Fatigue Damage Evaluation in Ferromagnetic Steel by Mechanical and Piezomagnetic Measurements

The objective of this research is to explore correlations between mechanical and piezomagnetic hysteresis of ferromagnetic steel samples with progressive changes that take place during the fatigue process. A series of strain controlled fatigue tests were carried out to study the piezomagnetic fields surrounding steel specimens in a quantitative way by measuring magnetic field variations resulting from the cyclic application of stress. It was observed that the piezomagnetic hysteresis traces change systematically with the progression of fatigue. The evolution of piezomagnetic hysteresis, when plotted as a function of the number of loading cycles, demonstrates the three principal stages of fatigue: initial accommodation, accretion of damage and terminal failure. The results derived from the piezomagnetic hysteresis traces are consistent with those obtained from mechanical hysteresis studies. However, when approaching failure, the piezomagnetic traces show more conspicuous changes than the corresponding mechanical stress-strain curves.