Fatigue, Magnetic and Mechanical Hysteresis

The objective of this research is to explore correlations between piezomagnetic signals of initially unmagnetised ferromagnetic steel samples of AISI 1018 steel with progressive changes that take place during the fatigue process. A series of strain-controlled fatigue tests were carried out to quantitatively study the piezomagnetic fields surrounding steel specimens by measuring magnetic field variations in the milligauss range resulting from the cyclic application of stress. It was observed that the piezomagnetic hysteresis traces change systematically with the progression of fatigue. The piezomagnetic hysteresis loop areas, when plotted as a function of the number of loading cycles, demonstrate 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. It was also observed that the piezomagnetic field surrounding a specimen subjected to cyclic loading behaves approximately like that of a bar magnet. This study has demonstrated that the piezomagnetic effects can be exploited to evaluate the progress of fatigue in ferromagnetic steels.