Ultrasonic Backscatter from Embedded Cylinders

Abstract

Advanced composite materials present unique challenges for nondestructive evaluation of their material properties and state of health or damage. Stitches and other through-the-thickness reinforcements are becoming much more common because of their ability to give substantially improved three-dimensional strength to finished components. They also allow textile preforms to hold their shapes before and during resin transfer molding, and can be important in automated tow-placement processes. Ultrasonic testing of advanced composites is desirable before, during and after consolidation and curing in order to monitor both the initial fabrication and the eventual inevitable structural degradation as the composite structure ages. Unfortunately, the very stitches that improve the properties of the composites interfere with the usual inspection methods. This difficulty can be overcome, however, if the interaction of the probing ultrasonic radiation with the through-the-thickness reinforcements can be understood and quantified. In this paper we present the results of our recent study which has considered theoretically and experimentally the scattering of elastic waves from embedded cylindrical elastic scatterers. In particular, we have derived an exact and analytic, closed-form solution for the oblique polar backscatter from these structures and then have compared our theoretical results with ultrasonic immersion tank experiments conducted on wires, fibers and tows embedded in polymeric materials.