Almost every weld contains something. Ripples, minor undercut, scattered pores, a trace of slag at a start-stop — a joint with no interruption whatever in its structure is rare, and no code requires one. The word for those interruptions is discontinuity, and it is deliberately neutral. A discontinuity becomes a defect only when it exceeds the acceptance criteria of the code governing that fabrication. The distinction is not pedantry. It determines whether a weld was rejectable, and the answer is set by the code and the inspection performed at least as much as by the weld.
Two words that are not interchangeable
A discontinuity is any interruption in the uniform structure of a weld or the metal around it: porosity, slag inclusions, lack of fusion, incomplete penetration, undercut, overlap, arc strikes, cracking. A defect is a discontinuity of a type, size, location or frequency that the applicable code will not accept, and which therefore has to be repaired or the component rejected. The same pore in two different fabrications can be either one.
The governing code is the first question
Acceptance criteria are not universal. AWS D1.1 governs structural steel welding; AWS D1.5 governs highway bridge welding to a noticeably more demanding standard; pressure work is evaluated against the relevant section of the ASME Boiler and Pressure Vessel Code, with ASME Section VIII for unfired pressure vessels and procedure and performance qualification under ASME Section IX.
Within a single code the criteria can differ again by service. AWS D1.1 applies different limits to statically loaded and cyclically loaded connections, because a discontinuity that is harmless under a steady load can be a fatigue initiation site under a fluctuating one. Establishing which code, which edition and which service category applied is the necessary first step, and it is frequently contested.
Planar and volumetric discontinuities are not comparable
Porosity and slag inclusions are volumetric: rounded voids that reduce load-bearing area without concentrating stress sharply. Lack of fusion, incomplete joint penetration and cracks are planar — flat, sharp-edged features that behave like pre-existing cracks in the structure.
The codes reflect this. Scattered porosity is tolerated within stated limits; cracks are generally not tolerated at all, in any size or location. Between those poles sit the planar fusion-type discontinuities, where the limits are tighter than for volumetric ones and where the orientation of the flaw relative to the applied stress matters as much as its size.
What the inspection could see
The examination method bounds what could have been found. Liquid penetrant testing under ASTM E1417 reveals only discontinuities open to the surface. Magnetic particle testing under ASTM E1444, with technique guidance in ASTM E709, finds surface and near-surface indications in ferromagnetic material and nothing deeper. Neither can detect a lack of fusion buried in the middle of a joint.
Radiography images volumetric discontinuities well and can miss a tight planar flaw whose plane is unfavourably oriented to the beam. Ultrasonic examination, with contact-method practice described in ASTM E164, is generally the more capable technique for planar discontinuities, but it depends far more heavily on operator technique, calibration and physical access.
Coverage matters as much as method
Many projects require volumetric examination of a stated proportion of joints rather than all of them, selected by category, criticality or sampling. A joint that was never examined has no inspection record and no finding, and the absence of a report is not evidence that the joint was sound. Reconstructing what proportion of the work was inspected, by which method, and how the sample was chosen is often more informative than the individual reports.
Acceptance was a snapshot in time
A weld accepted at fabrication was accepted in the condition it was in on the day it was examined. Hydrogen-assisted cold cracking develops after welding, which is why codes impose a delay before final examination on susceptible materials: inspect too early and a crack that will exist tomorrow does not exist yet. In-service mechanisms, meanwhile, may later initiate at a discontinuity that was entirely acceptable when it was made.
Rejectable is not the same as causal
This is the distinction most often lost in a dispute. Code acceptance criteria are workmanship and quality-control thresholds, set conservatively across a whole class of fabrications. They are not statements that a flaw exceeding them will fail, nor that one within them cannot.
Whether a particular discontinuity governed a particular failure is a fracture-mechanics and fitness-for-service question — flaw size and orientation, applied and residual stress, material toughness — answered by different methods from those that establish code compliance. A weld can be non-conforming and not causal, or fully compliant and still the origin.
The records that decide it
Beyond the weld itself: the contract documents identifying the governing code and edition, the examination procedures and technique sheets, calibration records and reference blocks, inspector qualification and certification, the radiographs or ultrasonic data as recorded rather than as summarised, weld maps identifying which joint is which, and the repair log. Where a weld was repaired, the repair is itself a weld, with its own procedure, its own thermal cycle and its own inspection requirement.
How this evidence is usually lost
Grinding out and re-welding a suspect joint is the ordinary, sensible response of a fabricator who wants to fix a problem, and it removes the discontinuity, its geometry, its position within the joint and the surrounding microstructure in a single operation. Radiographic film and ultrasonic data files have retention periods that expire. Preserving the joint as found, and requesting the raw inspection records early, costs very little at the point where it is still possible.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.