Issue 37

K. Yanase et alii, Frattura ed Integrità Strutturale, 37 (2016) 101-107; DOI: 10.3221/IGF-ESIS.37.14 105 According to the previously reported results [3-7],   0.18 k holds for carbon steels, Cr-Mo steel, ductile cast irons, and high tension brass. If the effect of biaxial stress is negligible (i.e.,  0 k ), the torsional fatigue limit is solely determined by the major principal stress as follows (cf. Fig. 6):      w w 1/6 1.43( 120) ( ) HV area (3) On the other hand, Schönbauer et al. proposed that the fatigue limit in the presence of a large defect or a long crack can be estimated by the following equation [2]:      th,lc w 2 0.65 K area (4) where the threshold stress intensity factor range of th,lc ΔK =6.7MPa m for the investigated 17-4PH steel at R = ˗ 1 was determined by Schönbauer et al. [9]. Fig. 7 shows the relationship between the shear stress amplitude  a and area . When a specimen endured the number of cycles N = 1.2  10 7 , it was regarded as a run-out specimen. As shown, the prediction for fatigue limit with k = 0 (Fig. 7(b)) renders higher correlation to the experimental data than using k = ˗ 0.18 (Fig. 7(a)). Further, for the torsional loading, fatigue limit can be reasonably estimated by the two prediction lines:    0.6 (1.6 ) a HV and the threshold for long crack (cf. Eq. (4)). 10 1 10 2 10 3 100 150 200 250 300 350 400  area (  m) Shear stress amplitude,   a  (MPa) "1‐hole defect" "2‐hole defect" "3‐hole defect"  a  = 0.6  (1.6HV) Threshold for long crack  area parameter model  with "k = ‐0.18" Failure Run‐out Prediction for fatigue limit (a) Prediction with k = ˗ 0.18 (b) Prediction with k = 0 Figure 7: Relationship between shear stress amplitude  a and area . In Fig. 8, fatigue limit for torsional loading and tension-compression loading is compared [2]. It is noted that, in the tension-compression data, various types of defects are considered (e.g., drilled hole, circumferential notch, corrosion pit). As shown, the respective fatigue limit can be reasonably estimated by considering the major principal stress (i.e., k = 0) and area . The torsional fatigue limit becomes insensitive to the defect when  100μm area , which is in strong contrast 10 1 10 2 10 3 100 150 200 250 300 350 400  area (  m) Shear stress amplitude,   a  (MPa) "1‐hole defect" "2‐hole defect" "3‐hole defect"  a  = 0.6  (1.6HV) Threshold for long crack  area parameter model with "k = 0"              Failure Run‐out Prediction for fatigue limit