Nondestructive Flaw Characterization in a Unidirectional Composite Plate

In this work, characterizing a flaw in a plate means sizing and localizing it. Rapid and accurate inspection of anisotropic plates using guided waves (known as Lamb waves) has been limited by the complex nature of these waves: they contain many modes and exhibit dispersion, i.e. their shape changes as they travel. Current practice typically simplifies the signals by restricting either the type of mode generated and/or detected and/or its frequency range. The objective of this work is to use broadband, multi-mode signals to characterize flaws in anisotropic plates. The plates of interest here are unidirectional Graphite-Epoxy slabs. The signals are obtained numerically, using the finite element software ABAQUS/Explicit; therefore, the signals are synthetic. The theoretical dispersion curves pertaining to the plates of interest here are first calculated. Then two digital signal-processing techniques, the 2DFFT and the reassigned spectrogram, are used for, respectively, sizing and locating the flaw. Both techniques are applied to the synthetic Lamb waves propagating through the graphite- epoxy plate. The 2DFFT output is compared to the theoretical dispersion curves. After agreement is obtained for a pristine plate, an energy coefficient can be calculated for a plate with a given notch and related to the depth of the flaw. Finally, the reassigned spectrogram is used to characterize the modal and frequency content of the synthetic signals as a function of time. The reassigned spectrogram can then be related to the analytical dispersion curves. This correlation enables the location of notches in the composite specimen