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Observations of crack initiation in low-cycle fatigue specimens of type 304 stainless steel
Scripta
METALLURGICA
Vol. ii, pp. 331-334, P r i n t e d in the U n i t e d
1977 States
Pergamon
Press,
Inc
OBSERVATIONS OF CRACK INITIATION IN LOW-CYCLE FATIGUE SPECIMENS OF TYPE 304 STAINLESS STEEL
P. S. Maiya Materials Science Division ARGONNE NATIONAL LABORATORY Argonne, Illinois 60439 [.Received
February
II,
1977)
The low-cycle fatigue fracture surfaces of Type 304 stainless steel tested under straincontrolled and fully reversed cyclic-loadlng conditions at total strain ranges between 0.5 and 2.0%, a strain rate of 4 x 10 -3 s-1, and temperature of 593°C in air show fatigue striations. Analysis of such fracture surfaces on the basis of fatlgue-strlatlon spacing measurements yields useful information on the portion of fatigue life expended in crack initiation (i). The crackinitiation llfe is defined as the period in the fatigue sequence required to generate a crack of specified length, usually of the order of one or two grain diameters. Thus, the crackinitiation llfe includes the number of cycles required to nucleate a crack as well as propagate the crack to a specified length. Several workers have discussed the usefulness of considering total fatigue llfe on the basis of a distinction between crack initiation and crack propagation (1-4), especially when investigating the effect of a specific variable [e.g., environment (i), surface coatings (5), and strain range (1,6,7)] on fatigue llfe. The procedure for obtaining crack-inltiatlon llfe is based on the plots of in a versus N (where a is the crack length after N strain cycles). Information on crack growth is obtained from fatlgue-striation spacing measurements with the assumption that each striation is caused by one strain cycle (8,9). Using this procedure , in Type 304 stainless steel specimens tested at 593°C under strain-controlled and fully reveraed cyclic-loading conditions at a total strain range of 1.0%, the life to crack initiation is observed to he 69% of total life and, at a strain range of 0.5%, 91% of total life consists of crack initiation (6,9). The purpose of the present note is to report some observations of surface damage in Type 304 stainless steel low-cycle fatigue specimens for which tests were interrupted after a significant fraction of llfe had elapsed and discuss whether the observations are consistent at least qualitatively with the crack-lnltlatlon llfe N O determined previously (6,9). The total fatigue llfe Nf of smooth hourglass-shape specimens of Type 304 stainless steel at a strain rate of 4 x 10-3 s-1, total strain ranges between 0.5 and 2.0%, and 593°C can be expressed by (6) Nf = N O + Np = 0.0122(ACp) -2"474 + 3.911(Aep) -I'I03 ,
(i)
where Ag_ is the plastic strain range, Np is the crack-propagatlon life, and N O is based on a crack length at initiation of ~i grain dlameter (~0.i mm). For a chosen plastic strai n range, it is thus possible to interrupt the test at a life close to N O and examine the surface damage around the circumference of the hourglass specimens at the minimum cross section by means of plastic-replica techniques. The technique used in the present study has been described in detail by Henry (i0). After interrupting the fatigue test, the specimen is cooled, removed from the fatigue machine, and mounted in a specimen Jig that enables rotation of the specimen about the longitudinal axis through a known angle. Thus, the assembly facilitates replication of the entire surface region of the hourglass s~ecimen at the minimum cross section. The plastic replicas are coated with gold (to ~i00 A in thickness) and e~m~ned by optical microscopy. The experimental variables for the interrupted fatigue tests are listed in Table I.
331
332
LOW-CYCLE
FATIGUE
OF
304
STAINLESS
STEEL
Vol.
ii,
No.
