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A-Scan Display - A 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.
Absolute Measurement - A Measurement made without a direct reference to a second signal or measurement.
Absorbed Dose - The amount of energy, imparted to matter by an ionizing particle per unit mass of irradiated material at the place of interest. It is expressed in rads.
Absorption - The taking up of energy from the medium through which it passes.
Absorption Coefficient - The ratio of energy absorbed by a medium or material to the energy incident on the surface. If a flux through a material decreases with distance x in proportion to e^-ax, then a is called the absorption coefficient. Also known as the absorption factor; absorption ratio; coefficient of absorption.
Absorption Coefficient, Linear - The fractional decrease in transmitted intensity per unit of absorber thickness. It is usually designated by the symbol µ and expressed in units of (cm^-1.2).
Accelerator - A device that accelerates charged atomic particles to high energies. An x-ray machine or a betatron is an accelerator.
Acceptance Standard - A controlled specimen containing natural or artificial discontinuities that are well defined and similar to the maximum acceptable discontinuity, in size and extent, in the product. - Also may refer to the document defining acceptable discontinuity size limits.
Acoustic Emission Testing (AE) - A nondestructive testing method that "listens" for transient elastic-waves generated due to a rapid release of strain energy caused by a structural alteration in a solid material.
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 Microscopy - A general term referring to the use of high resolution, high frequency ultrasonic techniques to produce images of surface or near surface features.
Acoustic Plane Wave - A disturbance of molecular matter (sound energy) for which the wave disturbance is distributed uniformly over a planar surface (same phase / same amplitude)
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 Reflectivity - The relative reflectivity of a specific material, that is, the tendency to deflect sound energy in a specific medium rather than absorb it.

ACPD - Acronym for Alternating Current Potential Drop.

Activation - The process by which neutron bombard is used to make normally stable atoms radioactive.

Activity - A measure of the radioactivity of a particular radioisotope. Activity is calculated by the number of atoms disintegrating per unit of time. Its unit of measurement is the curie.

Addendum - That portion of a gear tooth between the pitch line and the tip of the tooth. Plural is "addenda."

Air Gap - The free space that the magnetic flux must cross when a magnetic circuit contains a break. Cracks produce small air gaps on the surface of a part.

Algorithm - A set of steps for solving a specific problem. Algorithms can be expressed in many ways but are commonly expressed as one or more mathematical equations.

Alpha Particle - A positively charged particle emitted by certain radioactive material. It is made up of two neutrons and two protons; hence it is identical with the nucleus of a helium atom.

Alpha Radiation - A stream of fast-moving helium nuclei. This is a strongly ionizing radiation with very weak penetration (e.g. cannot penetrate a piece of paper).

Alternating Current (AC) - Electric current that reverses direction periodically (usually many times per second).

Ambient - Surrounding or background level. Usually used in relation to temperature, light or sound.

Ammeter - An instrument for measuring the magnitude of electric current in a circuit.

Amperage - A term used to describe the strength of electric current. The number of electrons moving past a fixed point in a conductor in one second. Measured in amperes. The term amps is often used for short.

Ampere - A standard unit of measure for electric current or the flow of electrons. The amount of current sent by one volt through a resistance of one ohm.

Ampere Turns - A measure of the magnetizing or demagnetizing strength of the coil, which is the product of the number of turns in a coil and the number of amperes of current flowing through it. For example: 500 amperes in a 6 turn coil = 500 x 6 = 3000 ampere turns.

Amplifier - A device to increase or amplify electric impulses.

Amplitude - (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.

Angle Beam Testing - An ultrasound testing technique that uses an incidence wave angle other than 90 degrees to the test surface. The refracted angle of the sound energy is calculated using Snell's law.

Angle Beam Transducers - A device used generated sound energy, send the energy into a material at angle other than 90 degrees to the surface, and receive reflected energy and convert it to electrical pulses.

Angle of Incidence - The angle between the direction of propagation of an electromagnetic or acoustic wave (or ray) incident on a body and the local normal to that body.

Angle of Reflection - The angle between the direction of propagation of an electromagnetic or acoustic wave (or ray) reflected by a body and the local normal to that body.

Angle of Refraction - The angle between the direction of propagation of an electromagnetic or acoustic wave (or ray) refracted by an optically homogeneous body and the local normal to that body.

Angstrom - A unit of length equal to 0.0000000001 or (1 x 10^-10) meter.

Angular Frequency - For any oscillation, the number of vibrations per unit time, multiplied by 2π. Also known as angular velocity and radian frequency.

Anisotropy - The characteristic of a substance for which a physical property, such as the elastic properties, varies with the direction along which the measurement is made.

Annealing - Any treatment of metal at an elevated temperature for the purpose of softening, removing residual stresses, recrystallization and other purposes.

Anode - (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.

Anodizing - An electrochemical process that thickens and toughens the naturally occurring protective oxide forming a surface coating for corrosion protection or aesthetic purposes on metal surfaces. It is Commonly used to produce an aluminum oxide coating on aluminum parts.

Antinode - A point in a standing wave where certain characteristics of the wave field have maximum amplitude.

Applied Stress - The stress applied to a part or assembly as a result of external forces or loads.

Arc Strike - The location where a welding electrode has contacted a metal surface, melting a small volume of metal.

Area-Amplitude Blocks - Calibration blocks in which there are a series of flat-bottomed holes of varying diameter.

Array Transducer - A transducer made up of several individually piezoelectric elements connected so that the signals they transmit or receive may be treated separately or combined as desired.

Artifact - In nondestructive inspection, and indication that may be interpreted.

Artificial Discontinuity - A feature, such as a notch, hole or crack, that is manufactured to closely resemble a natural defect.

Asperity - A term used in wear technology or tribology to describe a peak or projection from a surface.

Atom - The smallest particle of any element that still retains the characteristics of that element. All matter such as solids, liquids, and gases, is composed of atoms.

Atomic Number - A number representing the positive charge or number of protons in the nucleus of an atom.

Attenuation - The reduction in the level of a quantity, such as the intensity of a wave or radiation.

Attenuation Coefficient - A factor which is determined by the degree of reduction in sound wave energy per unit distance traveled. It is composed of two parts, one (absorption) proportional to frequency, the other (scattering) dependent on the ratio of grain size or particle size to wavelength.

Attenuator - A device for causing or measuring attenuation. Usually calibrated in decibels.

Austenite - A nonmagnetic solid solution of ferric carbide or carbon in iron. An elevated-temperature parent phase in ferrous metals from which all other low-temperature structures are derived. The normal structures are derived. The normal condition of certain types of stainless steels.

Autotransformer - A special type of single coil transformer in which the output voltage can be easily varied. The autotransformer is thus employed to adjust the primary voltage applied to the stepup transformer which produces the high voltage applied to the x-ray tube.

Axial - Longitudinal or parallel to the axis or centerline of a part.