Magnetic Particle Inspection

Magnetic Particle Inspection (manufacturing-quality/ndt/magnetic-particle-inspection)

Use when the task is executing magnetic particle inspection (MT) on a ferromagnetic part: selecting the magnetizing current for circular and longitudinal magnetization, verifying the field strength and coverage overlap, classifying the magnetic particles by sensitivity, and turning the observed indications into acceptance decisions.

Domain quick reference

All numbers below were computed and verified by running the logic module in scripts/.

• Head shot current (circular magnetization): I = amperes_per_inch * D, with D the part diameter in inches. Common practice is 300 to 800 A per inch of diameter. A 2.0 in shaft needs 1600 A at 800 A/in and 600 A at 300 A/in.
• Central conductor current: the same current-per-inch rule applies to the conductor diameter, because the bore field is set by the conductor radius. A 1.0 in conductor at 800 A/in carries 800 A and magnetizes the bore of any part OD it threads.
• Effective diameter of a hollow part: D_eff = sqrt(OD^2 - ID^2). A 2.0 in OD, 1.0 in ID sleeve behaves as a 1.732 in solid bar for L/D.
• Effective L/D: L/D = length / D_eff. An 8.0 in long sleeve has L/D = 4.62. Coil magnetization requires 2 <= L/D < 15; below 2, add pole pieces or stack parts; at or above 15, only the central portion is magnetized, so magnetize in sections or use another technique.
• Low fill factor coil: NI = 45000 / (L/D). At L/D = 4.62 the coil needs 9743 ampere-turns, and a 250 turn coil runs at 39 A (9743 / 250 = 38.97 A). At L/D = 4 the coil needs 11250 ampere-turns.
• High fill factor coil: NI = 35000 / (L/D + 2). At L/D = 4 the coil needs 5833 ampere-turns, far fewer than the low fill value of 11250, because the flux path is shorter when the part fills the coil.
• Field strength: H = NI / L for a long solenoid, in A/m. 1000 ampere-turns over a 0.25 m coil gives 4000 A/m, inside the wet fluorescent band of 2400 to 4800 A/m (30 to 60 oersted). The wet visible band is 2400 to 3200 A/m. Verdicts: 2000 A/m is low, 4000 A/m is adequate for fluorescent, 5000 A/m is high for fluorescent, and 4000 A/m is high for visible.
• Coverage: step = width * (1 - overlap), with 10 to 15 percent overlap common between adjacent shots. A 0.2 m magnetization zone at 15 percent overlap advances 0.17 m per pass, so a 0.5 m shaft needs 3 passes to cover its length.
• Particle classification: median diameter classes are extra-fine below 10 um, fine 10 to 20 um, medium 20 to 35 um, and coarse 35 um and above. Sensitivity follows size: high below 20 um, standard 20 to 35 um, low 35 um and above. An 8 um particle is extra-fine with high sensitivity; a 45 um particle is coarse with low sensitivity.
• Bath concentration: wet fluorescent 0.1 to 0.4 mL of concentrate per 100 mL of carrier, wet visible 1 to 2 mL per 100 mL. A 0.2 mL/100 mL fluorescent bath is within range; 1.5 mL/100 mL is within range only for the visible method.
• Indication class: linear when length / width >= 3. A 6.0 mm by 1.5 mm indication has ratio 4.0 and is linear; a 4.0 mm by 2.0 mm indication is not.
• Acceptance: a relevant 4.0 mm indication against a 3.0 mm limit is rejected, a 2.0 mm indication is accepted, and a non-relevant indication is evaluated, never auto-rejected.
• Residual field: demagnetize when the residual field exceeds the limit, commonly 3 A/m. A 5 A/m residual fails the check, 2 A/m passes.
• Defect to direction: a longitudinal (axial) defect is detected by circular magnetization; a transverse (circumferential) defect by longitudinal magnetization. Production parts are magnetized in both directions, 90 degrees apart, so every defect orientation is cut by a field.

Workflow

1. Confirm the part is ferromagnetic and name the defect class: a longitudinal (axial) defect needs circular magnetization, a transverse defect needs longitudinal magnetization.
2. For circular magnetization by head shot, compute the current with head_shot_current (300 to 800 A per inch of diameter typical). For a bore, use central_conductor_current on the conductor diameter.
3. For longitudinal magnetization by encircling coil, compute the effective diameter with effective_diameter_hollow (hollow parts), the L/D with effective_ld_ratio, the ampere-turns with coil_ampere_turns_low_fill or coil_ampere_turns_high_fill by fill factor, then the coil current with coil_current_from_turns.
4. Verify the field with solenoid_field_strength and tangential_field_verdict against the fluorescent band of 2400 to 4800 A/m (or 2400 to 3200 A/m visible), and plan the coverage with coverage_step at 10 to 15 percent overlap.
5. Classify the particle with particle_size_class and particle_sensitivity from the median particle size, and check the bath with bath_concentration_check.
6. Interpret the indications: classify linear versus rounded with indication_linear_ratio and indication_is_linear, mark each indication relevant or non-relevant, and disposition with acceptance_verdict against the engineering acceptance limit.
7. After testing, check the residual field with residual_field_verdict and demagnetize when required. Record the results under NAS 410 qualified personnel and the qualified procedure.

Pitfalls

• Confusing the two magnetization directions: a longitudinal (axial) defect is detected by circular magnetization, whose field circles the part; the encircling coil produces an axial field that detects transverse defects. Magnetizing in only one direction leaves every defect of the other orientation undetected.
• Using the part OD instead of the conductor diameter for central conductor magnetization: the bore field is set by the conductor radius, so the current-per-inch rule applies to the conductor, not to the part.
• Ignoring the L/D limits of coil magnetization: below L/D 2 the coil field is too weak and pole pieces or stacked parts are required, and at or above L/D 15 only the central portion is magnetized; the ampere-turns functions raise ValueError outside the band.
• Confusing MPI with liquid-penetrant-inspection: penetrant finds surface-breaking discontinuities in any material through capillary action and dwell time, while MPI finds surface and near-surface discontinuities in ferromagnetic materials only, driven by magnetization current and field, with no dwell time and no bleed-out sizing.
• Confusing MPI with eddy-current-inspection: eddy current works on any conductor, senses subsurface flaws through depth of penetration and the impedance plane, and needs no magnetizing current and no particle bath; MPI requires ferromagnetic material and produces visible particle indications.
• Treating a non-relevant indication as a reject: indications from threads, sharp section changes, or magnetic writing are recorded and evaluated; acceptance_verdict returns 'evaluate' for them, never 'reject' or 'accept' on length alone.
• Over-magnetizing: a field above the band collects particles into false background indications; tangential_field_verdict flags 'high' and the magnetizing current must be reduced before judging indications.
• Forgetting the residual field check: a part that still attracts chips or debris after testing must be demagnetized and re-checked; residual_field_verdict returns 'demagnetize' above the limit.

Behavior contract (gate 3)

The inspection math is exercised by the gate 3 contract test: scripts/test_magnetic_particle_inspection.py against scripts/magnetic_particle_inspection_logic.py (stdlib unittest, offline). Run: python3 scripts/test_magnetic_particle_inspection.py

Compliance

• Standards referenced, not reproduced: AS9100 clause 8.5.1.3 frames NDT as a special process requiring controlled procedures and qualified personnel, and NAS 410 sets the qualification and certification requirements for the NDT personnel who execute magnetic particle inspection; the current-per-inch, ampere-turns, field band, coverage, and acceptance calculations above are common MT methodology, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.