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Crack closure and residual stress effects in fatigue of a particle-reinforced metal matrix composite
Acta metall, mater. Vol. 41, No. 4, pp. 1189-1196, 1993
0956-7151/93 $6.00 + 0.00 Copyright© 1993Pergamon Press Ltd
Printed in Great Britain. All rights reserved
CRACK CLOSURE A N D RESIDUAL STRESS EFFECTS IN FATIGUE OF A PARTICLE-REINFORCED METAL MATRIX COMPOSITE D. M. KNOWLES, T. J. DOWNES and J. E. KING Department of Materials Science and Metallurgy, Pembroke Street, Cambridge CB2 3QZ, England (Received 13 August 1992)
AImtraet--A study of the influence of macroscopic quenching stresses on long fatigue crack growth in an aluminium alloy-SiC composite has been made. Direct comparison between quenched plate, where high residual stresses are present, and quenched and stretched plate, where they have I ~ n eliminated, has highlighted their r61e in crack closure. Despite similar strength levels and identical crack growth mechanisms, the stretched composite displays faster crack growth rates over the complete range of AK, measured at R = 0.1, with threshold being displaced to a lower nominal AK value. Closure levels are dependent upon crack length, but are greater in the unstretched composite, due to the effect of surface compressive stresses acting to close the crack tip. These result in lower values of AKar in the unstretched material, explaining the slower crack growth rates. Effective AKth values are measured at 1.7 MPax/m, confirmed by constant Km~ testing. In the absence of residual stress, closure levels of approximately 2.5 MPa~/m are measured and this is attributed to a roughness mechanism.
I. I N T R O D U C T I O N Particle reinforced composites are now being produced which show substantial increases in specific strength and stiffness compared with unreinforced alloys [1-4]. However, the main concern with the engineering application of these materials still lies with their low fracture toughness, often accompanied by poor fatigue crack propagation resistance at high growth rates [1, 3, 5, 6]. Both in order to allow property comparisons to be made and to examine ways in which to improve crack growth resistance in these composites it is essential to be able to generate consistent and valid test data. A previous paper by the authors has discussed the problems encountered in the testing of a SiC particle reinforced metal matrix composite (MMC) with a heat-treatable aluminium alloy matrix requiring solution treatment and cold water quenching [7]. The quench generates significant macroscopic residual stresses. The compressive surface stresses cause premature crack closure for cracks growing through near-surface material (i.e. close to the faces of beat-treated test pieces) leading to problems in generating reliable crack growth data because of the resulting curved and irregular crack fronts. Crackfront curvature can be significantly reduced by using specimens which have been cut from heat-treated plate in the T - S orientation. There is, however, concern that the residual stress distribution which still exists through the plate thickness, i.e. the stress variation along the S direction, can have a significant influence on crack closure levels and hence fatigue crack growth threshold values, even when straight Crack fronts are produced, because the residual stress
variation along the longitudinal (L) direction is small. This paper reports on an investigation into fatigue of a particle reinforced M M C in the as-quenched (in plate form) and as-quenched and stretched conditions. In the stretched condition macroscopic residual quenching stresses are, to a large extent, eliminated and hence closure levels are determined in a homogeneous stress field. For the unstretched plate, macroscopic residual stresses still exist. Closure levels have been compared directly for the two billet conditions. The effect of the quenching stresses on threshold values and on fatigue crack growth is described, along with a discussion on the factors which appear to be controlling the underlying closure mechanisms, i.e. those which physically prevent the full closure of the crack-tip.
2. EXPERIMENTAL
2.1. Material
The material used throughout this study consisted of an 8090 aluminium alloy matrix containing 2.61 Li-1.25 Cu-0.91 Mg-0.1 Zr (wt%), reinforced with 20 wt% F1200 (nominally 3/zm) grade particulate SiC. The composite was produced by BP Metal Composites Ltd, Farnborough, using a powder metallurgy route involving blending of the alloy powder and reinforcement, compaction, consolidation by hot isostatic pressing, hot forging and finally rolling to a plate thickness of approximately 15 mm. The material was solution treated at 530°C for 2 h and then cold water quenched. One billet was then stretched by 2%. Specimens were machined from
1189
1190
KNOWLES et al.: FATIGUE OF A METAL MATRIX COMPOSITE 30-
.o "6
20
10 Q
> 0
s 5ollm Fig. 1. Optical microstructure of composite material in plate form. plate either after quenching, or after quenching and stretching. The microstructure of the material is illustrated in Fig. 1. It can be seen that the SiC distribution is generally banded in the S - T and S - L planes and homogeneous in the L - T plane (directions defined relative to the plate in Fig. 2). There is a distinct range of SiC particle size present, with a large number of particles in the submicron to 1/~m range, and a continually decreasing number fraction up to approximately 7 g m in diameter. The volume percentage fraction can be seen to peak around a particle size of 3.73-5.28 gm, Fig. 3. 2.2. Mechanical testing
Tensile tests were performed in the transverse (T) orientation of the plate using Hounsfield specimens having a 10mm 2 cross-sectional area. Strain was measured using a clip gauge-type extensometer with a 10 mm gauge length. Fatigue testing was carried out on single edge notch bend (SENB) specimens with dimensions 12.5 x 12.5 x 100 mm. These were cut from the plate so as to grow the cracks in a T - S orientation (Fig. 2). After machining the specimens were aged for 5 h at 170°C, to produce an approximately peak aged condition. The ageing treatment was chosen from previous work on similar composites, so as to give high strength and straight crack fronts whilst still retaining large macroscopic residual stresses in the unstretched condition [7, 8]. Hardness studies indicated that stretching did not affect the time at which peak strength was attained.
''
"
Particle size (l~m) Fig. 3. Range of SiC particle size in the fabricated composites. Numbers under the columns indicate the maximum particle size detected in each band. Tests were initially performed in accordance with the guide-lines in ASTM standard E647-88 [9], under four point bending, with a sinusoidal loading cycle at a frequency of 20 Hz. Crack length was monitored by a d.c. potential drop method. An on-line interactive computer was used to control the loads and produce both threshold, constant load range (increasing AK) and constant AK data. Thresholds were approached by either one of two methods of load shedding: (i) stress intensity range reductions of 5% at a constant R-ratio (0.1) after the crack had grown through a distance equivalent to four times the maximum plastic zone size, and (ii) stress intensity range reductions of 5% after the crack had grown through a distance equivalent to four times the maximum plastic zone size, commencing with AK = 7.5 MPav/m and R = 0.1, but keeping Kmaxconstant and simply raising K~i~ to reduce AK (constant Km~xtests). Macroscopic crack closure measurements were made by means of a back face strain gauge located at the centre of the specimen, opposite the growing crack. Elastic compliance curves of relative strain vs load were monitored as a function of AK, allowing the crack to grow through a distance greater than 100/~m at a constant value of AK before measurements were recorded [10]. Crack opening stress intensity (Kop) was estimated as the point of deviation of the stress-strain curve from linearity during the increasing load half of the cycle. The effective stress intensity was then calculated from AK,fr = / ~ = -- Kop
Fig. 2. Orientation of single edge notch bend (SENB) specimens with respect to plate material: L longitudinal, T transverse and S short transverse.
