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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characteristic feature of these steels. The primary function of this material group is the minimization of magnetic reversal (magnetization) losses. This is achieved by alloying with aluminum and silicon. For reasons of punchability (formability), the silicon content is limited to a maximum of 3.5%. In addition to the high silicon content, the coarse grained structure (grain size between 20 µm and 200 µm) of electrical steels results in a reduced number of grain boundaries, which in turn minimizes the area of electromagnetic hysteresis during magnetization reversal and thereby improves efficiency (Wuppermann, 2005). The loading of the rotor results from a superposition of magnetic, electromagnetic, and centrifugal forces and is primarily determined by the driving behaviour of the user. Due to acceleration and braking events—that is, variations in the rotational speed of the rotor during operation—the loading must be considered time-dependent (Haefele et al., 2018). For the electromagnetic optimization of the rotor design, narrow and sharply notched saturation bridges are incorporated into the rotors, which promote material fatigue (Paul, 2014). To ensure operational reliability of the component throughout its service life, fatigue strength assessments are indispensable. In addition to fatigue tests used for the derivation of analytical and statistical models, fractographic and damage analyses provide valuable insights into the cyclic behaviour of this material group. In particular, the location of crack initiation and the mode of failure help to improve the understanding of the fatigue mechanisms in electrical steels.

Nomenclature d k

grain size

K t

notch concentration factor

t

sheet thickness

R ε R σ

strain ratio stress ratio

ε a

strain amplitude

σ an

nominal stress amplitude

1.1. Fatigue cracks in metallic materials A fatigue crack in metallic components refers to failure occurring below the static strength limits of the material under cyclic loading. Failure under cyclic loading can be divided into the following three phases: • Crack initiation • Crack propagation • Final fracture During the crack initiation phase, it must be considered that loads which do not cause macroscopic plastic deformation can still lead to microscopic slip phenomena. This involves the displacement of individual crystal planes and the activation of slip bands oriented approximately under ± 45° to the principal stress direction. During cyclic loading and unloading, these slip planes move back and forth (intrusion and extrusion), resulting in a roughening of the specimen surface. The repetition of this process leads to the formation of a crack nucleus, which gradually grows into a microcrack under continued cyclic loading. The microcrack propagates until it encounters an obstacle, such as a grain boundary. At this point, the crack propagation direction changes to 90° relative to the maximal principal stress, marking the end of the initiation phase. From this stage onward, the process is referred to as cyclic crack growth. In this phase, the crack is considered a macrocrack, which can grow continuously with each loading cycle. Crack propagation occurs along crystallographic cleavage planes. Once the crack reaches a critical length, the remaining cross-section is weakened to the extent that the applied load exceeds the static strength of the material. This leads to a sudden, failure, known as the fracture phase (Roesler et al., 2007). The distinctive feature of a fatigue fracture lies in its macroscopically deformation-free fracture surface, independent of the ductility of the material. The fracture surface is typically characterized by a relatively smooth appearance, on which a macroscopic distinction can be made between the fatigue crack growth zone (bright area) and the fracture zone (dark area). The crack initiation site is often clearly identifiable by a radiating pattern, which indicates