Very high cycle fatigue under tension/torsion loading of mold low alloy steel

Ultrasonic fatigue testing (UFT) is a recent fatigue methodology that applies resonance to achieve very high cyclic load frequencies. Its primary purpose is to study fatigue in the very high cycle fatigue regime. In this study, an UFT machine was used to conduct multiaxial tension/torsion fatigue tests at a 20-kHz frequency. The objective was to reach a reliable multiaxial fatigue test and methodology by modifying the geometry of the specimen, enabling also different shear to axial stress ratios. The study conducted numerical and experimental modal behavior analysis of the ultrasonic setup and specimens focused on achieving a consistent modal deformed shape and associating the measured displacement to the induced stresses thermographic imaging, laser and rosette strain gauges measurements were carried out to validate the results. A good numerical to experimental agreement was achieved. Tension/torsion fatigue test series were conducted to three combining dimension specimens with different shear/axial stress ratios. The crack initiation angles and fracture surfaces were analyzed and compared among each other and with tension-compression and pure torsion ultrasonic tested specimens. The results prove a clear trend in crack propagation details with the increase of the induced stress ratio.