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Effect of fibre spacing on the fatigue behaviour of metal matrix composites
Effect of fibre spacing on the fatigue behaviour of metal matrix composites S. J. HARRIS and R. E. LEE
Examination of existing fatigue data on metal matrix composites obtained in tension-tension and plain reverse bending shows that a considerable amount of variability exists between systems. Particular attention has been drawn to the rather large difference between the cyclic behaviour of aluminium-boron and aluminium-carbon fibre systems. The low fatigue strength obtained for the latter system is rather surprising since theory would suggest that small diameter fibres should give superior properties. Controlled fatigue experiments on the weakly bonded copper-tungsten system show that fibre diameter does not appear to effect significantly the fatigue behaviour of these composites. Closer examination of the data obtained indicates that very high fatigue stresses may be maintained at 106 cycles and that matrix hardening plays a significant part in maintaining this property level.
INTRODUCTION
A number of studies have been completed on the fatigue behaviour of fibre-reinforced metal matrix composites: Fig.l attempts to bring together a representative sample of cyclic stress-life curves which have been obtained on a number of these materials. It must be pointed out that two principle testing modes have been used to produce this data (a) reverse plain bending and (b) tension-tension - which means that the data is not strictly comparable because, for example, different criteria are taken for failure.. In tensiontension testing complete separation is the obvious failed condition, whilst in bending (which involves compressive and tensile loading) a change in flexural or compressive modulus is the end point. If the tension-tension results of Shimizu and Dolowy 1 for aluminium alloy 6061 reinforced with a volume fraction (Vf) of 0.40 of boron filaments are compared with those of Baker et al 2 in reverse bending for the same alloy reinforced with 0.47 Vf boron fibres, it can be seen that the tension-tension results produce the more optimistic values of fatigue strength. Table 1 shows that in these cases 61% of the tensile stress is reached in tensile-tensile fatigue conditions whilst 47% is reached in bending fatigue. Nevertheless, this difference in test method does not appear to affect the overall tenor of results. Hancock 3 has drawn attention to the main factors which appear to maximise the fatigue resistance of this type of composite, and he lists these as: brittle filaments, low yield strength ductile matrix, weak interfacial bonds and a large modulus difference between constituents. The data given
Department of Metallurgy and Materials Science, University of Nottingham, Nottingham, England. This paper was presented at the one-day symposium on Fatigue in Composites, held at Imperial College, London, on 15 November 1973. The symposium was organized by the Materials and Testing Group of the Institute of Physics.
COMPOSITES . MAY 1974
in Table 1 tend to support most of these arguments, with the exception of the data given for carbon fibrereinforced aluminium and to a certain extent the silica reinforced aluminium. Here it would be expected that the low yield strength, high purity aluminium matrix, brittle carbon fibres, limited interfacial strength and large modulus difference would give a fatigue stress in'reverse bending at 106 cycles close to 50% of the tensile strength, instead of the value of 32% that has been achieved. Such a percentage may even be inflated above its true level because of the low tensile strength obtained on these composites, that is 80% of the rule mixtures value. Hence if the experimental fatigue stress is compared with the rule of mixtures tensile strength then a drop to 25% occurs. The reason for these observations on aluminium-carbon may be either the incorporation of weaknesses into the composite during fabrication or the existence of special conditions in the composite when small diameter fibres are present. In connexion with the second of these two reasons it has been shown by several investigators 7--10 using monotonic tensile test data that the stress carried by the ductile metal matrix was considerably in excess of its yield and fracture stress when the fibre diameter approached 10 gin. This was particularly the case if the fibre volume fraction was sufficiently high, ie > 0.25. Lee and Harris lo have shown that there was a clear dependance of matrix stress on interfibre spacing. Mechanisms of fatigue failure
Baker 2 has categorized the mechanisms into matrix fatigue, interracial fatigue and fibre fatigue; he indicated the possibility of all of these mechanisms acting at the same time. Fractographic studies of reverse bending fatigue failures in aluminium reinforced with carbon and boron fibres have indicated that matrix fatigue cracking was a major contributor to the total fatigue. Attempts have been made to predict the fatigue behaviour of the matrix in terms of the relationship 11
101
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~
I
O i XI
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I0
Z
'
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I
Tension • 0.3Vf W-Ag m0.3Vf B e - A I
Bending O 0.47Vf B-b061AI [3 R R 5 8 A I
• 0.4Vf B-bO61AI
zx 0.35Vf C - A I
8 (~ ~)
0~. 0
~
•---
6,
°o O
0
0
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• --
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[] ~
10 2
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10 3
104
10 5
o
~
,
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I
10 6
10 7
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C y c l e s to failure
Fig.1 Tensile and bending fatigue data obtained by a number of sources on a number of metal matrices reinforced with metal wires and brittle fibres Table 1
Existing f a t i g u e d a t a o n m e t a l m a t r i x c o m p o s i t e s
Type of material
Composite tensile breaking strength GN/m 2
% Composite tensile strain to failure
Fatigue stress limit at 106 cycles GN/m 2
Fatigue stress at 106 cycles (percentage of tensile stress)
Reference
AI-0.47 Vf B
1.30
0.6
* 0.61
47%
(2)
A I - 0 . 4 0 Vf B
1.17
0.6
t
0.72
61%
(1)
A I - 0 . 4 5 VfSiO2
0.65
2.0
* 0.17
26%
(5)
A I - 0 . 3 0 Vf C
0.60
0.7
* 0.19
32%
(2)
A I - 0 . 3 3 Vf Be
0.53
4.0
t
0.25
47%
(4)
Ag-0.3 Vf W
0.71
1.8
t
0.38
53%
(6)
AI alloy
0.43
-
* 0.15
35%
(2)
* Tested in reverseplain bending
t
Tested in tension-tension
N f '/2 Aep = C
(1)
where N f is the number of cycles to failure and Aep is the plastic strain range endured. This kind of relationship could prove useful, because Aep could offer a means of measuring the amount of damage the matrix undergoes during cyclic loading. The cyclic plastic strain range for a composite has been predicted by Baker, 5 using the equation
Aep =
2o c - 2Oy [(Ef/Em) Vf + (1 - Vf)]
(2)
effective modulus of the yielded matrix and Ef and Em are the elastic moduli of the fibre and matrix. Baker s and Kelly and Bomford 12 have obtained values of ay and U to fit into an expression such as Equation (2}, assuming the matrix behaviour to be unaffected by the presence of fibres. If matrix strengthening in the composite does occur under fatigue cycling, the values of Oy and U in Equation (2) may increase resulting in a reduction in Aep. Thus an increase in the fatigue life of the composite should be expected if the interfibre spacing is small, which is more likely to occur in systems containing small diameter fibres, such as carbon.
