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Environmental fatigue of reinforced plastics
Environmental fatigue of reinforced plastics CJ. JONES, R.F. DICKSON, 7". ADAM, H. REITER and B. HARRIS This paper presents the results of a series of experiments on the fatigue behaviour of cross-plied (0/90) epoxy-based laminates reinforced with glass, carbon and Kevlar-49 fibres and, in particular, on the effects of environmental conditioning on this behaviour. Conditioning treatments used include drying, storage in an atmosphere of 65% relative humidity and boiling in water, prior to fatigue tests in repeated tension and in bending. The fatigue resistance of CFRPis unaffected by conditioning treatment and mode of stressing; for GRPthere is no significant difference between the behaviour of the dry material and that conditioned at 65% relative humidity. On the other hand, boiling in water always weakens GRPand KFRP, although the effect is small except for the case of GRP in the 0/90 orientation tested in tension. The large reduction in this case results directly from loss of fibre strength during pre-conditioning. Complete drying of KFRPlaminates is more damaging even than boiling.
Key words: composite materials; environmental testing; fatigue testing; glass fibres; carbon fibres; Kevlar fibres. Reinforced plastics laminates in aeronautical and other structures will often, in the course of normal service life, be exposed to an environment in which the temperature and humidity vary considerably with time. This may occur both on a short time scale, as a result of the diurnal cycle, or over longer, seasonal periods. For outdoors applications, the effects of temperature and humidity may be compounded by deterioration due to ultraviolet irradiation. Moisture from the atmosphere is absorbed by most resins used in reinforced plastics. The normal bulk diffusion processes that would operate in a block of unreinforced resin are naturally modified by the presence of reinforcing fibres or filler particles, and in anlsotropic laminates, as in wood, diffusion is also anisotropic, ingress of moisture being much more rapid when continuous interfacial diffusion paths are provided. In a laminated structure, provided cut edges are either avoided or resealed against moisture penetration, moisture uptake at ambient temperatures will be slow, especially if the laminate surfaces are also coated with a low permeability resin or paint. Under such circumstances, therefore, it may take many months for a composite structure to reach equilibrium with its surroundings, and its measured moisture content will reflect only the environmental average, rather than the short-term extremes of the diurnal c~cle. It must be borne in mind, however, that the effects of combined cycling of temperature and humidity, which in
a true diurnal cycle may vary between extreme values, may well be more severe than cycling either condition separately. Furthermore, the superposition of stress,
especially fluctuating stress, may also change the moisture uptake characteristics by inducing damage that can provide much more rapid ingress of moisture than that controlled by diffusion processes. The purpose of studies of environmental effects is therefore two-fold, being both to discover the extent of deterioration of materials properties as a result of environmental exposure, and to determine the degree to which this deterioration is aggravated by the presence of the environment under normal operating conditions. Environmental damage may also be seen to be of two kinds, that which has only a temporary effect, loss of properties being restored on drying out, and that which is permanent, irreversible damage to the reinforcing fibres or the interface having been sustained as a result of the exposure. The underlying mechanisms of such behaviour should, of course, be understood by the user of the material. In this paper, the results of some experiments to study the effects of moisture on the fatigue and failure of laminates of glass, carbon and Kevlar-49 reinforced epoxy resin are presented.
