When performing a visual inspection, what constitutes an unacceptable level of undercut in a weld?

Undercut is a welding defect characterized by a groove or channel melted into the base metal adjacent to the weld toe or weld root and left unfilled by weld metal. When performing a visual inspection, an unacceptable level of undercut is generally determined by its depth, length, sharpness, and the applicable code or standard. Codes and standards such as AWS D1.1 (Structural Welding Code - Steel) or ASME Section IX (Welding and Brazing Qualifications) specify allowable undercut limits. Generally, any undercut exceeding 1/32 inch (0.8 mm) in depth is considered unacceptable for dynamically loaded structures or components subjected to fatigue. For statically loaded structures, a slightly larger undercut, such as 1/16 inch (1.6 mm), may be acceptable depending on the code requirements. Undercut with sharp edges is more critical than smooth, rounded undercut because sharp edges create stress concentrations, which can lead to premature failure. Continuous undercut, extending along a significant length of the weld, is more detrimental than isolated, short segments of undercut. The location of the undercut also matters; undercut in highly stressed areas is more critical. To accurately measure undercut, a welding gauge is typically used. The acceptability of undercut always depends on the specific application, the material being welded, and the governing code or standard. For example, a weld on a critical lifting component would have stricter undercut limits than a weld on a non-structural bracket.