1、Designation: E 1781 08Standard Practice forSecondary Calibration of Acoustic Emission Sensors1This standard is issued under the fixed designation E 1781; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A
2、number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers requirements for the secondarycalibration of acoustic emission (AE) sensors. The secondarycalibration yields the f
3、requency response of a sensor to wavesof the type normally encountered in acoustic emission work.The source producing the signal used for the calibration ismounted on the same surface of the test block as the sensorunder testing (SUT). Rayleigh waves are dominant under theseconditions; the calibrati
4、on results represent primarily the sen-sors sensitivity to Rayleigh waves. The sensitivity of thesensor is determined for excitation within the range of 100 kHzto 1 MHz. Sensitivity values are usually determined at frequen-cies approximately 10 kHz apart. The units of the calibrationare volts per un
5、it of mechanical input (displacement, velocity,or acceleration).1.2 The values stated in SI units are to be regarded as thestandard. The values given in parentheses are for informationonly.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is
6、 theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E114 Practice for Ultrasonic Pulse-Echo Straight-BeamExamination by the Contact Met
7、hodE 494 Practice for Measuring Ultrasonic Velocity in Mate-rialsE 1106 Test Method for Primary Calibration of AcousticEmission SensorsE 1316 Terminology for Nondestructive Examinations3. Terminology3.1 DefinitionsRefer to Terminology E 1316, Section B,for terms used in this practice.3.2 Definitions
8、 of Terms Specific to This Standard:3.2.1 reference sensor (RS)a sensor that has had itsresponse established by primary calibration (also called sec-ondary standard transducer) (see Method E 1106).3.2.2 secondary calibrationa procedure for measuring thefrequency or transient response of anAE sensor
9、by comparisonwith an RS.3.2.3 test blocka block of homogeneous, isotropic, elasticmaterial on which a source, an RS, and a SUT are placed forconducting secondary calibration.4. Significance and Use4.1 The purpose of this practice is to enable the transfer ofcalibration from sensors that have been ca
10、librated by primarycalibration to other sensors.5. General Requirements5.1 Units for CalibrationSecondary calibration producesthe same type of information regarding a sensor as doesprimary calibration (Method E 1106). An AE sensor respondsto motion at its front face. The actual stress and strain at
11、thefront face of a mounted sensor depends on the interactionbetween the mechanical impedance of the sensor (load) andthat of the mounting block (driver); neither the stress nor thestrain is amenable to direct measurement at this location.However, the free displacement that would occur at the surface
12、of the block in the absence of the sensor can be inferred frommeasurements made elsewhere on the surface. Since AEsensors are used to monitor motion at a free surface of astructure and interactive effects between the sensor and thestructure are generally of no interest, the free motion is theappropr
13、iate input variable. It is therefore required that the unitsof calibration shall be volts per unit of free displacement or freevelocity, that is, volts per metre or volt seconds per metre.5.2 The calibration results may be expressed, in the fre-quency domain, as the steady-state magnitude and phaser
14、esponse of the sensor to steady-state sinusoidal excitation or,1This practice is under the jurisdiction of ASTM Committee E07 on Nonde-structive Testing and is the direct responsibility of Subcommittee E07.04 onAcoustic Emission Method.Current edition approved Dec. 15, 2008. Published January 2009.
