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


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character (Fig. 94). 
 
Fig. 94 Fracture appearance of a cold-worked type 316 stainless steel fatigue tested in vacuum of 593 °C 
(1100 °F) under a 1-min dwell time. Note the more intergranular nature of this fracture when compared to Fig. 
89(b), which shows the fracture appearance of the same alloy tested under the same conditions, except at zero 
dwell time. Source: Ref 242 
Example 22. 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, ramp strain rate = 6.7 × 10-3 s-1, with and without a 30-min 
dwell time at maximum tensile strain, air, at 600 and 700 °C (1110 and 1290 °F) (Ref 247). The results are given in Table 
2. At 600 and 700 °C (1110 and 1290 °F), the type 316 stainless steel showed about a 6.5-time decrease in the fatigue life, 
Nf, because of the imposition of a 30-min dwell period at maximum tensile strain. The type 321 stainless steel exhibited a 
tenfold decrease. Although the data are not shown, the behavior of type 304 stainless steel parallels that of type 316 
stainless steel, and the data for the type 347 stainless steel are similar to those of the type 321 stainless steel. 
Table 2 Effect of dwell time at maximum tensile strain on the fatigue life of type 316 and 321 stainless steels 
Alloy Dwell time, 
min 
Fatigue life 
(Nf, at 600 °C (1110 °F) 
Fatigue life 
(Nf, at 700 °C (1290 °F) 
Type 316 
(ASTM grain size = 2) 
0 
30 
2 × 103 
3 × 102 
1.3 × 103 
2 × 102 
Type 321 
(ASTM grain size = 1) 
0 
30 
2 × 103 
2 × 102 
1 × 103 
1 × 102 
Imposing a 30-min dwell time at the maximum tensile strain had an effect similar to that of decreasing the strain rate from 
6.7 × 10-3 to 6.7 × 10-5 s-1, except that the magnitude of the decrease in the fatigue life was greater for the dwell-time tests, 
and the decrease was greater for the type 32 1
3
47 stainless steels than for the type 304/316 stainless steels. This larger 
decrease in the fatigue life was reflected in the fracture appearance. All of the steels showed an intergranular appearance 
with cavitated intergranular facets reminiscent of creep rupture; however, the type 304/316 stainless steels exhibited more 
of a mixed intergranular fracture and transgranular fatigue with fatigue striations, while the type 32 1
3
47 stainless steel 
fractures were predominantly intergranular. 
Example 23. An IMI-685 titanium alloy (Ti-6Al-5Zr-0.5Mo-0.25Si and with 40 ppm H) with a microstructure 
consisting of aligned \u3b1 within prior-\u3b2 grains was tested under the following conditions: \u2206K = 22 to 40 MPa m (20 to 365 
ksi in ). R = 0.05, f = 1Hz, 5-min dwell time at maximum tensile load, air (relative humidity not specified), at room 
temperature (Ref 257). The tests were conducted by continuous fatigue cycling until the crack had grown to a length of 5 
mm (0.2 in.), the 5-min dwell was then applied, and the normal cycling was again resumed after the 5-min dwell. 
Because of appreciable scatter in the dwell test data, no specific numerical results will be given; however, the 5-min dwell 
at maximum tensile stress produced at least an order of magnitude increase in the fatigue crack growth rate. This increase 
was accompanied by changes in the fracture appearance. These changes to the predominantly fatigue-striated fracture 
included a large increase over a short distance immediately after the dwell period, changes in the local fracture direction 
(local cracks often propagated at right angles to the macroscopic fracture direction), and the introduction of quasi-
cleavage and cleavage fractures (Fig. 95). Although the nondwell fractures also exhibited some quasi-cleavage and 
cleavage regions, the dwell fractures contained more of these features. Both the local increase in the striation spacing and 
the changes in the local fracture direction resulted from the link-up of the fatigue crack with the dwell-induced nonfatigue 
cracked areas ahead of the main crack front. The increase in the fatigue crack growth rate and the changes in the fracture 
mode were due to the combined action of creep and hydrogen embrittlement ahead of the crack tip that occurred during 
the dwell period (Ref 257). 
 
