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Cost-effective high performance composites
Cost-effective high performance composites P.T. CURTIS and M. BROWNE (Defence ResearchAgency, UK) Received 1 July 1993, revised 30 July 1993 A hybrid composite material based on mixtures of ultra-high performance carbon fibres in the principal stress directions and cheaper standard fibres in the secondary stress directions has been developed. A wide range of mechanical properties have been measured on this material and, for comparison, also on the base materials. The hybrid material has been shown to have a mechanical performance similar to that of the ultra-high performance carbon fibre base material tested, a performance generally much improved over that of the standard fibre composite. This has been achieved together with a significant cost saving through the use of lower cost standard fibres in secondary, lower stressed layers. At present price levels, ultra-high performance fibres are typically 21½ times the cost of standard carbon fibres, although prepregs show just a 40% difference in cost. The philosophy of mixing standard and ultra-high performance carbon fibres, as described in this work, could lead to cost reductions of about 1 7% based on prepreg costs, but if alternative fabrication methods were used the savings could be much greater. Key w o r d s : hybrid material," mechanical properties; cost; ultra-high performance carbon fibres; standard carbon fibres; epoxy matrix Carbon fibre-reinforced plastics (CFRPS) are being increasingly used in the aerospace industry, principally because they combine high strength and stiffness with low density. The first generation CFRPs, although expensive in terms of raw material costs, saved money in the fabrication process through reduced part counts and could therefore compete reasonably well on cost with traditional metallic materials. In recent years, materials manufacturers have developed improved products in response to demands from aerospace manufacturers. Carbon fibres with 50% greater stiffness and twice the strength of those a decade ago are now obtainable. In addition, resin matrices with much improved toughness and increased environmental resistance are widely available. The costs of these new materials, however, remain high, since, although increased demand has reduced prices, this has been offset by the cost of materials' development. The older 'standard' materials have thus become substantially cheaper than the newer, high strength and high toughness CFRPs. In civil aerospace, composites have been used less widely, partly because it is more difficult to justify the higher raw material costs of CFRPs compared with metallic materials. Although their usage leads to weight savings, and thus to increased payloads and lower fuel costs, in the present economic climate this must be balanced against increased capital purchase costs which can be difficult to justify to airline customers.
For composites to be used more widely in the civil aerospace sector, the costs of materials and fabricated components must be reduced to compete more effectively with metallic components. It is worth noting, however, that advanced aluminium alloys, such as aluminiumlithium alloys, can be up to 10 times the cost of conventional metallic alloys so that composite materials can compete very favourably on cost with many of these advanced metallics. For these reasons, the civil aerospace usage of composite materials in Europe has concentrated on the standard, cheaper, carbon fibres but has frequently accepted using the slightly more expensive but tougher modified epoxy resin matrices. The higher cost of the new ultra-high performance carbon fibres, is perhaps, difficult to justify in the civil aerospace market at present. In the work described in this paper, an attempt has been made to develop hybrid composites with lower material costs than the ultra-high performance carbon fibre composites, but which maintain their exceptional mechanical properties. The novel step taken in developing these hybrid materials has been to recognize that, in a multidirectional laminated composite, the layers with fibres at angles other than the principal stress directions are not well utilized in terms of their load-carrying potential. There is thus little point in using ultra-high performance carbon fibres in these layers. The layers that are aligned
0010-4361/94/040273-08 @ 1994 Butterworth-Heinemann Ltd COMPOSITES. VOLUME 25. NUMBER 4. 1994 273
along the principal stress directions clearly would benefit from the use of the highest performing carbon fibres available, which would be expected to impart the best mechanical properties to the laminate. Thus, in this work, laminates were made with hybrid fibre combinations: ultra-high performance carbon fibres in the principal stress direction and cheaper standard fibres in the other layers. For comparison, non-hybrid panels were also made from the standard and high performance fibres alone. In all cases the same resin matrix material was employed so that no fabrication difficulties were introduced. Mechanical properties were measured and compared on these three material combinations. To make this a useful exercise from the design viewpoint, as well as measuring the standard 'data sheet' mechanical properties, composite design critical properties were also measured. These included notched compressive strength, compression strength after impact and tensile strength after impact. Results are compared and discussed, and recommendations on the usage of these materials are made. This work is the subject of a patent application ~.
EXPERIMENTAL DETAILS Materials and lay-ups
For this work T300 and T800 Toray fibres were chosen, the former being a standard high strength fibre and the latter a new ultra-high performance fibre. The T800 fibre currently costs 2'/2 times the price of the T300 fibre. The resin matrix selected was the Ciba-Geigy toughened epoxy system BSL924C. This particular resin is known to have good processing properties as a matrix resin and convey good all-round properties to the composite. There is, however, a significant added cost incurred through pre-impregnating the fibre with a resin system such as this into a prepreg, which is common for both materials. Thus, at current prices, the T800/924 prepreg material is 40% more expensive than the T300/924 material. Balanced and symmetrical laminates were laid up from manufacturers' pre-impregnated plies (prepreg), each 0.125 mm thick. The unidirectional (UD) prepregs used for this investigation were the ultra-high performance Toray T800 carbon fibre impregnated with the toughened epoxy Ciba-Geigy 924 resin and the standard Toray T300 fibre in the same resin.
