Understanding Elastic Wave Phenomena using Laser Doppler based Ultrasonic Imaging

A 3D Laser Doppler Vibrometer (LDV) shown below is being used to obtain images of ultrasonic wave propagation in structural components. The technique uses three independent laser beams for Doppler measurement on the test surface. Its measurement principle is based on the Doppler shift produced by an incident laser light and backscattered light by the vibrating surface of the test structure when excited by ultrasound. According to the Doppler shift, the change in wavelength of the backscattered light is a function of relative velocity of the vibrating surface. The velocity of the vibrating surface is obtained by measuring the change in wavelength of the backscattered light, which forms an interference fringe pattern. Thus, LDV works as a non-contact ultrasonic sensor with a long standoff distance. Motorized mirror on each of the three-laser beam paths enable velocity history measurement seamlessly over a software-controlled grid-points on the test surface. Distinct measurements at two very nearby points is possible as the spot size of the laser beam is less than 10 μm, whereas a typical piezoelectric transducer used in ultrasound imaging would occupy a much larger area and would require super-resolution scanning.

Various theoretical models are employed to obtain further insights into the complex wave phenomena on the surface. For example, a wave-filtering model with the measured data can predict the nature of wave propagation characteristics. Among various other interesting problems that are studied are interaction among various dissimilar wave modes, wave scattering due to material and structural defects which are of interest in non-destructive inspection, biomedical ultrasound, energy localization phenomena, spatio-temporal coherence, diffraction due to crystals and waveguide, and behavior of various photo-acoustic devices.

Remote monitoring of structures with laser Doppler ultrasound; image shows a 3D Laser scanning of a composite structural joint with hidden damage. Inset shows the hidden damage detected with the help of a new technique of ultrasound Doppler imaging and computation.

Reference:

Laser Doppler imaging of delamination in a composite T-joint with remotely located ultrasonic actuators, G. Kolappan Geetha, D. Roy Mahapatra, S. Gopalakrishnan, S. Hanagud, Composite Structures, 147, 197-210 (2016).