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


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in the fracture appearance, which can exhibit a change in the fatigue striation spacing or character 
and show the addition of such fractures as quasi-cleavage, cleavage, intergranular decohesion, areas of grain-boundary 
sliding, or cavitated fracture surfaces--fractures usually associated with embrittling environments and creep rupture (Ref 
242, 247, 253, 255, 256, 257). There is no significant difference in the effect on the fatigue crack growth rate between 
sinusoidal and triangular wave forms. 
The following examples illustrate the effect of frequency and wave form on fatigue properties. Example 16 to 20 discuss 
the effect of frequency; examples 21 to 27, the effect of wave form. 
Example 16. An AISI 4340 steel with an ultimate tensile strength of 2028 MPa (294 ksi) was tested under the following 
conditions: \u2206K = 25 MPa m (23 ksi in ), R = 0.1, moist air (water vapor pressure: 4.4 torr), at room temperature (Ref 
253). The results are listed below: 
 
Frequency, 
Hz 
da/dN, 
mm/cycle 
10 1 × 10-4 
1 3 × 10-4 
0.1 3 × 10-3 
The fatigue crack growth rate at 0.1 Hz was 30 times greater than at 10 Hz. At a frequency of 10 Hz, the fracture was 
primarily transgranular, typical of a normal fatigue fracture. As the frequency decreased to 1 Hz and lower, the fracture 
path gradually changed from transgranular to one consisting primarily of intergranular decohesion along the prior-
austenite grain boundaries. The increase in the fatigue crack growth rate and the change in the fracture appearance were 
attributed to the embrittling effect of the hydrogen liberated during the dissociation of the water vapor at the crack tip. 
Example 17. An AISI 4130 steel with an 0.2% offset yield strength of 1330 MPa (193 ksi) and a hardness of 43 HRC 
was tested under the following conditions: \u2206K = 10 to 40 MPa m (9 to 36.5 ksi in , R = 0.1, moist air (relative humidity 
not specified), at room temperature (Ref 251). The fatigue crack growth rate did not change significantly when tested at a 
frequency range of 1 to 50 Hz. The fracture exhibited a transgranular path and showed no evidence of embrittlement. At 
these test frequencies, there apparently was insufficient moisture in the air to permit hydrogen embrittlement to occur, 
even at 1 Hz. 
Example 18. Annealed type 304, 316, 321, and 347 stainless steels were tested under the following conditions: total 
strain range = 1%, mean strain = 0, triangular wave form, air, at 600 and 700 °C (1110 and 1290 °F) (Ref 247). The 
results are listed in Table 1. At 600 °C (1110 °F), the type 304/316 stainless steels and the type 32 1
3
47 stainless steels 
showed an essentially equal threefold decrease in the fatigue life when the strain rate was change from a relative high rate 
(6.7 × 10-3 s-1) to a low rate (6.7 × 10-5 s-1). At 700 °F (1290 °F), however, the fatigue life for the type 304/316 stainless 
steels decreased by a factor of two when the strain rate changed from a high to a low rate; for the type 32 1
3
47 stainless 
steels, the fatigue life decreased by a factor of five. The decrease in the fatigue life at 600 °C (110 °F) with decreasing 
strain rate for all of the stainless steels was associated with a change in the fracture appearance from one of principally 
transgranular with fatigue striations to one that was increasingly intergranular. 
Table 1 Effect of strain rate on the fatigue life of type 316 and 321 stainless steels 
600 °C (1110 °F) Alloy 
Strain rate 
( = s-1 
Fatigue life 
(Nf = cycles to failure) 
Fatigue Life, 
700 °C (1290 °F) 
(Nf = cycles to failure) 
Type 316(a) 
(ASTM grain size = 2) 
6.7 × 10-5 
6.7 × 10-5 
2 × 103 
6 × 102 
1.3 × 103 
7 × 102 
Type 321(a) 
(ASTM grain size = 1) 
6.7 × 10-3 
6.7 × 10-5 
2 × 103 
7 × 102 
1 × 103 
2 × 102 
(a) Although the data are not shown, the behavior of type 304 stainless steel parallels that of type 316 
stainless, and the behavior of type 347 stainless steel parallels that of type 321 stainless steel. 
At 700 °C (1290 °F), all the stainless steels exhibited a predominantly transgranular fracture at the high strain rate. At the 
low strain rate, the type 304/316 stainless steels still showed a mostly transgranular fracture with discernible fatigue 
striations, along with some intergranular fracture. However, the type 32 1
3
47 stainless steels exhibited a completely 
intergranular fracture. The difference in the high-temperature (700 °C, or 1290 °F) response between the two groups of 
stainless steels was attributed to the inability of the type 321/347 stainless steels to recover from the cyclic strain 
hardening to the same degree that the type 304/316 stainless steels could. With less recovery, the material within the 
grains is strengthened by cyclic strain hardening, making grain-boundary sliding an easier fracture process than 
transgranular fatigue. If recovery occurs, the material within the grains is strengthened less, and although some 
intergranular fracture still takes place, transgranular fatigue, which exhibits a lower fatigue crack growth rate than 
intergranular sliding, dominates (Ref 247). 
Example 19. An annealed Inconel X-750 alloy was tested under the following conditions: \u2206K = 35 Mpa m (32 
ksi in ), R = 0.05, triangular wave form, air and vacuum (better than 10-5torr), at 650 °C (1200 °F) (Ref 245). The results 
are listed below: 
 
