A study of the notch and frequency effects on fatigue life at high temperature is carried out using notched and unnotched plate specimens of SS 304 under stress-controlled testing conditions. Analysis of the $σ-Nf$ results obtained at 600°C under fatigue and creep-fatigue conditions allowed the generalization of the $σ-Nf-Kt$ relation proposed in an earlier study. Examinations of the experimental data with hold-time testing suggested that in these conditions, the frequency effect should be incorporated in the relationship. Results obtained from the modified relation are in agreement with the experimental data, within a factor of two. Finite element analysis was carried out to determine the state of stresses and strains at the notch root by simulating four creep-fatigue cycles. The computed results indicated that, under zero-to-tension cyclic loading with controlled nominal stress, the maximum local stress at the notch root relaxes; this results in a minimum local stress in compression, and as a consequence, the mean local stress is significantly reduced. The stress relaxation as well as the creep strain accumulation were found to occur only in the vicinity of the notch (within 0.75 mm). The numerical results concerning the local stress relaxation and the time-dependent strain accumulation are used to explain the notch-strengthening effect on life observed in the present study. [S0094-9930(00)00401-7]

1.
Wundt, B. M., 1972, “Effect of Notches on Low Cycle Fatigue,” A Literature Survey, ASTM STP 490.
2.
Glinka, G., 1988, “Relations Between the Strain Energy Density Distribution and Elastic-Plastic Stress-Strain Fields Near Cracks and Notches and Fatigue Life Calculation,” ASTM STP 942, pp. 1022–1047.
3.
Merah, N., 1994, “Effet d’entailles sur le comportement en fatigue et en fatigue-fluage d’un acier inoxydable,” PhD. thesis, Ecole Polytechnique of Montreal, Montreal, Canada.
4.
Taira
,
S.
, and
Ohtani
,
R.
,
1973
, “
Creep Crack Propagation and Creep Rupture of Notched Specimens
,”
ASME Conference
,
13
, pp.
1
213
.
5.
Ohnami, O., Asada, Y., Sakane, M., Kitagawa, M., and Sakon, T., 1988, “Notch Effect on Low Cycle Fatigue in Creep-Fatigue at High Temperatures: Experiment and Finite Element Method Analysis,” Low Cycle Fatigue, ASTM STP 942, ASTM, pp. 1066–1095.
6.
Sakane
,
M.
,
Ohnami
,
M.
,
Awaya
,
T.
, and
Shirafuji
,
N.
,
1989
, “
Frequency and Hold-Time Effects on Low Cycle Fatigue Life of Notched Specimens at Elevated Temperature
,”
ASME J. Eng. Mater. Technol.
,
111
, pp.
54
60
.
7.
Robinson
,
E. L.
,
1952
, “
Effect of Temperature Variation on the Long Term Rupture Strength of Steels
,”
Trans. ASME
,
74
, pp.
770
780
.
8.
Taira, S., 1962, “Lifetime of Structures Subjected to Varying Load and Temperature,” Creep in Structures, N. J., Hoff, ed., Springer Verlag, Berlin, Germany, pp. 96–119.
9.
Coffin, L. F., 1969, “Predictive Parameters and Their Application to High-Temperature Low-Cycle Fatigue,” Proceedings, 2nd International Conference on Fracture, Brighton, UK, pp. 643–654.
10.
Manson, S. S., 1973, “The Challenge to Unify Treatment of High Temperature Fatigue—A Partisan Proposal Based on Strain-Range Partitioning,” ASTM STP 520, American Society of Testing and Materials, pp. 744–782.
11.
ASME, 1986, Boiler and Pressure Vessel Code Case N-47, New York, NY.
12.
Merah
,
N.
,
Bui-Quoc
,
T.
, and
Bernard
,
M.
,
1994
, “
A Kt Based Method for Calculating LCF Life of Notched Specimens
,”
ASME J. Pressure Vessel Technol.
,
116
, pp.
171
179
.
13.
Merah, N., Bui-Quoc, T., and Bernard, M., 1995, “Calibration of DC Potential Drop Using an Optical Image Processing System in LCF Testing,” ASTM J. Testing and Evaluation, 23, 160–167.
14.
Conway, J. B., Berling, J. T., and Stentz, R. H., 1971, “Strain Rate and Hold Time Saturation in Low Cycle Fatigue Behavior,” Proceedings, International Conference on Thermal Stresses and Thermal Fatigue, London, UK, pp. 89–108.
15.
Sakane
,
M.
, and
Ohnami
,
M.
,
1983
, “
A Study of the Notch Effect on the Low Cycle Fatigue of Metals in Creep-Fatigue Interacting Conditions at Elevated Temperature
,”
ASME J. Eng. Mater. Technol.
,
105
, pp.
75
80
.
16.
Hibbit, Karlsson and Sorensen, Inc., 1996, ABAQUS, Version 5.5-1.
17.
Merah
,
N.
,
Bui-Quoc
,
T.
, and
Bernard
,
M.
,
1999
, “
Creep-Fatigue Crack Growth in Notched SS-304 Plates at 600°C
,”
Eng. Fract. Mech.
,
63
, pp.
39
55
.
18.
Neuber, H., 1961, “Research on the Distribution of Tension in Notched Construction Parts,” WADD TRGO-906.