(1)
KNOWLES et al.: FATIGUE OF A METAL MATRIX COMPOSITE
1191
Table 1. Composite 0.2% proof stresses Composite
0.2% proof stress (MPa)
Peak aged unstretched Peak aged stretched
10 -4
509 _+8 501 +_5
R=0.1 R=0.3
x •
j
10-5
Fractographic studies were carried out in a Camscan $2 SEM operated at 20-30 kV. 3. RESULTS
x Z
xA×A ×A
10-6
XA X
o3
10-7
Tensile results for the two composites are listed in Table 1. The variations in fatigue crack growth rate (da/dN) for the peak aged condition of both composites at R = 0.1, are shown as a function of cyclic stress intensity range (AK) in Fig. 4. Also plotted are the data obtained for the unstretched material tested at R = 0 . 3 . It is clear that for R = 0 . 1 the crack growth rates for the stretched material are always higher than those for the unstretched plate, at corresponding values of AK, by a factor of approximately 2-3, with nominal threshold values (AKth.... ) being reduced from around 3.9 to 3.5 MPa~/m by stretching. The curves lie approximately parallel to one another throughout the whole stress intensity range over which fatigue crack growth occurs. Increasing the R-ratio to 0.3 is seen to cause a significant increase in growth rates for the unstretched material such that da/dN values are consistently greater than those for the stretched plate at R = 0.1. The curve has a greater slope than that determined at R = 0.1 and turns prematurely up to stage III as a consequence of the higher Kmax value. Threshold was not determined precisely, but can be seen to lie at a lower value of AK than for either of the two composites tested at R = 0.1. Replotting the data for the unstretched composite, in terms of crack growth rate vs Kmax, produces the interesting effect of complete coincidence for curves from the tests at the two R-ratios. This has been done in Fig. 5, demonstrating that Kmax
I
I
I
,
3
4
I I J [ I I*l,l,l,,,I,,,llllhmd
5
6 7 8 910 12 1416 19
Kmax (MPa d'~) Fig. 5. Plot of K~ax against crack growth rate for the unstretched composite tested at R = 0.1 and R = 0.3. rather than the applied AK, is the major controlling factor over the whole crack growth curve, for these two R values. Figure 6 shows the crack opening stress intensity values as a function of AK and a/W for the two composites. For the stretched composite the closure levels can be seen to remain relatively constant with increasing AK, lying in the range 2.3-2.6 MPax/m. In the unstretched composite, the closure measurements ranged from Kop = 3.3 M P a x / m at AK = 7.6 M P a v / m (a/W = 0.36) to 2.7 M P a v / m at AK = 4.0 M P a x / m (a/W = 0.5), decreasing in an approximately linear fashion. It should be noted that the highest closure levels for the unstretched composite were recorded when the crack length was at its shortest (i.e. closest to the notch) and macroscopic quenched-in residual stresses would have had the least opportunity to relax through the specimen width. Figure 7 illustrates the results from a simple constant AK test (AK = 8.5 MPax/m, R = 0.1) pera/W
[] Unstretched R =0.1 10-4 _f- x Stretched R = 0.1 0 Unstretched R = 0.3 ~"
0.48 I
0
,t~xX~ xXX
0.46 I
0.44 I
0.42 I
0.40 I
0.38 I
[] Stretched x Unstretched
10-5
x
x
I~ 10-6 O
z
x
D
~
x
~
x
X [] [] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [] [] [] []
[]
10_7
0
2
I
I 3
I
J 4
I
I, Iml,l,ltlmllJllll 5 6 7 8 9 11 13 15
AK (MPa d'~) Fig. 4. Variations in fatigue crack growth rate (da/dN) for the peak aged conditions of the stretched and unstretched composites at R = 0. I, plotted as a function of AK. Also plotted are the results for the unstretched material at R =0.3.
I 4
I 5
I 6
I 7
AK (MP ax/-~) Fig. 6. Crack opening stress intensity values as a function of AK and a/W for the stretched and unstretched composites.
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KNOWLES et al.: FATIGUE OF A METAL MATRIX COMPOSITE
x l O -5
+ C o n s t a n t Kma x u n s t r e t c h e d * E f f e c t i v e AK s t r e t c h e d A C o n s t a n t Kma x s t r e t c h e d
m[]
2.5
[] [] []
10-4 ~
[]
• Effective AK unstretched
[]
"d
10-5
[]
.~ + + +
2.0 --
•~*4IA i~-
[] 10_6
[]
x.-
Z ~-~ 1.5--
[]
[]
z
~10 -7
[] [] 0.34
zxA A +
[]
[]
"+
i
I
I
I
i
I
I
0.36
0.38
0.40
0.42
0.44
0.46
I 0.48
a/W Fig. 7. Crack growth rate plotted a / W for a fatigue crack growing through the unstretched composite at AK = 8.5, R =0.1.
formed on the unstretched composite. It shows that as the crack grew through the specimen at a constant applied AK, the growth rates increased smoothly from 1 x 10 -5 to 2.7 x 10 -5 mm/cycle. This was felt to be a consequence of decreasing levels of closure as the macroscopic residual stresses were allowed to relax the further the crack grew. The effect of using the constant Km~ load shedding technique on near-threshold growth rates is shown in Fig. 8. Both the composites display growth plateaux starting from the stress intensity range 7.5 MPax/m where R = 0.1 ( K ~ = 7.8 MPax/m ). However, in the unstretched composite, the plateau persists for the longest range of AK (to below 5 MPax/m ). Below AK ~ 4.5 MPa~/m the two Km~x curves overlap one another, and remain coincident down to threshold which is at approximately 1.7 MPax/m. Also plotted on Fig. 8 are crack growth rate vs effective stress intensity range (AKe~) data for the two composites, determined using equation (1) and the results in Fig. 6. Below A K = 4 . 5 M P a ~ / m all the curves coincide, indicating that the constant ~ tests are producing closure-free growth rates below this stress intensity range. No differences in fracture morphology could be detected between the two composites at similar crack growth rates. Figure 9 shows typical fracture surfaces at low (4-5 MPax/m ) and high (9-10 MPa~/m) stress intensity ranges for the unstretched composite tested at R = 0.1. At the high AK range the fracture path is quite rough with some voids which are much larger than the reinforcement diameter ( ~ 3 #m). As with previous work on similar material [8] there was little evidence of particle fracture; a feature which has been cited as due to the high inherent strength of the fine reinforcement. At low AK the general appearance of the fatigue surface is that of a much smoother profile. Planar slip, which is a characteristic of near-threshold fatigue crack growth in aluminium-lithium alloys, and often leads to high closure levels, is totally suppressed [8]. Growth proceeds predominantly
I
,
I
,
I
, I ,l,l,l,l,,,h,,h,,.l
2
3 4 5 6 7 8 9 1113 16 AK (M Pax/-~) Fig. 8. Crack growth rate curves from constant K,~ (8.3 MPa~m) tests for the two composites. Also plotted are AK,e curves, calculated from the results in Figs 4 and 6. through the matrix and there are surface undulations of the order of 3 - 5 # m . A thin layer of oxide was found to cover the fracture surface completely (evident from charging problems under the SEM), indicative of a degree of fretting from asperity contact as a result of closure. 4. DISCUSSION 4.1. Rrle o f residual stresses
In the light of previous experience all the specimens tested here were cut from material which had been solution treated and quenched as a complete plate [7]. Though this served to eliminate the problems associated with crack bowing, for the unstretched plate it
Fig. 9. Typical fractographs of the unstretched composite at (a) low (4-5 MPaw/m) and (b) high (9-10 MPav/m) stress intensity factor ranges, R = 0.1.
