A B S T R A C T The relation is studied between crack initiation from micro-notches in a fully lamellar intermetallic -TiAl alloy and the local strain field. These micro-notches were introduced using femtosecond-laser ablation and had dimensions below the average colony size. The specimen under investigation was then subjected to fatigue loading. Continuous monitoring using a travelling optical microscope allowed detecting microcracks at an early stage. Prior to fatigue loading, a sustained load was applied and the local strain field was determined using digital image correlation. This was supplemented by a Finite Element analysis of the notches and their neighbourhood. It was found that a crack was initiated from a notch causing high normal strains in lamella direction, whereas no crack was initiated from notches with high shear strains. a = Notch depth [μm] b = Notch width [μm] c = Notch length [μm] da/dN = Crack growth rate [mm/cycle] DIC = Digital image correlation E i = values of elastic modulus [MPa] FE = Finite element method FSD = Forward scatter diode G ij = Values of shear modulus [MPa] R p0.2 = Yield strength [MPa] R p0.01 = Elastic limit [MPa] ΔK = Range of stress intensity factor [MPa√m] ε, ε ij = Strain tensor V ij = Values of Poisson's ratio σ = Stress [MPa]

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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.

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