Remote imaging of local resonance for inspection of honeycomb sandwich composite panels

We have developed and tested a new ultrasonic technique for rapid, remote inspection of honeycomb and similar sandwich composites based on imaging of local resonance behavior. A single pristine honeycomb cell can be approximately modeled as a hexagonal plate with appropriate boundary conditions on its six sides. With this in mind, we excite the entire honeycomb panel continuously with a single, steady tone at the resonance frequency of the single pristine cell using a fixed-position transducer. We then measure and visualize the full-field amplitude and phase response over the inspection area using a scanning laser Doppler vibrometer. We applied the technique to two imperfect aluminum honeycomb panels. By imaging the magnitude of the response at each pixel, we found that one can simultaneously visualize the high-amplitude response of the pristine cells, the lower-amplitude response of the out-of-spec cells, the near-zero response near the cell boundaries, and the non-zero response in the damaged cell walls.