CONDITIONS FOR THE INITIATION OF FATIGUE CRACKS IN STEEL UNDER CYCLIC LOADS, DEPENDING ON ITS STRENGTH

Currently, there are various, opposing points of view regarding the influence of non-metallic inclusions on the fatigue strength of steel. A number of studies by domestic and foreign metallurgists and materials scientists note the lack of correlation between the fatigue limit of steel and its total contamination with non-metallic inclusions. At the same time, numerous studies indicate that the presence of non-metallic inclusions has no practical effect on the cyclic fatigue strength of medium-strength steel. However, for steel with σв ≥ 1200 MPa, their negative effect is observed on transverse specimens, and for steel with σв ≥ 1700 MPa, also on longitudinal specimens. This article examines the conditions for fatigue crack initiation in steel under cyclic loads depending on its strength. It is shown that, under cyclic loading, the most dangerous stresses are tensile stresses, which form normal tensile stresses in the dislocation slip plane. A relationship has been obtained that allows one to determine the conditions under which the formation of crack nuclei from a surface defect or from non-metallic inclusions is most likely. It has been established that the influence of non-metallic inclusions on the possibility of fatigue crack formation is individual and depends on the morphology of non-metallic inclusions and their sizes. Large high-modulus non-metallic inclusions with a diameter of 5.0 – 7.0 μm or more can be responsible for the formation of cracks in the entire range of steel strength properties up to 500 to 2000 MPa. Ductile low-modulus aluminosilicate non-metallic inclusions with a Young's modulus no greater than that of the metallic matrix (200 – 210 GPa) do not cause the formation of cracks in the entire range of the ultimate tensile strength of steel. 

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