The role of phase cancellation in the ultrasonic NDE of stitched graphite-epoxy composites

Nondestructive evaluation (NDE) of stitched composite material with pulse-sensitive and phase-insensitive detection schemes to determine the degree of attenuation artifact produced by phase cancellation in stitched composite laminate panels is discussed. Phase cancellation is demonstrated in one example of stitched composite material. The effect of phase cancellation, as measured by the difference between phase-sensitive and phase-insensitive detection is found to be most prominent in the vicinity of stitches. This result is consistent with the hypothesis that the spatial variation in ultrasonic velocity caused by the stitch may result in distorted wavefronts which lead to phase cancellation at phase-sensitive detectors. The presence of such signal artifacts may preclude the use of conventional methods for careful characterization of composites with three-dimensional fiber architectures using the frequency-dependence of ultrasonic waves