PSI - Issue 19

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

8

511

Fail

Unfail

Unfailed but cracked

Unfailed and uncracked

Arrows indicate the run-out specimens

(a) S-N diagram

(b) Observed crack length of the run-out specimens

Fig. 12. Fatigue characteristics of Material A.

Fig. 12(a) shows an S-N diagram of Material A, in which the data points of failed specimens show a good linear relation (the Basquin relation) on a double logarithm graph. Fig. 12(b) displays the lengths of cracks in the run-out specimens measured at different stress levels. Many fatigue cracks were initiated and propagated at a limiting stress of  a = 550 MPa (the maximum stress applied to run-out specimens). Also, it should be noted that, as seen in Fig. 12(b), even at the stress amplitude much lower than the limiting stress (e.g.,  a = 400 MPa), the fatigue crack initiation was activated. Those characteristics, as shown in Figs. 12(a) and (b), are considered to be closely linked to the microstructure of the roll material.

Arrows indicate the run-out specimens

Fig. 13. A comparison of S-N diagram of Materials A and B.

To examine the influence of microstructure on the S-N diagram, the different type of roll steel, Material B, was additionally tested. This material contained a certain amount of graphite to attain the less sticking property (cf. Fig. 1). In practice, Material B is effectively used to reduce the cobble incident damage associated with sticking. Fig. 13 shows