KNOWLES et al.: FATIGUE OF A METAL MATRIX COMPOSITE
1193
(a)
Quenching residual stresses present in the machined specimens.
/
Compressive residual stresses closing crack faces togetheT as it grows. Fig. 10. Schematic diagram of (a) the compressive residual stresses which are present in the unstretchcd composite and (b) the manner in which these stresses act to clamp the crack faces together, leading to premature closure. did not eliminate the presence of quenching stresses, varying along the S direction. If one considers a specimen which has been cut from plate, the top and bottom surfaces will still be in compression, with the centre in tension, illustrated in Fig. 10. As the crack grows the local compressive stresses (initially as high as 190 MPa at the surface [7]) will be relieved, however to maintain equilibrium the compressive stresses on the opposite face will always act to try to clamp the crack surfaces back together again. This would be expected to cause premature closure of the crack faces, effectively adding a "compressive K" contribution to the applied (nominal) stress intensity. The results of Fig. 6 confirm that this is the case. Despite the scatter, it is clear that Kop values in the stretched composite are up to ~,0.6 MPa~/m lower than for the unstretched composite. A direct consequence of the higher closure levels for the unstretched composite is the higher threshold value recorded at R = 0.1, Fig. 4. The constant AK test provides a further insight into the influence of the residual stresses on growth rates. At AK = 8.5 MPa~/m, Km~nis equal to 0.94MPa~/m for an R-ratio of 0.1. It is therefore expected that closure will contribute significantly to growth rates. Km~ remains constant at 9.4 MPa~/m, but AK0~ is controlled by the Kop stress intensity. The expanded scale of Fig. 7 demonstrates that crack growth rates increase by a factor of approximately 2.5 whilst the crack propagates through the specimen at a nominally constant value of AK. If one assumes that the underlying closure level due to microstructural features remains constant over the range of AK in Fig. 6 (a conclusion which appears valid from the stretched billet data) comparison AM 41/4--N
with the crack lengths used during the closure measurements reveals that Kop falls by approximately 0.7 MPa~/m. The stretched composite fatigue crack growth curve in Fig. 4 illustrates that a change in AK of 0.7 MPa~/m around AK = 8.5 MPa~/m changes the crack growth rate by approximately 1.6 x 10 -5 mm/cycle. This is in good agreement with the growth rate change observed during the constant stress intensity range test. It is therefore concluded that this increase in growth rate is due to a reduction in the levels of crack closure experienced by the crack tip. The compressive stresses across the crack front, illustrated in Fig. 10, are always acting to cause premature closure of the crack faces. These forces are greatest when the crack length is shortest. As the crack grows, the compressive stresses experienced at the crack front are gradually relieved. It can thus be seen that as the crack penetrates the specimen, the levels of closure will decline steadily, causing the observed acceleration of crack growth rates for the same nominal value of applied AK. 4.2. Low growth rates and crack closure
The constant Km~ threshold results in Fig. 8 correlate very accurately with closure-corrected fatigue data, indicating that this form of test provides a relatively simple technique for evaluating AK~ threshold levels in these materials. The value of A / ~ does not appear to be influenced by the post quench stretch and is approximately 1.6-1.7 MPa~/m. The fractographic evidence indicates that the surface roughness and crack growth mechanism are the same for the two composites, and hence the difference in measured values of AKth observed at R = 0.1 is
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KNOWLES et al.: FATIGUE OF A METAL MATRIX COMPOSITE
a consequence of the macroscopic residual stresses influencing the closure levels. The convergence of the constant Km,xcurves, at the end of the plateau for the unstretched composite is thought to correspond to the elimination of closure effects, and hence the fact that K~, has risen above Kop. From Fig. 8 this appears to correspond to a AK value of 4.5 MPax/m, which correlates with a minimum stress intensity of 3.3 MPax/m. This is in good agreement with the Kop values measured in this regime ( ~ 3.1 MPax/m ) and supports the use of Kop as a criterion for determining the minimum effective stress intensity range. The plateaux observed in the higher AK regime of the constant Km~ tests are a consequence of the minimum stress intensity consistently lying below the closure level. Hence a decrease in the applied AK (with Km~ maintained constant) does not alter the stress intensity range experienced at the crack tip until K~. rises above the closure value. Consequently the initial section of the crack growth data is insensitive to changes in the applied stress intensity range. Figure 5 further illustrates the strong influence of closure on fatigue crack growth. The dominant effect of Km~ on the crack growth rates almost up to stage III occurs because of the fact that K~, always lies below Kop for both R --0.1 and R = 0.3. Hence changes in AK are only experienced at the crack-tip as alterations in the value o f Km~. Plotting fatigue data as a function of Km~ therefore, is analogous to producing a plot of AK,fr against crack growth rate. As discussed in the previous section, the constancy of closure level observed for the stretched composite, Fig. 6, does not appear to hold for that unstretched. In the unstretched condition closure levels appear to rise with AK and crack length. Given that there is no noticeable difference in fractography or deformation mechanism and the previously discussed observation of increasing crack growth rates at constant applied values of AK, this is confirmed as a manifestation of the macroscopic residual stresses. The Kop values were obtained with high AK measurements at the shortest crack lengths. Following from the results of the constant AK test it would therefore appear that the decrease in measured closure levels is due to a reduction in the compressive stress at the crack tip, rather than a change in the fracture surface roughness. 4.3. Mechanisms o f closure
Fractographic studies indicate the presence of a thin film of oxide on fracture surfaces, but no extensive local build up of oxide indicative of oxideinduced closure. Uncracked ligaments behind the advancing crack front have not been observed in previous research on similar material [11, 12] and the crack bridging/shielding mechanism extensively discussed by Shang and Ritchie [13] does not appear to operate in this system.
