Nondestructive Evaluation - Magnetic Particle Inspection Flashcards

Vocabulary flashcards covering the basic theory, equipment, and physics of Magnetic Particle Inspection (MPI) and its comparison to Liquid Penetrant Testing as discussed in Homework 2.

Magnetic particle inspection

A method of NDE that relies on visual inspection of surface and near-surface defects of a ferromagnetic article magnetized using internal or external fields.

ASTM A848 Type 1

A specific type of electrical iron that, when heat treated to Relay conditions, can have a relative permeability of $6500$ under a flux density of B-40 at $60\,hz$.

Permeability

A material property describing how flux density changes with applied magnetizing force; it is non-linear and for some iron alloys can reach values up to $10^5$.

Hysteresis

The lagging effect observed when flux density ($B$) is plotted against field intensity ($H$), caused by the energy required to demagnetize aligned magnetic domains.

Remanent induction ($B_r$)

The lower value of flux density to which a ferromagnetic material settles after the magnetizing force is removed and the domains remain partially aligned.

Coercive force

The negative external magnetic field required to overcome the internal forces keeping magnetic domains aligned, bringing the material's flux density to zero.

Yokes

External magnetization tools that induce a field between two legs; permanent versions require no power and avoid arcing, while electro-magnetic versions are adjustable and can demagnetize using AC power.

Coils

External magnetization devices that produce a longitudinal magnetic field to detect cracks oriented perpendicular to the field direction.

Central conductors

Devices passed through hollow components to produce a circumferential magnetic field, allowing detection of longitudinal cracks on both inner and outer surfaces.

Direct contact (Headshot)

An internal magnetization method where the article is pressed between two copper pads and current is passed through it to create a circumferential field.

Half-Wave Direct Current (HWDC)

A type of power source used in direct contact magnetization to provide optimal results for detecting defects located below the surface.

Prod contacts

A localized direct contact method using two copper prods to pass current through the area of interest, often used for large components that cannot fit in a headshot machine.

Flux leakage

The disturbance of magnetic field lines caused by a crack or discontinuity, which is detected by the collection of magnetic particles.

$$NI = \frac{45,000}{L/D}$$

The equation used to determine the necessary current ($I$) and number of turns ($N$) to produce a sufficient magnetic field in a coil based on the part's length ($L$) and diameter ($D$).

L/D Ratio

The length-to-diameter ratio of a test article; it is recommended to be at least $10$ (and ideally over $20$ by adding rods) to avoid demagnetization issues during inspection.

FOD

Foreign Object Debris; a risk associated with failing to demagnetize parts, as magnetic components may stick to the article after it is returned to service.

Type 1 error

A false positive in NDE, such as when residual magnetic particles on an undemagnetized part are misinterpreted as a defect during a follow-up inspection.

Liquid penetrant NDE

A visual inspection method suitable for non-ferromagnetic materials like chromium, which detects surface defects but cannot detect deep subsurface inclusions like magnetic particle inspection.