magnetic-particle-inspection

name	magnetic-particle-inspection
description	Magnetic particle inspection (MPI/MT) — magnetization methods (circular/longitudinal, AC/HWDC/FWDC), flux density requirements (B=1.5-2.5T for ferromagnetic), current level selection (π×D×800 A/m rule), wet vs dry particles, fluorescent vs visible, demagnetization (coil method), ASTM E1444/E709 standards, MIL-STD-1949, acceptance criteria, prod spacing, coil shot, yoke testing, discontinuity orientation sensitivity, field strength measurement (Hall probe, pie gauge).
metadata	{"priority":7,"promptSignals":{"phrases":["magnetic particle","MPI","magnetic testing","MT inspection","ASTM E1444"],"minScore":3}}

Magnetic Particle Inspection — Complete Skill

Fundamentals

Magnetization Physics

Principle: Ferromagnetic part magnetized → magnetic flux lines flow through part → surface/near-surface discontinuity interrupts flux → flux leaks at discontinuity → magnetic particles attracted to leakage field → visible indication

Leakage flux field (Hd at crack): H_d ≈ 2Bw×l/(μ₀×(l+w)²) [simplified; depends on crack aspect ratio, orientation, depth] Where: w = crack width; l = crack length (along field direction); B = flux density in part

Optimal field orientation: Field perpendicular to discontinuity gives maximum leakage → maximum particle attraction Minimum detectable: discontinuity ≥ 45° to applied field (sensitivity drops as angle decreases) Standard practice: magnetize in two perpendicular directions to ensure detection of all orientations

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Magnetization Methods

Circular Magnetization (Detects Longitudinal Flaws)

Direct current through part (head shot): I_circular = π × D × H_required For H_required = 2400 A/m (typical): I = π × D × 2400 [A; D in meters] Simplified rule: I = 300–800 A per 25 mm of diameter [lower end for solid bar; higher for hollow/complex]

Central conductor (for hollow parts/tubes): Current through central bar; circumferential field in tube wall I_central = π × D_OD × H_required [same formula as direct; field uniform through wall]

Prod technique: Prods placed on surface; current flows between prods → circular field around prod area Prod spacing: 75–200 mm (3–8 inches) [ASTM E1444 limit: ≤ 200 mm] Current: 3.5–4.5 A/mm of prod spacing [e.g., 100 mm spacing → 350–450 A] Limitation: arc burns at prod contact; not allowed on finished machined surfaces; not for pressure vessel internals where arc burn unacceptable

Longitudinal Magnetization (Detects Transverse Flaws)

Coil magnetization: Field inside coil (along coil axis): H_coil = N × I / L_coil [N = turns; I = current; L = coil length, m] Effective longitudinal range: within 6× coil radius of coil center For parts L/D > 3: use multiple shots with coil repositioned

Coil shot ampere-turns (ASTM E1444): High L/D (L/D > 4): NI = (35,000)/(L/D) [NI in A·turns; L = part length; D = part diameter] Low L/D (L/D 2–4): NI = 45,000/(L/D) Minimum NI = 1000 A·turns; Maximum NI = 10,000 A·turns (for most coil sizes)

Yoke (electromagnetic horseshoe): AC yoke: lifting force ≥ 4.5 kg (10 lb) [ASTM E1444 minimum] DC/HWDC yoke: lifting force ≥ 18 kg (40 lb) Yoke leg spacing: 75–200 mm; field coverage typically 25 mm each side of yoke axis

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Current Type Selection

Current Type	Penetration	Best For
AC (60 Hz)	Surface only (skin effect)	Surface cracks in smooth parts
HWDC	Medium (~3 mm)	Slightly subsurface
FWDC	Best (≥3 mm)	Subsurface, castings, forgings
Permanent magnet	Surface	Field use, no power available

Skin depth for AC in steel: δ_steel = 503/√(60 × 10 × 100) = 503/244.9 = 2.05 mm → AC confined to outer 2 mm → good for surface only HWDC: pulsing DC → deeper penetration due to lower effective frequency

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Magnetic Particles

Wet Method

Fluorescent particles (Type 1): Iron oxide particles coated with fluorescent dye; viewed under UV-A lamp (365 nm, min 1000 μW/cm²) Bath concentration: 0.1–0.4 mL/100 mL (ASTM E1444 per centrifuge tube) Sensitivity: superior — can detect tight cracks and fine defects; preferred for aerospace/critical applications Bath check: verify concentration per ASTM E1444 Appendix A1 (centrifuge tube settling)

Visible (non-fluorescent) particles (Type 2): Red/black iron oxide; viewed in white light ≥ 500 lux Concentration: 1.2–2.4 mL/100 mL Use when fluorescent not practical (outdoors, no UV lamp available)

