Non Destructive Testing (NDT) : Terms You Need to Understand to Use Correctly as NDT Professional !

Abstract 

Non-destructive tests (NDT) are methods that do not damage the parts being tested. NDT uses various inspection techniques to assess individual or group components. By employing different principles from physics, chemistry, and mathematics, NDT can test components without causing damage.

This glossary contains useful descriptions of industry specific words, terms, lingo, and phrases for NDT and it's methods 

Keywords 

Non Destructive Testing, Visual Testing, Magnetic Particle Test, Dye Penetrant Test, Ultrasonic Testing, Radiography, Infrared Scanning.

Learning Outcomes 

After undergoing this article you will be able to understand the useful NDT terms presented sequentially from A to Z 

Useful Non Destructive Testing Terms 

• A-scan: see Ultrasonic imaging
• Acoustic emission (AE): the class of phenomena whereby transient elastic waves are generated by the rapid release of energy from localised sources within a material.  The term is used collectively for the generation, propagation and detection of stress waves.  Loosely associated with non-destructive testing, although usually relying on monitoring the progress of damage.  Sometimes referred to as Stress Wave Emission (SWE) or microseismic activity. See ASTM E610-82 for a comprehensive set of definitions.
• Acoustic emission CARP codes: standard methods of acoustic emission testing as adopted by the Committee on Acoustic Emission from Reinforced Plastics.
• Acoustic emission count (N): the number of times the acoustic emission signal exceeds a preset threshold during any selected portion of a test.
• Acoustic emission count rate: the time rate at which AE counts occur.
• Acoustic emission dynamic clouds: use of factorial analysis and classification methods to analyse multivariate data from acoustic emission tests.
• Acoustic emission event: a local material change giving rise to AE.  One event would include all AE counts in a single signal envelope.
• Acoustic emission event energy: the total elastic energy released by an AE event.
• Acoustic emission EWGAE codes: standard methods of acoustic emission testing as adopted by the European Working Group on Acoustic Emission.
• Acoustic emission sensor: a detection device, generally piezoelectric, that transforms the particle motion produced by an elastic wave into an electric signal.
• Acoustic emission signal: an electrical signal obtained by detection of one or more acoustic emission events.
• Acoustic emission signal amplitude: the peak voltage of the largest excursion attained by the signal waveform from an AE event.
• Acoustic emission signature: a characteristic set of reproducible attributes of acoustic emission signals associated with a specific test article as observed with a particular instrumentation system under specified test conditions.
• Acoustic emission source location (AESL): see Source location.
• Acoustic emission stress wave factor: see Stress wave factor.
• Acoustic emission transducer: see Acoustic emission sensor.
• Acoustic emission wave propagation: see Wave propagation.
• Acoustic emission waveguide: a device that couples elastic energy from a structure or other test object to a remotely mounted sensor.  Usually a solid wire or rod..
• Acoustic microscopy: see Scanning acoustic microscopy.
• Acoustical holography: the recording of an acoustic pattern and its subsequent conversion to an optical analogue which may be reconstructed on a point-by-point basis with visible light to create a 3-D pattern corresponding to the interior of the slab.
• Acousto-ultrasonics (AU): a combination of acoustic emission methodology and ultrasonic materials characterisation, usually in the 500 kHz to 1 MHz frequency range.  Also called stress wave factor method, it is less concerned with overt flaw detection than with the integrated effect of a diffuse population of subcritical flaws.
• Acoustography: acousto-optical liquid crystal display for ultrasonic testing.
• AE: Acoustic Emission.
• AESL: Acoustic Emission Source Location.
• ALOK: (Amplituden und Laufzeit Orts Kuwen) - Amplitude and time transit locus curves.  Variation of ultrasonic echo delay time as a function of probe movement is used to select those parts of a pattern of reflected pulses which truly arrive from the defect.
• Amplitude distribution in acoustic emission: analysis of AE signals either as an occurrence count against pulse amplitude plot, or as a logarithmic cumulative distribution.  The latter is quantified as the 'b-value'.
• Applied potential tomography (APT): the use of a series of surface electrodes, causing current to flow in the body,and measurement of the potential between pairs of electrodes for the construction of tomographic images.
• APT: Applied Potential Tomography.
• Arbitrary units of light emission (AULE): arbitrary measurement for light emission with no study of spectral distribution.
• Arrival time interval (delta t): the time interval between detected arrivals of an acoustic emission wave at different sensors of an array.
• ATR: Attenuated Total Reflection.
• Attenuated total reflection (ATR): one operating mode for FTIR.
• Attenuation: decrease in signal levels with distance travelled.  The ratio between two signal levels is usually expressed in decibels.
• AU: Acousto-Ultrasonics.
• Audrey dielectrometer: abbreviation of AUtomatic DielectRomEtrY.
• AULE: Arbitrary Units of Light Emission.
• AUSS: Automatic Ultrasonic Scanning System.
• Automated fringe analysis: computer image processing of optical fringes.
• Automatic ultrasonic scanning system (AUSS): ultrasound water-squirter C-scan system developed for McDonnell Aircraft Co.
• Autoradiography: the use of a screen which is excited by neutrons and emits radiation to sensitize a photographic film for neutron radiography.  The screen is exposed to the neutrons and immediately transferred onto the film.

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• B-scan: see Ultrasonic imaging b-value: see Amplitude distribution in acoustic emission.
• Band selectable Fourier (ZOOM) analysis: concentration on a narrow range of vibration frequencies in the region of a resonant peak.
• Blockchain: "a collection of data that is shared across the supply chain using encryption technology.  From [source] to end product, blockchain provides secrity and transparency in business transactions by recording and stamping each component of the product, tracking each process within the supply chain.  Aiming to be tamper-proof, the information recorded at each part of the chain is very difficult to change". [Materials World, February 2020, 28(2), 9.
• Body waves: ultrasonic P-, SH- and SV-waves.
• Brittle lacquer: surface coating which cracks indicating the locality and magnitude of strains.
• Broadband sensors: acoustic emission/ultrasound transducers which can be used for spectrum analysis.
• Bulk waves: ultrasonic longitudinal (compression) and transverse (shear) waves.
• Burst emission: a qualitative description of the discrete signal related to an individual acoustic emission event.

