Fatigue damage assessment by the continuous examination of the magnetomechanical and mechanical behavior

Abstract

To evaluate the material degradation of ferritic steels caused by low cycle stress-induced fatigue, the continuous examination of changes in the magnetomechanical behavior during the cyclic mechanical loading is proposed, and this is validated by comparing with the continuous examination of changes in the mechanical stress-strain behavior. In this context two magnetomechanical examination methods are investigated, differing only in the magnetic field that is continuously applied to the sample during the stress-controlled cyclic mechanical loading, i.e., a constant magnetic field (method Hstat) or a time-varying magnetic field (method Hdyn), with the magnetic frequency significantly larger than the mechanical frequency. In both methods the magnetization variation M(σ,H) during each stress cycle due to the magnetomechanical effect and the strain ε(σ) are continuously measured throughout the complete cyclic mechanical loading test. When analyzing the fatigue-induced changes in the magnetization trajectory M(σ,H) determined by both methods (Hstat and Hdyn), several stages in the fatigue lifetime can be distinguished (i.e., a steady state and a final stage for as-received samples and an initial stage, a steady state and a final stage for annealed samples), which fully mimic the corresponding stages in the inelastic strain-stress behavior. All investigated magnetomechanical and mechanical parameters change significantly during the final fatigue stage (i.e., the last 2%–5% of the fatigue lifetime). This information can be used to estimate the remaining life of steel components.