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Hybrid carbon and glass fibre composites
Hybrid carbon and glass fibre composites A. R. BUNSELL and B. HARRIS
Hybrid composite materials, consisting of alternatively laminated layers of type 1 carbon fibres and glass fibres in an epoxy resin, have been studied. Two types of hybrid were made, with the alternate layers either unbonded or bonded together. It was considered that, by mixing two types of fibres in a resin to form such a hybrid, it would be possible to create a material having the combined advantages of the individual components whilst diminishing their disadvantages. The results obtained indicate that light engineering materials having controlled stress-strain behaviour can be produced, with properties designed to meet certain requirements. Such materials would allow a wider use of the more expensive components, such as carbon fibres.
INTRODUCTION
There is a steady increase both in the number of applications being found for fibre reinforced plastics and, concurrently, in the variety of fibre/resin systems that are available to designers. Some of these systems are useful, however, only in highly specialised situations where limitations such as high cost and brittle fracture behaviour are considered secondary to such qualities as low density, high rigidity and high strength. By mixing two or more types of fibre in a resin to form a hybrid composite it may be possible to create a material possessing the combined advantages of the individual components and simultaneously mitigating their less desirable qualities. It should, in addition, be possible to tailor the properties of such materials to suit specific requirements. There are many situations in which, for example, a high modulus material is required but in which the catastrophic brittle failure ususally associated with such a material would be unacceptable. In the case of a strut member, a high initial modulus followed by limited yielding of the material and accompanied by the smallest possible reduction of load carrying capacity is usually desirable. This type of behaviour (Fig. 1) can be produced by combining different fibres with suitable physical characteristics in a single resin system. Hybrid fibre reinforced materials can be made in two separate ways - either by intimately mingling the fibres in a common matrix, or by laminating alternate layers of each type of composite. In this work the latter technique has been used and the following considerations apply to this type of hybrid material. In princil~le several different types of fibre can be incorporated into a hybrid system, 1 ,z but in practice it is likely that a combination of only two types of fibre would be of most use. The hybrid material used in these experiments School of Applied Sciences, University of Sussex, Falmer, Brighton, Sussex, England
COMPOSITES. JU LY 1974
o,
Stroin Fig.1
Idealized load-strain diagram for a load-bearing strut
has been made from layers of type I carbon fibre, a high modulus fibre with low breaking strain, and E-glass fibre which has a considerably lower modulus and a greater extension to failure, both in an epoxy resin matrix. Fig.2 shows a typical arrangement of component layers to form a hybrid material. Hayashi I has proposed that such a hybrid structure behaves as a two-phase system the properties of which are obtained simply by the addition of the independent properties of each phase, although in practice he found a slight increase in breaking strain for the more brittle component. In Hayashi's model the brittle layers cease to take part in any load sharing after they have fractured, and th~ load-extension behaviour is solely that of the more extensible component. The authors consider that this need not always be so and that provided the layers are
157
well bonded a different kind of behaviour can be expected: the interlayer bond itself will contribute to the composite action, and the resulting propexties will not necessarily be a simple summation or average of the component properties. In a general two-fibre system with a component volume fraction ratio 1 : 1 the separate and combined elastic loading curves of the two types of fibre reinforced systems used would be as illustrated schematically in Fig.3. The coefficients of thermal expansion for the two type s of fibre will usually be different and in the system used in this study the high modulus material has the smaller coefficient. With an effective bond the high modulus fibres are put into compression after curing of the resin by the contraction which accompanies cooling from the hot-pressing temperature. The hybrid reaches a state of equilibrium in which the compressive forces in the high modulus material are just balanced by tensile forces induced in the less rigid material. For a composite in which the dimensions of the component layers are identical and for which the tensile and compressive moduli are assumed to be the same, t h e following relationship can be written: - E c A e c = EGAe G
(1)
in which E c and E G are the moduli, respectively, of the high and low modulus material (cfrp and grp) and AeG and --Ae c are, similarly, the strains in the two components. The initial load bearing capacity of the hybrid system is therefore, as Hayashi proposes, a simple addition of the strengths of the two systems and the initial modulus, EH, of the hybrid is given by the mixture rule
EH =EcVc +EcVc
(2)