EXPER/MEN TA L The materials used in this work are 0/90 laminates consisting, nominally, of 0.60 fibre volume fraction (l/f) of E-glass fibre, HTS carbon fibre, and Kevlar-49 fibre in Fotherglll and Harvey Code 69 resin. The composites were laminated at the Royal Aircraft Establishment, Famborough, UK, from 11-ply stacks of pro-impregnated fibres supplied by Rotorway Composites, Clevedon, Somerset. Autoclaving
0010-4361/83/030288-06$03.00 © 1983 Butterworth & Co (Publishers) Ltd. 288
COMPOSITES. VOL 14. NO 3. JULY 1983
procedures were as recommended by Fothergill and Harvey 1 except that pressure was maintained during cooling to prevent the development of thermal contraction cracking. The Vf of the final composites were 0.58, 0.54 and 0.52, respectively, for the GRP, CFRP and KFRP, but the composites were otherwise identical. Test samples were cut from the laminated plates, care being taken to avoid overheating the Kevlar composites by cooling the vice clamps with liquid nitrogen. Standard samples for mechanical testing were 10 mm wide, the length varying according to test requirements, and parallel-sided. Aluminium end-tabs were glued on for tensile and fatigue testing. The effect of environment on tensile and flexural strength, and tensile and flexural fatigue was studied. Samples were conditioned prior to testing by exposure to three standard treatments: 1) drying at 60°C for 4 weeks ('fully dried' state); 2) equilibrating at 65% relative humidity (RH) for at least three months at room temperature; and 3) boiling in water for 3 weeks, followed by storage in water at room temperature until testing. All axial fatigue tests were carried out under load control in repeated tension at a constant rate of stress application (RSA) rather than at constant frequency. This has been shown to be necessary2 to obtain a proper correlation between strength and the true fatigue behaviour of a material with rate-dependent mechanical properties. Tests on the GRP and KFRP laminates were carried out at a RSA of 100 kN/s, sufficiently low to avoid undue hysteretic heating, although the less sensitive CFRP laminates were tested at 200 kN/s to increase throughput. For testing the laminates in the -+ 45 ° orientation, a much lower RSA of 25 kN/s was used. The pre-conditioned specimens were wrapped in polymer film prior to fatigue testing to prevent any change in moisture content during the test. The machine used for the flexural fatigue work determined fatigue behaviour under constant deflection, pure bending conditions, by monitoring changes during cycling of'the flexural stiffness of the sample. It had a limited deflection range and was capable of generating high stresses only in very stiff samples. Stresses near to the monotonic flexural strength could not therefore be achieved in the GRP and
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KFRP laminates. However, since tensile failure rarely occurred during flexural cycling, this limitation was relatively unimportant, and the criterion of failure adopted relates to loss of stiffness rather than 'separation'.3 Acoustic emission (AE) monitoring techniques were used to study the change in character of the materials as a result of environmental exposure and fatigue. The system was an AETC 203 series amplitude distribution analyser backed by a MINC-11 minicomputer. RESUL TS AND DISCUSSION Moisture absorption
The uptake of water in laminate samples during different treatments is illustrated in Figs l(a), (b) and (c). It can be seen that in most cases saturation is not achieved at room temperature, even after 100 days, although saturation is reached in 10 to 20 days in boiling water. It should also be borne in mind that the composites as-received from the laminator already contain considerable amounts of moisture, as the drying results in Fig. 1(d) show. Although there are only relatively minor differences in apparent uptake in CFRP and GRP in boiling water and a 65% RH atmosphere, the as-received CFRP and GRP laminates contain substantially different amounts of water. In all cases, the organic fibre KFRP composites absorb much more water than the inorganic fibre composites, as would be expected. The general effect of absorbed moisture on the mechanical properties of these laminates is shown in Fig. 2. These results were obtained in 4-point bending, but interpretation is complicated by differences in failure modes among the three types of composite and, in the GRP, between samples of different moisture content. The CFRP and KFRP samples all failed in interlaminar shear, and the results, plotted in
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COMPOSITES . JULY 1983
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terms of shear strength, show only slight reductions in strength, even in the highly hydrophilic KFRP, with increasing moisture content. By contrast, the GFP failed in tension above about 1% moisture content, but in a mixed compression/shear mode below this, and the strength reduction with moisture content is much greater as a result of damage to the fibres in boiling water. Similarly, the tensile strengths of CFRP and KFRP are little affected by long-term exposure to boiling water, whereas the tensile strength and failure strain of the GRP laminates are irreversibly reduced by more than 50% after exposure to boiling water. These results indicate that, of the three reinforcing fibres used in this work, only the glass is significantly damaged by water. The effect of changes in moisture content (or exposure) on the behaviour of GRP is clearly reflected in the acoustic emission patterns observed during tensile testing. Transverse ply cracking in 0/90 non-woven laminates manifests itself as a peak in plots of AE rate vs stress at very low stress levels. 4 The onset of this cracking depends upon ply thickness and lay-up arrangement, and on the failure strain of resin, s and is therefore also affected by the plasticizing action of water. Fig. 3(a-e) shows characteristic hE rate vs stress curves for GRP laminates as-received, after the three standard conditioning treatments, and after completely redrying following boiling water exposure. The top line in each graph is the event count rate for all events monitored, and the numbered lines below indicate the numbers of counts/s in successively higher 12 dB wide bands, band 1 representing counts 0 to 12 dB above threshold, etc (threshold = 75 uV). In fully dried GRP the transverse cracking peak is at a stress level of abut 20% of (failure stress) and contains substantial numbers of high energy events. There is also a smaller peak at about 65% of caused by longitudinal splitting, largely contributed by lower amplitude events but also containing significant numbers of higher energy events in the range 36-60 dB above threshold. As the moisture content increases, the onset of transverse ply cracking is raised to significantly higher stress levels and the height of the peak, as a proportion of the overall count rate at failure, falls from about 94% to zero in samples exposed to boiling water. At this point, the longitudinal cracking is almost totally suppressed but becomes visible, no longer being registered by AE, at only 73% Or. The transverse ply cracking,
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which occurs on first tensile loading and does not register as AE on reloading, occurs in the first few cycles of a fatigue test and is therefore present throughout the life of any sample fatigued above 20% el. A sample cycled to a maximum stress level below the monotonic transverse ply cracking stress wiU also, after relatively little cycling, be found to have sustained complete transverse ply cracking, as evidenced by the AE behaviour. Axial fatigue behaviour
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Fig. 4 shows the variation of laminate tensile strengths with RSA. In the 0/90 orientation, only the tensile strength of the GRP laminate was significantly rate-dependent.