One can calculate the crack opening displacements occurring at the relatively low values of Km~ at which these composites are tested, from Knott [14] 2 Km~ ~ = - 2cryE
(2)
where 6~x is the maximum crack opening displacement. For Km~ = 7 MPax/m this gives a value of approximately 0.5 #m. With reference to the near threshold fractograph of Fig. 8 it can be seen that this is an order of magnitude less than the levels of surface roughness, where asperities of approximate height 5 #m frequently occur [12]. Subtracting the influence of residual stresses, it appears that although the presence of the reinforcement, high oxide levels and fine grain size significantly reduce facetted growth, leading to a reduction in nominal threshold levels for the composite when compared to 8090 produced by an ingot route [8], roughness is still the predominant closure mechanism. This produces the thin oxide layer most noticeable on the low growth rate failure surfaces through fretting contact. It should be noted that although the crack tip deformation is much more homogeneous in the composites, asymmetric deformation does still occur at the tip. This is sufficient to cause the mode II displacement necessary for roughness-induced closure to operate. In typical wrought aluminium-lithium alloys roughness levels usually increase as crack growth rates approach threshold. This is due to the plastic zone size ahead of the propagating crack decreasing below the grain size, leading to a transition from homogeneous to heterogeneous crack tip deformation [15]. Heterogeneous deformation along intense slip bands leads to facetted crack growth and a consequent rise in roughness associated with crack-tip deformation and hence closure levels. In the present composite, however, an opposite trend is observed. Facetted growth does not occur, and surface roughness is seen to rise with increasing AK. It is interesting to note that the levels of closure do not rise noticeably with increasing AK despite the obvious increase in surface roughness visible in Fig. 9, however. It is felt that this is due to the change in roughness profile not being of a sufficient magnitude to produce a measurable increase in the closure levels above their natural scatter range. 4.4. Threshold levels
The constant Km~ threshold tests provide a very reproducible technique by which to determine effective threshold stress intensity ranges for particulate reinforced MMCs, as has been previously reported [16]. The value of 1.6-1.7MPaw/m is remarkably consistent from one test to another. Rao and Ritchie [17] recorded lower effective thresholds than this in a range of aluminium-lithium alloys (0.8-1.0 MPa~m), but conversely Jata and Starke [18] measured higher values, in the range
KNOWLES et aL: FATIGUE OF A METAL MATRIX COMPOSITE
1195
MMCs, tested at low R-ratios. Even if a straight crack front is generated, compressive surface residual stresses, which always persist at the crack-tip, can cause premature contact of crack faces. These serve to lower the effective AK experienced at the crack tip, reducing growth rates and raising nominal threshold stress intensity levels. As a fatigue crack 4.5. Mid-range growth rates grows through the residual stress field, the residual In the mid-growth rate regime, Figs 4 and 8 stresses relax and the levels of closure are conseillustrate that, as with threshold, when plotted against quently reduced. This leads to crack growth rates AKar, the two composites possess very similar fatigue increasing as a crack grows through a specimen at crack growth rates. The differences observed between a constant applied value of AK. It also results in the growth rate curves in Fig. 4 are therefore largely crack length dependent threshold values which a consequence of varying closure levels at R = 0.1. causes problems with reproducibility and comparison The difference between the R = 0.3 and R = 0.1 data between test techniques even when crack fronts are is due to the Knox value for a corresponding AK being straight. Closure levels for the composite investigated here larger for the higher R-ratio, hence the effective stress intensity range which the crack tip experiences is lie in the range 2.3-3.3 MPax/m. Given the small greater and the crack therefore grows at a faster rate. range of AK over which fatigue crack growth occurs This is confirmed by the Kmax vs crack growth rate in these materials, owing to the low fracture toughplot of Fig. 5. Here it can be seen that growth in the ness of such composites, it is clear that closure plays mid AK regime is controlled by the value of Km~x. a significant r61e over almost the entire range of As previously proposed this is due to the value of the crack growth curve. Effective threshold stress K~an always lying below Kop, such that a change in intensity ranges lie in the range 1.6-1.7 MPax/m for the nominal AK only manifests itself as a change in these composites, confirmed by both closure measure/(max at the crack tip. The value of Km~xdictates AKin, ments and constant Km~x threshold tests. The latter and hence controls the growth rate for the two test has proved to produce very consistent results, eliminating the scatter often found when taking different R-ratios. For the two composites at R = 0.1 Fig. 6 clearly closure measurements. The intrinsic AKth values lie indicates that the unstretched composite possesses within the range recorded for typical unreinforced higher closure levels for all values of AK. As K~n lies aluminium-lithium alloys. This is as expected as below Kop in this regime, closure levels affect AKen, crack growth near threshold is entirely through which is larger for the stretched composite, and it the matrix. The threshold values are also in close agreement with those results for 7xxx/SiC particle therefore exhibits faster growth rates. reinforced systems. 4.6. High growth rates The principal mechanism of closure appears to The separation of the fatigue curves at high values be due to roughness. Even at threshold where, in of AK, Fig. 4, approaching fast fracture, occurs for contrast to wrought aluminium-lithium alloys, the fracture profiles are smoothest, maximum crack tip two reasons: opening is an order of magnitude less than that of the (i) at R = 0.3 the Km~ value reaches a critical asperity height, suggesting that this is not unexpected. fracture value K~ at a lower value of AK, and There does not appear to be a significant rise in (ii) stretching the composite reduces the fracture closure levels despite an increase in surface roughness toughness slightly from 13.9 to 12.0 MPa.v/m, at higher values of AK. This may be due to the scatter though the proof stresses are the same within in measured closure levels being greater than the experimental scatter. increase in their real value, or could be a consequence The plot of Fig. 5 confirms the effect of K~x on of the limited asymmetric crack tip deformation stage III crack growth. The coincidence of the curves which is possible in these materials. at low AK is due to closure effects, but approaching final failure static fracture modes, which are K ~ Acknowledgements--The authors wish to acknowledge controlled, start to dominate. It is their influence support from BP Metal Composites (DMK, TJD), SERC which maintains the overlap of the curves where (DMK, TJD), St Catharine's College, Cambridge (DMK). Material was supplied by BP Metal Composites Ltd. The Kmin values for the R = 0.3 data lie above Kop. authors are also grateful to Professor C. J. Humphreys for provision of research facilities.
2-3 MPav/m, for similar alloys, probably indicating a large variation in the interpretation of closure levels from conventional testing. In MMCs Shang and Ritchie [19] measure AKefr values around 1-2 MPav/m in 7xxx based systems which shows good agreement with the present results.