170°C and postcured. The high quality of the moulded panels was confirmed using an ultrasonic C-scanning facility, after which they were stored in an environment at 65% relative humidity (RH) until equilibrium moisture content was reached. Test methods
All the panels, apart from those destined for impact testing, were cut up using a milling machine into testpieces as recommended in the C R A G standard 2. The panels for the residual property tests were first impacted using the C R A G recommended method 2 and then sectioned as described above. Metal end-tabs were bonded onto coupons for all tests except compression after impact. Plain (i.e., unnotched) tension tests were carried out on the 8-ply UD panels, the 8-ply + 45 ° panels and the three 16-ply laminates. Coupons were 250 mm long and 20 mm wide and were tested according to the C R A G standard. Failure stresses were recorded and in some cases strain gauges were used to permit modulus to be measured. Plain compression tests were carried out on the three laminates using coupons of the same size, but an anti-buckling guide was employed to prevent macro-instability 2. Notched tension and compression tests were performed on the three multi-angular laminates, the former using coupons 250 mm long and 50 mm wide and the latter using coupons 250 mm long and 25 mm wide and using the same anti-buckling guide as used for the plain compression tests. In all cases, the coupons had 5 mm circular holes drilled at their centres. The impact tests involved clamping the as-moulded panels in a circular steel frame 100 mm in diameter. The impacts were performed using a Rosand instrumented drop-weight machine to levels of either 3.5 or 7 J. The impacts were repeated at 55 mm intervals across the panels, which were then cut up as described above. The impacted panels were scanned by through-transmission ultrasonics both before and after the impacts to permit the extent of the damage to be assessed. Some coupons were also X-rayed using conventional shadow radiography with a zinc iodide penetrant. These latter coupons were discarded after the X-radiography.
A standard 16-ply lay-up was used for this work with a ply stacking sequence of [(+ 45 °, 0°2)2]s. Laminates were made up in the following materials:
Residual property tests were carried out on impacted coupons 250 mm long and 50 mm wide cut from the impacted panels. The residual compression tests were performed using an anti-buckling guide to prevent macro-instability 3.
• • •
Failure stresses were determined using a minimum of five coupons, except for the notched tension tests for which three coupons were used.
all T800/924; all T300/924; and mixed fibre hybrids with 0 ° plies of T800/924 and 45 ° plies of T300/924.
Several panels of each material combination were made. In addition, panels 8 plies thick (with a moulded thickness of I mm) were made up from each material in both UD and all angle-ply ([-4-45 ° , - / + 45°]s) lay-ups. These were used for the determination of basic mechanical properties. The panels were cured in an autoclave to the manufacturer's recommended cure schedule to a temperature of
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RESULTS UD and Ez45 ° tests
The results of the tests on the plain unidirectional and + 45 ° coupons are summarized in Table 1. It is clear that the T800 fibre imparts much improved tensile strength and modulus to the UD material compared with the standard T300-reinforced material. Major Poisson's
Table 1. Basic mechanical properties f r o m UD and + / - 4 5 standard deviations)
° tensile tests (figures in brackets are
T300/924
T800/924
UD tensile strength (MPa) UD tensile modulus (GPa) Major Poisson's ratio
1432 (5) 138 (4) 0.328 (0.015)
2081 (132) 181 (24) 0.366 (0.017)
Shear strength (MPa) Shear modulus, Gu (GPa) + / - 45 ° Poisson's ratio
101 (2) 4.30 (0.26) 0.870 (0.015)
103 (1) 4.37 (0.19) 0.888 (0.035)
Table 2. Basic mechanical properties for the plain and notched laminates (figures in brackets are standard deviations)
T300/924
Hybrid
T800/924
Tensile strength (MPa) Tensile modulus (GPa) Notched tensile strength (MPa)
939 (17) 82.1 (7.7) 474 (14)
1406 (37) 136 (4) 724 (41)
1413 (37) 144 (6) 762 (25)
Compressive strength (M Pa) Notched compressive strength (M Pa)
813 (99) 475 (24)
809 (51) 389 (20)
853 (54) 382 (26)
Table 3. Residual strength data after d r o p - w e i g h t impact ( M P a ) (figures in brackets are standard deviations)
T300/924
Hybrid
T800/924
Tension after 7 J
481 (68)
813 (141)
906 (39)
Compression after 3.5 J Compression after 7 J
325 (13) 274 (7)
394 (26) 284 (22)
397 (47) 293 (18)
ratios for the two materials were very similar. The tests on the ± 45 ° laminates yielded shear data, which are also presented in Table 1. The shear properties for the two materials were very similar, primarily due to their having the same epoxy matrix material. Plain and notched laminate tests
The results of the tensile and compressive tests on the three plain and notched laminates are summarized in Table 2. Clearly the T800 and hybrid materials have significantly greater tensile strength and stiffness than the standard T300 material. Indeed both the hybrid and T800 materials showed similar tensile moduli and tensile strengths, both plain and notched. In compression, all three materials yielded similar plain strengths. When notched, however, the standard material performed better than the T800 or hybrid materials, in keeping with other work that has shown that the newer carbon fibres, with very high tensile strengths but small fibre diameters, are often poorer in notched compression tests4.t Residual property tests after impact
The results of the residual strength tests after dropweight impact are presented in Table 3 and shown schematically in Fig. 1 (note: straight lines have been used to link the points in the figure to show trends only and do not infer behaviour will follow these lines). In compression, the hybrid and T800/924 materials retained higher residual strengths than the T300/924 material after 3.5 J impact, but after 7 J impact all three materials
1,500 1,300
[]
1,100
HYBRID Tension
~ T300/924 Tension
900
~ T800/924
Compression
700
~ HYBRID
Compression
[~ T300/924
500
Compression
30(3 10C
0
3.5
7
Impact Energy J Fig. 1 Residual strength after impact for 924 e p o x y - b a s e d [ + / - 4 5 °, 0°2] 1 6 - p l y laminates (hybrid = T800 0 ° layers, T300 45 ° layers)
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had similar residual compressive strengths. After 7 J impact, the hybrid and the T800-reinforced materials were substantially stronger in tension than the T300 ~924 material, as observed with the undamaged laminates, but the T800/924 showed slightly greater residual strength than the hybrid material.
