AWS D1.1 · Table 6.1 · Criterion 6

Undercut — Service Category Dictates Limits

Undercut is a groove or notch in the base metal at the weld toe. AWS D1.1 acceptance limits differ for statically vs. cyclically loaded structures. Cyclically loaded structures (bridges, cranes) have stricter limits.

What is Undercut?

Undercut is a melted groove in the base metal at the weld toe, or toe line, where the weld meets the parent metal. The weld did not fill the recess, leaving a stress concentration.

Root undercut — Groove on the root (back) side of the weld, visible from the back side or via radiography.
Face undercut — Groove on the face (front) side of the weld, visible during face inspection.
Toe undercut — Groove at the toe (edge) where weld meets base metal. Most common and visually obvious.
Single-sided vs. both sides — Measure depth at worst point. If both toes are undercut, record both measurements.

Undercut Geometry — Groove at Weld Toe

FACE UNDERCUT ROOT UNDERCUT TOE UNDERCUT ┌─────────────────────────────────────────────────────────┐ │ Base ═╱ (groove) ╱ Weld | Weld ╱ (groove) ╱ Base │ │ ════════════════════════════════════════════════════════ │ │ At surface, visible At root, visible from back │ │ depth = 1/32" deep, etc. depth measured same way │ └─────────────────────────────────────────────────────────┘

AWS D1.1 Table 6.1 Acceptance Limits by Service Category

Criterion 6: Undercut — Different Limits for Different Service Categories

Statically Loaded Structures (Buildings, Columns, Platforms):

Base metal thickness ≤ 1 inch: max undercut = 1/32" (0.031")
Base metal thickness > 1 inch: max undercut = 1/16" (0.0625")

Cyclically Loaded Structures (Bridges, Cranes, Seismic SFRS):

Max undercut = 1/32" (0.031") regardless of thickness

Why the difference? Cyclically loaded welds experience stress reversals and fatigue crack initiation from stress concentrations. Undercut (especially deeper than 1/32") is a fatigue initiation site. Static structures tolerate slightly deeper undercut.

Common Undercut Causes

Excessive heat input / amperage — Arc melts the base metal faster than the weld puddle can fill. Base metal surface is "burned away."
High travel speed — Arc moves too fast over the base metal surface, melting it but not allowing the weld puddle to fill the groove.
Poor electrode angle — Electrode not aimed into the corner properly. Weld metal flows down the centerline, not up the toe.
Undersized weld / thin root opening — Weld puddle too small to reach and fill the toe groove.
Improper gun/torch manipulation — In GMAW, gun angle too steep melts base metal without filling. Backhand angle better for toe fill.

How to Inspect & Document for the Report

Visual measurement with depth gauge — Place depth gauge at weld toe. Measure perpendicular to the base metal surface. Document depth at each toe.
Replica tape / profile gauge — Press replica tape onto the weld toe, then remove and measure the depth imprint using a ruler or micrometer.
Cross-section measurement — Cut a sample through the undercut area and measure the groove depth under magnification.
Record for each location — Note left toe, right toe, root (if accessible). Compare to service category limits (static vs. cyclic).
Document repair decision — If undercut exceeds limit, mark for repair. If within limit, mark accept and record measurement.

CWI Inspection & Repair Tips

Always confirm service category FIRST — static vs. cyclic. This determines which limit applies.
Undercut is best repaired by grinding or butter pass (low-amperage pass without remelting base metal too deeply), then re-welding the toe area.
Prevention is simpler: reduce amperage, slow travel speed, improve electrode angle, and practice toe fill technique.
On critical seismic or bridge work (cyclic loading), be especially strict on the 1/32" limit. Fatigue cracks initiate at undercut.

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