ASNT NDT III UT key terms

A material property equal to density multiplied by sound velocity, describing how strongly a material resists sound transmission. Differences in this property at an interface control how much sound reflects versus passes through.

The speed at which sound travels through a material, determined by its elastic constants and density. It varies with material, wave mode, and temperature, and is used to convert transit time into distance.

The physical distance between successive compressions or rarefactions in a sound wave, equal to velocity divided by frequency. It governs sensitivity, beam spread, and the smallest detectable reflector size.

The region immediately in front of a transducer where sound pressure fluctuates along the beam axis and does not fall off smoothly. Flaw sizing and amplitude comparisons are unreliable within this zone.

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 valid in this zone.

The angle at which sound intensity falls to half its peak value as the beam diverges beyond the near field. It increases with longer wavelength and smaller transducer element diameter.

The fraction of sound energy reflected at a boundary between two materials, calculated from their acoustic impedances. Large impedance mismatches, such as metal to air, reflect nearly all energy.

A liquid or gel applied between the transducer and test surface to displace air and transmit sound into the part. Its impedance and thickness affect the amount of energy entering the material.

A crystal or ceramic that converts electrical pulses into sound and returning echoes back into electrical signals. Its thickness and properties determine the transducer's frequency and damping characteristics.

The backing material and electrical loading that stop the crystal from ringing after each pulse. Good damping shortens the pulse, improving axial resolution and permitting thin-section inspection.

The ability to separate two reflectors lying along the sound beam path at different depths. It improves with shorter wavelength and shorter pulse duration.

The shallow depth near the entry surface where echoes cannot be resolved because they merge with the initial pulse. It limits detection of near-surface flaws and depends on pulse length and instrumentation.

The large electrical and acoustic signal generated when the transducer is excited, appearing at zero depth on the display. It establishes the entry surface reference and contributes to the dead zone.

The horizontal sweep of the display that represents elapsed time or sound path distance. It must be calibrated against known thicknesses so that echo positions correspond to true depth.

A reference curve showing how echo amplitude from a known reflector decreases with sound path distance. It provides a baseline against which indications are compared for acceptance decisions.

A block of material with known dimensions, sound velocity, and machined reflectors used to calibrate equipment and establish sensitivity. Its material and surface finish should approximate the part being inspected.

The process of adjusting instrument settings so that displayed distance and amplitude accurately represent known reference reflectors. It is verified periodically during inspection to ensure valid results.

An inspection method in which sound enters the part perpendicular to the surface and travels parallel to the beam axis. It is used primarily to detect flaws parallel to the entry surface, such as laminations.

An inspection method in which a refracting wedge directs sound into the part at an angle, producing shear waves. It is used to detect flaws oriented at angles to the surface, such as lack of fusion.

The angle at which a sound beam bends as it passes from the wedge into the test material, governed by Snell's law. It is selected so the beam intersects the expected flaw orientation effectively.

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

The systematic path the transducer follows over the test surface to ensure full coverage of the inspection volume. Common patterns include raster, zigzag, and circumferential scans.

The proportion of the required inspection volume actually interrogated by the sound beam during scanning. It depends on beam dimensions, scan index spacing, and scanning speed.

A planar discontinuity parallel to the rolled or forged surface, typically from inclusions or gas porosity. It is readily detected with straight beam testing and can mask deeper reflectors.

Any signal on the display caused by a reflector, whether a true flaw, geometry, or electrical noise. Indications must be evaluated to determine whether they represent relevant discontinuities.

An actual interruption in material continuity, such as a crack, porosity, or inclusion. It becomes a defect only when it exceeds acceptance criteria.

An indication judged to originate from a genuine discontinuity requiring evaluation against acceptance criteria. Distinguishing relevant from non-relevant signals is a core interpretation skill.

The limits defined by code or specification that determine whether an indication is acceptable. They typically address amplitude, length, and number of indications relative to a reference.

The documented record of inspection scope, equipment, calibration, findings, and disposition. A complete report allows independent verification and supports the acceptance decision.

A controlled document describing equipment, materials, calibration, scanning, and acceptance steps for a specific inspection. It must be qualified and approved before use on production work.

Demonstration that a written procedure reliably detects the intended discontinuities under actual conditions. It is often required when codes mandate a performance demonstration or when no standard technique applies.

Formal recognition that an individual has met the training, experience, and examination requirements for a certification level. It is typically administered by a certifying body under a recognized scheme.

The highest certification level, authorizing the holder to develop and approve procedures, interpret codes, and direct the qualification of other personnel. It requires extensive experience and examination.

A colloquial term for the judgment and experience a certified inspector applies when interpreting ambiguous signals. It complements, but never replaces, documented procedures and acceptance criteria.

A machined feature of known size and shape, such as a side-drilled hole or flat-bottom hole, used to set sensitivity. Its response establishes the amplitude baseline for comparisons.

The loss of sound energy as it travels through a material due to scattering and absorption. High attenuation reduces echo amplitude and may require increased gain or lower frequency.

The number of wave cycles per second, expressed in hertz. Higher frequencies improve resolution and detect smaller flaws but penetrate less and attenuate more.