PSI - Issue 25

Pedro R. da Costa et al. / Procedia Structural Integrity 25 (2020) 445–453 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Figure 6 shows one fatigue failed tension/torsion specimen tested in ultrasonic fatigue at stress ratio R=-1 for both axial and torsional load with a stress shear/axial of 0.57. The fracture was achieved in the main throat (central throat) of the specimen as expected. The angle of the fractures surface is in agreement with similar fatigue tests carried out for the similar steel and loading conditions in classical servo-hydraulic fatigue testing machines.

Figure 6. Specimen failed by tension/torsion fatigue at VHCF Eight specimens were machined and tested to failure. The first specimen was subjected to several tests with a strain gage for stress-power calibration. For the remaining specimens the applied tension/torsion stresses were transformed into equivalent von Mises Stress and correlated to the obtained final fatigue life. Fatigue results (S-N data) were then plotted in a graph as shown in figure 7.

Figure 7. Ultrasonic tension/torsion stress life results

Observing the results presented in figure 7, for both uniaxial and multiaxial ultrasonic results the stresses increase for the same life and for run-out specimens for VHCF conditions. In uniaxial run-out conditions stress results have an increase of 11% when tested in ultrasonic fatigue at 20 kHz in relation to conventional 5 Hz. Regarding the multiaxial