| 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
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
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
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
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
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
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).
