Ultrasonic Testing Glossary: Important terminology related to ultrasonic testing

Ultrasonic Testing Glossary: Important terminology related to ultrasonic testing

ACOUSTIC IMPEDANCE

A material property defined as the product of sound velocity (C) and the material’s density (d).

ACOUSTIC INTERFACE 

The boundary between 2 media of different acoustic impedance.

ACOUSTIC ZERO

 Can be considered for practical purposes to be the point on the LCD display, which represents the entry surface of the specimen.

AMPLIFIER

 An electronic device which increases the strength of a signal fed into it, by obtaining power from a source other than the input signal.

AMPLITUDE 

Referring to an indication on the LCD screen, the vertical height of an indication measured from the lowest to the highest point on the indication. In wave motion, the maximum displacement of the particles of the material. 

ANGLE BEAM TESTING 

A testing technique in which the crystal is placed at an angle to the test surface so that the ultrasonic waves enter the material (via a plastic wedge) in a direction angular to the test surface.

ANGLE BEAM TRANSDUCER

Transducers that transmit or receive the acoustic energy at an angle to the surface to set up shear waves or surface waves in the part being inspected.

A-SCAN

 Pulse-echo format wherein the LCD display shows the pulse travel time in the horizontal direction (left to right) representing the corresponding sound paths. The vertical direction (bottom to top) displays the maximum value of the acoustic pressure echo amplitude received by the probe.

ATTENUATION

 The loss in acoustic energy, which occurs between any two points of travel. This loss may be due to absorption, reflection, etc.

BACK ECHO OR BACKWALL ECHO

The echo received from the side of the specimen opposite the side to which the transducer is coupled. This echo represents the thickness of the specimen at that point.

BACKGROUND NOISE 

Extraneous signals caused by sources within the Ultrasonic testing system and the material being tested.

BEAM INDEX POINT 

The point on the base of an angle beam probe’s wedge  from which the sound leaes the wedge and enters the specimen.

B-SCAN

 A sectional representation (side view) of a test specimen often performed in an immersion system. The sectional representation can be in the form of a photograph or recorder display.

COUPLANT 

A material (usually a liquid or gel) used between the transducer and the test specimen to eliminate air from this space and thus facilitate the passage of sound waves into and out of the specimen.

CRITICAL DEFECT 

Either the largest tolerable effect or the smallest intolerable defect. The critical defect size is usually given by a specification or code.

CROSS TALK

 An unwanted condition affecting dual element transducers in which acoustic energy travels from the transmitting crystals to the receiving crystal by routes other than the intended path through the material.

DAMPING (CONTROL)

 A variable resistance across the pulser circuit output which shapes the excitation pulse. Typically it is used to change pulse characteristics to optimize either. Penetration (low damping) or near surface resolution  (high damping). 

DAMPING MATERIAL 

Any gel, rubber-like substance, or other material which, when used in the transducer, results in a shorter ringing time of the piezoelectric crystal.

DELAY CONTROL 

Subcircuit of the sweep generator that allows a variably adjustable time period from the sending of the  trigger pulse to the start of the sweep across the LCD. 

DETECTABILITY 

The ability of a test system (instrument and transducer) to detect or “see” a given size reflector.

DISTANCE AMPLITUDE CORRECTION (DAC)

Circuit that automatically adjusts gain so that the echo amplitude of a given size reflector is displayed at a constant screen height regardless of the distance to that given size reflector.

DUAL-ELEMENT PROBE 

A probe containing two piezoelectric elements; one which transmits and one which receives.

DYNAMIC RANGE

 The ratio of maximum to minimum reflective areas that can be distinguished on the LCD (usually based on decibel ratios). 

ELECTRONIC ZERO 

The point in time it the fires the initial pulseto the transducer and the point on the LCD where the electron beam leaves the baseline due to the initial pulse signal coming from the transmitter. 

FIRST CRITICAL ANGLE

The minimum incident angle in the first medium at which the refracted longitudinal wave is eliminated from the test specimen.
FLAW A discontinuity which may be undesirable but does not ecessarily call for rejection.

FREQUENCY

The number of complete cycle undergone or produced by an oscillating body in one second.

GAIN 

A term used in electronics with reference to an increase in signal power; usually expressed as the ratio of the output power (for example, of an amplifier) to the input power in decibels.

GAIN (CONTROL) 

Selects the amount of calibrated gain (dB’s) available within the instrument. Usually consists of a coarse gain control (additions at 20dB increments), and a fine gain (additions at 1 or 2dB increments).

GATE 

A electronic baseline display used to electronically monitor portions of the displayed range with reference to distance or amplitude.

HERTZ

 The derived unit of frequency defined as the frequency of a periodic phenomenon of which the period is one second; equal to one cycle per second. Symbol Hz. 1 Kilohertz (KHZ) = 103 cycles per second.; 1 Megahertz (Mhz) = 106 cycles per second. Named after Heinrick Hertz (1857-94).

INDICATION

The signal displayed on the LCD signifying the presence of sound wave reflector in the part being tested.

