9.1 Relevant, Nonrelevant, and False Indications

The ASNT NDT Level II liquid penetrant general exam lists Inspection and Evaluation of Indications as official PT topic 3. Those items are not asking whether you can see a red smear. They are asking whether you can classify what you see before you size it, and whether you know which patterns a discontinuity can produce, which patterns geometry and roughness can produce, and which patterns the process itself can fake.

Liquid penetrant testing (PT) only reports what is open to the examination surface. Penetrant enters an opening by capillary action, excess is removed from the surface, and a developer draws the trapped liquid back out as a visible or fluorescent indication. That stain is evidence. It is not yet a defect. Evaluation starts by putting the stain in one of three classes the method standards and this guide use the same way magnetic particle testing does: relevant, nonrelevant, or false.

ASTM E1417 and ASME Section V, Article 6 tell you how to produce and view indications. They do not, by themselves, decide accept or reject. Classification is still the Level II's first duty, because a press-fit bleed and a fatigue crack can occupy the same square centimetre of developer.

Three classes, not two

Candidates who collapse the vocabulary into "real crack" versus "junk" miss items. The exam uses three boxes:

A discontinuity is an interruption in the typical structure of the material. An indication is what you see after developer dwell. A defect is a discontinuity that failed the acceptance standard. Those three words are not synonyms. Topic 3 punishes anyone who calls every red line a defect and anyone who calls every geometric collector false.

Classification is a process:

1. Confirm the process was valid — penetrant family, dwell, removal, developer, lighting, and temperature matched the procedure. A smear seen under a dim booth is not ready to classify.
2. Look at the surface with white light or a magnifier (an optical look) before you argue about the stain.
3. Ask whether the pattern follows a known feature for its entire length — thread helix, keyway corner, press-fit land, grit-blast texture.
4. If the class is still open, wipe and redevelop, then reclean and reprocess if needed.
5. Only then call the pattern relevant, nonrelevant, or false.

A nonrelevant collector can hide a real crack at the same location. Stamping "thread root" on every helix without looking for a radial extension is how fatigue cracks leave the booth.

Relevant indications the general exam expects you to name

Relevant means "caused by a discontinuity I must evaluate," not "automatically reject." The three families ASNT's PT outline and every shop procedure keep coming back to are cracks, pores open to the surface, and laps.

Cracks

Cracks are the primary PT target. Fatigue cracks start at a stress raiser — a weld toe, a fillet radius, a thread root, a corrosion pit — and grow under cyclic load. They are surface-initiated, often tight, and roughly perpendicular to the principal stress. Grinding cracks form when abusive grinding overheats a hardenable surface; they may appear as a network or as lines across the grind marks. Heat-treat (quench) cracks open at section changes during rapid cooling and may be oxide-filled until cleaning opens them. Crater cracks sit in weld stops. Heat-affected-zone and toe cracks sit along the fusion line.

Because PT requires an open path to the surface, a crack packed with oil, paint, peening smear, or oxide can be missed even though it is relevant in principle. Surface preparation is part of detectability, not a courtesy. A tight crack may first show as a faint rounded spot; that appearance problem belongs to the next section. For classification, a sharp, persistent, crack-like bleed that is not explained by geometry is relevant.

Pores open to the surface

Gas porosity, pinholes, and some sand holes in castings or weld metal produce relevant indications only when they break the examination surface. A spherical cavity sitting 2 mm under an intact skin is a radiographic or ultrasonic problem. When the pore is open, penetrant enters the cavity and developer draws out a rounded bleed. Isolated open pores are relevant and then compared with the rounded-indication table. A chain of open pores can become an aligned or accumulated condition the standard adds together.

Do not call subsurface porosity relevant just because the drawing mentions porosity. PT never saw that cavity.

Laps and other surface-open processing defects

A lap is folded metal that was forged or rolled shut without a true weld. When the fold reaches the surface, PT can enter the weak interface and show a linear or slightly ragged relevant indication, often along a die line or flow line. Related surface-open processing defects that behave the same way on a PT exam include open seams on bar, open cold shuts on castings, and surface-breaking hot tears. A lap that never reaches the surface is not a PT indication, relevant or otherwise.

Product form predicts the relevant family: weld toes and craters for cracks, weld metal and castings for open pores, forgings for laps, bar for seams, in-service shafts and fittings for fatigue. The specific exam writes stems that way.

Nonrelevant indications: geometry and permitted texture

Nonrelevant does not mean "ignore it and walk away." It means the cause is a known feature the acceptance standard does not treat as a discontinuity — unless a relevant extension leaves that feature.

Press fits. The interface of a press-fit, shrink-fit, or interference-fit assembly is a capillary. Penetrant enters the land and bleeds back as a continuous ring or band after developing. That band is nonrelevant if it stays in the fit and the procedure treats fit interfaces as geometric. A crack that leaves the land into the hub or shaft is a new, relevant indication.

Thread roots. A thread is a helix of sharp roots. Each root holds penetrant. After removal and developing you often see a helical or repeated-root pattern. That is nonrelevant geometric trapping when it follows the thread and stops at the thread. A radial or shank-wise extension that leaves the root is relevant. The same logic applies to tapped holes, spark-plug ports, and stud holes.

Keyways. The sharp corners and end radii of a keyway are classic collectors. Powder or penetrant sits in the corner for the length of the cut. Classify the corner pattern as nonrelevant when it does not leave the geometry. A crack at the keyway end radius that runs into the shaft is relevant — that radius is a fatigue site, so look twice.

