Ultrasonic imaging of a turbine blade model using a 360° synthetic-aperture-focusing-technique and reverberation suppression

In this contribution, we present a method to complement missing X-ray computed tomography (CT) data of irregular shaped metallic specimen, like turbine blades, by ultrasonic testing in immersion mode. Therefore, an adapted 360° synthetic-aperture-focusing-technique (SAFT) is used to obtain particular information of the inner pattern of the specimen. These structural information cannot be imaged by X-ray CT due to limited penetration in specific directions and consequently absent projections in the dataset. However, arising artifacts caused by refraction and signal reverberation impede the detection of the desired structural information by ultrasonic testing of those irregular metallic specimens. To cope with refraction effects caused by the immense discrepancies in speed of sound (SOS) at the interface of the couplant and the specimen, the approach is based on virtual source elements. Furthermore, the covering of near-surface structures by the first reflection signals is treated by a subspace based filtering approach and signal reverberation artifacts are suppressed by predictive deconvolution.