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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For the material NO30-15, both stress-controlled and strain-controlled tests are carried out in the rolling direction. The stress-controlled tests are performed on specimens with notch factors of K t = 1 and K t = 6.4, using different edge conditions: milled/polished, laser-cut, and shear-cut. The cyclic tests with constant amplitude are conducted under a stress ratio of R σ = 0.1 at ambient temperature. Additionally, a test series with unnotched specimens and polished edges is conducted at R σ = –1 using buckling supports to prevent buckling. For notched and unnotched specimens with milled/polished or shear-cut edges, variable amplitude loading tests are also performed using a Gaussian load spectrum under R σ = 0.1.

Table 1. Mechanical and metallographic properties of the investigated materials, in rolling direction.

Material NO30-15 NO30-19

R p0,2 in MPa

UTS in MPa

A 50mm in %

d k in mm

439 334

570 464

21.1 20.3

0.085 0.117

The execution and evaluation of the strain-controlled tests are carried out in accordance with ASTM E606/E606M: 2012 and the approach proposed by Thum (2019) at a strain ratio of R ε = –1. For the material NO30-19, stress-controlled tests with a stress ratio of R σ = 0.1 are performed on unnotched specimens and rotors (geometry according to Schwarz et al. (2024)) with laser-cut edges. After completion of the tests, all specimens and components are examined fractographically using both optical microscopy and scanning electron microscopy (SEM). 3. Results and discussion The focus of the study is on the fractographic examination of the fracture surfaces of specimens made from the materials NO30-15 and NO30-19. The results of the fatigue tests have already been published in the following For unnotched specimens with polished edges, the rolled surface represents the predominant site of crack initiation, regardless of the R-ratio or type of load control. At R σ = –1, approximately 18 % of the specimens failed from the polished edge, whereas at R σ = 0.1, about 10 %, and at R ε = –1, around 25 % of the specimens failed from the edge, see. No clear load dependence can be identified. The observation that the rolled surface exhibits higher damage susceptibility than the polished edge can be explained by several factors. First, the rolled surface has an average roughness approximately three times greater than that of the polished edge (Schwarz and Haefele (2025)). Second, the investigations of Dehmani et al. (2016) showed that polishing electrical steel induces significant compressive residual stresses at the edge. It is assumed that recrystallization annealing results in the elimination of residual stresses at the rolled surface. Under stresses below the yield strength, such as at R σ = –1, this results in a more favourable state at the polished edge compared to the rolled surface. Under cyclic loading exceeding the yield strength (R σ = 0.1 and R ε = –1), Schwarz and Haefele (2022) observed a load-dependent increase in surface roughness for both the edge and the rolled surface. Although this leads to insignificant differences in roughness between the two surfaces, the location of crack initiation remains unchanged. While cyclic plastic deformation is expected to reduce residual stresses, Schwarz and Haefele (2025) argue that the lattice distortion responsible for these residual stresses also leads to an increase in yield strength within the polished surface layer. The elevated yield strength prevents a complete relaxation of the compressive residual stresses, allowing the rolled surface to maintain its higher damage potential. Furthermore, the investigations of Bode et al. (2016) attribute the damage of the rolled surface to surface defects formed during the rolling process. Laser-cut specimens made from NO30-15 exhibit a load-dependent behaviour regarding the location of crack initiation under R σ = 0.1. At stress amplitudes below σ an = 230 MPa (approximately 1.16 times the yield strength), failure occurs exclusively at the laser-cut edge. At higher stress levels, however, there is an approximately 50% publications: Schwarz and Haefele (2022, 2023,2025) and Schwarz et al. (2024). 3.1. Influence of edge condition and control type on crack initiation at NO30-15