Identification of material damage precursors using nonlinear ultrasonics

The primary goal of this research effort is to develop nondestructive evaluation techniques capable of detecting material damage precursors mainly for turbine engine materials under low and high-cycle fatigue testing. The experimental results presented in this paper show a significant increase of the relative acoustic nonlinearity, βr, in aluminum and Ni-based superalloy fatigued specimens. While in agreement with the prior research, the main advantage of the current technique over the previous methods is that the ultrasonic beam may be focused to inspect the presence of damage precursors at localized stress concentrator site. For example, when the ultrasonic beam travelled through the root of the round-notched specimens, the acoustic nonlinearity exhibited an increase of approximately 450% as compared to the pristine specimens. This procedure will be further developed to detect damage precursors in propulsion components undergoing thermo-mechanically fatigue to quantify their remaining useful life.