AISI P20 steel under VHCF testing conditions

As more complex materials, structures and machines are created and applied in the modern world, the importance and complexity of fatigue testing methods become more indispensable for any mechanical project. This study focuses on an AISI P20 Steel in the Very High Cycle Fatigue (VHCF) regime, between 10(6) to 10(9) cycles, under different loading conditions: Tension-compression, pure torsion, and multiaxial tension/torsion. To reach such a considerable high number of cycles, no conventional fatigue testing method is energy and time reliable. Therefore, the present study conducted ultrasonic fatigue testing methods under 20kHz. Each stress type fatigue method was reached from its respective and different ultrasonic fatigue setup. Significant testing associated conditions and methodology, such as temperature, frequency and displacement, were carefully controlled, recorded, and made similar for all three methods. Stress-life results, material behaviour throughout fatigue testing and fracture surfaces for each stress type were compared for behaviour and damage correlation between pure torsion, tension-compression and the multiaxial combination of the two, tension-torsion. After all conducted fatigue tests, the obtained fatigue crack surfaces were analyzed by optical and scanning electron microscope (SEM) and compared between methods. Each tested method presented fatigue fracture surfaces with associated fracture morphologies similar to classic fatigue methods. The novel tension-torsion ultrasonic fatigue fractures showed both axial and torsion fatigue fracture dependencies and complexities, proving the induced stress field of interest.