Ef Vf + U(1 - Vf)
Present investigation where o c is the stress applied to the composite, Oy is the yield stress of the cyclically hardened matrix, U is the
102
The experimental work forms part of a programme which is examining the role of fibre spacing, fibre matrix bond
COMPOSITES . MAY 1974
strength and fibre properties in so much as they affect fatigue failure mechanisms in metal matrix composites. The particular system examined here is tungsten wire reinforced copper. Tungsten wires are available in a range of diameters between 10 and 50/am: this is in a size range below that which any other investigator has studied fatigue in a metal matrix, except of course the single set of results on carbonaluminium. 2 The mod.e of fabrication of these composites allows good fibre spatial distribution and produces a relatively low fibre-matrix bond strength. Hence fatigue testing of these composites should provide evidence of whether fibre spacing and diameter effect the ensurance limit in weakly bonded fibres with a small amount of ductility.
fibre distribution which can be achieved by this fabrication route.
Mechanical testing I. Fatigue testing Tests were carried out on a Mayes servo-hydraulic testing machine under load control conditions. T h e specimens were subjected to tensile.tensile cyclic conditions, with a frequency of cycling of 5 Hz. The number of cycles was counted until a clear break took place and the specimen could not support a load. 2. Tensile testing These tests were carried out on the machine stated above at a loading rate of 2.5 kN per minute. Strain in the specimen was measured with strain gauges bonded to its surface.
EXPERIMEN TA L
3. Fractography Care was taken in handling the fracture surface after failure so that possible evidence of the mechanism by which propagation took place could be sought. The samples in question were examined under a scanning electron microscope.
Materials Copper was unidirectionally reinforced with 0.37 Vf of asdrawn tungsten wire. Four different wire diameters were used in the range 20/am to 48/am. The diameter of the wire was measured under an opt!cal microscope with an image shearing eyepiece and the values are given in Table 2. The copper matrix was produced by electrodeposition from an acid sulphate bath, containing 188 g/1 of cupric sulphate and 74 g/1 sulphuric acid at a current density of 200 A/m 2 at 20°C.
Specimen preparation The composites were fabricated by a process involving filament winding and electrodeposition.13 The wire was wound on to a stainless steel mandrel at a predetermined spacing using a coil winding machine. It was then cathodically degreased in sodium hydroxide solution for 10 minutes, washed and immediately transferred to the plating bath. Copper was plated on to the wire for an appropriate time to give the required volume fraction. The sheet of material produced was then stripped from the mandrel. This warp sheet was cut up and several pieces, were stacked upon each other in a mould with the fibres aligJ ed parallel to the sides of the mould. After hot pressing at 700°C in vacuum, fatigue samples were cut from the resultant composite. The samples were 100 mm x 5 mm, with a thickness of 0.? mm. The ends were built up by sandwiching the ends in an epoxy resin contained in an outer copper tube. Fig.2 Photomicrographs of transverse sections cut from copper reinforced with (a) 10 p,m (top) and (b) 48 p,m (bottom) tungsten wires: both contain 0.37 volume fraction of wires
Checks were made on fibre volume fractions in randomly selected composites. Fig.2 shows examples of the good
Table 2
Tensile results on copper-tungsten composites
Type of material
Mean fibre diameter #m
Mean fibre breaking strength GN/m 2
Mean % fibre breaking strain
Mean breaking strength of composite GN/m 2
Ratio, experimental to rule of mixtures strength
Cu-0.37 Vf W
20
3.12
1.6
1.20
1.15
Cu-0.37 Vf W
30
2.68
2.5
1.10
1.10
Cu-0.37 Vf W
40
3.12
2.6
1.35
1.1.0
Cu-0.37 Vf W
48
3.41
3.1
1.35
1.04
AI-0,40 Vf B
100
2.75
0.6
1.17
-
COMPOSITES. MAY 1974
103
RESULTS
l
O
,~,
T~nsile strength The breaking strength of each of the four types of tungsten wire was determined and the values obtained are given in Table 2, together with their strain to failure. The same table includes the breaking stress of the four tungsten-copper composites. For comparison the results for boron fibres and a 0.4 Vf boron-aluminium composite are given. It'is to be noted that all the tungsten-copper composites exceed the rule of mixtures strength at the fibre breaking strain, ie it increases with decreasing fibre diameter.
The data obtained in these tests were plotted in the form of conventional S-N curves, see Fig.3. The curves are.flat and tend to indicate a fatigue limit. For comparison the tensiontension results of Shimizu and Dolowy ] for 6061 Aluminium alloy reinforced with 0.4 Ff boron fibres are included in Fig.3. It can be seen that all the tungsten-copper results are at stress levels above those for the B-AI, even though the tensile properties of the fibres and composites are similar. In Table 3 the fatigue stress at lO6 cycles for each composite is given and in addition this value is expressed as a percentage of the tensile breaking stress of the particular composite. For the tungsten-copper material the percentage lies between 80 and 90%, whilst for that of the B-A1 case cited, a value of 61% is reached. With the tungsten-copper composites the fatigue data indicate that the large diameter fibre (48/am) composites operated at a higher cyclic stress amplitude and there was a gradual reduction in stress amplitude as the diameter decreased, although the results for the 20/am and 30/am composites could, for practical purposes, be said to lie within the same population. From the data given in Fig.3 the cyclic plastic'strain range, Aep; was calculated and this is shown in Fig.4. The results for all the copper-tungsten material tend to be grouped together in a common population, with a slight indication that the strain range increases with fibre diameter. The data tends to level out at a Aep value of"~0.12, which implies a considerable energy absorption in the matrix, without failure.
