13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- Table 6. Ultimate tensile strength obtained experimentaly from tensile tests. Sperimen Ultimate tensile strength σt (MPa) Ph-42 Supra steel plasma nitrided 1174.3 Ph-42 Supra steel non-nitrided 1150.0 3.4. Basquin’s Equation The empiric law of Basquin for fatigue life [8] of PH-42 Supra steel plasma nitrided specimens obtained from the plotted mean curve in Fig. 5 is given by, (MPa) 2921 N 0.1024 f a − σ = (2) and for PH-42 Supra steel non-nitrided specimens, the Basquin’s equation is given by, (MPa) 2182.7 N 0.1159 f a − σ = (3) where σ a is the bending cyclic stress amplitude and Nf is the number of cycles to rupture. 3.5. Fractograph by SEM Fracture surface by SEM of PH-42 Supra steel plasma nitrided tensile test specimens are shown in Fig. 7a and 7b. Surface cracks initiated in the nitrided layer and at 90 degree to the load line are seen. Hence, the fracture mechanism of the nitrided layer was brittle fracture due to principal maximum tensile stress. The measured plasma nitrided layer thickness was 337.87µm. Figure 7. SEM photos of fractured PH-42 Supra steel plasma nitrided tensile test specimen: a) surface cracks and b) fracture surface showing the plasma nitrided layer thickness of 337.87µm. In Fig. 8, fracture surface of fatigue test specimen of PH-42 Supra steel non-nitrided are presented. Brittle fracture mechanism in Fig. 8a and 8b and the fatigue crack length near the specimen surface can be observed. In Fig. 9a and 9b, fracture surface of fatigue test specimen of PH-42 Supra steel plasma nitrided are shown. Instantaneous rupture by brittle fracture and a very small fatigue region a b
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