Second harmonic Rayleigh wave detection using a heterodyne laser interferometer
Abstract
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.