Fig. 95 Changes in the principally fatigue-striated fracture appearance of a Ti-6Al-5Zr-0.5Mo-0.25Si alloy as a 
result of a 5-min dwell at maximum tensile stress. The changes included a local increase in the striation spacing 
after the dwell period (a); a change in the local fracture direction, as indicated by the orientation of the fatigue 
striations (central portion of photograph), which are normal to the macroscopic fracture propagation direction 
(b); and the introduction of cleavage fracture shown in the stereo pair in (c). The macroscopic crack growth 
direction is vertical. Source: Ref 257 
Example 24. An IMI-685 titanium alloy (Ti-6Al-5Zr-0.5Mo-0.25Si and with 140 ppm H), which was \u3b2 annealed and 
stress-relieved, and a Ti-5Al-2.5Sn alloy with 90 ppm H, which was \u3b2 annealed and aged, were tested under the following 
conditions: load-controlled, maximum load = 95% of 0.2% offset yield strength, R = 0.3, f = 0.33 Hz, triangular wave 
form with and without a 5-min dwell at maximum load, air (relative humidity not specified), at room temperature (Ref 
141). 
Both alloys exhibited a drastic decrease in the fatigue life, Nf, as a result of the 5-min dwell at 95% of the 0.2% offset 
yield strength. The fatigue life of the IMI-685 alloy decreased from a no-dwell Nf of over 30,000 to 173 cycles, while the 
fatigue life of the Ti-5Al-2.5Sn alloy decrease from 12,300 to 103 cycles. The dwell fractures exhibited extensive areas of 
cleavage. The large decrease in the fatigue lives was attributed to hydrogen embrittlement, which produced cleavage in 
the phase. There was no evidence of a creep contribution to the fracture process (Ref 141). 
Example 25. A Ti-6Al-4V alloy containing 44 ppm H and a Ti-6Al-2Sn-4Zr-6Mo alloy containing 68 ppm H, which 
were annealed and basal textured, were tested to determine the influence of dwell time at maximum tensile stress on the 
fatigue crack growth rates of the alloys. The Ti-6Al-4V alloy was tested under the following conditions: three \u2206K ranges: 
12 to 14 MPa m (11 to 12.5 ksi in ), 21 to 25 MPa m (19 to 23 ksi in ), and 37 to 41 MPa m (33.5 to 37 ksi ); 
R = 0.01; f = 0.3 and 6 Hz; with and without a 45-min dwell at maximum tensile stress. The Ti-6Al-2Sn-4Zr-6Mo alloy 
was tested as follow: \u2206K = approximately 23 to 28 MPa m (21 to 25.5 ksi in ), and \u2206K = 38.5 MPa m (35 ksi in ), R 
= 0.01, f = 0.3, with and without a 45-min dwell at maximum tensile stress for the \u2206K = 23 to 28 MPa m (21 to 25.5 
ksi in ) tests, with and without a 10-min dwell for the \u2206K = 38.5 MPa m (35 ksi in ) test. All of the fatigue tests were 
conducted using displacement-controlled constant stress intensity (K), in air (relative humidity not specified), at room 
temperature. 
The imposition of a 45-min dwell period at maximum tensile stress resulted in a very slight increase in the fatigue crack 
growth rate for the Ti-6Al-4V alloy when tested at the 37- to 41-MPa m (33.5- to 37-ksi in ). \u2206K range. There was no 
significant increase in the fatigue crack growth rate at the lower values of \u2206K. The Ti-6Al-2Sn-4Zr-6Mo alloy showed a 
nominal two- to threefold increase in the total fatigue crack growth rate as a result of the 10-min dwell at \u2206K = 38.5 
MPa m (35 ksi in ), but there was little effect on the fatigue crack growth rate at the lower values of \u2206K. The small 
changes in the fatigue crack growth rate for both alloys were due to the crack advance by cleavage during the dwell 
periods. The cleavage fracture was the result of hydrogen embrittlement (Ref 259). 
Example 26. An annealed SUS 304 stainless steel (SUS 304 is the Japanese equivalent