5. S U M M A R Y
It has been demonstrated that macroscopic residual stresses arising from solution heat treatment and quenching can play a significant r61e in measured fatigue crack propagation rates of particle reinforced
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KNOWLES et al.: FATIGUE OF A METAL MATRIX COMPOSITE
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0956-7151/93 $6.00 + 0.00 Copyright© 1993Pergamon Press Ltd
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CRACK CLOSURE A N D RESIDUAL STRESS EFFECTS IN FATIGUE OF A PARTICLE-REINFORCED METAL MATRIX COMPOSITE D. M. KNOWLES, T. J. DOWNES and J. E. KING Department of Materials Science and Metallurgy, Pembroke Street, Cambridge CB2 3QZ, England (Received 13 August 1992)
AImtraet--A study of the influence of macroscopic quenching stresses on long fatigue crack growth in an aluminium alloy-SiC composite has been made. Direct comparison between quenched plate, where high residual stresses are present, and quenched and stretched plate, where they have I ~ n eliminated, has highlighted their r61e in crack closure. Despite similar strength levels and identical crack growth mechanisms, the stretched composite displays faster crack growth rates over the complete range of AK, measured at R = 0.1, with threshold being displaced to a lower nominal AK value. Closure levels are dependent upon crack length, but are greater in the unstretched composite, due to the effect of surface compressive stresses acting to close the crack tip. These result in lower values of AKar in the unstretched material, explaining the slower crack growth rates. Effective AKth values are measured at 1.7 MPax/m, confirmed by constant Km~ testing. In the absence of residual stress, closure levels of approximately 2.5 MPa~/m are measured and this is attributed to a roughness mechanism.
I. I N T R O D U C T I O N Particle reinforced composites are now being produced which show substantial increases in specific strength and stiffness compared with unreinforced alloys [1-4]. However, the main concern with the engineering application of these materials still lies with their low fracture toughness, often accompanied by poor fatigue crack propagation resistance at high growth rates [1, 3, 5, 6]. Both in order to allow property comparisons to be made and to examine ways in which to improve crack growth resistance in these composites it is essential to be able to generate consistent and valid test data. A previous paper by the authors has discussed the problems encountered in the testing of a SiC particle reinforced metal matrix composite (MMC) with a heat-treatable aluminium alloy matrix requiring solution treatment and cold water quenching [7]. The quench generates significant macroscopic residual stresses. The compressive surface stresses cause premature crack closure for cracks growing through near-surface material (i.e. close to the faces of beat-treated test pieces) leading to problems in generating reliable crack growth data because of the resulting curved and irregular crack fronts. Crackfront curvature can be significantly reduced by using specimens which have been cut from heat-treated plate in the T - S orientation. There is, however, concern that the residual stress distribution which still exists through the plate thickness, i.e. the stress variation along the S direction, can have a significant influence on crack closure levels and hence fatigue crack growth threshold values, even when straight Crack fronts are produced, because the residual stress
variation along the longitudinal (L) direction is small. This paper reports on an investigation into fatigue of a particle reinforced M M C in the as-quenched (in plate form) and as-quenched and stretched conditions. In the stretched condition macroscopic residual quenching stresses are, to a large extent, eliminated and hence closure levels are determined in a homogeneous stress field. For the unstretched plate, macroscopic residual stresses still exist. Closure levels have been compared directly for the two billet conditions. The effect of the quenching stresses on threshold values and on fatigue crack growth is described, along with a discussion on the factors which appear to be controlling the underlying closure mechanisms, i.e. those which physically prevent the full closure of the crack-tip.
2. EXPERIMENTAL
2.1. Material
The material used throughout this study consisted of an 8090 aluminium alloy matrix containing 2.61 Li-1.25 Cu-0.91 Mg-0.1 Zr (wt%), reinforced with 20 wt% F1200 (nominally 3/zm) grade particulate SiC. The composite was produced by BP Metal Composites Ltd, Farnborough, using a powder metallurgy route involving blending of the alloy powder and reinforcement, compaction, consolidation by hot isostatic pressing, hot forging and finally rolling to a plate thickness of approximately 15 mm. The material was solution treated at 530°C for 2 h and then cold water quenched. One billet was then stretched by 2%. Specimens were machined from
1189
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KNOWLES et al.: FATIGUE OF A METAL MATRIX COMPOSITE 30-
.o "6
20
10 Q
> 0
s 5ollm Fig. 1. Optical microstructure of composite material in plate form. plate either after quenching, or after quenching and stretching. The microstructure of the material is illustrated in Fig. 1. It can be seen that the SiC distribution is generally banded in the S - T and S - L planes and homogeneous in the L - T plane (directions defined relative to the plate in Fig. 2). There is a distinct range of SiC particle size present, with a large number of particles in the submicron to 1/~m range, and a continually decreasing number fraction up to approximately 7 g m in diameter. The volume percentage fraction can be seen to peak around a particle size of 3.73-5.28 gm, Fig. 3. 2.2. Mechanical testing
Tensile tests were performed in the transverse (T) orientation of the plate using Hounsfield specimens having a 10mm 2 cross-sectional area. Strain was measured using a clip gauge-type extensometer with a 10 mm gauge length. Fatigue testing was carried out on single edge notch bend (SENB) specimens with dimensions 12.5 x 12.5 x 100 mm. These were cut from the plate so as to grow the cracks in a T - S orientation (Fig. 2). After machining the specimens were aged for 5 h at 170°C, to produce an approximately peak aged condition. The ageing treatment was chosen from previous work on similar composites, so as to give high strength and straight crack fronts whilst still retaining large macroscopic residual stresses in the unstretched condition [7, 8]. Hardness studies indicated that stretching did not affect the time at which peak strength was attained.
''
"
Particle size (l~m) Fig. 3. Range of SiC particle size in the fabricated composites. Numbers under the columns indicate the maximum particle size detected in each band. Tests were initially performed in accordance with the guide-lines in ASTM standard E647-88 [9], under four point bending, with a sinusoidal loading cycle at a frequency of 20 Hz. Crack length was monitored by a d.c. potential drop method. An on-line interactive computer was used to control the loads and produce both threshold, constant load range (increasing AK) and constant AK data. Thresholds were approached by either one of two methods of load shedding: (i) stress intensity range reductions of 5% at a constant R-ratio (0.1) after the crack had grown through a distance equivalent to four times the maximum plastic zone size, and (ii) stress intensity range reductions of 5% after the crack had grown through a distance equivalent to four times the maximum plastic zone size, commencing with AK = 7.5 MPav/m and R = 0.1, but keeping Kmaxconstant and simply raising K~i~ to reduce AK (constant Km~xtests). Macroscopic crack closure measurements were made by means of a back face strain gauge located at the centre of the specimen, opposite the growing crack. Elastic compliance curves of relative strain vs load were monitored as a function of AK, allowing the crack to grow through a distance greater than 100/~m at a constant value of AK before measurements were recorded [10]. Crack opening stress intensity (Kop) was estimated as the point of deviation of the stress-strain curve from linearity during the increasing load half of the cycle. The effective stress intensity was then calculated from AK,fr = / ~ = -- Kop
Fig. 2. Orientation of single edge notch bend (SENB) specimens with respect to plate material: L longitudinal, T transverse and S short transverse.
