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


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a stress intensity range of \u2206K = 10 MPa m (9 ksi in ), the fatigue crack growth rate in vacuum was almost 1000 
times lower than that in air (Ref 239). Despite the large difference in the fatigue crack propagation rates, the fracture 
surfaces exhibited fairly similar appearances consisting of facets on crystallographic planes and transgranular fracture 
with some fatigue striations, although the vacuum fractures had a somewhat rougher topography with secondary cracking. 
It was concluded that the large decrease in the fatigue crack growth rate in vacuum was due to a number of factors, 
including enhanced slip reversibility, irregular fracture path with crack branching, mixed Model I and II crack loading, 
the absence of embrittlement at the crack tip, and the possibility of rewelding of the fracture surfaces on unloading (Ref 
239). 
Effect of Vacuum on Titanium Alloys. A substantial increase in the fatigue life in vacuum and a corresponding 
decrease in the fatigue crack growth rate were observed for a basal-transverse textured, solution treated and aged Ti-6Al-
4V alloy tested at room temperature in air (50% relative humidity) and in vacuum (10-6 torr) (Ref 240). The fatigue life 
tests were conducted at stress amplitudes of 670 to 950 MPa (97 to 138 ksi), a load ratio of R = -1, and a cyclic frequency 
of 80 Hz. The fatigue crack propagation tests were performed at a stress intensity range of about \u2206K = 7 to 15 MPa m 
(6.5 to 13.5 ksi in ), a load ratio of R = 0.2, and a cyclic frequency of 10 Hz. With the loading perpendicular to the basal 
planes in the alloy, the fatigue strength in vacuum was 875 MPa (127 ksi) versus 690 MPa (100 ksi) in air; at a constant 
stress amplitude of 875 MPa (127 ksi), more than 107 cycles were required to fail the material in vacuum, although only 
about 104 cycles were required to fail the alloy in air. The fatigue crack propagation rate exhibited a similar trend in 
vacuum. At a stress intensity range of \u2206K = 10 MPa m (9 ksi in ), the fatigue crack growth rate in vacuum decreased 
by about a factor of six compared to the rate in air. 
In fatigue life tests, any delay in fatigue crack initiation increases the fatigue life. Therefore, the orientation of the (0002) 
basal lattice planes with respect to the loading direction is important. In vacuum, when loading normal to the basal planes, 
fatigue crack initiation is delayed because of the high yield stress in the direction normal to the basal planes and because 
of the impeded slip reversibility on the active slip planes, which results in smaller slip steps at the specimen surface and 
more difficult fatigue crack nucleation. Once a crack is nucleated, it propagates more slowly because of a more ductile, 
less crystallographically controlled fracture. When loading normal to the basal planes in moist air, which is considered a 
corrosive medium, the fatigue life does not benefit from the increased yield strength and impeded slip reversal, because 
loading normal to the basal planes renders the material highly, susceptible to hydrogen embrittlement attack, which 
results in cleavage along the (0002) planes. This negates the beneficial characteristics of the textured structure observed in 
vacuum and results in a reduction in the fatigue life and an increase in the fatigue crack growth rate (Ref 240). 
Another Ti-6Al-4V alloy, except in the recrystallized annealed condition with a 0.2% offset yield strength of 862 MPa 
(125 ksi), showed a two- to threefold decrease in the fatigue crack growth rate in vacuum (10-5 torr) compared to humid 
air (50% relative humidity) when tested at a stress intensity range of \u2206K = 10 MPa m (9 ksi in ) at room temperature 
(Ref 241). The alloy was fatigue tested within a stress intensity range of about \u2206K = 7 to 15 MPa m (6.5 to 13.5 ksi in ) 
at a load ratio of R = 0.2 to 0.3 and a cyclic frequency range of 1 to 5 Hz. 
By directly observing the propagation of fatigue cracks in the vacuum environment of the scanning electron microscope, 
it was discovered that at low values of \u2206K the fatigue crack did not advance on each application of load. The specimen 
had to be cycled a considerable number of times to accumulate a critical amount of strain damage at the crack tip before 
crack advance occurred (Ref 241). Examination of the fracture surfaces showed no obvious evidence of crack arrest. The 
air fracture contained fatigue striations that were easy to identify, but the vacuum fractures were relatively featureless, 
with fatigue striations being difficult to discern. 
The objective of this investigation was to use such concepts as the crack-opening displacement and the state of strain at 
the tip of the crack to verify the ability of a crack tip geometric model to predict the spacing of resulting fatigue striations. 
Therefore, the differences between vacuum and air crack growth rates were examined from a standpoint of fracture 
mechanics and grain size rather than crack rewelding or ease of slip reversal. Despite the complex environmental effects 
on the material, the fatigue striation spacing could be fairly successfully predicted by using microstructural features as 
well as measured and calculated properties, such as the crack-opening displacement, \u2206K, strain, slip-line length, 
displacement per slip line, number of slip lines leaving the crack tip, and the cyclic work-hardening coefficient (Ref 241, 
243). 
Effect of Vacuum on Astroloy. The nickel-base superalloy Astroloy also exhibits a lower fatigue crack growth rate in 
vacuum (5 × 10-6 torr) than in air. Fatigue testing of Astroloy, aged at 845 °C (1555 °F) and having an average grain size 
of 5 \u3bcm, within a stress-intensity range of \u2206K = 20 to 50 MPa m (18 to 45.5 ksi in ), a load ratio of R = 0.05, and using 
a triangular wave form with a cyclic frequency of 0.33 Hz at 650 °C (1200 °F) reduced the fatigue crack growth rate in 
vacuum by a factor of three at \u2206K = 50 MPa m (45.5 ksi in ) and by a factor of ten at \u2206K = 20 MPa m (18 ksi in ) 
(Ref 244). Along with the reduction in the crack growth rate, the fracture path changed from predominantly intergranular 
in air to transgranular in vacuum. Figure 88 shows the typical fracture appearance when tested in air and vacuum. The 
decrease in the fatigue crack growth rate in vacuum was believed to be due to the change in the fracture path; in air, the 
loss of oxide-forming elements at the grain boundaries during high-temperature oxidation attack weakened or embrittled 
the boundaries, resulting in easier intergranular crack propagation (Ref 244). 
 
Fig. 88 Typical fatigue fracture appearance of Astroloy tested in air and vacuum at 650 °C (1200 °F). In air 
(upper portion of fractograph) the fracture exhibits a predominantly intergranular character; in vacuum (lower 
half of fractograph) the fracture is transgranular. Source: Ref 244 
Effect of Vacuum on Type 316 Stainless Steel. A different effect was observed when a 20% cold-worked type 
316 stainless steel was fatigue tested in air and vacuum (better than 10-6 torr) at a temperature of 593 °C (1100 °F), a 
stress intensity range of \u2206K = 28 to 80 MPa m (25.5 to 73 ksi in ), a load ratio of R = 0.05, and a cyclic frequency of 
0.17 Hz. In this stainless steel, the fatigue fractures in air were predominantly transgranular with distinct fatigue 
striations; the fractures in vacuum exhibited a more intergranular appearance with little evidence of striations (Fig. 89). 
No specific discussion on the fracture process in air or vacuum was presented because this work was principally 
concerned with the effect of temperature and hold times on the fatigue of the stainless steel. 
 
Fig. 89 Typical fatigue fracture appearance in a cold-worked type 316 stainless steel tested at 593 °C (1100 
°F) in air and vacuum. (a) The fracture in air was primarily transgranular with distinct fatigue striations. (b) The 
fracture in vacuum was