ULTRASONIC SPECTROSCOPY APPARATUS AND METHOD FOR DETERMININGTHICKNESS AND OTHER PROPERTIES OF MULTILAYER STRUCTURES

摘要

An ultrasonic apparatus and method based on location of resonance frequencie s, is provided for determining unknown parameters in a multilayer structure, such as thickness, elastic properties of individual layers, or bonding strength between layers. Ultrasonic waves reflected from the front and back surfaces of a thin-layere d medium (.notlessthan. 2 mm) overlap and interfere in the time domain. A spectroscopic approach is proposed, collecting and analyzing a large number of echoes together in the frequency domain to measure resonance frequencies and determine properties of the layered medium . With the presence of a fluid between layers, the resonance modes are mainly those of individual layers in vacuum and an inversion algorithm has been successfully applied to recover some properties. For adhesively bonded layers, the resonance modes o f the individual layers are strongly coupled and important frequency shifts are observed. A characteristic equation which includes a bonding parameter can be derived fo r a given type of multilayer structure and a numerical inversion technique is used to obtain thicknesses or other properties from measured resonance frequencies. The method is of particular interest when ultrasound is generated by a laser and detected by either a contact ultrasonic transducer ora laser interferometer.