Fractography Samples from the tensile tests, see Fig.ha, indicate that the fibres have necked down in failure and therefore show slight
I
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l
14
12 ~"~-
• D
•
w
== 8 .3E o3 O EZ
o
Wire Diameter • 20 p m
4
o 30 0 40 • 48
O~ n
Cyclic stress-life data
Table 3
~"
-
•
B-AI I
I
10 2
101
I
I
103 10' 10s Cycles to failure
I
106
Fig.3 Stress/number of cycles plots for copper-tungsten composites. Tests completed in tension-tension cyclic loading
A
0"20
•0
UJ
0
0
0.15
•
O• O • eO
O•
c"
o
Wire
O
="
•
13 •
0.10
•
•
0
O
O •
Diameter
• 20 ~.m o 30
o
O 40 • 48
0.05
I
101
102
I
I
I
103 104 10s Cycles to failure
Fig.4 P l o t of plastic s t r a i n r a n g e , for copper-tungsten composite
(Aep),
versus number
I
106 of cycles,
ductility. The matrix bridges between the fibres are obviously extended and have bloken in essentially a ductile manner. Although Fig.ha does not clearly indicate fibre pull out, this does occur on a reasonable scale over the fracture surface. On the fatigue fracture surface, see Fig.hb and 5c, it is apparent that pull out still occurs and that the matrix is considerably flatter on a microscopic scale. Occasional
Fatigue results on copper-tungsten composites (tension-tension) |
Composite fatigue stress limit at 106 cycles GN/m 2
Fatigue stress at 106 cycles (as percentage of tensile strength)
Tensile stress at which composite would break without matrix GN/m 2
20
0.98
81%
1.12
Cu-0.37 Vf W
30
0.98
89%
0.96
Cu-0.37 Vf W
40
1.12
83%
1.12
Cu-0.37 Vf W
48
1.24
91%
1.22
AI-0.40 Vf B
100
0.72
61%
-
Type of material Cu-0.37 Vf W
104
Mean fibre diameter /am
COMPOSITES . MAY 1974
steps exist on the fracture face and in these regions pull out is much more noticeable. It is apparent that the fibres have failed with some ductility: in fact, on a much wider scale no evidence could be found for fibre fatigue even at the extreme edges of the fracture surface.
DISCUSSION
In comparison with results obtained on systems such as aluminium-boron, the fatigue performance of coppertungsten composites seems to be superior. This comparison may not be totally fair because the nature of the bond in the two systems will be different, ie aluminium-boron will have the greater fibre/matrix bond strength, and hence its transverse and off-axis properties would be superior; furthermore, the tungsten reinforcement possesses the ability to deform plastically before failure, which could help considerably in preventing fibre fatigue. Of course, the use of tungsten would be prohibited in any situation where weight saving was at a premium. However, the results presented here do give an indication of the possible advantages in pure tension-tension fatigue of using slightly ductile fibres in a weakly bonded situation. The fatigue stress at 106 cycles of a pure copper matrix is raised by an order of magnitude above its tensile strength by the addition of 0.37 Vf of tungsten wires.
data obtained, see Fig.6, the values of Oy and U in Equation 2 are effectively increased by cyclic strain hardening. The results show that with the exception of the 48/am tungsten wire composites the cyclic work hardening rate is not dependent on fibre diameter as the monotonic hardening rate. 1° Hence it would be expected that the fatigue performance of a copper matrix would tend to be significantly improved by the presence of tungsten fibres without necessarily showing differences between composites containing similar volume fractions of dissimilar fibre diameters.
