Undercut Measurement Dispute on a Horizontal Fillet
During visual inspection of a horizontal fillet weld joining a 12 mm plate to a column flange, you measure undercut on the vertical leg (the plate side) at approximately 1.2 mm depth and 40 mm continuous length. The applicable code limits undercut for this member to 0.8 mm depth and 50 mm in any 300 mm length. The welder disputes your measurement, claiming the undercut is only 0.5 mm and that the "shadow" makes it look deeper. The weld cap is otherwise acceptable, with a leg size of 8 mm against a specified 8 mm. You are using a weld gauge and a magnifying glass. The lighting in the area is overhead fluorescent, casting a shadow into the undercut.
1. How do you resolve the measurement dispute, and what is the acceptance decision?
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Resolving the dispute:
- Improve the measurement conditions. Overhead fluorescent lighting casting a shadow into the undercut can exaggerate apparent depth. I would reposition a focused light source at a low angle to illuminate the undercut directly, and re-examine with the magnifier.
- Use the correct gauge technique. A weld undercut gauge or a dial/digital gauge should be placed perpendicular to the weld axis, with the reference feet resting on the undisturbed plate surface on both sides of the undercut. The measurement should be taken at the deepest point along the 40 mm length, not at a single arbitrary spot.
- Take multiple readings along the length and record the maximum. If the welder and I still disagree, a third-party or supervisor measurement with a calibrated gauge resolves it.
Acceptance decision:
- If the depth is confirmed at 1.2 mm, it exceeds the 0.8 mm limit and the weld is rejectable on depth, regardless of the 40 mm length being within the 50 mm allowance. Both criteria must be satisfied.
- If re-measurement shows 0.5 mm, the weld passes on depth and the 40 mm length is within 50 mm, so it passes.
The decision hinges on a properly lit, properly referenced measurement - not on who is more senior. I would document the method and the final reading.
2. The welder says the undercut is "only cosmetic" because the leg size is 8 mm as specified. Address this argument.
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The argument is incorrect. Undercut is evaluated as a separate acceptance criterion from leg size, because it reduces the effective cross-section of the base metal at the weld toe - the point of highest stress concentration.
- Meeting the 8 mm leg size confirms the weld throat, but does not restore the plate material removed by undercut. The undercut creates a notch at the toe, which acts as a stress raiser and a potential initiation site for fatigue or brittle fracture.
- The code sets an explicit undercut limit (0.8 mm depth, 50 mm length in 300 mm) precisely because undercut is a geometric discontinuity, not a cosmetic blemish. A weld can have a full leg and still fail on undercut.
- For cyclically loaded or low-temperature service, undercut is treated even more strictly, since the notch sensitivity increases.
My response: explain that leg size and undercut are independent criteria; the weld must satisfy both. If the undercut exceeds 0.8 mm, it is a repairable defect - typically by light grinding to a smooth contour (if within allowable thickness reduction) or by re-welding. I would issue the nonconformance, specify the repair method, and require re-inspection after repair.