Ultrasonic defect detection in multi-material, axis-symmetric devices with an improved synthetic aperture focusing technique (SAFT)

This paper describes an improved synthetic-aperture-focusing-technique (SAFT) for ultrasonic defect detection in multi-material devices with a circular cylindrical geometry. In conventional SAFT, defect imaging resolution in such specimen is limited, due to refraction at material interfaces. Hence, the improved technique allows to increase the probability of detection (POD) and the resolution of defects in multi-material, axis-symmetric devices. Especially, if defects are located in layers far away from the device surface and the discrepancies in the speed of sound (SOS) of the specimen materials are thoroughly substantial, it is indispensable to implicate refraction effects in the SAFT processing. Thereby, the examination is divided into three major steps. First, a specimen is scanned by rotating a spherically focused immersion ultrasound transducer around it. In a second step, a corrected time of flight (TOF) distribution is calculated by a fast marching method (FMM) with the `a-priori knowledge of the specimen layout and the different SOS in the materials. As third and last step, the improved SAFT is performed utilizing the TOF distribution. To verify improvements in comparison to a conventional SAFT, a specimen is inspected, consisting of a steel and a polyvinyl chloride (PVC) part.