Leakage field: what it means and why it matters

How it forms

In a magnetized ferromagnetic part the field runs through the metal, because metal is far easier for flux to travel through than air is. A crack or a seam breaks that path. The gap is mostly air, so the flux is forced to detour. Some of it goes around the bottom of the flaw, and some leaves the surface, crosses over the gap in the air and re-enters on the other side. The escaped flux is the leakage field. The two edges of the gap act like small magnetic poles, and fine particles are pulled onto them.

What makes it strong or weak

• Orientation. The strongest leakage comes from a flaw that lies at right angles to the field. A flaw lying along the field produces almost none, which is why direction matters so much in longitudinal vs circular magnetization.
• Depth. A flaw at the surface gives a strong, sharp field. A flaw below the surface gives a weaker, broader one that falls off quickly with depth, so MT is mainly for surface and near-surface flaws.
• Width and height of the gap. A tight, deep crack disturbs the field strongly. A shallow scratch disturbs it very little.
• Field strength. The field has to be strong enough to push flux out of the part, but not so strong that the whole part saturates and particles stick everywhere as background.

Not every leakage field is a flaw

Anything that disturbs the field can leak it: a sharp change in section, a keyway, a thread root, the edge of a weld, a boundary between hard and soft zones or a spot where two pieces of different metal join. These give nonrelevant indications, which have a real cause but are not a flaw. A different group, false indications, have nothing to do with the field at all, such as lint or particles trapped in a rough spot. The inspector has to sort these from relevant indications using the procedure.

Mix-ups to avoid

• Leakage field versus residual field. The residual field is what remains in the part after the current is switched off. The leakage field exists at a flaw while the part is magnetized. The residual field is covered in demagnetization.
• Subsurface does not mean invisible. It means a weaker indication that needs suitable conditions.
• Particles do not find flaws. They show where the field leaks.

How the exam uses it

Principles, flux fields and effects of discontinuities all come back to this idea. Typical questions ask which flaw will give the strongest indication, why a subsurface flaw gives a fuzzy one, or why a given geometry gave a nonrelevant indication.

Topic names here follow the MT General outline for exams before 15 December 2026; the MT outline change guide shows how the later list lines up.

Practice it

Test your picture of flux in the magnetic particle testing quiz, then use the sketch routine in the MT field direction and technique guide. Flaw direction is the next idea in longitudinal vs circular magnetization, and what stays in the part afterward is demagnetization. See every term in the NDT terms glossary.

Know the term? Check the rest of the MT General topics

Take the free 10-question check. It covers the MT General exam, needs no signup and explains every answer, so you see which topic to study next. The course then adds timed practice exams and a study plan built from your exam date.

Sources

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Background reading: PCC's magnetic particle section notes and ASNT's published topic headings. Outline checked 25 October 2026.

• Magnetic particle testing section notes, Portland Community College.
• ASNT NDT Level II examinations, published topic headings.

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