Load interaction effects during near-threshold fatigue crack growth under variable amplitude: theory, model, experiment

In this paper, we consider the loading interaction problems that arise in fatigue life prediction. The brief overview of recently most popular models is presented. Most of them explain this phenomenona by the crack closure effect with different nature. At the same time, cycle-sequence sensitivity can be observed at high stress ratio in the absence of crack closure. This fact calls into question the adequacy of these approaches. A new physically based model which can adequately predict fatigue life in a wide range of crack growth rates (the Paris region, near-threshold) is proposed. This model is based on the suggestion of a brittle fracture nature of the crack propagation in the near-threshold region. As a result it is shown that the threshold stress intensity, ΔKth, is not a material constant, but a variable that is extremely sensitive to load history. A numerical technique is proposed to estimate the near-tip stress for an arbitrary loading sequence including random loading spectra. This method is based on the constitutive equations with the combined (isotropic-kinematic) hardening rule and linear rule for strain prediction. The combined hardening can be interpreted as a simple modification of Frederick-Armstrong law and Chaboche model. The numerical integration of constitutive equations based on the return-mapping scheme (implicit Euler method) is performed. The experimental procedure for adjustment of models and its verification is proposed. We show the comparison of the experimental and calculated data with a constant amplitude loading under a variety of overloads and underloads and under spectral loading. In all cases, a satisfactory compliance with a high correlation factor can be observed.