Particle carrier fluid: Water-based (conditioned water + rust inhibitor + wetting agent): cheaper, less flammable Oil-based (petroleum distillate): slower evaporation; use where water may cause corrosion; lower flash point concern

Dry Method

Dry particles: iron filings; applied by blower; viewed in visible light Use for: rough surfaces (castings), elevated temperature parts (up to 316°C), welds in field Limitation: less sensitive than wet fluorescent; not suitable for smooth machined surfaces

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Field Strength Measurement

Hall Effect Probe (Gaussmeter)

Direct B measurement: Calibrated Hall probe placed against surface; reads tangential B (field parallel to surface above discontinuity) Target: B = 1.5–2.5 T in part (typical ferritic steel) → tangential surface field ≈ 30–60 gauss (3–6 mT) measured externally Use: verify magnetization level; map field uniformity

Quantitative Quality Indicator (QQI / Pie Gauge)

Pie gauge: thin foil with artificial crack pattern; placed on part surface; reveals field direction and strength ASTM E1444: pie gauge (8-sector) placed on part; particles should form indication on shim if field ≥ 1600 A/m tangential Field indicator: confirms direction and adequacy — cannot substitute for gaussmeter but quick field check

Magnetic flux density requirement: H_tangential ≥ 2400 A/m (30 Oe) minimum at part surface (ASTM E1444) H_tangential ≥ 9600 A/m (120 Oe) maximum (above this: saturation, particles can migrate under strong field) Sweet spot: 2400–9600 A/m

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Demagnetization

Why Demagnetize

Residual magnetism > 3 gauss causes: interference with instruments, attracts metal chips during machining, arc blow during welding Required when: residual field > 3 gauss (0.3 mT); always for machined surfaces and assemblies

Demagnetization Methods

Coil demagnetization (most effective): Pass part through AC coil while slowly withdrawing to ≥ 1 m from coil → Barkhausen domain cycling at decreasing amplitude → residual approaches zero Or: apply AC through coil and gradually reduce current to zero with part inside coil

DC step-down demagnetization: Apply DC (same current as magnetizing), reverse polarity, reduce amplitude step-wise to zero Each reversal + reduction → progressively smaller hysteresis loop → residual → 0

Verify: Hall probe residual field < 3 gauss after demagnetization

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Acceptance Criteria and Standards

Indications

Relevant vs. non-relevant: Relevant indication: caused by actual discontinuity (crack, seam, lap, cold shut, lack of fusion) Non-relevant: magnetic permeability variations (grain boundaries, heat treat lines), false geometry Inherent indications: part geometry changes (keyways, holes, threads) — mask with tape or compensate

ASTM E709 terminology: Linear indication: length ≥ 3× width Rounded indication: length < 3× width Critical indications: any linear indication, rounded indication > 5 mm (1/4 in) in aerospace, or as per code

ASTM E1444 acceptance (aerospace forgings, default): No linear indications > 1.6 mm (1/16 in) length No more than 10 rounded indications in any 15 cm² area; no rounded indication > 3.2 mm (1/8 in) Adjust per engineering drawing or customer specification

Standards

Standard	Scope
ASTM E709	Standard guide for MPI technique selection
ASTM E1444	Standard practice for MPI (aerospace)
MIL-STD-1949B	Military MPI requirements
ASME V Article 7	MPI for ASME pressure equipment
AWS D1.1	Weld inspection (includes MT acceptance)
SAE AMS 2641	Vehicle/bath requirements
ISO 9934-1	MPI general principles (EN equivalent)
ASNT SNT-TC-1A	MT operator certification (Levels I, II, III)

Output

Provide: part description (material: ferromagnetic? yes/no; geometry: solid/hollow; dimensions [mm]; surface condition: machined/as-cast/coated), magnetization method (circular: I = π×D×2400 [A] or prod spacing [mm] → I [A]; longitudinal: NI = 35,000/(L/D) [A·turns]; yoke: leg spacing [mm]; lifting force verified ≥ 4.5 kg AC / 18 kg DC), current type (AC/HWDC/FWDC and rationale: surface vs. subsurface target), particle type (wet fluorescent vs. visible; fluorescent UV-A intensity ≥ 1000 μW/cm²; bath concentration [mL/100 mL]; visible ambient ≥ 500 lux), field verification (pie gauge indication: yes/no at required positions; gaussmeter: H_tangential [A/m], in range 2400–9600 A/m?), demagnetization (residual field post-test [gauss]; method: coil withdrawal/step-down; verified < 3 gauss?), indications (location; linear or rounded; dimensions [mm]; relevant?; depth estimated from indication characteristics), acceptance (per ASTM E1444 / ASME V / drawing; accept or reject per applicable code), and applicable standard (ASTM E1444 for aerospace; ASME V Article 7 for pressure equipment; AWS D1.1 for welds).