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


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50 MPa m (45.5 
ksi in ) for both stainless steels. 
Example 8. A 7475 aluminum alloy, aged 6 h at 120 °C (250 °F), that had an average grain size of 18 m was tested 
under the following conditions: R = 0.1, f = 30 Hz, air (50% relative humidity) and vacuum (10-6 torr), at room 
temperature (Ref 239). The results are listed below: 
 
\u2206K Environment 
MPa m ksi in 
da/dN, 
mm/cycle 
4 3.5 4 × 10-6 Air 
12 11 4 × 10-4 
Vacuum 4 3.5 1 × 10-7 
The fatigue crack growth rate at \u2206K = 12 MPa m (11 ksi in ) was 100 times greater in air and 200 times greater in 
vacuum than at 4 MPa m (3.5 ksi in ). 
Example 9. A mill-annealed Ti-6Al-6V-2Sn alloy with a 0.2% offset yield strength of 1100 MPa (160 ksi) and an 
ultimate tensile strength of 1170 MPa (170 ksi) was tested under the following conditions: R = 0.1, f = 10 Hz, air (relative 
humidity not specified), at room temperature (Ref 254). The results are listed below: 
 
\u2206K 
MPa m ksi in 
da/dN, 
mm/cycle 
10 9 1 × 10-5 
40 36.5 1 × 10-2 
The fatigue crack growth rate at \u2206K = 40 MPa m (36.5 ksi in ) was 1000 times greater than at 10 MPa m (9 ksi in ). 
Example 10. A recrystallized annealed Ti-6Al-4V alloy was tested under the following conditions: R = 0.2 to 0.3, f = 1 
to 5 Hz, air (50% relative humidity) and vacuum (10-5 torr), at room temperature (Ref 244). The results are listed below: 
 
\u2206K Environment 
MPa m ksi in 
da/dN, 
mm/cycle 
7 6.5 1 × 10-8 Air 
15 13.5 7 × 10-8 
Vacuum 7 6.5 3 × 10-9 
The fatigue crack growth rate at \u2206K = 15 MPa m (13.5 ksi in ) was 7 times greater in air and 12 times greater in 
vacuum than at 7 MPa m (6.5 ksi in ). 
Example 11. Annealed IMI 155 (British designation for commercially pure titanium with 0.34% O) specimens were 
tested under the following conditions: R = 0.35, f = 130 Hz, air, (relative humidity not specified) and vacuum (2 × 10-6 
torr), at room temperature (Ref 236). the results are listed below: 
 
\u2206K Environment 
MPa m ksi in 
da/dN, 
mm/cycle 
11 10 3 × 10-6 Air(a) 
21 19 2 × 10-4 
Vacuum 11 10 3 × 10-5 
(a) There was considerable scatter in the data from 
four individual tests. The data shown represent 
the approximate average fatigue crack growth 
rate from the four tests. The air fatigue rates were 
smaller than those in vacuum because of irregular 
fatigue crack fronts and crack branching observed 
on air fatigue fractures. 
The fatigue crack growth rate at \u2206K = 21 MPa m (19 ksi in ) was 70 times greater in air and 7 times greater in vacuum 
than at 11 MPa m (10 ksi in ). 
Example 12. An annealed Inconel X-750 alloy was tested under the following conditions: R = 0.05, f = 10 Hz, 
triangular wave form, air (relative humidity not specified), at 25 and 650 °C (75 and 1200 °F) (Ref 245). The results are 
listed below: 
 
Temperature \u2206K 
°C °F MPa m ksi in 
da/dN, 
mm/cycle 
18 16.5 9 × 10-6 25 75 
50 45.5 1 × 10-3 
650 1200 18 16.5 1 × 10-4 
The fatigue crack growth rate at \u2206K = 50 MPa m (45.5 ksi in ) was 110 times greater at 25 °C (75 °F) and 30 times 
greater at 650 °C (1200 °F) than at 18 MPa m (16.5 ksi in ). The effect of \u2206K on the fracture appearance of Inconel X-
750 tested at 650 °C (1200 °F) in air can be seen by comparing Fig. 90(a) (\u2206K = 20 MPa m , or 18 ksi in ) with Fig. 92 
( K = 35 MPa m , or 32 ksi in ). 
 
