Magnetic Permeability
Magnetic permeability (μ) is a material property describing how easily it can be magnetized, equal to the ratio of magnetic flux density (B) to magnetizing force (H), and is a primary factor in MT current selection.
Definition
Ferromagnetic materials have permeability much greater than free space, enabling high flux densities at modest magnetizing currents. Austenitic stainless steels (paramagnetic) cannot be inspected by MT because their permeability is too low.
The principle
The measure of how readily a material carries magnetic flux. High-permeability (ferromagnetic) materials concentrate flux, which is what makes magnetic particle testing possible — and what makes eddy current testing of them difficult.
How it shows up in practice
Permeability varies with heat treatment, stress and prior magnetisation, so it is not a fixed material constant on real components. In ET it produces signals that swamp defect responses unless the part is magnetically saturated.
Where the codes address it
Underlies ASME Section V Articles 7 and 8; saturation techniques in the tubing appendices exist specifically to suppress permeability noise.
Where practitioners get caught
Assuming a stainless steel is non-magnetic by name. Cold-worked austenitic grades and duplex steels carry real permeability, and both MT applicability and ET behaviour change with it.
Where Magnetic Permeability fits in an inspection programme
A term is only useful when it connects to a decision. Magnetic Permeability appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.
Related terms
- Magnetic Flux — Magnetic flux (Φ) is the total magnetic field passing through a cross-sectional area, measured in Webers (Wb) or Maxwells, and is the physical phenomenon whose leakage at a flaw produces a magnetic particle indication.
- Hysteresis (B-H Curve) — Hysteresis is the lag of magnetization (B) behind the magnetizing force (H) in ferromagnetic materials, producing the characteristic B-H loop whose width is governed by coercive force and whose intercept is residual magnetism.
- Magnetic Particle Testing (MT) — Magnetic Particle Testing (MT) is a surface and near-surface NDT method for ferromagnetic materials that detects discontinuities by applying a magnetic field and observing the leakage flux attract magnetic particles to the flaw indication.
More physics terms
Back-Wall Echo · Beam Spread · Near-Field · Far-Field · Dead Zone · Shear Wave · Longitudinal Wave · Surface Wave · Lamb Wave · Mode Conversion
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.