Evolution of Fracture Mechanics Parameters near a Hole under Low-Cycle Fatigue According to Crack Simulation by Narrow Notches

A novel method for studying the effect of low-cycle fatigue on the evolution of fracture mechanics parameters under loading plane samples with stress concentrators has been proposed. Three programs of loading at a constant value of the stress range and different values of stress ratio (loading cycle asymmetry coefficient), as well as two programs with a constant value of the stress ratio and different stress range values, are considered. One of the programs is common for both cases. All the programs are based on uniaxial tension-compression loading. Each program has been implemented by testing a batch of the same samples, each of them involving from seven to nine samples. One sample from each batch has been assigned to assess the durability level. In the scope of each program, all the other samples have been brought to different stages of low-cycle fatigue. The experimental data have been obtained for cracks with different length simulated by a sequence of three narrow notches starting from a through hole in a rectangular sample. Each notch has been exposed to a constant external load at the same loading level. The strain response to a small increment in the notch length at a permanent external load has been measured at different stages of low-cycle fatigue by means of electronic speckle pattern interferometry (ESPI). The interference fringe patterns used as the initial experimental data have provided the determination of the tangential components of in-plane displacements directly on the notch banks. Thus, the values of notch opening have been determined on the basis of the results of direct measurements. The transition from measured displacement components to the values of the stress intensity factor and T -stress has been performed using relationships obtained for a modified variant of the crack growth compliance method based on the Williams formulation. The distributions of fracture mechanics parameters along the notch length have been obtained at different stages of cyclic loading. The crack mouth opening displacement, the stress intensity factor, and the T -stress depending on the number of loading cycles are obtained for the notches having a fixed length at different stages of low-cycle fatigue. It is shown that the experimental distributions of stress intensity factor values throughout the lifetime (durability level) almost coincide for all four combinations of the loading cycle parameters.