4
TABLE I InterruptedLLow-cycle fatigue Tests for Type 304 Stainless Steel at 593°C in Air*
Specimen Number
Total Strain Range
Plastic Strain Range
Percent of Total Life Expended in Crack Initiation**
Percent of Total Expended Life at Interruption
(%)
(%)
T382
0.504
0.285
91
~85
T383
0.50
0.283
91
~95
T385
1.0
0.691
74 t
~75
*Cyclic straln rate ~t = 4 x 10 -3
e-1"
**Calculated from Eq. (1). tEquatlon (i) slightly overpredicts the crack initiation llfe for this total strain range.
Figures 1 and 2 show the optical micrographs of shadowed plastic replicas taken at invervals of 45 ° around the circumference of the minimum cross section of the hourglass-shape speclmens for which tests were conducted at a total strain range Ac t of 0.5% and interrupted after 85 and 95% of llfe, respectively. A number of mlcrocracks and slip-band damage are vlslble. The dark areas indicate the presence of oxide. The mlcrographs in Pig. la and b reveal more surface damage than in other regions of this specimen as is evidenced by a higher density of microcracks. This suggests the possibility that the mlcrocracks in these regions (Fig. la and b) Join to form a major crack, which subsequently propagates to failure. Figure 3 shows the surface damage of the specimen cycled at a total strain range of 1% in a test interrupted at ~75% of the total llfe. No large cracks are observed in Figs. 1 and 2, but they are clearly discernible in Fig. 3. Additional examination of Figs. 1-3 reveals the following. Specimens cycled at a total strain range of 0.5% in tests interrupted after a number of cycles slightly less than or close to N O have surface mlcrocracks of length~l grain diameter, which indicates crack initiation [as defined in previous work (6,9)] has not occurred. The specimen cycled at a total strain range of 1% in tests interrupted after a number of cycles >N O contains surface cracks of lengths >_3 grain diameters, which shows that initiation has occurred. The above results are completely consistent with the crack-lnltiatlon life determined (6,9) from fatiguestriation spacing measurements assuming that each striation is caused by one cycle. Finally, the replica technique is useful for monitoring the progression of damage associated with continued cycling in elevated-temperature low-cycle fatigue studies and obtaining a better understanding of fatigue damage mechanisms. Acknowledgments The author is grateful to W. P. Burke and D. E. Busch for their assistance in the experimental work and A. P. L. Turner for helpful discussions. This work was supported by the U.S. Energy Research and Development Administration. References
1.
C. Laird and G. C. Smith, Phil. Mag. 8, 1945 (1963).
2.
J. A. Bennett, in Inter~tlonal Conference on Fatigue of Metals, p. 548. Mechanlcal Engineers, London (1956).
Institution of
Vol.
11,
No.
4
LOW-CYCLE FATIGUE OF 304 STAINLESS STEEL
333
3.
P. J. E. Forsyth, in Proceedings of the Crack Propagation Symposium, vol. i, p. 76, The College of Aeronautics, Cranfield, England (1962).
4.
S. S. Manson, Intl. J. Fracture Mechanics 2, 327 (1966).
5.
J. C. Grosskreutz, Met. Trans. 3, 1255 (1972).
6.
P. S. Maiya, Scripta Met. 9, 1141 (1975).
7.
C. Laird and A. R. Krause, Intl. J. Fracture Mechanics 4, 219 (1968).
8.
C. Laird, ASTM STP 415, Philadelphia, Pa., p. 131 (1967).
9. i0.
P. S. Maiya and D. E. Busch, Met. Trans. 6A, 1761 (1975). M. F. Henry, Report Number 71-C-338, Technical Information Series, @e~neral Electric Company, Schenectady, N.Y. (November 1971).
I
~m
I
FIG. 1 Optical micrographs of shadowed plastic replicas of Type 304 stainless steel specimen from a test interrupted after ~85% of total life (23352 cycles). Aet = 0.5%, ~t _ = 4 x 10 - 3 s - 1 , and t e m p e r a t u r e 593°C.
334
LOW-CYCLE
(d)
FATIGUE
135"
OF 304 S T A I N L E S S
(e} 180"
STEEL
Vol.
Ii, No,
Of)
c ~
I 200~m I
FIG. 2
Optical micrographs of shadowed plastic replicas of Type 304 stainless steel fatigue specimen from a test interrupted after ~95% of total life (26500 cycles). AE = 0.5%, ~t = 4 x 10-3 s-1, and temperature = 593°C. t
FIG. 3 Optical micrographs of shadowed plastic replicas of Type 304 stainless steel fatigue specimen interrupted after ~75% of total life (2700 cycles). A¢t = 1.0%, ~t = 4 x 10-3 s-lp and temperature = 593°C.
