The kinetics of fatigue crack growth has been studied in tensile testing of compact steel tensile specimens (S(T)-type) in the middle section of the kinetic diagram of fatigue fracture (fatigue crack growth diagram) under regular and irregular loading with different asymmetry and maximum load values. The samples were tested on a BISS Nano-25kN servo-hydraulic machine. Standard loading spectra typical for different technical objects exposed to alternating loading during operation were used. The values of the crack growth rate per cycle in the loading block were obtained. Parameters for assessing the character of irregular loading and crack closure, namely, the irregularity factor and crack closure coefficient were proposed. When calculating the effective value of the range of the stress intensity factor (SIF) at the crack mouth, we propose also to take into account the loading irregularity in addition to the closure coefficient. With this approach, the obtained fatigue crack growth diagrams can be grouped into one equivalent curve, which is characteristic of regular loading with R = 0. Moreover, grouping of the fatigue crack growth diagrams provided the use of unified parameters when calculating the crack growth kinetics, regardless of the type and parameters of loading, which rather simplified the crack growth determination. The fatigue crack growth life was predicted taking into account the crack «closure» and the nature of loading according both to the approach developed by the authors and by cyclic calculation method (cycle-by-cycle). All the data obtained are tabulated and classed according to the type of loading. The calculation results and experimental data showed good convergence, which was confirmed by the high values of the correlation coefficient.
Crack closure and fatigue crack growth near threshold of a metastable austenitic stainless steel