Environment Frequency, 
Hz 
da/dN, 
mm/cycle 
10 5 × 10-4 
0.1 1.5 × 10-3 
Air 
0.01 2 × 10-2 
10 2 × 10-4 Vacuum 
0.1 8 × 10-4 
In air, the fatigue crack growth rate at 0.01 Hz was 40 times greater than at 10 Hz. In vacuum, the fatigue crack growth 
rate at 0.01 Hz was only ten times greater than at 10 Hz. 
In air, the substantial increase in the fatigue crack growth rate with decreasing frequency was believed to result from the 
combined effect of creep and enhanced crack growth due to transgranular and intergranular oxidation by diffusion of 
oxygen in the air. Therefore, the fracture mode changed from a principally fatigue-dominated transgranular fracture with 
fatigue striations at the high frequencies (1 and 10 Hz to a primarily creep-dominated intergranular fracture at the low 
frequencies (0.1 and 0.01 Hz) (a similar change in fracture appearance in an IN-718 nickel-base alloy is illustrated in Fig. 
93). In vacuum, the increase in the fatigue crack growth rate with decreasing frequency was also due to a transition from a 
transgranular fatigue-dominated fracture to a primarily creep-dominated intergranular fracture. The smaller increase in the 
fatigue crack growth rate in vacuum may have been due to the absence of the additional embrittling effect of oxygen (Ref 
245). 
Example 20. An annealed Inconel 600 alloy was tested under the following conditions: \u2206K = 30 to 50 MPa m (27.5 to 
45.5 ksi in ), R = 0.05, air (relative humidity not specified), at room temperature (Ref 258). Frequency had no significant 
effect on the fatigue crack growth rate when tested in the range of 1 to 10 Hz. This is because room-temperature air is 
essentially an inert environment for Inconel 600 and because room temperature is too low for any significant creep to 
occur. 
Example 21. A 20% cold-worked type 316 stainless steel was tested under the following conditions: \u2206K = 30 MPa m 
(27.5 ksi in ), R = 0.05, f = 0.17 Hz, with and without a 1-min dwell time at Kmax, vacuum (better than 10-6 torr), at 593 
°C (1100 °F) (Ref 242). The results are listed below: 
 
Dwell time, 
min 
da/dN, 
mm/cycle 
0 4 ×10-4 
1 4 ×10-2 
The fatigue crack growth rate with a 1-min dwell time was 100 times greater than when no dwell time was present. The 
large increase in the fatigue crack growth rate was due to a creep interaction with the fatigue fracture. Consequently, the 
fracture acquired a more intergranular