U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590

Skip to content U.S. Department of Transportation/Federal Highway AdministrationU.S. Department of Transportation/Federal Highway Administration
NDE Logo

Federal Highway Administration Research and Technology
Nondestructive Evaluation (NDE) Web Manual

FHWA Home / Programs / Research & Technology

View/Search Glossary

This page allows the user to find the definition of a term found on the NDE Web site.
Item: Definition:
Print All
A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z   All
A-SCAN DISPLAYA data presentation method in which signal amplitude is plotted along the y-axis versus time on the x-axis. The horizontal distance between any two signals represents the material distance between the two conditions causing the signals. In a linear system, the vertical excursion is proportional to the amplitude of the signal
ABSORPTIONThe taking up of energy from the medium through which it passes.
ACCELERATORA device that accelerates charged atomic particles to high energies. An x ray machine or a betatron is an accelerator
ACOUSTIC IMPEDANCE (Z)The resistance of a material to the passage of sound waves. The value of this material property is the product of the material density and sound velocity. The acoustic impedance of a material determines how much sound will be transmitted and reflected when the wave encounters a boundary with another material. The larger the difference in acoustic impedance between two materials, the larger the amount of reflected energy will be.
ACOUSTIC PROPERTIES Intrinsic characteristics of any particular material that describe how sound travels through it. Such characteristics include the density, acoustic impedance, and sound velocity
ACOUSTIC REFLECTIVITYThe relative reflectivity of a specific material, that is, the tendency to deflect sound energy in a specific medium rather than absorb it.
ACOUSTIC WAVEA sound wave transmitted through material by elastic deformation
ALTERNATING CURRENT (AC)Electric current that reverses direction periodically (usually many times per second).
AMBIENTSurrounding or background level. Usually used in relation to temperature, light or sound.
  1. The maximum absolute value obtained by the disturbance of a wave or any quantity that varies periodically.
  2. The vertical height of a received signal on an A-scan. It is measured from peak to peak for an RF presentation or from base to peak for a video presentation.
ANALOG RECORDINGData represented as a continuous record of physical variables instead of discrete values, as in digital recording
ANISOTROPICHaving a physical property which varies with direction.
  1. The positive terminal or the collector of electrons in an electrical system (i.e. the positive terminal of a battery)
  2. The electrode at which oxidation or corrosion occurs. It is the opposite of cathode.
ANOMALYDeviation from uniformity in a physical property.
ATTENUATIONThe reduction in the level of a quantity, such as the intensity of a wave or radiation.
AUTOMATIC GAIN CONTROL (AGC) A process for increasing the amplitude of a trace with time, thus making all events on the trace appear to be of approximately the same amplitude. Various window lengths are used; the appearance of the data may be greatly affected by the window used in the calculation.
B-SCANA data presentation method applied to pulse echo techniques. It produces a two-dimensional view of a cross-sectional plane through the test object. The horizontal sweep is proportional to the distance along the test object and the vertical sweep is proportional to depth, showing the front and back surfaces and discontinuities between
BACK REFLECTION The signal received from the far boundary or back surface of a test object
BACKGROUND NOISE The extraneous signals caused by random signal sources within or exterior to the ultrasonic testing system, including the test material. Sometimes called grass or hash.
BOUNDARYThe edge, end or face of a finite medium
BOUNDARY ECHO Reflection of an ultrasonic wave from an interface
C-SCAN A data presentation method applied to pulse echo and transmission techniques. It yields a two-dimensional plan view of the object. No indication of depth is given unless special gating procedures are used
CALIBRATIONThe process whereby the magnitude of the output of a measuring instrument is related to the magnitude of the input force driving the instrument (i.e. Adjusting a weight scale to zero when there is nothing on it).
CATHODEA negatively charged electrode, as of an electrolytic cell, a storage battery, or an electron tube. The electrode at which reduction (and practically no corrosion) occurs. It is the opposite of anode.
COHERENCEA measure of the similarity of two oscillating functions
COMPRESSIONAL WAVE A wave in which the particle motion in the material is parallel to the wave propagation direction. Also called a longitudinal wave.
CONDUCTANCEThe product of conductivity and thickness
CONDUCTIVITYA measure of the ability of a material to conduct electrical current.
