Ultrasonic Testing

reject (suppression)

a control for minimizing or eliminating low amplitude signals (electrical or material noise) so that larger signals are emphasized.

pulse length

a measure of the duration of a signal as expressed in time or number of cycles.

linearity (amplitude)

a measure of the proportionality of the signals appearing on the time or distance axis of the display and the input signals to the receiver from a calibrated time generator or from multiple echoes from a plate of material of known thickness.

sensitivity

a measure of the smallest ultrasonic signal which will produce a discernible indication on the display of an ultrasonic system

A-scan

a method of data presentation utilizing a horizontal base line that indicates distance, or time, and a vertical deflection from the base line which indicates amplitude.

testing, ultrasonic

a nondestructive method of examining materials by introducing ultrasonic waves into, through, or onto the surface of the article being examined and determining various attributes of the material from effects on the ultrasonic waves.

shadow

a region in a body that cannot be reached by ultrasonic energy traveling in a given direction because of the geometry of the body or a discontinuity in it.

dual search unit

a search unit containing two elements, one a transmitter, the other a receiver.

pulse

a short wave train of mechanical vibrations.

mode conversion

phenomonon by which an ultrasonic wave that is propagating in one mode can reflect or refract at an interface to form ultrasonic wave(s) of other modes.

control echo

reference signal from a constant reflecting surface, such as a back reflection.

gap scanning

short fluid column coupling technique.

back reflection

signal response from the far boundary of the material under examination.

multiple reflections

successive echoes of ultrasonic energy between two surfaces.

multiple back reflections

successive reflections from the back surface of the material under examination.

test surface

that surface of a part through which the ultrasonic energy enters or leaves the part

indication

that which marks or denotes the presence of a reflector.

shear wave

wave motion in which the particle motion is perpendicular to the direction of propagation.

reference block

a block that is used both as a measurement scale and as a means of providing an ultrasonic reflection of known characteristics.

creeping wave

a compression wave that travels in a solid immediately adjacent to a boundary and generates a shear mode "headwave" travelling away from the boundary at the critical angle.

saturation

a condition in which an increase in input signal produces no increase in amplitude on the display.

normal incidence (see also straight beam)

a condition in which the axis of the ultrasonic beam is perpendicular to the entry surface of the part under examination.

continuous wave

a constant flow of ultrasonic waves, as opposed to pulsed.

dB control

a control that adjusts the amplitude of the display signal in dB units.

pulse tuning

a control used on some ultrasonic examination equipment to optimize the response of the search unit and cable to the transmitter by adjusting the frequency spectrum of the transmitted pulse.

area amplitude response curve

a curve showing the changes in amplitude at normal incidence from planar reflectors of different areas located at equal distances from the search unit in an ultrasonic-conducting medium.

attenuator

a device for altering the amplitude of an ultrasonic indication in known increments, usually decibels.

collimator

a device for controlling the size and direction of the ultrasonic beam.

bubbler

a device using a liquid stream to couple an ultrasonic beam to the test piece.

beam spread

a divergence of the ultrasonic beam as the sound travels through a medium.

attenuation

a factor that describes the decrease in ultrasound intensity with distance. Normally expressed in decibel per unit length.

impedance (acoustic)

a mathematical quantity used in computation of reflection characteristics at boundaries; product of wave velocity and material density.

B-scan presentation

a means of ultrasonic data presentation which displays a cross section of the specimen indicating the approximate length (as detected per scan) of reflectors and their relative positions.

dynamic range

a measure of the capability of a test system to accept input signals of varying magnitudes, given by the ratio of the maximum to minimum input signals which at constant gain will produce distortion-free outputs having discernible changes with incremental variations in input.

Lamb wave

a specific mode of propagation in which the two parallel boundary surfaces of the material under examination (such as a plate or the wall of a tube) establish the mode of propagation. The Lamb wave can be generated only at particular values of frequency, angle of incidence, and material thickness.

shear wave search unit (Y cut quartz search unit)

a straight beam search unit used for generating and detecting shear waves.

couplant

a substance used between the search unit and examination surface to permit or improve transmission of ultrasonic energy.

resonance method

a technique in which continuous ultrasonic waves are varied in frequency to identify resonant characteristics in order to discriminate some property of a part such as thickness, stiffness, or bond integrity.

contact testing

a technique in which the search unit makes contact directly with the test piece through a thin layer of couplant.

angle beam

a term used to describe an angle of incidence or refraction other than normal to the surface of the test object, as in angle beam examination, angle beam search unit, angle beam longitudinal waves, and angle beam shear waves.

straight beam

a vibrating pulse wave train traveling normal to the test surface.

headwave

a wave that is generated by mode conversion when a point source is located at the boundary of an elastic half-space.

echo dynamic

amplitude versus time of arrival relationship of ultrasonic signals relative to probe position.

delayed sweep

an A-scan or B-scan presentation in which an initial part of the time scale is not displayed.

loss of back reflection

an absence or significant reduction in the amplitude of the indication from the back surface of the part under examination.

search unit

an electro-acoustic device used to transmit or recieve ultrasonic energy, or both. The device generally consists of a nameplate, conector, case, backing, piezoelectric element, wearface, or lens, or wedge.

gate

an electronic means of selecting a segment of the time range for monitoring or further processing.

pulse echo method

an inspection method in which the presence and position of a reflector are indicated by the echo amplitude and time.

holography (acoustic)

an inspection system using the phase interface between the ultrasonic wave from an object and a reference signal to obtain an image of reflectors in the material under test.

reflector

an interface at which an ultrasonic beam encounters a change in acoustic impedance and at which at least part of the energy is reflected.

