PCN NDT Ultrasonic Testing Level 2 Exam key terms

The product of a material's density and the speed of sound through it, expressed in Rayls. Differences in acoustic impedance between two materials determine how much sound reflects at their boundary.

The speed at which an ultrasonic wave travels through a material, set by its density and elastic properties. It is used with frequency to calculate wavelength and with time-of-flight to size defects.

The physical distance between successive compressions or rarefactions in a wave, equal to velocity divided by frequency. It sets the smallest flaw that can be reliably resolved and the size of near-surface dead zones.

The number of wave cycles completed per second, measured in hertz. Higher frequencies improve resolution and detectability of small flaws but reduce penetration in coarse or attenuating materials.

The region directly ahead of the transducer where sound pressure fluctuates in intensity and the beam is not yet fully formed. Flaw sizing in this zone is unreliable, so its length is often calculated and noted.

The region beyond the near field where sound pressure decreases steadily with distance and the beam diverges predictably. Amplitude-based sizing and distance-amplitude corrections are typically applied here.

The widening of the sound beam as it travels through the far field, described by the half-angle to the edge of the beam. Larger elements and higher frequencies produce narrower, more directional beams.

The overall loss of sound energy as a wave travels through a material, caused by absorption, scattering, and beam spreading. It is measured in decibels per unit length and limits the depth of inspection.

The fraction of incident sound energy reflected at an interface, calculated from the acoustic impedances of the two materials. It explains why air-backed cracks return strong echoes while similar-impedance bonds may not.

The change of a wave from one type to another, such as longitudinal to shear, when it strikes an interface at an oblique angle. It affects refraction angles and can create unwanted echoes in angle-beam testing.

The relationship governing how a wave bends when crossing an interface at an angle, based on the sound velocities of the two media. It is used to set the refracted angle of angle-beam probes.

The bending of a sound beam as it passes obliquely from one material into another with a different sound velocity. The change in direction is predicted by Snell's law and is central to angle-beam inspection.

The property of certain crystals and ceramics to generate a voltage when deformed and to change shape when a voltage is applied. It is the basis for converting electrical pulses to sound and echoes back to signals.

The device containing a piezoelectric element that converts electrical energy into ultrasonic waves and returning echoes back into electrical signals. Its frequency, element size, and damping shape the inspection beam.

The suppression of ringing in a transducer element, usually by a backing block, which shortens the pulse and improves axial resolution. Heavy damping also broadens bandwidth at some cost in sensitivity.

A technique in which a short electrical pulse excites the transducer and the same transducer receives returning echoes. Flaw location is derived from the time between pulse and echo.

The elapsed time between transmission of a pulse and reception of its echo. Combined with sound velocity, it gives the distance to a reflector and is the basis for depth and surface-distance readings.

A liquid or gel applied between the transducer and the part to displace air and transmit sound across the gap. Its choice affects sensitivity and must match the surface finish and material.

An inspection method in which the transducer is placed directly on the part with a thin couplant layer. It allows single-sided access and is common for weld and casting examinations.

A method in which the part and transducer are submerged in a liquid, usually water, with the beam coupled through the fluid. It provides uniform coupling and easy mechanical scanning of complex shapes.

A probe configuration that directs sound into the part at a refracted angle, usually through a plastic wedge. It is used to detect flaws oriented perpendicular to the surface, such as lack of fusion in welds.

A wave in which particle motion is perpendicular to the direction of propagation. It travels at roughly half the longitudinal velocity in steel and is the primary mode used in angle-beam weld inspection.

A wave in which particle motion is parallel to the direction of propagation. It is the first mode generated by a straight-beam contact probe and travels fastest in solids and liquids.

A wave that travels along a free surface with energy concentrated near it, penetrating about one wavelength deep. It is useful for detecting surface-breaking cracks but is easily disrupted by coatings.

A display showing echo amplitude on the vertical axis against time or distance on the horizontal axis. The operator interprets peak height and position to locate and evaluate reflectors.

An electronic or displayed gain correction that compensates for the natural loss of echo amplitude with depth. It allows reflectors at different depths to be compared against a single reference sensitivity.

A calibrated block of known material and dimensions containing artificial reflectors such as holes or notches. It is used to set sensitivity, check beam angle, and establish distance-amplitude curves.

The process of adjusting the instrument and probe so that displayed distance and amplitude correspond to known values from a reference standard. It is repeated periodically and after equipment changes.

The systematic movement of the transducer over the part surface to cover the required inspection volume. Proper overlap and speed ensure that no reflectors are missed between passes.

The ability to distinguish two reflectors that are close together, either in depth or lateral position. It depends on pulse length, frequency, and beam width.

The ability to separate two reflectors lying along the beam path at different depths. It is improved by short pulses, high damping, and higher frequency.

The near-surface region where echoes from flaws cannot be distinguished from the transmit pulse and its ring-down. It is reduced by damping, higher frequency, and dual-element probes.

Any signal on the display caused by a reflector, whether a real flaw or a geometric or electrical artifact. Evaluation determines whether an indication is relevant, nonrelevant, or a defect.

An interruption in the normal physical structure of a part, such as a crack, porosity, or inclusion. It becomes a defect only when it exceeds the applicable acceptance criteria.

The specified limits defining which indications are acceptable and which require rejection or repair. They are set by codes, standards, or contract documents for the specific component.

The process of estimating the dimensions of a discontinuity, often using the 6 dB drop, 20 dB drop, or amplitude-based techniques. Accurate sizing supports fitness-for-service decisions.

The redirection of sound energy in many directions by grains, inclusions, or small interfaces within a material. It causes grain noise and attenuation, especially at higher frequencies in coarse-grained metals.