1、Designation: E1781 08E1781/E1781M 13Standard Practice forSecondary Calibration of Acoustic Emission Sensors1This standard is issued under the fixed designation E1781;E1781/E1781M; the number immediately following the designation indicatesthe year of original adoption or, in the case of revision, the
2、 year of last revision. A number in parentheses indicates the year of lastreapproval. A superscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope Scope*1.1 This practice covers requirements for the secondary calibration of acoustic emission (AE) sensors. The
3、 secondarycalibration yields the frequency response of a sensor to waves of the type normally encountered in acoustic emission work. Thesource 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
4、under these conditions; the calibration results represent primarily the sensors sensitivity toRayleigh waves. The sensitivity of the sensor is determined for excitation within the range of 100 kHz to 1 MHz. Sensitivity valuesare usually determined at frequencies approximately 10 kHz apart. The units
5、 of the calibration are volts per unit of mechanicalinput (displacement, velocity, or acceleration).1.2 UnitsThe values stated in either SI units or inch-pound units are to be regarded as the standard. The values given inparentheses are for information only. stated in each system may not be exact eq
6、uivalents; therefore, each system shall be usedindependently of the other. Combining values from the two systems may result in non-conformance with the standards.1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the
7、user of this standard to establish appropriate safety and health practices and determine the applicability of regulatorylimitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E114 Practice for Ultrasonic Pulse-Echo Straight-Beam Contact TestingE494 Practice for Measuring Ultrasonic Velo
8、city in MaterialsE1106 Test Method for Primary Calibration of Acoustic Emission SensorsE1316 Terminology for Nondestructive Examinations3. Terminology3.1 DefinitionsRefer to Terminology E1316, Section B, for terms used in this practice.3.2 Definitions of Terms Specific to This Standard:3.2.1 referen
9、ce sensor (RS)a sensor that has had its response established by primary calibration (also called secondarystandard transducer) (see Method E1106).3.2.2 secondary calibrationa procedure for measuring the frequency or transient response of an AE sensor by comparisonwith an RS.3.2.3 test blocka block o
10、f homogeneous, isotropic, elastic material 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 of calibration from sensors that have been calibrated by primarycalibration to other sensors
11、.1 This practice is under the jurisdiction ofASTM Committee E07 on Nondestructive Testing and is the direct responsibility of Subcommittee E07.04 on Acoustic EmissionMethod.Current edition approved Dec. 15, 2008June 1, 2013. Published January 2009June 2013. Originally approved in 1996. Last previous
12、 edition approved in 19982008 asE1781 - 98E1781 - 08.(2003)1. DOI: 10.1520/E1781-08.10.1520/E1781_E1781M-13.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the stand
13、ards Document Summary page on the ASTM website.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accuratel
14、y, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Driv
15、e, PO Box C700, West Conshohocken, PA 19428-2959. United States15. General Requirements5.1 Units for CalibrationSecondary calibration produces the same type of information regarding a sensor as does primarycalibration (Method E1106). An AE sensor responds to motion at its front face. The actual stre
16、ss and strain at the front face of amounted sensor depends on the interaction between the mechanical impedance of the sensor (load) and that of the mounting block(driver); neither the stress nor the strain is amenable to direct measurement at this location. However, the free displacement thatwould o
17、ccur at the surface of the block in the absence of the sensor can be inferred from measurements made elsewhere on thesurface. SinceAE sensors are used to monitor motion at a free surface of a structure and interactive effects between the sensor andthe structure are generally of no interest, the free
18、 motion is the appropriate input variable. It is therefore required that the units ofcalibration shall be volts per unit of free displacement or free velocity, that is, volts per metreunit or volt seconds per metre.unit.5.2 The calibration results may be expressed, in the frequency domain, as the st
19、eady-state magnitude and phase response of thesensor to steady-state sinusoidal excitation or, in the time domain, as the transient response of the sensor to a delta function ofdisplacement.5.3 Importance of the Test Block MaterialThe specific acoustical impedance (c) of the test block is an importa
20、nt parameterthat affects calibration results. Calibrations performed on blocks of different materials yield sensor sensitivities that are verydifferent. For example, a sensor that has been calibrated on a steel block, if calibrated on a glass or aluminum block, may havean average sensitivity that is
