Acoustic Emission Standard Terminology ASTM E1316-05

acoustic emission (AE)-the class of phenomena whereby transient elastic waves are generated by the rapid release of energy from localized sources within a material, or the transient waves so generated. Acoustic emission is the recommended term for general use. Other terms that have been used in AE literature include (1) stress wave emission, (2) microseismic activity, and (3) emission or acoustic emission with other qualifying modifiers.

acousto-ultrasonics (AU)-a nondestructive examination method that uses induced stress waves to detect and assess diffuse defect states, damage conditions, and variations of mechanical properties of a test structure. The AU method combines aspects of acoustic emission (AE) signal analysis with ultrasonic materials characterization techniques.

adaptive location-source location by iterative use of simu­lated sources in combination with computed location.

AE activity, n-the presence of acoustic emission during a test.

AE amplitude-see dB_AE.

AE rms, n-the rectified, time averaged AE signal, measured on a linear scale and reported in volts.

AE signal duration-the time between AE signal start and AE signal end.

AE signal end-the recognized termination of an AE signal, usually defined as the last crossing of the threshold by that signal.

AE signal generator-a device which can repeatedly induce a specified transient signal into an AE instrument.

AE signal rise time-the time between AE signal start and the peak amplitude of that AE signal.

AE signal start-the beginning of an AE signal as recognized by the system processor, usually defined by an amplitude excursion exceeding threshold.

array, n-a group of two or more AE sensors positioned on a structure for the purposes of detecting and locating sources. The sources would normally be within the array.

arrival time interval (∆t)-see interval, arrival time. attenuation, n-the decrease in AE amplitude per unit dis­tance, normally expressed in dB per unit length.

average signal level, n-the rectified, time averaged AE logarithmic signal, measured on the AE amplitude logarith­mic scale and reported in dB_ae units (where 0 dB_ae refers to 1 pV at the preamplifier input).

channel, acoustic emission-an assembly of a sensor, pream­plifier or impedance matching transformer, filters secondary amplifier or other instrumentation as needed, connecting cables, and detector or processor.

Note 2-A channel for examining fiberglass reinforced plastic (FRP) may utilize more than one sensor with associated electronics. Channels may be processed independently or in predetermined groups having similar sensitivity and frequency characteristics.

count, acoustic emission (emission count) (N)-the number of times the acoustic emission signal exceeds a preset threshold during any selected portion; of a test.

count, event (N,)-the number obtained by counting each discerned acoustic emission event once.

count rate, acoustic emission (emission rate or count rate) (N)-the time rate at which emission counts occur.

couplant-a material used at the structure-to-sensor interface to improve the transmission of acoustic energy across the interface during acoustic emission monitoring.

dB_AE a logarithmic measure of acoustic emission signal amplitude, referenced to 1 pV at the sensor, before amplifi­cation.

dead time-any interval during data acquisition when the instrument or system is unable to accept new data for any reason.

dynamic range-the difference, in decibels, between the overload level and the minimum signal level (usually fixed by one or more of the noise levels, low-level distortion, interference, or resolution, level) in a system or sensor.

effective velocity, n-velocity calculated on the basis of arrival times and propagation distances determined by artificial AE generation; used for computed location.

emission, burst-a qualitative description of the discrete signal related to an individual emission event occurring within the material.

Note 3-Use of the term burst emission is recommended only for describing the qualitative appearance of emission signals. Fig. 1 shows an oscilloscope trace of burst emission signals on a background of continuous emission.

emission, continuous-a qualitative description of the sus­tained signal' level produced by rapidly occurring acoustic ' emission from structural sources, leaks, or both.

Note 4- Use of the term continuous emission is recommended only for describing the qualitative appearance of emission signals. Fig. 2 and Fig. 3 show oscilloscope traces of continuous emission signals at two " different sweep rates.

energy, acoustic emission event-the total elastic energy released by an emission event.

energy, acoustic emission signal-the energy contained in a detected acoustic emission burst signal, with units usually reported in joules and values which can be expressed in logarithmic form (dB, decibels).

evaluation threshold-a threshold value used for analysis of the examination data. Data may be recorded with a system examination threshold lower than the evaluation threshold. For analysis purposes, dependence of measured data on the system examination threshold must be taken into consider­ation.

event, acoustic emission (emission event)-a local material change giving rise to acoustic emission.

examination area-that portion of a structure being monitored with acoustic emission.

examination region-that portion of the test article evaluated using acoustic emission technology.

Felicity effect-the presence of acoustic emission, detectable at a fixed predetermined sensitivity level at stress levels below those previously applied.

Felicity ratio-the ratio of the stress at which the Felicity effect occurs to the previously applied maximum stress.