in which Vc and VG are the volume fractions, respectively, of the cfrp and grp. On reaching an extension (ec + Aec), the stiffer material will fracture and the relationship given in Equation 2 will no longer hold. However, the load carried by the hybrid will not necessarily fall, as Hayashi proposes, to that which would be supported at this extension by the less rigid material alone: this would occur only if the bond between the layers was weak or non-existent. With a strong bond the high modulus material continues to play an important role in contributing to the rigidity and load-bearing capacity of the material. It is the strength of the bond as well as the properties of the low modulus material which determine behaviour of the hybrid after failure of the high modulus material. If the bond between the layers is good and little debonding occurs at the initial fracture of the less
/
/
,
/
~rp
Strain
I'--- Aeg--~
Fig.3 Load-strain diagram, obtained when combining cfrp and grp t o f o r m a hybrid fibre reinforced composite, showing the
effect of differential contraction between the layers
extensible component, there is a critical length, Le/2, which is the distance from the fracture at which the load carried by the high modulus layer would be unaffected by the fracture. This could lead to multiple fracture of the stiffer layers and, with a very good bond, the breaking of these layers into successive lengths between Lc/2 and Lc. If the more rigid material failed in a brittle manner completely across the specimen then, in the region of the fracture, the load would be taken by the less rigid lay6r and final failure "would also occur at this point. The final breaking extension of the hybrid specimen would thus be considerably less than that of the more extensible component since the extension of the specimen after first fracture would be restricted to a region much shorter than the overall specimen length. The final breaking extension of the hybrid cannot therefore be predicted simply by knowing the failure strains of the independent component layers since it is dependent on random and repeated fracture of the more brittle layers. In an unbonded layered'structure; or one in which severe delamination occurs, 3 the fracture of the individual layers would occur at their breaking strains as measured when tested independently: orily then would the final breaking strain be that of the more extensible layers tested separately. EXPERIMENTAL RESULTS
In order to investigate the behaviour of hybrid fibre rein9rP
¢frp
Fig.2
158
Symmetrically laminated hybrid composite
forced plastics, specimens were made by bonding together parallel layers of unidirectional cfrp and grp. The cfrp was epoxy resin containing type 1 carbon fibre (Vf = 0.40) and the grp was E-glass in epoxy resin (Vf = 0.60). The carbon fibre was surface-treated, continuous fibre supplied in preimpregnated sheet form by Ciba-Geigy and pro-coated with their 905 resin. The glass fibre was supplied, also in proimpregnated form, with a similar Shell resin, Epikote 828, by IMI Engineering Composites Division. The hybrid plates were made by the leaky mould method and cured for one hour at 180°C after which they were slowly cooled to room temperature. Two types of hybrid specimens were made in which the layers were either unbonded or bonded together.
COMPOSITES . JULY 1974
4n making the unbonded specimens pieces of silicone paper were placed between the grp and cfrp layers over the central portion of the plate. This resulted in specimens in which the two components were unbonded over the specimen gauge length but were bonded at their ends where they were later to be gripped during testing. The.layers of the bonded specimens were bonded over their whole length. To study the effect of the differing coefficients of expansion for the carbon and glass fibre a two-layer hybrid plate was made. Since glass fibres have a higher coefficient of thermal expansion than carbon fibres, the resulting plate was curved, with the.cfrp on the convex side. The exact dimensions of, and the curvature adgpted by, the hybrid sample were measured with a travelling microscope. Geometrical considerations of the natural curvature adopted by the unrestrained plate enabled estimates of the stresses induced in the layers of a symmetrical hybrid to be made It was found that in a three layer system consisting of one cfrp layer sandwiched between two grp layers the induced stress in the cfrp was 50 MN/m 2, while that in each of the grp layers was 25 MN/m 2. This corresponds to a compressive strain of 0.029% in the cfrp and a tensile strain of 0.05% in the grp. The cfrp was therefore compressed to roughly 10% of its tensile failure strain. Fig.4 shows two samples from a four-layer system, each layer being about 0.4 mm thick, in which two grp layers were sandwiched between carbon fibre layers. In one sample the layers are seen to be well bonded, whereas in the other the grp is not bonded to the cfrp and the buckling due to differential contraction is clearly shown. Before tensile tests were conducted on hybrid specimens, samples of the cfrp and grp were individually tested in an Instron UniversalTester. A rectangular specimen shape was used, for these as well as for subsequent tests, with a gauge length of 50 mm and width of 10 mm. Aluminium tabs were bonded on to the ends of the specimen to prevent grip damage. The rate of loading was kept constant at 0.5 mm/min and clip-on strain-gauge extensometers were used to measure deformation. The specimens were monitored by a Dunegan stress-wave emission apparatus which uses a ceramic piezoelectric transducer to detect fracture events. The properties