Comparison of the mechanical properties of CFRP made from round and non-round carbon fibres

COMPARISON OF THE MECHANICAL PROPERTIES OF CFRP MADE FROM R O U N D A N D N O N - R O U N D CARBON FIBRES

N. L. HANCOX

Materials Development Division, AERE, Harwell, Didcot O X I 10RA (Great Britain)

SUMMAR Y

The mechanical properties of epoxy resin matrix composites made from carbonfibres of two different shapes (round and dogbone) are compared. In no case was any significant difference found between the two materials and it is concluded that stress concentration between fibres does not depend strongly on fibre shape. The torsional work of fracture was measured and found to decrease rapidly with increasing fibre content because of the decreasing amount of resin available to undergo plastic deformation before failure. The non-round fibres allow a rather higher maximum volume loading to be achieved than the round ones.

INTRODUCTION

The cross-section of a carbon fibre depends on that of the precursor from which it is made. Most manufacturers use a circular polyacrylonitrile polymer as the starting material but Dauksys and Ray 1 reported the use of a non-round, dogbone shaped precursor. An advantage claimed for fibres made from this is that it is possible to achieve higher fibre volume fractions in a composite compared with those obtained with round fibres, because o f the way in which fibres pack together. As part of a study of carbon fibre manufacture carried out at Harwell, both round and dogbone shaped precursors have been converted into carbon fibres under a variety of conditions, and properties of the fibres and composites made from them determined. The present work was undertaken to establish if it was possible to get better fibre volume packing with the non-round fibres and if composite properties differed from those of a composite made with similar, round carbon fibres. The two types were prepared at Harwell from commercially available 179 Fibre Science and Technology (10) (1977)--© Applied Science Publishers Ltd, England, 1977 Printed in Great Britain

180

N.L. HANCOX

precursors, using a final heat treatment temperature of 1500°C. The precursors were Orlon, made by Du Pont, USA, from a polyacrylonitrile methacrylate copolymer spun to give a dogbone cross-section, and Euroacryl, made by ANIC, Italy, from a polyacrylonitrile copolymer and spun to give a round cross-section. Since the two precursors are spun in different ways they probably contain different types of defect. After conversion, but before making composites, the carbon fibres were subjected to a wet hypochlorite surface treatment to improve fibre resin bonding.

EXPERIMENTAL

A series of bars 110 mm x 7.5 mm x 1 mm and 110 mm × 6.35 mm x 6.35 mm with nominal volume loadings between 50~/o and 90V/o increasing in steps of 5~/o were prepared from either type of fibre by a wet lay-up technique. The epoxy resin matrix was made from 100 parts by weight liquid bisphenol A resin, 80 pbw methyl nadic anhydride hardener and 2 pbw benzyldimethylamine accelerator, cured for 2½ h at 120°C. Specimens were cut or ground to shape as required. The true volume loadings of the 60V/o and 90v/o materials were determined by an acid digestion technique and other loadings inferred from composite density measurements. Void contents were low, < V/o.

RESULTS AND DISCUSSION

As only small quantities ( ~ 100 g) of either fibre were available it was not possible to measure all the mechanical properties. Those that were determined were the flexural modulus at a span to depth ratio of 100:I, flexural strength at a span to depth ratio of 16:1, interlaminar shear strength and transverse flexurai strength, both at a span to depth ratio of 5:1, torsional shear modulus, strength and angular deflection at failure, measured by the method described by Hancox, 2 the stress concentration factor, fl, due to the presence of fibres in the matrix, the total work to cause failure in torsion, and an estimate of the work of fracture, G~tc, for torsional cracking. The results are shown in Figs. 1 to 10. The error bars represent the standard error of the mean. The flexural modulus and strength for either type of material is linearly related to the fibre volume loading over the volume range studied here. The straight lines in Figs. 1 and 2 have been made to pass through the points 0V/o, 3 GPa, and &/o, 71 MPa, respectively where these refer to the flexural modulus and strength of the pure resin. The highest volume loadings obtained were 85V/o for round fibres and 92~/o for non-round. The anomalously low strength readings in Fig. 2 are probably due to poor composites. Extrapolating the results to 100~/o fibres gives, for either type, a modulus of 172 GPa and a strength of 2040 MPa. If an approximate

181

COMPARISON OF CFRP FROM ROUND AND NON-ROUND FIBRES

160

o

non -round

•

round

150

1&O~ 130 120n

LL LU

110 100 -

'10 0

E 90L.. X

,-r

80-

70-

60-

5O

40 5O

I 55

I 60 Fibre

I 65

I 70

volume

Fig. 1.

