Influence of the machining conditions on the lifetime under cyclic loading of non-grain-oriented electrical steel strips

Different manufacturing processes are used in the development of rotors for electrical machines. The cyclic material tests are carried out on the polished edge. Prototypes are produced using laser cutting and series production done using shear cutting. Each of these processes produces characteristic edge states and changes the topological and mechanical properties of the specimens and components. As crack initiation of sheet metal specimens often occurs at the edge, the edge condition is crucial for the fatigue behaviour. To determine the edge influence in the material NO30-15, stress and strain controlled fatigue tests are performed on unnotched specimens. To determine the influence of the laser parameters, stress-controlled tests are carried out on NO30-19 with edges with optimised cutting parameters and incorrect cutting parameters. The edge conditions are polished, shear cut and laser cut (stress-controlled two variants). The edges are characterised by means of optical microscopy, tactile roughness measurement and hardness measurements. Stress-controlled tests are carried out with Rσ = 0.1. The results show that the influence of the edge condition on the limit life time regime decreases with increasing load amplitude. This is accompanied by a shift of the crack initiation location from the edge to the rolling surface in lasered specimens. The overelastic loading strongly influences the topology of edges and rolling surface. Strain-controlled tests are done under purely reversed loading Rε = -1. Laser edges yield a constant reduction of the limit life time regime in the LCF-regime (approx. 10 %), whereas shear-cut specimens show a stronger reduction in the dominant plastic range (approx. 33 %) than in the dominant elastic range (approx. 15 %). The comparison of optimised and defective laser parameters shows that incorrect laser parameters lead to strong fluctuations of the edge surfaces. These fluctuations lead to high scatter in the life expectancy of the specimen.