of the cfrp and grp specimens are given in Table 1 together with average results for a hybrid specimen containing equal volumes of the two types of fibre reinforced resin. An important difference in fracture mode which was observed was that the cfrp samples broke straight across in one brittle fracture but the grp failed only after extensive splitting. The behaviour of the individual components was first compared with that of a four-layer hybrid system consisting of two layers of one material sandwiched between two layers of the other. There was no obvious difference in strength or behaviour with either the carbon or the glass fibre layers on the outside, and both arrangements were used to study different aspects of fracture. The different ways in which the unbonded and bonded specimens behayed can be compared from Fig.5 and Fig.6. On loading, the grp layers in the unbonded specimens take all of the load initially, and only when the cfrp layers are straightned do these contribute to load bearing. The specimen is then loaded until the cfrp layers reach their breaking extension, whereupon they fail in a sudden, brittle manner
COMPOSITES. JULY 1974
Fig.4 Two hybrid samples, seen edge on, demonstrating the effect of differential thermal contraction and bonding. In the upper sample the buckled cfrp layers are not bonded to the central grp layers. The layers are bonded in the other sample and the cfrp has been put into compression
Table 1 Comparison between the hybrid and its component fibre reinforced layers. The hybrid contains equal volume fractions of cfrp and grp
Material cfrp grp Hybrid 1: 1
Initial modulus GN/m 2
Strain at first failure %
Strain at final failure (separation) %
Stress at first failure MN/m 2
142 41
0.26 1.25
0.26 1.75
400 520
89
0.37
1.25
295
which results in a sharp drop in load. The load falls to a level consistant with the elastic response of the grp at the breaking strain of the cfrp, and subsequent load/elongation behaviour is characteristic of the grp alone, as predicted by Hayashi. The carbon fibre layers broke straight across the specimen normal to the direction of pull and fibre alignment, whereas the grp layers failed only after extensive splitting. The final extension when the two parts of the samples were completely separated was the same as that at which the grp alone would have failed. The acoustic emission from these samples was also monitored during the tests. A gain of 60 db was used and the rate of emissions recorded over 10s intervals. The counter was adjusted to reset at 1000 counts and so for 10s periods in which this number was exceeded a counter saturation level of 1000 emissions is shown. This occurred with sudden severe damage and with the gross damage in the final stages. Little activity was detected prior to failure of the cfrp layers because of poor acoustic coupling between the unbonded layers, but a large burst of emission was recorded at their failure point. Thereafter the acoustic emission was identical with that obtained from grp when tested alone. In a bonded specimen the load was distributed over the whole cross-section from the beginning of extension although some gradual load redistribution may have occurred
159
500
400
$ --~ 3 0 0 C
O 0 .J
o E
._u O
V
I
I000
1.4 Strain ( % ) Load-strain curves for unbonded layered hybrid specimens consisting of two layers of cfrp and two of grp
Fig.5
500
400
C
ou
2 c
A ¢.
.o
500
E
o o
%1 u
.J
ou
200
-
000
I00
' Fig.6
160
0;2
'
0:4
'
0'6
O'B
1.4
I.'O
Stroin (O/o) Load-strain curves for bonded layered hybrid specimens consisting of two layers of cfrp and two of grp
COMPOSITES.
JULY 1974
Fig.7 A fractured layered system of cfrp and grp in which there is no bond between the layers. The cfrp has broken in a brittle manner and the grp has failed after splitting. The silicone paper used to prevent bonding can be seen between the two types of layers
Fig.8 A fractured layered hybrid system of cfrp and grp in which there is a good bond between the layers. The fracture of the cfrp layers has been greatly modified from its usual simple brittle manner
initially as a result of fibre misalignment. The initial modulus of these specimens was similar to that of unbonded samples and to the value calculated from Equation 2. However, more acoustic emission was detected in the early stages of loading because of the presence of a better bond between layers. A further increase in acoustic activity was detected just prior to the first load drop which occurred at a greater extension than would have been expected from the behaviour of cfrp tested alone. The load drop was smaller than that in unbonded samples and, as Fig.6 shows, subsequent extension caused repeated load increases followed by further sudden drops. Large bursts of acoustic emission were associated with these repeated drops. The final failure strain (at separation) of the bonded samples was somewhat less than that of unbonded samples. The modes of failure of these hybrids are shown in Fig.7 and Fig.8. The layers in the unbonded specimens broke independently while bonded specimens failed in a more complex manner. The cfrp layer in the bonded specimen has not broken straight across at one point but has failed in many places over the sample's length. Some debonding of the two types of layer has occurred but only to a small degree. When the layer arrangement is such that the cfrp is on the outside of the hybrid the multiple fracture of these layers is easily seen (Fig.9). The cfrp layers have broken repeatedly but usually not completely across the specimen.
