Frequency-wavenumber domain methods for analysis of incident and scattered guided wave fields

Full acoustic wavefield data were acquired from an aluminum plate with various structural discontinuities and artificial defects using an air-coupled transducer mounted on a scanning stage. Piezoelectric transducers permanently mounted on the specimen were used as wave sources. These source transducers were elements of a permanently attached sparse array. A time series of wavefield images clearly shows details of guided waves as they propagate outward from the source, reflect from specimen boundaries, and scatter from discontinuities within the structure. Distinct S0 and A0 Lamb wave modes are directly visible on constant time snapshots of the captured wavefield. However, the waves propagating outward from the source, and waves reflected from boundaries, obscure the weaker waves that are scattered from defects. To facilitate analysis of weaker scattered waves, source waves are removed from the full wavefield data using both time and frequency domain methods. The effectiveness of each method is evaluated in the wavenumber-wavenumber domain and results are fused to obtain images of scattered wavefields. The method is demonstrated on a through hole, to which a notch is added to simulate a crack. The angular dependence of the scattered wavefield is experimentally determined for source waves incident on the notch from two directions, one toward the side of the notch and the other toward the end of the notch.