The Risk of Over‑Reliance on Weld Overlays in Phased Array Ultrasonic Testing Interpretation

Phased array ultrasonic testing (PAUT) instruments often incorporate optional weld overlays that superimpose an assumed joint geometry on the sectorial scan (S‑scan). These overlays aid interpretation and scan planning by positioning signals relative to the weld profile based on index position. Their usefulness, however, depends on user defined weld geometry and stable index position. In practice, welds frequently deviate from the assumed geometry (e.g., land height, land offset, bevel angle), which can lead to significant interpretation errors. Incorrect overlays lead to placement and characterization errors and may reinforce confirmation bias when the S-Scan image is trusted over the A-Scan signal dynamics. This paper outlines common failure modes—misplacement, false negatives, and false positives—and offers mitigation: verification of weld preparation, emphasis on echo dynamics, and training that treats overlays as aids rather than authorities. The objective is balanced use: overlays remain valuable when accurate but are never a substitute for critical evaluation of ultrasonic data.