According to CSA W59, the maximum allowable depth of undercut for a weld under cyclic loading where the direction of load is transverse to the undercut is:

• A. 0.15 mm
• B. 0.25 mm
• C. 0.50 mm
• D. 0.75 mm

Answer: (B)

Undercut at the weld toe acts as a sharp notch that concentrates stress. Under cyclic loading, especially when the load is transverse to the undercut, this notch becomes a preferred site for crack initiation because the local stress concentration is higher and the material near the toe carries repeated loading cycles with less effective cross-section.

CSA W59 sets strict limits on how deep an undercut can be in fatigue-prone situations to keep the notch effect within acceptable levels. For a weld subjected to cyclic loading with the direction of load transverse to the undercut, the allowable depth is 0.25 mm. If the undercut were deeper, the stress concentration would be large enough to significantly shorten fatigue life, making crack initiation more likely during service.

Smaller depths, like 0.15 mm, would be more restrictive than necessary, while larger depths, such as 0.50 mm or 0.75 mm, would be too permissive for fatigue loading and could compromise joint durability.