PSI - Issue 59

Ivan Pidgurskyi et al. / Procedia Structural Integrity 59 (2024) 314–321 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Increasing the overload level to Q OL = 1.8 (Fig. 6) compared to Q OL = 1.67 (at R = 0.25) leads to a further increase in the number of delay cycles N D (more than twice) for all investigated cases of crack shape change.

Fig. 6. Dependence of the semi-elliptical surface crack growth retardation N D on the level of overload Q OL = 1.67 and Q OL = 1.8 and the shape aspect ratio a/c = 0.2; 0.5; 0.786.

The difference in the number of delay cycles N D of the surface cracks growth of different shapes is primarily related to the magnitude and distribution of the SIF along the curvilinear contour of each of the cracks and their interaction in the process of development. The change in the stiffness of the stress-strain state along the surface crack front also has an effect on the number of delay cycles. The interaction of both of these factors leads to changes in the surface cracks growth retardation (Figs. 5 and 6) and to changes in its front (Fig. 7). It can be seen that the curvature of the contour increases (the shape aspect ratio a/c increases) with an increase in the level of overload in comparison with the contour of a surface crack under regular cyclic loading. In this case, the contour of the cracks with the initial shape aspect ratio a/c = 0.2 was considered.

Fig. 7. Change in the geometry of the contour of the surface crack under the action of overload Q OL = 1.67 and Q OL = 1.8.

Analyzing the obtained data, we note that the level of overload and the stress ratio for the compared cases were the same. Also, in all cases, the crack front in the region of the minor axis tip was in plane strain conditions, and the crack front adjacent to the free surface was in conditions close to the plane stress state. The values that differ for the studied shape of contours of surface cracks are the magnitudes and gradient distribution of SIF along the contour of the crack. Obviously, the values of the range of SIF ∆ K , which are different for different shape of cracks under the same basic cyclic load and their distribution along the crack front, have the most significant effect on the increase in crack growth delay cycles under the action of overloads, in particular for low-alloy steels. To substantiate this conclusion, simulation modeling was carried out regarding the effect of the range of the SIF ∆ K BL of the main cyclic load mode on the surface cracks growth retardation after overloading.