Damage studies X-ray shadow radiography, ultrasonic C-scanning and visual photography were used to assess the levels of damage after impact and post failure in the three lami-
I
%1::¸ ¸
::i:!ii¸¸~I,~
!i i i ~
~
k~
Fig. 3 Failed notched compression coupons. Left to right: TSO0/ 924, T300/924, hybrid
Fig. 2 Failed plain compression coupons. Left to right: T800/924, T300/924, hybrid
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nates. Photographs of typical coupons after mechanical testing to failure are shown in Figs 2-6 for each of the three laminates. Fig. 2 shows typical failed coupons after plain compression testing, all three laminates exhibiting similar types of failure. In Fig. 3, typical notched compression failures are shown, with the T300/924 laminate exhibiting a slightly more extensive damage zone than the other two materials. Fig. 4 shows typical coupons after residual compression testing. It is clear that less damage occurred in the hybrid and T800/924 materials compared with the standard T300/924. The typical failed notched tensile coupons and tension after impact coupons presented in Figs 5 and 6 clearly show up a different failure mode in the T300/924 material than in the other two materials. The T300/924 failed fairly cleanly across
J
The basic tensile properties of the multi-angular laminates reflected the UD tensile behaviour with the T800/ 924 material consistently outperforming the T300/924, although the hybrid laminate just about matched the T800 material in all tensile properties (Table 2). The notched tension properties were particularly interesting, with the hybrid being 53% stronger than the standard T300/924 material and the T800/924 material being 60% stronger. Observation of the failed specimens (Fig. 5) helped explain this major improvement, since clearly the hybrid and T800 laminates failed in a different mode from the standard material. In the hybrid and T800 materials failure occurred at 45 ° to the test direction, but the standard material failed in a more brittle transverse mode and was clearly more notch-sensitive. The reason for this is unclear, but must be related to the sensitivities of the materials to delamination and longitudinal split initiation and growth. Materials that have increased tendency to splitting along the fibres are more able to blunt the stress concentrations at notches, leading to higher notched tensile strengths. In compression, the newer materials faired less well; all three laminates had similar plain compression strengths, but the standard material was stronger than either the hybrid or T800 material when notched (Table 2). Visual examination of the failures yielded no explanation for this behaviour (Fig. 3). However, other work 4,5 has found that the newer high performance carbon fibres often lead to poorer notched compressive strength in -laminates. This is attributed to the smaller diameter of the fibres, typically 5 gm rather than 7-8 I~m, which is assumed to
L .......................
Fig. 4 Failed compression after impact coupons. Left to right: T800/924, T300/924, hybrid
I
the width without the extensive 45 ° cracking apparent in the hybrid and T800/924 materials. Typical ultrasonic C-scans of areas of the three materials impacted to 7 J are shown in Fig. 7. This clearly shows that the T800/924 material had the smallest damage area and the T300/924 the largest, with the hybrid material in between. This trend is confirmed by the X-radiographs presented in Fig. 8, in which the T300/924 shows extensive delamination but also more severe damage at the impact site in the form of longitudinal splits and transverse damage. X-radiographs of typical coupons impacted to 3.5 J are shown in Fig. 9. In these, slightly different behaviour is apparent, with the hybrid and T800/924 materials showing similar levels of damage but the T300/924 material once again exhibiting more extensive delamination and damage at the impact site. DISCUSSION
As expected, the high performance Toray T800 fibre imparted very high basic tensile properties to laminates using this fibre (Table 1). Tensile strength and modulus were almost 50% greater than for the standard T300reinforced laminates, with no loss of shear properties. Clearly, in terms of these 'data sheet' properties, the T800/924 material is a very high performing composite which could be the automatic selection for high performance applications where its high cost is justified.
