What is the difference between a discontinuity and a defect under AWS D1.1?

A discontinuity is any interruption in the typical physical structure of the weldment — including porosity, undercut, incomplete fusion, or cracking. A defect is a discontinuity that exceeds the acceptance limit specified in the governing code. Every defect is a discontinuity, but not every discontinuity is a defect. The distinction determines whether you document it and move on, or reject it and require repair.

Does AWS D1.1 require documentation of acceptable discontinuities?

AWS D1.1 requires documentation of all NDE results, including acceptable discontinuities found during UT and RT. RT and UT reports must record indication type, location, and disposition. Visual inspection records note the inspection date, weld ID, inspector identity, and accept/reject decision. Rejected welds require a separate repair record and re-inspection sign-off before acceptance.

Are cracks ever acceptable under AWS D1.1?

No. AWS D1.1 treats cracks as absolute rejectable defects regardless of length or location. This applies to crater cracks, transverse cracks, longitudinal cracks, and heat-affected zone cracks. Any crack detected by visual, RT, UT, MT, or PT must be removed and the area repaired before the weld can be accepted. There is no engineering-judgment exception without a formal alternate acceptance evaluation by the Engineer of Record.

Who is responsible for documenting weld rejections under AWS D1.1?

The CWI or designated inspector is responsible for completing inspection records noting the weld ID, location, discontinuity type, measured dimensions, and disposition. The contractor performs the repair using a qualified WPS, then presents the weld for re-inspection. The inspector signs off on re-inspection as a separate record. Both the original rejection and the re-inspection acceptance must be traceable in the project inspection file.

Weld Discontinuity vs. Defect: AWS D1.1 Acceptance Guide

AWS D1.1 separates discontinuities from defects. Know the classification, acceptance criteria, and what CWIs must document when rejecting a structural weld.

Every CWI working under AWS D1.1 deals with weld discontinuities daily. But the moment a discontinuity exceeds a code limit, the terminology — and the required action — changes. Understanding exactly where that line falls, and what the documentation must capture on both sides of it, is what separates a disciplined inspection program from one that creates liability gaps.

What "Discontinuity" Means Under AWS D1.1

AWS D1.1 Chapter 8 sets the inspection acceptance criteria for structural welds. The code acknowledges that all welds contain some degree of variation — no weld is a perfect, uniform deposit of metal. What the code defines is the range of variation that is acceptable for the intended service.

A discontinuity is any interruption in the typical physical structure of the weldment. That includes:

Geometric discontinuities: undercut, overlap, incorrect weld profile, undersized weld
Soundness discontinuities: porosity, slag inclusions, incomplete fusion, incomplete joint penetration
Mechanical discontinuities: cracks, tears, cold laps

The presence of a discontinuity is simply a description — something is there to detect. Discontinuities are detectable by visual examination, RT, UT, MT, or PT depending on type and location. Finding one triggers measurement and evaluation against the applicable acceptance criteria. Nothing more.

When a Discontinuity Becomes a Defect

A defect is a discontinuity that fails the acceptance criteria in AWS D1.1 Chapter 8. Once that threshold is crossed, the weld is rejected and must be repaired or removed. The contractor cannot accept a code-failed weld by unilateral engineering judgment without a formal fitness-for-service (FFS) evaluation by the Engineer of Record — an alternate acceptance provision that AWS D1.1 allows, but only through a defined process.

The practical importance of this line: the same type of discontinuity, in the same joint, can be acceptable or rejectable depending on its dimensions and the loading category of the member. An undercut of 0.5 mm on a compression member may fall within the code limit. That same undercut on a primary tension member fails.

Before evaluating any discontinuity, confirm:

Loading category: statically loaded or cyclically loaded (fatigue loading)
Connection type: tubular or non-tubular
Weld category: AWS D1.1 assigns categories that affect acceptance for UT and some visual criteria

These variables determine which acceptance table in Chapter 8 applies.

Visual Inspection Acceptance Criteria

Visual inspection is the first and most universal inspection method under AWS D1.1. Chapter 8 sets the following limits for statically loaded non-tubular connections:

Cracks: Not permitted anywhere. No length threshold, no location exception. A crater crack at the weld termination is a defect. A HAZ crack from hydrogen embrittlement is a defect. This is absolute across all loading categories and connection types.

Undercut: The maximum allowable undercut depth at the weld toe is:

1/32 in (1 mm) for welds in primary tension members or tension flanges
1/16 in (2 mm) for compression members and other welds not subject to primary tension

For cyclically loaded structures, undercut limits are tighter. Always verify the loading category on the drawing before evaluating undercut.