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• C-band microwaves: electromagnetic radiation in the frequency range 4 - 8.2 GHz (wavelength in air typically 50 mm at 6 GHz).
• C-scan: see Ultrasonic imaging.
• CARP: Committee on Acoustic Emission from Reinforced Plastics.
• CAT: Computer-Aided Tomography.
• CAT-scan: Computer-Aided Tomography.
• Chemical shift anisotropy (CSA): nuclear magnetic resonance at different frequencies dependent on the orientation of a solid sample.
• Chemical spectroscopy: principally IR, Raman, NMR.
• Chemiluminescence (CL): the emission of light stimulated by chemical reactions.
• CL: Chemiluminescence.
• Clinking: acoustic emission from martensitic transformations.
• Coin-tap test: monitoring changes of resonant frequency and damping of vibrations in a small local area.  See also Wheel-tap test..
• Colour coding: the use of colour to distinguish differences in the magnitude of a parameter in NDT.
• Colour-level slicing: a colour coding technique in which all the data within a selected window is mapped to a single colour.
• Combination effect: see Raman effect.
• Committee on Acoustic Emission from Reinforced Plastics (CARP): initially a working group of the Corrosion Resistant Structures Committee of the Society of the Plastics Industry (USA) Reinforced Plastics/Composites Institute, this committee is now affiliated to the American Society for Nondestructive Testing.
• Compression waves: ultrasonic longitudinal bulk waves.
• Compton effect: the increase in wavelength of electromagnetic radiation on being scattered by interaction with weakly bound electrons in material objects.  It is assumed that the orbital electrons are free and therefore proportional to the first power of the electron density, causing diffuse scattering of X-rays.
• Compton scattering: interaction in which the incident photon causes an orbital electron to be ejected from an atom.The photon emerges at a scatter angle with lower energy.
• Computed tomography (CT): see Computer tomography.
• Computer tomography (CT): the determination of the internal information of a three dimensional body from the values of a set of line integrals at the boundaries.  Imaging of a slice at any arbitrary orientation through a body.
• Computer-aided tomography (CAT): see Computer tomography.
• Contact angle: the shape of a liquid droplet spread on a monofilament reflects the energetics of wetting at the solid fibre/liquid interface.
• Continuous emission: a qualitative description of the sustained signal level produced by rapidly occurring acoustic emission events.
• Continuous wave (CW): an uninterrupted signal, as distinct from pulses.
• Convolution function: filtering technique for image reconstruction.
• Corona discharge: NDT technique in which a high-intensity electric field is imposed across a dielectric material.  Gasses in voids in the body are ionised, and may be detected by the radiation emitted or by changes in the current in the energising coil.
• Couplant: a material used at the structure-to-sensor interface to improve the transmission of acoustic emission energy across the interface.
• CP: Cross polarisation.
• CP-MAS: Cross Polarisation with Magic Angle Spinning in NMR.
• Creeping waves: low intensity (near-)surface compression waves travelling parallel to the surface and not affected by surface irregularities.
• Critical angle: the angle, predicted by Snell's law, at which all radiation will be reflected, as is the case with light.
• Cross polarisation: The observation of NMR spins which may be completely obscured by homonuclear dipole-dipole coupling, by dilution in a sample with abundant spins, enhancing sensitivity through energy conserving transitions induced by heteronuclear dipole-dipole coupling.
• CSA: Chemical Shift Anisotropy.
• CT: Computed tomography, or Computer tomography.
• Cumulative amplitude distribution, F(V): the number of acoustic emission events with signals that exceed an arbitrary amplitude as a function of amplitude V.
• Cumulative threshold crossing distribution, Ft(V): the number of times an acoustic emission signal exceeds an arbitrary threshold as a function of the threshold voltage (V).
• CW: Continuous wave.

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• D-scan: see Ultrasonic imaging.
• Damping: the rate of decay of vibrations.
• DDA: Dynamic Dielectric Analysis.
• Delta T: see Arrival time interval.
• DETA: DiElectric Thermal Analysis.
• DFA: Diffuse Field Analysis.
• DIB: Diiodobutane (opaque penetrant for radiography).
• Dielectric constant: the real part of the complex relative permittivity of a dielectric.  The capacitance of the material in unit geometry in a static field.
• Dielectric loss factor: the product of the dielectric constant and the dissipation factor.
• Dielectric permittivity: the capacitance of a material in unit geometry.  In a static field this is the dielectric constant. In an alternating field it is a complex number combining the dielectric constant and the loss factor.
• Dielectric relaxation time: reciprocal of the angular frequency at maximum loss component.
• Dielectric spectroscopy: monitoring the response of dielectric materials at different frequencies.
• Dielectric thermal analysis (DETA): trade name for Polymer Laboratories instrumentation.
• Dielectrometry: the measurement of dielectric properties.
• Differential amplitude distribution, f(V): the number of acoustic emission events with signal amplitudes between Vi and Vj as a function of the amplitude, V.  f(V) is the absolute value of the derivative of the cumulative amplitude distribution, F(V).
• Differential scanning calorimetry (DSC): Direct measurement of the rate of heat release, indicating thermal state of cure, cure kinetics and glass transition effects.  Quantitative measurement of heat of reaction and heat capacity.
• Differential thermal analysis (DTA): measurement of the temperature difference between the sample and a reference material during a programmed thermal scan.
• Differential threshold crossing distribution, ft(V): the number of times the acoustic emission signal waveform has a peak between thresholds Vi and Vj as a function of the threshold, V.  ft(V) is the absolute value of the derivative of the cumulative threshold crossing distribution, Ft(V).
• Diffraction tomography: microwave technique to infer the internal structure of an object from multiple views of the diffraction shadow.
• Diffuse field analysis (DFA): the application of concepts of statistical room acoustics to the study of diffuse, short wavelength, reverberant ultrasound in solids.
• Diffuse reflectance (DR): one operating mode for FTIR.
• Digital radiography: computer-based image acquisition, image conditioning and image processing for radiography.
• Dispersion: change of sound velocity with wavelength causing component frequencies of a pulse to become separated in phase.
• DMTA: Dynamic Mechanical Thermal Analysis.
• Double through-transmission: ultrasound NDT method in which the signal passes through the object twice, being reflected either from the back wall or an external reflector.
• Double quantum decoupling: technique for the observation of high resolution proton NMR spectra in solids by isotopic dilution in a deuterated solid.
• DR: Diffuse Reflectance.
• DSC: Differential Scanning Calorimetry.
• DTA: Differential Thermal Analysis.
• Dunting: acoustic emission in ceramics.
• Dynamic dielectric analysis (DDA): trade name adopted by the Williamsburg Group.
• Dynamic mechanical thermal analysis (DMTA): method to isolate the storage and loss components of rheological response for supported or unsupported polymers.  Highly sensitive to both flow and glass transitions at all states of cure.