Fig. 5 Failed notched tension coupons. Left to right: T800/924, T300/924, hybrid
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tion and a large central damage region which clearly involved considerable fibre damage. The T800/924 material, however, showed a smaller area of damage, the central part being little greater than at 3.5 J impact, although there was more delamination visible. The hybrid material exhibited behaviour intermediate to the other two materials, with greater levels of delamination and concentrated fibre damage than the T800/924 material, but the latter was still considerably less than in the standard material. In impact, the greater stiffness and strength of the T800 fibres implies that increased incident loads are needed to deflect the material under impact and thus greater strain
Fig. 6 Failed tension after impact coupons. Left to right: T800/924, T300/924, hybrid
lead to microbuckling of the fibres and thus failure at lower applied stresses. This is possibly due to the applied stresses for column buckling of the fibres being reduced, but recent work has revealed that it is more likely be a result of the greater fibre waviness in moulded materials using smaller diameter fibres 6,7. However, the reduced compressive strength in these advanced materials is usually also apparent in unnotched compressive tests, but surprisingly, in the current work, this was not the case. After impact, the ultrasonic scans clearly showed that the T800 material had a much smaller area of damage compared with the T300/924 material, with the hybrid material lying between the two. Of particular note, the TS00/ 924 showed little of the extensive back-face spalling observed in the standard material, with the hybrid material exhibiting intermediate behaviour. The typical Xradiographs in Figs 8 and 9 clearly show these differences in overall area, but also provide information on the detailed damage produced during impact. After 3.5 J impact, the standard material showed quite extensive delamination plus some 0° splitting and a central, concentrated region of damage in which there was undoubtedly considerable fibre damage. The other two materials showed different behaviour, with reduced amounts of delamination and splitting, and little of the concentrated fibre damage apparent in the standard material. After 7 J impact, the damage in the standard material was considerable, with extensive back-face spalling, delamina-
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Fig. 7 Ultrasonic C-scans after 7 J impact: (a) T300/924 material, (b) hybrid material and (c) T800/924 material
Fig. 8
X-radiographs after 7 J impact ( x 1.3). Left to right: T300/924, hybrid, T800/924
Fig. 9
X-radiographs after 3.5 J impact. Left to right: T300/924 ( x 2), hybrid ( x 2), T800/924 ( x 1 3)
energies can be absorbed before the fibres fracture. Clearly then, with the same matrix material, the laminates with T800 fibre reinforcement would be expected to sustain less damage, particularly fibre damage, than the T300-reinforced material, as was observed experimentally. The hybrid material, although containing only 50% of the T800 fibres, was shown to possess plain and notched mechanical properties very similar to those of the all T800-reinforced material, reflecting the underusage of the angle plies in these loading conditions. In impact, however, where the angles of the fibres is less important than the stiffness and strength in determining impact response, the hybrid material exhibited damage levels roughly between those observed for the other two materials, very much in keeping with the ratio of 50% TS00 and 50% T300 fibres.
Despite the observations of damage just described, the materials did not always respond in terms of residual strength as would be expected from the type and extent of damage observed. In general, the hybrid material behaved much better than the damage areas would have suggested, generally performing almost as well as the T800/924 material. The T300/924 material showed much lower residual tensile strengths than the other two materials, but in compression the strength was perhaps greater than expected from the damage observed. Clearly damage types as well as the overall extent of damage are important. The tensile properties after impact very much paralleled the notched tensile behaviour, with the hybrid and T800 materials giving much greater residual strengths than the
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standard material; values were 69% and 89% higher respecti,vely! Again, this difference appeared to be associated with a different failure mode in the T800 and hybrid materials, the standard material exhibiting more brittle behaviour with relatively clean crack growth across the width. In addition, the much greater levels of concentrated fibre damage observed in the standard material after impact clearly resulted in reduced tensile strength, since fibre damage is probably the most important damage feature as far as tensile response is concerned. In compression, the standard material still exhibited poorer performance than the other two materials, but the differences were much less. After 3.5 J impact, the hybrid material was 21% stronger than the standard material and the T800/924 was 22% stronger. This is certainly in keeping with the damage observations after 3.5 J impact which showed smaller damage areas in the hybrid and T 8 0 0 materials compared with the standard material. After 7 J impact, however, the hybrid material was just 4% stronger than the standard material and the all T800 material just 7% stronger. The radiographs after 7 J showed that all three materials had fairly similar delamination areas, although there was an increasing level of concentrated central fibre damage and spalling along the rear-face 45 ° layer in going to the hybrid and standard materials. In compression, delamination is clearly a more important feature than fibre damage, since it leads to premature ply instability before the true strength of the fibres can be realized. Thus although the damage in the standard material was more concentrated with more fibre damage, the residual compressive strengths were very similar for all three materials. This may be because the extent of delamination was similar in all three materials, probably being controlled by the tough 924 epoxy system. At present price levels, the ultra-high performance fibres are 21/2 times the cost of standard carbon fibres, although prepregs show just a 40% difference in cost. The philosophy of mixing standard and ultra-high performance carbon fibres, as described in this work, could lead to cost reductions of about 17% based on prepreg costs, but if alternative fabrication methods were used the savings could be much greater.
