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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of up to 500 kN with an accuracy of ±1%. The pulsator is a valveless single -plunger pump designed to create variable unilateral cyclic loads on the test sample.

Fig. 1. Sample with a surface crack.

The surface crack was initiated from the notch under a load that did not exceed the load of subsequent tests. The notch was made with a thin disk cutter with diameter of 27 mm and thickness of 0.13 mm. The cutter was fixed in a specially made device. The depth of the surface defect (notch) varied in the range of (0.1 0.15) t , where t = 20 mm is the thickness of the sample. The length of the crack 2 c on the surface of the sample was measured visually using a portable microscope TQC LD6169 with ten times magnification. The propagation of the surface crack deep into the sample was investigated by marker lines formed as a result of a short-term increase of the lower level of cyclic load to R ~ 0.9 (Bezensek and Hancock (2004)). Another method of studying the kinetics of the surface crack shape was the use of a penetrating colouring liquid with a similar increase of the stress ratio R. The samples are made of low-alloy steel 09Mn2Si with mechanical characteristics R y = 375 MPa and R u = 534 MPa and parameters of cyclic crack resistance n = 3.08; C = 8.9·10 -12 (MPa) - n m (1 - n /2) ·cycle -1 . The chemical composition of 09Mn2Si steel is 0.08% C; 1.53% Mn; 0.78% Si; 0.021% P; 0.012% S; 0.04% Cr; 0.65% Cu; 0.12% Ni. Simulations were carried out using the specialized software AFGROW (Harter (2020)). The input data of the cyclic crack resistance of 09Mn2Si steel were obtained on the basis of the fatigue fracture kinetic diagram (FFKD), which was constructed based on the results of experimental studies of the surface cracks growth under cyclic loading. The accuracy of the predicted results of crack growth by the number of cycles, as well as the convergence of the data on the change in the contour shape during cyclic loading did not exceed 2% in comparison with the experimental data obtained under regular loading, as well as when assessing the impact of overloading. 3. Research results Fig. 2 presents comparative data on the growth of fatigue semi-elliptical surface cracks under an overload of Q OL = 1.67 and an identical crack under regular cyclic loading. The test was conducted at the maximum stress level of the main cycle σ max = 166.7 MPa and stress ratio R = 0.25. At the time of the overload, the size of the semi-elliptical surface fatigue crack was as follows: 2 c = 22 mm; a = 8.6 mm; a/c = 0.78 (Fig. 3). Fig. 2 presents comparative data on the growth of fatigue semi-elliptical surface cracks under an overload of Q OL = 1.67 and an identical crack under regular cyclic loading. The test was conducted at the maximum stress level of the main cycle σ max = 166.7 MPa and stress ratio R = 0.25. At the time of the overload, the size of the semi-elliptical surface fatigue crack was as follows: 2 c = 22 mm; a = 8.6 mm; a/c = 0.78 (Fig. 3).