PSI - Issue 2_A

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

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the tangent aligned with the straight section of the unloading curve in the strain-controlled test at ε ta = 2.0% (Figure 3 (a)) was drawn using Young’s modulus for SUS304 steel ( E = 193 GPa). The small discrepancy between the intersection of this gradient line at zero stress, and ε pa , represents the Bauschinger strain ( ε r ). In low cycle fatigue testing, ε r is typically negligible and ignored. The SUS304 steel exhibited decreasing stress and decreasing ε r with decreasing controlled strain. As shown in Figure 3(b), unlike SUS304 steel, the FMS alloy exhibited only a small decrease in maximum stress with the decrease in controlled strain. In all specimens, the unloading curve in the hysteresis loop bent inward as the strain began to decrease more significantly upon unloading, by about 30% from the maximum stress amplitude; a similar phenomenon to the pseudoelastic transformation strain observed in the tensile test (Figure 1). The gradient line of Young’s modulus for SUS304 steel is shown in Figure 3(b), and this line is in agreement with the tangent to the unloading curve, until the maximum stress amplitude decreases by about 30%. Therefore, the Young’s modulus for FMS alloy, which is a single-phase austenitic alloy, is nearly the same as that for SUS304 steel. Table 1 represents the intersection of the gradient line of Young’s modulus at zero stress as the PE point, and the difference between the PE point and the ε pa point as the pseudoelastic transformation strain, ε P E a The FMS alloy exhibited pseudoelastic transformation strain at all amplitudes in the low cycle fatigue test. Therefore, the pseudoelastic effect was still observed in the FMS alloy at half of the life to failure, under the low cycle fatigue test conditions. Figure 4 shows the relationship between ε ta and N f. The life to failure of the FMS alloy was 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%.

10 2 Total Strain amplitude, ε ta 10 -3 10 -2

FMS alloy SUS304 steel

10 3

10 4

Number of cyclics to failure, N f (cycles)

Fig. 4. The total strain amplitude ε ta plotted against the number of cycles to failure N f, ● mark FMS alloy, ○ mark SUS304 steel.

Normalized

steels

10 -3 Plastic strain amplitude, ε pa 10 -2

QT steels FMS alloy SUS304

10 1 10 2 10 3 10 4 10 5 10 6 10 7 Number of cyclics to failure, N f (cycles)

Fig. 5. The plastic strain amplitude ε pa plotted against the number of cycles to failure N f. Each marks showing ,● FMS alloy, ○ SUS304 steel, ＋ normalized steels ( S25C, S35C, S45C ( σ B =450-600MPa)), and × quench tempered steels ( S45C, SCr440, SCM435, SNCM439( σ B =900-1200MPa)).