Evolution of the fracture mechanics parameters in the vicinity of the hole in conditions of low-cycle fatigue according to the data of modeling a crack with narrow notches

We propose a new method for studying the effect of low-cycle fatigue on the evolution of the fracture mechanics parameters in conditions of loading plane specimens with stress concentrators. Three programs of loading with a constant value of the stress range and different values of the stress ratio, as well as two programs with a constant value of the stress ratio but different stress ranges are considered. One program is common for both cases. All the programs include uniaxial tension-compression. Each program was implemented by testing a batch of the same specimens from seven to nine in each. One specimen from each batch was assigned to assess the durability. The other specimens were brought to various stages of low-cycle fatigue in each program. Experimental data were obtained for cracks of different length which were modeled by a sequence of three narrow notches launched from a through hole in a rectangular specimen. Each notch was exposed to constant external load of the same level. The deformation response to a small increment in the notch length at a constant external load was measured at different stages of low-cycle fatigue using electronic speckle pattern interferometry. The interference fringe patterns used as initial experimental data provided determination of the tangential components of in-plane displacements directly on the notch sides and the values of notch opening were thus determined from the results of direct measurements. The transition from measured displacement components to the values of the stress intensity factor and T-stress was performed using the relationships of a modified version of the crack compliance method based on Williams formulation. Distributions of the fracture mechanics parameters along the notches were obtained at various stages of cyclic loading. The dependences of the crack mouth opening displacement, the stress intensity factor and the T -stress on the number of loading cycles are constructed for the notches of a fixed length at different stages of low-cycle fatigue. It is shown that experimental distributions of the stress intensity factor values over the life time practically coincide for all four combinations of the loading cycle parameters.