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The influence of strain aging on the fatigue crack propagation threshold
Scripta METALLURGICA
Vol. 17, pp. 49-52, 1983 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
THE INFLUENCE OF STRAIN AGING ON THE FATIGUE CRACK PROPAGATION THRESHOLD
J . I . Dickson, Mou-Chih Lu* and J.-P. Bailon D~p. de g~nie m~tallurgique, Ecole Polytechnique Montreal, Quebec, Canada H3C 3A7 *present address: Fansteel Corp. North Chicago, I l l i n o i s
(Received March 29, 1982)
I ntro ducti on While the influence of strain-aging on the endurance l i m i t and on the dislocation behaviour during cyclic deformation (l-6) has received some attention in the l i t e r a t u r e , almost no attention appears to have been paid to the influence of strain aging on fatigue crack propagation. An influence of strain aging on fatigue propagation can be expected to manifest i t s e l f most clearly at the very small propagation rates near AKth, the fatigue crack propagation threshold. The present study was therefore performed to determine whether an influence attributable to strain aging could be detected near AKth during tests carried out on two mild steels having d i f f e r e n t s e n s i t i v i t i e s to strain aging. Materials and experimental procedure The two steels studied, obtained as 15.3 mm thick hot-rolled plate, had the compositions indicated in table I . Steel A is an aluminum-killed mild steel; while steel B is a renitrogenized mild steel containing no aluminum and therefore much more sensitive to strain-aging effects. The crack propagation tests were carried out employing 12.7 mm thick compact tension samples of standard (ASTME399-78) geometry. Crack propagation was parallel to the h o t - r o l l i n g direction. During the testing, a load r a t i o , R = Kmin/Kmax, of O.l was employed at a frequency of 50 Hz. The value of AKth was approached by decreasing the cyclic stress intensity factor, AK, i n i t i a l l y in increments of 10% and near AKth, in increments of 5%. At any value of AK, cracks were propagated through lengths _> 0.2 mm to obtain the velocity measurements. The crack length was measured o p t i c a l l y using a method with a s e n s i t i v i t y of I0 ~m. The machined samples were annealed, generally f o r 0.5 hrsat 950°C, and cooled in a i r from this temperature. A few o f these samples were reannealed f o r 0.5 hrsat 700oc and waterquenched from this temperature. Both these heat treatments gave a f e r r i t i c grain size o f approximately 30 um f o r steel A and o f 20 um f or steel B. A few samples of steel A were annealed f o r 1 hr at 1200oc, furnace cooled to 950°C, and then a i r cooled to room temperature, which heat treatment produced a grain size o f 80 nm. Experimental results and discussion Figure 1 compares the threshold and near-threshold results obtained at 23°C in the a i r - c o o l e d and in the water-quenched conditions. For the purpose o f the present study, the crack propagation threshold w i l l be assumed to be the value of AK that corresponds to a crack propagation r a t e , da/dn, o f 4 X 10-7 mm/cycle, since in most o f the tests performed at leas t one measurement of da/dn near this value was obtained. The r e s u l t s , moreover, indicated t h a t , below 4 X 10-7 mm/cycle, the l o g - l o g p l o t o f da/dn versus AK corresponds to an almost v e r t i c a l l i n e . From figure l , i t can be seen that quenching from 700oc increases the value of aKth obtained at 23°C by approximately 25% in the case of steel A and 50% in the case o f material B, which steel is more s e n s i t i v e to s t r a i n aging. The present results therefore i n d i c a t e that the increase in
49
0036-9748/83/010049-04503.00/0 Copyright (c) 1983 Pergamon Press Ltd.
50
FATIGUE
TIIRESHOLD
Vol.
17, No.