C O M P O S I T E S . J U L Y 1974
Fig.9 Multiple fracture of the cfrp layer in a well bonded hybrid composite sample
These layers have been seen to break down into pieces about 1.5 mm long which supports the idea that there is a critical length, as discussed earlier. Typical values of initial modulus and strain to fracture for four-layer hybrid samples are given in Table 2. The strains at the first load drop, corresponding to at least partial frac-
161
ture of the cfrp layers, are higher than the average failure strain of the cfrp tested separately. The scatter in results with this material is inherently large and it is not possible to determine from these values whether the increase in strain was due solely to the residual compressive stresses. Some of the hybrid samples were unloaded several times during a test so that changes'in compliance could be studied. Fig. 10 shows the change of compliance of two such samples, one bonded and the other unbonded, and it can be seen that the latter loses its rigidity less rapidly than the former. There was little change of compliance after the first Signs of fracture were detected in the bonded sample, but a large sudden increase occurred in the unbonded sample. TaMe 2 Typical values of initial moduli and sUain to failure for four-layer hybrid samples Strain at
Material
Initial modulus GN/m 2
Four-layer bonded hybrid
first failure (%)
(separation) (%)
0.4
1.44
,,
93
0.3
1.09
,,
103
0.28
1.00
,,
94
0.38
1.25
0.3
1.68
,,
95
0.37
1.57
cfrp
142
0.26
1.26
grp
41
1.25
1.75
Table 3 Typical values of initial moduli and strain to failure for three layer hybrid samples consisting of a central layer of cfrp sandwiched between two layers of grp
Material
Initial modulus GN/m 2
Strain at first failure (%)
Strain at final failure (separation) (%)
Bonded
72
0.52
1.6
Carbon/ glass
77.5
0.4
1.35
Hybrid
68
0.53
1.3
Hybrid
69
0.46
1.45
Unbonded 63
0.31
1.84
Carbon/ glass
76
0.3
1.87
Hybrid
-
-
-
162
Unbon~
Z
0.06
~0"04 Q. E
002 0
,
I
I
0'2
i
0.4
I
I
i
I
0'6 0-8, Strain 1O/o)
I
I
1.0
I
I
I
1.2
Fig.lO Change of compliance with strain for an unbonded and a bonded hybrid sample consisting of two layers of cfrp sandwiched between two layers of grp. The large increase in compliance occurs in the unbonded sample when the cfrp layer fractures but is not nearly as dramatic in the bonded sample
Strain at final failure
90
Four-layer 100 unbonded hybrid
0"08
A three-layer hybrid system was also studied which consisted of one layer of cfrp sandwiched between two layers of grp. Some typical values of initial modulus and strain to fracture of these three-layer specimens are given in Table 3. Bonded and unbonded samples of this material were also made and typical load-strain relationships are shown in Fig. 11 and Fig. 12. The volume fractions of fibres in these composites are clearly different from the 0.50 overall value for the four-layer system since the three-layer material consisted of one third cfrp and two thirds grp by volume. The initial modulus of this material also obeys the mixture rule, and the load/extension behaviour was similar to that of the four-layer system.
DISCUSSION
The hybrid materials described in this paper all bohave on first loading as predicted by Hayashi's model. The values of elastic modulus are equal to those given by the mixture rule up to the point of initial fracture of the carbon fibre component. As observed by Hayashi~ the extension at this point is greater than the breaking extension of the cfrp tested alone as a result of the residual compressive stress induced in the cfrp during manufacture of the hybrid plate. After initial fracture, which occurs at the weakest point of the cfrp layer, the well-bonded samples behaved in a manner quite unlike the unbonded material. The load/elongation curves were not like those predicted by any model that considers only the independent properties of the separate constituents. The bond between layers ensures that the cfrp continues to carry a share of the applied load and to contribute to the overall stiffness of the hybrid. Because of the bond, load transfer occurs between the grp and cfrp and there is a critical length for effective reinforcement by the stiffer layers similar to that in the ordinary theory of composites. At a distance of half the critical length from the point of fraqture, the load in the cfrp would be that which would be carried if no fracture had occurred and multiple fracture of the stiffer layers therefore occurs (Fig.9). The bursts of acoustic emission accompanying the repeated load drops during extension of the bonded hybrids signal the occurrence of this multiple fracture of the cfrp layers. The extension at the point of final failure (sap-
COMPOSITES. JULY 1974
so°/ 400h
c
u0
300
? o
0 0 _.1
200
,U 0
,ooo
IOC
1.4
0 Fig.11
Strain (O/o) Load-strain curves for unbonded layered hybrid specimens consisting of one central layer between two layers of grp
500
400
u
10
0 0 .J
i-
9 E u 0 u
I000
0 Fig.12
1.4 Strain (O/o) Load-strain curves for bonded layered hybrid specimens consisting of one central cfrp layer between two layers of grp
COMPOSITES.
JULY 1974
163
aration) was considerably less than the breaking the grp, either alone or in unbonded specimens. expected, since the interlaminar bond limits the the grp to short regions around fractfires in cfrp
extension of This is as extension of layers.
It is therefore possible, with a good bond between the different layers of fibre reinforced materials, to produce light engineering materials with controlled stress-strain behaviour. This is in addition to the use of high modulus fibre reinforced strips to stiffen low modulus fibre reinforced plastic structure. 4 The load elongation curves obtained for the hybrid materials described here were similar to that proposed as being desirable for a strut (Fig. 1). It is clearly possible that the design of three or even four-phased composites along these lines could permit the creation of materials with properties tailored to suit specific requirements in which a wider and more subtle use is made than hitherto of the more expensive varieties of fibre.
164
A CKNOWL EDGMEN TS
The authors would like to thank the Science Research Council for their sponsorship and IMI Engineering Composites Group for supplying samples of preimpregnated GRP. REFERENCES
1 Hayashi,T. 'On the improvement of mechanical properties of composites by hybrid composition', Proceedings o f the Eighth International Reinforced Plastics Conference (October 1972) paper 22 2 Hofer,.K.E., Rao, N. and Stander, M. 'Fatigue behaviour of graphite-glass-epoxycomposites', Proceedings o f the Second International Carbon Fibres Conference (February 1974) paper 31 3 Hancox, N. L. and Wells, H. "Izodimpact properties of carbon fibre sandwich structures', Composites 4 No 1 (January 1973) pp 26-30 4 Hancox, N. L. and Wells, H. 'Hybrid carbon/glass fibre resin composites', Proceedings o f the Second International Carbon Fibres Conference (February 1974) paper 24
COMPOSITES. JULY 1974
Hybrid composite materials, consisting of alternatively laminated layers of type 1 carbon fibres and glass fibres in an epoxy resin, have been studied. Two types of hybrid were made, with the alternate layers either unbonded or bonded together. It was considered that, by mixing two types of fibres in a resin to form such a hybrid, it would be possible to create a material having the combined advantages of the individual components whilst diminishing their disadvantages. The results obtained indicate that light engineering materials having controlled stress-strain behaviour can be produced, with properties designed to meet certain requirements. Such materials would allow a wider use of the more expensive components, such as carbon fibres.
