrid, epoxy, laminate, glass, carbon, diffusion, fracture
1 INTRODUCTION
It has been shown that under certain conditions the addition of a second reinforcing fibre to form a hybrid composite can enable the failure strain of the first (more brittle) plies to be increased above their normal value. Incorporation of a second phase into a glass composite subjected to water or acid attack might be expected to provide a means of restricting crack growth. Carbon fibre reinforced epoxy resins are themselves prone to brittle cracks, but not to the same extent as that demonstrated by glass/epoxy laminates under the special conditions of acid-induced stress corrosion. Carbon fibres are resistant to water and dilute acid attack and, provided that the energetic conditions are satisfied, there is a possibility that hybridisation might reduce the crack growth rate at a given strain. This investigation was concerned with the extent of the improvement in stress corrosion failure times obtainable when carbon plies are added to a unidirectional glass composite, and with the importance of the position of the carbon plies used. An account has already been given elsewhere of the behaviour of the same laminates
Unidirectional glass reinforced epoxy laminates normally fail in tension to give 'brush-like' failure surfaces, with some of the energy being absorbed by the process of pulling fibres out of one or both fractured pieces. However, the tensile strength of glass fibre reinforced composites is seriously reduced by hot, wet conditions or by acid. Unidirectional glass laminates held under a tensile stress in wet or acidic environments can fail catastrophically at stresses far below the short term tensile strength, 1,2 with failure times increasing as the applied stress is decreased. At low strains especially, the failure process is dominated by the growth of brittle cracks in a direction transverse to the applied load. These cracks are uninterrupted by the interfaces between fibre and matrix, and they therefore lead to a characteristically planar fracture surface. * Present address: British Aerospace, Stevenage, UK.
Composites Science and Technology 0266-3538/92/$05.00 © 1992 Elsevier Science Publishers Ltd. 257
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Mark A. French, Geoffrey Pritchard
Table 1. Reference codes for lay-ups Code
Stacking sequence
A1 C G-G-G-G-G-G-G-G-G-G-G-G-G-G C A2 G-G-G C G-G-G-G-G-G-G-G C G-G-G A3 G-G-G-G-G-G C G-G C G-G-G-G-G-G C = carbon ply; -G- -- glass ply.
immersed without edge protection in the same environments with no applied stress. 3
2 EXPERIMENTAL 2.1 Specimen fabrication Unidirectional all-glass, all-carbon and hybrid glass/carbon/epoxy laminates were manufactured at Westland Helicopters Ltd, Yeovil, Somerset, by stacking sheets of two kinds of prepreg (Fibredux ® 913C/XAS, containing 60% by volume of Grafil type II high tensile strength carbon fibres, and Fibredux 913G-E, containing 55% by volume of E glass fibres, both in an epoxy resin matrix, from Ciba Geigy) to form 16 ply (2 mm) sheets. The sheets were then cured for two hours at 90°C and one hour at 120°C. To make the hybrids, two carbon plies were interspersed with 14 glass ones and located symmetrically at various positions in the laminates (Table 1). The sheets were checked for void content by ultrasonic C-scanning and machined into parallelsided strip specimens approximately 2 0 0 m m l o n g × 1 5 m m wide, by means of a diamond wheel. The cut edges were sanded with carborundum paper and the strips were dried over silica gel at 50°C in a vacuum desiccator for 14 days. Aluminium alloy end-tabs, 2 m m thick, were bonded to the specimen ends with Redux 403, an aluminium filled epoxy adhesive (from Ciba Geigy). Static tensile characterization of each type of laminate was obtained by using a Zwick 1484 200 kN machine at a crosshead speed of 5 mm/min.
placed in wet atmospheres. All the jigs were fitted with load cells. Those jigs designed for placing in humidity chambers at 50°C/95% relative humidity were of stainless steel construction, and their load cells were protected with water-resistant putty (Fig. 1). The others were essentially small loading frames of mild steel, with glass jackets for surrounding the specimen with water or acid while it was under load. The loads were applied by tightening the nut on a threaded section attached to one of the grips. The creep rate of the unidirectional laminates was negligible. In one set of experiments box-shaped acrylic cells 6 0 m m by 13 m m by 11 mm, containing 0.5 M solutions of sulphuric acid, were attached to the specimen faces so that the liquid did not directly contact the cut edges. (This practice was adopted after simpler methods such as edge sealing with various sealants had proved unsuccessful.) Physical changes were observed and the specimens were photographed as failure approached. The times at which failure occurred could be correlated with reductions in the load sustained by using chart recorders connected to the load cells.