1
AKth caused by quenching is related to the greater amount of dynamic s t r a i n aging occurring in the v i c i n i t y of the crack t i p during very slow fatigue crack growth in the quenched samples. In order to obtain f u r t h e r evidence of the role of s t r a i n aging, two other sets of tests were performed on the a i r - c o o l e d samples of steel A. A f t e r the threshold had been reached at 23°C, two samples of steel A were aged f o r 20 hrs under a s t a t i c load corresponding to the value of the mean stress i n t e n s i t y f a c t o r that had been employed at the threshold. Aging temperatures of 23 and 55°C were employed with the sample cooled to 23oc under load a f t e r aginq at 55°C. The new value of AK required to cause a crack growth rate of approximately 4 X lO-7-mm/cycle was then measured. From table I I , i t can be noted that t h i s new value is l i t t l e above that required to cause the same propagation rate in the absence of s t a t i c s t r a i n aging, i n d i c a t i n g a weak influence of s t a t i c s t r a i n aging on the threshold i n this a i r -cooled material. The second procedure was one of c y c l i c s t r a i n aging a f t e r the influence of s t a t i c s t r a i n aging had been measured. The two samples were aged again e i t h e r at 23°C or at 55°C for a given period, during which c y c l i n g was performed continuously at AK = 0.85 AKth and R : 0 . I . A f t e r t h i s c y c l i c s t r a i n aging, the new AK value required to give a propagation rate of approximately 4 X 10-7 ram/cycle was measured e i t h e r at 23°C or 55oc. Table 3 summarizes the results obtained. I t can be noted that c y c l i c s t r a i n aging at 23oc f o r 20hrs had l i t t l e e f f e c t on AKth; however, c y c l i c s t r a i n aging at 55°C had a stronger influence and produced new AKth values s i g n i f i c a n t l y higher than the reference value obtained in the absence of a deliberate s t r a i n aging procedure. The AKth values measured at 23 and 55°C in the absence of any s t r a i n aging were also compared f o r air-cooled samples of steel A, but having a grain size of 80 values were e s s e n t i a l l y i d e n t i c a l ( I 0 . I MPa m at 23oc, ]0.0 MPa m at 55°C). This of AKth compared to that obtained f o r the 30 ~m grain size confirms the increase of increasing grain size f o r low carbon steels noted by previous authors (7-10).
sequences um. The higher value AKth with
Discussion The r e s u l t s presented i n table 2 and 3 can therefore be i n t e r p r e t e d as i n d i c a t i n g that dynamic s t r a i n aging occurring at 55°C during stress cycling at 0.85 &Kth can increase both the 23°C and 55°C values of AKth f o r air-cooled steel A by = 15-25%. On the other hand, n e i t h e r 20 hrs of such stress c y c l i n g at 23oc nor 20 hrs of s t a t i c s t r a i n aging at 55oc s i g n i f i c a n t l y increased the 23°C threshold value of t h i s steel in the a i r - c o o l e d condition. These results are in agreement with a strong influence of stress c y c l i n g on increasing the size of s t r a i n - a g i n g y i e l d points indicated by studies on niobium ( I I ) and on mild steel (12). The weak influence of s t a t i c s t r a i n aging at 55°C on AKth can also be related to the strong unpinning e f f e c t a n d s i g n i f i c a n t decrease of the y i e l d point obtained i f , a f t e r a s t a t i c s t r a i n aging at a high stress, the stress is even momentarily decreased to a low stress (13, 14). Suresh and Ritchie (15) recently have also carried out tests on a Cr-Mo pressure vessel steel having a b a i n i t i c microstructure in which periods of stress cycling performed s l i g h t l y below AKth caused crack retardation when AK was reincreased s l i g h t l y above the threshold. They i n t e r p r e t e d such crack r e t a r d a t i o n as being caused by crack closure effects associated with a build-up of f r e t t i n g corrosion product during c y c l i n g s l i g h t l y below AKth. The r e l a t i v e l y small influence of c y c l i n g s l i g h t l y below AKth when carried out at 23°C compared to 55°C observed i n the present study appears to be much more i n keeping with a dynamic s t r a i n - a g i n g e f f e c t , p a r t i c u l a r l y since s i m i l a r AKth values f o r air-cooled steel A (80 ~m grain size) were obtained at 23 and 55oc and the presence of f r e t t i n g oxide would be expected (16) to influence the AKth value for an R-ratio of 0 . I . In material not quenched to r e t a i n i n t e r s t i t i a l solute atoms in s o l u t i o n , the d i s s o l u t i o n of f i n e carbide or n i t r i d e p r e c i p i t a t e s may be required before important dynamic s t r a i n - a g i n g effects can occur near the crack t i p . Such d i s s o l u t i o n of f i n e precipitates is suggested from low-cycle fatigue studies (2, 3, 6, 17). On the other hand, with this explan a t i o n , the distance over which subsequent crack growth is influenced by c y c l i n g below AKth appears large compared to the expected p l a s t i c zone sizes. In one case, an e f f e c t over a distance of approximately 0.7 mm was observed compared to the calculated plane s t r a i n monotonic or larger p l a s t i c zone size of approximately 0.3 mm. In the higher strength steel employed by Suresh and Ritchie (15), there is stronger discrepancy between the distance over which crack retardation occurred and the p l a s t i c zone s i z e , which discrepancy supports t h e i r proposed explanation. I t is possible t h a t the build-up of a f r e t t i n g corrosion product also may be
Vol.