stress directions has been developed. A wide range of mechanical properties have been measured for this material and, for comparison, also on the base materials. This material has been shown t o have a similar mechanical performance to that of the ultra-high performance carbon fibre base material tested, a performance generally much improved over that of the standard fibre composite. This has been achieved together with a significant cost saving through the use of lower cost standard fibres in the secondary, lower stressed layers. A CKNO WL ED GEMENTS The authors acknowledge financial support given by D Science (Air) 1, Ministry of Defence, and DTI Air Division. The authors also thank Mr A. Desport for performing some of the tests, Mr J. Coleman for fabricating the panels tested in this work and Mr~B. Clarke for performing many of the ultrasonic scans. © Crown Copyright 1993. REFERENCES 1 Curtis, P.T. and Browne, M. 'Hybrid composite materials' Patent Application 01/P2131 (1992) 2 Curtis, P.T. 'CRAG test methods for the measurement of the engineering propertiesof fibre reinforcedplastics' RAE Tech Rep 88012 (Royal AerospaceEstablishment, Farnborough, 1988) 3 Dorey,G., Sigety, P., Stellbrink, K. and CHart, W.G.J. 'Garteur/ TP-007 -- impact damage of a carbon fibre composite laminate' RAE Tech Rep 84049 (Royal Aircraft Establishment, Farnborough, 1984) Curtis, P.T. 'An investigation of the mechanical properties of improved carbon fibre composite materials' RAE Tech Rep 86021 (Royal Aircraft Establishment, Farnborough, 1986) 5 Curtis, P.T. 'An investigation of the mechanical properties of improved carbon fibre compositematerials' J Composite Mater 21 (1987) p 1118 6 Jell', P.M. and Fleck, N.A. ~Compression failure mechanisms in Unidirectionalcomposites'J Composite Mater 27 (1992) 7 Soutis, C., Curtis, P.T. and Fleck, N.A. "Compressivefailure of notched carbon fibrecomposites'Proc Roy Soc London A440(1993) p 241 AUTHORS
CONCL USIONS A hybrid composite material based on mixtures of ultrahigh performance carbon fibres in the principal stress directions and cheaper standard fibres in the secondary
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The authors are with the Materials and Structures Department, Defence Research Agency, Farnborough, Hants GU14 6TD, UK. Correspondence should be addressed to Dr Curtis.
For composites to be used more widely in the civil aerospace sector, the costs of materials and fabricated components must be reduced to compete more effectively with metallic components. It is worth noting, however, that advanced aluminium alloys, such as aluminiumlithium alloys, can be up to 10 times the cost of conventional metallic alloys so that composite materials can compete very favourably on cost with many of these advanced metallics. For these reasons, the civil aerospace usage of composite materials in Europe has concentrated on the standard, cheaper, carbon fibres but has frequently accepted using the slightly more expensive but tougher modified epoxy resin matrices. The higher cost of the new ultra-high performance carbon fibres, is perhaps, difficult to justify in the civil aerospace market at present. In the work described in this paper, an attempt has been made to develop hybrid composites with lower material costs than the ultra-high performance carbon fibre composites, but which maintain their exceptional mechanical properties. The novel step taken in developing these hybrid materials has been to recognize that, in a multidirectional laminated composite, the layers with fibres at angles other than the principal stress directions are not well utilized in terms of their load-carrying potential. There is thus little point in using ultra-high performance carbon fibres in these layers. The layers that are aligned
0010-4361/94/040273-08 @ 1994 Butterworth-Heinemann Ltd COMPOSITES. VOLUME 25. NUMBER 4. 1994 273
along the principal stress directions clearly would benefit from the use of the highest performing carbon fibres available, which would be expected to impart the best mechanical properties to the laminate. Thus, in this work, laminates were made with hybrid fibre combinations: ultra-high performance carbon fibres in the principal stress direction and cheaper standard fibres in the other layers. For comparison, non-hybrid panels were also made from the standard and high performance fibres alone. In all cases the same resin matrix material was employed so that no fabrication difficulties were introduced. Mechanical properties were measured and compared on these three material combinations. To make this a useful exercise from the design viewpoint, as well as measuring the standard 'data sheet' mechanical properties, composite design critical properties were also measured. These included notched compressive strength, compression strength after impact and tensile strength after impact. Results are compared and discussed, and recommendations on the usage of these materials are made. This work is the subject of a patent application ~.
EXPERIMENTAL DETAILS Materials and lay-ups
For this work T300 and T800 Toray fibres were chosen, the former being a standard high strength fibre and the latter a new ultra-high performance fibre. The T800 fibre currently costs 2'/2 times the price of the T300 fibre. The resin matrix selected was the Ciba-Geigy toughened epoxy system BSL924C. This particular resin is known to have good processing properties as a matrix resin and convey good all-round properties to the composite. There is, however, a significant added cost incurred through pre-impregnating the fibre with a resin system such as this into a prepreg, which is common for both materials. Thus, at current prices, the T800/924 prepreg material is 40% more expensive than the T300/924 material. Balanced and symmetrical laminates were laid up from manufacturers' pre-impregnated plies (prepreg), each 0.125 mm thick. The unidirectional (UD) prepregs used for this investigation were the ultra-high performance Toray T800 carbon fibre impregnated with the toughened epoxy Ciba-Geigy 924 resin and the standard Toray T300 fibre in the same resin.
170°C and postcured. The high quality of the moulded panels was confirmed using an ultrasonic C-scanning facility, after which they were stored in an environment at 65% relative humidity (RH) until equilibrium moisture content was reached. Test methods
All the panels, apart from those destined for impact testing, were cut up using a milling machine into testpieces as recommended in the C R A G standard 2. The panels for the residual property tests were first impacted using the C R A G recommended method 2 and then sectioned as described above. Metal end-tabs were bonded onto coupons for all tests except compression after impact. Plain (i.e., unnotched) tension tests were carried out on the 8-ply UD panels, the 8-ply + 45 ° panels and the three 16-ply laminates. Coupons were 250 mm long and 20 mm wide and were tested according to the C R A G standard. Failure stresses were recorded and in some cases strain gauges were used to permit modulus to be measured. Plain compression tests were carried out on the three laminates using coupons of the same size, but an anti-buckling guide was employed to prevent macro-instability 2. Notched tension and compression tests were performed on the three multi-angular laminates, the former using coupons 250 mm long and 50 mm wide and the latter using coupons 250 mm long and 25 mm wide and using the same anti-buckling guide as used for the plain compression tests. In all cases, the coupons had 5 mm circular holes drilled at their centres. The impact tests involved clamping the as-moulded panels in a circular steel frame 100 mm in diameter. The impacts were performed using a Rosand instrumented drop-weight machine to levels of either 3.5 or 7 J. The impacts were repeated at 55 mm intervals across the panels, which were then cut up as described above. The impacted panels were scanned by through-transmission ultrasonics both before and after the impacts to permit the extent of the damage to be assessed. Some coupons were also X-rayed using conventional shadow radiography with a zinc iodide penetrant. These latter coupons were discarded after the X-radiography.
