PSI - Issue 2_A

Nobuo Nagashima et al. / Procedia Structural Integrity 2 (2016) 1435–1442 Author name / Structural Integrity Procedia 00 (2016) 000–000

1441

7

contributes to the cyclic hardening. As shown in Figure 7(a), the SUS304 steel undergoes lesser strain hardening at ε ta = 0.6% and 0.9%, likely because almost no α’ phase is produced. The cyclic strain hardening at ε ta = 1.4% and 2.0% is likely attributable to the increase in the α’ phase due to cyclic loading . As shown in Figure 1, in the FMS alloy, pseudoelastic strain appears as an inelastic deformation component of springback, with 0.2% or more plastic deformation during unloading, in monotonic tension deformation. As shown in Figure 3, pseudoelastic strain also appears when cyclic strain is applied. The XRD results in Figure 10 show the presence of diffraction peaks of the ε phase, which is the cause of the pseudoelastic strain at ε ta = 2.0-0.6%. Comparison of Figures 7 and 8 shows that the hardening stage in the middle of the cyclic loading corresponds to the increase in elastic strain. For example, at ε ta = 0.6%, the ratio ε ea /ε ta increases from 45% to 80%. The increase in the ratio of elastic strain to total strain is obviously due to the pseudoelastic de formation of the ε phase. The cyclic hardening due to the increase in pseudoelastic strain suggests that the variants causing the reversible deformation of the ε phase increased in this deformation stage. These results show that the increase in the cumulative damage strain is mitigated by the pseudoelastic strain due to reversible deformation of the ε phase ; and thus the FMS alloy exhibits excellent low cycle fatigue properties under cyclic strain-controlled loading.

30

SUS304 steel fatigue test

0 Volume fraction of α ' (vol %) 0 10 20

1

2

Total strain amplitude , ε ta (%)

Fig. 9. The volume fraction o f α’ martensite in SUS304 steel after fatigue failure.

Fig. 10. The XRD profiles taken on FMS alloy after fatigue failure.

4. Conclusions In this study, a low cycle fatigue test was performed on an austenitic FMS alloy possessing pseudoelastic strain, one of the characteristics of the shape memory effect, and on SUS304 steel possessing an austenitic structure; and the effect of pseudoelastic strain on fatigue behavior was evaluated. The following is a summary of the results. (1) Comparison of the FMS alloy and SUS304 steel showed that the life to failure of the FMS alloy is comparable to that of SUS304 steel at ε ta = 2.0%, two times higher at 1.4% and 0.9%, and four times higher at 0.6%. (2) The ε pa - N f relationship is linear for both the FMS alloy and SUS304 steel. ε pa and N f are well correlated, and the Manson-Coffin law holds. The FMS alloy has high strength, but the K p for the alloy is 0.45, which is less than the value of 0.49 for the low-strength N steel. Therefore, K p is not suitable for use as an index to differentiate high- and low-strength steels. (3) The ε ea - N f relationship for the FMS alloy is plotted above that for other steels. A large cyclic elastic strain means that the plastic strain is reduced at constant total strain amplitude, and consequently the accumulated plastic strain is reduced, resulting in longer low cycle fatigue life. The ε ea - N f relationship is categorized by material, and therefore suitable for use as an index to evaluate the low cycle fatigue properties of pseudoelastic FMS shape memory alloys. (4) The XRD results showed the presence of diffraction peaks of the ε phase, which is the cause of the pseudoelastic strain in the FMS alloy at ε ta = 2.0-0.6%. This shows that pseudoelastic strain due to