4
METALLURGICA
Vol. ii, pp. 331-334, P r i n t e d in the U n i t e d
1977 States
Pergamon
Press,
Inc
OBSERVATIONS OF CRACK INITIATION IN LOW-CYCLE FATIGUE SPECIMENS OF TYPE 304 STAINLESS STEEL
P. S. Maiya Materials Science Division ARGONNE NATIONAL LABORATORY Argonne, Illinois 60439 [.Received
February
II,
1977)
The low-cycle fatigue fracture surfaces of Type 304 stainless steel tested under straincontrolled and fully reversed cyclic-loadlng conditions at total strain ranges between 0.5 and 2.0%, a strain rate of 4 x 10 -3 s-1, and temperature of 593°C in air show fatigue striations. Analysis of such fracture surfaces on the basis of fatlgue-strlatlon spacing measurements yields useful information on the portion of fatigue life expended in crack initiation (i). The crackinitiation llfe is defined as the period in the fatigue sequence required to generate a crack of specified length, usually of the order of one or two grain diameters. Thus, the crackinitiation llfe includes the number of cycles required to nucleate a crack as well as propagate the crack to a specified length. Several workers have discussed the usefulness of considering total fatigue llfe on the basis of a distinction between crack initiation and crack propagation (1-4), especially when investigating the effect of a specific variable [e.g., environment (i), surface coatings (5), and strain range (1,6,7)] on fatigue llfe. The procedure for obtaining crack-inltiatlon llfe is based on the plots of in a versus N (where a is the crack length after N strain cycles). Information on crack growth is obtained from fatlgue-striation spacing measurements with the assumption that each striation is caused by one strain cycle (8,9). Using this procedure , in Type 304 stainless steel specimens tested at 593°C under strain-controlled and fully reveraed cyclic-loading conditions at a total strain range of 1.0%, the life to crack initiation is observed to he 69% of total life and, at a strain range of 0.5%, 91% of total life consists of crack initiation (6,9). The purpose of the present note is to report some observations of surface damage in Type 304 stainless steel low-cycle fatigue specimens for which tests were interrupted after a significant fraction of llfe had elapsed and discuss whether the observations are consistent at least qualitatively with the crack-lnltlatlon llfe N O determined previously (6,9). The total fatigue llfe Nf of smooth hourglass-shape specimens of Type 304 stainless steel at a strain rate of 4 x 10-3 s-1, total strain ranges between 0.5 and 2.0%, and 593°C can be expressed by (6) Nf = N O + Np = 0.0122(ACp) -2"474 + 3.911(Aep) -I'I03 ,
(i)
where Ag_ is the plastic strain range, Np is the crack-propagatlon life, and N O is based on a crack length at initiation of ~i grain dlameter (~0.i mm). For a chosen plastic strai n range, it is thus possible to interrupt the test at a life close to N O and examine the surface damage around the circumference of the hourglass specimens at the minimum cross section by means of plastic-replica techniques. The technique used in the present study has been described in detail by Henry (i0). After interrupting the fatigue test, the specimen is cooled, removed from the fatigue machine, and mounted in a specimen Jig that enables rotation of the specimen about the longitudinal axis through a known angle. Thus, the assembly facilitates replication of the entire surface region of the hourglass s~ecimen at the minimum cross section. The plastic replicas are coated with gold (to ~i00 A in thickness) and e~m~ned by optical microscopy. The experimental variables for the interrupted fatigue tests are listed in Table I.
331
332
LOW-CYCLE
FATIGUE
OF
304
STAINLESS
STEEL
Vol.
ii,
No.
