3.5 Geometric Discontinuities: Undercut, Overlap, and Profile
Key Takeaways
- Undercut is a groove at the weld toe — limited to 0.01" for cyclically loaded, 1/32" for statically loaded (AWS D1.1)
- Overlap is weld metal flowing over unfused base metal — generally not acceptable (indicates lack of fusion)
- Excessive reinforcement: ≤ 1/8" for material up to 1" thick, ≤ 3/16" for material over 1" (AWS D1.1)
- Theoretical fillet weld throat = 0.707 × leg size — undersized throats are rejectable
- Causes of undercut: excessive current, arc length, travel speed, or improper electrode angle
- Causes of overlap: slow travel speed, insufficient amperage, improper electrode angle
3.5 Geometric Discontinuities: Undercut, Overlap, and Profile
Geometric discontinuities are visible surface conditions that affect the weld profile. While less severe than cracks or lack of fusion, they can still cause rejection when they exceed code limits.
Undercut
Undercut is a groove melted into the base metal adjacent to the weld toe or weld root and left unfilled by weld metal. It creates a notch that acts as a stress concentrator, particularly dangerous under fatigue (cyclic) loading.
Types:
- Weld toe undercut — groove at the toe where the weld face meets the base metal surface
- Weld root undercut — groove at the root side of the weld
Causes:
- Excessive current (amperage too high)
- Excessive arc length (voltage too high)
- Travel speed too fast
- Improper electrode angle
- Excessive weave width
AWS D1.1 Acceptance Criteria (Table 8.1 — Visual):
| Loading Condition | Undercut Limit |
|---|---|
| Cyclically loaded (nontubular) | ≤ 0.01" (0.25 mm) depth |
| Statically loaded (nontubular) | ≤ 1/32" (1 mm) for lengths up to 2" in any 12" of weld; ≤ 1/16" for isolated locations |
Overlap (Cold Lap)
Overlap is the protrusion of weld metal beyond the weld toe or weld root. The weld metal flows over the base metal surface without fusing to it — creating a notch-like condition.
Causes:
- Travel speed too slow
- Insufficient amperage (metal flows ahead of the arc without fusion)
- Improper electrode angle
- Wrong electrode size for the joint
Code Treatment: Overlap is generally not acceptable because it creates a crack-like notch at the toe and indicates probable lack of fusion beneath the overlap.
Excessive Reinforcement (Crown Height)
Excessive reinforcement is weld metal built up above the surface of the base metal beyond what is needed. While some reinforcement is normal, excessive reinforcement:
- Creates a stress concentration at the weld toe
- Wastes filler metal
- May indicate overheating of the joint
AWS D1.1 limits for groove weld reinforcement:
| Base Metal Thickness | Maximum Reinforcement (Face) |
|---|---|
| Up to 1" (25 mm) | 1/8" (3 mm) |
| Over 1" (25 mm) | 3/16" (5 mm) |
Insufficient Throat (Fillet Welds)
A fillet weld with a concave profile may have insufficient throat dimension. The theoretical throat is the minimum distance from the root to the face of the fillet weld:
- For an equal-leg fillet weld: theoretical throat = 0.707 × leg size
- If the actual throat is less than the specified minimum, the weld is undersized and rejectable
Other Profile Issues
| Issue | Description | Significance |
|---|---|---|
| Insufficient leg size | Fillet weld leg does not meet minimum specified size | Rejectable — weld is undersized |
| Excessive convexity | Weld face too convex on fillet weld | Stress concentration at toe |
| Excessive concavity | Weld face too concave on fillet weld | Insufficient throat, undersized weld |
| Misalignment (high-low) | Base metal edges not properly aligned | Creates stress concentration, affects penetration |
| Weld spatter | Droplets of molten metal deposited outside the weld | Generally should be removed; may mask other discontinuities |
For the Exam: Undercut and overlap are the most commonly tested geometric discontinuities. Know the AWS D1.1 limits: 0.01" for cyclically loaded, 1/32" for statically loaded (with length limits). Know that overlap is generally not acceptable because it indicates lack of fusion.
Per AWS D1.1, what is the maximum allowable undercut depth for cyclically loaded nontubular structures?
Why is overlap (cold lap) considered unacceptable by most codes?
The theoretical throat of an equal-leg fillet weld is calculated as: