Ultrasonic reflection spectrum analysis for NDT of periodic layered media

This report investigates in detail the normal incidence acoustic reflection spectra from periodic, layered media and emphasizes applications of the results to nondestructive testing (NDT) of composite materials. In contrast to a previous study which derived a number of exact but iterative techniques for predicting the acoustic spectrum for a given laminate, this study derives a number of general, closed form expressions for both finite and infinite periodic laminae which defines measurable properties independent of the number laminae. Using these results, composite materials are modeled as two component laminae with one component being a thin interface layer. Interesting features of this model are low frequency velocities which are dependent on composition but not lamina thickness and an expression for boundary layer thickness in terms of band gaps in the frequency spectrum. Comparisons of experimental and theoretical results show that although effects of periodic structures are measurable ultrasonically, other effects associated with material dispersion and inconsistency in material properties make it difficult to get consistent ultrasonic measurements from some graphite epoxy composites.