Characterization and Sizing of Crack and Porosity Weld Defects Using Phased Array Ultrasonic Testing

Abstract

The accurate characterization and sizing of weld defects are critical for ensuring the structural integrity and safety of industrial fabrications. Mischaracterizing a planar crack as volumetric porosity can lead to catastrophic failure, while mistaking benign porosity for a critical crack result in unnecessary and costly repairs. The study addresses this critical gap by systematically investigating the distinct acoustic signatures of planar cracks and volumetric porosity using Phased Array Ultrasonic Testing (PAUT). Experimental PAUT inspections were performed on 9.50 mm thick steel V-groove butt welds containing induced crack and porosity defects. Data were analyzed using Omni PC software, comparing A-scan, S-scan, and C-scan responses, and evaluating the accuracy of amplitude-based sizing against visual spatial measurements. The results demonstrate that cracks produce linear, well-defined S-scan patterns with sharp A-scan signals and corner trap effects, while porosity generates diffuse, cloud-like S-scan patterns with multiple, overlapping A-scan peaks. Amplitude-based sizing was found to be unreliable for cracks, significantly underestimating crack height (1.69 mm vs. 7.80 mm for a toe crack), whereas it is more appropriate for defining porosity cluster boundaries when using the -6 dB drop method. A comprehensive classification matrix was developed, achieving 100% accuracy in distinguishing these defect types based on S-scan pattern, weld zone location, amplitude behavior, and the relationship between depth amplitude (DA^) and visual amplitude (ViA^). The study concludes that a multi-feature approach, which prioritizes spatial imaging over amplitude alone and advocates for the use of tip-diffraction techniques for crack sizing, is essential for reliable PAUT weld inspections. The findings provide a practical, validated framework to improve defect classification, enhance sizing accuracy, and support informed fitness-for-service decisions in structural welding.