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• EATF: Externally Applied Thermal Field.
• ECD: Electrostatic Charge Decay.
• Eddy current: NDT technique utilising electromagnetic induction.  Only practical for conducting fibres (eg carbon).
• Eddy current tomography: imaging of an arbitrary slice using eddy current methods.
• Edge replication: technique for making replicas of specimen edges to enhance X-ray NDT of fatigue specimens.
• EE: Electron Emission in fracto-emission monitoring.
• Electric screening effectiveness (ESE): measurement (in decibels) of the attenuation of a microwave or radiofrequency signal at a specified frequency.
• Electrical imaging: use of electrified particles, potential mapping or applied potential tomography to image the variation in electrical response of a body.
• Electrified particle imaging: use of charged particles to detect surface discontinuities.
• Electron emission: the emission of electrons both during and following fracture.
• Electron spin resonance (ESR): the interaction of the magnetic moment on an unpaired electron with the magnetic moment on the nucleus.
• Electron spin resonance imaging: computed tomography using EAR.
• Electron spin resonance spectroscopy: chemical spectroscopic technique involving strong magnetic fields and microwave frequency fields.
• Electron stimulated desorption (ESD): the release of ions and neutral particles (fracto-emission) as a result of emitted electrons bombarding the surface when pulled back by the static charge on the fracture surface.
• Electronic speckle pattern interferometry (ESPI): the use of video recording and electronic image comparison for speckle interferometry instead of photographic processing.
• Electrostatic charge decay (ECD): NDT technique providing an indication of location, lateral dimension and shape of flaws.
• Embedded optical fibre strain gauge: the use of monomode optical fibres within the matrix of a composite as part of an interferometer to infer strains from minute changes of length.
• Embedded optical strain gauge: any strain sensor contained within the matrix of a composite, whether connected to systems outside the material (eg optical fibres), or interrogated by laser beams (eg Kevlar fibres have strain sensitive bands in the Raman spectrum).
• Embedded sensors: transducers wholly or partially enclosed by the matrix of the composite.
• EMI: Electro-Magnetic Interference.
• ESD: Electron Stimulated Desorption.
• ESE: Electric Screening Effectiveness.
• ESPI: Electronic Speckle Pattern Interferometry.
• EAR: Electron Spin Resonance.
• European Working Group on Acoustic Emission (EWGAE): a pan-European committee which defines standards and organises an annual conference.
• Event count (Ne): the number obtained by counting each discerned acoustic emission event once.
• Event duration: the time from first positive threshold crossing of an acoustic emission burst signal to the final threshold crossing within the ringdown envelope.
• EWGAE: European Working Group on Acoustic Emission.
• EWGAE calibrator: a particular configuration of the pencil lead break method for calibrating acoustic emission systems.
• Externally applied thermal field (EATF): the use of external heat sources for thermography (rather than stress generated thermal fields).

