ASNT NDT Level III key terms

A break or irregularity in a material's normal structure, such as a crack, porosity, or inclusion. It may be inherent, processing-related, or service-induced, and is not automatically a rejectable defect.

A discontinuity, or group of discontinuities, that exceeds the acceptance criteria of the applicable code or specification and therefore makes the part or weld rejectable. Every defect is a discontinuity, but most discontinuities are not defects.

The band of base metal beside a weld that was altered by welding heat but did not melt. It can become harder, softer, or more crack-susceptible than the surrounding metal and often requires its own inspection.

A weld flaw in which deposited metal fails to bond to the base metal or to a previous pass. It forms a planar, crack-like discontinuity that is hard to detect when oriented unfavorably to the test beam.

Incomplete fusion at the root of a joint, where weld metal does not extend through the full joint thickness or reach the root face. It is common in single-sided welds made without backing.

Progressive cracking caused by the simultaneous action of a tensile stress and a corrosive environment, often at stresses below yield strength. The cracks are usually branched and require both conditions to exist at once.

Rounded cavities in weld or cast metal formed by gas trapped during solidification. Acceptance depends on the size, quantity, and distribution of the pores rather than on their mere presence.

A planar separation parallel to the surface of laminated plate, clad material, or a composite, caused by poor bonding, inclusions, or impact. It strongly reflects ultrasound and is a frequent rejection cause.

Direction-dependent properties produced by grain orientation during rolling, forging, or casting. In ultrasonic testing it changes sound velocity and attenuation with test direction, affecting sensitivity and flaw sizing.

The response obtained from a nondestructive test that must be interpreted, such as a dark spot on a radiograph or a signal on an ultrasonic display. It may come from a real discontinuity or from a harmless geometric or instrumental cause.

The loss of ultrasonic or radiation energy as it travels through material, caused by absorption, scattering, and beam spreading. High attenuation reduces penetration and forces changes in technique or equipment.

The product of a material's density and its sound velocity. Differences in acoustic impedance across an interface determine how much sound is reflected and how much is transmitted, which controls echo amplitude.

The relationship describing how a sound beam bends when it crosses an interface between materials of different sound velocity. It predicts refracted angles and the angle at which mode conversion begins.

The change of an ultrasonic wave from one vibration type to another, such as longitudinal to shear, on oblique reflection or refraction at an interface. It can produce confusing extra signals during interpretation.

A liquid or gel placed between the transducer and the test surface to carry ultrasound across the air gap. Poor or uneven coupling produces weak, unstable signals and unreliable results.

A curve built from echoes of equal-size reflectors at different depths, used to compensate for beam spread and attenuation. It lets indications at different depths be compared against one reference level.

The property of certain crystals and ceramics to develop a voltage when mechanically strained and to change shape when a voltage is applied. It is the working principle of ultrasonic transducers.

Blurring of a radiographic image caused by the finite size of the radiation source and the source-to-object and object-to-film distances. It grows with larger focal spots and shorter source-to-object distance.

The degree of darkening of a processed radiograph, expressed as optical density. It must fall within specified limits before the image is acceptable for interpretation.

A device such as a wire or hole penetrameter placed on the part to demonstrate the achieved radiographic sensitivity. Its image must be visible on the radiograph or digital image at the required level.

In eddy current testing, the concentration of induced currents near the part surface, with current density falling off with depth. Higher frequency and higher conductivity increase the effect and reduce penetration.

The gap between an eddy current coil and the test surface. Changes in lift-off alter the signal and can mask or imitate discontinuities, so it must be controlled or compensated.

The tendency of a liquid penetrant to be drawn into narrow surface openings by surface tension forces. It is the mechanism that carries penetrant into tight cracks during penetrant testing.

The limits stated in a code, standard, or specification that define whether an indication or measured feature is acceptable. They may be written as size, count, density, or signal amplitude thresholds.

A procedure or process parameter whose change can affect the resulting quality and therefore requires requalification of the procedure. Examples include base material group, thickness range, and process type.

A document recording the actual parameters used and the test results obtained when a welding or NDT procedure was qualified. It provides the objective evidence supporting the written procedure.

A stage in fabrication or inspection at which work may not continue until a designated party, such as the inspector or client representative, has witnessed or reviewed the activity.

The ability to follow a material, instrument, or record back to its origin and history through documented identification. It is required for calibration records, heat numbers, and personnel certifications.

A document prepared by a recognized body through agreement among interested parties that gives rules or characteristics for activities and results. It is voluntary until a code, contract, or regulation makes it mandatory.

The statistical likelihood that a discontinuity of a given size, type, and orientation will be found by a specific technique under defined conditions. It is used to compare methods and judge inspection reliability.

How closely the measured dimensions of a discontinuity match its true dimensions. It depends on technique, orientation, and the reference used, and it drives decisions about acceptance and repair.

An engineering assessment of whether a component containing a known discontinuity can still perform safely for its intended service. It is used when code limits are impractical or to justify continued operation.

The discontinuity size at which the remaining ligament can no longer carry the design load, leading to failure. It is estimated by fracture mechanics and sets the target for detection sensitivity.

A signal or image that appears to come from a discontinuity but actually arises from geometry, surface condition, or equipment. Recognizing them prevents unnecessary repair and supports correct interpretation.

The process of evaluating a person's training, experience, and examination results to establish the ability to perform a specific NDT task. It comes before certification and is documented as evidence.

Written confirmation by an authorized body that a person has met the qualification requirements for a stated method and level. It may be issued by the employer or by a national certifying organization.

The employer's documented description of training, experience, examination, and certification requirements for NDT personnel. It must meet or exceed the minimums of the standard the employer follows.

A certified person authorized to select techniques, interpret codes and standards, and approve procedures and acceptance criteria. In employer-based programs the Level III may also examine and certify other personnel.

The near-vision and color-perception checks that NDT personnel must pass initially and at stated intervals. Near vision is usually tested with a Jaeger or equivalent chart and color vision with plates.

The periodic revalidation of an existing certification by re-examination or by documented continuing practice and education before it expires. Intervals and methods are set by the certifying standard or written practice.