Monitoring thickness deviations in planar multi-layer elastic structures using impedance signatures

In this letter, a low frequency ultrasonic resonance technique that operates in the 20 - 80 - kHz regime is presented that demonstrates detection of thickness changes on the order of ± 10 μ m . This measurement capability is a result of the direct correlation between the electrical impedance of an electro-acoustic transducer and the mechanical loading it experiences when placed in contact with a layered elastic structure. The relative frequency shifts of the resonances peaks can be estimated through a simple one-dimensional transmission model. Separate experimental measurements confirm this technique to be sensitive to subtle changes in the underlying layered elastic structure.