Pidgurskyi I. Simulation of fatigue surface cracks growth in low-alloy steels after overloads

The scientific problem of identifying the fundamental laws of surface cracks development in the course of their coalescence under cyclic loading is solved in the dissertation. A simulation model of coalescence of identical coplanar surface cracks has been developed. Based on the definition of SIF for contours which are modeling the gradual propagation of cracks in the coalescence zone, the proposed model was tested to compare the calculated durability with experimental data. Based on regression analysis within the boundaries of the proposed simulation model, a statistical mathematical model was developed, which describes the change of SIF in the zone of coalescence of two identical surface cracks. Based on the developed models a technique has been proposed to determine SIF of normal separation along a contour of a surface crack with a saddle-shaped front under uniaxial tension. The influence of single tensile overloads on the growth retardation of surface semielliptical cracks with energy stable contour of low-alloy steel 09Г2С was estimated. It is established that raising the level of single overload increases the number of cycles of growth retardation of the surface fatigue crack. Increasing the amplitude of the SIF of the base level of cyclic loading leads to a decrease in the number of delay cycles with material thickness t = 20 mm. The influence of the compression parameter a/c of the semielliptical crack on the retardation of the number of cycles of its growth under tensile overload was investigated. It is established that a decrease in the compression parameter a/c leads to an increase in the number of delay cycles. The results of research of the development of surface cracks with a complex geometry contour after application of one-time overloads have been presented. It is established that the number of cycles of growth retardation of a non-canonical surface crack after the effect of tensile overload increases significantly (by several times) in comparison with a surface semielliptical crack of similar size at the same overload. Another peculiarity of cracks of non-canonical shape under the action of overload is a change in the shape of the contour, which approximates a rectangle. Approaches for estimating the growth retardation of non-canonical surface cracks have been proposed.