The effect of low-cycle fatigue on evolution of fracture mechanics parameters in residual stress field caused by cold hole expansion

Localized displacement measurements based on electronic speckle-pattern interferometry are used to obtain crack mouth opening displacement (CMOD), stress intensity factor (SIF) and T-stress values during crack growth around cold-expanded holes. The specimens with a central open hole are made from 2024 aluminium alloy. The expansion level is 5% of nominal interference. The results are obtained for the same stress range Delta sigma = 350 MPa, but different stress ratio R = -0.4 and R = -1.0. A sequence of narrow notches, inserted under the constant external load, serves for crack modelling at different stages of cyclic loading. Initial experimental data represent in-plane displacement component values measured in the vicinity of the crack tip. The transition from in-plane displacement components to SIF and T-stress values follows from the relationships of modified version of the crack compliance method. The crack length curves of CMOD, SIF and T-stress profiles are obtained for different stages of cyclic loading. These data provide the construction of dependencies of fracture mechanics parameters for cracks of fixed lengths from the loading cycle number.