Fatigue crack growth kinetic on steels under variable amplitude loading

The fatigue crack growth kinetics on samples from four types of steels under constant and variable amplitude cycling loading with different asymmetry ratio is studied. The loading parameters were chosen in such a way that the resulting fatigue crack growth rate curves fit onto the middle region of the fatigue crack growth diagram. Standard compact specimens with an edge crack were used for the tests. Loading was carried out on a modern servo-hydraulic test machine, which allows you to specify different laws of stroke movement, record all parameters. The readout of the crack opening displacement gauge was recalculated into the crack length by the compliance method using software supplied with the test equipment. The available methods aimed at predicting the fatigue crack growth life with a variable amplitude cyclic loading are not capable of taking into account the peculiarities of random loading. In this study, a new approach is proposed to estimate the fatigue crack growth life with a constant and variable amplitude loading with different loading parameters, taking into account the phenomenon of a "crack closing" and nature of random loading. The "crack closing" is taken into account by the introduction of an effective stress intensity factor (SIF) into the calculation, which, unlike the well-known SIF, is calculated using the closing factor. The nature of random loading is determined by the irregularity factor of the random loading block. Calculation of the fatigue crack growth life according to the proposed model and according to the known "cycle-by-cycle" calculation method. A comparison of the calculated data with the experimental ones showed similar results. At the same time, the proposed model aimed at calculating the fatigue crack growth supposes the presence of a small number of initial parameters and requires considerably less resources for calculation.