Fig. 92 Fatigue fracture appearance of Inconel X-750 tested in air at 650 °C (1200 °F) at \u2206K = 35 MPa m (32 
ksi in ). The fracture path is transgranular with ductile fatigue striations. The crack propagation direction is 
indicated by arrow. Compare with the fracture appearance of the same alloy shown in Fig. 90(a), which was 
tested at \u2206K = 20 MPa m (18 ksi in ). Source: Ref 245 
Example 13. A nickel-base superalloy (Astroloy), aged at 845 °C (1555 °F), was tested under the following conditions: 
R = 0.05, f = 0.33 Hz, triangular wave form, air and vacuum (better than 5 × 10-6 torr), at 650 °C (1200 °F) (Ref 244). The 
results are listed below: 
 
\u2206K Environment 
MPa m ksi in 
da/dN, 
mm/cycle 
20 18 5 × 10-4 Air 
50 45.5 5 × 10-3 
Vacuum 20 18 5 × 10-5 
The fatigue crack growth rate at \u2206K = 50 MPa m (45.5 ksi in ) was 10 times greater in air and 40 times greater in 
vacuum than at 20 MPa m (18 ksi in ). 
Example 14. An Inconel 718 alloy, which was heat treated at 925 °C (1700 °F) for 10 h, air cooled, then aged at 730 °C 
(1345 °F) for 48 h, was tested under the following conditions: R = 0.1, f = 0.1 Hz, air and dry helium gas, at 650 °C (1200 
°F) (Ref 227). The results are listed below: 
 
\u2206K Environment 
MPa m ksi in 
da/dN, 
mm/cycle 
40 36.5 2 × 10-3 Air 
60 54.5 1 × 10-2 
Helium 40 36.5 4 × 10-4 
The fatigue crack growth rate at \u2206K = 60 MPa m (54.5 ksi in ) was five times greater in air and ten times greater in 
helium than at 40 MPa m (36.5 ksi in .). 
Example 15. An annealed Incoloy 800 alloy was tested under the following conditions: R = 0.1, f = 0.1 Hz, air and dry 
helium gas, at 427 °C (800 °F) (Ref 228). The results are listed below: 
 
\u2206K 
MPa m ksi in 
da/dN, 
mm/cycle 
28 25.5 3 × 10-4 
44 40 2 × 10-3 
In both air and helium, the fatigue crack growth rate at \u2206K = 44 MPa m (40 ksi in ) was seven times greater than at 28 
MPa m (25.5 ksi in ). 
Effect of Frequency and Wave Form. Frequency, expressed as hertz (Hz = cycles/s), is the cyclic rate at which a 
fatigue load is applied. Strain rate, expressed as (s-1), is a form of frequency that indicates the rate at which a material is 
strained. 
In general, the effect of testing frequency is similar to the effect of \u2206K; that is, if a material is susceptible to 
environmental attack, the effect of the environment is greatest at the lower frequencies when the longer cycle times allow 
the environment to affect the material ahead of the crack tip. This environment effect is generally manifested as an 
increase in the fatigue crack growth rate. The environmental contribution to the fatigue crack growth rate and its effect on 
the fracture appearance are similar to that observed at low \u2206K values, namely the addition of nonfatigue fracture modes to 
the fatigue fracture, as shown in Fig. 93 (see also the section "Effect of Stress Intensity Range, \u2206K" in this article). When 
the environment is not a factor, frequency usually has little effect on the fatigue crack growth rate. 
 
Fig. 93 Effect of frequency on the fracture appearance of an IN-718 nickel-base superalloy that was creep 
fatigue tested at 650 °C (1200 °F): R = 0.1, Kmax = 40 MPa m 36.5 ksi in ). (a) Striation formation at 10 Hz. 
(b) Mixture of intergranular and transgranular cracking at 0.5 Hz. (c) Fully intergranular cracking at 0.001 Hz 
The wave form, which is the shape of the load or strain versus time curve, can affect the fatigue crack growth rate and the 
fracture appearance. As in the case of \u2206K and frequency, any time the environment can affect the material, there is a 
change, generally an increase, in the fatigue crack growth rate. The wave form affords this opportunity by imposing low 
ramp rates (ramp is the slope of the increasing and decreasing segments of the cyclic load curve) and long dwell or hold 
times (period when the cyclic loading is stopped at any position along the load curve), especially at maximum tensile 
loads. 
In addition to the usual fatigue crack growth rate accelerating effects of an aggressive environment, the addition of creep 
effects can complicate the fracture process, especially at elevated temperature and long dwell times. The effect of creep is 
unique because it occurs even in inert environments. When the wave form does affect the fatigue crack growth rate, the 
effect is often reflected