1、Designation: E 1106 07Standard Test Method forPrimary Calibration of Acoustic Emission Sensors1This standard is issued under the fixed designation E 1106; 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 (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the requirements for the abso-lute calibration of acoustic emission (AE) sensors. The cali-bration yields the
3、 frequency response of a transducer to waves,at a surface, of the type normally encountered in acousticemission work. The transducer voltage response is determinedat discrete frequency intervals of approximately 10 kHz up to1 MHz. The input is a given well-established dynamic dis-placement normal to
4、 the mounting surface. The units of thecalibration are output voltage per unit mechanical input (dis-placement, velocity, or acceleration).1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to
5、 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 MethodE 494 Practice for Measuring Ultrasonic Velocit
6、y in Mate-rialsE 650 Guide for Mounting Piezoelectric Acoustic EmissionSensorsE 1316 Terminology for Nondestructive Examinations3. Terminology3.1 Refer to Terminology E 1316 for terminology used inthis test method.4. Significance and Use4.1 Transfer StandardsOne purpose of this test method isfor the
7、 direct calibration of displacement transducers for use assecondary standards for the calibration ofAE sensors for use innondestructive evaluation. For this purpose, the transfer stan-dard should be high fidelity and very well behaved andunderstood. If this can be established, the stated accuracysho
8、uld apply over the full frequency range up to 1 MHz.NOTE 1The stated accuracy applies only if the transfer standardreturns to quiescence, following the transient input, before any wavereflected from the boundary of the calibration block returns to the transferstandard (;100 s). For low frequencies w
9、ith periods on the order of thetime window, this condition is problematical to prove.4.2 Applications SensorsThis test method may also beused for the calibration ofAE sensors for use in nondestructiveevaluation. Some of these sensors are less well behaved thandevices suitable for a transfer standard
10、. The stated accuracy forsuch devices applies in the range of 100 kHz to 1 MHz andwith less accuracy below 100 kHz.5. General Requirements5.1 A primary difficulty in any calibration of a mechanical/electrical transduction device is the determination of themechanical-motion input to the device. Using
11、 this calibrationprocedure, the motional input may be determined by twodifferent means: theoretical calculation and measurement withan absolute displacement transducer.5.2 Theoretical CalculationElasticity theory has beenused to calculate the dynamic displacement of the surface of aninfinite half-sp
12、ace due to a normal point-force step function intime. The solutions give the displacement of any point on thesurface as a function of time, yielding a waveform for thedisplacement called the seismic surface pulse.5.2.1 This calibration test method uses an approximation tothis theoretical solution. S
13、ee also Breckenridge3and Hsu andBreckenridge4. The half-space is approximated by a largemetal block in the form of a circular cylinder and the pointforcestep function is closely approximated by the breaking of a glasscapillary against the plane surface of the block. The displace-ment as a function o
14、f time should be calculated for the locationof the device under test (on the same surface of the block as theinput). This calculation should be performed using a measuredvalue of the step function force and the elastic constants thatare determined by speed of sound measurements on the block.1This te
15、st method is under the jurisdiction of ASTM Committee E07 onNondestructive Testing and is the direct responsibility of Subcommittee E07.04 onAcoustic Emission Method.Current edition approved July 1, 2007. Published July 2007. Originally approvedin 1986. Last previous edition approved in 2002 as E 11
16、06 - 86(2002)e1.2For 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 Summary page onthe ASTM website.3Breckenridge, F. R., “Acoustic Emission Tran
17、sducer Calibration by Means ofthe Seismic Surface Pulse,” Journal of Acoustic Emission Vol 1, pp. 8794.4Hsu, N. N., and Breckenridge, F. R., “Characterization and Calibration ofAcoustic Emission Sensors,” Materials Evaluation, Vol 39, 1981, pp. 6068.1Copyright ASTM International, 100 Barr Harbor Dri
