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On the basis of direct measurement of the opening displacement of the notch contour the procedure of the evaluation of local elastoplastic strains range, De à , is developed. The experimental values of De à are compared with the calculational data using known equations by Neuber, Glinka, etc. It is shown that for De à < 2% the experimental and calculational values are in good agreement but only in the case when the gradient of strains near the notch tip is taken into account. The proposed experimental method is used for the prediction of period N i to fatigue macrocrack initiation in specimens of a complicated shape.

It is shown that with a cyclic loading in the vicinity of the stress concentrator within the static and cyclic plastic zones, a specific volume of material is gradually formedthe fatigue process zone. Its size d * determines the length of non-propagated cracks or safety defects in the material and can be a determining factor in the diagnosis of material damage. For these materials, the size d * does not depend a lot on the type and geometric characteristics of the stress concentrator. It can be determined by laboratory conditions. While detecting the defects which are longer than d * , it is decided to reduce the intervals between inspections of the construction element, repair or replace it. Keywords: fatigue, durability, fatigue process zone, crack initiation and growth, defects of materials OSOBLIWOŚCI TWORZENIA STREFY PROCESU ZMĘCZENIOWEGO W MATERIAŁACH KONSTRUKCYJNYCH Streszczenie Pokazano, że cykliczne obciążenie w pobliżu koncentratora naprężeń w statycznych i cyklicznych strefach plastycznych stopniowo tworzy specyficzną objętość materiału-strefę procesu zmęczeniowego. Jej rozmiar d * określa długość nierosnoncych pęknięć lub innych bezpiecznych defektów i może być czynnikiem decydującym w diagnozie uszkodzeń materiału. Dla badanych materiałów rozmiar d * mało zależy od rodzaju i geometrycznych charakterystyk koncentratora naprężeń. Jego można wyznaczyć w warunkach laboratoryjnych. Po ujawnieniu defektów, dłóższych niż d * , podejmuje się decyzja o skróceniu czasu między przeglądami elementa konstrukcji, jego remoncie lum zamianie. Słowa kluczowe: zmęczenie; trwałość; strefa procesu zmęczeniowego; inicjacja i propagacja pęknięcia; defekty materiałowe

A high resolution sensor of linear displacements has been elaborated for diagnosing the size of the crack tip opening in standard test specimens. Based on the data measured during fatigue crack growth resistance tests of a wheel steel heat treated after various modes, calculations of the strain and energy parameters of local fracture were performed for the corresponding variants of treatment, and fatigue crack growth rate diagrams were constructed using different approaches of fatigue fracture mechanics. It was established that the diagrams obtained using the strain and energy approaches are more sensitive to material microstructure and its mechanical properties than the traditional diagrams obtained using the stress approach. Based on the parameters of the fatigue crack growth resistance estimated using the strain and energy approaches, the effective diagnostics and the best heat treatment mode for the wheel steel was developed. It comprises the stages of isothermal quenching and tempering at a temperature of 500 °С, providing the highest fatigue fracture toughness of the steel.

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