PSI - Issue 2_A

Nobuo Nagashima et al. / Procedia Structural Integrity 2 (2016) 1435–1442

1440

Author name / Structural Integrity Procedia 00 (2016) 000–000

6

10 0 Stress amplitude, σ a (MPa ) 200 300 400 500 600 700 800

10 0 Stress amplitude, σ a (MPa) 200 300 400 500 600 700 800 (a)

(b) FMS alloy

2.0%

SUS304 steel

1.4%

0.9%

2.0%

1.4%

0.6%

0.9%

0.6%

10 1

10 2

10 3

10 4

10 4

10 3

10 2

10 1

Number of cycles, N (cycles)

Number of cycles, N (cycles)

Fig. 7. The stress amplitude σ a plotted as a function of the number of cycles N , for four different values of the total strain amplitude ε ta , (a) SUS304 steel, and (b) FMS alloy.

1

solid marks : FMS alloy open marks : SUS304

0.8

0.6

ε ea / ε ta

0.4

ε ta = 2.0% ε ta = 1.4% ε ta = 0.9% ε ta = 0.6%

0.2

0

10 0

10 1

10 2

10 3

10 4

Number of cycles, N (cycels)

Fig. 8. It shows each strain change in the low-cycle fatigue testing, ε ea / ε ta – N , solid mark FMS alloy and open mark SUS304 steel.

Figure 8 shows the ε ea /ε ta - N relationship at different strain amplitudes. For the SUS304 steel (indicated with open marks), the ε ea /ε ta ratio in the range of 10-30% shows little change during the period from the initial stage of cyclic loading to the time of failure. For the FMS alloy (indicated with solid marks), the ε ea /ε ta ratio increases with the increase in the number of cycles. The increase in the ratio is greater at smaller strain amplitudes. At ε ta = 0.6%, the elastic strain is 45% of the total strain in the initial stage of cyclic loading, and about 80% at 1,000 cycles. These results indicate that, in contrast to the SUS304 steel, at constant total strain amplitude, the FMS alloy begins to exhibit an increase in elastic strain, and a decrease in plastic strain, in the middle of the cyclic loading. It is likely that the cyclic softening and hardening curves in Figure 7 are correlated to the deformation structure caused by low cycle fatigue. The structural factors that have a significant effect on the cyclic softening and hardening behavior of the FMS alloy and SUS304 ste el are the ε phase and the α’ phase formed during cyclic deformation. Specifically, as discussed above, the ε phase in the FMS alloy is a cause of the characteristic cyclic deformation behavior due to pseudoelasticity. The α’ phase, ε phase, and γ phase in the cross-section of each specimen tested were analyzed with a ferrite meter and X-ray diffractometry (XDR), and the relationship between the deformation structure and the cyclic loading characteristics was examined. Figure 9 shows the ferrite meter measurements of the failed specimens after the test. At ε ta = 0.6% and 0.9%, only a small amount of ferrite (or α’ ) was present in the SUS304 steel. In the high strain test, the average amount of ferrite increased by 12% and 23% at ε ta = 1.4% and 2.0%, respectively. No ferrite was present in the FMS alloy specimens. Figure 10 shows the XRD results for the FMS alloy specimens. Peaks of the γ phase and the hcp ε phase were observed in each peak curve. The peaks of the ε phase appeared at the same position. The mechanism behind the cyclic softening and hardening behavior of these two austenitic steels is discussed below, based on the results of the analysis. SUS304 steel undergoes cyclic deformation- induced martensitic (α’) transformation at higher strain levels. It is likely that in SUS304 steel, dislocations accumulate due to cyclic loading, and function as dislocation cells or slip bans to increase the deformation resistance, and the increase in the α’ phase