Influence of mean stresses on fatigue life and damage of a turbine blade steel in the VHCF-regime

In the present study, the fatigue behavior of a martensitic steel (X10CrNiMoV12-2-2) has been investigated at different load ratios ranging from R = −1 to R = 0.7 at room temperature in air. Tests were conducted under uniaxial stress at 20 kHz using an ultrasonic fatigue testing machine (version BOKU Wien) up to 2 × 109 cycles. To investigate the frequency influence, additional samples were tested on an electromagnetic resonance testing machine at frequencies around 100 Hz. A transition from surface crack initiation to volume cracks starting at inclusions is observed at a fatigue life between 2 × 107 and 4 × 107 cycles irrespective to the load ratio. The fracture surfaces at numbers of cycles to failure exceeding this limit show the classical fish-eye morphology with the crack originating from CaO/MgO–Al2O3 inclusions with diameters between 16 and 45 μm. The diameter of crack initiating inclusions follows a normal distribution. A decrease of the slope of the S–N curves with increasing load ratio was observed. Furthermore, no significant frequency dependency (100 Hz compared to 20 kHz) was observed at R = −1. In addition, it could be demonstrated that the √area-parameter proposed by Murakami is well applicable for these steels at load ratios between −1 and 0.5.