Ultrasonic backscatter measurements using phased array ultrasonic transducers

Diffuse ultrasonic backscatter techniques are used to characterize microstructures and work well when grain scattering is confined within the single-scattering regime. When effects from higher-order scattering are present within the measurements, grain sizes are often overestimated. These effects have been primarily examined with single-element transducers, but phased array ultrasonic transducers (PAUTs) offer several measurement advantages, particularly for samples with complex geometry. In this presentation, the influence of higher-order scattering on PAUT experiments is discussed for samples of weakly scattering aluminum and strongly scattering steel. Spatial variance measurements from PAUT signals show differences in scattering content that are dependent on focal depth and material scattering strength when compared with experiments using single-element transducers. We hypothesize that the multiple transducer elements within PAUTs increase the probability of double or multiple scattering effects within backscatter measurements. These effects are dependent on the material single-crystal anisotropy, the grain size, frequency, experimental configuration, and specifics of the PAUT. Models to describe the PAUT experimental results are also discussed. Accurate theoretical models are critical for validation in order to provide insight into the limits of such measurements for microstructure quantification.