ASNT NDT III ET key terms

Electric current induced in a conductive material when it is exposed to a changing magnetic field; it flows in closed loops and generates its own opposing magnetic field.

The production of an electromotive force in a conductor exposed to a changing magnetic field, or the production of a magnetic field by a changing current.

The induced voltage in a circuit equals the negative rate of change of magnetic flux through that circuit; a faster or larger change produces a larger induced voltage.

An induced current flows in a direction such that its magnetic field opposes the change that produced it, explaining the phase and direction relationships in eddy current testing.

The total opposition of a coil to alternating current, combining resistance and inductive reactance; changes in impedance are the primary signal measured in eddy current inspection.

The opposition a coil offers to alternating current because of its inductance, increasing with frequency and inductance according to the relationship two pi times frequency times inductance.

The property of a coil that resists changes in current by storing energy in a magnetic field; it depends on coil geometry, turns, and nearby magnetic materials.

A display plotting the real (resistive) component against the imaginary (inductive) component of coil impedance, used to interpret how test variables move the signal point.

The angular difference between the resistive and inductive components of coil impedance, measured on the impedance plane; it helps separate defect signals from unwanted variables.

The gap between the inspection coil and the test surface; increasing it weakens coupling and changes impedance, usually producing a signal that must be suppressed or monitored.

The ratio of the cross-sectional area of the test part to the cross-sectional area of the coil's inside opening, describing how well an internal coil couples to a tube.

The degree of electromagnetic interaction between a coil and the test object; stronger coupling gives larger signals and depends on spacing, fill factor, and material properties.

The tendency of alternating current and eddy currents to concentrate near the surface of a conductor, with current density decreasing exponentially with depth.

The depth at which eddy current density has fallen to about 37 percent of its surface value; roughly three such depths define the practical limit of inspection.

The rate of alternation of the coil current, in hertz; higher frequency reduces penetration depth and increases sensitivity to surface features.

A material property describing how readily it carries electric current, usually expressed as a percentage of the International Annealed Copper Standard; it strongly affects eddy current response.

A material property describing how easily it supports a magnetic field; magnetic materials concentrate flux and can dominate eddy current signals unless saturated.

The condition in which a magnetic material cannot be further magnetized because its magnetic domains are aligned; used to reduce permeability effects during eddy current testing of ferromagnetic parts.

A coil arrangement that measures the total impedance of the coil relative to a reference, responding to both the test material and any reference standard.

Two coils connected in opposition so that common signals cancel while differences between them, such as a defect at one coil, produce a signal.

A coil used to apply a steady magnetic field that saturates or magnetically biases a ferromagnetic part, reducing permeability variations in the eddy current signal.

The distortion of eddy current flow near the end or edge of a part, where the current path is interrupted, producing signals that can mask or mimic defects.

A specimen with known, documented features or properties used to set equipment sensitivity, verify performance, and compare signals during eddy current inspection.

A reference piece, often with machined notches or holes of specified size, used to establish and check the response of an eddy current system.

The ratio of the amplitude of a meaningful indication to the amplitude of background interference; higher values make detection and sizing of defects more reliable.

Examination of materials or components to find discontinuities or measure properties without impairing their future usefulness.

An interruption or irregularity in the normal structure of a material, such as a crack, void, or inclusion; it is not necessarily a defect unless it exceeds acceptance criteria.

A signal or response from a test that requires interpretation; it may come from a real discontinuity or from an irrelevant source.

An indication caused by something other than a genuine discontinuity, such as geometry, material variation, or equipment artifacts.

A discontinuity or flaw that exceeds applicable acceptance criteria and makes a part unsuitable for its intended service.

Specified limits defining the size, type, or number of indications permitted for a part to be considered acceptable.

The process of adjusting and verifying instrument response against known standards so measurements are accurate and traceable.

Demonstrated evidence that personnel, equipment, or procedures meet specified requirements, often through documented training, examination, and performance records.

Written confirmation that a person has met the required qualification criteria for a given NDT method and level.

The ability of a test system to detect small discontinuities or small changes in a measured property, set through gain, frequency, and reference standards.

The ability of a test system to distinguish between two indications or features that are close together or similar in size.

A documented, detailed set of instructions describing how a test is to be performed, including equipment, parameters, and acceptance references.