The use of composite materials has increased steadily during the past two decades, particularly for aerospace, underwater and automotive structures. This is largely because many composite materials exhibit high strength-to-weight and stifihess-to-weight ratios, which make them ideally suited for use in weight-sensitive structures. The elastic properties of composite materials may be significantly different in specimens manufactured under the same general specifications and the bulk material properties may be different from those of the lamina. The elastic properties degrade as a result of aging, environmental and other effects (e.g., matrix cracking) resulting in overstress and eventual failure of the material. The elastic properties determine the performance of the material and it is necessary to assure the conformance of these properties with design requirements. Conventional destructive techniques for determining the elastic stiffness constants can be costly and often inaccurate. This is particularly true for the through-the-thickness properties. Nondestructive determination of these properties offers a better alternative for material characterization and for assuring structural performance