I 75 loading

I 80

Vf

I 85

I 9O

V/o

Flexura] modulus.

correction is made for shear deflection, Maliick and Broutman, 3 over the entire range of volume loadings, the extrapolated modulus becomes 175 GPa. The results indicate that the type and distribution of defects in the two sorts of fibre are similar.

182

N.L.

HANCOX

o non-round • round

1800-

~/,

1700

n

1600

~r It uO

1500

JZ:

it..

1400

ol

t_

1300

x Q; It

12oo 11oo

1°°° t goo 50

I

55

I

I

1

I

I

60 65 70 75 80 Fibre votume loading Vf V/o

I

85

90

Fig. 2. Flexuralstrength. Single fibre properties were not measured but for 1500°C carbon fibre will most probably lie in the range 228-255 GPa for modulus and 2060--2760 MPa for strength, for a round fibre. For the non-round material the strength will be about the same and the modulus slightly less. On the basis of these figures the shortfall in modulus and strength, estimated from composite properties, is 25 per cent to 33 per cent for modulus and up to 24 per cent for strength. The shear strength measured by the short beam shear test, Fig. 3, goes through a gentle maximum between 70V/o and 85v/0 with no significant difference between results for the two types of fibre. Transverse flexurai strength, Fig. 4, decreases steadily with increasing fibre volume loading. In both cases the decrease at higher

183

COMPARISON OF CFRP FROM ROUND AND NON-ROUND FIBRES

110

-

100

-

o

non

-round

0

n

F90-

+

C L_ J

8O ..G

70 E i 0

6O

t_

50

I

55

I

I

I

1

[

60 65 70 75 80 Fibre volume loading Vf Vlo

I

I

85

gO

Fig. 3. Interlaminar shear strength. fibre volume loadings is presumably due to the rapidly increasing stress concentration between fibres. Anomalous, low points are again believed to be due to difficulties in fabricating good composites at the higher volume loadings. Torsional shear properties are illustrated in Figs. 5 to 7. The solid line in Fig. 5 was taken from the work of Heaton, 4 who calculated the variation of shear modulus with fibre percentage for fibres arranged in a square array, up to a loading of 78.5V/o. The line shown is for the case where the ratio of fibre to matrix shear modulus is 12:1. In view of the deviation of fibres in real composites from a square array the agreement is good. Torsional shear strengths were calculated using the Nadai correction to allow for non-linearity in the torque twist cu~rve (see reference 2). This correction resulted in a reduction of about 20 per cent in the shear strength for a 50V/o composite, falling to about 3-5 per cent for a 80V/o-90V/o composite. The shear strengths in Fig. 6 are lower, more peaked and more scattered than the results obtained from the short beam test. These differences most probably arise because in the latter failure was initiated by compression under the centre roller, whilst in the torsion

184

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0 •

+

HANCOX

non - r o u n d round

1 2 0 ~

I cL

70 t.. X ~O

60

0

>

0 tI--

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40-

30-

20

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L

55

60 Fibre

l

I

[

I

65 70 75 80 v o l u m e l o a d i n g Vf V/o

Fig.4. Transverseflexuralstrength.

I

85

9O

I

COMPARISON OF CFRP FROM ROUND AND NON-ROUND FIBRES

16-

o

non -round

•

round

185

15

t

14-

13t~

n" CD (D

Theory Heaton

12-

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v)

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¢o

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1 55

I 60

Fibre Fig. 5.

