Fatique behaviour of electrical steel

Abstract

Electrical steel comes into focus with the development of electrically powered cars. In contrast to electrical motors used stationarily (e.g. conveyer belt drives in industrial applications), electrical steel in a car engine is subjected to cyclic loading due to vibrations caused by the imbalance of the rotor and start and stop driving events. For a safe and reliable design of an electrical motor the fatigue behaviour of electrical steel needs to be analysed. To minimize eddy current losses, a rotor consists of several hundred electrical steel sheets with a typical thickness of less than 1 mm. Due to optimal electrical and magnetic properties a very coarse microstructure of electrical steel is required. Only one to three grains are distributed along sheet thickness. Regarding the grain size and sheet thickness the material behaviour is governed by the reaction of single grains and grain-grain-interaction to external cyclic loading. Fatigue experiments with a load ratio of R = 0.005 and R = 0.1 were carried out. They give a very flat S-N-curve where the fatigue limit is close to the yield strength of this electrical steel. Crack initiation is observed at surface roughness and areas of stress concentration resulting from manufacturing processes.