NDT Glossary: 100+ Essential Terms Defined

Comprehensive reference guide to nondestructive testing terminology, from fundamental NDT concepts to method-specific technical terms. Covers all six major NDT methods and industry standards.

Nondestructive testing employs specialized terminology that can be challenging for those new to the field. This comprehensive glossary defines over 100 essential NDT terms, from foundational concepts to method-specific vocabulary. Whether you're preparing for ASNT certification, learning NDT technology, or simply need quick reference to technical terminology, this glossary provides clear definitions of key concepts.

The terms are arranged alphabetically and cover all major NDT methods including ultrasonic testing, radiography, magnetic particle testing, liquid penetrant testing, and eddy current testing. Additional terms address API standards, ASME codes, and industry-specific applications. Understanding this terminology is essential for NDT professionals seeking certification, for engineers specifying NDT inspections, and for anyone involved in asset integrity management. Use this glossary alongside formal training programs and standards study to build comprehensive knowledge of NDT concepts and terminology.

A-Scan

Amplitude-scan display in ultrasonic testing showing signal amplitude versus time, used to evaluate defect characteristics and distance. A-scans provide quantitative data for defect sizing.

ASME

American Society of Mechanical Engineers. Develops standards for pressure vessel design, inspection, and fabrication (Section VIII, Section V). ASME codes are widely adopted globally.

ASNT

American Society for Nondestructive Testing. Primary professional organization in NDT, administers certification programs and develops recommended practices. SNT-TC-1A is the primary certification standard.

Attenuation

Decrease in ultrasonic signal strength as it travels through material, caused by scattering and absorption. Used to detect defects and evaluate material condition in thickness measurements.

B-Scan

Two-dimensional cross-sectional display in ultrasonic testing showing material depth versus horizontal distance, creating a profile view of inspected material. B-scans provide spatial visualization.

Calibration

Process of adjusting and verifying NDT equipment to ensure accuracy and compliance with applicable standards. Periodic calibration is mandatory for all inspection equipment.

Certification

Official qualification granted by ASNT after successful completion of training, practical experience, and examination. Levels include Level I, II, and III based on expertise.

Couplant

Liquid substance applied between ultrasonic transducer and inspected surface to facilitate acoustic signal transmission. Common couplings include mineral oil and water-soluble gels.

C-Scan

Three-dimensional color map display in ultrasonic testing showing signal amplitude at specific depth, useful for visualizing defect locations and sizing. C-scans provide top-down visualization.

Defect

Discontinuity or imperfection in material that may affect safety or performance. May include cracks, voids, inclusions, or corrosion. Defects are evaluated against acceptance criteria.

Discontinuity

Interruption in the physical structure or configuration of material. Not all discontinuities are defects; some may not affect performance and may be acceptable.

Eddy Current Testing (ET)

Electromagnetic NDT method that induces circular electrical currents in conductive materials to detect surface and near-surface defects without contact.

Flaw

Undesirable condition in material that reduces serviceability or may lead to failure. Similar to defect but often implies severity or rejection criteria.

Frequency

Number of oscillations per unit time in an ultrasonic or electromagnetic wave, measured in megahertz (MHz). Higher frequencies provide better resolution but less penetration.

Gain

Electronic amplification of signal in NDT equipment. Increased gain amplifies weak signals but also amplifies noise. Must be properly controlled for accurate results.

Gating

Electronic process of focusing on specific time interval or material depth in ultrasonic testing to isolate signals of interest and reject unwanted signals.

Gray Scale

Display format in radiography and ultrasonic testing using shades of gray from black to white to represent signal intensity or image density.

Half-Skip Distance

Distance traveled by ultrasonic wave from first back-surface reflection to surface in angled beam testing, used for angle verification and calibration.

Hardenable Steel

Steel capable of significant hardness increase through heat treatment, typically containing alloying elements like manganese, chromium, or molybdenum.

Heat Affected Zone (HAZ)

Area adjacent to a weld where material properties change due to heat during welding, often susceptible to cracking or property degradation.

Impedance

Property of material resistance to ultrasonic wave transmission. Impedance mismatch at material boundaries causes signal reflection used in ultrasonic detection.

Indication

Signal or response detected by NDT equipment. Not all indications represent defects; some may be noise or legitimate material features requiring interpretation.

Sensitivity

Ability of NDT equipment to detect small defects. Higher sensitivity can detect smaller flaws but may increase false indications if not properly controlled.

Lateral Wave

Ultrasonic wave traveling along surface of material at velocity of shear waves. Used in surface wave inspection for surface-breaking crack detection.

Level I Technician

Entry-level NDT certification demonstrating basic knowledge and ability to perform inspections under supervision. Requires 30 hours training and 200 hours experience.

Level II Technician

Intermediate NDT certification demonstrating ability to conduct and interpret inspections independently. Requires Level I training plus 2 years experience minimum.

Level III Technician

Expert NDT certification demonstrating ability to develop procedures, train technicians, and make final interpretation decisions. Requires 5 years experience with 2 at Level II.

Liquid Penetrant Testing (PT)

NDT method using low-viscosity liquid to penetrate surface-breaking defects, followed by removal and application of developer to make indications visible.

Magnetic Particle Testing (MPT)

NDT method magnetizing ferromagnetic material and applying iron powder particles to reveal surface and near-surface defects through particle agglomeration.

NADCAP

National Aerospace and Defense Contractors Accreditation Program. Aerospace industry program for qualifying testing and inspection service providers to rigorous standards.

