Monitoring the Mechanical Properties of Adhesives and Coatings via Ultrasonic Shear Wave Techniques
Donald L. Hunston
Nondestructively characterizing the mechanical properties of adhesives and coatings is a difficult challenge. There are many instances, however, where it would be useful to measure these properties and to monitor how they change while the sample is being fabricated or when it is exposed to environmental attack or aging. One of the few methods that has been successful in this regard uses ultrasonic shear waves to measure the dynamic shear modulus [1-6]. Shear waves are used because they provide a measure of shear viscosity and elasticity, which are often the most informative mechanical properties obtainable by non-destructive testing. Direct measurement of shear wave propagation in the samples of interest is not possible because the attenuation is too high. Consequently, an indirect technique is used where the wave is generated in a low loss material (substrate) and propagated toward an interface coated with the sample. The presence of the sample changes the wave reflected back from the interface. These changes can be measured and used to determine the mechanical properties of the sample. This method has been known for many years, and although it offers unique capabilities, it has not been widely used because the measurements are very tedious and often require custom build electronics. The advent of modern digital electronics has greatly simplified such measurements so this technology is now often used in the fields of sonics and ultrasonics. The purpose in the work here is to apply this approach to the shear wave propagation technique. A digital electronics system was assembled. A program was written to control data acquisition so the system could automatically monitor time dependent processes like during and drying. A data analysis program was developed to calculate shear properties from the measured information. Finally, a series of experiments was performed to test both the system and the software.