Roughness bleed. Grit blast, coarse grind, heavy machine marks, and cast texture hold penetrant in the valleys. After removal you get a speckled or veined background that can look like a rash of tiny rounded indications. That roughness bleed is nonrelevant (or is treated as background the procedure says you must reduce) when it maps the texture and has no discrete, persistent discontinuity. The fix is process control — better pre-clean, a different remover method, or a smoother surface if the procedure allows — not a mass reject of every sparkle.

Other common nonrelevant collectors: the edge of a drilled hole, an abrupt section change, the toe radius of a sound weld with no crack-like extension, scribe lines, and the edge of a permitted coating. Magnetic writing has no PT analogue; do not import that MT term onto a penetrant booth.

False indications: the process lied

A false indication is not caused by a discontinuity and is not explained by intended geometry. The process, the soil, or a contaminant made a stain that looks like evaluation work.

Lint. Fibers from rags, clothing, or paper stick to a damp or developer-coated surface, pick up fluorescent or red penetrant, and draw a linear mark. Under UV-A a lint line can look more crack-like than a real tight crack. The optical look is decisive: you will see a fiber, or the line will move or vanish when the fiber is lifted.

Developer clumps. Dry powder that was allowed to cake, or a nonaqueous wet developer sprayed too close, leaves lumps. Each lump can look like a rounded pore. Clumps sit on the surface, often with a three-dimensional look the optical exam catches. They wipe away and do not return in the same spots after a controlled redevelop unless you spray the same clump again.

Incomplete removal. If water-washable or post-emulsified penetrant is under-washed, or if a solvent wipe is too timid, residual penetrant stays in surface texture and then blooms through the developer as streaks, smears, or a glowing background. That is false (or uncontrolled background), not a field of relevant pores. The opposite error — over-wash or aggressive solvent — can strip penetrant from a real crack and create a missed relevant indication, which is a detection failure rather than a false call.

Scale. Mill scale, heat-treat scale, rust flakes, and loose oxide hold penetrant in cracks of the scale, not of the metal. After developing you see linear or network stains that map the scale plates. An optical look and a clean-to-bare-metal reprocess distinguish scale false patterns from a quench crack in the steel. Scale can also mask a relevant crack, so the correction is reclean, not a shrug.

Related false sources: fingerprints that fluoresce, drain lines of wet developer on a horizontal face, paint chips, grease streaks that the pre-clean missed, and fluorescent contamination transferred from a dirty booth or from a previous part.

How to confirm a classification

The general exam likes "what should the Level II do next?" stems. Use this confirmation ladder. Stop when the class is clear; do not skip to a full reprocess if a fiber is sitting on the developer.

Optical look

Inspect the site with the lighting the procedure allows for visual work — often white light plus a 5× to 10× magnifier, or the optical aid the procedure names. You are looking for a fiber, a developer lump, a scale plate, a thread helix, a keyway corner, a pore mouth, or a tight crack opening. Many false and nonrelevant calls die here. A Level II who never takes the UV lamp away from a Type I indication is classifying glow, not metal.

Wipe and redevelop

If the procedure allows a local check, gently wipe the indication with a dry or slightly solvent-dampened cloth as the procedure specifies, then reapply developer and watch the same spot. A relevant discontinuity bleeds again in the same place. Lint and clumps do not rebuild a discontinuity path. Incomplete-removal smear may reappear as a broad blush rather than a sharp, repeatable line. Do not scrub a tight crack so hard that you extract all residual penetrant and then declare the part clean — the wipe is a check, not a second remover station.

Reclean and reprocess

If wipe-and-redevelop is still ambiguous, or if scale, soil, or incomplete removal is suspected across a larger area, reclean to the procedure's surface-prep rules and run the full PT sequence again: penetrant, dwell, removal, developer, lighting, evaluation window. A relevant crack, open pore, or lap returns. A false pattern that came from last cycle's leftover penetrant or from a clump does not. A nonrelevant geometric collector returns in the geometry, which confirms the class rather than promoting the pattern to relevant.

If a geometric collector might hide a crack, the procedure may allow a light blend or local machine of the radius (within remaining-thickness rules) and a reprocess. That is still classification work. It is not permission to grind away a relevant indication so the report can say "none."

Realistic exam scenarios

A helical fluorescent line follows a bolt thread for 360° and stops at the last thread. No radial tail enters the shank. Optical look shows the root, not a fiber. Classification: nonrelevant geometric trapping. If a sharp line leaves the root into the shank and returns after wipe-and-redevelop, that extension is relevant.

A rounded red spot sits on a weld face. Under white light you see a developer lump, not a pore mouth. The spot wipes off and does not return after a controlled redevelop. Classification: false developer clump.

A linear stain on a forged boss follows a die line, returns after reclean-and-reprocess, and optical exam shows a fold, not lint. Classification: relevant lap. Disposition waits for the acceptance table in the next sections.

A grit-blasted plate is a field of tiny sparkles that map the blast texture and fade into background. Classification: nonrelevant roughness bleed (or uncontrolled background you must reduce). Do not reject "a thousand pores" you cannot resolve as discrete discontinuities.

Topic 3 rewards slow language: classify, confirm, then measure. Speed that skips the optical look is how lint becomes a crack on a report.