8.3 Indication Classification: Relevant, Non-Relevant, and False

8.3 Indication Classification: Relevant, Non-Relevant, and False

Systematic Three-Way Indication Classification Framework

In Magnetic Particle Testing (MT), any visible accumulation of magnetic particles held upon the surface of a magnetized component is defined as an indication. However, the mere presence of a particle accumulation does not automatically indicate a flaw, material defect, or reason for component rejection.

To ensure consistent and accurate non-destructive evaluation, governing engineering standards (including ASTM E1444, ASTM E709, ASME Boiler and Pressure Vessel Code Section V Article 7, and NAVSEA T9074-AS-GIB-010/271) establish a rigorous three-way classification framework:

1. Relevant Indications: Particle accumulations produced by magnetic flux leakage originating from unintentional mechanical discontinuities, structural flaws, or material separations (e.g., fatigue cracks, quench cracks, forging laps, lack of fusion). Relevant indications must be evaluated against engineering acceptance criteria to determine if they constitute rejectable defects.
2. Non-Relevant Indications: Particle accumulations produced by genuine magnetic flux leakage, but where the leakage field originates from intentional design geometry, cross-sectional changes, or localized metallurgical variations in magnetic permeability rather than structural flaws or mechanical voids (e.g., sharp keyways, magnetic writing, cold-worked stamping, dissimilar weld buttering, grain flow lines).
3. False Indications: Particle accumulations that are NOT caused by magnetic flux leakage. These accumulations result entirely from mechanical entrapment, surface friction, gravity sagging, electrostatic attraction, or chemical contamination (e.g., particle buildup in deep machining grooves, mill scale crevices, drainage puddles, or fluorescent lint).

                                  MAGNETIC PARTICLE INDICATION
                                                │
                       Is the accumulation held by magnetic flux leakage?
                                         ┌──────┴──────┐
                                        YES            NO
                                         │              │
                         Is leakage caused by a         ▼
                         mechanical discontinuity?  FALSE INDICATION
                                  ┌──────┴──────┐   (Mechanical entrapment,
                                 YES            NO   rough grooves, lint, sag)
                                  │              │
                                  ▼              ▼
                         RELEVANT INDICATION   NON-RELEVANT INDICATION
                         (Cracks, laps, seams,  (Magnetic writing, fillets,
                          lack of fusion)        cold work, cladding)
                                  │
                      Does it exceed code criteria?
                                  ┌──────┴──────┐
                                 YES            NO
                                  │              │
                                  ▼              ▼
                                DEFECT       ACCEPTABLE
                             (Rejectable)   DISCONTINUITY

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Relevant Indications: Signatures and Evaluation

Physical Nature

A relevant indication occurs when a mechanical interruption in the physical continuity of the material diverts magnetic flux lines across an air or non-metallic gap. Because the relative magnetic permeability of air ($\mu_r = 1$) is hundreds to thousands of times lower than that of ferromagnetic steel ($\mu_r = 200\text{ to }2000$), flux lines bridge the gap through the air above the surface, forming an intense leakage field with opposing north and south magnetic poles.

Distinctive Signatures

• Particle Retention: Ferromagnetic particles are held tightly to the surface by magnetic dipole forces ($F \propto \nabla(B^2)$). Relevant indications resist gentle air blowing (dry powder) or light carrier wash (wet suspension).
• High Sharpness & Contrast: The indication forms a sharp, distinct, well-defined outline that accurately mirrors the underlying surface-breaking profile of the discontinuity.
• Repeatability: If the part is wiped clean and re-magnetized under the same technique, the indication reappears identically in location, orientation, and intensity.
• Microscopic Confirmation: Under optical magnification ($10\times\text{ to }30\times$), a true mechanical separation, crack mouth, or fissure is visible in the metal matrix.