INDICATION (DEFECT) LEVEL

 The number of decibels of calibrated gain, which must be set on the instrument to bring the indication (defect) echo signal to peak at the reference line on the LCD screen.

INITIAL PULSE (IP)

 The pulse of electrical energy sent by pulser to the transducer. 

LINEARITY, VERTICAL OR AMPLITUDE

 The characteristics of an ultrasonic test system indicating is ability to respond in a proportional manner to a range of echo amplitudes produced by specified reflectors.

LINEARITY, HORIZONTAL OR DISTANCE

 The characteristics of an ultrasonic test system indicating its ability to respond in a proportional manner to a range of echo signals produced by specified reflectors, variable in time, usually a series of multiple back reflections. 

LONGITUDINAL WAVE

 Mode of wave propagation characterized by particle movement parallel to the direction of wave travel.

“MAIN BANG” 

The slang term used to describe the combination of 

1. Initial Pulse Voltage (as displayed on the LCD screen) 
2. Electric zero
3. Acoustic Zero on a straight beam contact test
4. Ringing of the crystal.

MATERIAL LOSS ATTENUATION (M.L.A.)

 The loss of sound pressure in a traveling waveform caused by the scattering of some of the wave’s sound pressure by the grain structure and/or porosity of the. medium, and by absorption, a conversion of sound energy into heat.

MODE CONVERSION 

The changing of a portion of a sound beam’s energy into a wave of a different mode due to refraction at incident angles other than zero degrees. In NDT this usually involves conversion of longitudinal waves into. shear waves or surface waves.  

PENETRATION 

The ability of the test system to overcome material loss attenuation; i.e. the ability of the sound beam to by-pass small reflectors such as grain boundaries and porosity in the specimen.

PIEZOELECTRIC ELEMENTS

A family of elements (such as lead metaniobate, quartz, lithium sulfate) which possess the characteristic ability to produce a) A voltage differential across their faces when deformed by a externally applied mechanical force and b) A change in their own physical configuration (dimensions) when an external voltage is applied to them.

PROBE 

Another name for transducer or search unit.

PULSE REPETITION RATE

 The frequency with which the clock circuit sends its trigger pulses to the sweep generator and the transmitter, usually quoted in terms of pulses per second (pps).

RANGE 

The distance represented by the entire horizontal LCD screen display.

RECEIVER 

The circuit of a flaw detector that receives both the.initial pulse voltage from the transmitter and the returning echoes (as voltage) from the transducer. By passing these incoming signals through certain subcircuits, the signals are rectified, filtered and amplified with the results send to the LCD screen for display. 

REJECT (CONTROL) 

Also known as suppression, it limits the input sensitivity of the amplifier in the receiver. “Grass” or scattering noise can be reduced or eliminated from the LCD screen by its use. On most analog instruments. It also destroys the vertical linearity relationship between echo heights.

SENSITIVITY

 The ability of the test system (instrument and transducer) to detect a given size reflector at a given distance. 

SHEAR (TRANSVERSE) WAVE

 Mode of wave propagation characterized by particle movement perpendicular to the direction of wave travel.

SIGNAL TO NOISE RATIO

 The ratio of amplitudes and indication from the smallest defect considered significant and those caused by random factors, such as grain scattering or instrument noise.

SINGLE ELEMENT PROBE

 A probe containing only one piezoelectric element, which is used to both transmits and receives sound.

SOUND BEAM

 The characteristic shape of the ultrasonic wave sent into the material.

SOUND PATH DISTANCE

 The distance from the transducer beam index point to the reflector located in the specimen, measured along the actual path that the sound travels. Sometimes referred to as angular distance in angle beam testing.

STRAIGHT BEAM PROBE (NORMAL BEAM PROBE)

 A probe which transmits the sound in to the material perpendicular to the entry surface.

SURFACE WAVE 

 Mode of wave propagation characterized by an elliptical movement of the particles (molecules) on the surface of the specimen as the wave front moves forward, this movement penetrating the specimen to a depth of one wavelength.

THROUGH TRANSMISSION
 A test method in which the vibrations emitted by one search unit are directed toward, and received by, another search unit. The ratio between quantity of vibration sent and received is a measure of the integrity, quality of the material being tested.

TRANSDUCER 
A device that transforms one form of energy into another.

TRANSMITTER

 Circuit of the flaw detector that sends the initial pulse voltage to both the transducer and receiver.

ULTRASONIC 

Of or relating to frequencies above the human audible range; e.g. above 20,000 cycles/sec. (Hertz).

ULTRASONICS

 The study of pressure waves which are of the same nature as sound waves, but which have frequencies above the human audible limit, i.e. above 20,000 cycles/sec (Hertz).

V-PATH 

The angular distance sound travels, measured from the top surface of the material to the bottom, and reflecting back up to the top surface.

WAVELENGTH

 The distance between like points on successive wavefronts; i.e. the distance between any two successive particles of the oscillating medium that are in the same phase. It is denoted by the Greek letter Lamda

Ultrasonic Testing Machine

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