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• f(V): see Differential amplitude distribution.
• F(V): see Cumulative amplitude distribution.
• F-map: Feature map in ultrasonic imaging.
• F-scan ultrasonics: see Ultrasonic imaging.
• Fast Fourier transform (FFT): the computer algorithm used to calculate the Fourier transform, which simplifies earlier methods by a system of pairing, resulting in a shorter calculation.
• FD: Fractal dimension
• FDI: Frequency Domain Interferometry.
• FE: Fracto-Emission.
• Feature mapping: detection and identification of anomalies in a material from features of the received wave, and display thereof (aka F-map).
• Felicity effect: the breakdown of the Kaiser effect in acoustic emission.  Emissions occur on reloading at a specific fraction of the previous maximum load.
• Felicity percentage: the ratio of the load at which acoustic emissions occur on reloading to the previous maximum load, as a percentage Felicity ratio: the ratio of the load at which acoustic emissions occur on reloading to the previous maximum load.
• FFT: Fast Fourier transform.
• Fibre optic Raman spectroscopy: the use of fibre optics to propagate the laser beam for Raman spectroscopy.  Used as embedded cure sensors.
• Fibre percolation model (FPM): model to predict the threshold volume fraction above which the composite becomes electrically conductive.
• FID: Free Induction Decay.
• Filmless radiography: use of fluorescent screens or xeroradiography in place of photographic film for X-ray inspection.
• Fluorescent screen: method of direct imaging for soft x-rays.
• Fokker bond tester: resonance continuous wave instrument for mechanical impedance examination of bonding in honeycomb structures and laminates.  Frequency and amplitude of transducer vibration can be related to bond strength.
• Fourier transform: the basic mathematics necessary to express an interferogram pattern in terms of the individual frequencies, and the interconversion process.
• Fourier transform infrared spectroscopy (FTIR): technique using the Michelson interferometer for infrared spectroscopy.  The infrared beam is split equally and reflected from a stationary plane mirror or a moveable mirror.  After one beam has passed through the specimen the beams combine either constructively or destructively, depending on the relationship between the path difference and the wavelength.
• Fourier transform nuclear magnetic resonance (FT-NMR): use of the fast Fourier transform electronics to enhance data capture rates in NMR spectroscopy and tomography.
• FPM: Fibre Percolation Model.
• Fracto-emission (FE): class of phenomena (including AE, EE, NE, NIE, PIE, phE) where charged or neutral particles, light and sound are emitted before, during and after fracture.
• Free induction decay (FID): the exponentially decaying interferogram recorded in pulsed NMR..
• Frequency analysis: use of broadband acoustic emission sensors to allow correlation of specific emission frequencies and damage modes.
• Frequency domain imaging: use of Fourier transform techniques in ultrasound NDT to show fibre orientation (high frequencies) or delamination data (lower frequencies).
• Frequency domain interferometry (FDI): microwave technique with improved depth resolution by combining the characteristics of interferometry and radar.
• ft(V): see Differential threshold crossing distribution.
• Ft(V): see Cumulative threshold crossing distribution.
• FT-NMR: Fourier Transform Nuclear Magnetic Resonance.
• FTIR: Fourier transform infrared spectroscopy.
• FTIR operating modes: FTIR may be operated in a number of modes, namely attenuated total reflection (ATR), diffuse reflectance (DR), internal reflection spectroscopy (IRS), multiple internal reflection (MIR) and transmission.
• FTIR microspectrometry: combination of Fourier transform infrared spectrometry and microscope optics to resolve spectra from small spatial areas.

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• g(V): see Logarithmic amplitude distribution.
• Gamma rays: high-energy radiation with discrete wavelengths, generated by radioactive isotopes.
• GBP: Generalised weighted back projection.
• Generalised weighted back projection (GBP): microwave imaging technique using a numerically efficient inversion procedure with experimental swept-frequency data from penetrable dielectric objects.
• Geometric fuzziness: lack of clarity due to the finite size of the focal spot for an X-ray source.
• Graham's multiparameter acoustic emission: use of a series of bandpass filters for partial frequency analysis.
• Gray (grey) - scale images: images in which a gradation of gray levels from white through to black are assigned according to the value of a specified parameter.
• Gyromagnetic ratio: the ratio of the magnetic moment and the angular momentum for a charged particle describing a circular orbital.

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• Hartman-Hahn matching condition: the matching which is required for solid state NMR involving adjustment of the amplitudes of two radio frequency transmitters so that the precession frequency of both proton and carbon nuclei are the same, hence allowing cross polarisation.
• Head waves: ultrasonic P-waves.
• Heterodyne holography: use of a slight difference in light frequency for reconstructing the two exposures, such that fringes are made to sweep across the field, allowing quantitative assessment of strain.
• Holographic interferometry: recording of two holograms on the same photographic plate, with deformation of the object surface between exposures.
• Holography: technique for recording and reconstructing amplitude and phase distributions of a disturbed coherent wave by recording the pattern of interference between the unknown object wave and a reference beam.
• Hounsfield number: density scale from -1000 (vacuum), through zero (water) to +1000 (bone) used in medical tomography.  A change of one integer on the scale is a 0.1% change in attenuation value.

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• Image processing: enhancement techniques for improved interpretation of graphic images.
• Image quality indicator (IQI): most commonly a set of wires with different diameters made of the material to be radiographed.  A measure of the radiographic quality is the ratio of the smallest wire detected by the radiograph to the object thickness. For composites it has been proposed that a step wedge of the same material be used as an IQI.
• Impedance method: vibration NDT technique using measurements of the point impedance of a structure.  The point impedance is defined as the ratio of the harmonic force input to the resultant velocity of the structure at the same point.
• Infrared camera: device using specially sensitised photographic film, or latterly electronic sensors, to provide an image of the near-infrared (thermal) part of the electromagnetic spectrum.
• Infrared photography: imaging of the near-infrared (thermal) part of the electromagnetic spectrum.
• Infrared spectroscopy: use of the absorption of the infrared spectrum to determine the structure of polymer molecules from the amplitude of the absorbed or reflected signal as a function of frequency.
• Infrared thermography: measurement and imaging of temperature distributions using infrared cameras.
• Interferometric holography: see Holographic interferometry.
• Interferometry: techniques in which the peaks and troughs of a waveform interfere constructively or destructively to produce patterns, such as light and dark line fringes in optics.
• Internal reflection spectroscopy (IRS): one operating mode for FTIR.
• Ion-graphing: measurement of the current carrying capability of a sample to infer the state-of-cure or moisture content.
• IQI: Image Quality Indicator.
• IRS: Internal Reflection Spectroscopy.

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• K-band microwaves: electromagnetic radiation in the frequency range 18 - 26.5 GHz (wavelength in air typically 15 mm at 20 GHz).
• Ka-band microwaves: electromagnetic radiation in the frequency range 26.5 - 40 GHz (wavelength in air typically 10 mm at 30 GHz).
• Kaiser effect: the absence of detectable acoustic emission at a fixed sensitivity level, until previously applied stress levels are exceeded.
• Ku-band microwaves: electromagnetic radiation in the frequency range 12.4 - 18 GHz (wavelength in air typically 20 mm at 15 GHz).

