The problem of the measurement of the thickness of the individual acoustically thin layers in multilayered structures has received considerable attention. In most cases, the difficulties of the measurements are due to the multiple reflection from the boundaries; hence the problem is usually solved in the frequency domain where the resonance structure of the frequency response is analyzed [1]. In the case of polymer structures the problem of the inversion of the ultrasonic data for the thickness of all layers is complicated by the high ultrasonic absorption and weak reflection from the internal boundaries. The other difficulty is the curvature of the specimen affected by the measurement [2]. The difficulties in carrying out such characterizations stem from the fact that the received signal is dependent not only on the acoustic properties of the layers, but also on the orientation, shape, surface properties, location and dimensions of the interfaces [3]. Distortion of the ultrasonic beam will take place if the interface is curved within the ultrasonic beam width. This distorts the geometry of the displayed structure, smears the resolution, and in the extreme case, can rise to duplicate image artifacts [4]. In the inspection of objects with a curved surface, the pulse interacts with complicated environments, the pulse spectral decomposition and synthesis is inapplicable because of difficulties associated with the response to each spectral component. Therefore, the interface between the different high absorptive layers in multilayered polymer composite, are not easy to observe and measure. The purpose of this study is to investigate the limits of the ultrasonic pulse-echo method in the frequency range between 10 MHz and 25 MHz for the measurement of the internal layer thickness in a curved multilayered polymer structure.
展开▼
