PSI - Issue 19

Keiji Yanase et al. / Procedia Structural Integrity 19 (2019) 504–512 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Results and discussion

Fig. 9 shows the shear-mode fatigue crack growth behavior in a run-out specimen, which was observed with the replica method. Unexpectedly, many shear-mode fatigue cracks were initiated and propagated accompanying some kinking. Fig. 10 shows the SEM images of a failed specimen. It revealed that the crack penetrated the carbides avoiding the hard matrix, which resulted in the tortuous crack propagation behavior, as observed in Fig. 9. Further, it shows that a relatively long, shear-mode surface-crack is reproducible with the present testing method. According to Figs. 9 and 10, it is expected that the present testing method can account for the effect of microstructure on the shear-mode fatigue crack propagation behavior and the fatigue strength. Fig. 11 shows the fatigue crack growth behavior of two major cracks in the run-out specimen (Fig. 9). According to this figure, though it was defined as run-out, the shear-mode cracks did not stop their propagations; instead, they showed a tendency of acceleration, at N = 1.2  10 7 cycles (Fig. 11(a)). The typical low growth rate of shear-mode fatigue crack observed in a JIS SUJ-2 bearing steel near the threshold level (cf. Matsunaga et al. (2009)) was also confirmed for the present roll material (Fig. 11(b)). Figs. 9-11 prove the effectiveness of the testing method (Fig. 8) for evaluating the shear-mode fatigue crack growth behavior of the roll material. Further fatigue tests were thus performed to gain more insights.

Air

Air

Air

N = 2.0 × 10 6 cycles 2 a = 104 µm

N =2.5 × 10 5 cycles 2 a = 51 µm

N = 4.0 × 10 6 cycles 2 a = 211 µm

Specimen axis

N = 9.0 × 10 6 cycles 2 a = 274 µm

N = 7.0 × 10 6 cycles 2 a = 252 µm

N = 1.2 × 10 7 cycles 2 a = 296 µm

100µm

Fig. 9. Growth behavior of a shear-mode fatigue crack in a run-out specimen of Material A, observed by the replica method. (  a = 550 MPa,  static = -1000 MPa and run-out at N = 1.2  10 7 cycles. Arrows indicate the crack tips)