PSI - Issue 59

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

317

4

Fig. 2. Experimental results of the growth of identical fatigue semi-elliptic surface cracks under a regular load σ max = 166.7 MPa (I) and under the influence of an overload Q OL = 1.67 (II).

a)

b)

Fig. 3. Fractures of semi-natural samples with fatigue surface semi-elliptical cracks: (a) under regular loading; (b) under the influence of overload Q OL = 1.67.

The choice of the initial parameters of the semi-elliptical surface crack under overloading was justified by the possibility of studying the effect of overloading under the conditions of an energetically stable crack contour, when the SIF for points along the contour of the surface crack are almost the same. With such aspect ratio a/c ≈ 0.8, the ef fect of compression of the stress-strain state along the front of a surface crack on the delay of fatigue crack growth under the action of overloading can be established. The effect of overloading is characterized by a delay in the number of cycles N D of surface crack growth along the semi-elliptical contour of the surface crack. Usually, surface cracks growth retardation is investigated for characteristic points of the front: on the surface (point C) and in the deepest point of the front (point A). Studies show that in the direction of depth, the delay in crack growth after the application of tensile overload is less than on the surface of the sample. This is confirmed by a number of studies (Pokrovskii et al. (1996), Fleck et al. (1983)), and is explained by a change in the deformation conditions along the front of the surface crack from plane deformation at the maximum depth point to a plane stress state on the surface of the plate. Since the delay in the surface crack growth at the deepest point is smaller, then under conditions of plane deformation the deepest point A and the area of the contour close to it will advance at a higher speed. This leads to a slight change in the aspect ratio a / c (up to 5%) in the zone affected by tensile overload (Fig. 3). Similar data were obtained in works (Pokrovskii et al. (1996), Fleck et al. (1983)). Depending on the features of initiation or coalescence the contours of surface cracks can acquire different semi elliptical shapes (Fig. 4).