Magnetic nondestructive evaluation of fatigue damage of ferromagnetic steels for nuclear fusion energy systems

An inspection method is proposed for structural metal components of fusion reactors based on magnetic properties measurement. This integrity inspection is very important because structural materials are fully damaged by neutron radiation, heat load, thermal and mechanical stresses. Present work shows that magnetic hysteresis is sensitive to microstructural changes caused by high cycle fatigue, even at a stress level below the yield stress. The fatigue-induced degradation in the microstructure mainly has its fingerprint on the upper part of the hysteresis curve, where the rotation of magnetization is predominant. The changes in magnetic properties are well represented by the decreasing remanent induction with fatigue cycling. A microscopic study was also performed using transmission electron microscopy (TEM) to investigate the microstructural changes in the fatigued specimen. The TEM images show an increase in dislocation density in the ferrite phase.