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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Fig. 4. Semi-elliptical surface cracks with the same major axis of the ellipse 2c and variable aspect ratio a/c = 0.2; 0.5; 0.786.

Shape aspect ratio a/c of a semi-elliptical crack is one of the most important factors in assessing the stress intensity factors, which change significantly along the crack contour (Coules (2016), Brennan et al. (2008)). In the case when shape aspect ratio a/c is large (for example, a/c = 2.0) the maximum values of SIF are observed at the surface points of the contour (point C). Conversely, for small values of a/c = 0.2 ÷ 0.6, the maximum SIF values are observed for the deepest point of the contour (point A). During cyclic loading, such a significant change in the force parameter of the SIF along the front of the surface crack leads to a change in the contour geometry. In the range of values of a/c = 0.7 ÷ 0.95 the contour of a fatigue surface crack acquires an energetically stable shape (Coules (2016), Brennan et al. (2008)), when the shape of the surface crack stabilizes at almost the same values of SIF. It is in this range of a/c that most studies of the kinetics of the shape of fatigue surface cracks under the action of overloads have been carried out (Pokrovskii et al. (1996), Fleck et al. (1983)). However, there are cases when surface cracks merge and a macrocrack with a small aspect ratio a/c = 0.2 ÷ 0.5 is formed. The difficulty of analyzing and interpreting the results obtained for such cracks under the action of overloading lies in the absence of a generalized geometric characteristic, by changing of which it would be possible to establish the kinetics of its growth (Panasiuk et al. (1991)). Another feature, which is difficult to interpret, is the redistribution of SIF and the change in the stiffness of the stress state along the crack front during its propagation. It is too difficult to assess the impact of these factors using experimental methods, so the modeling method using the specialized AFGROW software was applied. An assessment of the surface cracks growth retardation N D was made while modeling the influence of the following factors: surface crack shape aspect ratio; stress ratio; overload level. The research methodology consisted in determining the number of delay cycles after the action of a single tensile overload of a certain level for semi-elliptical surface cracks with a fixed length on the surface of 2c = 22 mm and a variable shape aspect ratio a/c = 0.2; 0.5; and 0.786 compared to the same contour under regular loading. Fig. 5 presents the results of crack growth retardation N D during overload Q OL = 1.67 for the investigated configurations of the crack front: a/c =0.2; 0.5; 0.786 (Fig. 4). It is established that with a decrease of a/c crack growth retardation increases exponentially by 1.16 and 1.71 times respectively. Decreasing the stress ratio from R = 0.25 to R = 0 led to 2.3-2.4 times decrease in retardation N D for each of the studied cases, and the trend of changes in N D is similar to the data at R = 0.25.

Fig. 5. Dependence of the semi-elliptical surface crack growth retardation N D under single overload Q OL = 1.67 from the shape of its contour and the stress ratio (steel 09Mn2Si).