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### Scattering Measurements and In Situ Imaging with Sparse Guided Wave Arrays

#### Abstract

Knowledge of how guided waves are expected to scatter from typical defects is useful for detection, localization, and characterization of damage. Structural health monitoring using sparse guided wave transducer arrays has been frequently proposed, but scattering information is rarely incorporated as part of the analysis methodology. If scattering characteristics are represented as a matrix indexed by incident angle and scattered angle, sparse array measurements essentially sample this matrix. Prior work by the authors has demonstrated a methodology for estimating the entire scattering matrix from these sampled angles. Here we improve this methodology and apply it to the generic defect geometry of a crack growing from a fastener hole, where a notch is used to simulate a crack. First, a controlled experiment is performed to obtain guided wave data for estimating scattering matrices for notches of different lengths. Second, a similar notch is introduced in a different specimen that is geometrically more complex and thus closer to being representative of an actual part. The scattered signals from the second specimen are computed and then analyzed via both delay-and-sum imaging and minimum variance imaging, utilizing the previously estimated scattering characteristics for damage detection, localization, and characterization.