Investigation of the Synthetic Aperture Focusing Technique resolution for heavy rotor forging ultrasonic inspection

This contribution determines the resolution achievable by Synthetic Aperture Focusing Technique (SAFT) processing ultrasonic inspection data acquired along circular scan paths. If the inspection is conducted using a transducer with a narrow divergence angle, the lateral resolution after SAFT reconstruction becomes highly depth dependent. In Nondestructive Testing (NDT), such a configuration is typical for the ultrasonic testing of heavy rotor steel forging blanks for energy industry components, such as rotor shafts and turbine discs. There, the inspection is performed in contact mode over specimen´s mantle surface with longitudinal waves. For defect classification and quantification, knowledge of the resolution is essential. The radial dependency of the resolution is determined by SAFT processing simulated far field point spread functions at different depths. The simulation results are discussed and compared with actual inspection data of turbine disc forgings containing reference reflectors and natural indications.