Improving the understanding of how biaxiality ratios correlate on cruciform fatigue test specimens for VHCF ultrasonic fatigue testing

Very High Cycle Fatigue (VHCF) using ultrasonic machines is a subject that is receiving growing attention. Recent developments focus on biaxial stresses which are of interest to industries such as the aeronautical where plane stresses appear in the fuselage and wings. It has been shown before that in-plane cruciform specimens can be changed so that different biaxiality ratios are achieved from equibiaxial to pure shear. This paper analyses how these biaxiality ratios relate to one another between the in-plane orthogonal directions x and y in cruciform specimens. Specimens in this study are composed of Aluminum 6082-T651, a medium strength alloy used in many highly stressed engineering applications, including trusses, cranes, bridges, and transportation. These asymmetric models are purposely designed to develop orthogonal biaxial stresses with non-unitary biaxiality ratios. Comparing the simulation results with the experimental data shows that the strain rates can provide acceptable prediction of biaxiality ratios. Moreover, it was observed that the biaxiality ratios obtained from stress, displacement and strain are not equal and, in fact, can be correlated by an expression that was derived during this research.