INTRODUCTION
There is a steady increase both in the number of applications being found for fibre reinforced plastics and, concurrently, in the variety of fibre/resin systems that are available to designers. Some of these systems are useful, however, only in highly specialised situations where limitations such as high cost and brittle fracture behaviour are considered secondary to such qualities as low density, high rigidity and high strength. By mixing two or more types of fibre in a resin to form a hybrid composite it may be possible to create a material possessing the combined advantages of the individual components and simultaneously mitigating their less desirable qualities. It should, in addition, be possible to tailor the properties of such materials to suit specific requirements. There are many situations in which, for example, a high modulus material is required but in which the catastrophic brittle failure ususally associated with such a material would be unacceptable. In the case of a strut member, a high initial modulus followed by limited yielding of the material and accompanied by the smallest possible reduction of load carrying capacity is usually desirable. This type of behaviour (Fig. 1) can be produced by combining different fibres with suitable physical characteristics in a single resin system. Hybrid fibre reinforced materials can be made in two separate ways - either by intimately mingling the fibres in a common matrix, or by laminating alternate layers of each type of composite. In this work the latter technique has been used and the following considerations apply to this type of hybrid material. In princil~le several different types of fibre can be incorporated into a hybrid system, 1 ,z but in practice it is likely that a combination of only two types of fibre would be of most use. The hybrid material used in these experiments School of Applied Sciences, University of Sussex, Falmer, Brighton, Sussex, England
COMPOSITES. JU LY 1974
o,
Stroin Fig.1
Idealized load-strain diagram for a load-bearing strut
has been made from layers of type I carbon fibre, a high modulus fibre with low breaking strain, and E-glass fibre which has a considerably lower modulus and a greater extension to failure, both in an epoxy resin matrix. Fig.2 shows a typical arrangement of component layers to form a hybrid material. Hayashi I has proposed that such a hybrid structure behaves as a two-phase system the properties of which are obtained simply by the addition of the independent properties of each phase, although in practice he found a slight increase in breaking strain for the more brittle component. In Hayashi's model the brittle layers cease to take part in any load sharing after they have fractured, and th~ load-extension behaviour is solely that of the more extensible component. The authors consider that this need not always be so and that provided the layers are
157
well bonded a different kind of behaviour can be expected: the interlayer bond itself will contribute to the composite action, and the resulting propexties will not necessarily be a simple summation or average of the component properties. In a general two-fibre system with a component volume fraction ratio 1 : 1 the separate and combined elastic loading curves of the two types of fibre reinforced systems used would be as illustrated schematically in Fig.3. The coefficients of thermal expansion for the two type s of fibre will usually be different and in the system used in this study the high modulus material has the smaller coefficient. With an effective bond the high modulus fibres are put into compression after curing of the resin by the contraction which accompanies cooling from the hot-pressing temperature. The hybrid reaches a state of equilibrium in which the compressive forces in the high modulus material are just balanced by tensile forces induced in the less rigid material. For a composite in which the dimensions of the component layers are identical and for which the tensile and compressive moduli are assumed to be the same, t h e following relationship can be written: - E c A e c = EGAe G
(1)
in which E c and E G are the moduli, respectively, of the high and low modulus material (cfrp and grp) and AeG and --Ae c are, similarly, the strains in the two components. The initial load bearing capacity of the hybrid system is therefore, as Hayashi proposes, a simple addition of the strengths of the two systems and the initial modulus, EH, of the hybrid is given by the mixture rule
EH =EcVc +EcVc
(2)
in which Vc and VG are the volume fractions, respectively, of the cfrp and grp. On reaching an extension (ec + Aec), the stiffer material will fracture and the relationship given in Equation 2 will no longer hold. However, the load carried by the hybrid will not necessarily fall, as Hayashi proposes, to that which would be supported at this extension by the less rigid material alone: this would occur only if the bond between the layers was weak or non-existent. With a strong bond the high modulus material continues to play an important role in contributing to the rigidity and load-bearing capacity of the material. It is the strength of the bond as well as the properties of the low modulus material which determine behaviour of the hybrid after failure of the high modulus material. If the bond between the layers is good and little debonding occurs at the initial fracture of the less
/
/
,
/
~rp
Strain
I'--- Aeg--~
Fig.3 Load-strain diagram, obtained when combining cfrp and grp t o f o r m a hybrid fibre reinforced composite, showing the
effect of differential contraction between the layers
