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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the origin of crack initiation. On the macroscopic scale, beach marks (also referred to as arrest lines) can be observed on the fracture surface of fatigue cracks. These marks are oriented orthogonally to the crack propagation direction and are formed due to changes in the stress amplitude or temporary retardation of crack growth often combined with a corrosive attack at the crack tip during that period. When examined using a scanning electron microscope (SEM), fatigue striations can be observed on the fracture surface. These striations are formed during each loading cycle, as the crack opens and advances incrementally. The occurrence of these fatigue striations correlates with the number of load cycles, and their orientation is likewise perpendicular to the direction of crack growth (Grosch, 2017). 1.2. Fatigue cracks in electrical steel Microscopically, the fatigue crack in electrical steel can be divided into three distinct zones. Dehmani et al. (2016) describe these as the initiation zone, the crystalline zone, and the ductile zone (see Fig. 1). Investigations by Schwarz and Haefele (2022) show that in the initiation zone, both transcrystalline and intercrystalline crack initiation can occur. Comparable observations can be made during the crack propagation phase (crystalline zone). The initiation zone corresponds to the phase of crack initiation. The fracture surface in this region appears relatively rough, often showing a radial pattern that indicates the crack origin. According to Dehmani et al. (2016) and Schwarz and Haefele (2022), this zone typically extends across one to three grains. In the case of intercrystalline initiation, the fracture surface of the grain is divided into two regions: a rough initiation area with an orientation deviating from the remaining grain, and a smooth intercrystalline fracture surface (Schwarz and Haefele, 2025). The crystalline zone corresponds to the crack growth phase, or stable crack propagation. In the transcrystalline case, the fracture surface is characterized by cleavage facets with fan-shaped loops or flow patterns. The orientation of the fracture surface is aligned with the maximum normal stress direction. Schayes et al. (2016) report that the crystallographic orientation plays only a secondary role during this phase. Within the crystalline zone, Schwarz and Haefele (2022) observed for the electrical steel NO30-15 that, even during transcrystalline crack growth, individual grains can exhibit intercrystalline fracture features. They describe that the fracture surface (grain boundary) is oriented orthogonally to the normal stress, and therefore related to the grain orientation. Although Dehmani et al. (2016) equates the fracture with the ductile zone, Schwarz and Haefele (2022) report that brittle cleavage features can still be found between highly deformed grains. The topology of the brittle fracture does not differ significantly from that of the crack propagation region. In addition to the typical dimple morphology of the final ductile fracture, regions of severe local deformation are observed, resulting in knife-edge features in the remaining cross-section. This phenomenon is attributed to the absence of precipitates or inclusions within the material.

Fig. 1. Stages of fatigue failure of NO30-15 from Schwarz and Haefele (2022).

2. Materials and test setup The two investigated materials are non-grain-oriented electrical steels, NO30-15 (high-strength grade) and NO30 19 (low-strength grade), each with a thickness of t = 0.3 mm. The mechanical–technological properties as well as the grain size d k are summarized in Table 1.