17, No,
1
FATIGUE THRESIIOLD
51
TABLE 1 Chemical Composition (wt %) Mn
Cu
Ni
Steel A
0.05
O. 32
0.012
0.06
0.004
0.03
Steel B
O.lO
0.58
0.013
0.06
O.OlO
0.06
Cr
Mo
Al
Nb
Steel A
0.01
O. 004
0.007
Steel B
0.05
O.Ol 7
O.OOl
0.003
0.0026 0.0104
TABLE 2 Influence o f s t a t i c s t r a i n aging on AKth f o r steel A
SAMPLE A1 A2
STRAIN-AGING CONDITIONS (temperature oc/time hr)
~Kth MPa :¢m-
no aging
9,3
55 / 2O
9.7
no agi ng
9.5
23 / 20
9.9
TABLE 3 Influence o f c y c l i c s t r a i n aqing on AKth f o r steel A SAMPLE
A2
Al
STRAI N-AGI NG CONDITIONS (temperature °C/time hr)
TEMPERATURE
(oc)
AKth MPa
no aging
23
9.5
23 / 20
23
9.9
55 / lO
55
I0.9
55 / lO
55
I I .5
55 / I0
55
I I .5
55 /
23
I I .3
no aging
23
9.3
55 / 15
23
I I .4
5
52
FATIGUE THRESHOLD
Vol.
17, No. 1
influencing the present measurements of the new value of AKth a f t e r cycling at 0.85 AKth and that the increases in AKth observed in such tests resulted from a combination of dynamic strain aging and of corrosion product wedging. Indeed in tests of this type performed on steels in the near-threshold region, the p o s s i b i l i t y of both dynamic strain aging and crack product wedging produced by the f r e t t i n g corrosion product must be taken into consideration. Acknowledgements Financial support from the Ouebec Ministry of Education (FCAC program) and from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. The authors are indebted to Dr. Jacques rlasounave for help~ful discussions. The steels employed were donated by Sidbec-Dosco Ltd. and obtained through the assistance of Dr. Paul Hastings. References I. 2. 3. 4. 5. 6. 7. 8. 9. I0. II. 12. 13. 14. 15. 16. 17.
J.D. Baird, Metall. Reviews, 16, 149 (1971). D.V. Wilson, Met. Sci., I I , 321 (1977). D.V. Wilson, Acta Met., 21, 673 (1973). D.V. Wilson and J.K. Tromans, Acta Met., 18, 1197 (1970). B. Mintz and D.V. Wilson, Acta Met., 13, 947 (1965). J.T. McGrath and W.J. Bratina, Czech. J. Phys., BI9, 284 (1969). J. Masounave and J.-P. Bailon, Scripta Met., I0, 165 (1976). S. Taira, K. Tanaka and ft. Hoshina, Fatigue Mechanisms, ASTM STP 675, p. 135, ASTM, Philadelphia (1979). J.P. Benson and D.V. Edn~nds, rletals Sci., 12, 223 (1978). J.P. Benson, Metals Sci., 13, 535 (1979). D. Shah and C. A l t s t e t t e r , J. Less Common Metals, 31, 195 (1973). R. Thibau, M.A.Sc. Thesis, Ecole Polytechnique de Montreal (1977). J . l . Dickson, R. Desrosiers and L.M. Malik, Scripta Met., 14, 311 (1980). D.E. MacDonald and W.A. Wood, J. Inst. Metals, I00, 73 (1972). S. Suresh and R.O. Ritchie, Mater. Sci. Eng., 51, 61 (1981). R.O. Ritchie, C.M. Moss and S. Suresh, J. Engng. Mater. Tech., HI02, 293 (1980). I. Uribe, M.A. Sc. Thesis, Ecole Polytechnique de Montreal, (1980). 10-4 R=O.I f
= 5 0 Hz
Lab.
air
Figure 1 Comparison of the near-threshold fatigue crack propagation rates for steels A and B in the a i r cooled (AC) and water-quenched (WQ) conditions.
rj t.)
/,o , . ; iO-e
E E
i;,i..
!'°:i.