15、Originallyapproved in 1996. Last previous edition approved in 1998 as E 1781 - 98(2003)1.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summ
16、ary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.in the time domain, as the transient response of the sensor to adelta function of displacement.5.3 Importance of the Test Block MaterialThe specificacoustica
17、l impedance (rc) of the test block is an importantparameter that affects calibration results. Calibrations per-formed on blocks of different materials yield sensor sensitivi-ties that are very different. For example, a sensor that has beencalibrated on a steel block, if calibrated on a glass or alum
18、inumblock, may have an average sensitivity that is 50 % of the valueobtained on steel and, if calibrated on a polymethyl methacry-late block, may have an average sensitivity that is 3 % of thevalue obtained on steel.35.3.1 For a sensor having a circular aperture (mountingface) with uniform sensitivi
19、ty over the face, there are frequen-cies at which nulls in the frequency response occur. These nullsoccur at the zeroes of the first order Bessel function, J1(ka),where k =2pf/c, f = frequency, c = the Rayleigh speed in thetest block, and a = the radius of the sensor face.3Therefore,calibration resu
20、lts depend on the Rayleigh wave speed in thematerial of the test block.5.3.2 For the reasons outlined in 5.3 and 5.3.1, all secondarycalibration results are specific to a particular material; asecondary calibration procedure must specify the material ofthe block.46. Requirements of the Secondary Cal
21、ibration Apparatus6.1 Basic SchemeA prototype apparatus for secondarycalibration is shown in Fig. 1.Aglass-capillary-break device orother suitable source device (A) is deployed on the upper faceof the steel test block (B). The RS (C) and the SUT (D) areplaced at equal distances from the source and i
22、n oppositedirections from it. Because of the symmetry of the sensorplacement, the free surface displacements at the locations ofthe RS and SUT are the same. Voltage transients from the twosensors are recorded simultaneously by digital waveformrecorders (E) and processed by a computer.6.1.1 Actual dy
23、namic displacements of the surface of thetest block at the locations of the RS and SUT may be differentbecause the RS and SUT may present different load imped-ances to the test block. However, consistent with the definitionsused for primary and secondary calibration, the loading effectsof both senso
24、rs are considered to be characteristics of thesensors themselves, and calibration results are stated in termsof the free displacement of the block surface.6.2 Qualification of The Test BlockThe prototype second-ary calibration apparatus was designed for sensors intended foruse on steel. The test blo
25、ck is therefore made of steel (hotrolled steelA36 material). For a steel block, it is recommendedthat specification to the metal supplier require that the block bestress relieved at 566C (1050F) or greater and that the stressrelief be conducted subsequent to any flame cutting.6.2.1 For a steel test
26、block, there must be two parallel faceswith a thickness, measured between the faces, of at least 18 cm.The volume of the block must contain a cylinder that is 40 cmin diameter by 18-cm long, and the two faces must be flat andparallel to within 0.12-mm overall (60.06 mm).6.2.2 For a steel test block,
27、 the top surface of the block (theworking face) must have a RMS roughness value no greaterthan 1 m (40 in.), as determined by at least three profilometertraces taken in the central region of the block. The bottomsurface of the block must have a RMS roughness value nogreater than 4 m (160 in.). The r
28、eason for having aspecification on the bottom surface is to ensure reasonableability to perform time-of-flight measurements of the speed ofsound in the block.6.2.3 For blocks of materials other than steel, minimumdimensional requirements, dimensional accuracies, and theroughness limitation must be s
29、caled in proportion to thelongitudinal sound speed in the block material relative to thatin steel.6.2.4 The top face of the block shall be the working face onwhich the source, RS, and SUT are located. These locationsshall be chosen near the center so as to maximize the distancesof source and receive
30、rs to the nearest edge of the face. For atest block of any material, the distance from the source to theRS and the distance from the source to the SUT must each be100 6 2 mm (the same as that specified for primary calibra-tion).6.2.5 The block must undergo longitudinal ultrasonic ex-amination for in
31、dications at some frequency between 2 and 5MHz. The guidelines of Practice E114should be followed.The block must contain no indications that give a reflectiongreater than 12 % of the first back wall reflection.6.2.6 The material of the block must be highly uniform, asdetermined by pulse-echo, time-o
32、f-flight measurements of bothlongitudinal and shear waves. These measurements must bemade through the block at a minimum of seven locationsspaced regularly over the surface. The recommended methodof measurement is pulse-echo overlap using precisely con-trolled delays between sweeps. See Practice E 4
