Vira V. Determination of the fatigue durability of notched bodies on crack initiation and propagation stages

The materials fatigue and durability of the structural elements with stress concentrators under cyclic loading is the object of investigation. Conduct experimental substantiation provisions known unified model of fatigue fracture of materials entered on the basis of its new parameters to describe the nucleation and growth of fatigue cracks and to develop a methodology for predicting the fatigue life of the bodies with structural stress concentrators with the influence of asymmetry cycle loading. This work is based on experimental and computational methods, including experimental determination of local displacements (strain) using mechanical sensors, laser interferometry and optical analysis, mechanical testing under cyclic loads on fatigue crack growth resistance and durability calculation model of the stress-strain state and fatigue durability bodies with stress concentrators. The main provisions of the known unified model of fatigue fracture are experimentally proved and new methods of local fatigue fracture parameters determining are developed, particularly, the two new methods (protected by patents of Ukraine) of experimental determination of the fatigue process zone size d* are proposed. This parametr determines the effective stress concentration factor, the range of local strain and local strain energy per loading cycle at different stages of materials fatigue. A comparative analysis of the applicability of the force, strain and energy parameters in the cracks initiation and growth stages is provided and consequently heat treatment of railway wheels steels optimization is done. The influence of stress ratio R on the fatigue fracture is investigated and new invariant parameters to describe the initiation (relative local maximum stress) and growth (relative maximum stress intensity factor Kmax/d*(R)) of the fatigue macrocrack are proposed. The new method of durability prediction for the cyclically loaded bodies with the stress concentrators of different shape based only on fatigue crack growth diagrams is developed and experimentally confirmed. The results and practical recommendations are the basis of new methods for evaluating the fatigue life of aircraft structural elements SE "Antonov".