Ultrasonic Spectrum Analysis for NDT of Layered Composite Materials

Abstract
The structural use of advanced composite materials has increased in recent years and a number of problems have arisen associated with assuring the integrity of composite structures. Because ultrasonics has emerged as an important tool for inspecting composite materials, it has become necessary to understand more about the interaction of acoustic waves with composites and what information can be obtained by monitoring this interaction. In this paper a simple model is presented which predicts the ultrasonic frequency spectra for a broad class of layered composite materials having a finite number of laminae. Experimental verification of this model is demonstrated for arrays exhibiting both periodic and aperiodic frequency spectra. The relationship between the spectra of periodic arrays having both finite and infinite numbers of layers is discussed. Important results relating to NDT which have emerged from this study include methods for predicting the results of spectrum analysis studies on layered materials and techniques for mapping small changes in the modulus and thickness of composite materials. Also discussed is the existence of forbidden frequency bands at which ultrasound cannot be transmitted through thick layered composites.

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