PSI - Issue 19

Fumiyosi Yoshinaka et al. / Procedia Structural Integrity 19 (2019) 214–223 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 5. Comparison of the cyclic hardening and softening behaviors at Δ ε t of (a) 6%, (b) 4%, (c) 2%, and (d) 1% .

Table 3. The maximum stress in the first cycle σ max, N = 1 . Material σ max, N = 1 at Δ ε t = 6%

σ max, N = 1 at Δ ε t = 4%

σ max, N = 1 at Δ ε t = 2%

σ max, N = 1 at Δ ε t = 1%

Fe-15Mn-10Cr-8Ni-4Si

343 MPa 376 MPa 396 MPa 289 MPa

317 MPa 348 MPa 262 MPa 347 MPa

273 MPa 296 MPa 277 MPa 288 MPa

254 MPa 240 MPa 272 MPa 266 MPa

Fe-28Mn-5Cr-6Si

SUS304 LY225

3.3. S-N diagram

Figure 6 shows the relationship between the stress amplitude σ a and the number of cycles to failure N f ( S - N diagram) obtained in the HCF tests. The fatigue limit σ w , which is roughly assessed by the average value of the minimum σ a , where the specimen was fractured ( σ a = 275 MPa), and σ a , where the specimen was not fractured for up to 10 7 cycles ( σ a = 250 MPa), was calculated to be 262.5 MPa. The ratio of σ w to the 0.2% proof stress σ 0.2 , i.e., σ w / σ 0.2, was 0.96. A high fatigue limit compared to the yield stress is generally reported also for other austenitic steels (Hatanaka and Shimizu (1982)). However, compared to these studies that showed σ w / σ 0.2 over 1, the fatigue limit of Fe-15Mn-10Cr-8Ni-4Si was not so high. Fig. 7 shows the specimen surface observed using a digital microscope after testing at 250 MPa for 10 7 cycles without failure. As shown in the figure, the surface relief was well developed The straight morphologies reflected the planar slip nature of this alloy. The details of the deformed microstructure are reported in Section 3.4. Some darker straight patterns indicated by the arrow in Fig. 7 were recognized in some places, and there is a possibility that these were fatigue cracks; however, it was difficult to affirm that. No cracks with a size of several grains were found. The nature of the crack initiation in this alloy will be investigated as part of our future work.