ASNT NDT Level II key terms

Alternating currents induced in a conductive material by a nearby changing magnetic field. Their magnitude and distribution shift when the coil passes over a crack or over a change in conductivity or permeability.

The depth below the surface where eddy current density has dropped to about 37 percent of its surface value. It shrinks as frequency, conductivity, or permeability rises, so higher frequencies concentrate testing near the surface.

A display that plots the changing electrical impedance of an eddy current coil as a moving point or trace. Inspectors read the shape, direction, and angle of the trace to sort defects from harmless variables.

The fraction of a coil's cross-sectional area that is occupied by the test part. A high fill factor gives tighter coupling and better sensitivity when tubing or bar is inspected with an encircling coil.

The small gap between an eddy current coil and the test surface, caused by coating, clearance, or wobble. Changing lift-off alters coil impedance and can create signals that hide or imitate flaws.

The angular position of an eddy current signal on the impedance plane. It relates to the depth and type of feature producing the signal, helping separate surface conditions from subsurface ones.

An eddy current method for ferromagnetic tubing in which exciter and detector coils sit well apart. The field passes through the tube wall, giving through-wall sensitivity to both inner and outer defects.

Flux that leaves a magnetized part at a discontinuity such as a crack or slag stringer. Particles gather at this leakage field and build the visible indication used in magnetic particle testing.

Magnetizing a part by passing current through it, which sets up a circular field around the current path. This orientation is best at revealing discontinuities running roughly parallel to the part's long axis.

Magnetizing a part along its long axis with a coil or yoke so flux runs lengthwise through the part. This orientation reveals discontinuities lying transverse to that axis.

The amount of residual magnetism left in a material after the magnetizing force is removed. High retentivity leaves a part strongly magnetized after inspection, which may call for demagnetization before service or machining.

Reducing leftover magnetism in a part to an acceptable level, usually by applying a reversing field that steadily decreases. It is needed when residual magnetism would disturb machining, welding, or sensitive instruments.

A unit expressing magnetic flux density, used to state the field applied to a part during magnetic particle testing and to confirm the required level was reached.

The tendency of a liquid to be drawn into narrow openings by surface tension. In penetrant testing it pulls penetrant into tight surface-breaking discontinuities so it can later bleed back out.

The period a penetrant is left on the part surface so it can soak into openings. Too short a dwell gives weak indications; the allowed time is set by the governing specification and material.

A powder or coating applied after excess penetrant is removed. It draws trapped penetrant back to the surface and provides a contrasting background that makes the bleed-out easy to see.

A liquid applied after lipophilic penetrant so the excess penetrant can be rinsed away with water. Its contact time must be controlled, because over-emulsification can wash penetrant out of genuine defects.

The return of penetrant from a surface-breaking discontinuity to the part surface after development. It forms the indication the inspector evaluates for size, shape, and probable cause.

An ultraviolet lamp emitting near 365 nanometres that makes fluorescent penetrant or magnetic particle indications glow against a dark background. Its intensity at the inspection surface must be checked before testing.

The loss of radiation intensity as it passes through matter, caused by absorption and scattering. It depends on density and thickness and sets the exposure needed for a useful radiograph.

The thickness of a given material that cuts radiation intensity to half its original value. It is a simple way to estimate how a thickness change or added shielding will affect exposure.

Blurring on a radiograph caused by the finite size of the source and by the source-to-object and object-to-film distances. A small source placed far away reduces the blur.

A measure of how dark a processed radiograph is, taken as the logarithm of the ratio of incident to transmitted light. Codes specify minimum and maximum values for an acceptable image.

A device such as wires or holes of known size placed on the part or cassette. It proves the radiograph has enough sensitivity and contrast to show the detail required.

Radiation scattered back toward the film from objects behind the cassette, such as walls or fixtures. It adds fog and is detected by a lead letter placed on the back of the cassette.

The product of a material's density and the speed of sound in it. Differences in acoustic impedance across a boundary control how much sound energy is reflected there.

The property of certain crystals and ceramics that makes them change shape when a voltage is applied and produce a voltage when deformed. Transducers use it to send and receive ultrasound.

The region close to a transducer where the beam is narrow but pressure varies sharply along its length. Sizing is unreliable there, so testing often uses the far field or a focused probe.

The bending of an ultrasonic wave as it crosses at an angle from the wedge into the test part. Snell's law links the angles to the sound speeds, and the resulting shear wave finds angled defects.

A liquid or gel placed between the transducer and the part to exclude air and carry sound across the gap. Weak coupling causes large signal losses and inconsistent results.

A curve built from known reflectors showing how echo amplitude falls off with distance in a given material. Signals are compared with this curve to judge whether a reflector matters.

A plot of echo amplitude against time or depth along a single beam path. Reflector position is read from the horizontal axis and signal strength from the vertical axis.

The ability of a viewing system to show two closely spaced features as separate. It depends on the instrument, the lens, and the viewing distance and limits the smallest flaw that can be seen.

The ratio of an image's apparent size to the object's actual size. Higher magnification reveals finer detail but reduces the area that can be seen at one time.

The area of the part visible through the viewing instrument at one time. It shrinks as magnification increases, so the inspector must plan overlapping views to cover the whole surface.

The light falling on the inspection surface, described by its level and uniformity. Sufficient illumination and contrast are needed for reliable detection and evaluation of surface indications.

A tubular optical instrument used for remote visual inspection of internal surfaces such as bores and tubes. Rigid or flexible versions carry light in and return an image to the eye or a camera.