18、ve, PO Box C700, West Conshohocken, PA 19428-2959, United States.5.3 Absolute Displacement MeasurementAn absolutemeasurement of the dynamic normal surface displacement ofthe block is required for this calibration test method. Thetransducer used for this measurement is a standard transduceragainst wh
19、ich the device under test is compared. The standardtransducer should meet or exceed the performance of thecapacitive transducer described by Breckenridge andGreenspan5. The important characteristics of the standardtransducer include high fidelity, high sensitivity, and operatingcharacteristics amena
20、ble to theoretical calculation. It shouldalso present no appreciable dynamic loading to the surface it ismeasuring.5.3.1 For a calibration, the standard transducer and thedevice to be calibrated are both placed on the same surface ofthe block as the mechanical input and equidistant in oppositedirect
21、ions from it. This guarantees that both experience thesame displacement-time history. Comparison of the output ofthe transfer standard or AE sensor with the output of thestandard transducer yields a calibration of the device undertest.5.3.2 Other relative geometries for the input and transducersare
22、possible, but results from other geometries should only beused to supplement results from the “same surface” geometry.AE waves in structures are most frequently dominated bysurface wave phenomena, and the calibration should be basedon the transducers response to such waves.5.4 Units for the Calibrat
23、ionAn AE sensor may beconsidered to respond to either stress or strain at its front face.The actual stress and strain at the front face of a mountedsensor depend on the interaction between the mechanicalimpedance of the sensor (load) and that of the mounting block(driver). Neither the stress nor the
24、 strain is amenable to directmeasurement at this location. However, the free displacementthat would occur at the surface of the block in the absence ofthe sensor can be inferred from either elasticity theory calcu-lations or from measurements made elsewhere on the surface.Since AE sensors are used t
25、o monitor motion at a free surfaceof a structure and interactive effects between sensor andstructure are generally of no interest, the free surface motion isthe appropriate input variable. It is, therefore, recommendedthat the units of calibration should be voltage per unit of freemotion; for exampl
26、e, volts per metre.5.5 Block Material:5.5.1 Since the calibration depends on the interaction of themechanical impedance of the block and that of the AE sensor,a calibration procedure must specify the material of the block.Calibrations performed on blocks of different materials willyield transducer s
27、ensitivity versus frequency curves that aredifferent in shape and in average magnitude. The amount bywhich such averages differ may be very large. A transducercalibrated on a glass or aluminum block will have an averagesensitivity that may be from 50 to 100% of the value obtainedon steel, and will h
28、ave an average sensitivity that may be aslittle as 3 % of the value obtained on steel if calibrated on apolymethyl methacrylate block. In general, the sensitivity willbe less if the block is made of a less rigid or less densematerial.5.5.2 The Rayleigh speed in the material of the block affectssurfa
29、ce wave calibrations. For a sensor having a circularaperture (mounting face) with uniform sensitivity over theface, the aperture effect predicts nulls at the zeroes of J1(ka),where k=2pf/c, and f = frequency, c = Rayleigh speed, anda = radius of the sensor face. Hence, the frequencies at whichthe nu
30、lls occur are dependent upon the Rayleigh speed.6. Apparatus6.1 A typical basic scheme for the calibration is shown inFig. 1. A glass capillary, B, of diameter about 0.2 mm, issqueezed between the tip of the loading screw, C, and the upperface of the large steel transfer block, A. When the capillary
31、breaks, the sudden release of force is a step function whoserisetime is of the order of 0.1 s. The magnitude of the forcestep is measured by the combination of the PZT disc, D, in theloading screw and a charge amplifier, E, connected to a storageoscilloscope, F. The standard capacitive transducer, G
32、, and thedevice under test, H, are placed equally distant (usually 100mm) from the source and in opposite directions from it. It isobvious from the symmetry that the surface displacementswould be the same at the two transducer locations if it were notfor the loading effects of the transducers. The l
33、oading effect ofthe standard capacitive transducer is negligible and the loadingeffect of the unknown sensor is part of its calibration.6.1.1 Voltage transients from the two transducers are re-corded simultaneously by digital recorders, I, and the informa-tion is stored for processing by the compute
