ASM Metals HandBook Volume 12 - Fractography
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ASM Metals HandBook Volume 12 - Fractography

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of AISI type 304 stainless steel) 
with ultimate tensile strengths of 648 MPa (94 ksi) and 356 MPa (52 ksi) at room temperature and 600 °C (1110 °F), 
respectively, was tested to determine the effect of the type of cyclic load on the fatigue crack growth rate (Ref 255). 
Through-thickness cracks in thin-wall cylindrical test specimens were grown to half crack lengths ranging from 0.55 to 
1.23 mm (0.02 to 0.048 in.) using fast-fast strain cycling. At the predetermined half length, one slow-fast cycle was 
introduced, and the fast-fast strain cycling was resumed. The fatigue test wave form is illustrated in Fig. 96. The wave 
form was triangular, fully reversed, with a total strain range of 0.8 to 1.5%. The fast-fast cycles were applied at a strain 
rate of ±50 × 10-5 s-1, and the slow-fast cycle consisted of a 1.4 × 10-5 s-1 rising strain ramp and 28 × 10-1 s-1 declining 
strain ramp. All test were conducted in air at a temperature of 600 °C (1110 °F). 
Fig. 96 Fatigue test strain wave from. A, strain, rate = 50 × 10-5 s-1 (fast-fast cycles); B, strain rate = 1.4 × 
10-5 s-1 (the slow-rising strain ramp of the slow-fast cycle); C, strain rate = 28 × 10-5 s-1 (the fast-declining 
strain ramp of the slow-fast cycle). Source: Ref 255 
The introduction of one slow-fast cycle always increased the fatigue crack growth rate, sometimes by up to a factor of 
two. When the fast-fast cycling was resumed after the slow-fast cycle, the crack propagated by a fatigue striation forming 
mechanism, in which case there was a local increase in the striation spacing, or by intergranular fracture (Fig. 97). Where 
the crack propagated by a striation-forming mechanism or by intergranular fracture was determined by the ratio \u2206Jc/\u2206Jf, 
where \u2206Jc = range of the creep J-integral and \u2206Jf = range of the cycle J-integral. The values of \u2206Jc and \u2206Jf can be 
determined from plots of load versus crack-opening displacement at half crack length for the slow-fast cycle, \u2206Jc, and the 
fast-fast cycle, \u2206Jf (Ref 255). When the value of \u2206Jc/\u2206Jf is less than 0.1, the crack propagates by a striation-forming 
mechanism; if the value exceeds 0.1, the crack propagates by intergranular fracture when fast-fast cycling is resumed . In 
general, fatigue striations were formed when half crack lengths were less than about 0.85 mm (0.033 in.) and the total 
strain range was less than 1%. Intergranular fractures occurred at the longer half crack lengths and the higher total strain 
Fig. 97 Effect of one slow-fast cycle on the fracture appearance of an SUS 304 stainless steel that was fatigue 
tested in air at 600 °C (1110 °F). (a) Increase in fatigue striation spacing after the introduction of the slow-fast 
cycle at a half crack length = 0.97 mm (0.038 in.) (b) Intergranular fracture after the application of one slow-
fast cycle at a half crack length of 1.0 mm (0.039 in.). Crack growth direction in both figures is right to left. 
Source: Ref 255 
From a material standpoint, the factors that determined whether the cracking occurred by striation fatigue or intergranular 
fracture were the degree of creep damage and the size of the damaged zone produced during the slow-fast cycle. If the 
creep-damaged zone ahead of the crack tip was smaller than the stain-deformed region resulting from the fast-fast cycling 
before the slow-fast cycle, the crack propagated by striation-forming fatigue. However, if the creep-damaged zone was 
larger than that of the fast-fast cycling and if wedge cracks were produced at the grain boundaries adjacent to the crack 
tip, intergranular fracture resulted. Regardless of the cracking mechanism, the fatigue crack growth rate is accelerated 
until the fatigue crack escapes the creep-damaged zone (Ref 255). 