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C O M P O S I T E S • J U L Y 1983
Fig. 5 shows the SIN curves obtained for the three laminates in the 0/90 orientation and the various environmental conditions. The CFRPlaminates are largely insensitive to the effect of moisture. The GRP fatigue response is also unaffected by moisture content unless the fibres are damaged by an extreme treatment like boiling. In KFRP, boiling lowers the SIN curve slightly, but beyond 10 s cycles there is a cross-over. The most unusual feature ofKFRP, from first results, is that the completely dried out composite
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subsequent fatiguing. As Fig. 7(a) shows, the uptake of moisture during boiling is only marginally greater in the damaged composite, despite the transverse ply cracking. The SIN curves of pre-loaded samples conditioned at 65% RH and in boiling water coincide with results for undamaged samples (Fig. 7(b)).
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In the -+ 45 ° orientation, the resin naturally exerts a more dominant effect on mechanical properties, but there is nevertheless an effect due to the fibres since the KFRP SIN curve falls significantly below those, almost coincident, for the GRP and CFgP (Figs 8 and 9). As in the case of the 0/90 orientation, the CFRP laminates are unaffected by environmental treatments though the performance of the GRP and KFRP is weakened by treatment in water at IO0°C, albeit to a smaller extent than in the 0/90 orientation. For the GRP there is no difference between the results for dried material and that conditioned at 65% RH.
Flexural fatigue In all cases, the flexural stiffness falls linearly, and more or less rapidly depending on material and stress level, until some local failure, such as interply delamination or splitting
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appears to be less fatigue resistant than that containing moisture. A disturbing feature of the fatigue of KFRP is the sharp downward curvature of the SIN curves, by comparison w i t h those of the GRP and CFRP (Fig. 6). To examine the effect of transverse ply cracking on conditioning and fatigue, a number of GRP samples were preloaded to 0.4 GPa prior to environmental treatments and
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COMPOSITES. JULY 1983
causes a sudden change in flexural stiffness (Fig. 10(a)). This 'knee' in the curve or some similar criterion, such as the number of cycles for a given reduction (15%, in this case) of stiffness serves as a criterion of 'failure'.
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A french version of this paper was presented at the Troisidmes Journ6es Nationales sur les Composites, Pads, France, 21-23 September 1982.
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This work has been carried out with fmancial support from the Procurement Executive of the Ministry of Defence. The authors are grateful to Dr G. Dorey and Dr P. Curtis of the Royal Aircraft Establishment for their guidance and encouragement. The support of the Polymer Engineering Directorate of the SERC is acknowledged for providing acoustic emission facilities.
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REFERENCES
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Fig. 10 Schematic illustration o f f l e x u r a l fatigue curves and definition o f a failure c r i t e r i o n , (b)-(d) Flexural fatigue behaviour of conditioned laminates in 0 / 9 0 ° o r i e n t a t i o n : (b) CFRP; (c) GRP; and (d) KFRP
COMPOSITES. JULY1983
'Catboform Technical Data Sheet' (Fothergill & Harvey Limited) Sims, G.D. and Gladman, D.G. 'Plastics & Rubber: Materials and Applications' (May 1978) pp 41-48 Beaumont, P.W.R. and Harris, B. in 'Carbon Fibres, their Composites and Applications' (Plastics Institute, London, UK, 1972) pp 283-291 Harris,B., Guild, F.J. and Brown, C.R. Y Phys D: Appl Phys 12 (1979) pp 1385-1407 Garrett, K.W. and Bailey, J.E. JMater Sci 12 (1977) pp 157-168 & 2189-2194
AUTHORS 7
Mr T. Adam is with the School of Engineering at the University of Bath. The other authors are with the School of Materials Science, University of Bath, Claverton Down, Bath BA2 7AY, UK. Inquiries should be addressed to Professor Harris in the first instance.