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• L-band microwaves: electromagnetic radiation in the frequency range 1 - 2 GHz (wavelength in air typically 300 mm at 1 GHz).
• L-scan: see Ultrasonic imaging.
• Lamb waves: ultrasonic plate waves, consisting of a standing wave that exists between the top and bottom surfaces of the plate and a travelling wave that propagates parallel to the midplane of the plate with a velocity which is a function of the wavelength and the plate thickness.  The distance between the free surfaces is of the same order as the compression mode wavelength in the material.
• Lame constants: the elastic moduli (E, μ, ν and λ) used to specify the complete linear elastic behaviour of an isotropic material.
• Larmor frequency: the characteristic frequency of any particle with spin behaving as an orbiting body, particularly nuclei in NMR.
• Lead screens: safety barriers for the absorption of X-rays.
• Leaky Lamb waves (LLW): ultrasonic waves excited in solid layers bounded by fluid.  Leaky Lamb waves appear on both sides of the plate when insonified by transducers in the fluid.
• Liquid crystals: materials which appear to share properties common to both liquids and crystals.  Three types of structure are recognised, the nematic mesophase with turbidity and low viscosity, the cholesteric mesophase with higher viscosity and optically active, and the smectic mesophase which more closely resembles a solid.  Liquid crystals are normally thermotropic (phase changes occur at specific temperatures).  Nematic liquid crystals find application in electro-optical and mechanical-optical transducers.  Cholesteric liquid crystals are sensitive to changes in temperature, pressure and vapour impurities.  Dramatic colour changes can be obtained from mixtures of cholesteric liquid crystals.
• LLW: Leaky Lamb Waves.
• Local vibration: vibration NDT techniques in which an exciting force is applied at single point to energise a small area of the structure (as distinct from global vibration).
• LOCAN: LOCation ANalyser.
• Location analyser (LOCAN): acoustic emission source location system (TM of Physical Acoustics Corporation).
• Logarithmic amplitude distribution, g(V): the number of acoustic emission events with signal amplitudes between V and alpha-V (where alpha is a constant multiplier) as a function of the amplitude. This is a variant on the differential amplitude distribution, appropriate for logarithmically windowed data.
• Longitudinal mode wave propagation: compression waves, where the speed of propagation is given by the square root of (Young's modulus/density).  Usually the fastest mode of wave propagation in solids, and the only mode of propagation supported in liquids and gases.
• Longitudinal relaxation time: the delay time (T1) characteristic of the spin-lattice interaction in NMR.
• Loss factor: the frequency dependent (imaginary) part of the complex relative permittivity of a dielectric material in an alternating current field.
• Loss tangent: the fractional energy absorbed by a dielectric in each cycle is measured as the dissipation factor (tan δ), where the loss angle δ has the value (90-θ), where θ is the phase angle.
• Love waves: ultrasonic surface waves, analogous to a surface compression waves, without a vertical component of displacement.
• Luminescence: class of phenomena including CL and ML.

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• Magic angle rotation (MAR): see Magic angle spinning.
• Magic angle spinning (MAS): a technique to overcome chemical shift anisotropy in NMR.  Any shielding anisotropy falls to zero when the orientation of the molecule with respect to the applied magnetic field is 54.7°, the so-called magic angle.  In practice the sample is spun around an axis exactly parallel to the magic angle at a rate comparable to the CSA.
• Magnetic resonance: class of phenomena including EAR and NMR.
• Magnetic resonance imaging (MRI): use of NMR (or perhaps EAR) signals for computer tomography.
• Magnetoresistance (MR): the percentage change in electrical resistivity as a function of the applied magnetic field.
• MAR: Magic Angle Rotation.
• MAS: Magic Angle Spinning.
• MDE: MicroDiElectrometry.
• Mechanoluminescence (ML): the emission of light stimulated by mechanical stress.
• Medical resonance imaging (MRI): computed tomography using NMR signals.
• Membrane resonance: vibration in which the motion is similar to that of a diaphragm.
• Microdielectrometry (MDE): use of microfabricated interdigitated electrodes instead of parallel plates for dielectric measurements.
• Microimaging by NMR: computer tomography at microscopic scale using NMR signals.
• Microseismic activity: alternative term for Acoustic Emission.
• Microwave diffraction tomography: inference of the internal structure of an object from multiple views of the microwave diffraction shadows.
• Microwave holography: holography using coherent microwave radiation.
• Microwave interferometry: interferometry using coherent microwave radiation.
• Microwave thermography: use of centimetre/millimetre wavelength radiation for temperature measurement in medical diagnosis.
• Miniature tensile tester (Minimat): small (1 kN maximum force) tensile testing frame for mounting on a microscope stage (TM of Polymer Laboratories).
• Minimat: Miniature Tensile Tester.
• MIR: Multiple Internal Reflection.
• ML: Mechanoluminescence.
• mm-band: electromagnetic radiation in the frequency range around 100 GHz (typical wavelength in air of 3 mm).
• Modal analysis: examination of the relative positions of the vibration spectral peaks to infer structural variations.
• Mode conversion: the exchange of energy between different propagation modes upon reflection at an interface or surface.
• Moire interferometry: use of gratings which modulate the phase rather than the amplitude of the light in combination with overlapping laser beams to generate Moire fringes.
• MR: Magnetoresistance.
• MRI: Magnetic Resonance Imaging, or Medical Resonance Imaging.
• Multiple internal reflection (MIR): one operating mode for FTIR.
• Multiple pulse line narrowing: the application of many RF pulses combined with suitable time delays to remove homogeneous dipolar broadening in the NMR spectra of solids.