extensible component, there is a critical length, Le/2, which is the distance from the fracture at which the load carried by the high modulus layer would be unaffected by the fracture. This could lead to multiple fracture of the stiffer layers and, with a very good bond, the breaking of these layers into successive lengths between Lc/2 and Lc. If the more rigid material failed in a brittle manner completely across the specimen then, in the region of the fracture, the load would be taken by the less rigid lay6r and final failure "would also occur at this point. The final breaking extension of the hybrid specimen would thus be considerably less than that of the more extensible component since the extension of the specimen after first fracture would be restricted to a region much shorter than the overall specimen length. The final breaking extension of the hybrid cannot therefore be predicted simply by knowing the failure strains of the independent component layers since it is dependent on random and repeated fracture of the more brittle layers. In an unbonded layered'structure; or one in which severe delamination occurs, 3 the fracture of the individual layers would occur at their breaking strains as measured when tested independently: orily then would the final breaking strain be that of the more extensible layers tested separately. EXPERIMENTAL RESULTS
In order to investigate the behaviour of hybrid fibre rein9rP
¢frp
Fig.2
158
Symmetrically laminated hybrid composite
forced plastics, specimens were made by bonding together parallel layers of unidirectional cfrp and grp. The cfrp was epoxy resin containing type 1 carbon fibre (Vf = 0.40) and the grp was E-glass in epoxy resin (Vf = 0.60). The carbon fibre was surface-treated, continuous fibre supplied in preimpregnated sheet form by Ciba-Geigy and pro-coated with their 905 resin. The glass fibre was supplied, also in proimpregnated form, with a similar Shell resin, Epikote 828, by IMI Engineering Composites Division. The hybrid plates were made by the leaky mould method and cured for one hour at 180°C after which they were slowly cooled to room temperature. Two types of hybrid specimens were made in which the layers were either unbonded or bonded together.
COMPOSITES . JULY 1974
4n making the unbonded specimens pieces of silicone paper were placed between the grp and cfrp layers over the central portion of the plate. This resulted in specimens in which the two components were unbonded over the specimen gauge length but were bonded at their ends where they were later to be gripped during testing. The.layers of the bonded specimens were bonded over their whole length. To study the effect of the differing coefficients of expansion for the carbon and glass fibre a two-layer hybrid plate was made. Since glass fibres have a higher coefficient of thermal expansion than carbon fibres, the resulting plate was curved, with the.cfrp on the convex side. The exact dimensions of, and the curvature adgpted by, the hybrid sample were measured with a travelling microscope. Geometrical considerations of the natural curvature adopted by the unrestrained plate enabled estimates of the stresses induced in the layers of a symmetrical hybrid to be made It was found that in a three layer system consisting of one cfrp layer sandwiched between two grp layers the induced stress in the cfrp was 50 MN/m 2, while that in each of the grp layers was 25 MN/m 2. This corresponds to a compressive strain of 0.029% in the cfrp and a tensile strain of 0.05% in the grp. The cfrp was therefore compressed to roughly 10% of its tensile failure strain. Fig.4 shows two samples from a four-layer system, each layer being about 0.4 mm thick, in which two grp layers were sandwiched between carbon fibre layers. In one sample the layers are seen to be well bonded, whereas in the other the grp is not bonded to the cfrp and the buckling due to differential contraction is clearly shown. Before tensile tests were conducted on hybrid specimens, samples of the cfrp and grp were individually tested in an Instron UniversalTester. A rectangular specimen shape was used, for these as well as for subsequent tests, with a gauge length of 50 mm and width of 10 mm. Aluminium tabs were bonded on to the ends of the specimen to prevent grip damage. The rate of loading was kept constant at 0.5 mm/min and clip-on strain-gauge extensometers were used to measure deformation. The specimens were monitored by a Dunegan stress-wave emission apparatus which uses a ceramic piezoelectric transducer to detect fracture events. The properties of the cfrp and grp specimens are given in Table 1 together with average results for a hybrid specimen containing equal volumes of the two types of fibre reinforced resin. An important difference in fracture mode which was observed was that the cfrp samples broke straight across in one brittle fracture but the grp failed only after extensive splitting. The behaviour of the individual components was first compared with that of a four-layer hybrid system consisting of two layers of one material sandwiched between two layers of the other. There was no obvious difference in strength or behaviour with either the carbon or the glass fibre layers on the outside, and both arrangements were used to study different aspects of fracture. The different ways in which the unbonded and bonded specimens behayed can be compared from Fig.5 and Fig.6. On loading, the grp layers in the unbonded specimens take all of the load initially, and only when the cfrp layers are straightned do these contribute to load bearing. The specimen is then loaded until the cfrp layers reach their breaking extension, whereupon they fail in a sudden, brittle manner
COMPOSITES. JULY 1974