(1)
KNOWLES et al.: FATIGUE OF A METAL MATRIX COMPOSITE
1191
Table 1. Composite 0.2% proof stresses Composite
0.2% proof stress (MPa)
Peak aged unstretched Peak aged stretched
10 -4
509 _+8 501 +_5
R=0.1 R=0.3
x •
j
10-5
Fractographic studies were carried out in a Camscan $2 SEM operated at 20-30 kV. 3. RESULTS
x Z
xA×A ×A
10-6
XA X
o3
10-7
Tensile results for the two composites are listed in Table 1. The variations in fatigue crack growth rate (da/dN) for the peak aged condition of both composites at R = 0.1, are shown as a function of cyclic stress intensity range (AK) in Fig. 4. Also plotted are the data obtained for the unstretched material tested at R = 0 . 3 . It is clear that for R = 0 . 1 the crack growth rates for the stretched material are always higher than those for the unstretched plate, at corresponding values of AK, by a factor of approximately 2-3, with nominal threshold values (AKth.... ) being reduced from around 3.9 to 3.5 MPa~/m by stretching. The curves lie approximately parallel to one another throughout the whole stress intensity range over which fatigue crack growth occurs. Increasing the R-ratio to 0.3 is seen to cause a significant increase in growth rates for the unstretched material such that da/dN values are consistently greater than those for the stretched plate at R = 0.1. The curve has a greater slope than that determined at R = 0.1 and turns prematurely up to stage III as a consequence of the higher Kmax value. Threshold was not determined precisely, but can be seen to lie at a lower value of AK than for either of the two composites tested at R = 0.1. Replotting the data for the unstretched composite, in terms of crack growth rate vs Kmax, produces the interesting effect of complete coincidence for curves from the tests at the two R-ratios. This has been done in Fig. 5, demonstrating that Kmax
I
I
I
,
3
4
I I J [ I I*l,l,l,,,I,,,llllhmd
5
6 7 8 910 12 1416 19
Kmax (MPa d'~) Fig. 5. Plot of K~ax against crack growth rate for the unstretched composite tested at R = 0.1 and R = 0.3. rather than the applied AK, is the major controlling factor over the whole crack growth curve, for these two R values. Figure 6 shows the crack opening stress intensity values as a function of AK and a/W for the two composites. For the stretched composite the closure levels can be seen to remain relatively constant with increasing AK, lying in the range 2.3-2.6 MPax/m. In the unstretched composite, the closure measurements ranged from Kop = 3.3 M P a x / m at AK = 7.6 M P a v / m (a/W = 0.36) to 2.7 M P a v / m at AK = 4.0 M P a x / m (a/W = 0.5), decreasing in an approximately linear fashion. It should be noted that the highest closure levels for the unstretched composite were recorded when the crack length was at its shortest (i.e. closest to the notch) and macroscopic quenched-in residual stresses would have had the least opportunity to relax through the specimen width. Figure 7 illustrates the results from a simple constant AK test (AK = 8.5 MPax/m, R = 0.1) pera/W
[] Unstretched R =0.1 10-4 _f- x Stretched R = 0.1 0 Unstretched R = 0.3 ~"
0.48 I
0
,t~xX~ xXX
0.46 I
0.44 I
0.42 I
0.40 I
0.38 I
[] Stretched x Unstretched
10-5
x
x
I~ 10-6 O
z
x
D
~
x
~
x
X [] [] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [] [] [] []
[]
10_7
0
2
I
I 3
I
J 4
I
I, Iml,l,ltlmllJllll 5 6 7 8 9 11 13 15
AK (MPa d'~) Fig. 4. Variations in fatigue crack growth rate (da/dN) for the peak aged conditions of the stretched and unstretched composites at R = 0. I, plotted as a function of AK. Also plotted are the results for the unstretched material at R =0.3.
I 4
I 5
I 6
I 7
AK (MP ax/-~) Fig. 6. Crack opening stress intensity values as a function of AK and a/W for the stretched and unstretched composites.
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KNOWLES et al.: FATIGUE OF A METAL MATRIX COMPOSITE
x l O -5
+ C o n s t a n t Kma x u n s t r e t c h e d * E f f e c t i v e AK s t r e t c h e d A C o n s t a n t Kma x s t r e t c h e d
m[]
2.5
[] [] []
10-4 ~
[]
• Effective AK unstretched
[]
"d
10-5
[]
.~ + + +
2.0 --
•~*4IA i~-
[] 10_6
[]
x.-
Z ~-~ 1.5--
[]
[]
z
~10 -7
[] [] 0.34
zxA A +
[]
[]
"+
i
I
I
I
i
I
I
0.36
0.38
0.40
0.42
0.44
0.46
I 0.48
a/W Fig. 7. Crack growth rate plotted a / W for a fatigue crack growing through the unstretched composite at AK = 8.5, R =0.1.
formed on the unstretched composite. It shows that as the crack grew through the specimen at a constant applied AK, the growth rates increased smoothly from 1 x 10 -5 to 2.7 x 10 -5 mm/cycle. This was felt to be a consequence of decreasing levels of closure as the macroscopic residual stresses were allowed to relax the further the crack grew. The effect of using the constant Km~ load shedding technique on near-threshold growth rates is shown in Fig. 8. Both the composites display growth plateaux starting from the stress intensity range 7.5 MPax/m where R = 0.1 ( K ~ = 7.8 MPax/m ). However, in the unstretched composite, the plateau persists for the longest range of AK (to below 5 MPax/m ). Below AK ~ 4.5 MPa~/m the two Km~x curves overlap one another, and remain coincident down to threshold which is at approximately 1.7 MPax/m. Also plotted on Fig. 8 are crack growth rate vs effective stress intensity range (AKe~) data for the two composites, determined using equation (1) and the results in Fig. 6. Below A K = 4 . 5 M P a ~ / m all the curves coincide, indicating that the constant ~ tests are producing closure-free growth rates below this stress intensity range. No differences in fracture morphology could be detected between the two composites at similar crack growth rates. Figure 9 shows typical fracture surfaces at low (4-5 MPax/m ) and high (9-10 MPa~/m) stress intensity ranges for the unstretched composite tested at R = 0.1. At the high AK range the fracture path is quite rough with some voids which are much larger than the reinforcement diameter ( ~ 3 #m). As with previous work on similar material [8] there was little evidence of particle fracture; a feature which has been cited as due to the high inherent strength of the fine reinforcement. At low AK the general appearance of the fatigue surface is that of a much smoother profile. Planar slip, which is a characteristic of near-threshold fatigue crack growth in aluminium-lithium alloys, and often leads to high closure levels, is totally suppressed [8]. Growth proceeds predominantly
I
,
I
,
I
, I ,l,l,l,l,,,h,,h,,.l
2
3 4 5 6 7 8 9 1113 16 AK (M Pax/-~) Fig. 8. Crack growth rate curves from constant K,~ (8.3 MPa~m) tests for the two composites. Also plotted are AK,e curves, calculated from the results in Figs 4 and 6. through the matrix and there are surface undulations of the order of 3 - 5 # m . A thin layer of oxide was found to cover the fracture surface completely (evident from charging problems under the SEM), indicative of a degree of fretting from asperity contact as a result of closure. 4. DISCUSSION 4.1. Rrle o f residual stresses
In the light of previous experience all the specimens tested here were cut from material which had been solution treated and quenched as a complete plate [7]. Though this served to eliminate the problems associated with crack bowing, for the unstretched plate it
Fig. 9. Typical fractographs of the unstretched composite at (a) low (4-5 MPaw/m) and (b) high (9-10 MPav/m) stress intensity factor ranges, R = 0.1.
