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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Table 2. Coefficients for the Coffin – Manson ( α in , C in ) and Basquin ( α e , C e ) laws. Material α in C in

α e

C e

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

0.36 0.43 0.34 0.46

0.40 0.41 0.11 0.33

0.22 0.15 0.09 0.13

0.045 0.024 0.011 0.008

Fe-28Mn-5Cr-6Si

SUS304 LY225

3.2. Cyclic hardening and softening

To investigate the cyclic hardening and softening behavior, the relationship between the maximum stress σ max in each cycle and the number of cycles N was obtained. Fig. 4 shows the σ max - N relationship for Fe-15Mn-10Cr-8Ni 4Si at Δ ε t = 1%, 2%, 4%, and 6%. The horizontal axis represents the number of cycles in the semi-log scale. According to the figure, this material showed cyclic hardening up to the fatigue fracture stage at all strain levels. At lower Δ ε t values of 1% and 2%, σ max gradually and monotonically increased with the increase of N on the semi-log plot. On the other hand, σ max rapidly increased in the early stage of the tests for higher Δ ε t values of 4% and 6%. In the case of Δ ε t = 6%, significant hardening was recognized at the stage near the final fracture.

Fig. 4. Relationship between the maximum stress σ max in each cycle and the number of cycles N .

The cyclic hardening and softening behaviors of the reference materials is shown in Fig. 5 compared to the result of Fe-15Mn-10Cr-8Ni-4Si. To compare the different materials on one graph, the ratio between the maximum stress σ max in each cycle and that in the first cycle σ max, N = 1 , that is σ max / σ max, N = 1 , is used. The values of σ max, N = 1 for each material are listed in Table 3. Fe-28Mn-5Cr-6Si, which exhibited a long N f , though not as long as that of Fe-15Mn 10Cr-8Ni-4Si, showed nearly the same hardening tendency for all strain levels; it became hard in the early stage, after which it took a roughly constant σ max value up to the fracture appearance. SUS304 demonstrated a tendency similar to that of Fe-15Mn-10Cr-8Ni-4Si at Δ ε t = 6%, where significant hardening occurred at the stage near the fracture appearance. Considering these facts, the gradual and mild cyclic hardening nature is one of the reasons for the prolonged N f of Fe-15Mn-10Cr-8Ni-4Si.