I 65

volume

I 70

load

I 75

ng

80 Vf V/o

Torsional shear modulus.

,I 85

I 90

186

N . L . HANCOX

100~

•o n o n - r o u n d round

g0 ~ 0

n :E

f J

80

/ 70-

/

t,,-

J

\

/

\

60¢,-

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l 55

I

i

60 Fibre

I

J

65 70 75 volume Iooding

I 80 Vf V/o

I

I

80

90

Fig. 6. Torsional shear strength.

test failure was due to shear between fibres on a plane parallel to the long axis of the specimen. The angular deflection at failure, 0, for a specimen 100 mm long and 6.2 mm in diameter decreases rapidly with increasing fibre volume fraction (see Fig. 7). The high values of 0, at low volume loadings, reflect the large amount of plastic deformation occurring in the resin matrix before failure finally takes place. The stress concentration factor, ]~, due to the fibres in the matrix was deduced assuming that the fibre resin bond was sufficiently good for failure to occur in the resin (Reynolds and HancoxS). Values are shown in Fig. 8. The variation up to a fibre loading of 78.5V/o, for square and hexagonal arrays, is taken from reference 4. Most of the experimental results are for loadings above 78-5V/o and so cannot be compared directly with theory, although the general trend is reasonable. It is not possible to establish any significant difference between sU:ess concentrati.'on factors for the two types of fibre, despite their very different shapes, This is in agreement with the observation that all other torsional measurements are independent of the fibre shape. The total work to cause failure in a torsion specimen was determined from the

187

COMPARISON OF CFRP FROM ROUND AND NON-ROUND FIBRES

100

0

nor1 - r o u n d

•

round

90

80 LGr~

\

\

70-

\

L.

60-

C O

50!-

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I

\

•

/.0

3

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~,

I---

20

lO

I

50

55

I

I

I

I

I

60 65 70 75 80 Fibre volume loading Vf Vlo

I

I

85

90

Fig. 7. Torsional angular deflection at failure. area under the torque twist curve, assuming that were the specimens slowly unloaded the load would return linearly to zero, at zero deflection, or at the deflection corresponding to the permanent set in the specimen (see Fig. 11). Failure was only quasi-controlled so the work will be overestimated. From Fig. 9 it can be seen that the work falls rapidly with increasing fibre volume loading. The high values at around 50v/. are due to the large amount of plastic deformation which takes place in the resin matrix before failure. As the fibre volume increases, this deformation--and hence the work of failure--decreases.

188

N . L . HANCOX

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o

non -round

•

round

Square.

Heaton

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L

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2,0

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1.5

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I

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I

1

t

I

I

I

60

65

70

75

80

85

Fibre

volume

looding

VfVlo

Fig. 8.

Stress concentration factor.

I go

189

COMPARISON OF CFRP FROM ROUND AND NON-ROUND FIBRES

A calculation of the work to failure can be made as follows. The strain energy stored in a volume dV of resin is .~2

~-dav where T and G are the strength and modulus of the resin. The shear stress in the resin is assumed to be constant throughout as the resin is behaving plastically as it approaches failure. If I'm is the volume fraction of resin, the shear strain energy stored at failure, IV, is given by:

~2 rcd2l W = 2--'G~

V.,

(1)

Taking ~2/'G = 1.68 MPa, d = 6-2 x 10- 3 m, l = 10- 1 m and various values of V,, gives the lower straight line shown in Fig. 9. This agrees reasonably well with

~,

o

non -round

•

round

ul

0

0

v

Equation {1"~" ~

E; 0

~

~

/P ~

- , q

50

55

50

Fibre

55

volume

70

75

80

loading

Vf V/o

Fig. 9. Torsional work to failure.