A standard 16-ply lay-up was used for this work with a ply stacking sequence of [(+ 45 °, 0°2)2]s. Laminates were made up in the following materials:
Residual property tests were carried out on impacted coupons 250 mm long and 50 mm wide cut from the impacted panels. The residual compression tests were performed using an anti-buckling guide to prevent macro-instability 3.
• • •
Failure stresses were determined using a minimum of five coupons, except for the notched tension tests for which three coupons were used.
all T800/924; all T300/924; and mixed fibre hybrids with 0 ° plies of T800/924 and 45 ° plies of T300/924.
Several panels of each material combination were made. In addition, panels 8 plies thick (with a moulded thickness of I mm) were made up from each material in both UD and all angle-ply ([-4-45 ° , - / + 45°]s) lay-ups. These were used for the determination of basic mechanical properties. The panels were cured in an autoclave to the manufacturer's recommended cure schedule to a temperature of
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RESULTS UD and Ez45 ° tests
The results of the tests on the plain unidirectional and + 45 ° coupons are summarized in Table 1. It is clear that the T800 fibre imparts much improved tensile strength and modulus to the UD material compared with the standard T300-reinforced material. Major Poisson's
Table 1. Basic mechanical properties f r o m UD and + / - 4 5 standard deviations)
° tensile tests (figures in brackets are
T300/924
T800/924
UD tensile strength (MPa) UD tensile modulus (GPa) Major Poisson's ratio
1432 (5) 138 (4) 0.328 (0.015)
2081 (132) 181 (24) 0.366 (0.017)
Shear strength (MPa) Shear modulus, Gu (GPa) + / - 45 ° Poisson's ratio
101 (2) 4.30 (0.26) 0.870 (0.015)
103 (1) 4.37 (0.19) 0.888 (0.035)
Table 2. Basic mechanical properties for the plain and notched laminates (figures in brackets are standard deviations)
T300/924
Hybrid
T800/924
Tensile strength (MPa) Tensile modulus (GPa) Notched tensile strength (MPa)
939 (17) 82.1 (7.7) 474 (14)
1406 (37) 136 (4) 724 (41)
1413 (37) 144 (6) 762 (25)
Compressive strength (M Pa) Notched compressive strength (M Pa)
813 (99) 475 (24)
809 (51) 389 (20)
853 (54) 382 (26)
Table 3. Residual strength data after d r o p - w e i g h t impact ( M P a ) (figures in brackets are standard deviations)
T300/924
Hybrid
T800/924
Tension after 7 J
481 (68)
813 (141)
906 (39)
Compression after 3.5 J Compression after 7 J
325 (13) 274 (7)
394 (26) 284 (22)
397 (47) 293 (18)
ratios for the two materials were very similar. The tests on the ± 45 ° laminates yielded shear data, which are also presented in Table 1. The shear properties for the two materials were very similar, primarily due to their having the same epoxy matrix material. Plain and notched laminate tests
The results of the tensile and compressive tests on the three plain and notched laminates are summarized in Table 2. Clearly the T800 and hybrid materials have significantly greater tensile strength and stiffness than the standard T300 material. Indeed both the hybrid and T800 materials showed similar tensile moduli and tensile strengths, both plain and notched. In compression, all three materials yielded similar plain strengths. When notched, however, the standard material performed better than the T800 or hybrid materials, in keeping with other work that has shown that the newer carbon fibres, with very high tensile strengths but small fibre diameters, are often poorer in notched compression tests4.t Residual property tests after impact
The results of the residual strength tests after dropweight impact are presented in Table 3 and shown schematically in Fig. 1 (note: straight lines have been used to link the points in the figure to show trends only and do not infer behaviour will follow these lines). In compression, the hybrid and T800/924 materials retained higher residual strengths than the T300/924 material after 3.5 J impact, but after 7 J impact all three materials
1,500 1,300
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1,100
HYBRID Tension
~ T300/924 Tension
900
~ T800/924
Compression
700
~ HYBRID
Compression
[~ T300/924
500
Compression
30(3 10C
0
3.5
7
Impact Energy J Fig. 1 Residual strength after impact for 924 e p o x y - b a s e d [ + / - 4 5 °, 0°2] 1 6 - p l y laminates (hybrid = T800 0 ° layers, T300 45 ° layers)
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had similar residual compressive strengths. After 7 J impact, the hybrid and the T800-reinforced materials were substantially stronger in tension than the T300 ~924 material, as observed with the undamaged laminates, but the T800/924 showed slightly greater residual strength than the hybrid material.
