Second harmonic Rayleigh wave detection using a heterodyne laser interferometer
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Nonlinear acoustic measurements, including the generation of higher harmonics caused by nonlinear material behavior, haves proven to be a useful technique to detect changes in the microstructure of a material, and thus, nondestructively characterize material state. Optical detection of second harmonic Rayleigh waves by means of a heterodyne laser interferometer has advantages over traditional detection techniques such as contact transducers. Laser detection is a non-contact point measurement, which provides absolute readings of the surface displacements and the particle velocity of the Rayleigh surface waves. Frequency or phase modulation of the laser is caused by the Doppler effect on the velocity and displacement amplitudes of a vibrating specimen. This modulation can be recovered with suitable demodulators. This research explores digital demodulation techniques and their effects on higher harmonic measurements. A 9 % Cr ferretic martensitic steel is used to demonstrate the accuracy of the non-contact laser measurement. The results show that the second harmonic increase is hidden in wideband noise for this measurement setup and both, quadrature demodulation and zero-cross detection provide similar results for the amplitude of the first and second harmonic in this experimental arrangement.