\

85

90

190

N. L. HANCOX

measured values above 80V/o but underestimates the work for smaller volume loadings. There are two factors contributing to this discrepancy. First, failure is not completely controlled so that experimentally determined values of the work to cause failure will be overestimates. Secondly, both r and G are functions of 0, and r2/G for the resin should be evaluated allowing for this. Approximating the torque twist curve to a series of linear segments and putting the strain energy in the form Gr202/2, where 0 is the angle of twist/unit length, gives Gr202/2 = 7.9 MPa. Now the shear stress in the rod is assumed to vary linearly up to its maximum value at the rod surface rather than be constant throughout the material, and so: W=

1 Gr202 ~d21 2

2

4

V,,

(2)

Evaluating this gives the upper line in Fig. 9 which agrees much better with measurements at the lower volume fractions but overestimates the effect at the higher ones. This may be due to the close fibre packing constraining the resin matrix in some way so as to reduce the energy stored. It may be thought that this analysis will influence the stress concentration results of Fig. 8. However, it is found that when the above method of obtaining the stored shear strain energy is applied to composites, as well as to the resin, the former results are increased in an approximately similar proportion to that for the resin, so that the ratio of the two, and hence fl, is little affected. Attempts to calculate the work of fracture Guc (equal to 2y, where ~ is the fracture surface energy) for the materials were not really successful because of the difficulty of determining the area over which cracking occurred. Sectioned specimens often showed a complex network of cracks and it was decided that this (visual) approach would not yield meaningful results. Hence the following method was adopted. It was assumed that the crack ran the length of the specimen between grips (100 ram), that after failure the specimen consisted of an inner undamaged core, diameter d:, and an outer damaged layer with no torsional stiffness, and that the final shear modulus, Gy, based on the overall diameter, d;, would be the same as the initial shear modulus, Gi, if the diameter of the undamaged core was used in its calculation. Since the shear modulus of a rod is inversely proportional to the fourth power of the diameter: G: _ d/4 (3)

Gi

di 4

from which d / a n d hence the crack depth (d~ - d:)/2 can be obtained. Values of Gnc based on areas determined as above are shown in Fig. 10. The scatter is considerable and results fall into two distinct groups, both containing values for either type of fibre. This effect is probably due to errors in determining the crack area. The fall off in Gnc with increasing fibre volume is very rapid as is to be expected with a diminishing amount of resin between fibres.

~.

0

o

~D 0

131o 0

o

Torsional

I

work

of

A v

fracture

I o

GII C K J m - 2 0

I

0

0 C

0 C

m

0

•

I

t~

7

0

,z

0

Z

Z

0 Z

Z 0

0

E

L. 0 I---

Z

Lrinear

/

Fig. 11.

Permanent

Schematic torque twist curve for composite.

Angutar deftection, degrees

set

J

Work to foil. ure

ox

> 7:

Z r~

COMPARISONOF CFRP FROM ROUND AND NON-ROUNDFIBRES

193

CONCLUSION A wide range of mechanical measurements have been made on composites manufactured from non-round and round carbon fibres both produced at the same temperature. In no case, including torsional measurements which are sensitive to stress concentration between fibres and which might be expected to vary with fibre shape, have any significant differences in behaviour been noted between the two types of fibre. It can therefore be concluded that as regards mechanical properties either shape gives a satisfactory composite. The only advantage of the non-round fibre is that it enables composites of a rather higher volume loading (90V/o versus 85V/o) to be made, because of the ease with which fibres can pack. This might be useful if the maximum flexural or tensile strength or modulus, or maximum shear modulus was required for a given type of fibre. However, the torsional work of fracture is very much lower at the higher volume fractions, because of the small amount of matrix available to undergo plastic deformation, and so very high volume loading composites should be used with care.

REFERENCES

i. 2. 3. 4. 5.

R. J. DAUKSYSand J. D. RAY,J. Comp. Mat., 3 (1969) pp. 684-98. N. L. HANCOX,J. Mat. Sci., 7 (1972) pp. 1030-6. P. K. MALLICKand L. J. BROUTMAN,30th SPI Conference, Washington, USA, 18 F 1-13, 1975. M. D. HEATON,J. Phys. D., 2 (1968) pp. 1039-40 and J. Phys. D., 3 0970) pp. 672-7. W. N. REYNOLDSand N. L. HANCOX,J. Phys. D., 4 (1971) pp. 1747-53.