Undercut weakens a welded joint in two distinct ways: it reduces the effective cross-sectional area of the connection, and it creates a sharp groove that concentrates stress right where the weld meets the base metal. That combination makes the joint more likely to crack under repeated loading and less capable of carrying its designed load. For structural and fatigue-critical work, even a small undercut can be the starting point of a failure.
What Undercut Actually Is
Undercut is a groove or channel melted into the base metal along the edge of a weld bead that doesn’t get filled back in with weld metal. It looks like a narrow trench running parallel to the weld toe. Instead of a smooth transition from the weld to the surrounding plate, you get a sharp notch. That notch is the root of nearly every problem undercut causes.
How Undercut Weakens a Joint
The damage from undercut works through two mechanisms. First, the groove physically removes base metal from the joint area. That means the remaining cross-section is thinner than designed, and the actual stress on the material under load is higher than the engineer calculated. The net stress increase is directly proportional to how much material is missing.
Second, and usually more dangerous, is the stress concentration effect. When a load passes through a smooth surface, stress distributes relatively evenly. A sharp notch like an undercut forces the stress to funnel through a smaller area at the root of the groove. This multiplied stress at the notch tip is what engineers call a stress concentration factor (SCF). Research on butt welds with undercut has shown that the severity depends on the three-dimensional shape of the groove, the overall weld bead geometry, the material properties, and the stress range the joint sees in service. A shallow, rounded undercut concentrates far less stress than a deep, V-shaped one.
This stress concentration is why fatigue cracks most commonly start at weld imperfections like undercut. Under cyclic loading, where a structure is loaded and unloaded repeatedly (think bridges, crane supports, or pressure vessels), that notch tip sees stress levels well above what the rest of the joint experiences. Over thousands or millions of load cycles, a crack initiates at the undercut and propagates through the joint. A weld that would have lasted decades without undercut can fail years earlier because of it.
Why Thin Material Is More Vulnerable
The thinner the plate, the worse the same undercut becomes. On a 25 mm thick plate, a 0.5 mm deep undercut represents a small fraction of the total thickness. On a 3 mm plate, that same groove takes up a much larger percentage and creates a proportionally bigger stress riser. Research has confirmed that weld quality matters significantly more in plates under 5 mm thick, where undercuts reduce fatigue strength to a much greater degree than in thicker sections. If you’re welding thin sheet metal, auto body panels, or lightweight structural tubing, undercut tolerance is essentially zero.
How Much Undercut Is Allowed
Structural welding codes set specific limits. Under AWS D1.1, the standard for structural steel, undercut running along the full length of a weld can be no deeper than 1/32 inch (about 0.8 mm). There is a limited exception: undercut up to 1/16 inch (1.6 mm) deep is permitted if it occurs in stretches no longer than 2 inches within any 12-inch run of weld. That means you can have a slightly deeper groove, but only in short, well-spaced patches, not continuously. These limits apply to visual inspection, and inspectors measure undercut depth using specialized gauges like bridge cam gauges or V-WAC gauges that sit across the weld toe.
For fatigue-critical applications like bridges, offshore platforms, or components subject to vibration, the acceptance criteria are often tighter than D1.1’s general allowances. Some project specifications call for zero visible undercut.
What Causes Undercut
Undercut happens when the arc melts into the base metal at the weld toe but the filler metal doesn’t flow back in to fill the groove. Several welding parameters push you toward this problem:
- Travel speed too fast. Moving the torch quickly doesn’t give the weld pool enough time to fill the edges. The arc melts the base metal, but the puddle has already moved on before it can fill in behind itself.
- Voltage or amperage too high. Excessive heat creates a wider, more fluid arc that digs into the base metal beyond where the filler can reach. If you’re getting long, continuous runs of undercut, your heat input is likely too high.
- Wrong torch angle. Directing too much arc energy toward one side of the joint melts the plate edge without depositing enough filler there. Keeping your torch at a 10 to 15 degree angle from vertical helps distribute the heat evenly across the joint.
- Arc length too long. Holding the torch too far from the workpiece spreads the arc out, producing more spatter and undercutting the edges. A shorter, tighter arc keeps the heat focused where the filler metal is going.
- Overheated base metal. When the plate itself is too hot from previous passes or inadequate cooling time, the edges melt more readily and undercut forms even with otherwise correct settings.
How to Prevent It
The fix depends on your process, but the underlying principle is the same: get enough filler metal to the edges of the weld before the puddle moves on. For MIG welding with shielding gas, push the torch (angle it over the pool, moving toward unwelded metal). For gasless flux-core wire, pull instead (drag the torch in front of the pool). Getting this direction wrong is one of the most common causes of undercut for newer welders.
If undercut keeps showing up, try reducing voltage to lower heat input, or increase wire feed speed so more filler metal is available to fill the edges. Slowing your travel speed gives the pool more time to wet out to the toes. On weave beads, pausing briefly at each edge of the weave lets filler flow into the corners where undercut typically forms. New wire welders especially tend to weave too fast, which leaves the edges unfilled.
How to Fix Existing Undercut
Under AWS D1.1, the approved repair method is straightforward: clean the undercut area and deposit additional weld metal to fill the groove. You can grind the notch smooth first to create a better surface for the repair pass, then lay a thin bead over the affected area. Any grinding or gouging to remove the defect has to be done carefully so you don’t nick or gouge the adjacent base metal or good weld metal, and you shouldn’t remove more base metal than necessary.
Simply grinding undercut smooth without adding filler might seem tempting, but it further reduces the cross-sectional area and isn’t code-approved unless an engineer specifically signs off on it. The only recognized repair for most structural work is to put metal back where it’s missing.