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Weld Discontinuity vs Weld Defect — D1.1:2025 Explained
Per D1.1:2025, a discontinuity becomes a defect only when it exceeds the acceptance criteria in Table 8.1. Most discontinuities are acceptable — the code provides specific limits for each of the 8 categories. Section §8.10.1 defines linear vs rounded classification for RT and UT evaluation.
Discontinuity vs Defect — The D1.1 Distinction
D1.1 deliberately uses two different terms. A discontinuity is any interruption in the typical structure of a material — it is a neutral term that describes an imperfection without implying rejection. A defect is a discontinuity that exceeds the acceptance criteria in Table 8.1 and therefore requires repair or removal.
This distinction is central to how D1.1 works: inspectors evaluate discontinuities against specific criteria, and only those that exceed the limits become defects requiring action. A small amount of piping porosity in a fillet weld on a statically loaded connection may be a discontinuity that is acceptable. The same porosity in a CJP groove weld transverse to tensile stress is a defect. The imperfection is the same; the code classification differs based on weld type and loading.
D1.1:2025 §8.10.1 — Classification
Section 8.10.1 classifies discontinuities by shape: linear (length greater than 3 times width) and rounded (length 3 times width or less). A cluster of rounded discontinuities is evaluated by the total area of the cluster rather than individual pore sizes.
This classification determines which acceptance criteria row in Table 8.1 or 8.2 applies to the discontinuity. Slag stringers, linear porosity, and elongated lack-of-fusion areas are linear discontinuities. Individual pores and compact inclusions are rounded discontinuities. The shape test is applied to each individual discontinuity before the size and frequency limits are checked.
Table 8.1 — The 8 Categories
Table 8.1 (Visual Inspection Acceptance Criteria) covers 8 categories, each with separate limits for statically loaded (A) and cyclically loaded (B/C) connections:
| Item | Category | Key Criterion |
|---|---|---|
| (1) | Cracks | Zero tolerance — any size, any location |
| (2) | Fusion | Complete fusion required to root and sides |
| (3) | Craters | Filled to full weld cross-section before arc extinguished |
| (4) | Weld profiles | Per Clause 7.23 (face geometry, convexity, concavity limits) |
| (5) | Inspection timing | Per Clause 8.9 (ambient temp; 48-hr delay for A514/A517/A709 HPS 100W) |
| (6) | Undersized welds | Limited underrun by weld size range; underrun not in excess of 10% of weld length |
| (7) | Undercut | Limits by thickness and loading type (static vs cyclic) |
| (8) | Piping porosity | Limits by connection type, weld type, and load category |
Which Discontinuity Has Zero Tolerance
Only cracks. Table 8.1 item (1) states that any crack shall be unacceptable regardless of size or location for both statically and cyclically loaded connections. This is the only discontinuity category in Table 8.1 with an absolute rejection criterion.
All other discontinuities in Table 8.1 permit some level of imperfection within defined limits. Undercut up to 1/32 in in certain locations is acceptable. Piping porosity in fillet welds on statically loaded connections is acceptable within the frequency and size limits of item (8)(A)(2). Weld profile deficiencies are evaluated against the dimensional limits of Clause 7.23.
Inspector note: The practical consequence of the discontinuity/defect distinction is that inspectors must evaluate, not just observe. Finding a pore does not automatically mean rejection — the inspector applies the specific criteria for that weld type, loading category, and joint configuration. Documentation should record both the discontinuity observed and the acceptance decision with the relevant Table 8.1 item applied.
All Defect Sub-Pages
Each discontinuity type in Table 8.1 has a dedicated reference page with the specific acceptance criteria and field guidance:
Frequently Asked Questions
Per D1.1:2025, a discontinuity is any interruption in the typical structure of the weld — it is a neutral term for any imperfection. A defect is a discontinuity that exceeds the acceptance criteria in Table 8.1 and therefore requires repair or removal. Not all discontinuities are defects: for example, small amounts of piping porosity in fillet welds on statically loaded connections are acceptable under Table 8.1 item (8)(A)(2). Only discontinuities that exceed the specific limits become defects requiring action.
D1.1:2025 §8.10.1 classifies discontinuities by shape. A linear discontinuity has a length greater than 3 times its width — this applies to slag stringers, linear porosity, and elongated lack-of-fusion areas. A rounded discontinuity has a length of 3 times its width or less — this applies to individual pores and compact inclusions. A cluster of rounded discontinuities is evaluated by the total area of the cluster. This shape classification determines which row of the acceptance criteria applies.
Cracks. D1.1:2025 Table 8.1 item (1) states that any crack shall be unacceptable regardless of size or location, for both statically loaded connections (Category A) and cyclically loaded connections (Categories B and C). Cracks are the only discontinuity in Table 8.1 with an absolute zero-tolerance criterion. All other discontinuity types — undercut, porosity, incomplete fusion, weld profiles — permit some level of imperfection within defined limits.
D1.1:2025 Clause 8.9 requires visual inspection of all production welds. Inspection shall be performed after the weld has cooled to ambient temperature, except for A514, A517, and A709 HPS 100W steels where inspection shall be delayed a minimum of 48 hours. The inspector applies the acceptance criteria in Table 8.1 to each weld. Subsurface discontinuities such as inclusions and internal porosity require radiographic or ultrasonic testing per Clause 8.11 or 8.13 when specified in the contract documents.