Rather than to extol the virtues of copper-tungsten in tension-tension fatigue the purpose of this work has been to explore failure mechanisms in metal matrix composites particularly in respect of changing fibre diameter. According to theory (see Section on 'Mechanisms of fatigue failure') at small fibre diameters (at constant volume fraction) the value of the plastic strain range, Aep, should fall and thus the life to failure increase. The data obtained here does not bear out this conclusion; for instance, the S-N curves in Fig.3 show a reverse trend with the larger fibre diameters showing improved lives at a given stress. Analysis in terms of the plastic strain range Aep indicates little difference between the composites. The fractographic examination provides evidence of cracks which propagate in the matrix resulting in transfer of load on to the fibres which appear to fail in tension. The failure was very sudden and complete, ie instant specimen separation occurred. Further evidence of this failure procedure is given by the values of the fracture stress of the fibres assuming the matrix to be supporting no load, see Table 3. The results of this analysis are comparable with the fatigue stress values at 106 cycles extracted from Fig.3 and at 106 cycles given in Table 3. Such an interpretation means that the matrix is contributing effectively to the fatigue strength of these weakly bonded composites. This means that cracks initiate and grow in the matrix without propagating through the fibres. By the stage at which the matrix bears virtually no load, final failure occurs by tensile fracture of the fibres. Such an explanation precludes the operation of a matrix induced fibre fatigue mechanism. Hence, the order of fatigue performance given by the composites tested here may be explained by the variation of the fibre tensile breaking stress, see Tables 2 and 3, thus indicating that fibre diameter in itself is of little consequence. Further support for this argument is given by Lee and Harris lo who have examined the cyclic stress-strain behaviour in these copper-tungsten composites. From the
COMPOSITES . MAY
1974
Fig.5 Scanning electron micrographs o f fracture produced (a) in tensile tests on a composite reinforced with 40 #m fibres, (top); (b) in a fatigue test on composite reinforced with 40 # m fibres, (centre); (c) as (b) but at a higher magnification ( b o t t o m )
105
Relevance of this investigation to metal matrix composites in general
I.t is worth while C'onsidering the results obtained by Baker, Jackson and Braddick 2 on carbon fibre reinforced aluminium. A consideration of the results obtained with these small 8/am fibres where the fatigue stress was low (~25% of rule of mixture tensile strength) and the results obtained in the present paper suggest that the small diameter fibres in themselves do not bring about the poor fatigue results. Whilst some allowance ought to be made for the brittle nature of carbon fibres, the existence of some form of bonding between fibre and aluminium matrix and the presence of oxide in the aluminium which may alter its plastic behaviour under cyclic conditions, nevertheless the large drop in fatigue properties would require a better explanation than just the summation of all of these effects. One explanation of these results may be given if one considers the information given by Marsden and Harris 14 on premature tensile failure in carbon fibre reinforced copper alloys, particularly at Vf values > 0.30. Here it is suggested that fibre damage in the form of transverse cracks can occur depending on the prior thermal history of the material. Such damage does not intrinsically reduce the strengthening efficiency of the fibres and matrix, which are loaded up to a proportion of the fibre failure strain. However, at this stage premature crack propagation occurs. Under fatigue conditions with a severely work hardened matrix, the incidence of fibre damage can considerably increase the chances of crack propagation through broken fibres and matrix. The surprising result given in Table 1 is the rather low percentage of tensile strength, reflected by the fatigue at 106 cycles in beryllium wire reinforced aluminium and tungsten reinforced silver (47% and 53% respectively) compared with the 80-90% obtained in the present experiments. Explanations of this discrepancy may be rather different in the two cases cited. For the aluminium-beryUium system, the fibres may have been subjected to fatigue damage, the fibres being deformed cyclically to a relatively large amount of plastic deformation. In addition, the matrix-fibre bond strength may be greater than that existing in the coppertungsten samples used in the present investigation thus allowing matrix cracks to propagate across into the fibres. With the silver-tungsten System, the method of fabrication (molten metal infiltration) may have recrystallized the tungsten wires thus embrittling them and bringing about a greater interfacial bond strength. Similar evidence may be found from the work of Ham and Place as for infiltrated copper0.23 Vf tungsten composites. Further, the fabrication technique may not produce such well ordered composites as those presently described: in fact infiltration may tend to encourage fibre misalignment. All of these factors may combine to reduce the effective tension-tension fatigue strength. Therefore it is important that a series of experiments be carried out to examine fibre diameter effects in a composite with a good interfacial bond strength before it is possible to refute the theoretical argument about better fatigue lives with decreasing fibre diameter.
4O
0
E
30
Z 09 CO
48
0 .m
-6 / / / J r
0 t-
Fatigue failure between
//////
X
,o, an''O°c'c'e' OCC°'S
1 y / O~ 0
~,hiscyclic strain ran( I 0.1
I 02
I 0.3
I 04
Cyclic
total
I 05
I 06
I 07
strain (%)
Fig.6 Derived cyclic stress-strain curves for the matrix in coppertungsten composites, assuming the rule of mixtures to apply. The stress-strain curve for as-pressed copper is shown for comparison. The numbers refer to the tungsten wire diameters in #m
which was an order of magnitude greater than the UTS of copper. The fatigue strengths of weakly bonded c o p p e r - 0 3 7 Vf tungsten composites do not change appreciably as the fibre diameter and spacing decrease. The copper matrix appears to be capable of significan~ amounts of cyclic hardening in the presence of tungsten fibres and thus enhancing fatigue strength. The tungsten fibres have apparently suffered little fatigue damage and fail with a certain amount of ductility. A CKNOWL EDGEMENTS
The authors are indebted to the Science Research Council for providing finance to support this work. They would lik to extend their thanks to Professor J. S. L1. Leach for proriding laboratory facilities and to Mr C. E. Jagger for preparing the materials for this investigation. REFERENCES
l
Shimizu,H. and Dolowy, J. F. 'Composite materials: testing and design' (ASTM STP 460, 1969) p 192
2
Baker, A. A., Braddick, D. M. and Jackson, P. W. J M a t Sci
3
Hancock, J. R. 'Composite materials: testing and design
7 (1972) p 747 4 5
6 7 8
9 10 11
(second conference) (ASTM STP 497, 1972) p 483 Toy, A. Journal o f Materials 3 (1968) p 43 Baker, A. A. J M a t Sci 3 (1968) p 412 ' Morris,A. W. H. and Steigetwald, E. A. T r a n s A I M E 2 3 9 (1967) p 730 Kelly, A. and Lilholt, H. PhilMag 20 (1969) p 311 Garmong, E. and $heppard, L. A. Metal Trans 2 (1971) p 17 Chawla, K. K. Metallography 6 (1973) p 155 Lee, R. E. and Harris, S. J. J M a t Sci (in press) Tavernelli, J. F. and Coffin, L.F. TransASM 51 (1959)
p438 12
CONCLUSIONS
The results of the investigation may be summarized thus: I Copper-0.37 Vf tungsten wire composites existed unbroken after 106 cycles of a peak to peak stress
106
13 14 15
Kelly, A. and Bamford, M. J. 'Physics of strength and plasticity', (MIT Press, 1969) p 339 Harris,S. J. et al. Trans l Met Fin 49 (1971) p 205 Harris,S. J. and Marsden, A. L. Conference on Practical
Metal Matrix Composites, (Institution of Metallurgists, 1974) Ham, R. K. and Place, T. A. JMech Phys Solids 14 (1966) p 271
COMPOSITES. MAY 1974
Examination of existing fatigue data on metal matrix composites obtained in tension-tension and plain reverse bending shows that a considerable amount of variability exists between systems. Particular attention has been drawn to the rather large difference between the cyclic behaviour of aluminium-boron and aluminium-carbon fibre systems. The low fatigue strength obtained for the latter system is rather surprising since theory would suggest that small diameter fibres should give superior properties. Controlled fatigue experiments on the weakly bonded copper-tungsten system show that fibre diameter does not appear to effect significantly the fatigue behaviour of these composites. Closer examination of the data obtained indicates that very high fatigue stresses may be maintained at 106 cycles and that matrix hardening plays a significant part in maintaining this property level.
INTRODUCTION
A number of studies have been completed on the fatigue behaviour of fibre-reinforced metal matrix composites: Fig.l attempts to bring together a representative sample of cyclic stress-life curves which have been obtained on a number of these materials. It must be pointed out that two principle testing modes have been used to produce this data (a) reverse plain bending and (b) tension-tension - which means that the data is not strictly comparable because, for example, different criteria are taken for failure.. In tensiontension testing complete separation is the obvious failed condition, whilst in bending (which involves compressive and tensile loading) a change in flexural or compressive modulus is the end point. If the tension-tension results of Shimizu and Dolowy 1 for aluminium alloy 6061 reinforced with a volume fraction (Vf) of 0.40 of boron filaments are compared with those of Baker et al 2 in reverse bending for the same alloy reinforced with 0.47 Vf boron fibres, it can be seen that the tension-tension results produce the more optimistic values of fatigue strength. Table 1 shows that in these cases 61% of the tensile stress is reached in tensile-tensile fatigue conditions whilst 47% is reached in bending fatigue. Nevertheless, this difference in test method does not appear to affect the overall tenor of results. Hancock 3 has drawn attention to the main factors which appear to maximise the fatigue resistance of this type of composite, and he lists these as: brittle filaments, low yield strength ductile matrix, weak interfacial bonds and a large modulus difference between constituents. The data given
Department of Metallurgy and Materials Science, University of Nottingham, Nottingham, England. This paper was presented at the one-day symposium on Fatigue in Composites, held at Imperial College, London, on 15 November 1973. The symposium was organized by the Materials and Testing Group of the Institute of Physics.
COMPOSITES . MAY 1974
in Table 1 tend to support most of these arguments, with the exception of the data given for carbon fibrereinforced aluminium and to a certain extent the silica reinforced aluminium. Here it would be expected that the low yield strength, high purity aluminium matrix, brittle carbon fibres, limited interfacial strength and large modulus difference would give a fatigue stress in'reverse bending at 106 cycles close to 50% of the tensile strength, instead of the value of 32% that has been achieved. Such a percentage may even be inflated above its true level because of the low tensile strength obtained on these composites, that is 80% of the rule mixtures value. Hence if the experimental fatigue stress is compared with the rule of mixtures tensile strength then a drop to 25% occurs. The reason for these observations on aluminium-carbon may be either the incorporation of weaknesses into the composite during fabrication or the existence of special conditions in the composite when small diameter fibres are present. In connexion with the second of these two reasons it has been shown by several investigators 7--10 using monotonic tensile test data that the stress carried by the ductile metal matrix was considerably in excess of its yield and fracture stress when the fibre diameter approached 10 gin. This was particularly the case if the fibre volume fraction was sufficiently high, ie > 0.25. Lee and Harris lo have shown that there was a clear dependance of matrix stress on interfibre spacing. Mechanisms of fatigue failure
Baker 2 has categorized the mechanisms into matrix fatigue, interracial fatigue and fibre fatigue; he indicated the possibility of all of these mechanisms acting at the same time. Fractographic studies of reverse bending fatigue failures in aluminium reinforced with carbon and boron fibres have indicated that matrix fatigue cracking was a major contributor to the total fatigue. Attempts have been made to predict the fatigue behaviour of the matrix in terms of the relationship 11
101
I"
I
•
I
~
I
O i XI
E
I0
Z
'
I
I
Tension • 0.3Vf W-Ag m0.3Vf B e - A I
Bending O 0.47Vf B-b061AI [3 R R 5 8 A I
• 0.4Vf B-bO61AI
zx 0.35Vf C - A I
8 (~ ~)
0~. 0
~
•---
6,
°o O
0
0
o •~
~
'
~
~
0 0 .__. O~
4 I \
"0
-- 0 " ~
• --
~ 1 - 1 ~ . ' ' ~ ' " - - Z % - - - - . - - ~ , . Z~ A
~ 13_
2
[] ~
10 2
I
I
I
10 3
104
10 5
o
~
,
o
ii--,." A
I
I
10 6
10 7
\
C y c l e s to failure
Fig.1 Tensile and bending fatigue data obtained by a number of sources on a number of metal matrices reinforced with metal wires and brittle fibres Table 1
Existing f a t i g u e d a t a o n m e t a l m a t r i x c o m p o s i t e s
Type of material
Composite tensile breaking strength GN/m 2
% Composite tensile strain to failure
Fatigue stress limit at 106 cycles GN/m 2
Fatigue stress at 106 cycles (percentage of tensile stress)
Reference
AI-0.47 Vf B
1.30
0.6
* 0.61
47%
(2)
A I - 0 . 4 0 Vf B
1.17
0.6
t
0.72
61%
(1)
A I - 0 . 4 5 VfSiO2
0.65
2.0
* 0.17
26%
(5)
A I - 0 . 3 0 Vf C
0.60
0.7
* 0.19
32%
(2)
A I - 0 . 3 3 Vf Be
0.53
4.0
t
0.25
47%
(4)
Ag-0.3 Vf W
0.71
1.8
t
0.38
53%
(6)
AI alloy
0.43
-
* 0.15
35%
(2)
* Tested in reverseplain bending
t
Tested in tension-tension
N f '/2 Aep = C
(1)
where N f is the number of cycles to failure and Aep is the plastic strain range endured. This kind of relationship could prove useful, because Aep could offer a means of measuring the amount of damage the matrix undergoes during cyclic loading. The cyclic plastic strain range for a composite has been predicted by Baker, 5 using the equation
Aep =
2o c - 2Oy [(Ef/Em) Vf + (1 - Vf)]
(2)
effective modulus of the yielded matrix and Ef and Em are the elastic moduli of the fibre and matrix. Baker s and Kelly and Bomford 12 have obtained values of ay and U to fit into an expression such as Equation (2}, assuming the matrix behaviour to be unaffected by the presence of fibres. If matrix strengthening in the composite does occur under fatigue cycling, the values of Oy and U in Equation (2) may increase resulting in a reduction in Aep. Thus an increase in the fatigue life of the composite should be expected if the interfibre spacing is small, which is more likely to occur in systems containing small diameter fibres, such as carbon.
Ef Vf + U(1 - Vf)
Present investigation where o c is the stress applied to the composite, Oy is the yield stress of the cyclically hardened matrix, U is the
102
The experimental work forms part of a programme which is examining the role of fibre spacing, fibre matrix bond
COMPOSITES . MAY 1974
strength and fibre properties in so much as they affect fatigue failure mechanisms in metal matrix composites. The particular system examined here is tungsten wire reinforced copper. Tungsten wires are available in a range of diameters between 10 and 50/am: this is in a size range below that which any other investigator has studied fatigue in a metal matrix, except of course the single set of results on carbonaluminium. 2 The mod.e of fabrication of these composites allows good fibre spatial distribution and produces a relatively low fibre-matrix bond strength. Hence fatigue testing of these composites should provide evidence of whether fibre spacing and diameter effect the ensurance limit in weakly bonded fibres with a small amount of ductility.
fibre distribution which can be achieved by this fabrication route.
Mechanical testing I. Fatigue testing Tests were carried out on a Mayes servo-hydraulic testing machine under load control conditions. T h e specimens were subjected to tensile.tensile cyclic conditions, with a frequency of cycling of 5 Hz. The number of cycles was counted until a clear break took place and the specimen could not support a load. 2. Tensile testing These tests were carried out on the machine stated above at a loading rate of 2.5 kN per minute. Strain in the specimen was measured with strain gauges bonded to its surface.
EXPERIMEN TA L
3. Fractography Care was taken in handling the fracture surface after failure so that possible evidence of the mechanism by which propagation took place could be sought. The samples in question were examined under a scanning electron microscope.
Materials Copper was unidirectionally reinforced with 0.37 Vf of asdrawn tungsten wire. Four different wire diameters were used in the range 20/am to 48/am. The diameter of the wire was measured under an opt!cal microscope with an image shearing eyepiece and the values are given in Table 2. The copper matrix was produced by electrodeposition from an acid sulphate bath, containing 188 g/1 of cupric sulphate and 74 g/1 sulphuric acid at a current density of 200 A/m 2 at 20°C.
Specimen preparation The composites were fabricated by a process involving filament winding and electrodeposition.13 The wire was wound on to a stainless steel mandrel at a predetermined spacing using a coil winding machine. It was then cathodically degreased in sodium hydroxide solution for 10 minutes, washed and immediately transferred to the plating bath. Copper was plated on to the wire for an appropriate time to give the required volume fraction. The sheet of material produced was then stripped from the mandrel. This warp sheet was cut up and several pieces, were stacked upon each other in a mould with the fibres aligJ ed parallel to the sides of the mould. After hot pressing at 700°C in vacuum, fatigue samples were cut from the resultant composite. The samples were 100 mm x 5 mm, with a thickness of 0.? mm. The ends were built up by sandwiching the ends in an epoxy resin contained in an outer copper tube. Fig.2 Photomicrographs of transverse sections cut from copper reinforced with (a) 10 p,m (top) and (b) 48 p,m (bottom) tungsten wires: both contain 0.37 volume fraction of wires
Checks were made on fibre volume fractions in randomly selected composites. Fig.2 shows examples of the good
Table 2
Tensile results on copper-tungsten composites
Type of material
Mean fibre diameter #m
Mean fibre breaking strength GN/m 2
Mean % fibre breaking strain
Mean breaking strength of composite GN/m 2
Ratio, experimental to rule of mixtures strength
Cu-0.37 Vf W
20
3.12
1.6
1.20
1.15
Cu-0.37 Vf W
30
2.68
2.5
1.10
1.10
Cu-0.37 Vf W
40
3.12
2.6
1.35
1.1.0
Cu-0.37 Vf W
48
3.41
3.1
1.35
1.04
AI-0,40 Vf B
100
2.75
0.6
1.17
-
COMPOSITES. MAY 1974
103
RESULTS
l
O
,~,
T~nsile strength The breaking strength of each of the four types of tungsten wire was determined and the values obtained are given in Table 2, together with their strain to failure. The same table includes the breaking stress of the four tungsten-copper composites. For comparison the results for boron fibres and a 0.4 Vf boron-aluminium composite are given. It'is to be noted that all the tungsten-copper composites exceed the rule of mixtures strength at the fibre breaking strain, ie it increases with decreasing fibre diameter.
