Flux Leakage and Indication Behaviour

Outcomes

After this chapter, the learner should be able to:

• explain how a discontinuity generates a leakage field;
• predict how orientation, size, shape, depth, permeability and coating affect an indication;
• distinguish false, non-relevant and relevant indications;
• use indication morphology as evidence without over-claiming depth or type; and
• plan confirmation steps for uncertain patterns.

3.1 Creation of a leakage field

Flux follows the lower-reluctance ferromagnetic path until an interruption forces it to divert. At a suitably oriented discontinuity, some flux leaves the surface on one side and re-enters on the other. The resulting local poles attract magnetisable particles. Particle chains bridge the region and provide a visible pattern that is usually wider than the physical opening.

A surface-breaking crack often presents a strong air gap and a concentrated leakage field. A shallow buried discontinuity can also produce a surface field, but the field spreads before reaching the surface. The corresponding indication tends to be broader and less sharply defined.

3.2 Variables controlling leakage response

A useful indication depends on several variables acting together. Orientation is normally the strongest controllable influence. The response is greatest when the major axis of a discontinuity is close to 90 degrees to the magnetic field, becomes weaker as the angle closes, and may disappear when the feature lies parallel to the field. Magnetising level also matters: too little field produces weak leakage, while excessive field can load corners and geometry changes with background particles.

Discontinuity geometry and depth

A larger interruption generally diverts more flux than a smaller one when the remaining conditions are comparable. Tight planar features often create a steeper local gradient than rounded volumetric conditions. As a discontinuity moves farther below the surface, its field spreads before reaching the particles, so the visible response normally becomes broader and weaker.

Material, surface and coating effects

Permeability controls how readily the component carries flux and how the field redistributes at local changes in material or section. Rough surfaces trap particles and reduce definition. A coating increases the separation between the particles and the leakage field, making fine or shallow conditions more difficult to reveal.

Particle access and mobility

Particles must be able to reach the leakage site, move into the local gradient and remain there long enough to form a stable indication. Media concentration, carrier condition, application timing and surface wetting therefore influence the final pattern as directly as the magnetic variables.

Orientation is the most important controllable variable. Best sensitivity normally occurs near a right-angle intersection. A discontinuity at about 45 degrees may still respond, but the indication is weaker. A straight feature parallel to the field may be missed entirely.

3.3 False indications

False indications are patterns produced by the inspection process rather than a magnetic leakage field associated with the component. Examples include:

• hair, lint or fibre holding particles;
• dirt, scale or dried carrier residue;
• tracks produced by dragging equipment;
• poor rinsing or uncontrolled aerosol application;
• fluorescent contamination on gloves or the viewing area; and
• particle streaks formed while the field is absent.

Cleaning and repeating the controlled sequence should remove or relocate a false pattern. The technician should never erase an uncertain pattern without first recording its position and deciding how it will be investigated.

3.4 Non-relevant indications

Non-relevant indications are produced by a magnetic response that is not caused by a discontinuity requiring evaluation. Common origins include:

• abrupt changes in section;
• corners, threads, keyways, splines and sharp radii;
• weld toes and geometric transitions;
• machining marks or cold-worked regions;
• boundaries between different alloys or microstructures;
• forging flow, hard-facing and heat-treatment transitions;
• magnetic writing from local contact with a cable or magnetised object;
• consequent poles; and
• furring caused by excessive magnetisation at edges.

These patterns can be repeatable and strongly held. Their repeatability does not make them relevant. Confirmation may require a changed field direction, lower magnetising level, demagnetisation and retest, surface examination, comparison with geometry, or another NDT method.

3.5 Relevant indications

A relevant indication is caused by a discontinuity. Examples within MT capability include fatigue cracking, grinding cracks, quench cracks, forging laps and bursts, casting hot tears, seams, surface-connected weld lack of fusion and some shallow in-service cracking. The correct label should reflect the evidence available. A technician may report "linear relevant indication at the weld toe" when the precise metallurgical mechanism is not proven.

The commonly used geometric distinction is:

• linear: length greater than three times width; and
• rounded or non-linear: length not greater than three times width.

That ratio is only a classification rule when the controlling document adopts it. It does not by itself determine acceptance.

3.6 Indication investigation ladder

When a pattern is uncertain:

1. preserve its location with a photograph, sketch or temporary mark;
2. verify lighting and media application;
3. clean the area and repeat the same shot;
4. change the field direction and observe whether the pattern responds as expected;
5. reduce excessive field if geometry is furring;
6. inspect the surface visually under magnification where permitted;
7. compare with drawings, manufacturing history and known transitions;
8. apply a complementary NDT method if required; and
9. escalate unresolved interpretation.

3.7 Worked interpretation examples

Example A - sharp line at 90 degrees to a yoke axis. The line repeats after cleaning and becomes strongest when the line joining the poles is perpendicular to the indication. This behaviour supports a relevant linear condition because the yoke field between the poles is broadly parallel to the pole-to-pole line, and a crack at 90 degrees interrupts it strongly.

Example B - fuzzy band beside a change in diameter. The band follows the full circumference and increases dramatically with current. It is likely a geometry or permeability response. Reduce the magnetising level, change technique, and confirm the transition before classifying it.

Example C - fine fluorescent line that moves after wiping. If the line changes position or disappears after controlled cleaning, investigate fibre or fluorescent contamination before reporting a discontinuity.

3.8 Common traps

• Inferring actual crack width from indication width.
• Assuming a broad indication is harmless; a buried relevant discontinuity can be broad.
• Treating every repeatable pattern as relevant.
• Removing the pattern before recording its position.
• Classifying from a photograph without knowing field direction and geometry.
• Increasing current until a weak parallel discontinuity appears instead of changing orientation.

Chapter summary

An MT indication is a magnetic and visual response shaped by the discontinuity, component, applied field, surface and particles. Interpretation is a controlled investigation. The Level 2 technician classifies the evidence, records uncertainty and applies acceptance rules only after determining relevance.