Estimation of Fatigue Crack Growth Rate in Heat-Resistant Steel by Processing of Digital Images of Fracture Surfaces

The micro- and macroscopic fatigue crack growth (FCG) rates of a wide class structural materials were analyzed it was concluded that both coincide either during high-temperature tests or at high stress intensity factor (SIF) values. Their coincidence requires level cyclic deformation the metal along entire front as necessary condition for formation striations (FS). Based on analysis digital fractographic images fracture surfaces, method quantitative assessment spacing FS has been developed. includes detection by binarization image based principle local minima, rotation highlighted fragments using Hough transform, calculation distances between continuous lines. tested 34KhN3M steel in initial state after long-term operation (~3 × 105 h) rotor disk steam turbine thermal power plant (TPP). Good agreement confirmed FCG (both macro microscopic, determined manually imaging techniques) SIF ranges their noticeable discrepancy low ranges. Possible reasons values are analyzed. It also noted is easier to detect surface degraded steel. Hydrogen embrittlement promotes secondary cracking FS, making them quantify. shown invariable value microscopic rate range operated lower than observable state. Secondary may have contributed typical pattern unoperated