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 H-stat) or a time-varying magnetic field (method H-dyn), with the magnetic frequency significantly larger than the mechanical frequency. In both methods the magnetization variation M(sigma, H) during each stress cycle due to the magnetomechanical effect and the strain epsilon(sigma) are continuously measured throughout the complete cyclic mechanical loading test. When analyzing the fatigue-induced changes in the magnetization trajectory M(sigma, H) determined by both methods (H-stat and H-dyn), 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

Detection of stress concentration and early plastic deformation by monitoring surface weak magnetic field change

“…The results demonstrated the validity of the weak magnetic field monitoring on nondestructive evaluation of the starting of plastic deformation. And works in [22] displayed similar direction of research using this method. The stress induced magnetization of ferromagnetic samples is mainly determined by the max stress has been exerted on the samples and has little relation with the increase of cycle index under cyclic stress, which is not consistent with the cyclic increasing model of magnetoelastic energy.…”

Monitoring fatigue crack growth using magnetic stray field measurements

“…Several methods to detect and quantify fatigue crack growth have been investigated over the years. A first class of methods involves the generation of a relatively strong magnetic field in the vicinity of the specimen to magnetise the material during testing [1,2]. From a practical perspective, these active magnetic methods are less attractive, as the constant generation of these magnetic field is unfeasible in terms of power consumption and required equipment.…”