293
Key words: composite materials; environmental testing; fatigue testing; glass fibres; carbon fibres; Kevlar fibres. Reinforced plastics laminates in aeronautical and other structures will often, in the course of normal service life, be exposed to an environment in which the temperature and humidity vary considerably with time. This may occur both on a short time scale, as a result of the diurnal cycle, or over longer, seasonal periods. For outdoors applications, the effects of temperature and humidity may be compounded by deterioration due to ultraviolet irradiation. Moisture from the atmosphere is absorbed by most resins used in reinforced plastics. The normal bulk diffusion processes that would operate in a block of unreinforced resin are naturally modified by the presence of reinforcing fibres or filler particles, and in anlsotropic laminates, as in wood, diffusion is also anisotropic, ingress of moisture being much more rapid when continuous interfacial diffusion paths are provided. In a laminated structure, provided cut edges are either avoided or resealed against moisture penetration, moisture uptake at ambient temperatures will be slow, especially if the laminate surfaces are also coated with a low permeability resin or paint. Under such circumstances, therefore, it may take many months for a composite structure to reach equilibrium with its surroundings, and its measured moisture content will reflect only the environmental average, rather than the short-term extremes of the diurnal c~cle. It must be borne in mind, however, that the effects of combined cycling of temperature and humidity, which in
a true diurnal cycle may vary between extreme values, may well be more severe than cycling either condition separately. Furthermore, the superposition of stress,
especially fluctuating stress, may also change the moisture uptake characteristics by inducing damage that can provide much more rapid ingress of moisture than that controlled by diffusion processes. The purpose of studies of environmental effects is therefore two-fold, being both to discover the extent of deterioration of materials properties as a result of environmental exposure, and to determine the degree to which this deterioration is aggravated by the presence of the environment under normal operating conditions. Environmental damage may also be seen to be of two kinds, that which has only a temporary effect, loss of properties being restored on drying out, and that which is permanent, irreversible damage to the reinforcing fibres or the interface having been sustained as a result of the exposure. The underlying mechanisms of such behaviour should, of course, be understood by the user of the material. In this paper, the results of some experiments to study the effects of moisture on the fatigue and failure of laminates of glass, carbon and Kevlar-49 reinforced epoxy resin are presented.
EXPER/MEN TA L The materials used in this work are 0/90 laminates consisting, nominally, of 0.60 fibre volume fraction (l/f) of E-glass fibre, HTS carbon fibre, and Kevlar-49 fibre in Fotherglll and Harvey Code 69 resin. The composites were laminated at the Royal Aircraft Establishment, Famborough, UK, from 11-ply stacks of pro-impregnated fibres supplied by Rotorway Composites, Clevedon, Somerset. Autoclaving
0010-4361/83/030288-06$03.00 © 1983 Butterworth & Co (Publishers) Ltd. 288
COMPOSITES. VOL 14. NO 3. JULY 1983
procedures were as recommended by Fothergill and Harvey 1 except that pressure was maintained during cooling to prevent the development of thermal contraction cracking. The Vf of the final composites were 0.58, 0.54 and 0.52, respectively, for the GRP, CFRP and KFRP, but the composites were otherwise identical. Test samples were cut from the laminated plates, care being taken to avoid overheating the Kevlar composites by cooling the vice clamps with liquid nitrogen. Standard samples for mechanical testing were 10 mm wide, the length varying according to test requirements, and parallel-sided. Aluminium end-tabs were glued on for tensile and fatigue testing. The effect of environment on tensile and flexural strength, and tensile and flexural fatigue was studied. Samples were conditioned prior to testing by exposure to three standard treatments: 1) drying at 60°C for 4 weeks ('fully dried' state); 2) equilibrating at 65% relative humidity (RH) for at least three months at room temperature; and 3) boiling in water for 3 weeks, followed by storage in water at room temperature until testing. All axial fatigue tests were carried out under load control in repeated tension at a constant rate of stress application (RSA) rather than at constant frequency. This has been shown to be necessary2 to obtain a proper correlation between strength and the true fatigue behaviour of a material with rate-dependent mechanical properties. Tests on the GRP and KFRP laminates were carried out at a RSA of 100 kN/s, sufficiently low to avoid undue hysteretic heating, although the less sensitive CFRP laminates were tested at 200 kN/s to increase throughput. For testing the laminates in the -+ 45 ° orientation, a much lower RSA of 25 kN/s was used. The pre-conditioned specimens were wrapped in polymer film prior to fatigue testing to prevent any change in moisture content during the test. The machine used for the flexural fatigue work determined fatigue behaviour under constant deflection, pure bending conditions, by monitoring changes during cycling of'the flexural stiffness of the sample. It had a limited deflection range and was capable of generating high stresses only in very stiff samples. Stresses near to the monotonic flexural strength could not therefore be achieved in the GRP and
