PSI - Issue 76

Afshin Khatammanesh et al. / Procedia Structural Integrity 76 (2026) 115–122

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with compressed air, and the appropriate pulse-pause ratio was chosen depending on the applied stress amplitude to ensure that the temperature in the gauge section does not exceed 30 °C. For more details about ultrasonic fatigue testing see Mayer (2016) and Schönbauer et al . (2023).

Fig. 1. Geometry of ultrasonic fatigue test specimens (dimensions in mm).

3. Results and discussion 3.1. S-N test results

The results of S-N experiments are summarised in Fig. 2, where open symbols mark specimens that failed from surface inclusions and solid symbols represent failures from interior inclusions. Regardless of the steel sheet investigated, all surface failures were in the HCF regime, i.e., below 10 7 cycles, while interior failure was observed between 10 6 and 10 10 cycles. The lowest stress amplitude at which surface failure occurred with sheet steel B (thickness of 2.3 mm) is σ a = 700 MPa. For sheet steels A and C (thicknesses of 1.8 mm and 3.1 mm, respectively), the lowest stress amplitude at which surface failure was observed is σ a = 750 MPa. As no surface failure occurred at lower stress amplitudes, it can be concluded that there is a fatigue limit in the HCF regime associated with surface fracture. No fatigue limit associated with interior failure could be determined. However, it is evident that sheet steel A exhibits the highest VHCF strength, with lifetimes that are up to two orders of magnitude longer than those of sheet steel B. The VHCF lifetimes of sheet steel C lie in between.

Fig. 2. S-N test results.