KNOWLES et al.: FATIGUE OF A METAL MATRIX COMPOSITE
1193
(a)
Quenching residual stresses present in the machined specimens.
/
Compressive residual stresses closing crack faces togetheT as it grows. Fig. 10. Schematic diagram of (a) the compressive residual stresses which are present in the unstretchcd composite and (b) the manner in which these stresses act to clamp the crack faces together, leading to premature closure. did not eliminate the presence of quenching stresses, varying along the S direction. If one considers a specimen which has been cut from plate, the top and bottom surfaces will still be in compression, with the centre in tension, illustrated in Fig. 10. As the crack grows the local compressive stresses (initially as high as 190 MPa at the surface [7]) will be relieved, however to maintain equilibrium the compressive stresses on the opposite face will always act to try to clamp the crack surfaces back together again. This would be expected to cause premature closure of the crack faces, effectively adding a "compressive K" contribution to the applied (nominal) stress intensity. The results of Fig. 6 confirm that this is the case. Despite the scatter, it is clear that Kop values in the stretched composite are up to ~,0.6 MPa~/m lower than for the unstretched composite. A direct consequence of the higher closure levels for the unstretched composite is the higher threshold value recorded at R = 0.1, Fig. 4. The constant AK test provides a further insight into the influence of the residual stresses on growth rates. At AK = 8.5 MPa~/m, Km~nis equal to 0.94MPa~/m for an R-ratio of 0.1. It is therefore expected that closure will contribute significantly to growth rates. Km~ remains constant at 9.4 MPa~/m, but AK0~ is controlled by the Kop stress intensity. The expanded scale of Fig. 7 demonstrates that crack growth rates increase by a factor of approximately 2.5 whilst the crack propagates through the specimen at a nominally constant value of AK. If one assumes that the underlying closure level due to microstructural features remains constant over the range of AK in Fig. 6 (a conclusion which appears valid from the stretched billet data) comparison AM 41/4--N
with the crack lengths used during the closure measurements reveals that Kop falls by approximately 0.7 MPa~/m. The stretched composite fatigue crack growth curve in Fig. 4 illustrates that a change in AK of 0.7 MPa~/m around AK = 8.5 MPa~/m changes the crack growth rate by approximately 1.6 x 10 -5 mm/cycle. This is in good agreement with the growth rate change observed during the constant stress intensity range test. It is therefore concluded that this increase in growth rate is due to a reduction in the levels of crack closure experienced by the crack tip. The compressive stresses across the crack front, illustrated in Fig. 10, are always acting to cause premature closure of the crack faces. These forces are greatest when the crack length is shortest. As the crack grows, the compressive stresses experienced at the crack front are gradually relieved. It can thus be seen that as the crack penetrates the specimen, the levels of closure will decline steadily, causing the observed acceleration of crack growth rates for the same nominal value of applied AK. 4.2. Low growth rates and crack closure
The constant Km~ threshold results in Fig. 8 correlate very accurately with closure-corrected fatigue data, indicating that this form of test provides a relatively simple technique for evaluating AK~ threshold levels in these materials. The value of A / ~ does not appear to be influenced by the post quench stretch and is approximately 1.6-1.7 MPa~/m. The fractographic evidence indicates that the surface roughness and crack growth mechanism are the same for the two composites, and hence the difference in measured values of AKth observed at R = 0.1 is
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KNOWLES et al.: FATIGUE OF A METAL MATRIX COMPOSITE
a consequence of the macroscopic residual stresses influencing the closure levels. The convergence of the constant Km,xcurves, at the end of the plateau for the unstretched composite is thought to correspond to the elimination of closure effects, and hence the fact that K~, has risen above Kop. From Fig. 8 this appears to correspond to a AK value of 4.5 MPax/m, which correlates with a minimum stress intensity of 3.3 MPax/m. This is in good agreement with the Kop values measured in this regime ( ~ 3.1 MPax/m ) and supports the use of Kop as a criterion for determining the minimum effective stress intensity range. The plateaux observed in the higher AK regime of the constant Km~ tests are a consequence of the minimum stress intensity consistently lying below the closure level. Hence a decrease in the applied AK (with Km~ maintained constant) does not alter the stress intensity range experienced at the crack tip until K~. rises above the closure value. Consequently the initial section of the crack growth data is insensitive to changes in the applied stress intensity range. Figure 5 further illustrates the strong influence of closure on fatigue crack growth. The dominant effect of Km~ on the crack growth rates almost up to stage III occurs because of the fact that K~, always lies below Kop for both R --0.1 and R = 0.3. Hence changes in AK are only experienced at the crack-tip as alterations in the value o f Km~. Plotting fatigue data as a function of Km~ therefore, is analogous to producing a plot of AK,fr against crack growth rate. As discussed in the previous section, the constancy of closure level observed for the stretched composite, Fig. 6, does not appear to hold for that unstretched. In the unstretched condition closure levels appear to rise with AK and crack length. Given that there is no noticeable difference in fractography or deformation mechanism and the previously discussed observation of increasing crack growth rates at constant applied values of AK, this is confirmed as a manifestation of the macroscopic residual stresses. The Kop values were obtained with high AK measurements at the shortest crack lengths. Following from the results of the constant AK test it would therefore appear that the decrease in measured closure levels is due to a reduction in the compressive stress at the crack tip, rather than a change in the fracture surface roughness. 4.3. Mechanisms o f closure
Fractographic studies indicate the presence of a thin film of oxide on fracture surfaces, but no extensive local build up of oxide indicative of oxideinduced closure. Uncracked ligaments behind the advancing crack front have not been observed in previous research on similar material [11, 12] and the crack bridging/shielding mechanism extensively discussed by Shang and Ritchie [13] does not appear to operate in this system.