2.2 Immersion procedure Special jigs were used to apply a tensile stress while the samples were immersed in liquids or
Fig. 1. Stainless steel jig, with 16 ply A2 laminate after failure.
Environmental stress corrosion of hybrid fibre composites
259
3 RESULTS A N D DISCUSSION 3.1 Stress corrosion in water and humid air 1.5
The failure time was arbitrarily defined as the time to reach a 10% reduction in load, since once the failure process had initiated, the load fell progressively over a period of time. The process involved such events as longitudinal splitting at one or both edges, delamination, and some transverse cracking, particularly at lower loads. In all-glass specimens, transverse cracks were joined by longitudinal ones, so that the fracture surfaces appeared stepped, resembling those of unstressed, immersed specimens after tensile testing 3 but with far less longitudinal splitting. The stress rupture times were very variable in water and at high humidities, as shown in Fig. 2. Data points with arrows indicate that no failure occurred by the end of the experiment. The scatter in water and humid air was much greater than could be expected from variations in the strength or quality of the laminates, as reflected in the static tensile tests. It will be noted later that the scatter was also very much larger than it was in dilute sulphuric acid, and therefore the crucial factor must be environment-related. The cause must involve one of three parameters: (i) the number, distribution, and nature (size or shape) of existing flaws in the glass fibres; (ii) the rate of formation of new flaws; or (iii) the rate of 1011 90 nm.io,,
80 70
•
6O
• •
na~ O~
•
h~
!
li
AAinA
05
°
a
AA •
•
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;
i 4
i 5
LOG TIME (MINUTES)
Fig. 3. Loglo (failure time) as a function of applied tensile strmn for (A) 16 ply glass-epoxy laminates compared with
( i ) equivalent laminates having one carbon ply on each major surface, but with no edge protection in either case. Environment: 0.5 M sulphuric acid at 30°C.
flaw and crack growth. The number of pre-existing flaws is unlikely to be responsible since they must be statistically distributed throughout all the fibre surfaces. Flaw propagation is thought to involve the bond rupture of strained silicon-oxygen bonds by water molecules at the tips of flaws in the glass fibres, 4,5 and unless many flaws are too small for water to enter, this too is unlikely to be the dominant factor. In contrast, the formation of new flaws involves the exchange of acidic hydrogen ions, present in water or dilute acid (but much more numerous in dilute acid), with larger cations in the glass fibre surfaces, causing surface shrinkage. The surface shrinkage in turn creates tensile stresses which eventually lead to new surface flaws. 6.7 3.2 Stress corrosion in dilute sulphuric acid
11-4,
~ 5o
¢
mln 30 20 I0 0 0.5
l
1.5 2 L O G T I M E (DAYS)
2.5
3
3.5
Fig. 2. Log10 (failure time) as a function of the applied tensile stress, expressed as a percentage of the mean tensile strength. 16 ply A1 laminates (symbol: Q) with one carbon ply on each outer surface, the remainder being glass, are compared with equivalent laminates (A2) having 14 glass plies and two carbon plies in positions 4 and 13 (symbol: II). Environment: 96% relative humidity at 50°C. The specimen edges were not protected. Arrows indicate no failure.
