PSI - Issue 33

A. Kansy et al. / Procedia Structural Integrity 33 (2021) 1173–1180 A. Kansy, M. Kaynak, C. Bleicher, H. Kaufmann / Structural Integrity Procedia 00 (2019) 000–000

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Both curves reach the same fatigue strength with a stress amplitude at the knee point σ a,k (N k ) = 104 MPa and a nominal stress amplitude at N lim = 1ꞏ10 7 of σ a,N,lim = 98. Thus, the slight differences in graphite morphology show no effect on fatigue strength. Scatter bands of T σ = 1:1.10 and T σ = 1:1.18 were determined. Buthmann determined for EN-GJL-250 and specimens with a diameter of 12 mm and a HSV 90% of 9547 mm³ a nominal stress amplitude of σ a,N,lim = 86 MPa and a scatter band of T σ = 1:1.30. Figure 4 shows the S-N curves for the unnotched specimens with 6 mm diameter (HSV 90% of 378 mm³) removed from the component (red) and cast samples (blue). The curves show a similar nominal stress amplitude σ a,N,lim = 86 MPa and 84 MPa which are about 13 MPa lower in comparison to the large unnotched specimens. Scatter bands of T σ = 1:1.36 and T σ = 1:1.31 were determined and are much higher than the scatter bands of the large unnotched specimens. In case of the component, the specimens were removed at a large distance from each other, so that an influence of the cooling conditions can be seen in the fatigue strength, resulting in a wider scatter band. Buthmann determined for EN-GJL-250 and unnotched specimens with a diameter of 4 mm and a HSV 90% of 615 mm³ a nominal stress amplitude of σ a,N,lim = 77,5 MPa and a scatter band of T σ = 1:1.20.

Figure 4. S-N curves of unnotched specimens with 6 mm diameter, HSV 90% of 378 mm³; red curve: component; blue curve: cast samples

Figure 5 shows the S-N curves for the notched specimens with 6 mm diameter (HSV 90% of 0.1 mm³) removed from the component (red) and cast samples (blue). The nominal stress amplitudes of σ a,N,lim = 68 MPa and 62 MPa differ by approximately 10 %. The low effect of the sharp notch on the fatigue strength can be explained by the high micro supporting effect of the material [7]. Scatter bands of T σ = 1:1.48 and T σ = 1:1.43 were determined and are even higher compared to the scatter bands of the unnothed samples. Graphite lamellae, which serve as inner notches, lead to a local stress intensification. If these lamellae are located unfavorably in the area of the outer notch, the stress intensification is overlaid by the outer notch and by the lamella. For some specimens, this leads to very early failure and thus to a wide scatter band. Buthmann determined for EN-GJL-250 and specimens with a diameter of 4 mm. notch factor of K t = 3 and a HSV 90% of 0.014 mm³ a nominal stress amplitude of σ a,N,lim = 59 MPa and a scatter band of T σ = 1:1.35.