7D-5 Detection and Sizing of Delaminations in Composites Using Modally-Selective Lamb-Wave Transducers

This work addresses several aspects of damage detection and evaluation in composite laminates, in the general context of structural health monitoring. A group delay measurement technique is proposed using modally-selective Lamb-wave transducers for the detection and sizing of delaminations in woven quasi-isotropic and carbon-epoxy cross- ply composites. Unlike amplitude or energy based Lamb-wave methods, this method is insensitive to transducer coupling. The first asymmetric Lamb mode was used in the investigation because of its extended low-dispersion zone and its relatively short wavelength, at any given frequency. The transducers used to excite and detect the Lamb waves are array-type transducers able to select a specific mode. First, inserts of different sizes and different types introduced in the mid-plane during fabrication in a carbon-epoxy plain-weave quasi-isotropic composite panel were investigated. A simple "ray" analysis taking into account the observed change in group velocity and the measured time delay induced geometrically by the defect accounts accurately for the size of the defects. The same technique was applied to the detection of impact damage in cross-ply carbon-epoxy laminates. The naturally produced defects are more complex as an impact produces delaminations distributed at multiple lamina interfaces throughout the thickness of the panel, as well as matrix damage and fiber cracking. The cumulative effect of the complex damage upon the propagating wave is, again, to lower its group velocity. The time-delay measured vs. the accumulation of impact damage is significant and it correlates with the damage extent. The results are repeatable and consistent, suggesting time-delay as a reliable damage parameter for quantitative monitoring of delaminations and impact damage in composites.