Structural imaging through local wavenumber estimation of guided waves

This paper describes a method to effectively image structural features and defects using local estimates of the wavenumber of propagating guided Lamb waves at a fine grid of spatial sampling points. The guided waves are rapidly excited at each grid point using a scanning Q-switched laser system and sensed by a single fixed ultrasonic transducer. Through reciprocity, this produces a full-wave-field time history of a virtual wave being excited from the transducer. Using frequency-wavenumber processing, localized wavelength estimates are obtained by isolating each wave mode, narrowband filtering to one or more high-energy frequency bands, and identifying the center-wavelength of a sliding wavenumber band-pass filter that maximizes the energy at each grid point. The approach was capable of producing detailed images of hidden wall-thinning in an aluminum plate and a steel pipe section and local impact delamination in a complicated composite component.