Effect of monotonic and cyclic loading on the magnetic properties of non-oriented and grain-oriented electrical steels

The effect of monotonic and cyclic tensile loadings on the magnetic properties of a non-oriented electrical steel (NOES) and grain-oriented electrical steel (GOES) was studied using a single strip tester to measure the magnetic properties of both materials after mechanical loading. In tensile tests, specimens were stressed up to different load levels below the yield stress and then unloaded. For NOES, only the highest investigated load level, with a maximum stress (σ max ) of 350 MPa, produced a measurable shift in the knee point of the magnetization curve. This was attributed to microyielding and the respective formation of heterogeneous dislocation structures, that generated long-range internal stresses. Cyclic loading was performed under tension-tension conditions with R σ = 0.1 at σ max = 200, 320, and 350 MPa, followed by magnetic single strip tester measurements at fatigue states of 0.1%, 1%, 10%, 50%, and 100% of the fatigue life. At σ max = 350 MPa, ratcheting was detected after 1% of the expected lifetime, whereas no ratcheting occurred at lower stresses. Nevertheless, magnetic degradation was observed not only after cyclic loading at 350 MPa, but also at 320 MPa, as indicated by increased field demand of the magnetic field strength and higher specific losses. This is attributed to dislocation accumulation and the associated internal stresses. Additionally, domain imaging from MOKE microscopy, before and after cyclic loading, revealed refined structures after 10% of the fatigue life at σ max = 350 MPa, while for the lower load levels, no clear trend in domain evolution was observed. GOES tested at a single load level of 40% of the yield stress showed no systematic influence of monotonic or cyclic loading on magnetic properties could be identified, mainly due to scatter arising from large grain size and small specimen geometry.