Noise

Unwanted electronic or acoustic signals that interfere with legitimate NDT indications. Proper setup and filtering minimizes noise and improves signal quality.

Phased Array Ultrasonic Testing (PAUT)

Advanced ultrasonic method using electronically-controlled transducer arrays enabling rapid beam steering and focusing without mechanical movement.

Probe

General term for transducer or sensor in NDT equipment. In ultrasonic testing, the device that generates and receives sound waves.

Radiography

NDT method using X-rays or gamma rays to create images of material internal structure, widely used for weld inspection and defect detection.

Rayleigh Wave

Surface wave in ultrasonic testing traveling along material surface useful for surface-breaking crack detection without need for coupling.

Refraction

Change in direction of ultrasonic wave as it enters material with different velocity. Occurs at angle-beam interfaces and enables inspection at angles.

Rejection Criteria

Specified standards or limits for determining whether indications are acceptable or require remedial action, typically based on applicable standards.

Resolution

Ability to distinguish between two closely-spaced defects or features. Higher frequency ultrasonic provides better resolution but less penetration depth.

Scattering

Ultrasonic wave dispersion when traveling through material with multiple small reflective boundaries, causing signal attenuation and noise.

Shear Wave

Ultrasonic wave where particle motion perpendicular to wave direction. Penetrates steel more effectively than compression waves for deep inspection.

Signal-to-Noise Ratio

Relative strength of legitimate signal versus background noise. High ratio indicates clean signals; low ratio indicates noisy conditions requiring adjustment.

Skip Distance

Distance traveled by ultrasonic beam in angled-beam contact inspection, including initial entry and reflection from back surface.

Specification

Documented requirements defining inspection procedures, standards, acceptance criteria, and documentation requirements for specific applications.

Surface Wave

Ultrasonic wave traveling along material surface at velocity approximately 90% of shear wave velocity, used for surface-breaking defect detection.

Thickness Gauging

Ultrasonic measurement of remaining material thickness, commonly used to assess corrosion or erosion of pipes and vessels.

Time-Base

Horizontal axis on ultrasonic A-scan display representing time (or distance), used to measure defect location and evaluate signals.

Transducer

Device converting electrical energy to mechanical energy (or vice versa) for ultrasonic or eddy current testing. Quality transducers are critical for reliable inspection.

Velocity

Speed at which ultrasonic waves travel through material, varying by wave mode and material properties. Used to calculate distance measurements accurately.

Visual Inspection

NDT method using direct observation (with or without magnification) to detect surface defects, dimensional issues, or structural problems.

Wavelength

Distance between successive peaks in ultrasonic wave, calculated as velocity divided by frequency. Smaller wavelength provides better resolution.

Weld Metal

Material deposited during welding process, typically having different properties than base material. Prone to defects like porosity and lack of fusion.

Zyglo

Trade name for a high-visibility penetrant dye used in liquid penetrant testing, providing excellent contrast for defect visualization.

API Standards

American Petroleum Institute standards governing oil and gas operations. Includes API 510 (pressure vessels), API 570 (piping), API 653 (tanks), API 580 (RBI).

Acoustic Emission

Advanced NDT technique detecting stress waves generated by crack growth. Used for monitoring structural integrity and detecting active defects.

Backup Ring

Support ring used during welding to prevent molten metal penetration. Backup ring removal inspection is often required in critical applications.

Beam Angle

Angle of ultrasonic transducer relative to material surface, typically 45°, 60°, or 70°. Angle selection depends on geometry and defect type.

Coverage

Percentage of equipment or structure actually inspected. Higher coverage provides more confidence in asset condition assessment.

Degrees Celsius

Temperature measurement unit. Important for understanding material properties and environmental conditions during inspection.

Electroslag Welding (ESW)

Welding process using electrical resistance heating. ESW joints often require additional inspection scrutiny due to process characteristics.

False Positive

Detection of indication that is not actually a defect. False positives can lead to unnecessary repairs and increased costs.

Full Volume Inspection

Inspection technique that examines entire volume of material, typically using focused scanning patterns to ensure complete coverage.

Gas Tungsten Arc Welding (GTAW)

Manual welding process commonly used for pressure equipment. Welds must meet strict inspection requirements per standards.

Hardfacing

Application of hard, wear-resistant material to surfaces. Hardfaced components require specialized inspection techniques.

Inclination Angle

Angle of weld defect relative to material surface. Critical for assessing defect severity and fracture potential.

Inspection Interval

Time period between consecutive inspections. Intervals determined by standards, risk assessment, or regulatory requirements.

Joint Efficiency

Ratio of weld strength to base material strength, expressed as percentage. Joint efficiency affects pressure vessel design thickness.

Expand Your NDT Knowledge

This glossary provides quick reference to essential NDT terminology. For deeper understanding of specific methods and applications, explore our comprehensive resource guides and professional training programs.

Industry Standards References: Terms in this glossary are drawn from ASNT standards, ASTM International standards, and API Recommended Practices that govern NDT practice.

Certification Preparation: Understanding these terms is essential for ASNT certification exam success. Review this glossary while studying standards and attending certification preparation courses.

Method-Specific Learning: Each NDT method (ultrasonic, radiography, penetrant, magnetic particle, eddy current) has specialized terminology. This glossary covers cross-method terms relevant to all methods.

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This glossary is designed as a quick reference guide. For comprehensive training on NDT concepts and methods, formal training programs from Atlantis NDT provide in-depth education and preparation for professional certification.