Engineering Acceptance Evaluation

Once verified as relevant, indications are categorized dimensionally:

• Linear Indications: Indications whose length is greater than three times their width ($L > 3W$). Examples: cracks, seams, laps, lack of fusion. Most engineering codes (e.g., ASME Section VIII, ASTM E1444) enforce a zero-tolerance policy for surface-breaking linear cracks in critical tension zones.
• Rounded Indications: Indications that are circular or elliptical with a length equal to or less than three times their width ($L \le 3W$). Examples: gas porosity, deep pinholes, small slag inclusions. Evaluated against maximum allowable diameter and cluster density rules.
• Aligned Indications: Four or more rounded indications separated by $1.5\text{ mm}$ ($1/16\text{ in}$) or less edge-to-edge in a line are classified as linear and evaluated accordingly.

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Non-Relevant Indications: Causes, Physics, and Verification

Non-relevant indications present the greatest challenge to Level II and Level III personnel. Because non-relevant indications are produced by genuine magnetic flux leakage, they attract and hold particles with real magnetic force, often closely mimicking true crack indications.

1. Magnetic Writing

• Mechanism: Occurs when a magnetized component comes into frictional contact with another piece of steel, or when a steel tool (screwdriver, scraper, wrench) is scraped across the magnetized part. Localized contact generates extreme friction and localized magnetic fields, establishing localized magnetic poles on the surface.
• Morphology: Appears as faint, fuzzy, irregular, wavy, or meandering lines. They frequently resemble pencil squiggles, broad bands, or zig-zags that do not align with known stress directions or grain flow.
• Level III Verification Technique:
  1. Completely demagnetize the component (verifying residual field $<3\text{ Gauss}$).
  2. Clean the surface.
  3. Re-magnetize the part under the identical current and waveform and re-apply particles.
  4. Disposition: If the indication was magnetic writing, it will completely disappear. If a true crack was present, the indication will reform crisp and sharp.

2. Cold Working and Plastic Deformation

• Mechanism: Severe localized plastic deformation—such as die-stamped part serial numbers, sheared edges, cold-bent tube radii, or roller-burnished fillets—alters the crystalline lattice. High dislocation densities and localized residual stresses substantially alter the material's local magnetic permeability ($\mu$).
• Morphology: Particle accumulations form directly around stamped numbers, letter profiles, or sheared edges.
• Level III Verification Technique: Lightly polish the surface with 400-grit emery cloth to remove surface burrs, demagnetize, and re-test. Examine under $10\times$ optical magnification to verify that the particle line is driven by permeability boundaries rather than micro-tearing.

3. Sharp Geometric Transitions and Section Changes

• Mechanism: When a component exhibits an abrupt change in cross-sectional area (sharp internal shoulder fillets, deep keyway corners, splines, gear teeth roots), magnetic flux lines traveling through the steel become severely constricted. If the narrow section approaches magnetic saturation, excess flux lines spill out into the air to bypass the constriction, creating geometric flux leakage.
• Morphology: Broad, fuzzy, symmetrical particle accumulations that hug the exact contour of the fillet or internal shoulder.
• Level III Verification Technique:
  1. Field Strength Modulation: Reduce the magnetizing amperage to the lower allowable threshold of the technique sheet (e.g., dropping surface tangential field from 60 Gauss to 30 Gauss). Geometric leakage will disperse, whereas true crack indications will remain sharply defined.
  2. Field Reorientation: Alter the direction of the magnetic field so that flux lines do not encounter the geometric constriction orthogonally.

4. Dissimilar Metal Boundaries & Cladding

• Mechanism: Occurs at the metallurgical boundary between materials possessing different magnetic permeabilities ($\mu_1 \ne \mu_2$). Classic examples include ferritic steel clad with austenitic stainless steel ($\mu_{\text{ferritic}} \gg \mu_{\text{austenitic}} \approx 1$), or nickel-base alloy weld buttering on carbon steel.
• Morphology: Distinct, straight linear indications following the exact fusion boundary between the dissimilar alloys.
• Level III Verification Technique: Review engineering drawings to identify cladding/buttering boundaries. Measure magnetic permeability across the boundary or apply chemical etching / copper sulfate testing to verify that the indication aligns with the dissimilar metallurgical interface.

