Bulk Wave Characterization of Laminated Composites

Composite materials are currently seeing wider use in the aerospace and automobile industries. Composites offer many advantages over conventional materials, such as a greater strength to weight ratio and the ability to engineer their mechanical properties to a specific task. The major problems associated with composites are cost and reliability. Like virtually all engineering materials, composites can have flaws which may compromise their strength and reliability. The ability to detect these flaws in a reliable, cost effective fashion is significantly essential in the utilization of composite materials in critical structural areas. Currently, nondestructive evaluation using ultrasonic wave amplitude analysis, is most often used to inspect materials for flaws. This method can detect gross macroscopic flaws such as delamination or cracks, but more subtle flaws in the individual layers of a composite such as incomplete cure or low fiber volume ratio, cannot be found using conventional inspection techniques. Full stiflhess modulus reconstruction, using acoustic wave velocities, is an alternative way to nondestructively determine the exact mechanical properties of a given composite part. Much research has been done in the area of modulus reconstruction of single layered composites [1-3]. The objective of this paper is to develop schemes for modeling multi-layered composites commonly seen in practice. Two basic methods of modeling composites are presented here; the layered method and the averaged method. The layer method treats each ply as a separate material. The averaged method consists of taking all the layers and averaging their material properties together. This paper will look at the differences between these two methods and will show how the relationship between the wavelength and the ply thickness determines which theory will apply.