CONTACT METHOD The testing method in which the transducer face makes direct contact with the test object through a thin film of couplant
CONTACT TRANSDUCERS An ultrasonic transducer that is designed to be used in direct contact with the surface of the test article.
CONTRAST (RADIOGRAPHIC)The measure of difference in the film or detector density (darkness) from one area to another, resulting from various X-ray intensities interacting with the detector.
CORROSIONDeterioration of a metal by chemical or electrochemical reaction with its environment.
COUPLANTA substance (usually liquid) used between the transducer and the test surface to permit or improve transmission of ultrasonic energy into the test object.
  1. The physical connect of an ultrasonic transducer to the test component.
  2. The interaction of the coil's magnetic field with the test sample, which results in the generation of eddy currents in the sample and, in turn, a change in probe impedance.
CRACKA long, narrow discontinuity.
DAMPINGLimiting the duration or decreasing the amplitude of vibrations
DEFECTA discontinuity or other imperfection causing a reduction in the quality of a material or component.
DELAMINATIONA laminar discontinuity such as an area of unbonded materials.
DIELECTRIC CONSTANT Dielectric constant or relative permittivity is the factor by which the capacitance of a capacitor filled with a particular material is decreased relative to the same filled with vacuum.
DIELECTRIC PERMITTIVITYDescribes the charge separation or polarization in a medium.
DIGITAL LOGA log recorded as a series of discrete numerical values (compared to analog recording).
DISPERSIONThe variation of phase velocity with frequency. In general, any process separating radiation into components having different frequencies, energies, velocities, or other characteristics.
DISPERSIVE MEDIUMA medium in which the propagation velocity depends on the wave frequency.
ECHOA signal indicating reflected acoustic energy.
EDDY CURRENTSCircular induced currents that are generated by an alternating current in the nearby coil.
ELASTICAble to return immediately to the original size and shape after being stretched or squeezed; springy.
ELASTIC MODULUSThe ratio of the stress applied to an elastic body to the change in its shape.
ELASTICITYA term that describes how quickly molecules return to their original positions.
ELECTRICAL CONTACTContact of two conductors allowing current to pass.
ELECTRICAL CURRENTThe movement of electrons between atoms.
ELECTRICAL IMPEDANCEThe total opposition that a circuit presents to an alternating current.
ELECTROCHEMICALPertaining to combined electrical and chemical action. Deterioration (corrosion) of a metal occurs when an electrical current flows between cathodic and anodic areas on metal surfaces.
ELECTRODEA piece of metallic material that acts as an electric contact with a non-metal. It often refers to an instrument designed to measure an electrical response that is proportional to the condition being assessed (e.g. pH, resistivity).
ELECTROMAGNETIC FIELDA field that is created when energy from a power source such as a battery is applied to a circuit, making the electrons flow through a conductor, a new type of field is developed around the wire.
ELECTRONA negatively charged particle that is in constant motion and generally orbits the nucleus of an atom. The electron is the lightest known particle that possesses a charge.
FATIGUEThe phenomenon leading to fracture under repeated or fluctuating stresses having a maximum values less than the tensile strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stresses.
FATIGUE CRACKS Cracks in a material formed from repeated stress, none of which exceeded the maximum stress of the material
FILLED CRACK A crack-like discontinuity, open to the surface, but filled with some foreign material-oxide, grease, etc.- which tends to prevent penetrants from entering.
FILTERINGThe attenuation of a signal's components based on a measurable property (usually frequency). Filtering usually involves a numerical operation that enhances only a portion of the signal.
FINE CRACKA discontinuity in a solid material with a very fine opening to the surface, but possessing length and depth greater than the width of this opening; usually depth is many time the width.
FLAWA defect
FLEXURAL WAVES Flexural waves occur in bars or plates and refers to the flexing, or bending, of a bar or plate. The velocity of flexural waves depends on their frequency or wavelength, and the bending mode (shape).
FLUX LEAKAGEFlux, or lines of force, leaking from pole to pole outside a magnet.
FREQUENCYThe number of waves that pass a given point in a specified unit of time.
FREQUENCY DOMAINRefers to measurements analyzed according to their constituent frequencies. The usual alternative is time domain measurements.
