PSI - Issue 82

Peter Haefele et al. / Procedia Structural Integrity 82 (2026) 174–181 Peter Haefele and Patrick Schwarz / Structural Integrity Procedia 00 (2026) 000–000

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residual ductility. It should be noted that the inhomogeneous hardness of shear-cut electrical steel edges is determined by both the deformation capacity of the material and the cutting parameters (Gottwalt et al., 2024). The absence of intergranular crack initiation is attributed to the cold deformation occurring during the shearing process. The directional plastic deformation reduces the crystallographic anisotropy between grains and thereby minimizes their orientation differences (Gottwalt et al., 2024).

Fig. 3. Fracture images of shear edges NO30-15 R σ = 0.1; (a) crack in the fracture portion; (b) crack in the smooth-cut portion.

3.2. Comparison of fracture surfaces between laser-cut K t = 1 specimens and rotors made of NO30-19 For unnotched specimens made from NO30-19 with laser-cut edges, similar load-dependent trends in crack initiation sites are observed as for NO30-15. Crack initiation at the rolled surface occurs only at stress levels exceeding 1.13 times the yield strength. Notably, all cracks initiated at the laser-cut edge exhibit transgranular crack initiation, see Figs. 4(a) and 4(b). The occurrence of intergranular crack initiation under dominant plastic deformation is approximately 44 %, and thus slightly lower than in NO30-15. The load dependence of intergranular cracking is comparable to the observations reported by Schayes et al. (2016). For rotors with laser-cut edges, the fraction of intergranular crack initiation is lower, around 29 %, compared to that of the material specimens. This can be explained by the fact that for unnotched specimens, intergranular cracks originate from the rolled surface in three out of four cases (with one crack propagating along a grain boundary inside the specimen). The inhomogeneous stress distribution in the saturation bridge of the rotor confines crack initiation to the notch root (laser-cut edge), resulting in lower stresses at the rolled surface. Due to the high stress concentration associated with the rotor geometry, no load dependence of grain boundary failure is observed, since even low applied stresses cause plastic deformation at the notch root. The site of crack initiation in rotors is therefore, regardless of load level, located at the notch root surface and the laser exit side, see Figs. 4(c) and 4(d). No correlation is observed between the number of cycles to failure and the crack initiation site. Although the laser exit region shows small welding bead adhesions (see Fig. 4(d)), their damaging effect is presumed to be mitigated by the local stress and strain conditions at the notch. Overall, with respect to the type and location of crack initiation, a high degree of similarity can be established between unnotched specimens and rotor components.

Fig. 4. Laser edges NO30-19 R σ = 0.1; (a) Kt = 1 sample notch laser edge; (b) Kt = 1 sample notch roller surface; (c) rotor notch surface notch base; (d) rotor notch laser exit.