6.1 Discontinuity Classes

Discontinuity, Defect, Flaw, and Indication

A discontinuity is any interruption in the normal physical structure or configuration of a part — a void, a crack, an inclusion, or a change in geometry. It is a neutral, descriptive term. A discontinuity becomes a defect only when its type, size, location, or number causes the part to fail the applicable acceptance criteria of a code or specification. A flaw is treated as a synonym for discontinuity in many standards, while an indication is the response an NDT method produces — a bleed-out in penetrant testing, a signal on an ultrasonic screen — that must then be interpreted and evaluated.

A Level III must keep these terms distinct. Reporting a rounded pore as a "defect" pre-judges the acceptance decision that belongs to the code, and treating every "indication" as a real discontinuity without confirmation leads to false rejects. The chain is always indication → discontinuity → (evaluate against criteria) → defect or accept.

Classifying by Origin: Inherent, Processing, Service

The classic ASNT origin scheme groups discontinuities by when in the material's life they form. This matters because origin predicts a discontinuity's likely location, orientation, and shape, which in turn narrows the method choice.

Inherent discontinuities

Inherent discontinuities split into inherent wrought and inherent cast. Inherent wrought discontinuities live in the ingot that will later be rolled or forged: pipe (a shrinkage cavity that forms at the top center of the ingot as it freezes), blowholes, and nonmetallic inclusions. If the ingot is not cropped, these are carried into and elongated by the working operation. Inherent cast discontinuities form in the shaped casting itself as the molten metal freezes — gas porosity, shrinkage, cold shut, and hot tears.

Processing discontinuities

Processing discontinuities divide into primary (bulk working — rolling, forging, extrusion) and secondary (finishing and joining). A key exam nuance: a lamination is really an inherent condition (pipe, blowhole, inclusion, or segregation) that is flattened and spread out parallel to the surface during rolling — so it is frequently listed under both inherent origin and primary processing. Welding is a secondary/joining source with its own family: gas porosity, slag inclusions, lack of fusion, incomplete penetration, undercut, and cracks (longitudinal, transverse, crater, and heat-affected-zone).

Service discontinuities

Service discontinuities appear only after the component is put to work. Fatigue cracks initiate at stress concentrations under cyclic loading and are usually surface-originating. Stress-corrosion cracking (SCC) is branched cracking from the combination of sustained tensile stress and a corrosive environment. Creep is slow deformation and grain-boundary cavitation at high temperature. Hydrogen-induced cracking and general corrosion/erosion crack or thin the wall over time.

Classifying by Geometry: Surface vs Subsurface, Planar vs Volumetric

The second axis — geometry — is what actually decides which method can see the discontinuity.

• Surface (surface-connected) discontinuities open to an accessible surface: fatigue cracks, grinding cracks, seams, surface porosity. They are reachable by VT, PT, MT, and ET.
• Subsurface (internal) discontinuities lie wholly below the surface: internal porosity, shrinkage, forging bursts, flakes, mid-wall lack of fusion. Only the volumetric methods UT and RT reach them.
• Planar (two-dimensional) discontinuities — cracks, laminations, lack of fusion — have negligible thickness. They reflect sound strongly when a beam strikes them squarely, so UT excels, but they can be nearly invisible to RT when oriented parallel to the beam.
• Volumetric (three-dimensional / rounded) discontinuities — porosity, shrinkage, slag — have width in all directions and produce a measurable density change, so RT excels.

The linear vs rounded shape call feeds directly into acceptance criteria: a common code rule labels an indication linear when its length-to-width ratio is 3:1 or greater, and rounded below that. Codes usually treat linear indications more severely because cracks and lack of fusion are far more likely to propagate than isolated pores.

Putting Origin and Geometry Together

Origin tells you where to look and how the discontinuity is likely oriented; geometry tells you which method physics can detect it. A forging lap (primary-processing origin, surface-connected, tight and oblique to the surface) is best found with MT on ferromagnetic steel, the field crossing the fold — not with RT, which is defeated by the tight planar geometry. A casting's internal shrinkage (inherent-cast, subsurface, volumetric) is a natural RT target. Keeping this two-axis map in mind prevents the classic Level III trap of choosing a method that the discontinuity's geometry defeats.

A Common Naming Trap

Several discontinuities look similar but come from different origins, and the exam rewards precise naming. A cold shut is an inherent-cast fusion failure between two streams of molten metal in a mold, whereas lack of fusion is a welding discontinuity at the fusion face between weld metal and base metal. A lap is a forging or rolling fold of surface metal, while a lamination is an inherent flaw flattened parallel to the rolled surface. Same-sounding words, different origins, different orientations — and therefore different best-detection methods.