Z "0 IO"7
AC
0 "0
B
IO-O
5
,~,
L , ,~ . . . . . . io
&K
( MPa
I 20
V'~')
•
,
wo
I 30
,I 40
50
Vol. 17, pp. 49-52, 1983 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
THE INFLUENCE OF STRAIN AGING ON THE FATIGUE CRACK PROPAGATION THRESHOLD
J . I . Dickson, Mou-Chih Lu* and J.-P. Bailon D~p. de g~nie m~tallurgique, Ecole Polytechnique Montreal, Quebec, Canada H3C 3A7 *present address: Fansteel Corp. North Chicago, I l l i n o i s
(Received March 29, 1982)
I ntro ducti on While the influence of strain-aging on the endurance l i m i t and on the dislocation behaviour during cyclic deformation (l-6) has received some attention in the l i t e r a t u r e , almost no attention appears to have been paid to the influence of strain aging on fatigue crack propagation. An influence of strain aging on fatigue propagation can be expected to manifest i t s e l f most clearly at the very small propagation rates near AKth, the fatigue crack propagation threshold. The present study was therefore performed to determine whether an influence attributable to strain aging could be detected near AKth during tests carried out on two mild steels having d i f f e r e n t s e n s i t i v i t i e s to strain aging. Materials and experimental procedure The two steels studied, obtained as 15.3 mm thick hot-rolled plate, had the compositions indicated in table I . Steel A is an aluminum-killed mild steel; while steel B is a renitrogenized mild steel containing no aluminum and therefore much more sensitive to strain-aging effects. The crack propagation tests were carried out employing 12.7 mm thick compact tension samples of standard (ASTME399-78) geometry. Crack propagation was parallel to the h o t - r o l l i n g direction. During the testing, a load r a t i o , R = Kmin/Kmax, of O.l was employed at a frequency of 50 Hz. The value of AKth was approached by decreasing the cyclic stress intensity factor, AK, i n i t i a l l y in increments of 10% and near AKth, in increments of 5%. At any value of AK, cracks were propagated through lengths _> 0.2 mm to obtain the velocity measurements. The crack length was measured o p t i c a l l y using a method with a s e n s i t i v i t y of I0 ~m. The machined samples were annealed, generally f o r 0.5 hrsat 950°C, and cooled in a i r from this temperature. A few o f these samples were reannealed f o r 0.5 hrsat 700oc and waterquenched from this temperature. Both these heat treatments gave a f e r r i t i c grain size o f approximately 30 um f o r steel A and o f 20 um f or steel B. A few samples of steel A were annealed f o r 1 hr at 1200oc, furnace cooled to 950°C, and then a i r cooled to room temperature, which heat treatment produced a grain size o f 80 nm. Experimental results and discussion Figure 1 compares the threshold and near-threshold results obtained at 23°C in the a i r - c o o l e d and in the water-quenched conditions. For the purpose o f the present study, the crack propagation threshold w i l l be assumed to be the value of AK that corresponds to a crack propagation r a t e , da/dn, o f 4 X 10-7 mm/cycle, since in most o f the tests performed at leas t one measurement of da/dn near this value was obtained. The r e s u l t s , moreover, indicated t h a t , below 4 X 10-7 mm/cycle, the l o g - l o g p l o t o f da/dn versus AK corresponds to an almost v e r t i c a l l i n e . From figure l , i t can be seen that quenching from 700oc increases the value of aKth obtained at 23°C by approximately 25% in the case of steel A and 50% in the case o f material B, which steel is more s e n s i t i v e to s t r a i n aging. The present results therefore i n d i c a t e that the increase in
49
0036-9748/83/010049-04503.00/0 Copyright (c) 1983 Pergamon Press Ltd.
50
FATIGUE
TIIRESHOLD
Vol.
17, No.