21、 50 % of the value obtained on steel and, if calibrated on a polymethyl methacrylate block, may havean average sensitivity that is 3 % of the value obtained on steel.35.3.1 For a sensor having a circular aperture (mounting face) with uniform sensitivity over the face, there are frequencies atwhich n
22、ulls in the frequency response occur. These nulls occur at the zeroes of the first order Bessel function, J1 (ka), where k= 2pif/c,f = frequency, c = the Rayleigh speed in the test block, and a = the radius of the sensor face.3 Therefore, calibrationresults depend on the Rayleigh wave speed in the m
23、aterial of the test block.5.3.2 For the reasons outlined in 5.3 and 5.3.1, all secondary calibration results are specific to a particular material; a secondarycalibration procedure must specify the material of the block.46. Requirements of the Secondary Calibration Apparatus6.1 Basic SchemeA prototy
24、pe apparatus for secondary calibration is shown in Fig. 1. A glass-capillary-break device or othersuitable source device (A) is deployed on the upper face of the steel test block (B). The RS (C) and the SUT (D) are placed at equaldistances from the source and in opposite directions from it. Because
25、of the symmetry of the sensor placement, the free surfacedisplacements at the locations of the RS and SUTare the same.Voltage transients from the two sensors are recorded simultaneouslyby digital waveform recorders (E) and processed by a computer.6.1.1 Actual dynamic displacements of the surface of
26、the test block at the locations of the RS and SUT may be different becausethe RS and SUT may present different load impedances to the test block. However, consistent with the definitions used for primaryand secondary calibration, the loading effects of both sensors are considered to be characteristi
27、cs of the sensors themselves, andcalibration results are stated in terms of the free displacement of the block surface.6.2 Qualification of The Test BlockThe prototype secondary calibration apparatus was designed for sensors intended for useon steel. The test block is therefore made of steel (hot ro
28、lled steel A36 material). For a steel block, it is recommended thatspecification to the metal supplier require that the block be stress relieved at 566C (1050F)1050F or greater and that the stressrelief be conducted subsequent to any flame cutting.3 Breckenridge, F. R., Proctor, T. M., Hsu, N. N., a
29、nd Eitzen, D. G.,“ Some Notions Concerning the Behavior of Transducers,” Progress in Acoustic Emission III, JapaneseSociety of Nondestructive Inspection, 1986, pp. 675684.4 Although this practice addresses secondary calibrations on test blocks of different materials, the only existing primary calibr
30、ations are performed on steel test blocks.To establish a secondary calibration on another material would also require the establishment of a primary calibration for the same material.FIG. 1 Schematic of the Prototype Secondary Calibration Apparatus: A = a Capillary-Break Source, B = a 41 by 41 by 19
31、-cm 16 by 16by 7.5 in. Steel Block, C = the RS, D = the SUT, and E = the Two-Channel Waveform Recorder SystemE1781/E1781M 1326.2.1 For a steel test block, there must be two parallel faces with a thickness, measured between the faces, of at least 18 cm.cm 7 in. The volume of the block must contain a
32、cylinder that is 40 cm 16 in. in diameter by 18-cm 7 in. long, and the twofaces must be flat and parallel to within 0.12-mm 0.12 mm 0.005 in. overall (60.06 mm).mm 0.0025 in.).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 tha
33、n1 m (40 in.),40 in., as determined by at least three profilometer traces taken in the central region of the block. The bottomsurface of the block must have a RMS roughness value no greater than 4 m (160 in.).160 in. The reason for having aspecification on the bottom surface is to ensure reasonable
34、ability to perform time-of-flight measurements of the speed of soundin the block.6.2.3 For blocks of materials other than steel, minimum dimensional requirements, dimensional accuracies, and the roughnesslimitation must be scaled in proportion to the longitudinal sound speed in the block material re
35、lative to that in steel.6.2.4 The top face of the block shall be the working face on which the source, RS, and SUT are located. These locations shallbe chosen near the center so as to maximize the distances of source and receivers to the nearest edge of the face. For a test blockof any material, the
36、 distance from the source to the RS and the distance from the source to the SUT must each be 100 6 2 mm4 6 0.1 in. (the same as that specified for primary calibration).6.2.5 The block must undergo longitudinal ultrasonic examination for indications at some frequency between 2 and 5 MHz. Theguideline
37、s of Practice E114 should be followed. The block must contain no indications that give a reflection greater than 12 % ofthe first back wall reflection.6.2.6 The material of the block must be highly uniform, as determined by pulse-echo, time-of-flight measurements of bothlongitudinal and shear waves.