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• N: acoustic emission count (qv).
• Ne: acoustic emission event count (qv).
• Natural frequency: the frequency with which a structure will vibrate most readily if unconstrained.
• Natural vibration modes: the modes of vibration adopted by a structure which is unconstrained.
• NDE: Non-Destructive Evaluation, or Non-Destructive Examination.
• NDI: Non-Destructive Inspection.
• NDT: Non-Destructive Testing.
• NE: Neutral Emission.
• Negative ion emission (NIE): the emission of negative particles both during and following fracture.
• Neutral emission (NE): the emission of neutral particles both during and following fracture.
• Neutron capture: absorption of a neutron into a nucleus to create an isotope of increased mass number.
• Neutron radiography: transmission of a neutron beam through a material and production of a film radiograph.  Attenuation is a function of two effects: neutron capture and neutron scatter.  Neutrons are readily absorbed by materials containing hydrogen (such as polymers).
• Neutron scatter: deflection of a neutron path by collision with the nucleus of an atom.
• NIE: Negative ion emission.
• Nielsen source: the pencil lead break technique adopted by EWGAE for calibration of acoustic emission systems.
• NMR: Nuclear Magnetic Resonance.
• Non-destructive evaluation (NDE): assessment of the structural integrity of a component by non-destructive means.
• Non-destructive examination (NDE): inspection of a component by non-destructive means.
• Non-destructive inspection (NDI): examination of a component by non-destructive means.
• Non-destructive testing (NDT): testing a material or component without impairing the function thereof.
• b see Non-destructive.
• NQR: Nuclear Quadrupole Resonance.
• Nuclear magnetic resonance (NMR): nuclear magnetic resonance in a complex molecule is generally achieved by placing the material in a strong constant magnetic field, and then applying a perpendicular alternating RF magnetic field.  The nuclei absorb or emit energy at characteristic frequencies of the RF field indicative of the chemical environment of the nucleus.
• Nuclear quadrupole resonance (NQR): the nuclear quadrupole moment is connected with the spatial orientation of the nuclear spin and gives a measure of the lack of sphericity in the distribution of electric charge within the nucleus.  NQR is similar to NMR.
• Nyquist plot: if damping of a vibration is increased, the resonant frequency is not changed but maximum response is reduced.  If the phase angle between force and response is measured, the results may be presented as a Nyquist plot, a circular locus of the response at any frequency as a vector of length at a reference angle.

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• OFS: Optical Fibre Sensors.
• Opaque penetrants: penetrating fluids with high X-ray absorption for improved contrast in imaging of cracks in low absorption materials.  Typical fluids include tetrabromoethane (TBE), diiodobutane (DIB) and zinc iodide solution.
• Optical caustics: the curved surface in the region of a singularity may be projected by reflection of light illuminating the area.
• Optical fibre sensors (OFS): optical fibres used to sense changes in any physical parameter.
• Orthogonal Arrays: of which Latin Squares is one example, are used in Taguchi Methods, to design experiments which will provide the maximum information on the effect of variables from the minimum number of trials..
• Orthotropic photoelasticity: photoelasticity in birefringent orthotropic materials such as some GRPs.

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• P-scan (ultrasound tomography): see Ultrasonic imaging.
• P-waves: geophysicist nomenclature for ultrasonic longitudinal waves, where particle displacements are parallel to the direction of wave propagation.
• Passive heating: the use of external heaters to generate thermal images.
• Phase gratings: gratings which modulate the phase rather than the amplitude of light.
• phE: Photon Emission.
• Photoelastic coating: use of surface coatings to produce photoelastic images of the stress at the surface of opaque materials.
• Photoelasticity: the velocity of light through some (birefringent) materials is altered according to the stress in the material and the polarisation of the light, giving rise to fringes which indicate the stress levels and directions.
• Photon emission (phE): the emission of light both during and following fracture.
• Photothermal testing: see Temperature visualisation.
• Pick-off process: the slower of the two positron annihilation routes in a medium, which includes the effects of interactions with the surrounding material.
• PIE: Positive Ion Emission.
• Plate waves: ultrasonic Lamb or rod waves.
• Polar scan attenuation: ultrasound NDT technique for layer orientation and stacking sequence.
• Positive ion emission (PIE): the emission of positive ions during and following fracture.
• Positron annihilation: the positron is the antiparticle of the electron and is annihilated upon entry into a molecular medium.  The pick-off process (qv) is useful as it responds sensitively to property changes produced by diffusing water in a medium.
• Precession: the rotation of the spin axis of a charged particle (eg the nucleus in NMR) around a second reference direction, as for a gyroscope.
• Projected fringes: Moire fringe technique in which the specimen is recorded on film with a master grating held just in front of the film plane.  Less troublesome than shadow Moire Proof testing: loading a component to a predetermined stress or pressure (usually in excess of the working level) to confirm the structural integrity.
• Proton enhanced nuclear induction spectroscopy: see Cross polarisation.
• Pulse-echo: operating an ultrasound transducer as a transmitter for a short period, followed by a longer period as a receiver, to examine the material immediately below the transducer.
• Pulsed video thermography (PVT): use of video-compatible infrared cameras for thermography.
• PVT: Pulsed video thermography.