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KFRP laminates. However, since tensile failure rarely occurred during flexural cycling, this limitation was relatively unimportant, and the criterion of failure adopted relates to loss of stiffness rather than 'separation'.3 Acoustic emission (AE) monitoring techniques were used to study the change in character of the materials as a result of environmental exposure and fatigue. The system was an AETC 203 series amplitude distribution analyser backed by a MINC-11 minicomputer. RESUL TS AND DISCUSSION Moisture absorption
The uptake of water in laminate samples during different treatments is illustrated in Figs l(a), (b) and (c). It can be seen that in most cases saturation is not achieved at room temperature, even after 100 days, although saturation is reached in 10 to 20 days in boiling water. It should also be borne in mind that the composites as-received from the laminator already contain considerable amounts of moisture, as the drying results in Fig. 1(d) show. Although there are only relatively minor differences in apparent uptake in CFRP and GRP in boiling water and a 65% RH atmosphere, the as-received CFRP and GRP laminates contain substantially different amounts of water. In all cases, the organic fibre KFRP composites absorb much more water than the inorganic fibre composites, as would be expected. The general effect of absorbed moisture on the mechanical properties of these laminates is shown in Fig. 2. These results were obtained in 4-point bending, but interpretation is complicated by differences in failure modes among the three types of composite and, in the GRP, between samples of different moisture content. The CFRP and KFRP samples all failed in interlaminar shear, and the results, plotted in
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(c) H20 - 100°C; and (d) air - 60°C
COMPOSITES . JULY 1983
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100
COMPOSI T E S . JU LY 1983
terms of shear strength, show only slight reductions in strength, even in the highly hydrophilic KFRP, with increasing moisture content. By contrast, the GFP failed in tension above about 1% moisture content, but in a mixed compression/shear mode below this, and the strength reduction with moisture content is much greater as a result of damage to the fibres in boiling water. Similarly, the tensile strengths of CFRP and KFRP are little affected by long-term exposure to boiling water, whereas the tensile strength and failure strain of the GRP laminates are irreversibly reduced by more than 50% after exposure to boiling water. These results indicate that, of the three reinforcing fibres used in this work, only the glass is significantly damaged by water. The effect of changes in moisture content (or exposure) on the behaviour of GRP is clearly reflected in the acoustic emission patterns observed during tensile testing. Transverse ply cracking in 0/90 non-woven laminates manifests itself as a peak in plots of AE rate vs stress at very low stress levels. 4 The onset of this cracking depends upon ply thickness and lay-up arrangement, and on the failure strain of resin, s and is therefore also affected by the plasticizing action of water. Fig. 3(a-e) shows characteristic hE rate vs stress curves for GRP laminates as-received, after the three standard conditioning treatments, and after completely redrying following boiling water exposure. The top line in each graph is the event count rate for all events monitored, and the numbered lines below indicate the numbers of counts/s in successively higher 12 dB wide bands, band 1 representing counts 0 to 12 dB above threshold, etc (threshold = 75 uV). In fully dried GRP the transverse cracking peak is at a stress level of abut 20% of (failure stress) and contains substantial numbers of high energy events. There is also a smaller peak at about 65% of caused by longitudinal splitting, largely contributed by lower amplitude events but also containing significant numbers of higher energy events in the range 36-60 dB above threshold. As the moisture content increases, the onset of transverse ply cracking is raised to significantly higher stress levels and the height of the peak, as a proportion of the overall count rate at failure, falls from about 94% to zero in samples exposed to boiling water. At this point, the longitudinal cracking is almost totally suppressed but becomes visible, no longer being registered by AE, at only 73% Or. The transverse ply cracking,