Damage studies X-ray shadow radiography, ultrasonic C-scanning and visual photography were used to assess the levels of damage after impact and post failure in the three lami-
I
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Fig. 3 Failed notched compression coupons. Left to right: TSO0/ 924, T300/924, hybrid
Fig. 2 Failed plain compression coupons. Left to right: T800/924, T300/924, hybrid
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nates. Photographs of typical coupons after mechanical testing to failure are shown in Figs 2-6 for each of the three laminates. Fig. 2 shows typical failed coupons after plain compression testing, all three laminates exhibiting similar types of failure. In Fig. 3, typical notched compression failures are shown, with the T300/924 laminate exhibiting a slightly more extensive damage zone than the other two materials. Fig. 4 shows typical coupons after residual compression testing. It is clear that less damage occurred in the hybrid and T800/924 materials compared with the standard T300/924. The typical failed notched tensile coupons and tension after impact coupons presented in Figs 5 and 6 clearly show up a different failure mode in the T300/924 material than in the other two materials. The T300/924 failed fairly cleanly across
J
The basic tensile properties of the multi-angular laminates reflected the UD tensile behaviour with the T800/ 924 material consistently outperforming the T300/924, although the hybrid laminate just about matched the T800 material in all tensile properties (Table 2). The notched tension properties were particularly interesting, with the hybrid being 53% stronger than the standard T300/924 material and the T800/924 material being 60% stronger. Observation of the failed specimens (Fig. 5) helped explain this major improvement, since clearly the hybrid and T800 laminates failed in a different mode from the standard material. In the hybrid and T800 materials failure occurred at 45 ° to the test direction, but the standard material failed in a more brittle transverse mode and was clearly more notch-sensitive. The reason for this is unclear, but must be related to the sensitivities of the materials to delamination and longitudinal split initiation and growth. Materials that have increased tendency to splitting along the fibres are more able to blunt the stress concentrations at notches, leading to higher notched tensile strengths. In compression, the newer materials faired less well; all three laminates had similar plain compression strengths, but the standard material was stronger than either the hybrid or T800 material when notched (Table 2). Visual examination of the failures yielded no explanation for this behaviour (Fig. 3). However, other work 4,5 has found that the newer high performance carbon fibres often lead to poorer notched compressive strength in -laminates. This is attributed to the smaller diameter of the fibres, typically 5 gm rather than 7-8 I~m, which is assumed to
L .......................
Fig. 4 Failed compression after impact coupons. Left to right: T800/924, T300/924, hybrid
I
the width without the extensive 45 ° cracking apparent in the hybrid and T800/924 materials. Typical ultrasonic C-scans of areas of the three materials impacted to 7 J are shown in Fig. 7. This clearly shows that the T800/924 material had the smallest damage area and the T300/924 the largest, with the hybrid material in between. This trend is confirmed by the X-radiographs presented in Fig. 8, in which the T300/924 shows extensive delamination but also more severe damage at the impact site in the form of longitudinal splits and transverse damage. X-radiographs of typical coupons impacted to 3.5 J are shown in Fig. 9. In these, slightly different behaviour is apparent, with the hybrid and T800/924 materials showing similar levels of damage but the T300/924 material once again exhibiting more extensive delamination and damage at the impact site. DISCUSSION
As expected, the high performance Toray T800 fibre imparted very high basic tensile properties to laminates using this fibre (Table 1). Tensile strength and modulus were almost 50% greater than for the standard T300reinforced laminates, with no loss of shear properties. Clearly, in terms of these 'data sheet' properties, the T800/924 material is a very high performing composite which could be the automatic selection for high performance applications where its high cost is justified.
Fig. 5 Failed notched tension coupons. Left to right: T800/924, T300/924, hybrid
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tion and a large central damage region which clearly involved considerable fibre damage. The T800/924 material, however, showed a smaller area of damage, the central part being little greater than at 3.5 J impact, although there was more delamination visible. The hybrid material exhibited behaviour intermediate to the other two materials, with greater levels of delamination and concentrated fibre damage than the T800/924 material, but the latter was still considerably less than in the standard material. In impact, the greater stiffness and strength of the T800 fibres implies that increased incident loads are needed to deflect the material under impact and thus greater strain
Fig. 6 Failed tension after impact coupons. Left to right: T800/924, T300/924, hybrid
lead to microbuckling of the fibres and thus failure at lower applied stresses. This is possibly due to the applied stresses for column buckling of the fibres being reduced, but recent work has revealed that it is more likely be a result of the greater fibre waviness in moulded materials using smaller diameter fibres 6,7. However, the reduced compressive strength in these advanced materials is usually also apparent in unnotched compressive tests, but surprisingly, in the current work, this was not the case. After impact, the ultrasonic scans clearly showed that the T800 material had a much smaller area of damage compared with the T300/924 material, with the hybrid material lying between the two. Of particular note, the TS00/ 924 showed little of the extensive back-face spalling observed in the standard material, with the hybrid material exhibiting intermediate behaviour. The typical Xradiographs in Figs 8 and 9 clearly show these differences in overall area, but also provide information on the detailed damage produced during impact. After 3.5 J impact, the standard material showed quite extensive delamination plus some 0° splitting and a central, concentrated region of damage in which there was undoubtedly considerable fibre damage. The other two materials showed different behaviour, with reduced amounts of delamination and splitting, and little of the concentrated fibre damage apparent in the standard material. After 7 J impact, the damage in the standard material was considerable, with extensive back-face spalling, delamina-
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Fig. 7 Ultrasonic C-scans after 7 J impact: (a) T300/924 material, (b) hybrid material and (c) T800/924 material
Fig. 8
X-radiographs after 7 J impact ( x 1.3). Left to right: T300/924, hybrid, T800/924
Fig. 9
X-radiographs after 3.5 J impact. Left to right: T300/924 ( x 2), hybrid ( x 2), T800/924 ( x 1 3)
energies can be absorbed before the fibres fracture. Clearly then, with the same matrix material, the laminates with T800 fibre reinforcement would be expected to sustain less damage, particularly fibre damage, than the T300-reinforced material, as was observed experimentally. The hybrid material, although containing only 50% of the T800 fibres, was shown to possess plain and notched mechanical properties very similar to those of the all T800-reinforced material, reflecting the underusage of the angle plies in these loading conditions. In impact, however, where the angles of the fibres is less important than the stiffness and strength in determining impact response, the hybrid material exhibited damage levels roughly between those observed for the other two materials, very much in keeping with the ratio of 50% TS00 and 50% T300 fibres.
