A Working Glossary of NDT and Inspection Vocabulary — From Indication and Couplant Through DAC Curves and Acceptance Criteria
A practical glossary explaining the core vocabulary of non-destructive testing and inspection — indication, acceptance criteria, couplant, probe of curvature, DAC curve, and related terms — written for purchasers and engineers who want genuine technical grounding when discussing NDT results and inspection reports with a supplier. Shivam Forge, Rajkot, India. Call +91-9265772827.
NDT and inspection terminology is specific vocabulary describing real, physical aspects of how an inspection method actually works and how its results get evaluated, and purchasers or engineers who only encounter inspection reports occasionally often find those reports harder to interpret than necessary simply because terms like 'indication,' 'couplant,' or 'DAC curve' appear without explanation, on the assumption that anyone reading an NDT report already understands the underlying methodology. This glossary exists to close that specific gap: each term below describes a real, physical concept or a defined evaluation criterion used in ultrasonic, radiographic, magnetic particle, or penetrant testing, and understanding these terms genuinely changes how useful an inspection report becomes to read — turning it from an opaque pass/fail statement into a document whose actual technical content (what was found, how it was evaluated, and against what standard) becomes accessible. This glossary is deliberately scoped to inspection and NDT-specific vocabulary alone, distinct from our companion glossaries covering general forging process terminology and heat treatment terminology — the terms defined here are specifically the vocabulary of the inspection and evaluation process itself, applicable across whichever NDT method a given component's specification actually calls for.
Any detected signal or visible response produced during an inspection — a UT signal reflection, an MPI powder buildup, a PT dye bleed-out — that requires evaluation to determine whether it represents an actual discontinuity and, if so, whether that discontinuity is acceptable under the applicable criteria.
The defined standard, specified by an applicable code, industry standard, or customer drawing, against which a detected indication is evaluated to determine whether the component passes or fails inspection — acceptance criteria define both what indication characteristics are relevant and what magnitude is tolerable.
A relevant indication is one confirmed to correspond to an actual material discontinuity requiring evaluation against acceptance criteria; a false (or non-relevant) indication results from a source other than a genuine discontinuity — surface condition, geometry, or equipment artifact — and is dispositioned as not requiring further evaluation once identified as such.
The final determination made about a component or indication following inspection and evaluation — accept, reject, repair, or requires further evaluation — representing the actual decision an inspection process exists to support.
The liquid medium (typically a gel or water) applied between an ultrasonic transducer and the component surface to transfer sound energy efficiently into the material, eliminating the air gap that would otherwise reflect nearly all ultrasonic energy before it enters the part.
A reference curve used in ultrasonic testing that corrects for the natural reduction in signal amplitude as sound travels farther into a material, allowing indications detected at different depths to be evaluated on a consistent, depth-corrected basis against acceptance criteria.
In eddy current and some ultrasonic applications, a reference describing how a transducer or probe's contact geometry is matched to a curved component surface, affecting coupling quality and, in eddy current testing, the calibration reference used for accurate measurement on curved parts.
A block of known material and geometry containing artificial reference reflectors or discontinuities of documented size, used to calibrate inspection equipment sensitivity before actual component inspection, ensuring the equipment's response to a known reflector is verified and consistent.
NDT and inspection vocabulary can feel opaque to purchasers and engineers who only encounter inspection reports periodically, but nearly every term in standard use describes something specific and physically concrete about how an inspection method actually works or how its results get formally evaluated — not an arbitrary jargon layer. Understanding this vocabulary genuinely changes how useful an inspection report becomes: rather than reading as an opaque pass or fail statement, a report becomes legible as a document describing exactly what was detected, how it was evaluated, and against what defined standard the accept/reject decision was actually made.
The evaluation-process vocabulary is the first and arguably most important set of terms to understand, because it clarifies what an inspection report is actually telling you at each stage. An indication is simply anything the inspection method detected — a reflected ultrasonic signal, a buildup of magnetic particle powder, a bleed-out of penetrant dye — and on its own, an indication says nothing about whether a genuine problem exists. That determination requires evaluation: confirming whether the indication is relevant (corresponds to an actual material discontinuity) or false (arises from surface condition, part geometry, or an equipment artifact with no real discontinuity behind it), and then, for relevant indications, comparing the finding against defined acceptance criteria — the standard, drawn from an applicable code, industry standard, or the customer's own drawing, that actually determines whether the finding is tolerable. The final disposition — accept, reject, repair, or requiring further evaluation — is the practical conclusion this entire evaluation chain exists to produce.
A second set of terms describes the technical mechanics specific inspection methods depend on to actually function correctly and produce reliable, comparable results. Couplant is essential to ultrasonic testing specifically because air transmits ultrasonic energy so poorly that any air gap between transducer and part would reflect away nearly all the sound energy before it entered the material at all — couplant provides the acoustic bridge that makes UT physically possible. The DAC curve addresses a related but distinct UT challenge: because signal amplitude naturally attenuates with travel distance through material, an identical-size discontinuity produces a weaker signal the deeper it sits in a component, and a DAC curve corrects for this so indications at different depths can be evaluated on a fair, consistent basis. Calibration blocks, meanwhile, provide the reference standard that confirms inspection equipment sensitivity is verified and working correctly before it's ever trusted to evaluate an actual unknown indication on a real component.
For customers or engineers who want to discuss inspection reports, NDT method selection, or acceptance criteria using accurate, shared vocabulary, Shivam Forge's quality engineering team is glad to walk through any of these terms as they apply to your specific inspection requirement. Contact us at +91-9265772827 or sales@shivamforge.com with your component drawing or inspection question to discuss your requirement.
An indication is simply a detected signal or response from an inspection method — it hasn't yet been evaluated. A defect (or rejectable discontinuity) is an indication that has been evaluated against acceptance criteria and determined to exceed the tolerable limit. Not every indication is a defect; many indications are evaluated and found acceptable, or determined to be false (non-relevant) indications with no actual material discontinuity behind them.
Acceptance criteria is the defined standard — from an applicable code, industry standard, or the customer's own drawing — that a detected, relevant indication is measured against to determine pass or fail. Different applications and industries specify different acceptance criteria for the same NDT method, which is why the same physical indication could be acceptable on one component and rejectable on another depending on the applicable specification.
Air is an extremely poor transmitter of ultrasonic energy compared to solid materials, so any air gap between the transducer and the component surface would reflect nearly all the sound energy before it ever entered the part being tested. Couplant — typically a gel or water — fills that gap, providing an efficient acoustic path for sound energy to actually transfer from the transducer into the material.
A DAC (distance-amplitude correction) curve accounts for the fact that ultrasonic signal amplitude naturally decreases the farther the sound travels through material, even from an identical-size reflector. Without this correction, an identical discontinuity would appear to produce a weaker signal simply because it's located deeper in the part. The DAC curve lets an inspector evaluate indication amplitude on a depth-corrected, consistent basis.
A calibration block is a reference standard of known material and geometry containing artificial reflectors of documented size, used before actual inspection to verify and calibrate the inspection equipment's sensitivity — confirming the equipment produces a known, expected response to a known reflector before it's relied on to evaluate real, unknown indications on the actual component.
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