Fig.4 Two hybrid samples, seen edge on, demonstrating the effect of differential thermal contraction and bonding. In the upper sample the buckled cfrp layers are not bonded to the central grp layers. The layers are bonded in the other sample and the cfrp has been put into compression
Table 1 Comparison between the hybrid and its component fibre reinforced layers. The hybrid contains equal volume fractions of cfrp and grp
Material cfrp grp Hybrid 1: 1
Initial modulus GN/m 2
Strain at first failure %
Strain at final failure (separation) %
Stress at first failure MN/m 2
142 41
0.26 1.25
0.26 1.75
400 520
89
0.37
1.25
295
which results in a sharp drop in load. The load falls to a level consistant with the elastic response of the grp at the breaking strain of the cfrp, and subsequent load/elongation behaviour is characteristic of the grp alone, as predicted by Hayashi. The carbon fibre layers broke straight across the specimen normal to the direction of pull and fibre alignment, whereas the grp layers failed only after extensive splitting. The final extension when the two parts of the samples were completely separated was the same as that at which the grp alone would have failed. The acoustic emission from these samples was also monitored during the tests. A gain of 60 db was used and the rate of emissions recorded over 10s intervals. The counter was adjusted to reset at 1000 counts and so for 10s periods in which this number was exceeded a counter saturation level of 1000 emissions is shown. This occurred with sudden severe damage and with the gross damage in the final stages. Little activity was detected prior to failure of the cfrp layers because of poor acoustic coupling between the unbonded layers, but a large burst of emission was recorded at their failure point. Thereafter the acoustic emission was identical with that obtained from grp when tested alone. In a bonded specimen the load was distributed over the whole cross-section from the beginning of extension although some gradual load redistribution may have occurred
159
500
400
$ --~ 3 0 0 C
O 0 .J
o E
._u O
V
I
I000
1.4 Strain ( % ) Load-strain curves for unbonded layered hybrid specimens consisting of two layers of cfrp and two of grp
Fig.5
500
400
C
ou
2 c
A ¢.
.o
500
E
o o
%1 u
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ou
200
-
000
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' Fig.6
160
0;2
'
0:4
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0'6
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Stroin (O/o) Load-strain curves for bonded layered hybrid specimens consisting of two layers of cfrp and two of grp
COMPOSITES.
JULY 1974
Fig.7 A fractured layered system of cfrp and grp in which there is no bond between the layers. The cfrp has broken in a brittle manner and the grp has failed after splitting. The silicone paper used to prevent bonding can be seen between the two types of layers
Fig.8 A fractured layered hybrid system of cfrp and grp in which there is a good bond between the layers. The fracture of the cfrp layers has been greatly modified from its usual simple brittle manner
initially as a result of fibre misalignment. The initial modulus of these specimens was similar to that of unbonded samples and to the value calculated from Equation 2. However, more acoustic emission was detected in the early stages of loading because of the presence of a better bond between layers. A further increase in acoustic activity was detected just prior to the first load drop which occurred at a greater extension than would have been expected from the behaviour of cfrp tested alone. The load drop was smaller than that in unbonded samples and, as Fig.6 shows, subsequent extension caused repeated load increases followed by further sudden drops. Large bursts of acoustic emission were associated with these repeated drops. The final failure strain (at separation) of the bonded samples was somewhat less than that of unbonded samples. The modes of failure of these hybrids are shown in Fig.7 and Fig.8. The layers in the unbonded specimens broke independently while bonded specimens failed in a more complex manner. The cfrp layer in the bonded specimen has not broken straight across at one point but has failed in many places over the sample's length. Some debonding of the two types of layer has occurred but only to a small degree. When the layer arrangement is such that the cfrp is on the outside of the hybrid the multiple fracture of these layers is easily seen (Fig.9). The cfrp layers have broken repeatedly but usually not completely across the specimen.
C O M P O S I T E S . J U L Y 1974
Fig.9 Multiple fracture of the cfrp layer in a well bonded hybrid composite sample
These layers have been seen to break down into pieces about 1.5 mm long which supports the idea that there is a critical length, as discussed earlier. Typical values of initial modulus and strain to fracture for four-layer hybrid samples are given in Table 2. The strains at the first load drop, corresponding to at least partial frac-
161
ture of the cfrp layers, are higher than the average failure strain of the cfrp tested separately. The scatter in results with this material is inherently large and it is not possible to determine from these values whether the increase in strain was due solely to the residual compressive stresses. Some of the hybrid samples were unloaded several times during a test so that changes'in compliance could be studied. Fig. 10 shows the change of compliance of two such samples, one bonded and the other unbonded, and it can be seen that the latter loses its rigidity less rapidly than the former. There was little change of compliance after the first Signs of fracture were detected in the bonded sample, but a large sudden increase occurred in the unbonded sample. TaMe 2 Typical values of initial moduli and sUain to failure for four-layer hybrid samples Strain at
