Measurement and calculation of stable growth of fatigue cracks

Comprehensive multiscale study of fatigue crack kinetics (FCK) in aircraft engine disks and samples from heat-resistant titanium and nickel alloys is presented. The aim of the studies was to determine the possibility of microfractographic reconstitution and calculated prediction of stable fatigue crack growth based on the measurement and calculation of fatigue striation spacing S. The studies included micro- and macrofractographic measurements of stable crack growth (according to fatigue striation spacing, measured by scanning electron microscopy with high resolution, and to the marker lines determining positions of the crack macrofront after a certain number of loading cycles) and comparing their results, finite element modeling of cracks and calculating the stress intensity factor (SIF) range ΔK, calculated prediction of FCK using kinetic equation S(ΔK), obtained earlier on the basis of physical and mathematical modeling of stable fatigue crack growth mechanism, comparison of prediction results with fractographic data. Because of the study, it is shown that the average value of S characterizes fatigue crack growth rate (FCGR) during the entire stage of stable growth of the fatigue crack, corresponding to the second section of the kinetic diagram «FCGR – SIF range». The use of equation S(ΔK) makes it possible to predict the stable growth of fatigue cracks of different configurations in parts and specimens from structural materials with different crystal structure and microstructure under different loading conditions. The obtained results are of particular importance for ensuring safe operation of high-stress rotary parts of aero engines, in which the duration of stable growth of low cycle fatigue cracks reaches 70 % of the total cyclic life, and fatigue striation spacing is the only measurable characteristic of stable growth rate under service conditions.

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