Piezomagnetic hysteresis as a non-destructive measure of the metal fatigue process

The primary goal of this research is to investigate the piezomagnetic fields surrounding AISI 1018 steel specimens under repeated loads. 11 strain controlled fatigue tests were carried out to study the fatigue behavior of AISI 1018 steel samples. Experimental results show that the piezomagnetic hysteresis traces change systematically with the progression of fatigue. Extensive analysis of the test data show that the evolution of the magnetic hysteresis traces demonstrates the three principal stages of fatigue: initial accommodation, accretion of damage and terminal failure. These three stages can be marked by two transitional points N2 and N3. Analyzed results show that although the specific N2 and N3 values may fluctuate widely from test to test, their ratios to fatigue life Nf, i.e., N2/Nf and N3/Nf, tend to be stable. Results also show that the piezomagnetic traces show more conspicuous changes than the corresponding stress–strain traces. This research has demonstrated that the piezomagnetic hysteresis can be usefully exploited as a non-destructive measure of the fatigue process in ferromagnetic steels.