Fourier transformation for nondestructive evaluation

Abstract

Fourier transformation of the time domain signal has been used for a variety of applications in the area of nondestructive evaluation of both isotropic and composite materials. Reflection pulses, which are close together in time domain, can be separated out in the frequency domain. The information from the reflected or refracted pulses was deconvolved and the wave velocity/sample thickness and damping for very thin samples could be measured [V. K. Kinra and V. Dayal, Exp. Mech. 28(3), 288–297 (1988)]. The added advantage of these techniques is that they immediately warn the user of the dispersion in the material. Fourier transform of pulses was applied to the measurement of the wave velocity of Lamb waves. Lamb waves or plate waves are affected by the in-plane material properties and, for platelike structures, this measurement is very useful in making velocity and damping measurements [V. Dayal, Rev. Prog. QNDE 13A, 189–194]. The technique has since been used for the measurements of mechanical properties such as elastic constants of composite materials, automated simultaneous measurement of wave speed and thickness, and measurement of very thin coatings. Various applications will be reviewed.