5. Forging Flow Lines (Grain Flow)

• Mechanism: In heavy forgings (crankshafts, connecting rods, large hooks), directional working creates macrostructural grain flow. Segregation of alloying elements or ferrite/pearlite banding during solidification creates subtle, parallel ribbons of varying magnetic permeability.
• Morphology: Broad, faint, parallel striations following the contour of the forging grain flow lines under high magnetizing field strengths.
• Level III Verification Technique: Reduce magnetizing current; inspect under white light; verify that lines follow flow contours without sharp micro-fissuring.

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False Indications: Mechanical Trapping and Artifacts

False indications do not involve magnetic fields or flux leakage. They are purely physical, mechanical, or optical artifacts that trap particles or fluorescent liquid on the surface.

1. Mechanical Entrapment in Surface Roughness

• Causes: Heavy rough-machining tool marks (lathe feeds $>0.4\text{ mm/rev}$), coarse milling scallops, electrical discharge machining (EDM) recast layers, coarse grit-blast profiles, mill scale pits, and weld toe ripples mechanically trap magnetic particles.
• Verification: Demagnetize the part completely and re-apply particles without any magnetizing field. If the particle accumulation still forms in the machining groove, it is 100% false entrapment.

2. Gravity Drainage Sagging and Pooling

• Causes: When inspecting complex geometric shapes or vertically oriented parts, wet suspension drains downward under gravity. Excess bath fluid pools in pockets, undercut grooves, or along the lower bottom edges of hanging components, depositing heavy particle lines ("drip lines" or "tide marks").
• Verification: Ensure parts are positioned with adequate drainage angles. Use gentle carrier wash or air blow while the field is active to disperse pooled fluid before evaluating.

3. Fluorescent Lint and Fiber Contamination

• Causes: Cotton rags, paper towels, shop clothing fibers, or airborne lint soaked in fluorescent carrier fluid adhere to the component surface. Under UV-A black light, these fibers fluoresce with extreme brightness, closely masquerading as linear micro-cracks.
• Verification: Inspect under white light. A fiber or thread can be readily identified and picked off with tweezers or wiped away with a solvent cloth.

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Step-by-Step Level III Adjudication Protocol

When an inspector discovers an indication on a critical or high-value component, the Level III must ensure that the part is not scrapped prematurely based on non-relevant or false indications. The following 5-step adjudication protocol must be executed:

                                 ADJUDICATION PROTOCOL
                                           │
┌──────────────────────────────────────────┴──────────────────────────────────────────┐
│ STEP 1: Surface Cleanliness & False Indication Check                                │
│ - Wipe indication with solvent-dampened lint-free cloth.                            │
│ - Inspect under white light at 10x magnification.                                   │
│ - Re-apply bath WITHOUT magnetic field. Does accumulation re-form?                  │
│   --> YES: False indication (mechanical entrapment). Dress surface / clear.         │
│   --> NO: Proceed to Step 2.                                                        │
└──────────────────────────────────────────┬──────────────────────────────────────────┘
                                           │
┌──────────────────────────────────────────┴──────────────────────────────────────────┐
│ STEP 2: Demagnetization & Re-Test Cycle                                             │
│ - Demagnetize part to < 3 Gauss. Clean thoroughly.                                  │
│ - Re-magnetize under qualified technique and re-apply suspension.                   │
│ - Did indication disappear?                                                         │
│   --> YES: Non-relevant magnetic writing confirmed. Clear component.                │
│   --> NO: Proceed to Step 3.                                                        │
└──────────────────────────────────────────┬──────────────────────────────────────────┘
                                           │
┌──────────────────────────────────────────┴──────────────────────────────────────────┐
│ STEP 3: Field Level & Direction Modulation                                          │
│ - Is indication at a sharp section fillet, keyway, or dissimilar weld boundary?     │
│ - Reduce magnetizing current to lower code threshold (e.g. 30 Gauss tangential).    │
│ - Reorient field direction by 45 to 90 degrees.                                     │
│ - Does particle band disperse into background?                                      │
│   --> YES: Non-relevant geometric / permeability leakage confirmed. Clear.          │
│   --> NO: Indication is persistent and RELEVANT. Proceed to Step 4.                 │
└──────────────────────────────────────────┬──────────────────────────────────────────┘
                                           │
┌──────────────────────────────────────────┴──────────────────────────────────────────┐
│ STEP 4: Optical & Complementary NDT Verification                                    │
│ - Perform localized light dressing (400-grit abrasive, < 0.025 mm stock removal).   │
│ - Examine under 20x to 30x binocular microscope.                                    │
│ - Cross-verify with high-frequency Eddy Current (ET) or Liquid Penetrant (PT).      │
│ - Confirm mechanical crack mouth or void separation.                                │
└──────────────────────────────────────────┬──────────────────────────────────────────┘
                                           │
┌──────────────────────────────────────────┴──────────────────────────────────────────┐
│ STEP 5: Engineering Acceptance & Fracture Mechanics Disposition                     │
│ - Measure precise length, width, aspect ratio, and location relative to stress.     │
│ - Compare against governing code acceptance criteria (e.g. ASME, ASTM, API, AWS).   │
│ - Final Disposition: Conforming, Blend-out repair permitted, or Reject/Scrap.       │
└─────────────────────────────────────────────────────────────────────────────────────┘

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Summary Comparison Table: Indication Classification Matrix

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Practical Level III Engineering Scenario and Exam Traps

Scenario: During final acceptance inspection of a high-value, finish-ground forged alloy steel aircraft landing gear cylinder, a Level II technician identifies three separate indications on three identical parts:

• Part A exhibits several faint, wavy, irregular squiggly lines crossing the polished cylindrical outer diameter.
• Part B exhibits a heavy, continuous linear indication running precisely inside the sharp $0.8\text{ mm}$ ($1/32\text{ in}$) internal radius of a dynamic seal retaining groove.
• Part C exhibits a razor-sharp, dense, tightly bound $3.0\text{ mm}$ ($1/8\text{ in}$) linear indication on the outer cylinder wall running perpendicular to the principal dynamic tensile axis.

The technician rejects all three cylinders as defective. The production manager appeals the rejection to the Level III.

Level III Adjudication and Final Disposition:

1. Evaluation of Part A (Magnetic Writing): The wavy, irregular squiggles do not align with any stress axis or grain flow. The Level III demagnetizes Part A to $<2\text{ Gauss}$, cleans the surface, re-magnetizes under the approved shot, and re-applies fluorescent suspension. The indications completely vanish. Root cause: technicians used unmagnetized steel calipers that dragged across the cylinder while measuring OD. Disposition: Non-relevant magnetic writing confirmed; Part A ACCEPTED.
2. Evaluation of Part B (Geometric Flux Leakage): The indication sits directly at a sharp internal seal groove transition. The Level III reduces the magnetizing current by 25% (maintaining a verified 30 Gauss tangential field with a Hall probe and confirming QQI response). The broad particle accumulation completely disperses. Optical inspection at $20\times$ confirms smooth, sound metal with no micro-cracking. Disposition: Non-relevant geometric indication confirmed; Part B ACCEPTED.
3. Evaluation of Part C (True Quench/Grinding Crack): The razor-sharp linear indication on Part C reforms with identical sharpness after demagnetization and re-testing. It persists under reduced amperage. Under $20\times$ optical magnification, a genuine mechanical crack fissure is confirmed. The governing aerospace specification permits zero linear cracks on flight-critical cylinders. Disposition: Relevant defect confirmed; Part C REJECTED.