Note 5-The fixed sensitivity level will usually be the same as was used for the previous loading or test.

first hit location-a zone location method defined by which a channel among a group of channels first detects the signal.

floating threshold-any threshold with amplitude established by a time average measure of the input signal.

hit-the detection and measurement of an AE signal on a channel.

interval, arrival time-the time interval between the detected arrivals of an acoustic emission wave at the i^th and j^th sensors of a sensor array.

Kaiser effect-the absence of detectable acoustic emission at a fixed sensitivity level, until previously applied stress levels are exceeded.

location accuracy, n-a value determined by comparison of the actual position of an AE source (or simulated AE source) to the computed location.

location, cluster, n-a location technique based upon a speci­fied amount of AE activity located within a specified length or area, for example: 5 events within 12 linear inches or 12 square inches.

location, computed, n-a source location method based on algorithmic analysis of the difference in arrival times among sensors.

Note 6-Several approaches to computed location are used, including linear location, planar location, three dimensional location, and adaptive location.

(a) linear location, n-one dimensional source location requiring two or more channels.

(b) planar location, n-two dimensional source location requiring three or more channels.

(c) 3D location, n- three dimensional source location requiring five or more channels.

(d) adaptive location, n-source location by iterative use of simulated sources in combination with computed location.

location, continuous AE signal, n-a method of location based on continuous AE signals, as opposed to hit or difference in arrival time location methods.

Note 7-This type of location is commonly used in leak location due to the presence of continuous emission. Some common types of continu­ous signal location methods include signal attenuation and correlation analysis methods.

(a) signal attenuation-based source location, n-a source location method that relies on the attenuation versus distance phenomenon of AE signals. By monitoring the AE signal magnitudes of the continuous signal at various points along the object, the source can be determined based on the highest magnitude or by interpolation or extrapolation of multiple readings.

(b) correlation-based source location, n-a source location method that compares the changing AE signal levels (usually waveform based ampli­tude analysis) at two or more points surrounding the source and deter­mines the time displacement of these signals. The time displacement data can be used with conventional hit based location techniques to arrive at a solution for the source site.

location, source, n-any of several methods of evaluating AE data to determine the position on the structure from which the AE originated. Several approaches to source location are used, including zone location, computed location, and con­tinuous location.

location, zone, n-any of several techniques for determining the general region of an acoustic emission source (for example, total AE counts, energy, hits, and so forth).

Note 8-Several approaches to zone location are used, including independent channel zone location, first hit zone location, and arrival sequence zone location.

(a) independent channel zone location, n-a zone location technique that compares the gross amount of activity from each channel.

(b) first-hit zone location, n-a zone location technique that compares only activity from the channel first detecting the AE event.

(c) arrival sequence zone location, n-a zone location technique that compares the order of arrival among sensors.

overload recovery time-an interval of nonlinear operation of an instrument caused by a signal with amplitude in excess of the instrument's linear operating range.

pressure, design-pressure used in design to determine the required minimum thickness and minimum mechanical properties.

processing capacity-the number of hits that can be processed at the processing speed before the system must interrupt data collection to clear buffers or otherwise prepare for accepting additional data.

processing speed-the sustained rate (hits/s), as a function of the parameter set and number of active channels, at which AE signals can be continuously processed by a system without interruption for data transport.

rate, event count (Ne)-the time rate of the event count. rearm delay time-see time, rearm delay.

ring-down count-see count, acoustic emission, the pre­ferred term.

sensor, acoustic emission-a detection device, generally pi­ezoelectric, that transforms the particle motion produced by an elastic wave into an electrical signal.

signal, acoustic emission (emission signal)-an electrical signal obtained by detection of one or more acoustic emission events.

signal amplitude, acoustic emission-the peak voltage of the largest excursion attained by the signal waveform from an emission event.

signal overload level-that level above which operation ceases to be satisfactory as a result of signal distortion, overheating, or damage.

signal overload point-the maximum input signal amplitude at which the ratio of output to input is observed to remain within a prescribed linear operating range.

signal strength-the measured area of the rectified AE signal with units proportional to volt-sec.

Discussion-The proportionality constant is specified by the AE instrument manufacturer.

signature, 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.

stimulation-the application of a stimulus such as force, pressure, heat, and so forth, to a test article to cause activation of acoustic emission sources.

system examination threshold-the electronic instrument threshold (see evaluation threshold) which data will be detected.

verification, AE system (performance check, AE system)-­the process of testing an AE system to assure conformance to a specified level of performance or measurement accuracy. (This is usually carried out prior to, during and/or after an AE examination with the AE system connected to the examination object, using a simulated or artificial acoustic emission source.)

voltage threshold-a voltage level on an electronic compara­tor such that signals with amplitudes larger than this level will be recognized. The voltage threshold may be user adjustable, fixed, or automatic floating.

waveguide, acoustic emission-a device that couples elastic energy from a structure or other test object to a remotely mounted sensor during AE monitoring. An example of an acoustic emission waveguide would be a solid wire of rod that is coupled at one end to a monitored structure, and to a sensor at the other end.