Figure 3 shows the effect of using one carbon ply on each outer face of a glass laminate, without protecting the edges. Rupture times of all-glass laminates, as a function of strain, are compared with those of hybrid lay-up type A1 in 0.5 M sulphuric acid at 30°C. The log~o rupture times were linearly related to the applied strain, and the variability was very much less than that previously noted in water. Since there was no edge protection, the acid could attack the glass fibres regardless of the carbon cladding. There was very little difference in stress rupture times for any of the samples containing glass fibres
6
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Mark A. French, Geoffrey Pritchard
when the edges were exposed. (Therefore, for clarity, A2 and A3 data are not shown in Fig. 3.) The only difference was observed in the all-carbon samples, which remained intact even after a month at the highest stresses applied, i.e. 95% of their ultimate tensile strength. Figure 4 compares the failure times of A1, A2 and A3 series laminates in 0.5 M sulphuric acid at 20°C. In this case the edges were protected against acid attack and the acid applied by means of small cells attached to the larger specimen surfaces, as mentioned earlier. Failure times of A1 laminates were generally now very much longer, as expected, because the glass was not directly in contact with the acid. But it was also noted that the failure times were also very much more variable than those of the A3 laminates, at a given strain. The variability between A2 specimens was intermediate between those of A1 and A3. So the time required for acid-induced rupture in A3 laminates, with several outer glass plies, was relatively reproducible, and depended only on the applied strain. In contrast, in the case of carbon-clad A1 laminates, the time required for penetration of the liquid through the outer carbon layer, and consequent rupture, varied a great deal from one specimen to another. Such variability can be explained most easily by assuming the formation of very small microcracks in the carbon plies. The time to failure of A1 laminates was nearly always longer than that of laminates without any carbon cladding, so it is
1.5
•
,,OA
•
0.5
0
I
J
1
2
I 3 LOG TIME (MINUTES)
i
i
4
5
Fig. 4. Log~o (failure time) as a function of applied tensile strain for 16 ply unidirectional glass/epoxy laminates containing 2 carbon plies as substitutes for 2 glass ones. ( , ) : A1 lay-up (carbon on the outside surfaces); (O): A2 lay-up (carbon in positions 4 and 13); (A) A3 lay-up: (carbon in positions 7 and 10). Environment: 0-5 M sulphuric acid at 20°C. All edges protected.
• O0 AA" • 0.5
O"
~)
i 1
i 2
3 LOG TIME (MINUTES)
I
t i
i
4
5
Fig. 5. Loglo (failure time) as a function of applied tensile strain for 16 ply unidirectional glass/epoxy laminates ( i ) with and (/k) without edge protection, compared with equivalent carbon-clad laminates (O). Environment: 0.5 M sulphuric acid, at 20°C.
unlikely that these cracks were present at the outset. It is also improbable that they formed by purely mechanical means, as the imposed strains were mostly far below the tensile failure strains of the dry laminates, which were, for example, about 1.6% in the case of A1 lay-ups. One possibility is that the microcracks could be caused by sorption, as a result of swelling strains in the resin rich zones between carbon fibre bundles, but the reason for such variable crack formation times is unclear. Microcracks have previously been detected and demonstrated in similar unidirectional epoxy laminates as a result of hygrothermat conditioning, both with all-glass s and all-carbon 9 reinforced materials. Interestingly, protection of the specimen edges achieved a small increase in failure time, even in the case of all-glass laminates (Fig. 5). We assume that acid access through the edges was more rapid than through the faces because the cut edges had no surface veneer of epoxy resin. Data for A1 carbon-clad laminates are reproduced here again alongside those for the two sets of glass experiments. They show that the benefits of carbon cladding vary from very little, up to about a one hundredfold increase in lifetime. No such benefit was seen with the specimens exposed to acid at their cut edges. 3.3 Fracture surfaces
The fracture surface morphology of all-glass specimens was dependent on the applied strain, as reported by Hogg & Hull. 1° High strains
Environmental stress corrosion of hybrid fibre composites (1"0-1.3%) resulted in sufficiently large stress concentrations at the tips of transverse cracks to induce fibre debonding, delamination and longitudinal cracking. Intermediate strains of 0.61.0% produced numerous crack nuclei, which coalesced by transverse and longitudinal cracking to result in irregular, 'stepped' fracture surfaces. At strains of around 0.3%, transverse crack growth dominated, leading to a planar fracture surface, with no evidence of fibre pull-out. The failure of glass plies during stress rupture experiments on hybrids at relatively low stresses did not necessarily lead to failure of the carbon plies, and in those cases where the carbon did not fail, the environmentally induced cracks travelled through the glass plies producing planar fracture surfaces. Glass plies on the outside of laminates sometimes failed separately, leaving the innermost glass plies, enclosed by carbon, apparently intact. In other cases, peeling the outer carbon plies off after failure revealed numerous planar fractures in the glass core (see Fig. 6). This suggests that the interface between the glass and carbon plies survived the first complete fracture of the glass plies, thus facilitating load transfer via the carbon from one part of the glass core to another. Three types of failure process were observed, regardless of the presence of edge protection:
Type /:Single transverse crack in the acidimmersed region. Type 2: Single transverse crack initiated at the acid-air interface. Type 3: Combination of transverse and longitudinal cracks. The Type 1 fracture surfaces of A3 laminates were often planar, in line with the model of
Fig. 6. The carbon-clad (A1) specimentshown here had not apparently failed, but when the outer carbon layer was removed, the glass core showed multiple transverse fractures.
