Regularities of fatigue crack initiation and growth in two-phases titanium alloy after beach mark formation under VHCF regime

The paper introduces the results of fatigue crack growth (FCG) tests under high-frequency loading (20 kHz). The FCG tests are performed in stepwise decreasing of stress intensity factor (SIF) value. The morphology of the obtained fracture surfaces is investigated by scanning electron microscopy (SEM). The study is focused on the analysis of local areas located just ahead from crack tip (beach marks). These marks are reflecting the forced stop of the cyclic loading during the FCG test for correction the stress intensity factor value. Results of SEM observations shows the fatigue crack ‘re-initiation’ just ahead of these beach marks in the bulk of material after the start of new cyclic loading. This new crack nucleation takes place from the boundary junction of neighboring alpha-phase or cleaved single alpha-phase. The discovered mechanism of fatigue crack growth was implemented in the mathematical model. The numerical simulation of crack front shape under high-frequency loading shows a suitable agreement between the prediction and experimental results.