Investigation of the influence of global stresses and strains on the magnetic properties of electrical steels with varying alloying content and grain size

The process of manufacturing iron cores for electric machines out of electrical steel can drastically influence the magnetic properties of the material. It is well known that the local soft magnetic properties are degraded by the cutting process. In the production process the material also experiences tensile and compressive stress, i.e. when pressing the stator core into motor housing. In this work, the influence of these stresses on the soft magnetic properties of six different 0.35 mm thick fully processed non-oriented electrical steels with varying alloy content and varying grain size is studied on 30 mm wide and 300 mm long strip samples. A single strip testing setup with force in field direction applied to the specimen is used. This field metric method allows for quantitative analysis of the magnetic properties and ac iron losses as a function of the applied stress. For the specific case of 1.0 T and 400 Hz measured in rolling direction on one material the application of tensile stress first leads to a decrease in the losses, up to a stress of 30 MPa. For higher stresses the losses increase again by 11 % at an applied strain of 100 MPa. The results for compressive stress look significantly different: the application of stress always leads to an increase of the losses. At -40 MPa, the iron losses increase by 64 %. These effects are stronger the lower the polarization or frequency of the applied magnetic field is chosen. The Bertotti model is used to separate the static (hysteresis) and dynamic (eddy current and excess) losses. We discuss the observed deteriorations as an effect of the inverse magnetostriction, which leads to changes in the magnetoelastic energy.