The analysis of the three-dimensional elastoplastic stress state at the top of surface cracks is made using the example of steel 40X for various types of biaxial loading. Results of fatigue tests of cruciform specimens with a surface crack at biaxial loading are presented in the paper. The change in the normal stresses before the crack front is investigated when the samples are loaded to the maximum load and then unloaded to zero. Relation of the type of loading of the samples with crack opening is installed. It is proposed to characterize the fracture at the crack tip by the sum of normal stresses. The characteristic of the stressed state at the crack tip correlating with the rate of crack growth at various loading types is proposed. Formulas are obtained for calculating the rate of growth of fatigue cracks developing as normal detachment under uniaxial and biaxial loading of metal in the region of the surface crack.
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