1、Designation: E 1781 98 (Reapproved 2003)e1Standard 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
2、of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.e1NOTEEditorial changes made throughout the standard in July 2003.1. Scope1.1 This practice covers requirements for the second
3、arycalibration of acoustic emission (AE) sensors. The secondarycalibration yields the frequency 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 sens
4、orunder testing (SUT). Rayleigh waves are dominant under theseconditions; the calibration 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 a
5、t frequen-cies approximately 10 kHz apart. The units of the calibrationare volts per unit 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 n
6、ot purport to address all of thesafety concerns, if any, associated with its use. It is 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 Standar
7、ds:E114 Practice for Ultrasonic Pulse-Echo Straight-BeamExamination by the Contact Method2E 494 Practice for Measuring Ultrasonic Velocity in Mate-rials2E 1106 Method for Primary Calibration of Acoustic Emis-sion Sensors2E 1316 Terminology for Nondestructive Examinations23. Terminology3.1 Definition
8、sRefer to Terminology E 1316, Section B,for terms used in this practice.3.2 Definitions 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
9、calibrationa procedure for measuring thefrequency or transient response of anAE sensor 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
10、of this practice is to enable the transfer ofcalibration from sensors that have been calibrated 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 11
11、06). An AE sensor respondsto motion at its front face. The actual stress and strain at 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 meas
12、urement at this location.However, the free displacement that would occur at the surfaceof 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
13、the sensor and thestructure are generally of no interest, the free motion is theappropriate 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 resul
14、ts may be expressed, in the fre-quency domain, as the steady-state magnitude and phaseresponse 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 onAc
15、oustic Emission Method.Current edition approved July 10, 2003. Published September 2003. Originallyapproved in 1996. Last previous edition approved in 1998 as E 1781 - 98.2Annual Book of ASTM Standards, Vol 03.03.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, P
16、A 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 specificacoustical impedance (rc) of the test block is an importantparameter that affects calibration results. Calibrations per-form
17、ed 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 aluminumblock, may have an average sensitivity that is 50 % of the valueobtained on steel and, if calibrated on a polym
18、ethyl 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 sensitivity over the face, there are frequen-cies at which nulls in the frequency response occur. These nullsoccur at the ze
19、roes 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 results depend on the Rayleigh wave speed in thematerial of the test block.5.3.2 For the reasons outlined in 5.3 and 5.
20、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 Calibration Apparatus6.1 Basic SchemeA prototype apparatus for secondarycalibration is shown in Fig. 1.Aglass-capillar
21、y-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 in oppositedirections from it. Because of the symmetry of the sensorplacement, the free surface displacements at the
22、 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 dynamic displacements of the surface of thetest block at the locations of the RS and SUT may be differentbecause the
23、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 sensors are considered to be characteristics of thesensors themselves, and calibration results are stated in termsof the
24、 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 block is therefore made of steel (hotrolled steelA36 material). For a steel block, it is recommendedthat specification
25、 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 block, there must be two parallel faceswith a thickness, measured between the faces, of at least 18 cm.The volume o
26、f 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, the top surface of the block (theworking face) must have a RMS roughness value no greaterthan 1 m (40 in.), as det
27、ermined 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 reason for having aspecification on the bottom surface is to ensure reasonableability to perform time-of-flight meas
28、urements 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 scaled in proportion to thelongitudinal sound speed in the block material relative to thatin steel.6.2.4 The top fac
29、e 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 receivers to the nearest edge of the face. For atest block of any material, the distance from the source to theRS and the
30、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 indications at some frequency between 2 and 5MHz. The guidelines of Practice E114should be followed.The block must co
31、ntain 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-of-flight measurements of bothlongitudinal and shear waves. These measurements must bemade through the block at a mi
32、nimum 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 494.Itisrecommended that the pulse-echo sensors have their mainresonances in the range between 2 and 5 MHz. For the
33、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 Nondestructive Inspection, 1986, pp. 675684.4Although this practice addresses secondary calibrations on test blocks of
34、different materials, the only existing primary calibrations are performed on steel testblocks. To establish a secondary calibration on another material would also requirethe establishment of a primary calibration for the same material.FIG. 1 Schematic of the Prototype Secondary CalibrationApparatus:
35、 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 SystemE 1781 98 (2003)e12(or more) longitudinal measurements, the maximum differencebetween the individual values of the measurements must be nomore than 0.3 % of the
36、 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 by drawing down 6-mm pyrex tubing to a diameter of0.1 to 0.25 mm. Source events are generated by squeezi
37、ng 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 device that can provide sufficient energy tomake the calibration measurements conveniently at all frequen-
38、cies within the range of 100 kHz to 1 MHz. Depending on thetechnique of the calibration, the source could be a transientdevice such as a glass-capillary-break apparatus, a sparkapparatus, a pulse-driven transducer, or a continuous wavedevice such as a National Institute for Standards and Technol-ogy
39、 (NIST) Conical Transducer driven by a tone burst genera-tor. If the RS and SUT are to be tested on the block sequentiallyinstead of simultaneously, then it must be established that thesource is repeatable within 2 %.6.4 Reference SensorThe RS in the prototype secondarycalibration system is an NIST
40、Conical Transducer.6.4.1 In general, the RS must have a frequency response, asdetermined by primary calibration, that is flat over 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 R
41、S 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 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
42、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 use. 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 Re
43、cording and Processing EquipmentFor meth-ods using transient sources, the instrumentation would includea computer and two synchronized 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 2
44、0 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 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 D
45、ata Processing7.1 Raw DataIn the prototype secondary calibration sys-tem, the triggering event is the Rayleigh spike of the referencechannel. 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 tw
46、o channelscomprises 2048 ten-bit data with a sampling interval t = 102.4s. Therefore, the total record has a length of T = 102.4 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 re
47、flections presentin the captured waveforms because the reflected rays arrive atthe sensors from directions that are different from 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 av
48、erage of the last tenvalues in the record prior to the 55 s cutoff.7.2 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:NOTE 1The nulls in the response curves are predicted by the apertureeff
49、ect 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 SystemE 1781 98 (2003)e13Sfm! 5(j 5 0n21sjexp i2pmj/n!, (1)Ufm! 5(j 5 0n21ujexp i2pmj/n! (2)where:n = 2048,j = 0, 1, 2, ., n 1,sj= jthsample value in the RS channel,uj= jthsample value in the SUT ch
