Development of a three-point bending fatigue testing methodology at 20 kHz frequency

A novel testing methodology on three-point bending fatigue test at 20 kHz has been developed based on ultrasonic vibration theory to determine the bending fatigue strength of structural materials in very high cycle regime. In this paper, the working feature of the testing machine has been introduced. On the basis of vibration theory analysis, the differential equation of longitudinal and flexural vibration were given, finite element model was introduced to design and calculate the amplifier, specimen and the connector in the testing system. Furthermore, to produce the shapes of the components necessary to achieve the required displacement and stress states. Longitudinal cyclic load was transferred to transverse vibration of specimen by the tip in the testing system. In order to understand the effect of the tip on the testing system, matching analysis between amplifier and tip in the bending vibration system has been discussed. Finally, a successful application of the methodology is demonstrated by the experimental results from TiAl alloy specimens subjected to three-point bending stress states.