The data obtained in these tests were plotted in the form of conventional S-N curves, see Fig.3. The curves are.flat and tend to indicate a fatigue limit. For comparison the tensiontension results of Shimizu and Dolowy ] for 6061 Aluminium alloy reinforced with 0.4 Ff boron fibres are included in Fig.3. It can be seen that all the tungsten-copper results are at stress levels above those for the B-AI, even though the tensile properties of the fibres and composites are similar. In Table 3 the fatigue stress at lO6 cycles for each composite is given and in addition this value is expressed as a percentage of the tensile breaking stress of the particular composite. For the tungsten-copper material the percentage lies between 80 and 90%, whilst for that of the B-A1 case cited, a value of 61% is reached. With the tungsten-copper composites the fatigue data indicate that the large diameter fibre (48/am) composites operated at a higher cyclic stress amplitude and there was a gradual reduction in stress amplitude as the diameter decreased, although the results for the 20/am and 30/am composites could, for practical purposes, be said to lie within the same population. From the data given in Fig.3 the cyclic plastic'strain range, Aep; was calculated and this is shown in Fig.4. The results for all the copper-tungsten material tend to be grouped together in a common population, with a slight indication that the strain range increases with fibre diameter. The data tends to level out at a Aep value of"~0.12, which implies a considerable energy absorption in the matrix, without failure.
Fractography Samples from the tensile tests, see Fig.ha, indicate that the fibres have necked down in failure and therefore show slight
I
I
I
l
14
12 ~"~-
• D
•
w
== 8 .3E o3 O EZ
o
Wire Diameter • 20 p m
4
o 30 0 40 • 48
O~ n
Cyclic stress-life data
Table 3
~"
-
•
B-AI I
I
10 2
101
I
I
103 10' 10s Cycles to failure
I
106
Fig.3 Stress/number of cycles plots for copper-tungsten composites. Tests completed in tension-tension cyclic loading
A
0"20
•0
UJ
0
0
0.15
•
O• O • eO
O•
c"
o
Wire
O
="
•
13 •
0.10
•
•
0
O
O •
Diameter
• 20 ~.m o 30
o
O 40 • 48
0.05
I
101
102
I
I
I
103 104 10s Cycles to failure
Fig.4 P l o t of plastic s t r a i n r a n g e , for copper-tungsten composite
(Aep),
versus number
I
106 of cycles,
ductility. The matrix bridges between the fibres are obviously extended and have bloken in essentially a ductile manner. Although Fig.ha does not clearly indicate fibre pull out, this does occur on a reasonable scale over the fracture surface. On the fatigue fracture surface, see Fig.hb and 5c, it is apparent that pull out still occurs and that the matrix is considerably flatter on a microscopic scale. Occasional
Fatigue results on copper-tungsten composites (tension-tension) |
Composite fatigue stress limit at 106 cycles GN/m 2
Fatigue stress at 106 cycles (as percentage of tensile strength)
Tensile stress at which composite would break without matrix GN/m 2
20
0.98
81%
1.12
Cu-0.37 Vf W
30
0.98
89%
0.96
Cu-0.37 Vf W
40
1.12
83%
1.12
Cu-0.37 Vf W
48
1.24
91%
1.22
AI-0.40 Vf B
100
0.72
61%
-
Type of material Cu-0.37 Vf W
104
Mean fibre diameter /am
COMPOSITES . MAY 1974
steps exist on the fracture face and in these regions pull out is much more noticeable. It is apparent that the fibres have failed with some ductility: in fact, on a much wider scale no evidence could be found for fibre fatigue even at the extreme edges of the fracture surface.
DISCUSSION
In comparison with results obtained on systems such as aluminium-boron, the fatigue performance of coppertungsten composites seems to be superior. This comparison may not be totally fair because the nature of the bond in the two systems will be different, ie aluminium-boron will have the greater fibre/matrix bond strength, and hence its transverse and off-axis properties would be superior; furthermore, the tungsten reinforcement possesses the ability to deform plastically before failure, which could help considerably in preventing fibre fatigue. Of course, the use of tungsten would be prohibited in any situation where weight saving was at a premium. However, the results presented here do give an indication of the possible advantages in pure tension-tension fatigue of using slightly ductile fibres in a weakly bonded situation. The fatigue stress at 106 cycles of a pure copper matrix is raised by an order of magnitude above its tensile strength by the addition of 0.37 Vf of tungsten wires.
data obtained, see Fig.6, the values of Oy and U in Equation 2 are effectively increased by cyclic strain hardening. The results show that with the exception of the 48/am tungsten wire composites the cyclic work hardening rate is not dependent on fibre diameter as the monotonic hardening rate. 1° Hence it would be expected that the fatigue performance of a copper matrix would tend to be significantly improved by the presence of tungsten fibres without necessarily showing differences between composites containing similar volume fractions of dissimilar fibre diameters.
