Application of Sparse Synthetic Aperture Focusing Techniques to Ultrasound Imaging in Solids Using a Transducer Wedge

This article discusses the application of sparse synthetic aperture focusing techniques (SAFTs) for fast and accurate ultrasonic nondestructive testing (NDT) imaging of solids in cases where a wedge is required between the transducer array and the test medium. A wedge is often used to appropriately direct the ultrasonic beams when testing for structural defects at particular orientations or when inspecting parts with particular geometries (e.g., waveguides). Both the virtual element (VE) and the plane-wave (PW) modalities of sparse-firing SAFT are examined for the wedge case that requires particular considerations in the beamforming algorithms for the wave refractions and mode conversions occurring at the wedge-medium interface. The method of wave mode compounding is also examined for this application to increase the array gain without increasing its physical aperture. Numerical simulations and experimental tests demonstrate the potential improvements in speed and accuracy obtainable by sparse SAFT adapted to wedge-transducer cases compared to a traditional full matrix capture (FMC) imaging mode. A practical implementation of the imaging of transverse defects in rail tracks is also presented.