4
TABLE I InterruptedLLow-cycle fatigue Tests for Type 304 Stainless Steel at 593°C in Air*
Specimen Number
Total Strain Range
Plastic Strain Range
Percent of Total Life Expended in Crack Initiation**
Percent of Total Expended Life at Interruption
(%)
(%)
T382
0.504
0.285
91
~85
T383
0.50
0.283
91
~95
T385
1.0
0.691
74 t
~75
*Cyclic straln rate ~t = 4 x 10 -3
e-1"
**Calculated from Eq. (1). tEquatlon (i) slightly overpredicts the crack initiation llfe for this total strain range.
Figures 1 and 2 show the optical micrographs of shadowed plastic replicas taken at invervals of 45 ° around the circumference of the minimum cross section of the hourglass-shape speclmens for which tests were conducted at a total strain range Ac t of 0.5% and interrupted after 85 and 95% of llfe, respectively. A number of mlcrocracks and slip-band damage are vlslble. The dark areas indicate the presence of oxide. The mlcrographs in Pig. la and b reveal more surface damage than in other regions of this specimen as is evidenced by a higher density of microcracks. This suggests the possibility that the mlcrocracks in these regions (Fig. la and b) Join to form a major crack, which subsequently propagates to failure. Figure 3 shows the surface damage of the specimen cycled at a total strain range of 1% in a test interrupted at ~75% of the total llfe. No large cracks are observed in Figs. 1 and 2, but they are clearly discernible in Fig. 3. Additional examination of Figs. 1-3 reveals the following. Specimens cycled at a total strain range of 0.5% in tests interrupted after a number of cycles slightly less than or close to N O have surface mlcrocracks of length~l grain diameter, which indicates crack initiation [as defined in previous work (6,9)] has not occurred. The specimen cycled at a total strain range of 1% in tests interrupted after a number of cycles >N O contains surface cracks of lengths >_3 grain diameters, which shows that initiation has occurred. The above results are completely consistent with the crack-lnltiatlon life determined (6,9) from fatiguestriation spacing measurements assuming that each striation is caused by one cycle. Finally, the replica technique is useful for monitoring the progression of damage associated with continued cycling in elevated-temperature low-cycle fatigue studies and obtaining a better understanding of fatigue damage mechanisms. Acknowledgments The author is grateful to W. P. Burke and D. E. Busch for their assistance in the experimental work and A. P. L. Turner for helpful discussions. This work was supported by the U.S. Energy Research and Development Administration. References
1.
C. Laird and G. C. Smith, Phil. Mag. 8, 1945 (1963).
2.
J. A. Bennett, in Inter~tlonal Conference on Fatigue of Metals, p. 548. Mechanlcal Engineers, London (1956).
Institution of
Vol.
11,
No.
4
LOW-CYCLE FATIGUE OF 304 STAINLESS STEEL
333
3.
P. J. E. Forsyth, in Proceedings of the Crack Propagation Symposium, vol. i, p. 76, The College of Aeronautics, Cranfield, England (1962).
4.
S. S. Manson, Intl. J. Fracture Mechanics 2, 327 (1966).
5.
J. C. Grosskreutz, Met. Trans. 3, 1255 (1972).
6.
P. S. Maiya, Scripta Met. 9, 1141 (1975).
7.
C. Laird and A. R. Krause, Intl. J. Fracture Mechanics 4, 219 (1968).
8.
C. Laird, ASTM STP 415, Philadelphia, Pa., p. 131 (1967).
9. i0.
P. S. Maiya and D. E. Busch, Met. Trans. 6A, 1761 (1975). M. F. Henry, Report Number 71-C-338, Technical Information Series, @e~neral Electric Company, Schenectady, N.Y. (November 1971).
I
~m
I
FIG. 1 Optical micrographs of shadowed plastic replicas of Type 304 stainless steel specimen from a test interrupted after ~85% of total life (23352 cycles). Aet = 0.5%, ~t _ = 4 x 10 - 3 s - 1 , and t e m p e r a t u r e 593°C.
334
LOW-CYCLE
(d)
FATIGUE
135"
OF 304 S T A I N L E S S
(e} 180"
STEEL
Vol.
Ii, No,
Of)
c ~
I 200~m I
FIG. 2
Optical micrographs of shadowed plastic replicas of Type 304 stainless steel fatigue specimen from a test interrupted after ~95% of total life (26500 cycles). AE = 0.5%, ~t = 4 x 10-3 s-1, and temperature = 593°C. t
FIG. 3 Optical micrographs of shadowed plastic replicas of Type 304 stainless steel fatigue specimen interrupted after ~75% of total life (2700 cycles). A¢t = 1.0%, ~t = 4 x 10-3 s-lp and temperature = 593°C.
4