Volume 20, Issue 4 (July 1992)
Time-Domain Ultrasonic Measurement of the Thickness of a Sub-Half-Wavelength Elastic Layer
This work is motivated by the need to measure the thickness of extremely thin adhesive layers in adhesively bonded joints. The classical method for ultrasonic determination of the thickness of a plate is the so-called “time-of-flight” method. One launches a longitudinal wave normally incident upon an immersed specimen and records the successive reflections from the two walls. The temporal separation between two successive reflections corresponds to the spatial distance between the two walls via the elastic wavespeed. This method breaks down when the specimen thickness (h) is less than three wavelengths (3λ). By combining conventional ultrasonics with the theory of Fourier transforms, Kinra and Dayal developed a technique that worked for h < 3λ. However, their technique runs into a convergence problem when h/λ < 0.5. More recently. Kinra and Iyer have explained the reasons for these convergence problems and refined this technique to the point at which it works satisfactorily down to h/λ = 0(10−2).
The objective of this paper is to report a new technique which accomplishes all of the foregoing objectives without the use of Fourier transforms. The entire ultrasonic nondestructive evaluation (NDE) is carried out in the time domain. Whereas the implementation of the previous technique would require an engineer with a college education, the new technique can be confidently used by a trained technician with no more than a high school education. It is envisioned that the use of the new technique will expand by at least an order of magnitude the technology base in which ultrasonics can be used for thickness measurement.