Schlieren system

an optical system used for visual display of an ultrasonic beam passing through a transparent medium.

C-scan

an ultrasonic data presentation which provides a plan view of the test object, and discontinuities therein.

immersion testing

an ultrasonic examination method in which the search unit and the test part are submerged (at least locally) in a fluid, usually water.

Rayleigh wave

an ultrasonic surface wave in which the particle motion is elliptical and the effective penetration is approximately one wavelength.

noise

any undesired signal (electrical or acoustic) that tends to interfere with the reception, interpretation, or processing of the desired signal.

focused beam

converging energy of the sound beam at a specified distance

DGS (distance gain size-German AVG)

distance amplitude curves permitting prediction of reflector size compared to the response from a back surface reflection.

frequency (inspection)

effective ultrasonic wave frequency of the system used to inspect the material.

DAC (distance amplitude correction)

electronic change of amplification to provide equal amplitude from equal reflectors at different depths.

scattered energy

energy that is reflected in a random fashion by small reflectors in the bath of a beam of ultrasonic waves.

grazing incidence

immersion inspection with the beam directed at a glancing angle to the test surface.

skip distance

in angle beam examination, the distance along the test surface, from sound entry point tothe point at which the sound returns to the same surface. It can be considered the top surface distance of a complete vee path of sound in the test material.

frequency (fundamental)

in resonance testing, the frequency at which the wave length is twice the thickness of the examined material.

echo

indication of reflected energy.

damping, search unit

limiting the duration of a signal from a search unit subject to a pulsed input by electrically or mechanically decreasing the amplitude of successive cycles.

resolution

the ability of ultrasonic equipment to give simultaneous, separate indications from discontinuities having nearly the same range and lateral position with respect to the beam axis.

beam axis

the acoustic centerline of a search unit's beam pattern as defined by the locus of points of maximum sound pressure in the far field, and its extension into the near field.

interface

the boundary between two materials.

distance amplitude compensation (electronic)

the compensation or change in receiver amplification necessary to provide equal amplitude on the display of the ultrasonic flaw detector for reflectors of equal area which are located at different depths in the material.

scattering

the dispersion, deflection, or redirection of the energy in an ultrasonic beam caused by small reflectors in the material being examined.

radio frequency (r-f) display

the display of an unrectified signal on the CRT or recorder.

dead zone

the distance in the material from the surface of the test object to the depth at which a reflector can first be resolved under specified conditions. It is determined by the characteristics of the search unit, the ultrasonic test instrumentation, and the test object.

scanning index

the distance of the search unit is moved between scan paths after each traverse of the part.

acoustic birefringence

the effect observed when orthogonally polarized SH-shear waves are propagated through the same material thickness and the difference in phase velocity is related to components of plane stress.

markers

the electronically generated time pulses or other indicators that are used on the instrument display to measure distance or time.

back surface

the end of a reference black that is opposite the entry surface.

critical angle

the incident angle of the ultrasonic beam beyond which a specific refracted wave no longer exists.

insonification

the introduction or application of ultrasonic energy to a volume of material for the purpose of ultrasonic examination. Also spelled ensonification.

penetration depth

the maximum depth in a material from which usable ultrasonic information can be obtained and measured.

range

the maximum sound path length that is displayed.

scanning

the movement of a search unit relative to the test piece in order to examine a volume of the material.

frequency (pulse repetition)

the number of times per second an electro-acoustic search unit is excited by the pulse generator to produce a pulse of ultrasonic energy. This is also called pulse repetition rate.

apparent attenuation

the observed ultrasound energy loss. In addition to the true loss, the apparent attenuation may also include losses attributable to instrumentation, specimen configuration, beam divergence, interface reflections, and measurement procedure.

crystal (see transducer)

the piezoelectric element in an ultrasonic search unit. The term is used to describe single crystal piezoelectrics as well as polycrystalline piezoelectrics, such as ferroceramics.

distance linearity range

the range of horizontal deflection in which a constant relationship exists between the incremental horizontal displacement of vertical indications on the A-scan presentation and the incremental time required for reflected waves to pass through a known length in a uniform transmission medium.

signal-to-noise ratio

the ratio of the amplitude of an ultrasonic indication to the amplitude of the maximum background noise

corner effect

the reflection of an ultrasonic beam directed at normal incidence to the line of intersection of two perpendicular planes.

near field

the region of the ultrasonic beam adjacent to the transducer and having complex beam profiles. Also known as the Fresnel zone.

initial pulse

the response of the ultrasonic system display to the transmitter pulse (sometimes called main bang).

cross talk

the signal leakage (acoustic or electric) across an intended acoustic or electric barrier.

diffraction

the spreading or bending of waves as they pass through an aperture or around the edge of a barrier.

base line

the time of flight or distance trace (horizontal) across the A-scan CRT display (for no signal condition).

mode

the type of ultrasonic wave propagating in the materials as characterized by the particle motion (for example, longitudinal, transverse, and so forth).

sweep

the uniform and repeated movement of an electron beam across the CRT.

amplitude

the vertical pulse height of a signal, usually base to peak, when indicated by an A-scan presentation.

far field

the zone of the beam where equal reflectors give exponentially decreasing amplitudes with increasing distance.

harmonics

those vibrations which are integral multiples of the fundamental frequency.

longitudinal wave

those waves in which the particle motion of the material is essentially in the same direction as the wave propagation.

decibel (dB)

twenty times the base ten logarithm of the ratio of two ultrasonic signal amplitudes, dB=20log10 (amplitude ratio)

bi-modal technique

ultrasonic examination method that utilizes both the longitudinal (L-wave) and shear (S-wave) modes of propagation in order to estimate or measure flaw height.