ASTM E1781-98(2003)e1

Standard Practice for Secondary Calibration of Acoustic Emission Sensors

SIGNIFICANCE AND USE
The purpose of this practice is to enable the transfer of calibration from sensors that have been calibrated by primary calibration to other sensors.
SCOPE
1.1 This practice covers requirements for the secondary calibration of acoustic emission (AE) sensors. The secondary calibration yields the frequency response of a sensor to waves of the type normally encountered in acoustic emission work. The source producing the signal used for the calibration is mounted on the same surface of the test block as the sensor under testing (SUT). Rayleigh waves are dominant under these conditions; the calibration results represent primarily the sensor's sensitivity to Rayleigh waves. The sensitivity of the sensor is determined for excitation within the range of 100 kHz to 1 MHz. Sensitivity values are usually determined at frequencies approximately 10 kHz apart. The units of the calibration are volts per unit of mechanical input (displacement, velocity, or acceleration).  
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.  
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

General Information

• Status: Historical
• Publication Date: 09-Jul-2003
• ICS: 17.140.01 - Acoustic measurements and noise abatement in general
• Technical Committee: E07 - Nondestructive Testing
• Drafting Committee: E07.04 - Acoustic Emission Method

Relations

• Replaces: ASTM E1781-98 - Standard Practice for Secondary Calibration of Acoustic Emission Sensors
• Effective Date: 10-Jul-2003
• Replaced By: ASTM E1781-08 - Standard Practice for Secondary Calibration of Acoustic Emission Sensors
• Effective Date: 15-Dec-2008
• Referred By: ASTM E2863-17 - Standard Practice for Acoustic Emission Examination of Welded Steel Sphere Pressure Vessels Using Thermal Pressurization
• Effective Date: 10-Jul-2003
• Referred By: ASTM E1930/E1930M-17 - Standard Practice for Examination of Liquid-Filled Atmospheric and Low-Pressure Metal Storage Tanks Using Acoustic Emission
• Effective Date: 10-Jul-2003
• Referred By: ASTM E1067/E1067M-18 - Standard Practice for Acoustic Emission Examination of Fiberglass Reinforced Plastic Resin (FRP) Tanks/Vessels
• Effective Date: 10-Jul-2003
• Referred By: ASTM E2374-16(2021) - Standard Guide for Acoustic Emission System Performance Verification
• Effective Date: 10-Jul-2003
• Referred By: ASTM E1316-23b - Standard Terminology for Nondestructive Examinations
• Effective Date: 10-Jul-2003
• Referred By: ASTM E1888/E1888M-17 - Standard Practice for Acoustic Emission Examination of Pressurized Containers Made of Fiberglass Reinforced Plastic with Balsa Wood Cores
• Effective Date: 10-Jul-2003
• Referred By: ASTM E543-21 - Standard Specification for Agencies Performing Nondestructive Testing
• Effective Date: 10-Jul-2003
• Referred By: ASTM E1118/E1118M-16(2020) - Standard Practice for Acoustic Emission Examination of Reinforced Thermosetting Resin Pipe (RTRP)
• Effective Date: 10-Jul-2003
• Referred By: ASTM E2661/E2661M-20e1 - Standard Practice for Acoustic Emission Examination of Plate-like and Flat Panel Composite Structures Used in Aerospace Applications
• Effective Date: 10-Jul-2003

