Ultrasonic Scattering from Complex Crack Morphology Features

In a typical ultrasound measurement, only the scattering cross-section features are considered involving the overall crack profile. A realistic crack, however, can have local curvature and sharpness features, which can have an effect on the ultrasonic scattering response and detected signal content. A study of crack morphology effects on ultrasonic scattering in the near-and far-field is described. Finite element models, scanning laser vibrometry, and optical microscopy measurements are used to study ultrasound scattering from realistic cracks in superalloy materials, where three primary length scale effects are considered -1) overall crack profile, 2) local variations in crack profile, and 3) local roughness features. Ultrasonic frequency/wavelength and crack length scale features are considered for both near-field and far-field regions, where direct comparisons of model predictions and experimental measurements are made. Local crack roughness features produced very localized and complex wave scattering effects in the extreme near-field of the crack site, while crack profile variations on the order of, or larger than, the ultrasonic wavelength produced coherent scattered field patterns that diverged, were directional in nature, and extended well into the far-field.