PSI - Issue 13

Yoshimasa Takahashi et al. / Procedia Structural Integrity 13 (2018) 1010–1013 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

1012

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indicate that the specimens tested in ambient air achieves the highest  a value. Since the (  a , N ) plots achieved at the semi-final step in ambient air are located eminently above the original S - N curve (see Fig. 2), the pre-cycled specimens are clearly strengthened. Such a coaxing effect (i.e. time-dependent strengthening) is relatively weaker in dry air and nitrogen. It should be also noted that the coaxing effect was not observed for the base alloy.

200

200

200

Dry air

N 2

Ambient air

10 5 Stress amplitude  a , MPa 50 100 150

150

150

100

100

50

50

10 6

10 7

10 8

10 5

10 6

10 7

10 8

10 5

10 6

10 7

10 8

Number of cycles to failure N f , cycles

Fig. 2. Comparison of S - N curves of new alloy fatigued in different environments.

0.001 1

120

Ambient air

Loading direction

N 2 Dry air

l

110

90 Stress amplitude  a , MPa 100

0.0005 Crack length l , mm .

Semi-final  a achieved

Ambient air (90 MPa)

N 2 (85 MPa) Dry air (80 MPa)

0 0.2 0.4 0.6 0.8 1 (  10 8 ) Number of cycles N , cycles

80

10 7

10 8

Number of cycles N , cycles

Fig. 3. Crack growth curves for run-out specimens.

Fig. 4. Results of  a step-up tests (dotted lines: fatigue limit).

Figure 5 shows the photographs of the small fatigue cracks in each environment observed during the  a step-up tests. The crack growth direction is nearly perpendicular to the loading axis (i.e. mode I) in ambient air while oblique to the loading axis (i.e. shear type) in dry air and nitrogen. The present comparative experiments indicate that i) the existence of humidity (water vapor) plays a key role in the shift of small crack growth mode and ii) the effect of additional Mg on fatigue properties (i.e. fatigue limit, coaxing effect) is most eminent when the small crack growth tends to be mode I in character. Since the mode I crack growth can be attributed to the activation of multiple slip systems, the effect of Mg may be related to the amount of local plasticity at the crack tip. Further discussion on the interrelation of crack tip local slip mode, water vapor and solute Mg will be presented elsewhere.