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
e1
Designation:E1781–98 (Reapproved 2003)
Standard Practice for
Secondary Calibration of Acoustic Emission Sensors
This standard is issued under the fixed designation E1781; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Editorial changes made throughout the standard in July 2003.
1. Scope 3. Terminology
1.1 This practice covers requirements for the secondary 3.1 Definitions—Refer to Terminology E1316, Section B,
calibration of acoustic emission (AE) sensors. The secondary for terms used in this practice.
calibration yields the frequency response of a sensor to waves 3.2 Definitions of Terms Specific to This Standard:
of the type normally encountered in acoustic emission work. 3.2.1 reference sensor (RS)—a sensor that has had its
The source producing the signal used for the calibration is response established by primary calibration (also called sec-
mounted on the same surface of the test block as the sensor ondary standard transducer) (see Method E1106).
undertesting(SUT).Rayleighwavesaredominantunderthese 3.2.2 secondary calibration—aprocedureformeasuringthe
conditions; the calibration results represent primarily the sen- frequencyortransientresponseofanAEsensorbycomparison
sor’s sensitivity to Rayleigh waves. The sensitivity of the with an RS.
sensorisdeterminedforexcitationwithintherangeof100kHz 3.2.3 test block—a block of homogeneous, isotropic, elastic
to1MHz.Sensitivityvaluesareusuallydeterminedatfrequen- material on which a source, an RS, and a SUT are placed for
cies approximately 10 kHz apart. The units of the calibration conducting secondary calibration.
are volts per unit of mechanical input (displacement, velocity,
4. Significance and Use
or acceleration).
1.2 The values stated in SI units are to be regarded as the 4.1 The purpose of this practice is to enable the transfer of
calibration from sensors that have been calibrated by primary
standard. The values given in parentheses are for information
only. calibration to other sensors.
1.3 This standard does not purport to address all of the
5. General Requirements
safety concerns, if any, associated with its use. It is the
5.1 Units for Calibration—Secondary calibration produces
responsibility of the user of this standard to establish appro-
the same type of information regarding a sensor as does
priate safety and health practices and determine the applica-
primary calibration (Method E1106). An AE sensor responds
bility of regulatory limitations prior to use.
to motion at its front face. The actual stress and strain at the
2. Referenced Documents
front face of a mounted sensor depends on the interaction
2.1 ASTM Standards: between the mechanical impedance of the sensor (load) and
that of the mounting block (driver); neither the stress nor the
E114 Practice for Ultrasonic Pulse-Echo Straight-Beam
Examination by the Contact Method strain is amenable to direct measurement at this location.
However,thefreedisplacementthatwouldoccuratthesurface
E494 Practice for Measuring Ultrasonic Velocity in Mate-
rials of the block in the absence of the sensor can be inferred from
measurements made elsewhere on the surface. Since AE
E1106 Method for Primary Calibration of Acoustic Emis-
sion Sensors sensors are used to monitor motion at a free surface of a
structure and interactive effects between the sensor and the
E1316 Terminology for Nondestructive Examinations
structure are generally of no interest, the free motion is the
appropriateinputvariable.Itisthereforerequiredthattheunits
ofcalibrationshallbevoltsperunitoffreedisplacementorfree
This practice is under the jurisdiction of ASTM Committee E07 on Nonde-
structive Testing and is the direct responsibility of Subcommittee E07.04 on
velocity, that is, volts per metre or volt seconds per metre.
Acoustic Emission Method.
5.2 The calibration results may be expressed, in the fre-
Current edition approved July 10, 2003. Published September 2003. Originally
quency domain, as the steady-state magnitude and phase
approved in 1996. Last previous edition approved in 1998 as E1781-98.
Annual Book of ASTM Standards, Vol 03.03. response of the sensor to steady-state sinusoidal excitation or,
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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E1781–98 (2003)
sensors are recorded simultaneously by digital waveform
recorders (E) and processed by a computer.
6.1.1 Actual dynamic displacements of the surface of the
test block at the locations of the RS and SUT may be different
because the RS and SUT may present different load imped-
ancestothetestblock.However,consistentwiththedefinitions
used for primary and secondary calibration, the loading effects
of both sensors are considered to be characteristics of the
sensors themselves, and calibration results are stated in terms