38、 These measurements must be made through the block at a minimum of seven locations spacedregularly over the surface. The recommended method of measurement is pulse-echo overlap using precisely controlled delaysbetween sweeps. See Practice E494. It is recommended that the pulse-echo sensors have thei
39、r main resonances in the rangebetween 2 and 5 MHz. For the seven (or more) longitudinal measurements, the maximum difference between the individual valuesof the measurements must be no more than 0.3 % of the average value. The shear measurements must satisfy the same criterion.6.3 SourceThe source u
40、sed in the prototype secondary calibration system is a breaking glass capillary. Capillaries are preparedby drawing down 6-mm pyrexPyrex tubing to a diameter of 0.1 to 0.25 mm. Source events are generated by squeezing the capillarytubing against the test block using pressure from the side of a 4-mm
41、diameter glass rod held in the hand. Since the capillary is aline source, its length must be oriented at 90 degrees to the direction of propagation to the sensosrs.56.3.1 In general, a secondary calibration source may be any small aperture (less than 3 mm) mm 0.12 in.) device that canprovide suffici
42、ent energy to make the calibration measurements conveniently at all frequencies within the range of 100 kHz to 1MHz. Depending on the technique of the calibration, the source could be a transient device such as a glass-capillary-breakapparatus, a spark apparatus, a pulse-driven transducer (with puls
43、e rise time less than one (1) micro-second), or a continuous wavedevice such as a National Institute for Standards and Technology (NIST) Conical Transducer driven by a tone burst generator. Ifthe RS and SUT are to be tested on the block sequentially instead of simultaneously, then it must be establi
44、shed that the sourceis repeatable within 2 %.6.4 Reference SensorThe RS in the prototype secondary calibration system is an NIST Conical Transducer.6.4.1 In general, the RS must have a frequency response, as determined by primary calibration, that is flat over the frequencyrange of 100 kHz to 1 MHz
45、within a total overall variation of 20 dB either as a velocity transducer or a displacement transducer.For a valid calibration the RS must have been calibrated on the same material as the material that the SUT is to be used on. It ispreferred that the RS be of a type that has a small aperture and th
46、at its frequency response be as smooth as possible. See 5.3.1 andFig. 2Figs. 2 and 3 concerning the aperture effect.6.5 Sensor Under TestingThe SUT must be tested under conditions that are the same as those intended for the SUT when inuse. The couplant, the electrical load applied to the SUT termina
47、ls, and the hold-down force must all be the same as those that willbe applied to the SUT when in use. The preferred couplant is low-viscosity machine oil, and the preferred hold-down force is 9.8N. N 2.2 lbf. These conditions are all the same as for primary calibration.6.6 Data Recording and Process
48、ing EquipmentFor methods using transient sources, the instrumentation would include acomputer and two synchronized transient recorders, one for the RS channel and one for the SUT channel. The transient recordersmust be capable of at least eight-bitten-bit accuracy and a sampling rate of 20 MHz, or a
49、t least ten-bittwelve-bit accuracy and asampling rate of 10 MHz. They must each be capable of storing data for a time record of at least 55 s. The data are transferredto the computer for processing and also stored on a permanent device, for example, floppy disc, as a permanent record.7. Calibration Data Processing7.1 Raw DataIn the prototype secondary calibration system, the triggering event is the Rayleigh spike of the referencechannel. By means of pre-triggering, the data sequence in both channels is made to begin 25 s before
