Learning Objectives

By the end of this lesson, the technician will be able to:

• Objective 1 (Cognitive/Understanding): Explain how liquid penetrant testing (PT) and magnetic particle testing (MT) work, what each can and cannot find (surface vs. near-surface, material restrictions), and where the other NDT methods (UT, RT) take over.
• Objective 2 (Cognitive/Understanding): Describe the PT process steps — clean, apply, dwell, remove, develop, evaluate — and the MT principle of flux leakage, well enough to support certified examinations and to perform informational screening correctly.
• Objective 3 (Field/Practical): Apply NDT in the technician's lane — informational screening of suspect areas (with honest limits), evidence preservation, supporting certified NDT personnel — and state the governance rule: code-required examinations are performed and interpreted by certified NDT personnel (SNT-TC-1A/ASNT framework), never by field screening.

Field Scenario

The Crack That Was Three Times Longer Than It Looked: The toe crack from Lesson 9.7's screen frame — the 2-inch hairline found with raking light — is scheduled for repair. Before the engineering disposition arrives, a technician runs an informational dye penetrant check to give the repair planning honest dimensions (authorized as screening; the formal pre-repair examination will be done by certified NDT).

The process is humble and exact. Clean: solvent-wipe the area bare — penetrant cannot enter a crack full of paint, oil, or brushing debris (and here the 9.7 warning pays off: nobody wire-wheeled the crack, which would have smeared metal over its mouth and blinded the test). Apply: red dye penetrant sprayed over the area. Dwell: ten minutes by the watch — capillary action needs time to pull dye into the flaw. Remove: careful solvent-dampened wipes — never flooding solvent onto the surface, which washes dye back out of the flaw. Develop: a thin, even coat of white developer, which acts as a blotter, pulling trapped dye back out to bloom against the white.

The bloom redraws the map: the "2-inch" crack bleeds out as a red line just over 6 inches long — its ends had tightened to widths the eye and pick couldn't catch, but capillary action could. The repair scope, the excavation length, and the engineering conversation all change. The certified examination later confirms the PT picture and adds MT's contribution on this magnetic steel: slight subsurface continuation past one visible end.

The lesson layered on 9.7's: the eye finds the middle of cracks; NDT finds their ends — and repairs scoped to visible length excavate the middle of a flaw and weld its tips back in. Screening informs; certification decides; and the cheap red-and-white spray cans, used with discipline, are among the highest-value tools a technician carries.

Concept Overview

Where PT and MT Sit in the NDT Family

Visual inspection (9.7) reads the surface the eye can resolve. The flaw population continues below that threshold and below the surface:

• PT — Liquid Penetrant Testing: finds surface-breaking flaws only — but finds them far finer than the eye, on any non-porous material (steel, stainless, aluminum, bronze — the non-ferrous world MT can't touch).
• MT — Magnetic Particle Testing: finds surface and slightly subsurface flaws — but only on ferromagnetic materials (carbon/alloy steel yes; austenitic stainless 300-series and aluminum no — 9.1's magnet lesson deciding test selection).
• UT — Ultrasonic Testing: sound waves interrogate the interior — wall thickness (9.4's corrosion surveys), laminations, and the buried LOF/LOP and subsurface cracks that 9.7 admitted it cannot see. Certified-operator territory, heavy on interpretation skill.
• RT — Radiographic Testing: X-ray/gamma imaging of the interior — the CJP weld's classic examination (9.6). Licensed, controlled-area, certified-personnel work; the technician's involvement is exclusion-zone respect and support.

How PT Works — Capillary Action as Instrument

A low-surface-tension dye is drawn by capillary action into any opening that breaks the surface; after surface dye is removed, a developer wicks the trapped dye back out, spreading it into an indication far wider than the flaw — which is the amplification that makes invisible flaws visible. Two families: visible (red dye) — field-portable spray cans, white developer, daylight evaluation; fluorescent — UV-evaluated, more sensitive, mostly shop/controlled-condition work by certified personnel. The discipline points, each tied to a failure mode of the method:

• Pre-cleaning is the test: contaminated or smeared-over flaws don't take penetrant — and mechanical cleaning (grinding, wire-wheeling, sandblasting) can close flaw mouths by smearing metal (peening effect); solvent cleaning is the default, and surfaces that were mechanically worked may need etching (certified-level call) to reopen flaws.
• Dwell time is non-negotiable (typically 5–15+ minutes per the penetrant's data sheet and the temperature — cold surfaces slow capillary action; very hot surfaces flash the carrier off): impatience reads as "no flaws."
• Removal technique decides sensitivity: wipe with solvent-dampened cloth toward the flaw-free direction; flooding solvent over the surface rinses flaws empty and manufactures clean results.
• Developer thin and even: too thick masks fine indications; the bloom is watched as it develops (timing per data sheet), and indication shape speaks: a continuous line = crack-like; a dotted line = porosity string or a tight crack surfacing intermittently; rounded spots = pores.
• Temperature and product compatibility: penetrant families have working temperature ranges, and on nickel alloys/stainless in certain services, low-sulfur/low-halogen penetrants are specified (contamination chemistry — the spec or NDT procedure governs).

