The experimental and theoretical study of heat dissipation at fatigue crack tip under biaxial loading

Abstract

In this work we have studied both experimentally and theoretically the evolution of a heat flux at a fatigue crack tip during biaxial loading. The goal is to elucidate the dependence of the heat flux on the crack growth rate for different loading conditions. Tests were carried out on titanium alloy (Grade 2) and stainless steel (AISI 304) flat samples weakened by notches to initiate fatigue cracks at their centers. The dissipated thermal energy was monitored using an infrared camera and the original Seebeck effect-based contact heat flux sensor. Samples were subjected to constant stress amplitude cyclic loading at different biaxial coefficients. The experimental results obtained in this study confirmed our previous conclusions about two stages of energy dissipation at the tip of a fatigue crack propagating in the Paris regime. At the first stage, the crack growth rate is proportional to the heat flux multiplied by the crack length. The second stage is characterized by a traditional linear relationship between heat flux and crack growth rate. The theoretical approach proposed here for calculating energy dissipation at a fatigue crack tip is based on the relation between elasto-plastic and elastic strain fields near a crack tip in terms of Young’s and secant plasticity moduli. This relation was examined by applying a digital image correlation (DIC) method to measure a strain field and through numerical simulations. DIC can also be used to determine the size of plastic area near a fatigue crack tip.