Porosity: Surface porosity must be evaluated for individual indication diameter and for cluster frequency. The code sets maximum individual pore diameter and a maximum number of pores per unit length of weld. Both the size of individual indications and the density of clustering matter — a single acceptable pore in isolation may be fine; the same size pore appearing every inch of weld is a pattern that may fail the frequency limit.

Overlap and cold lap: These are always rejectable. An overlap is an extension of weld metal beyond the weld toe that is not fused to the base metal. Cold lap has the same appearance and consequence. Neither has an acceptable threshold — both require repair.

Weld size and profile: The deposited weld must meet the size shown on the weld symbol. Undersized fillet welds are defects. Profile requirements — limits on weld convexity and concavity — are also set in Chapter 8. Excessive convexity creates stress concentration; excessive concavity reduces the effective throat.

A full breakdown of dimensional tolerances for weld profile appears in the Visual Acceptance Criteria for AWS D1.1 article.

Volumetric NDE: What Changes for Internal Discontinuities

When volumetric NDE is required — for complete joint penetration groove welds in specified categories, or when mandated by contract or the Engineer — the acceptance criteria become more detailed than visual examination allows.

RT classifies indications by type: porosity clusters, linear slag inclusions, incomplete fusion, or cracks. Each type has dimensional limits that depend on the weld category, the base metal thickness, and the loading type. Cracks are always rejectable on RT film; slag and porosity have size-and-frequency thresholds.

UT acceptance under AWS D1.1 uses a decibel (dB) attenuation approach. Indications are evaluated against a calibration reference level — the combination of dB rating and indication length, compared to the weld's category and thickness, determines accept or reject. Unlike visual inspection, UT can detect subsurface incomplete fusion and cracks that look fine on the surface.

MT detects surface and near-surface discontinuities in ferromagnetic materials. PT detects only surface-breaking discontinuities, making it a secondary choice when MT is not practical.

Documentation When You Reject a Weld

When a discontinuity fails the applicable acceptance criterion, the inspection record must capture:

Weld identification: weld number, joint designation, drawing reference
Discontinuity description: type, measured dimensions (length, depth, diameter as applicable), location within the joint
The criterion violated: cite the specific code provision — for example, "undercut exceeds 1/32 in (1 mm) limit for tension member per AWS D1.1:2025 Chapter 8"
Disposition: rejected — requires repair
Inspector identification and date

After repair, re-inspection records must reference the original rejection. The NDE audit packet documentation article covers how to organize this for third-party audits and owner submittals.

Repairs must use a qualified WPS — a weld that already failed inspection is not exempt from WPS requirements. In some cases, the repair geometry differs enough from the original joint that a separate repair WPS is needed. See Repair Weld WPS Requirements for the qualification path.

Common Defects by Process

SMAW: Porosity from damaged or improperly stored low-hydrogen electrodes is the most common rejection cause. Excessive amperage combined with fast travel speed causes undercut at the weld toe. Multi-pass builds with cold restarts that lack proper interpass cleaning or preheat maintenance risk lack of fusion between passes.

FCAW-G: Shielding gas disruption from wind — even a light breeze through a bay door — is the leading cause of porosity in gas-shielded FCAW. Excessive travel speed produces slag inclusions. Undercut is common when operators push voltage above the WPS range to increase deposition.

FCAW-S: Self-shielded wire handles wind well but generates significant slag volume. Inadequate interpass cleaning before subsequent passes is the primary defect driver. The inspector should verify that the slag has been completely removed and that the weld surface is clean before each pass is accepted.

GMAW: Short-circuit transfer at low heat input settings carries higher risk of incomplete fusion, particularly at the root of a CJP groove weld. The root pass on a tight-tolerance joint with short-circuit transfer requires careful voltage and travel speed control.

SAW: The flux blanket virtually eliminates atmospheric porosity. The primary SAW defect mode is centerline solidification cracking in wide, deep beads — a depth-to-width ratio exceeding roughly 1:1 produces solidification stress that the metal can't accommodate. Proper bead geometry control is the prevention.

Rule library based on AWS D1.1:2025; verify against your governing edition. The authority having jurisdiction or your contract may specify an earlier edition with different acceptance criteria.

Managing rejection records across a project — keeping them tied to the weld map, WPS, welder ID, and repair history — is where paper-based inspection systems break down. WPS Welding's inspection documentation tools connect all of these records in one audit-ready file.