A Rayleigh wave technique to measure the acoustic nonlinearity parameter of materials

Nonlinear ultrasonic techniques have shown great potential for evaluating accumulated damage early in the fatigue life, and ultimately for predicting remaining lifetime of a structural component. The acoustic nonlinearity parameter, a direct measure of the accumulated fatigue damage, is determined from the second harmonic amplitude in finite amplitude sinusoidal ultrasonic waves transmitted through the material. An absolute determination of the acoustic nonlinear parameter is notoriously difficult for several reasons. In this paper, a new experimental technique based on Rayleigh surface waves is presented for determining the absolute acoustic nonlinearity parameter of a relatively thin material specimen. Rayleigh waves are efficiently generated in a specimen by exciting at its edge, and the surface normal velocity of the propagating Rayleigh waves is measured with a laser interferometer system. The high efficiency of the excitation method allows us to drive the transmitting piezoelectric transducer as low as 60 Vpp, and thus to avoid the inherent harmonic distortion from the transducer. The absolute acoustic nonlinearity parameter is then determined from the measured magnitudes of the fundamental and second harmonic surface normal velocities. This technique is applied to determining the acoustic nonlinearity parameters of aluminum alloys 2024 and 6061; the results are compared with those available in the literature. The present technique is especially well-suited for relatively thin components, and much simpler and efficient than the traditional longitudinal wave technique.