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Fig. 7. Type 1 failure of an A3 laminate in acid at 0-5% strain, by means of a single planar fracture, with small steps induced by limited delamination.
confined acid attack of Hogg & Hull.~° The acid was confined to the crack region, with a limited amount of delamination which resulted in small steps. Figures 7 and 8 show A3 specimens which were both subjected to 0-5% strain; they failed by Type 1 and Type 2 methods, respectively. Type 2 failures began with numerous small transverse cracks on both sides of the specimens. Complete failure of the glass core in A1 lay-ups, and of the glass plies in A2 and A3 lay-ups, occurred at the acid-air interface. Specimens of A3 type showed delamination of the outer glass plies from the still intact carbon plies on both sides of the specimens. In Fig. 8, the level of acid has been raised slightly after the failure, to allow the crack to be seen more clearly. Jones et al.ll has explained Type 2 failures in terms of stress concentrations, arising from the formation of products such as calcium sulphate as a result of acid leaching of the glass. The salts then crystallize inside the composite; planar fracture results. This explanation suggests that such failures would not occur if acids with soluble salts were to be used. Figure 9 shows an A1 lay-up with Type 2 failure. The existence of salt crystals on fibres is not conclusive proof that they
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Mark A. French, Geoffrey Pritchard
Fig. 8. Type 2 failure of an A3 laminate in acid at 0.5% strain. The transverse crack occurred at the acid-air interface, but the level of liquid was subsequently raised slightly, to enable the fracture region to be seen.
acted as stress raisers and precipitated failure. There is no evidence as to whether the crystals formed before failure or afterwards. A later paper will show some of the damage features developing after the failure of the glass fibres. Type 3 failures consistent with the Hogg and Hull mechanism occurred typically as follows. An A2 lay-up specimen held at 0-9% strain showed two widely separated nucleation sites for horizontal cracks (i.e. at 90 ° to the fibres) after 35 minutes. Twenty minutes later, one crack had grown from 3 to 7 m m long and the other (near the acid-air interface) had grown by the coalescence of several small cracks, with shear cracking along the fibres at the crack tips, producing a slightly stepped appearance. Other cracks were also beginning. Three minutes later, limited delamination had taken place around the stepped crack, and the inner carbon ply became visible as a dark region. Almost immediately afterwards, at 63 minutes, the first two growing cracks had joined by means of a longitudinal crack extending several centimetres. The carbon core remained intact holding the specimen together after failure of the outer glass plies. Colour changes which had previously been observed at this stage in failed, unstressed samples were not noticeable until about an hour after failure had occurred. 3.4 The stress corrosion mechanism
Fig. 9. Type 2 failure of an A1 laminate in acid. The carbon cladding has been removed to show the planar fracture in the glass core. The fracture occurred at the air-acid interface.
