Rotors for electric motors are composed of numerous laminations made from thin electrical steel sheets. To optimize magnetic properties, magnets are embedded within the rotor, resulting in lamination designs with narrow saturation bridges. Due to centrifugal forces and sharp notch radii, these saturation bridges constitute the component’s critical failure area.
Electrical steel strips are iron-silicon alloys with sheet thicknesses ranging from 0.1 mm to 1 mm, characterized by a large-grained microstructure (grain sizes between 20 µm and 200 µm). When determining material characteristics, tests with standard specimen geometries result in a transfer problem to the small bridge. To investigate size effects in electrical steel strips, quasi-static and fatigue tests (stress-controlled with R = 0.1) are conducted on NO30-15 (t = 0.3 mm) and NO20-13 (t = 0.2 mm) steel strips. Three different specimen geometries are tested, with test area widths ranging from 10 mm to 0.5 mm. Additionally, metallographic analysis is performed to examine the edge conditions and microstructures within the test areas.
Test results for NO30-15 indicate that the material’s mechanical properties are less sensitive to changes in specimen size, showing only a increase in tensile strength of approximately 5%, compared to a more significant 21% increase in fatigue strength. In fatigue tests, there is a superposition of size effects with reduced surface influences, which positively impacts service life as specimen size decreases.
Factographic analysis has shown that, in larger specimens and with increasing load amplitudes, failure is more likely to occur along the rolled surface. At load levels below the yield strength, failure tends to occur along the specimen edges. This effect is not observed in smaller specimens, suggesting that the reduction of the rolled surface has a greater impact on the results than a shorter specimen edge. The thinner material (N20-13) exhibits a reduced sensitivity to size effects under cyclic loading, with an approximate 9% increase in fatigue strength.