Q

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• Radiography: techniques using high-energy radiation to obtain a shadow picture, due to differential absorption of the waves.
• Radon algorithm: the mathematical function used to determine the internal information of a 3-D object from the values of a set of line integrals at the boundaries.  The basis of computer tomography.
• Raman effect: Scattering of light such that it undergoes a change in frequency and a random alteration in phase.  Usually referred to as 'combination effect' in Soviet literature.
• Raman optomechanical strain gauge: use of the proportional change in Raman frequency of vibrational modes with tensile strain in reinforcing fibres to monitor local strains in composites.
• Raman scattering: see Raman effect.
• Raman spectroscopy: laser-based spectroscopy using the Raman effect in the optical and near infrared region.
• Rayleigh scattering: scattering of light such that it retains the frequency and a definite phase relationship to the incident light.
• Rayleigh waves: most surface waves are associated with the name of Lord Rayleigh.  Rayleigh waves travel with a velocity less than that of transverse waves in the same medium.
• Reconstruction techniques: numerical techniques for image generation.
• Resin cure monitoring: following the state-of-cure of a resin system from liquid state through gel to the solid state.
• Resonance Raman scattering (RRS): Raman scattering characterised by an increase of two or three orders of magnitude in intensity relative to normal Raman scattering. The absolute frequencies of RRS shift in direct proportion to any shift in the exciting frequency.
• Resonance spectroscopy: coin-tapping with electronic analysis of the frequency spectrum.
• Resonant frequency: the frequency at which a structure or resonant transducer will "ring" when excited by external forces.
• Resonant sensors: transducers which have peak sensitivity at a characteristic resonant frequency, rather than a wide frequency response.
• Ring-down count: see Acoustic emission count, the preferred term.
• Ring-down sensors: transducers which have a characteristic frequency and undergo an exponential decay in output amplitude after excitation.
• Rise time: the time from first positive threshold crossing to peak amplitude in acoustic emission.
• RMS: Root Mean Square.
• Rod waves: ultrasonic longitudinal waves, where the mode of propagation can be compared to a pressure pulse with particle displacement parallel to the direction of propagation.
• Rontgen rays: early name for X-rays.
• Root mean square (RMS) method: acoustic emission instrumentation using the integral of the square of the RMS voltage over the time of interest.
• RRS: Resonance Raman Scattering.
• Russell effect: formation of developable images on photographic plates in the absence of light or ionising radiation.

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• S-band microwaves: electromagnetic radiation in the frequency range 2 - 4 GHz (wavelength in air typically 100 mm at 3 GHz).
• S-waves: geophysicists nomenclature for ultrasonic shear waves, with particle displacement transverse to the direction of wave propagation and polarised either horizontally (SH-waves) or vertically (SV-waves).
• S/N: Signal/noise ratio.
• SAFT: Synthetic Aperture Focusing Technique.
• SAM: Scanning Acoustic Microscopy.
• Scanning acoustic microscopy (SAM): Microstructural examination using line-by-line scanning with a very fine ultrasonic beam at frequencies of the order of 100 MHz to 1 GHz.
• Schlieren technique: see Ultrasound Schlieren.
• SEA: Surface Energy Analysis.
• SGTF: Stress Generated Thermal Field.
• SH-waves: see S-waves.
• Shadow Moire: measurement of shape using a grating held just clear of the surface and illuminated obliquely.
• Shakedown: continuation of acoustic emission during the load hold period effectively purging all the emissions up to the proof stress, such that the Kaiser response is achieved on reloading.
• Shear waves: transverse ultrasonic bulk waves.
• Shearography: a particular form of speckle-shearing interferometry.
• Signal/noise (S/N): the ratio of the signal strength to the underlying background noise from the environment and the electronic systems.  For optimum response signal strength should be high relative to noise.
• Simultaneous thermal analysers (STA): combined instrument offering DSC, DTA and TGA (TM of Polymer Laboratories).
• Source location: triangulation from the relative arrival times of acoustic emission signals to determine to position of the AE event.
• SPATE: Stress Pattern Analysis by Thermal Emission.
• Specific damping capacity: the ratio of energy dissipation at each cycle to the total energy per cycle.
• Speckle: a phenomenon associated with scattered coherent light, in which any diffusely reflecting surface appears to be covered with myriad bright and dark points, lending the appearance of a roughened bright metallic surface.
• Speckle-shearing interferometry (SSI): recording of two coherent images of an object surface, with one image shifted with respect to the other.
• Specklegram: speckle photograph.
• Specklography: use of the speckle phenomenon associated with scattered coherent light.
• Spectroscopy: a range of chemical (EAR, IR, NMR, Raman, UV) and ultrasound techniques using frequency dependence of material response.
• Spin-lattice interaction: the longitudinal relaxation time (T1) in NMR.
• Spin-spin interaction: the transverse relaxation time (T2) in NMR.
• SSI: Speckle-Shearing Interferometry.
• STA: Simultaneous Thermal Analysers.
• Stereoradiography: production of two X-ray images from slightly different angles, which can be optically recombined to produce an apparent 3-D view.
• Stimulation: the application of a stimulus such as force, pressure, heat etc, to a test article to cause activation of acoustic emission sources.
• Stonely waves: ultrasonic interfacial waves between solids, where the particle displacement is elliptical.
• Stress generated thermal field (SGTF): the use of mechanical (vibration) stresses to generate heat spots near discontinuities in materials.
• Stress pattern analysis by thermal emission (SPATE): visualisation of temperature changes due to the thermoelastic effect (also known as TSA).
• Stress wave emission (SWE): alternative term for Acoustic Emission.
• Stress wave factors (SWF): a general measure of the efficiency of stress wave propagation.  This is a generic term, which may be defined and calculated in a number of ways (see NDT of FRPC volume 2 page 26).
• Surface energy analysis (SEA): automatic measurement of advancing and receding contact angles on fibres to indicate bonding ability.
• Surface waves: ultrasonic wave propagation in the surface layers.  May be creeping waves, Love waves, Rayleigh waves or Stonely (interfacial) waves.
• SV-waves: see S-waves.
• SWE: Stress Wave Emission.
• SWF: Stress Wave Factors.
• Synthetic Aperture Focusing Technique (SAFT): computer code which sharpens the recorded image from ultrasound scans.