Examples 27. A fully heat treated IN-738 nickel-base superalloy was tested to determine the effect of wave form on 
fatigue life (Ref 256). Seven strain wave forms (Fig. 98) were used to conduct fatigue life tests at a total strain range of 
0.4 to 1.2%. The strain rates for the fast-fast and the slow-slow wave forms were 10-2 and 10-5 s-1, respectively. For the 
slow-fast and fast-slow sawtooth wave forms, the slow strain rate was 10-5 s-1 and the fast rate was 10-2 s-1. The ramp rate 
for the truncated wave forms was 10-2 s-1, and the dwell times at maximum tensile or compressive strains varied from 400 
to 700 s. All tests were performed in air at a temperature of 850 °C (1560 °F). 
Fig. 98 Strain wave forms used in fatigue testing. Source: Ref 256 
The fatigue life to crack initiation, defined as the number of cycles required to initiate a 0.5-mm (0.02-in.) long crack, was 
lower for the SS, TW-TC, SF, and TW-T wave forms than for the FF wave form (see Fig. 98 for explanation of 
abbreviations). The FS and TW-C types were also lower than FF, but generally not by as much as the SS, TW-TC, SF, 
and TW-T types. 
Crack initiation and propagation for the FF wave form were transgranular, the SS, TW-TC, SF, and TW-T forms showed 
intergranular crack initiation but a mixed propagation. For the FS and TW-C wave forms, fatigue crack initiation was 
either intergranular or transgranular; the determining factor was the total strain range. The higher range favored 
intergranular initiation and mixed propagation, while the lower ranges promoted transgranular initiation and propagation. 
In addition to the wave forms influencing the fracture path, the dwell and ramp rates were also shown to affect the mean 
stress. It was discovered that a dwell on one side of the strain wave developed a mean stress on the opposite side of the 
wave. For example, a dwell at maximum tensile strain (maximum tensile stress) developed a compressive mean stress in 
the fatigue load cycle. This shift in the mean stress was apparently due to creep during dwell at the maximum tensile (or 
compressive) strain so that returning the specimen to the maximum compressive (or tensile) strain resulted in a shift in the 
initial zero-mean stress to the compressive (or tensile) side of the fatigue curve. Although not as large as in the dwell 
wave forms, mean stresses sometimes developed in wave forms having unequal ramp rates, with the mean stress 
occurring on the side of the faster ramp rate. 
The one clear trend in the fatigue life tests showed that the FF wave forms exhibited the longest fatigue lives. Because of 
scatter in the data, no clear trends were established for the other wave forms, although a few individual specimens had 
longer fatigue lives than some of the shorter-life FF specimens. Because FF loading always produced the optimum (low 
fatigue crack growth rate) transgranular crack path, the occasional longer fatigue lives of individual non-FF wave form 
specimens were attributed to such factors as the presence of a compressive mean stress, multiple crack propagation, and 
crack branching, all of which would contribute to an increase in the fatigue life (Ref 256). 
References cited in this section 
15. R.O. Ritchie, in Environment-Sensitive Fracture of Engineering Materials, Z.A. Foroulis, Ed., The 
Metallurgical Society, 1979, p 538-564 
19. R.M.N. Pelloux, Trans. ASM, Vol 62, 1969, p 281-285 
25. D.A. Meyn, Trans. ASM, Vol 61 (No. 1), 1968, p 42 
31. P.C. Paris and F. Erdogan, J. Basic Eng., (Trans. ASME), D, Vol 85, 1963, p 528 
32. H.H. Johnson and P.C. Paris, Eng. Fract. Mech., Vol 1, 1968, p 3 
38. C.D. Beachem, Metall. Trans. A, Vol 3A, 1972, p 437 
40. C.A. Zapffe and C.E. Sims, Trans. AIME, Vol 145, 1941, p 225 
89. D.A. Meyn and E.J. Brooks, in Fractography and Material Science, STP 733, L.N. Gilbertson and R.D. 
Zipp, Ed., American Society for Testing and Materials, 1981, p 5-31 
92. M.F. Stevens and I.M. Bernstein, Metall. Trans. A, Vol 16A, 1985, p 1879 
95. M.B. Whiteman and A.R. Troiano, Corrosion, Vol 21, 1965, p 53 
96. M.L. Holtzworth, Corrosion, Vol 25, 1969, p 107 
97. R. Langenborg, J.