of the free displacement of the block surface.
6.2 Qualification of The Test Block—The prototype second-
FIG. 1 Schematic of the Prototype Secondary Calibration
arycalibrationapparatuswasdesignedforsensorsintendedfor
Apparatus: A=a Capillary-Break Source, B=a 41 by 41 by
use on steel. The test block is therefore made of steel (hot
19-cm Steel Block, C=the RS, D=the SUT, and E=the Two-
rolledsteelA36material).Forasteelblock,itisrecommended
Channel Waveform Recorder System
thatspecificationtothemetalsupplierrequirethattheblockbe
stress relieved at 566°C (1050°F) or greater and that the stress
relief be conducted subsequent to any flame cutting.
in the time domain, as the transient response of the sensor to a
6.2.1 For a steel test block, there must be two parallel faces
delta function of displacement.
withathickness,measuredbetweenthefaces,ofatleast18cm.
5.3 Importance of the Test Block Material—The specific
The volume of the block must contain a cylinder that is 40 cm
acoustical impedance (rc) of the test block is an important
in diameter by 18-cm long, and the two faces must be flat and
parameter that affects calibration results. Calibrations per-
parallel to within 0.12-mm overall (60.06 mm).
formed on blocks of different materials yield sensor sensitivi-
ties that are very different. For example, a sensor that has been 6.2.2 For a steel test block, the top surface of the block (the
working face) must have a RMS roughness value no greater
calibratedonasteelblock,ifcalibratedonaglassoraluminum
than1µm(40µin.),asdeterminedbyatleastthreeprofilometer
block,mayhaveanaveragesensitivitythatis50%ofthevalue
traces taken in the central region of the block. The bottom
obtained on steel and, if calibrated on a polymethyl methacry-
surface of the block must have a RMS roughness value no
late block, may have an average sensitivity that is 3% of the
value obtained on steel. greater than 4 µm (160 µin.). The reason for having a
specification on the bottom surface is to ensure reasonable
5.3.1 For a sensor having a circular aperture (mounting
face) with uniform sensitivity over the face, there are frequen- ability to perform time-of-flight measurements of the speed of
sound in the block.
ciesatwhichnullsinthefrequencyresponseoccur.Thesenulls
occur at the zeroes of the first order Bessel function, J (ka),
6.2.3 For blocks of materials other than steel, minimum
where k =2pf/c, f =frequency, c =the Rayleigh speed in the dimensional requirements, dimensional accuracies, and the
test block, and a =the radius of the sensor face. Therefore,
roughness limitation must be scaled in proportion to the
calibration results depend on the Rayleigh wave speed in the longitudinal sound speed in the block material relative to that
material of the test block.
in steel.
5.3.2 Forthereasonsoutlinedin5.3and5.3.1,allsecondary
6.2.4 The top face of the block shall be the working face on
calibration results are specific to a particular material; a
which the source, RS, and SUT are located. These locations
secondary calibration procedure must specify the material of
shall be chosen near the center so as to maximize the distances
the block.
of source and receivers to the nearest edge of the face. For a
test block of any material, the distance from the source to the
6. Requirements of the Secondary Calibration Apparatus
RS and the distance from the source to the SUT must each be
100 6 2 mm (the same as that specified for primary calibra-
6.1 Basic Scheme—A prototype apparatus for secondary
calibrationisshowninFig.1.Aglass-capillary-breakdeviceor tion).
other suitable source device (A) is deployed on the upper face
6.2.5 The block must undergo longitudinal ultrasonic ex-
of the steel test block (B). The RS (C) and the SUT (D) are
amination for indications at some frequency between 2 and 5
placed at equal distances from the source and in opposite
MHz. The guidelines of Practice E114 should be followed.
directions from it. Because of the symmetry of the sensor
The block must contain no indications that give a reflection
placement, the free surface displacements at the locations of
greater than 12% of the first back wall reflection.
the RS and SUT are the same. Voltage transients from the two
6.2.6 The material of the block must be highly uniform, as
determinedbypulse-echo,time-of-flightmeasurementsofboth
longitudinal and shear waves. These measurements must be
3 made through the block at a minimum of seven locations
Breckenridge, F. R., Proctor, T. M., Hsu, N. N., and Eitzen, D. G.,“ Some
spaced regularly over the surface. The recommended method
Notions Concerning the Behavior of Transducers,” Progress in Acoustic Emission
III, Japanese Society of Nondestructive Inspection, 1986, pp. 675–684.
of measurement is pulse-echo overlap using precisely con-
Although this practice addresses secondary calibrations on test blocks of