Material
Initial modulus GN/m 2
Four-layer bonded hybrid
first failure (%)
(separation) (%)
0.4
1.44
,,
93
0.3
1.09
,,
103
0.28
1.00
,,
94
0.38
1.25
0.3
1.68
,,
95
0.37
1.57
cfrp
142
0.26
1.26
grp
41
1.25
1.75
Table 3 Typical values of initial moduli and strain to failure for three layer hybrid samples consisting of a central layer of cfrp sandwiched between two layers of grp
Material
Initial modulus GN/m 2
Strain at first failure (%)
Strain at final failure (separation) (%)
Bonded
72
0.52
1.6
Carbon/ glass
77.5
0.4
1.35
Hybrid
68
0.53
1.3
Hybrid
69
0.46
1.45
Unbonded 63
0.31
1.84
Carbon/ glass
76
0.3
1.87
Hybrid
-
-
-
162
Unbon~
Z
0.06
~0"04 Q. E
002 0
,
I
I
0'2
i
0.4
I
I
i
I
0'6 0-8, Strain 1O/o)
I
I
1.0
I
I
I
1.2
Fig.lO Change of compliance with strain for an unbonded and a bonded hybrid sample consisting of two layers of cfrp sandwiched between two layers of grp. The large increase in compliance occurs in the unbonded sample when the cfrp layer fractures but is not nearly as dramatic in the bonded sample
Strain at final failure
90
Four-layer 100 unbonded hybrid
0"08
A three-layer hybrid system was also studied which consisted of one layer of cfrp sandwiched between two layers of grp. Some typical values of initial modulus and strain to fracture of these three-layer specimens are given in Table 3. Bonded and unbonded samples of this material were also made and typical load-strain relationships are shown in Fig. 11 and Fig. 12. The volume fractions of fibres in these composites are clearly different from the 0.50 overall value for the four-layer system since the three-layer material consisted of one third cfrp and two thirds grp by volume. The initial modulus of this material also obeys the mixture rule, and the load/extension behaviour was similar to that of the four-layer system.
DISCUSSION
The hybrid materials described in this paper all bohave on first loading as predicted by Hayashi's model. The values of elastic modulus are equal to those given by the mixture rule up to the point of initial fracture of the carbon fibre component. As observed by Hayashi~ the extension at this point is greater than the breaking extension of the cfrp tested alone as a result of the residual compressive stress induced in the cfrp during manufacture of the hybrid plate. After initial fracture, which occurs at the weakest point of the cfrp layer, the well-bonded samples behaved in a manner quite unlike the unbonded material. The load/elongation curves were not like those predicted by any model that considers only the independent properties of the separate constituents. The bond between layers ensures that the cfrp continues to carry a share of the applied load and to contribute to the overall stiffness of the hybrid. Because of the bond, load transfer occurs between the grp and cfrp and there is a critical length for effective reinforcement by the stiffer layers similar to that in the ordinary theory of composites. At a distance of half the critical length from the point of fraqture, the load in the cfrp would be that which would be carried if no fracture had occurred and multiple fracture of the stiffer layers therefore occurs (Fig.9). The bursts of acoustic emission accompanying the repeated load drops during extension of the bonded hybrids signal the occurrence of this multiple fracture of the cfrp layers. The extension at the point of final failure (sap-
COMPOSITES. JULY 1974
so°/ 400h
c
u0
300
? o
0 0 _.1
200
,U 0
,ooo
IOC
1.4
0 Fig.11
Strain (O/o) Load-strain curves for unbonded layered hybrid specimens consisting of one central layer between two layers of grp
500
400
u
10
0 0 .J
i-
9 E u 0 u
I000
0 Fig.12
1.4 Strain (O/o) Load-strain curves for bonded layered hybrid specimens consisting of one central cfrp layer between two layers of grp
COMPOSITES.
JULY 1974
163
aration) was considerably less than the breaking the grp, either alone or in unbonded specimens. expected, since the interlaminar bond limits the the grp to short regions around fractfires in cfrp
extension of This is as extension of layers.
It is therefore possible, with a good bond between the different layers of fibre reinforced materials, to produce light engineering materials with controlled stress-strain behaviour. This is in addition to the use of high modulus fibre reinforced strips to stiffen low modulus fibre reinforced plastic structure. 4 The load elongation curves obtained for the hybrid materials described here were similar to that proposed as being desirable for a strut (Fig. 1). It is clearly possible that the design of three or even four-phased composites along these lines could permit the creation of materials with properties tailored to suit specific requirements in which a wider and more subtle use is made than hitherto of the more expensive varieties of fibre.
164
A CKNOWL EDGMEN TS
The authors would like to thank the Science Research Council for their sponsorship and IMI Engineering Composites Group for supplying samples of preimpregnated GRP. REFERENCES
1 Hayashi,T. 'On the improvement of mechanical properties of composites by hybrid composition', Proceedings o f the Eighth International Reinforced Plastics Conference (October 1972) paper 22 2 Hofer,.K.E., Rao, N. and Stander, M. 'Fatigue behaviour of graphite-glass-epoxycomposites', Proceedings o f the Second International Carbon Fibres Conference (February 1974) paper 31 3 Hancox, N. L. and Wells, H. "Izodimpact properties of carbon fibre sandwich structures', Composites 4 No 1 (January 1973) pp 26-30 4 Hancox, N. L. and Wells, H. 'Hybrid carbon/glass fibre resin composites', Proceedings o f the Second International Carbon Fibres Conference (February 1974) paper 24
COMPOSITES. JULY 1974