Use of Guided Waves for Detection of Interior Flaws in Layered Materials

Abstract

Another method of ultrasonic NDE of composites involves plate, or Lamb, wave inspection. This method uses an angled incident beam to generate a plate-type mode in a plate or shell shaped composite structure. Usually, a receiving transducer is used to detect the signal. This method is well suited for detecting large or extended flaws or for determining bulk material properties; however the long wavelengths of the modes typically used limit their ability to precisely localize a flaw or to detect small flaws. [2-6] In the proposed technique, we attempt to exploit the waveguide properties of a composite laminate. A unidirectional fiber reinforced composite lamina consists of fibers or fiber tows (bundles) running parallel to one another in a single direction. A number of these laminae are stacked at various orientations, thus forming the composite. The speed of sound in a given direction in a lamina is dependent upon the orientation of the fibers relative to the direction of propagation of the sound wave. To obtain an order of magnitude approximation of the velocity in a given layer, a spring model of a composite was used to determine Young's Modulus, E, in the directions parallel and perpendicular to the fiber directions, and the density, p. The velocity in these two directions was determined (by c - (E/p )112 ) to be approximately 2,000 mls perpendicular to the fibers and 10,000 mls parallel to the fibers. Using