trolled delays between sweeps. See Practice E494.Itis
differentmaterials,theonlyexistingprimarycalibrationsareperformedonsteeltest
recommended that the pulse-echo sensors have their main
blocks. To establish a secondary calibration on another material would also require
the establishment of a primary calibration for the same material. resonances in the range between 2 and 5 MHz. For the seven
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E1781–98 (2003)
FIG. 3 Waveform of the RS from a Calibration Performed on the
Prototype Secondary Calibration System
NOTE 1—The nulls in the response curves are predicted by the aperture
6.5 Sensor Under Testing—The SUT must be tested under
effectdescribedin5.3.1.Theworstcaseerrorisapproximately3.6dBand
conditions that are the same as those intended for the SUT
occurs at the first aperture null (0.3 MHz). Most of the data agree within
when in use. The couplant, the electrical load applied to the
1 dB.
SUT terminals, and the hold-down force must all be the same
FIG. 2 Comparison of Primary and Secondary Calibration Results
as those that will be applied to the SUT when in use. The
for Another SUT Having a Nominal Diameter of 0.5 in.
preferred couplant is low-viscosity machine oil, and the pre-
ferred hold-down force is 9.8 N. These conditions are all the
(or more) longitudinal measurements, the maximum difference same as for primary calibration.
between the individual values of the measurements must be no 6.6 Data Recording and Processing Equipment—For meth-
morethan0.3%oftheaveragevalue.Theshearmeasurements ods using transient sources, the instrumentation would include
a computer and two synchronized transient recorders, one for
must satisfy the same criterion.
6.3 Source—The source used in the prototype secondary the RS channel and one for the SUT channel. The transient
recorders must be capable of at least eight-bit accuracy and a
calibration system is a breaking glass capillary. Capillaries are
preparedbydrawingdown6-mmpyrextubingtoadiameterof sampling rate of 20 MHz, or at least ten-bit accuracy and a
samplingrateof10MHz.Theymusteachbecapableofstoring
0.1 to 0.25 mm. Source events are generated by squeezing the
capillary tubing against the test block using pressure from the data for a time record of at least 55 µs.The data are transferred
to the computer for processing and also stored on a permanent
side of a 4-mm diameter glass rod held in the hand.
6.3.1 In general, a secondary calibration source may be any device, for example, floppy disc, as a permanent record.
small aperture device that can provide sufficient energy to
7. Calibration Data Processing
makethecalibrationmeasurementsconvenientlyatallfrequen-
7.1 Raw Data—In the prototype secondary calibration sys-
cies within the range of 100 kHz to 1 MHz. Depending on the
technique of the calibration, the source could be a transient tem, the triggering event is the Rayleigh spike of the reference
device such as a glass-capillary-break apparatus, a spark channel. By means of pre-triggering, the data sequence in both
apparatus, a pulse-driven transducer, or a continuous wave channels is made to begin 25 µs before the trigger event. The
device such as a National Institute for Standards and Technol- raw captured waveform record of one of the two channels
ogy (NIST) Conical Transducer driven by a tone burst genera- comprises 2048 ten-bit data with a sampling interval t=102.4
tor.IftheRSandSUTaretobetestedontheblocksequentially µs. Therefore, the total record has a length of T=102.4 µs.
instead of simultaneously, then it must be established that the Reflectionsfromthebottomoftheblockappearapproximately
source is repeatable within 2%. 60 µs after the beginning of the record in both channels (see
6.4 Reference Sensor—The RS in the prototype secondary Figs. 3 and 4). It is undesirable to have the reflections present
calibration system is an NIST Conical Transducer. in the captured waveforms because the reflected rays arrive at
6.4.1 In general, the RS must have a frequency response, as the sensors from directions that are different from those
determined by primary calibration, that is flat over the fre- intendedforthecalibration.Therecordistruncatedandpadded
quency range of 100 kHz to 1 MHz within a total overall as follows: data corresponding to times greater than 55 µs are
variation of 20 dB either as a velocity transducer or a replaced by values, all equal to the average of the last ten
displacementtransducer.ItispreferredthattheRSbeofatype values in the record prior to the 55 µs cutoff.
that has a small aperture and that its frequency response be as 7.2 Complex Valued Spectra—Using a fast fourier trans-
smooth as possible. See 5.3.1 and Fig. 2 concerning the form (FFT), complex valued spectra S(f ) and U (f ) derived
m m
aperture effect. from the RS a
...