Mechanical and functional fatigue of non-oriented and grain-oriented electrical steels

The mechanical fatigue and functional fatigue of a non-oriented electrical steel (NOES) and a grain-oriented electrical steel (GOES) are investigated under stress-control in the high cycle fatigue regime. The deformation behavior of the two electrical steels grades under cyclic loading with a ratio of R = 0.1 is characterized by strongly pronounced ratcheting. For NOES ratcheting starts in the first cycle due to stresses above the yield strength. GOES shows an incubation period with cyclic propagation of Lüders bands due to stresses close to the yield strength. The functional fatigue is investigated at one stress level for each grade with intermittent fatigue tests followed by subsequent ex-situ measurements of the magnetic properties. The functional degradation results in extremely poor magnetic properties due to plastic deformation even at the 1% fatigue state. While the degradation for NOES correlates with a pronounced ratcheting right from the beginning, GOES also showed strongly increased power losses in the early stages of fatigue life during Lüders band propagation. For GOES in particular, the sensitivity of the magnetic properties towards mechanical stress was demonstrated, with small plastic strains leading to a large increase in power loss.