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which occurs on first tensile loading and does not register as AE on reloading, occurs in the first few cycles of a fatigue test and is therefore present throughout the life of any sample fatigued above 20% el. A sample cycled to a maximum stress level below the monotonic transverse ply cracking stress wiU also, after relatively little cycling, be found to have sustained complete transverse ply cracking, as evidenced by the AE behaviour. Axial fatigue behaviour
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C O M P O S I T E S • J U L Y 1983
Fig. 5 shows the SIN curves obtained for the three laminates in the 0/90 orientation and the various environmental conditions. The CFRPlaminates are largely insensitive to the effect of moisture. The GRP fatigue response is also unaffected by moisture content unless the fibres are damaged by an extreme treatment like boiling. In KFRP, boiling lowers the SIN curve slightly, but beyond 10 s cycles there is a cross-over. The most unusual feature ofKFRP, from first results, is that the completely dried out composite
291
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subsequent fatiguing. As Fig. 7(a) shows, the uptake of moisture during boiling is only marginally greater in the damaged composite, despite the transverse ply cracking. The SIN curves of pre-loaded samples conditioned at 65% RH and in boiling water coincide with results for undamaged samples (Fig. 7(b)).
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In the -+ 45 ° orientation, the resin naturally exerts a more dominant effect on mechanical properties, but there is nevertheless an effect due to the fibres since the KFRP SIN curve falls significantly below those, almost coincident, for the GRP and CFgP (Figs 8 and 9). As in the case of the 0/90 orientation, the CFRP laminates are unaffected by environmental treatments though the performance of the GRP and KFRP is weakened by treatment in water at IO0°C, albeit to a smaller extent than in the 0/90 orientation. For the GRP there is no difference between the results for dried material and that conditioned at 65% RH.
Flexural fatigue In all cases, the flexural stiffness falls linearly, and more or less rapidly depending on material and stress level, until some local failure, such as interply delamination or splitting
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appears to be less fatigue resistant than that containing moisture. A disturbing feature of the fatigue of KFRP is the sharp downward curvature of the SIN curves, by comparison w i t h those of the GRP and CFRP (Fig. 6). To examine the effect of transverse ply cracking on conditioning and fatigue, a number of GRP samples were preloaded to 0.4 GPa prior to environmental treatments and
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COMPOSITES. JULY 1983
causes a sudden change in flexural stiffness (Fig. 10(a)). This 'knee' in the curve or some similar criterion, such as the number of cycles for a given reduction (15%, in this case) of stiffness serves as a criterion of 'failure'.
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A french version of this paper was presented at the Troisidmes Journ6es Nationales sur les Composites, Pads, France, 21-23 September 1982.
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This work has been carried out with fmancial support from the Procurement Executive of the Ministry of Defence. The authors are grateful to Dr G. Dorey and Dr P. Curtis of the Royal Aircraft Establishment for their guidance and encouragement. The support of the Polymer Engineering Directorate of the SERC is acknowledged for providing acoustic emission facilities.
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REFERENCES
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Fig. 10 Schematic illustration o f f l e x u r a l fatigue curves and definition o f a failure c r i t e r i o n , (b)-(d) Flexural fatigue behaviour of conditioned laminates in 0 / 9 0 ° o r i e n t a t i o n : (b) CFRP; (c) GRP; and (d) KFRP
COMPOSITES. JULY1983
'Catboform Technical Data Sheet' (Fothergill & Harvey Limited) Sims, G.D. and Gladman, D.G. 'Plastics & Rubber: Materials and Applications' (May 1978) pp 41-48 Beaumont, P.W.R. and Harris, B. in 'Carbon Fibres, their Composites and Applications' (Plastics Institute, London, UK, 1972) pp 283-291 Harris,B., Guild, F.J. and Brown, C.R. Y Phys D: Appl Phys 12 (1979) pp 1385-1407 Garrett, K.W. and Bailey, J.E. JMater Sci 12 (1977) pp 157-168 & 2189-2194
AUTHORS 7
Mr T. Adam is with the School of Engineering at the University of Bath. The other authors are with the School of Materials Science, University of Bath, Claverton Down, Bath BA2 7AY, UK. Inquiries should be addressed to Professor Harris in the first instance.
293