1
AKth caused by quenching is related to the greater amount of dynamic s t r a i n aging occurring in the v i c i n i t y of the crack t i p during very slow fatigue crack growth in the quenched samples. In order to obtain f u r t h e r evidence of the role of s t r a i n aging, two other sets of tests were performed on the a i r - c o o l e d samples of steel A. A f t e r the threshold had been reached at 23°C, two samples of steel A were aged f o r 20 hrs under a s t a t i c load corresponding to the value of the mean stress i n t e n s i t y f a c t o r that had been employed at the threshold. Aging temperatures of 23 and 55°C were employed with the sample cooled to 23oc under load a f t e r aginq at 55°C. The new value of AK required to cause a crack growth rate of approximately 4 X lO-7-mm/cycle was then measured. From table I I , i t can be noted that t h i s new value is l i t t l e above that required to cause the same propagation rate in the absence of s t a t i c s t r a i n aging, i n d i c a t i n g a weak influence of s t a t i c s t r a i n aging on the threshold i n this a i r -cooled material. The second procedure was one of c y c l i c s t r a i n aging a f t e r the influence of s t a t i c s t r a i n aging had been measured. The two samples were aged again e i t h e r at 23°C or at 55°C for a given period, during which c y c l i n g was performed continuously at AK = 0.85 AKth and R : 0 . I . A f t e r t h i s c y c l i c s t r a i n aging, the new AK value required to give a propagation rate of approximately 4 X 10-7 ram/cycle was measured e i t h e r at 23°C or 55oc. Table 3 summarizes the results obtained. I t can be noted that c y c l i c s t r a i n aging at 23oc f o r 20hrs had l i t t l e e f f e c t on AKth; however, c y c l i c s t r a i n aging at 55°C had a stronger influence and produced new AKth values s i g n i f i c a n t l y higher than the reference value obtained in the absence of a deliberate s t r a i n aging procedure. The AKth values measured at 23 and 55°C in the absence of any s t r a i n aging were also compared f o r air-cooled samples of steel A, but having a grain size of 80 values were e s s e n t i a l l y i d e n t i c a l ( I 0 . I MPa m at 23oc, ]0.0 MPa m at 55°C). This of AKth compared to that obtained f o r the 30 ~m grain size confirms the increase of increasing grain size f o r low carbon steels noted by previous authors (7-10).
sequences um. The higher value AKth with
Discussion The r e s u l t s presented i n table 2 and 3 can therefore be i n t e r p r e t e d as i n d i c a t i n g that dynamic s t r a i n aging occurring at 55°C during stress cycling at 0.85 &Kth can increase both the 23°C and 55°C values of AKth f o r air-cooled steel A by = 15-25%. On the other hand, n e i t h e r 20 hrs of such stress c y c l i n g at 23oc nor 20 hrs of s t a t i c s t r a i n aging at 55oc s i g n i f i c a n t l y increased the 23°C threshold value of t h i s steel in the a i r - c o o l e d condition. These results are in agreement with a strong influence of stress c y c l i n g on increasing the size of s t r a i n - a g i n g y i e l d points indicated by studies on niobium ( I I ) and on mild steel (12). The weak influence of s t a t i c s t r a i n aging at 55°C on AKth can also be related to the strong unpinning e f f e c t a n d s i g n i f i c a n t decrease of the y i e l d point obtained i f , a f t e r a s t a t i c s t r a i n aging at a high stress, the stress is even momentarily decreased to a low stress (13, 14). Suresh and Ritchie (15) recently have also carried out tests on a Cr-Mo pressure vessel steel having a b a i n i t i c microstructure in which periods of stress cycling performed s l i g h t l y below AKth caused crack retardation when AK was reincreased s l i g h t l y above the threshold. They i n t e r p r e t e d such crack r e t a r d a t i o n as being caused by crack closure effects associated with a build-up of f r e t t i n g corrosion product during c y c l i n g s l i g h t l y below AKth. The r e l a t i v e l y small influence of c y c l i n g s l i g h t l y below AKth when carried out at 23°C compared to 55°C observed i n the present study appears to be much more i n keeping with a dynamic s t r a i n - a g i n g e f f e c t , p a r t i c u l a r l y since s i m i l a r AKth values f o r air-cooled steel A (80 ~m grain size) were obtained at 23 and 55oc and the presence of f r e t t i n g oxide would be expected (16) to influence the AKth value for an R-ratio of 0 . I . In material not quenched to r e t a i n i n t e r s t i t i a l solute atoms in s o l u t i o n , the d i s s o l u t i o n of f i n e carbide or n i t r i d e p r e c i p i t a t e s may be required before important dynamic s t r a i n - a g i n g effects can occur near the crack t i p . Such d i s s o l u t i o n of f i n e precipitates is suggested from low-cycle fatigue studies (2, 3, 6, 17). On the other hand, with this explan a t i o n , the distance over which subsequent crack growth is influenced by c y c l i n g below AKth appears large compared to the expected p l a s t i c zone sizes. In one case, an e f f e c t over a distance of approximately 0.7 mm was observed compared to the calculated plane s t r a i n monotonic or larger p l a s t i c zone size of approximately 0.3 mm. In the higher strength steel employed by Suresh and Ritchie (15), there is stronger discrepancy between the distance over which crack retardation occurred and the p l a s t i c zone s i z e , which discrepancy supports t h e i r proposed explanation. I t is possible t h a t the build-up of a f r e t t i n g corrosion product also may be
Vol.