33、94.Itisrecommended that the pulse-echo sensors have their mainresonances in the range between 2 and 5 MHz. For the seven3Breckenridge, F. R., Proctor, T. M., Hsu, N. N., and Eitzen, D. G.,“ SomeNotions Concerning the Behavior of Transducers,” Progress in Acoustic EmissionIII, Japanese Society of Non
34、destructive Inspection, 1986, pp. 675684.4Although this practice addresses secondary calibrations on test blocks ofdifferent materials, the only existing primary calibrations are performed on steel testblocks. To establish a secondary calibration on another material would also requirethe establishme
35、nt of a primary calibration for the same material.FIG. 1 Schematic of the Prototype Secondary CalibrationApparatus: A = a Capillary-Break Source, B = a 41 by 41 by19-cm Steel Block, C = the RS, D = the SUT, and E = the Two-Channel Waveform Recorder SystemE1781082(or more) longitudinal measurements,
36、the maximum differencebetween the individual values of the measurements must be nomore than 0.3 % of the average value. The shear measurementsmust satisfy the same criterion.6.3 SourceThe source used in the prototype secondarycalibration system is a breaking glass capillary. Capillaries areprepared
37、by drawing down 6-mm pyrex tubing to a diameter of0.1 to 0.25 mm. Source events are generated by squeezing thecapillary tubing against the test block using pressure from theside of a 4-mm diameter glass rod held in the hand.6.3.1 In general, a secondary calibration source may be anysmall aperture (l
38、ess than 3 mm) device that can providesufficient energy to make the calibration measurements conve-niently at all frequencies within the range of 100 kHz to 1MHz. Depending on the technique of the calibration, the sourcecould be a transient device such as a glass-capillary-breakapparatus, a spark ap
39、paratus, a pulse-driven transducer (withpulse rise time less than one (1) micro-second), or a continuouswave device such as a National Institute for Standards andTechnology (NIST) Conical Transducer driven by a tone burstgenerator. If the RS and SUT are to be tested on the blocksequentially instead
40、of simultaneously, then it must be estab-lished that the source is repeatable within 2 %.6.4 Reference SensorThe RS in the prototype secondarycalibration system is an NIST Conical Transducer.6.4.1 In general, the RS must have a frequency response, asdetermined by primary calibration, that is flat ov
41、er the fre-quency range of 100 kHz to 1 MHz within a total overallvariation of 20 dB either as a velocity transducer or adisplacement transducer. It is preferred that the RS be of a typethat has a small aperture and that its frequency response be assmooth as possible. See 5.3.1 and Fig. 2 concerning
42、 theaperture effect.6.5 Sensor Under TestingThe SUT must be tested underconditions that are the same as those intended for the SUTwhen in use. The couplant, the electrical load applied to theSUT terminals, and the hold-down force must all be the sameas those that will be applied to the SUT when in u
43、se. Thepreferred couplant is low-viscosity machine oil, and the pre-ferred hold-down force is 9.8 N. These conditions are all thesame as for primary calibration.6.6 Data Recording and Processing EquipmentFor meth-ods using transient sources, the instrumentation would includea computer and two synchr
44、onized transient recorders, one forthe RS channel and one for the SUT channel. The transientrecorders must be capable of at least eight-bit accuracy and asampling rate of 20 MHz, or at least ten-bit accuracy and asampling rate of 10 MHz. They must each be capable of storingdata for a time record of
45、at least 55 s. The data are transferredto the computer for processing and also stored on a permanentdevice, for example, floppy disc, as a permanent record.7. Calibration Data Processing7.1 Raw DataIn the prototype secondary calibration sys-tem, the triggering event is the Rayleigh spike of the refe
46、rencechannel. By means of pre-triggering, the data sequence in bothchannels is made to begin 25 s before the trigger event. Theraw captured waveform record of one of the two channelscomprises 2048 ten-bit data with a sampling interval t = 102.4s. Therefore, the total record has a length of T = 102.4
47、 s.Reflections from the bottom of the block appear approximately60 s after the beginning of the record in both channels (seeFigs. 3 and 4). It is undesirable to have the reflections presentin the captured waveforms because the reflected rays arrive atthe sensors from directions that are different fr
48、om thoseintended for the calibration. The record is truncated and paddedas follows: data corresponding to times greater than 55 s arereplaced by values, all equal to the average of the last tenvalues in the record prior to the 55 s cutoff.NOTE 1The nulls in the response curves are predicted by the a
49、pertureeffect described in 5.3.1. The worst case error is approximately 3.6 dB andoccurs at the first aperture null (0.3 MHz). Most of the data agree within1 dB.FIG. 2 Comparison of Primary and Secondary Calibration Resultsfor Another SUT Having a Nominal Diameter of 0.5 in.FIG. 3 Waveform of the RS from a Calibration Performed on thePrototype Secondary Calibration SystemE17810837.2 Complex Valued SpectraUsing a fast fourier trans-form (FFT), complex valued spectra S(fm) and U (fm) derivedfrom the RS and SUT, respectively, are calculated:Sfm! 5(j 5 0n21
