Introduction to Non-Destructive Testing Techniques - Magnetic Particle Testing

Key vocabulary terms and definitions for Magnetic Particle Testing in NDT.

Magnetic Particle Testing

A nondestructive method using magnetic fields and iron particles to detect flaws in ferromagnetic components.

Ferromagnetic material

A material that can be magnetized and retain magnetic properties, e.g., iron, nickel, cobalt and certain alloys.

Flux leakage field

Magnetic field that leaks out through a crack or defect, making leakage detectable by particle accumulation.

Magnetic pole

Locations where magnetic field lines exit or enter a magnet; poles are at the ends of magnets.

Dipole

An object with two equal but opposite magnetic poles; bar magnets act as dipoles.

Magnetic flux leakage (MFL)

Leakage of magnetic flux at discontinuities, which enables visualization with magnetic particles.

Bar magnet

A simple magnet used to illustrate magnetic field lines with a north and a south pole.

Crack edge poles

When a crack forms, a north and a south pole form at each edge, causing flux leakage at the crack.

Flux leakage field location

Where the magnetic field leaks out due to a defect, creating an indication for inspection.

Magnetic field orientation

Direction of the magnetic field within a part; affects defect detectability and indication strength.

Longitudinal magnetic field

Field lines run along the length of the part; produced by coils, solenoids, or magnets.

Circular magnetic field

Field lines encircle a conductor; produced by current flowing through the component or a surrounding conductor.

Direct magnetization

Magnetization achieved by passing current directly through the component.

Indirect magnetization

Magnetization achieved via external magnetic sources (yokes, permanent magnets, coils).

Electromagnetic yoke

A device that generates a strong localized magnetic field; can be AC or DC.

Prods

Handheld electrodes used to pass current through the component; risk of arcing if not used properly.

Permanent magnets

Magnets used as a source of magnetism; strong and sometimes difficult to remove, but useful in certain environments (e.g., underwater).

Coil shot

Longitudinal magnetization using a coil or solenoid surrounding the part.

Rectified current

AC converted to pulsating DC using rectification methods (HWAC, FWAC, or three-phase).

Half-wave rectified AC (HWAC)

AC rectified to unidirectional pulsating current; often used to power electromagnetic yokes.

Full-wave rectified AC (FWAC)

Rectified AC producing a continuous pulsating DC with smoother polarity changes.

Three-phase full-wave rectified

Three-phase rectified current that closely resembles DC for uniform magnetization.

Skin effect

AC magnetic field penetrates only a thin surface layer, limiting subsurface detection in ferromagnetic materials.

Demagnetization

Process to remove residual magnetism; methods include heating above Curie temperature or reversing the magnetic field with AC.

Curie temperature

Temperature above which a ferromagnetic material loses its permanent magnetism (e.g., ~770°C for low-carbon steel).

Field indicator

A device with a soft iron vane deflected by a magnetic field to indicate field strength/direction.

Hall-effect (gauss) meter

Electronic device that measures magnetic field strength using the Hall effect; gives a digital readout.

QQI (Quantitative Quality Indicator)

A standard artificial flaw used in wet method testing to verify field direction and strength.

Pie gauge

A permeable disk with non-ferromagnetic divisions; indicates field orientation when magnetizing.

Slotted strips

Highly permeable strips with slots used to infer field strength and distribution during inspection.

Wet vs dry particle inspection

Dry uses dry powder; wet uses a suspended liquid. Wet generally offers better mobility and sensitivity for small defects.

Dry particle characteristics

Fine (~50 μm) and coarse (~150 μm) particles; mix of shapes to balance flow and sensitivity; concentration important.

Fluorescent vs visible particles

Wet particles can be visible or fluorescent; fluorescent particles require UV light for indications.

Water-based vs oil-based carriers

Carriers affect corrosion protection, hydrogen embrittlement, and indication clarity; water-based are common and easier to clean.

Water break test

A daily check of water-based carrier wetting performance to ensure proper surface wetting.