Enhancement of the POD of flaws in the bulk of highly attenuating structural materials by using SAFT processed ultrasonic inspection data
The determination of defects in materials and components using nondestructive testing and evaluation techniques is widely applied in many industrial areas. The safe operation of transportation vehicles such as airplanes, trains and ferries requires permanent inspection of relevant parts and components. Depending on the size of flaws to be detected and depending on the material to be inspected the choice of the proper nondestructive method is made. Generally, the capability of the chosen nondestructive evaluation (NDE) system has to be assessed, especially in safety-relevant areas such as those named above. In this report we address the determination of the size of flaws in the bulk of Duplex stainless steels and the determination of POD from ultrasonic data. Duplex steels are increasingly used as structural materials because of their favourable strength and toughness properties. Due to the large thicknesses of the relevant components the NDE method of choice is usually ultrasonic inspection. However, the ferritic-austenitic mixed microstructure of these steels causes a strong attenuation of the ultrasonic waves. Thus, ultrasonic inspection has to be optimized in order to increase the least detectable defect sizes. In this respect, we have applied the Synthetic Aperture Focusing Technique SAFT which leads to a reduction of the microstructural noise signals and thus to an improvement of defect detection. Based on the ultrasonic rf-data acquired on a test block which has been supplied with model defects of various dimensions and depths, we have applied an â versus a-approach t o determine the POD for the inspected Duplex specimen. Following the Department of Defense Handbook Nondestructive Evaluation System Reliability Assessment in its updated version (MIL-HDBK-1823, 2007) we have determined POD curves applying both the raw and the SAFT-processed ultrasonic data. The latter has lead to a remarkable improvement of defect detectability which encourages a standard application of ultrasonic inspection to ensure the integrity and to increase the reliability of Duplex components.