17, No,
1
FATIGUE THRESIIOLD
51
TABLE 1 Chemical Composition (wt %) Mn
Cu
Ni
Steel A
0.05
O. 32
0.012
0.06
0.004
0.03
Steel B
O.lO
0.58
0.013
0.06
O.OlO
0.06
Cr
Mo
Al
Nb
Steel A
0.01
O. 004
0.007
Steel B
0.05
O.Ol 7
O.OOl
0.003
0.0026 0.0104
TABLE 2 Influence o f s t a t i c s t r a i n aging on AKth f o r steel A
SAMPLE A1 A2
STRAIN-AGING CONDITIONS (temperature oc/time hr)
~Kth MPa :¢m-
no aging
9,3
55 / 2O
9.7
no agi ng
9.5
23 / 20
9.9
TABLE 3 Influence o f c y c l i c s t r a i n aqing on AKth f o r steel A SAMPLE
A2
Al
STRAI N-AGI NG CONDITIONS (temperature °C/time hr)
TEMPERATURE
(oc)
AKth MPa
no aging
23
9.5
23 / 20
23
9.9
55 / lO
55
I0.9
55 / lO
55
I I .5
55 / I0
55
I I .5
55 /
23
I I .3
no aging
23
9.3
55 / 15
23
I I .4
5
52
FATIGUE THRESHOLD
Vol.
17, No. 1
influencing the present measurements of the new value of AKth a f t e r cycling at 0.85 AKth and that the increases in AKth observed in such tests resulted from a combination of dynamic strain aging and of corrosion product wedging. Indeed in tests of this type performed on steels in the near-threshold region, the p o s s i b i l i t y of both dynamic strain aging and crack product wedging produced by the f r e t t i n g corrosion product must be taken into consideration. Acknowledgements Financial support from the Ouebec Ministry of Education (FCAC program) and from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. The authors are indebted to Dr. Jacques rlasounave for help~ful discussions. The steels employed were donated by Sidbec-Dosco Ltd. and obtained through the assistance of Dr. Paul Hastings. References I. 2. 3. 4. 5. 6. 7. 8. 9. I0. II. 12. 13. 14. 15. 16. 17.
J.D. Baird, Metall. Reviews, 16, 149 (1971). D.V. Wilson, Met. Sci., I I , 321 (1977). D.V. Wilson, Acta Met., 21, 673 (1973). D.V. Wilson and J.K. Tromans, Acta Met., 18, 1197 (1970). B. Mintz and D.V. Wilson, Acta Met., 13, 947 (1965). J.T. McGrath and W.J. Bratina, Czech. J. Phys., BI9, 284 (1969). J. Masounave and J.-P. Bailon, Scripta Met., I0, 165 (1976). S. Taira, K. Tanaka and ft. Hoshina, Fatigue Mechanisms, ASTM STP 675, p. 135, ASTM, Philadelphia (1979). J.P. Benson and D.V. Edn~nds, rletals Sci., 12, 223 (1978). J.P. Benson, Metals Sci., 13, 535 (1979). D. Shah and C. A l t s t e t t e r , J. Less Common Metals, 31, 195 (1973). R. Thibau, M.A.Sc. Thesis, Ecole Polytechnique de Montreal (1977). J . l . Dickson, R. Desrosiers and L.M. Malik, Scripta Met., 14, 311 (1980). D.E. MacDonald and W.A. Wood, J. Inst. Metals, I00, 73 (1972). S. Suresh and R.O. Ritchie, Mater. Sci. Eng., 51, 61 (1981). R.O. Ritchie, C.M. Moss and S. Suresh, J. Engng. Mater. Tech., HI02, 293 (1980). I. Uribe, M.A. Sc. Thesis, Ecole Polytechnique de Montreal, (1980). 10-4 R=O.I f
= 5 0 Hz
Lab.
air
Figure 1 Comparison of the near-threshold fatigue crack propagation rates for steels A and B in the a i r cooled (AC) and water-quenched (WQ) conditions.
rj t.)
/,o , . ; iO-e
E E
i;,i..
!'°:i.
Z "0 IO"7
AC
0 "0
B
IO-O
5
,~,
L , ,~ . . . . . . io
&K
( MPa
I 20
V'~')
•
,
wo
I 30
,I 40
50