One can calculate the crack opening displacements occurring at the relatively low values of Km~ at which these composites are tested, from Knott [14] 2 Km~ ~ = - 2cryE
(2)
where 6~x is the maximum crack opening displacement. For Km~ = 7 MPax/m this gives a value of approximately 0.5 #m. With reference to the near threshold fractograph of Fig. 8 it can be seen that this is an order of magnitude less than the levels of surface roughness, where asperities of approximate height 5 #m frequently occur [12]. Subtracting the influence of residual stresses, it appears that although the presence of the reinforcement, high oxide levels and fine grain size significantly reduce facetted growth, leading to a reduction in nominal threshold levels for the composite when compared to 8090 produced by an ingot route [8], roughness is still the predominant closure mechanism. This produces the thin oxide layer most noticeable on the low growth rate failure surfaces through fretting contact. It should be noted that although the crack tip deformation is much more homogeneous in the composites, asymmetric deformation does still occur at the tip. This is sufficient to cause the mode II displacement necessary for roughness-induced closure to operate. In typical wrought aluminium-lithium alloys roughness levels usually increase as crack growth rates approach threshold. This is due to the plastic zone size ahead of the propagating crack decreasing below the grain size, leading to a transition from homogeneous to heterogeneous crack tip deformation [15]. Heterogeneous deformation along intense slip bands leads to facetted crack growth and a consequent rise in roughness associated with crack-tip deformation and hence closure levels. In the present composite, however, an opposite trend is observed. Facetted growth does not occur, and surface roughness is seen to rise with increasing AK. It is interesting to note that the levels of closure do not rise noticeably with increasing AK despite the obvious increase in surface roughness visible in Fig. 9, however. It is felt that this is due to the change in roughness profile not being of a sufficient magnitude to produce a measurable increase in the closure levels above their natural scatter range. 4.4. Threshold levels
The constant Km~ threshold tests provide a very reproducible technique by which to determine effective threshold stress intensity ranges for particulate reinforced MMCs, as has been previously reported [16]. The value of 1.6-1.7MPaw/m is remarkably consistent from one test to another. Rao and Ritchie [17] recorded lower effective thresholds than this in a range of aluminium-lithium alloys (0.8-1.0 MPa~m), but conversely Jata and Starke [18] measured higher values, in the range
KNOWLES et aL: FATIGUE OF A METAL MATRIX COMPOSITE
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MMCs, tested at low R-ratios. Even if a straight crack front is generated, compressive surface residual stresses, which always persist at the crack-tip, can cause premature contact of crack faces. These serve to lower the effective AK experienced at the crack tip, reducing growth rates and raising nominal threshold stress intensity levels. As a fatigue crack 4.5. Mid-range growth rates grows through the residual stress field, the residual In the mid-growth rate regime, Figs 4 and 8 stresses relax and the levels of closure are conseillustrate that, as with threshold, when plotted against quently reduced. This leads to crack growth rates AKar, the two composites possess very similar fatigue increasing as a crack grows through a specimen at crack growth rates. The differences observed between a constant applied value of AK. It also results in the growth rate curves in Fig. 4 are therefore largely crack length dependent threshold values which a consequence of varying closure levels at R = 0.1. causes problems with reproducibility and comparison The difference between the R = 0.3 and R = 0.1 data between test techniques even when crack fronts are is due to the Knox value for a corresponding AK being straight. Closure levels for the composite investigated here larger for the higher R-ratio, hence the effective stress intensity range which the crack tip experiences is lie in the range 2.3-3.3 MPax/m. Given the small greater and the crack therefore grows at a faster rate. range of AK over which fatigue crack growth occurs This is confirmed by the Kmax vs crack growth rate in these materials, owing to the low fracture toughplot of Fig. 5. Here it can be seen that growth in the ness of such composites, it is clear that closure plays mid AK regime is controlled by the value of Km~x. a significant r61e over almost the entire range of As previously proposed this is due to the value of the crack growth curve. Effective threshold stress K~an always lying below Kop, such that a change in intensity ranges lie in the range 1.6-1.7 MPax/m for the nominal AK only manifests itself as a change in these composites, confirmed by both closure measure/(max at the crack tip. The value of Km~xdictates AKin, ments and constant Km~x threshold tests. The latter and hence controls the growth rate for the two test has proved to produce very consistent results, eliminating the scatter often found when taking different R-ratios. For the two composites at R = 0.1 Fig. 6 clearly closure measurements. The intrinsic AKth values lie indicates that the unstretched composite possesses within the range recorded for typical unreinforced higher closure levels for all values of AK. As K~n lies aluminium-lithium alloys. This is as expected as below Kop in this regime, closure levels affect AKen, crack growth near threshold is entirely through which is larger for the stretched composite, and it the matrix. The threshold values are also in close agreement with those results for 7xxx/SiC particle therefore exhibits faster growth rates. reinforced systems. 4.6. High growth rates The principal mechanism of closure appears to The separation of the fatigue curves at high values be due to roughness. Even at threshold where, in of AK, Fig. 4, approaching fast fracture, occurs for contrast to wrought aluminium-lithium alloys, the fracture profiles are smoothest, maximum crack tip two reasons: opening is an order of magnitude less than that of the (i) at R = 0.3 the Km~ value reaches a critical asperity height, suggesting that this is not unexpected. fracture value K~ at a lower value of AK, and There does not appear to be a significant rise in (ii) stretching the composite reduces the fracture closure levels despite an increase in surface roughness toughness slightly from 13.9 to 12.0 MPa.v/m, at higher values of AK. This may be due to the scatter though the proof stresses are the same within in measured closure levels being greater than the experimental scatter. increase in their real value, or could be a consequence The plot of Fig. 5 confirms the effect of K~x on of the limited asymmetric crack tip deformation stage III crack growth. The coincidence of the curves which is possible in these materials. at low AK is due to closure effects, but approaching final failure static fracture modes, which are K ~ Acknowledgements--The authors wish to acknowledge controlled, start to dominate. It is their influence support from BP Metal Composites (DMK, TJD), SERC which maintains the overlap of the curves where (DMK, TJD), St Catharine's College, Cambridge (DMK). Material was supplied by BP Metal Composites Ltd. The Kmin values for the R = 0.3 data lie above Kop. authors are also grateful to Professor C. J. Humphreys for provision of research facilities.
2-3 MPav/m, for similar alloys, probably indicating a large variation in the interpretation of closure levels from conventional testing. In MMCs Shang and Ritchie [19] measure AKefr values around 1-2 MPav/m in 7xxx based systems which shows good agreement with the present results.
5. S U M M A R Y
It has been demonstrated that macroscopic residual stresses arising from solution heat treatment and quenching can play a significant r61e in measured fatigue crack propagation rates of particle reinforced
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