Issue 37

K. Yanase et alii, Frattura ed Integrità Strutturale, 37 (2016) 101-107; DOI: 10.3221/IGF-ESIS.37.14 104 In Fig. 4, the fatigue crack growth rate (FCGR) under torsional loading is compared to that under tension-compression loading [9]. It is noted that, for torsional loading, the stress intensity factor range is calculated by assuming a semi-circular crack. As shown, FCGR can be uniquely characterized irrespective of the difference of loading condition. However, the crack shape is not necessarily semi-circular, as was noted for a medium carbon steel (JIS S35C) (Fig. 5, [10]). Accordingly, a further study is necessary to inspect the evolution of crack shape and its effect on the torsional fatigue behavior of 17- 4PH. Figure 5: Crack profile propagating from an artificial defect in a medium carbon steel (JIS S35C) under torsional loading ( R = ˗ 1) [10]. Fatigue Limit By considering the major principal stress 1  and the minor principal stress 2  on the surface of material, the fatigue limit can be expressed as [3]:        1 2 w 1/6 1.43( 120) ( ) k HV area (1) where HV signifies the Vickers hardness. Under torsional loading, the principal stresses are rendered as    1 and     2 (Fig. 6). Therefore, Eq. (1) can be rewritten to yield:                w w 1/6 1/6 1.43( 120) 1.43( 120) 1 (1 ) 1 ( ) ( ) HV HV k k area area (2)      Two‐hole defect Applied torque Applied torque Figure 6: Stress transformation on the specimen surface (cf. Fig. 3).