ASNT NDT Level III MT key terms

A measure of how readily a material accepts and conducts magnetic flux, equal to the ratio of flux density to magnetizing force. High-permeability steels magnetize easily and carry strong internal fields.

The opposition a magnetic circuit offers to the passage of flux, analogous to electrical resistance. Air gaps and low-permeability regions have high reluctance and divert flux, causing leakage fields at discontinuities.

The ability of a material to keep magnetism after the magnetizing force is removed. High retentivity makes the residual technique practical but also makes parts harder to demagnetize.

The reverse magnetizing force required to bring residual magnetism back to zero. It indicates how hard a material is to demagnetize; hard, high-coercivity materials require stronger reversing fields.

The condition in which a ferromagnetic material has aligned essentially all of its domains, so further increases in magnetizing force produce little or no increase in flux density.

A material such as iron, nickel, or cobalt that is strongly attracted by a magnet and can concentrate flux within itself. Only ferromagnetic materials can be inspected by magnetic particle testing.

A microscopic region inside a ferromagnetic material where atomic magnetic moments point the same way. Magnetization proceeds by growth and rotation of domains, and domain boundaries matter at discontinuities.

The quantity of magnetic flux passing through a unit area perpendicular to the flux direction, expressed in tesla or gauss. It determines whether a discontinuity produces a detectable leakage field.

The strength of the applied magnetic field, expressed in amperes per meter or oersteds, produced by current in a conductor or coil. It is the cause; flux density in the part is the effect.

Magnetic flux that leaves and re-enters the surface of a magnetized part at a discontinuity, crack, or sharp section change. Particles collect at this field and form the visible indication.

The temperature above which a ferromagnetic material loses its ferromagnetism and behaves as a paramagnetic material. Parts above this temperature cannot be effectively inspected by magnetic particle testing.

Magnetism that remains in a part after the magnetizing current is switched off. It can be used deliberately in the residual technique or may be an unwanted side effect requiring demagnetization.

A graph of flux density versus magnetizing force as the applied field is cycled in both directions. Its shape reveals saturation, retentivity, and coercive force for the material.

A portable magnetizing device with a laminated core and coil that induces a longitudinal field between two contact poles. Articulated legs let it fit curved surfaces, and it is widely used for field inspection.

A hand-held contact electrode used to pass magnetizing current through a localized area of a part, creating a circular field around the contact points. Prods risk arc burns if contact is poor.

A helical winding that produces a longitudinal magnetic field in a part placed inside or near it. Field strength rises with ampere-turns and falls rapidly with distance from the coil.

A conductive bar inserted through the bore of a hollow part so that current passing through it magnetizes the part circumferentially. It is the standard approach for internal surfaces of rings and tubes.

Fine magnetic particles carried in a liquid vehicle such as water or light oil and applied by flow or spray. Wet methods give good coverage and high sensitivity to fine surface discontinuities.

Magnetic particles applied as a dry powder, often from a bulb or spray, usually while the magnetizing current is on. Dry methods suit rough surfaces, portable work, and elevated temperatures.

Magnetic particles treated with a fluorescent dye that emits visible light under ultraviolet radiation. They give bright indications against a dark background and provide the highest sensitivity.

The number of magnetic particles per unit volume of a wet suspension. It is checked by a settling test because low concentration reduces sensitivity while excessive concentration causes a hazy background.

An ultraviolet-A lamp, typically emitting at about 365 nm, used to make fluorescent particles glow. Its intensity at the inspection surface must be measured before use.

Rectified alternating current that flows in one direction in pulses, producing a field with good surface sensitivity and somewhat deeper penetration than plain alternating current. It is often used with prods.

A device containing known artificial defects, such as a pie-shaped gauge, Ketos ring, or flux indicator strip, used to confirm that a technique produces adequate field strength and direction.

A meter with a Hall-effect probe that measures magnetic flux density. It is used to check yoke strength, verify magnetizing levels, and confirm that demagnetization is complete.

Equipment that removes residual magnetism by applying a reversing field that decreases gradually to zero. Common forms include coils, yokes, and tunnel units for production parts.

A technique in which current passes through the part or a central conductor, producing flux lines that circle the current path. It finds discontinuities lying roughly parallel to the current.

A technique that drives flux lines lengthwise through the part, usually with a coil, yoke, or electromagnetic core. It detects discontinuities that lie across the flux path.

Applying two or more magnetizing fields in different directions, typically at about right angles, so that discontinuities in any orientation are found in one inspection sequence.

A technique in which the magnetizing current stays on while particles are applied, giving particles maximum freedom to migrate to leakage fields. It is the usual choice for wet fluorescent inspection.

A technique in which particles are applied after the magnetizing force is removed, relying on residual magnetism. It requires material with enough retentivity and a strong enough residual field.

A periodic check, often daily, confirming that equipment, particles, and the light source work correctly, usually by inspecting a reference standard with known defects. Results are documented.

A written document describing how a specific part or part family will be inspected, including equipment, magnetization technique, current values, particles, and acceptance criteria.

An indication caused by a discontinuity that must be judged against the acceptance criteria of the applicable standard. Being relevant does not mean rejectable; size and type are still evaluated.

An indication produced by something other than a discontinuity, such as leakage at a keyway or section change, magnetic writing, or residual fields. It must be recognized and set aside.

The step in which the inspector decides whether an observed particle build-up is relevant, nonrelevant, or false, before any judgment of acceptance is made.

The step in which a relevant indication is compared with the acceptance criteria of the applicable code to decide whether the part is acceptable, repairable, or rejectable.

An indication whose length is at least three times its width, typical of cracks and lack of fusion. Most codes apply stricter limits to linear than to rounded indications.

An indication whose length is less than three times its width, typical of porosity or slag. Acceptance limits for rounded indications are usually less severe than for linear ones.

The rules in the applicable code or specification that define the size, type, and number of indications allowed. They must be agreed before inspection and applied during evaluation.