Experimental characterization of fatigue damage in a nickel-base superalloy using nonlinear ultrasonic waves

This research develops a robust experimental procedure to track the evolution of fatigue damage in a nickel-base superalloy with the acoustic nonlinearity parameter, β, and demonstrates its effectiveness by making repeatable measurements of β in multiple specimens, subjected to both high- and low-cycle fatigue. The measurement procedure developed in this research is robust in that it is based on conventional piezoelectric contact transducers, which are readily available off the shelf, and it offers the potential for field applications. In addition, the measurement procedure enables the user to isolate sample nonlinearity from measurement system nonlinearity. The experimental results show that there is a significant increase in β linked to the high plasticity of low-cycle fatigue, and illustrate how these nonlinear ultrasonic measurements quantitatively characterize the damage state of a specimen in the early stages of fatigue. The high-cycle fatigue results are less definitive (the increase in β is not as substantial) due to increased uncertainties involved in the high-cycle fatigue tests, but still show a clear relationship between β and remaining fatigue life. One application of the measured β versus fatigue-life data is to potentially serve as a master curve for life prediction based on nonlinear ultrasonic measurements.