GAMMA RADIATIONhigh-energy, short wavelength electromagnetic radiation emitted by a nucleus. Energies of gamma rays are usually between 0.010 and 10 mev. X-rays also occur in this energy range, but are non-nuclear origin. Gamma radiation usually accompanies alpha and beta emissions and always accompanies fission. Gamma rays are very penetrating and are best attenuated by dense materials like lead and depleted uranium.
IN-PHASEThat part of a periodic signal that has zero phase shift with a reference signal.
INDUCED MAGNETIZATIONMagnetization caused by an applied magnetic field, in contrast to remanent magnetization.
INTERPOLATIONA method to determine intermediate values from surrounding known values.
INTERPRETATIONTransforming NDE measurements into the subsurface structure or description of evaluated material or element and detected anomalies.
INVERSION, INVERTINGThe process of deriving a model of the subsurface that is consistent with the NDE or geophysical data obtained. Generally refers to a more specific methodology than interpretation.
LONGITUDINAL WAVESCommonly used term for compressional wave.
MAGNETIC FIELDThe space in which a magnetic force is exerted. This space exists within and around a magnetized material and a conductor carrying electrical current.
MAGNETIC FLUXA measure of quantity of magnetism, taking account of the strength and the extent of a magnetic field. The total number or lines of force existing in a magnetic circuit.
MAGNETIC PERMEABILITYA characteristic of a material, it is proportional to the magnetism induced in that material divided by strength of the magnetic field used.
MAGNETIC SUSCEPTIBILITYA measure of the extent to which a substance may be magnetized; it represents the ratio of magnetization to magnetic field strength.
MAGNETOMETERA device for measuring the magnetic field. Variations in the field strength may indicate changes in magnetic properties of the tested material or presence of ferrous metals.
MAPPINGLocating information from NDE or geophysical measurements in space (as opposed to time, which is monitoring). The results are usually summarized as maps.
MHOA unit of electrical conductance that is the reciprocal of ohm
MIGRATIONA seismic/radar term whose general meaning is the correction of the recorded image for the effects of reflector dip. A very typical result of migration is the removal of hyperbolic features on the record resulting from reflections from objects and other discontinuities and more accurate description of the reflectors.
MONITORINGObserving the change in a NDE or geophysical measurement with time
MULTIPLE BACK REFLECTIONSRepetitive echoes from the far boundary of the test object.
NOISEAny unwanted signal; a disturbance that is not part of signal from specified and unknown sources.
OHM (Ω)The unit of electrical resistance through which 1 amp of current will flow when the potential difference is 1 V.
OHM-METER(Ω-M) Unit of electrical resistivity; the resistivity of 1 m3 of material, which has a resistance of 1 ohm when electrical current flows between opposite faces; the standard unit of measurement for resistivity.
PENETRANTA fluid (usually a liquid but can be a gas) which possesses unique properties that render it highly capable of entering small openings, a characteristic which makes this fluid especially suitable for use in the detection of surface discontinuities which may be present.
PERMITTIVITYThe property which enables a three-dimensional material to store electrical current; i.e. its capacitivity.
PHASE ANGLEThe difference in phase between two sinusoidal varying quantities.
PHASE ARRAYA mosaic of transducer elements in which the timing of the elements’ excitation can be individually controlled to produce certain desired effects, such as steering the beam axis or focusing the beam.
PHASE SHIFTA measure of the offset between two periodic signals of the same frequency. Measured in degrees or radians.
PHASE VELOCITYThe velocity of a single frequency continuous wave.
PHYSICAL PROPERTIESThe properties of a material that are relatively insensitive to structure and can be measured without the application of force. Examples are density, melting temperature, damping capacity, thermal conductivity, thermal expansion, magnetic properties, and electrical properties.
PITCH-CATCHA term describing a test method in which the ultrasonic energy is emitted by one transducer and received by another on the same or opposite surface.
POROSITYThe property of being porous.
PRECISIONThe reproducibility of a measurement; the closeness of each of a set of similar measurements to the arithmetic mean of that set.
PRESENTATIONThe method used to show information. This may include A-scans, B-scans, C-scans, or D-scans.
PROCESSINGTo change data so as to emphasize certain aspects or correct for known influences, thereby facilitating interpretation.
PROPAGATIONAdvancement of a wave through a medium
PULSEA transient electrical or ultrasonic signal.
