Ultrasonic fatigue testing is a recent fatigue methodology that applies resonant principles to induce cyclic stresses in a modally designed specimen enabling the application of fatigue damage at very high frequencies. Its primary purpose is to study fatigue life in the Very High Cycle Fatigue regime between 10E07 and 10E09 cycles with a higher performance of time and energy wise compared to conventional servo-hydraulic fatigue testing machines. In this study, an ultrasonic fatigue testing machine was used to carry out multiaxial tension/torsion fatigue tests at a frequency of 20 kHz. The objective was to reach a reliable multiaxial fatigue testing method by modifying only the specimen, with the ability to induce different shear to axial stress ratios. The improved design/methodology of the testing ultrasonic multiaxial method focused first on the innovative design specimens and setup modal behaviour by conducting both numerical and experimental analyses. Laser displacement measurements, thermographic imaging, and rosette strain gauges applied to the specimen’s fatigue stress region were carried out to validate the new design concept and compared with the computed results obtained by finite element. A good numerical to experimental results agreement was achieved. A series of fatigue tests were carried out in tension/torsion fatigue to three different combining dimension specimens allowing three different shear/axial stress ratios. The resulting crack initiation angles and fracture surfaces were analysed and compared not only between each other but also with tension-compression and pure torsion ultrasonic tested specimens.