Despite the observations of damage just described, the materials did not always respond in terms of residual strength as would be expected from the type and extent of damage observed. In general, the hybrid material behaved much better than the damage areas would have suggested, generally performing almost as well as the T800/924 material. The T300/924 material showed much lower residual tensile strengths than the other two materials, but in compression the strength was perhaps greater than expected from the damage observed. Clearly damage types as well as the overall extent of damage are important. The tensile properties after impact very much paralleled the notched tensile behaviour, with the hybrid and T800 materials giving much greater residual strengths than the
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standard material; values were 69% and 89% higher respecti,vely! Again, this difference appeared to be associated with a different failure mode in the T800 and hybrid materials, the standard material exhibiting more brittle behaviour with relatively clean crack growth across the width. In addition, the much greater levels of concentrated fibre damage observed in the standard material after impact clearly resulted in reduced tensile strength, since fibre damage is probably the most important damage feature as far as tensile response is concerned. In compression, the standard material still exhibited poorer performance than the other two materials, but the differences were much less. After 3.5 J impact, the hybrid material was 21% stronger than the standard material and the T800/924 was 22% stronger. This is certainly in keeping with the damage observations after 3.5 J impact which showed smaller damage areas in the hybrid and T 8 0 0 materials compared with the standard material. After 7 J impact, however, the hybrid material was just 4% stronger than the standard material and the all T800 material just 7% stronger. The radiographs after 7 J showed that all three materials had fairly similar delamination areas, although there was an increasing level of concentrated central fibre damage and spalling along the rear-face 45 ° layer in going to the hybrid and standard materials. In compression, delamination is clearly a more important feature than fibre damage, since it leads to premature ply instability before the true strength of the fibres can be realized. Thus although the damage in the standard material was more concentrated with more fibre damage, the residual compressive strengths were very similar for all three materials. This may be because the extent of delamination was similar in all three materials, probably being controlled by the tough 924 epoxy system. At present price levels, the ultra-high performance fibres are 21/2 times the cost of standard carbon fibres, although prepregs show just a 40% difference in cost. The philosophy of mixing standard and ultra-high performance carbon fibres, as described in this work, could lead to cost reductions of about 17% based on prepreg costs, but if alternative fabrication methods were used the savings could be much greater.
stress directions has been developed. A wide range of mechanical properties have been measured for this material and, for comparison, also on the base materials. This material has been shown t o have a similar mechanical performance to that of the ultra-high performance carbon fibre base material tested, a performance generally much improved over that of the standard fibre composite. This has been achieved together with a significant cost saving through the use of lower cost standard fibres in the secondary, lower stressed layers. A CKNO WL ED GEMENTS The authors acknowledge financial support given by D Science (Air) 1, Ministry of Defence, and DTI Air Division. The authors also thank Mr A. Desport for performing some of the tests, Mr J. Coleman for fabricating the panels tested in this work and Mr~B. Clarke for performing many of the ultrasonic scans. © Crown Copyright 1993. REFERENCES 1 Curtis, P.T. and Browne, M. 'Hybrid composite materials' Patent Application 01/P2131 (1992) 2 Curtis, P.T. 'CRAG test methods for the measurement of the engineering propertiesof fibre reinforcedplastics' RAE Tech Rep 88012 (Royal AerospaceEstablishment, Farnborough, 1988) 3 Dorey,G., Sigety, P., Stellbrink, K. and CHart, W.G.J. 'Garteur/ TP-007 -- impact damage of a carbon fibre composite laminate' RAE Tech Rep 84049 (Royal Aircraft Establishment, Farnborough, 1984) Curtis, P.T. 'An investigation of the mechanical properties of improved carbon fibre composite materials' RAE Tech Rep 86021 (Royal Aircraft Establishment, Farnborough, 1986) 5 Curtis, P.T. 'An investigation of the mechanical properties of improved carbon fibre compositematerials' J Composite Mater 21 (1987) p 1118 6 Jell', P.M. and Fleck, N.A. ~Compression failure mechanisms in Unidirectionalcomposites'J Composite Mater 27 (1992) 7 Soutis, C., Curtis, P.T. and Fleck, N.A. "Compressivefailure of notched carbon fibrecomposites'Proc Roy Soc London A440(1993) p 241 AUTHORS
CONCL USIONS A hybrid composite material based on mixtures of ultrahigh performance carbon fibres in the principal stress directions and cheaper standard fibres in the secondary
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The authors are with the Materials and Structures Department, Defence Research Agency, Farnborough, Hants GU14 6TD, UK. Correspondence should be addressed to Dr Curtis.