P-WAVESee compressional wave
RADIATIONEnergy traveling in the form of electromagnetic waves/photons.
RAYLEIGH WAVESAn ultrasonic wave that propagates along the surface of a test object. The particle motion is elliptical in a plane perpendicular to the surface, decreasing rapidly with depth below the surface. The effective depth of penetration is considered to be about one wavelength.
RECEIVERThe section of the instrument that amplifies echoes returning from the test object. Also, the transducer that picks up the echoes.
REFLECTION COEFFICIENTthe ratio of the reflected to incident amplitudes of a pulse reflected from an interface.
REMANENT MAGNETIZATION (REMANENCE)Magnetization remaining after the application of magnetic field has ceased.
REPEATABILITYAbility to repeatedly produce a measurement or detectable indication with separate measurements or inspections.
RESISTIVITYElectrical resistance to the passage of a current, expressed in ohm-meters; Reciprocal of conductivity.
RESOLUTIONThe ability to clearly distinguish signals obtained from two reflective surfaces with a minimum difference in depth. Near surface resolution is the ability to clearly distinguish a signal from a reflector at a minimum distance under the near surface without interference from the initial pulse signal. Far surface resolution is the ability to clearly distinguish signals from the back surface when the sound beam is normal to that back surface.
R-WAVESee Rayleigh wave.
SCANNINGMovement of the transducer over the surface of the test object in a controlled manner so as to achieve complete coverage. May be either contract or immersion method.
SENSITIVITYA measure of the ability to detect small signals. Limited by the signal-to-noise ratio.
SHALLOW DISCONTINUITYA discontinuity open to the surface of a solid object which possesses little depth in proportion to the width of this opening. A scratch or nick may be a "shallow discontinuity" in this sense.
SHEAR WAVESWaves which move perpendicular to the direction the wave propagates.
SHEAR WAVE TRANSDUCERAn angle beam transducer designed to cause converted shear waves to propagate at a nominal angle in a specified test medium.
SLAG INCLUSIONSNonmetallic solid material entrapped in weld metal or between weld metal and base metal.
SOUNDMechanical vibrations transmitted by an elastic medium
SPACINGThe distance between sources or transmitters and detectors or receivers on a testing device.
SPALLINGThe cracking and flaking of particles out of a surface.
SPECTRUMThe amplitude distribution of frequencies in a signal.
STRAIGHT BEAMAn ultrasonic wave traveling normal to the test surface.
STRAINA measure of relative change in the size or shape of a body. “Linear strain” is change (increase or decrease) in a linear dimension. Usually expressed in inches per inch (in. /in.), or millimeters per millimeter (mm/mm).
STRESSForce per unit area, often thought of as a force acting through a small area within a plane. It can be divided into components, perpendicular and parallel to the plane, called normal stress and shear stress, respectively. Usually expressed as pounds per square inch (psi), or megapascales (MPa).
SUBSURFACE DISCONTINUITYAny discontinuity which does not open onto the surface of the part in which it exists.
SURFACE-BREAKING CRACKSCracks formed on the surface of an object.
SURFACE WAVESSee Rayleigh wave.
S-WAVESee shear wave
TARGETThe object at which a NDE survey detection and characterization is aimed.
TIME DOMAINIn NDE and geophysics refers to measurements analyzed according to their behavior in time. The usual alternative is frequency domain measurements.
TOMOGRAPHYA method for determining the distribution of physical properties within the evaluated medium or element by inverting the results of a large number of measurements made in three dimensions (e.g., seismic, radar, resistivity, EM) between different source and receiver locations.
TRANSDUCERAny device that converts an input signal to an output signal of a different form; it can be a transmitter or receiver in some cases.
TRANSIENTOccurring when the system is changing with time; i.e., a steady state has not been attained.
TRANSMITTERThe transducer that emits ultrasonic energy.
ULTRASONICA term referring to acoustic vibration frequencies greater than about 20,000 Hz.
ULTRASONIC TESTINGThe transmission of high-frequency sound waves into a material to detect imperfections or to locate changes in material properties.
VIBRATIONA rapid back and forth motion of a particle or solid.
WAVELENGTHThe distance needed in the propagation direction for a wave to go through a complete cycle.
Page last modified on November 3, 2015.