How MT Works — Flux Leakage as Instrument

Magnetize the part; magnetic flux flows through the steel like current through a conductor. A crack (or near-surface void) interrupts the flux, forcing some of it to leak out of the surface at the flaw — and finely divided iron particles (dry powder or wet fluorescent bath) pile up at the leakage like filings at a magnet's pole, drawing the flaw on the surface. Field realities:

• Equipment: the electromagnetic yoke (the field-standard tool — two legs, energized between them), prods (current injection — arc-burn risk makes them disfavored on finished surfaces), permanent-magnet yokes for no-power situations.
• Direction matters — the method's defining discipline: flux leakage is strongest when the flaw lies perpendicular to the flux lines; a crack parallel to the flux barely leaks and barely shows. Therefore proper MT examines every area twice, with the field rotated ~90° between shots (yoke legs repositioned). A single-orientation MT pass is a half-test that can miss a major crack completely.
• Sensitivity and depth: excellent on surface cracks (often through thin paint films, though clean metal is better); detects slightly subsurface flaws (the Field Scenario's continuation) — a capability PT entirely lacks; AC fields favor surface sensitivity, DC reaches marginally deeper.
• Material restriction is absolute: ferromagnetic only. The 9.1 magnet check is literally the method-selection test.
• Demagnetization: parts can retain magnetism after MT — a problem for bearings, machining, and instrumentation neighborhoods (Modules 4/6); the procedure includes demag where residual fields matter.

The Governance Layer — Certification Is the Method's Other Half

NDT's value is only as good as its interpretation, which is why the framework exists: ASNT SNT-TC-1A / CP-189 certification levels (Level I performs under supervision, Level II interprets and accepts/rejects to procedure, Level III owns procedures), written procedures per method, and code-required examinations (the "MT all CJP welds" notes of 9.6, pressure-work exams of 9.4) executed by certified personnel with documented reports. The technician's screening — the Field Scenario's informational PT — is legitimate and valuable as screening: it scopes problems, preserves evidence, sharpens engineering questions. It never substitutes for, and is never represented as, the certified examination.

How the Principle Works: Method Selection and the Screening Discipline

Selection logic (the two questions):

1. Is the material magnetic? (9.1's check) — No → PT (or UT/RT for interior). Yes → MT available, PT also valid.
2. Where could the flaw live? Surface-breaking suspicion (toe cracks, fatigue, grinding checks) → PT/MT. Interior suspicion (CJP soundness, LOF/LOP, corrosion wall loss) → UT/RT — certified work by definition.

The screening protocol (technician's lane, done right):

1. Authorization and purpose stated: screening to inform — recorded as such (10.8), with the formal examination's place in the plan untouched.
2. Evidence preserved: no grinding/wheeling the suspect area (9.7's rule, now with the smearing mechanism explained); photographs before chemicals.
3. Process executed per the product's data sheet — the steps of Section 3, with the dwell timed and the removal disciplined; MT screening includes the two-orientation rule or it's noted as a single-axis look.
4. Indications documented, not adjudicated: photographed with scale while developed (PT blooms fade and smear — shoot promptly), extents marked on the metal beyond the indication ends, dimensions recorded. The report language is "indication observed," with interpretation/acceptance left to the certified examination and engineering (a relevant indication vs. a non-relevant one — weld toe geometry, scratches — is itself a Level II call on code work).
5. Cleanup with intent: penetrant residues removed per product guidance (and per service compatibility — food/oxygen service has its own rules); MT demag where downstream equipment cares.
6. Boundaries threaded through: chemicals per SDS (1.9 — solvents, aerosols, ventilation); hot surfaces vs. product temperature ranges; the equipment's own safe condition (isolation where applicable — Module 1); and on anything in the 9.10 categories, the finding routes exactly as 9.7 prescribed, now with better dimensions attached.

NDT Method Reference Matrix

Field Application Checklist: Informational PT/MT Screening of a Suspect Area

1. Confirm the lane: screening authorized and labeled as informational; code-required examination identified separately in the plan (certified personnel, procedure, report). Findings on 9.10-category items follow 9.7's routing regardless of screening results.
2. Boundaries: SDS review for the chemical kit (1.9), ventilation/ignition awareness with aerosols and solvents (hot-work adjacency — 9.5), equipment in a safe state per Module 1, surface temperature vs. product range.
3. Select the method by material and flaw hypothesis (Section 4's two questions; the 9.1 magnet check in hand).
4. Preserve before testing: photos as-found, no abrasive prep on the suspect zone; solvent-clean only.
5. Execute per data sheet — PT's five steps with the dwell timed; MT with two orientations and particle application per the yoke procedure.
6. Record indications as evidence: scaled photos during development, extents marked beyond indication ends, dimensions and locations logged (10.8); language disciplined to "indication," not verdicts.
7. Clean up per product and service requirements; demag where applicable; restore the area's condition notes.
8. Deliver the package — as-found photos, screening method/products used, indication record — to the engineering/certified-NDT chain, and capture the whole event in the work order with the formal examination's reference when it lands. The screening's value is measured by how much sharper it made the real examination's scope.

Knowledge Check

Source List

• ASTM E165 / E1417 — liquid penetrant examination standard practices
• ASTM E709 / E1444 — magnetic particle examination standard practices
• ASNT SNT-TC-1A / CP-189 — NDT personnel qualification and certification framework
• AWS D1.1 / ASME Section V — code examination requirements invoking PT/MT/UT/RT (with 9.4/9.6/9.7)
• Lessons 1.9, 9.1, 9.7, 9.10, 10.8 cross-references — chemical safety, material identification, visual screening, repair limits, and evidence documentation