Rather than to extol the virtues of copper-tungsten in tension-tension fatigue the purpose of this work has been to explore failure mechanisms in metal matrix composites particularly in respect of changing fibre diameter. According to theory (see Section on 'Mechanisms of fatigue failure') at small fibre diameters (at constant volume fraction) the value of the plastic strain range, Aep, should fall and thus the life to failure increase. The data obtained here does not bear out this conclusion; for instance, the S-N curves in Fig.3 show a reverse trend with the larger fibre diameters showing improved lives at a given stress. Analysis in terms of the plastic strain range Aep indicates little difference between the composites. The fractographic examination provides evidence of cracks which propagate in the matrix resulting in transfer of load on to the fibres which appear to fail in tension. The failure was very sudden and complete, ie instant specimen separation occurred. Further evidence of this failure procedure is given by the values of the fracture stress of the fibres assuming the matrix to be supporting no load, see Table 3. The results of this analysis are comparable with the fatigue stress values at 106 cycles extracted from Fig.3 and at 106 cycles given in Table 3. Such an interpretation means that the matrix is contributing effectively to the fatigue strength of these weakly bonded composites. This means that cracks initiate and grow in the matrix without propagating through the fibres. By the stage at which the matrix bears virtually no load, final failure occurs by tensile fracture of the fibres. Such an explanation precludes the operation of a matrix induced fibre fatigue mechanism. Hence, the order of fatigue performance given by the composites tested here may be explained by the variation of the fibre tensile breaking stress, see Tables 2 and 3, thus indicating that fibre diameter in itself is of little consequence. Further support for this argument is given by Lee and Harris lo who have examined the cyclic stress-strain behaviour in these copper-tungsten composites. From the
COMPOSITES . MAY
1974
Fig.5 Scanning electron micrographs o f fracture produced (a) in tensile tests on a composite reinforced with 40 #m fibres, (top); (b) in a fatigue test on composite reinforced with 40 # m fibres, (centre); (c) as (b) but at a higher magnification ( b o t t o m )
105
Relevance of this investigation to metal matrix composites in general
I.t is worth while C'onsidering the results obtained by Baker, Jackson and Braddick 2 on carbon fibre reinforced aluminium. A consideration of the results obtained with these small 8/am fibres where the fatigue stress was low (~25% of rule of mixture tensile strength) and the results obtained in the present paper suggest that the small diameter fibres in themselves do not bring about the poor fatigue results. Whilst some allowance ought to be made for the brittle nature of carbon fibres, the existence of some form of bonding between fibre and aluminium matrix and the presence of oxide in the aluminium which may alter its plastic behaviour under cyclic conditions, nevertheless the large drop in fatigue properties would require a better explanation than just the summation of all of these effects. One explanation of these results may be given if one considers the information given by Marsden and Harris 14 on premature tensile failure in carbon fibre reinforced copper alloys, particularly at Vf values > 0.30. Here it is suggested that fibre damage in the form of transverse cracks can occur depending on the prior thermal history of the material. Such damage does not intrinsically reduce the strengthening efficiency of the fibres and matrix, which are loaded up to a proportion of the fibre failure strain. However, at this stage premature crack propagation occurs. Under fatigue conditions with a severely work hardened matrix, the incidence of fibre damage can considerably increase the chances of crack propagation through broken fibres and matrix. The surprising result given in Table 1 is the rather low percentage of tensile strength, reflected by the fatigue at 106 cycles in beryllium wire reinforced aluminium and tungsten reinforced silver (47% and 53% respectively) compared with the 80-90% obtained in the present experiments. Explanations of this discrepancy may be rather different in the two cases cited. For the aluminium-beryUium system, the fibres may have been subjected to fatigue damage, the fibres being deformed cyclically to a relatively large amount of plastic deformation. In addition, the matrix-fibre bond strength may be greater than that existing in the coppertungsten samples used in the present investigation thus allowing matrix cracks to propagate across into the fibres. With the silver-tungsten System, the method of fabrication (molten metal infiltration) may have recrystallized the tungsten wires thus embrittling them and bringing about a greater interfacial bond strength. Similar evidence may be found from the work of Ham and Place as for infiltrated copper0.23 Vf tungsten composites. Further, the fabrication technique may not produce such well ordered composites as those presently described: in fact infiltration may tend to encourage fibre misalignment. All of these factors may combine to reduce the effective tension-tension fatigue strength. Therefore it is important that a series of experiments be carried out to examine fibre diameter effects in a composite with a good interfacial bond strength before it is possible to refute the theoretical argument about better fatigue lives with decreasing fibre diameter.
4O
0
E
30
Z 09 CO
48
0 .m
-6 / / / J r
0 t-
Fatigue failure between
//////
X
,o, an''O°c'c'e' OCC°'S
1 y / O~ 0
~,hiscyclic strain ran( I 0.1
I 02
I 0.3
I 04
Cyclic
total
I 05
I 06
I 07
strain (%)
Fig.6 Derived cyclic stress-strain curves for the matrix in coppertungsten composites, assuming the rule of mixtures to apply. The stress-strain curve for as-pressed copper is shown for comparison. The numbers refer to the tungsten wire diameters in #m
which was an order of magnitude greater than the UTS of copper. The fatigue strengths of weakly bonded c o p p e r - 0 3 7 Vf tungsten composites do not change appreciably as the fibre diameter and spacing decrease. The copper matrix appears to be capable of significan~ amounts of cyclic hardening in the presence of tungsten fibres and thus enhancing fatigue strength. The tungsten fibres have apparently suffered little fatigue damage and fail with a certain amount of ductility. A CKNOWL EDGEMENTS
The authors are indebted to the Science Research Council for providing finance to support this work. They would lik to extend their thanks to Professor J. S. L1. Leach for proriding laboratory facilities and to Mr C. E. Jagger for preparing the materials for this investigation. REFERENCES
l
Shimizu,H. and Dolowy, J. F. 'Composite materials: testing and design' (ASTM STP 460, 1969) p 192
2
Baker, A. A., Braddick, D. M. and Jackson, P. W. J M a t Sci
3
Hancock, J. R. 'Composite materials: testing and design
7 (1972) p 747 4 5
6 7 8
9 10 11
(second conference) (ASTM STP 497, 1972) p 483 Toy, A. Journal o f Materials 3 (1968) p 43 Baker, A. A. J M a t Sci 3 (1968) p 412 ' Morris,A. W. H. and Steigetwald, E. A. T r a n s A I M E 2 3 9 (1967) p 730 Kelly, A. and Lilholt, H. PhilMag 20 (1969) p 311 Garmong, E. and $heppard, L. A. Metal Trans 2 (1971) p 17 Chawla, K. K. Metallography 6 (1973) p 155 Lee, R. E. and Harris, S. J. J M a t Sci (in press) Tavernelli, J. F. and Coffin, L.F. TransASM 51 (1959)
p438 12
CONCLUSIONS
The results of the investigation may be summarized thus: I Copper-0.37 Vf tungsten wire composites existed unbroken after 106 cycles of a peak to peak stress
106
13 14 15
Kelly, A. and Bamford, M. J. 'Physics of strength and plasticity', (MIT Press, 1969) p 339 Harris,S. J. et al. Trans l Met Fin 49 (1971) p 205 Harris,S. J. and Marsden, A. L. Conference on Practical
Metal Matrix Composites, (Institution of Metallurgists, 1974) Ham, R. K. and Place, T. A. JMech Phys Solids 14 (1966) p 271
COMPOSITES. MAY 1974