Introduction to Non-Destructive Testing Techniques - Magnetic Particle Testing (MT) - Glossary

Glossary-style flashcards covering key terms and definitions from Magnetic Particle Testing (MT) within NDT, based on the provided lecture notes.

Magnetic Particle Testing (MT)

An NDT method that uses magnetic fields and iron particles to detect surface and near-surface flaws in ferromagnetic components by visualizing flux leakage fields.

Ferromagnetic material

Materials that can be magnetized and retain magnetic properties (e.g., iron, nickel, cobalt and their alloys).

Flux leakage field

The magnetic field that leaks out of a material at a defect due to a crack or discontinuity, causing particle accumulation and an observable indication.

Magnetic pole

Locations where magnetic lines of force enter or exit a magnet; ends of a magnet where poles concentrate.

Dipole

An object with paired opposite magnetic poles (north and south); magnets are modeled as dipoles.

Magnetic moment

The strength of the magnetic field produced by a source; related to electron motion, spin, and alignment of domains.

Diamagnetic materials

Materials with weak negative susceptibility that are repelled by a magnetic field and do not retain magnetization (e.g., copper, silver, gold).

Paramagnetic materials

Materials with small positive susceptibility that are slightly attracted by a magnetic field and do not retain magnetization (e.g., magnesium, molybdenum, lithium).

Ferromagnetic materials

Materials with large positive susceptibility that are strongly attracted to magnetic fields and can retain magnetization due to magnetic domains.

Magnetic domains

Regions within a ferromagnetic material where many atomic moments are aligned; alignment increases magnetization when magnetized.

Saturation (magnetic saturation)

Point at which further increases in magnetizing force produce little or no increase in magnetic flux density (B).

Retentivity

Residual magnetism or flux that remains in a material after magnetizing force is removed at saturation.

Residual magnetism

Magnetic flux that remains in a material when the magnetizing force is zero.

Coercive force (coercivity)

The reverse magnetic field required to reduce the magnetic flux in a material to zero.

Permeability (μ)

Property describing how easily a magnetic flux is established in a material; μ = B/H.

Reluctance

Opposition to establishing a magnetic field in a material; analogous to electrical resistance.

Hysteresis loop (B–H loop)

Plot of magnetic flux density (B) versus magnetizing force (H); reveals properties like retentivity, coercivity, and permeability.

Flux density (B)

Amount of magnetic flux per unit area; measured in tesla.

Magnetic field strength (H)

Magnetizing field intensity; measured in amps per meter (A/m).

Flux (Φ)

Total number of lines of magnetic force in a material; unit is the weber (Wb).

Magnetization (M)

Magnetic dipole moment per unit volume of a material; measures how magnetized a material is.

Magnetic field orientation

The direction of magnetic lines of force relative to a flaw; optimal detection occurs when field is at 45–90 degrees to the flaw.

Longitudinal magnetic field

Field lines run parallel to the part’s long axis; produced by coils/solenoids or magnets.

Circular magnetic field

Field lines run circumferentially around a conductor; produced by current through the conductor or surrounding conductor.

Direct magnetization

Magnetizing a component by passing current directly through it (circular field); requires good electrical contact and may leave residual magnetism.

Indirect magnetization

Establishing a magnetic field within the component via an external source (permanent magnets, yokes, central conductors, coils).

Yoke

An electromagnet device used to create a strong, localized magnetic field; can be powered by AC or DC.

Prods

Handheld electrodes used to pass current through the part for magnetization; can be a single or dual setup.

Coil shot

Longitudinal magnetization achieved by placing a component inside a coil or solenoid with multiple turns.

AC vs DC magnetization

DC penetrates deeper into ferromagnetic material; AC yields surface-limited fields due to skin effect.

Skin effect

If AC is used, the magnetic field penetrates only a shallow surface layer, reducing subsurface detectability.

Rectified currents (HWAC, FWAC, 3ϕ FWAC)

Converting AC to DC-like current to optimize penetrations and particle mobility: Half-Wave AC (HWAC), Full-Wave AC (FWAC), and Three-Phase Full-Wave Rectified.

Ampereturns

Product of current (amperes) and number of turns in a coil used to quantify magnetizing force.

Field indicators

Mechanical devices with a deflectable vane to give a qualitative indication of field strength and direction.

Gauss meter (Hall-effect meter)

Electronic device that measures magnetic field strength (Gauss or Tesla) using Hall effect.

Quantitative Quality Indicator (QQI)

A thin artificial metal strip with a etched pattern used to verify field direction and adequacy in wet MT.

Pie gage

Permeable disk divided into segments that reveal field direction by how indications radiate from the center.

Slotted strips

Permeable strips with slots used with MT to give a general indication of field strength in an area.

Dry magnetic particles

Solid powders used in MT; small sizes (rough surfaces or shallow defects); particle sizes range roughly 50–150 μm.

Wet suspension magnetic particles

Particles suspended in water or oil; smaller (≈10 μm) and more mobile; can be fluorescent or visible.

Fluorescent particles

Particles that fluoresce under UV light to enhance visibility of indications; used with wet MT.

Water break test

A test of water-based suspensions to ensure adequate surface wetting by observing film formation.

Demagnetization

Process to remove residual magnetism, via heating above Curie temperature or reversing/decaying magnetic field; sometimes using AC yokes.

Curie temperature

Temperature at which ferromagnetic materials lose permanent magnetization (e.g., ~770°C for low-carbon steel).

arc blow

Welding issue where residual magnetic fields distort the arc or cause metal to be repelled.

Demagnetization check (Gauss/field measurement)

Verification that residual magnetic flux has been reduced to acceptable levels, often < 3 Gauss (3x10^-4 Tesla).