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• T1: longitudinal relaxation time (spin-lattice interaction) in NMR.
• T2: transverse relaxation time (spin-spin interaction) in NMR.
• Tapometer: commercial coin-tapping system.
• TBE: Tetrabromoethane (opaque penetrant for radiography).
• Temperature imaging: use of liquid crystals, infrared cameras or electronic sensors to build up a picture of a thermal field.
• Temperature visualisation: use of liquid crystals, infrared cameras or electronic sensors to build up a picture of a thermal field.
• TGA: Thermo-Gravimetric Analysis.
• Thermo-gravimetric analysis (TGA): measurement of weight changes with changing temperature with high precision and sensitivity.  Indicative of chemical reactions.
• Thermal mechanical analysis (TMA): measurement of thermal expansion with changing temperature and high displacement sensitivity.
• Thermoelastic effect: the thermodynamic relationship whereby a change of stress in a solid is accompanied by a change in temperature, as described by Lord Kelvin in 1853.  The basis for SPATE.
• Thermoelastic stress analysis (TSA): visualisation of temperature changes due to the thermoelastic effect (also known as SPATE) and the inference of stress therefrom.
• Thermography: temperature imaging.
• Thermovision cameras: infrared imaging systems (TM of Agema Systems).
• Through-transmission: ultrasound technique using separate transducers, mounted on either side of the plate, to send and receive the signal.
• Time-of-flight diffraction (TOFD): use of the time delay between various ultrasonic signals to characterise defects.  Time delay of echoes from defect extremities is used to provide an estimate of defect depth.
• Tin cry: acoustic emission from twinning in tin.
• TMA: Thermal Mechanical Analysis.
• TMR: Topical Magnetic Resonance.
• TOFD: Time of Flight Diffraction.
• Topical magnetic resonance (TMR): nuclear magnetic resonance examination at specific internal locations, using carefully contoured field gradients to create a small spatial volume of homogeneous field.
• Transmission: one operating mode for FTIR.
• Transverse relaxation time: the decay time (T2) characteristic of the spin-spin interaction in NMR.
• TSA: Thermoelastic Stress Analysis.
• Tyndall scattering: scattering of light such that it retains the frequency and a definite phase relationship to the incident light.

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• Ultrasonic Imaging:
  • A-scan: the raw data from ultrasonic inspection, usually oscilloscope display of pulse amplitude against time.
  • B-scan: size and position against probe movement on the surface.
  • C-scan: size and position on an area parallel to the surface.
  • D-scan: size and position on an area normal to the surface.
  • F-scan: Feature mapping.
  • L-scan: Time-of-flight imaging.
  • P-scan: Projection scan. Ultrasound tomography.
  Ultrasonic spectroscopy: use of the A-scan and Fast Fourier Transform to obtain the frequency spectra for ultrasound signals.
• Ultrasonic wave propagation: see Wave propagation modes.
• Ultrasonics: high frequency sound waves, typically in the range 500 kHz to 15 MHz.
• Ultrasound Schlieren technique: visualisation of ultrasonic waves by a Schlieren optical system.
• Ultrasound tomography (P-scan): see Ultrasonic imaging.
• Ultraviolet spectroscopy: use of the absorption of the ultraviolet spectrum to determine the structure of polymer molecules from the amplitude of the absorbed signal as a function of frequency.

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• Van der Pauw (VDP) technique: four-probe measurement technique for the average resistivity of a sample.
• VDP: Van der Pauw technique.
• Velocimetry: techniques utilising the effects of flaws on the wavespeed and wave-path length between transmitting and receiving piezoelectric transducers.  Developed in the Soviet Union using frequencies in the range 25 - 60 kHz.
• Vibration Pattern Imager (VPI): scanning laser vibrometry based on a Michelson interferometer.  The Doppler frequency is related to surface velocity.
• Vibrothermography: use of cyclic stresses together with an infrared camera to locate hot spots around defects.
• Visual inspection: use of human sight for fault detection.
• Volume fraction: the proportion of one component relative to the whole sample on a volume/volume basis.
• Voxel: volume element, in computed tomography.  The 3-D equivalent of the 2-D pixel.
• VPI: Vibration Pattern Imager.

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• Wave impedance (Z): the ratio of the electric and magnetic microwave field vectors.
• Wave propagation modes: bulk, plate or surface waves (qv).
• Wavenumber: the ratio of 2π to the wavelength in ultrasonics.
• Waveguide: connector for high-frequency microwave examination of materials.
• Weight fraction: the proportion of one component relative to the whole sample on a weight/weight basis.
• Wheel-tap test: monitoring changes in the resonant frequency and damping of vibrations in a structure as a whole.  See also Coin-tap test.
• Woodpecker: hand-held tapping unit for detection of exfoliation in sandwich structures (TM of Mitsui).

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• X-band microwaves: electromagnetic radiation in the frequency range 8.2 - 12.4 GHz (wavelength in air typically 30 mm at 10 GHz).
• X-ray diffraction: technique using the atomic planes as a diffraction grating for the measurement of interatomic spacing and stresses.
• X-rays: high-energy radiation with a broad band of wavelengths, generated electrically.
• Xeroradiography: filmless radiography, using a plate, the surface of which becomes radiation sensitive when charged by static electricity.  The plate is discharged by x-rays according to the amount of incident radiation, and an image is developed using oppositely charged particles.

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• Young's fringes: optical fringes formed by diffraction.

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• Z: microwave wave impedance.
• Zeugmatography: NMR imaging by tomography.
• ZIPSCAN: general purpose digitally-based data storage and data analysis device for ultrasonic testing, developed at Harwell NDT Centre.
• ZOOM: see Band selectable Fourier analysis.

References

Principal sources:

Standard definitions of terms relating to Acoustic Emission, American Society for Testing and Materials designation E610-82, Philadelphia PA, June 1982.

R Halmshaw, Non-destructive Testing, Edward Arnold, London, 1988.

W Kemp, NMR in Chemistry: a Multinuclear Approach, Macmillan Education, London, 1986.

J Summerscales, Non-destructive Testing of Fibre Reinforced Plastics Composites, Elsevier Applied Science Publishers, London, Vol 1 (1987), Vol 2 (1990).