The stress rupture times and the appearances of the fracture surfaces were quite different in acid from those in water and humid air. Dilute acid produced failure at much lower stresses than water, and resulted in a different fracture morphology. The factors producing a different mechanism may be" (i) the greater availability in dilute sulphuric acid of hydrogen ions for initiating flaws in the glass surface; (ii) differences between the rate of diffusion into the resin matrix of ions from the sulphuric acid, whether anions or cations, and ions from water. Previous work suggests that dilute sulphuric acid cannot diffuse into highly cross-linked thermosetting resins ~2a3 probably because of the large size of the sulphate anion. Regester ~2 has shown that sulphate ions do not diffuse through polyester resins, although chloride ions can diffuse to some extent. Our measurements of the diffusion of 36S labelled sulphuric acid into epoxy
Environmental stress corrosion of hybrid fibre composites
resins confirmed that sulphate ion migration was negligible. Therefore for reasons of electrical neutrality there would be little migration of the hydrogen ions from the acid unless microcracks developed in the matrix. Acid would contact the glass fibres and because of the high hydrogen ion concentration, would rapidly induce new and substantial cracks in the fibres. If the induced cracks did not lead to immediate fibre failure, the cracks produced would still be extended and sharpened by hydrolysis. This process is consistent with relatively short stress rupture times. Long times to failure can be expected in the absence of matrix microcracks in the outer plies. The degradation processes could also be slowed down by a protective surface layer such as a gelcoat, but water would still diffuse through all except completely impermeable outer layers, and would eventually induce hydrolysis and glass fibre failure by r a n d o m flaw growth, Matrix cracks would then develop, allowing acid ingress, with further fibre attack and eventual specimen failure.
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ACKNOWLEDGEMENTS The authors acknowledge the financial assistance of the Science and Engineering Research Council by the provision of a C A S E studentship to M.A.F. The National Physical Laboratory (NPL), Teddington, Middlesex, was the collaborating C A S E organisation. John M. Sillwood and John Aveston of NPL took part in several useful discussions. Westland Helicopters Ltd provided fabrication facilities.
REFERENCES 1. Hayashi, T., On the improvement of mechanical properties of composites by hybrid composition. Proc. 8th
International Reinforced Plastics Conference,
Brighton, UK. Paper 22, 1972, p. 149. 2. Bunsell, A. R., & Harris, B., Hybrid carbon and glass fibre composites. Composites, 5 (1974) 157. 3. French, M. A. & Pritchard, G., Proc. Symposium on Durability of Polymer Based Composite Systems for Structural Applications, Free University of Brussels,
4 CONCLUSIONS (1) Stress rupture times in water and humid air were longer and more variable than in acid. (2) Failure times in acid became shorter as the applied stresses increased. (3) Low strains favoured planar fracture surfaces. (4) Hybrid laminates A1 and A2 showed longer failure times than all-glass laminates, provided that the specimen edges were protected. (5) Edge protection and carbon cladding together did not completely prevent rupture in acid. (6) While the results do not constitute direct evidence of the mechanism of environmental attack on the glass fibres, they are nevertheless consistent with mechanistic theories recently reported in the literature. (7) It is likely that microcracks provide access to the glass surfaces in edge-protected, carbon-clad hybrids.
4. 5. 6. 7.
August 1990, ed. A. H. Cardon & G. Verchery. Elsevier Science Publishers, Barking, Essex, 1991, p. 345. Michalske, T. A. & Freiman, S. W., Nature, 295 (1982) 511. Michalske, T. A. & Bunker, B. C., J. Amer. Ceram. Soc., 70 (1987) 780. Metcalfe, A. G. & Schmitz, G. K., Glass Technology, 13 (1972) 5. Barker, H. A., Baird-Smith, I. G. & Jones, F., Symposium on Reinforced Plastics: Anti-corrosion Applications, National Engineering Laboratory, East
Kilbride, UK, 1979, Paper 12. 8. Kasturiarachchi, K. A. & Pritchard, G., J. Mater. Sci. Letters, 3 (1984) 283. 9. Stansfield, K. E., The effects of stress and thermal spiking on the hygrothermal response of carbon fibre reinforced plastics. PhD thesis, Kingston Polytechnic, UK, 1989. 10. Hogg, P. J. & Hull, D., Metal Science, 14 (1980) 120. 11. Jones, F. R., Rock, J. W. & Bailey, J. E., J. Mater. Sci., 18 (1983) 1059. 12. Regester, R. F., Behaviour of fibre reinforced plastic materials in chemical service. J. Comp. Mater., 10 (1976) 2. 13. Caddock, B. D., Evans, K. E. & Hull, D., Proc. 2nd Conference on Fibre Reinforced Composites, Liverpool University, 1986. Institute of Mechanical Engineers, Mech. Eng. Publications Ltd, Paper C25-86, 55-62.