34、r, J.6.1.2 With such a system, it is possible to do the necessarycomparison between the signal from the unknown sensor andthat from the standard transducer or with the displacementwaveform calculated by elasticity theory. A similar resultshould be obtained either way.6.2 The Transfer BlockThe transf
35、er block must be madefrom specially chosen material. It should be as defect-free aspossible and should undergo an ultrasonic longitudinal exami-nation at 2.25 MHz. The method described in Practice E114should be used. The block should contain no flaws which givea reflection larger than 10 % of the fi
36、rst back wall reflection.The material should also be highly uniform as determined bypulse-echo time of flight measurements through the block at aminimum of 15 locations regularly spaced over the surface (seePractice E 494). The individual values of the longitudinal andshear wave speed should differ
37、from the average by no morethan 61 part and 63 parts in 103, respectively.Atransfer blockand calibration apparatus is shown in Fig. 2.6.3 The Step Function SourceThe step function forceevents are to be made by breaking glass capillary tubing (Fig.3). The capillaries are drawn down from ordinary labo
38、ratoryglass tubing made of borosilicate glass. Sizes of the capillarymay range from about 0.1 mm to 0.3 mm outside diameter,with 0.2 mm being typical. A bore size equal to the wallthickness gives the best results. The force obtained is usuallybetween 10 N and 30 N, with 20 N being typical.6.3.1 The
39、capillary is to be laid horizontally on a piece ofmicroscope cover glass (0.08 by 1.5 by 1.5 mm) which has5Breckenridge, F. R., and Greenspan, M., “Surface-Wave Displacement: Abso-lute Measurements Using a Capacitive Transducer,” Journal, Acoustic Society ofAmerica, Vol 69, pp 11771185.E1106072Astee
40、l transfer blockBcapillary sourceCloading screwDPZT discEcharge amplifierFstorage oscilloscopeGstandard transducerHtransducer under testItransient recordersJcomputerFIG. 1 Schematic Diagram of the ApparatusFIG. 2 Photograph of the Steel Block with the Calibration Apparatus in PlaceE1106073been cemen
41、ted to the top face of the steel block with salol(phenyl salicylate) or cyanoacrylate cement. The force isapplied to the capillary by a solid glass rod (2 mm in diameter)which has been laid horizontally on top of the capillary and atright angles to it. The rod is forced downward by the loadingscrew
42、until the capillary breaks. The loading screw is to bethreaded through a yoke above the calibration surface. Theloading screw should contain a ceramic force transducer whichhas been calibrated by dead weights. Thus, although the size ofa source event cannot be predicted in advance, its magnitudemay
43、be measured and used for the elasticity theory calculationof the surface displacement.6.3.2 Ideally, the capillary should rest directly on the steelwith no cover glass interposed. It may be found necessary touse the cover slide to prevent damage to the block surface. Thepresence of the cover glass d
44、oes alter the waveform veryslightly; a slight ringing occurs due to reflections at itsboundaries. The ringing contains only frequencies above 2MHz. Furthermore, the effects on both standard transducer andunknown sensor are the same; therefore, the calibration is notaffected.6.4 The Standard Transduc
45、erThe standard transducer tobe used for the absolute measurement of displacement in thecalibration is to have characteristics at least as good as thecapacitive transducer described by Breckenridge andGreenspan.5This device, shown in Figs. 4 and 5, essentiallyconsists of an inertial mass (about 40 g)
46、 mounted on compliantsupports and separated from the top surface of the steel blockby an air gap of about 4 m. This gap is determined bymeasuring the capacitance between the transducer and thetransfer block using a three-terminal ratio arm bridge asdescribed by Breckenridge and Greenspan.5The inerti
47、al massis a brass cylinder with its axis horizontal. When the blocksurface moves at frequencies above the natural resonance ofthe mass on its compliant supports (approximately 1 kHz), thebrass cylinder remains approximately stationary. The brasscylinder is polarized to 100 Vdc through a large valued
48、 resistor.The large resistance causes the capacitor to operate essentiallyin a fixed charge condition so that the voltage varies inverselywith capacitance for the frequencies of interest.6.4.1 For use as a primary standard, it is essential that thesensitivity of the transducer be calculable. To make
49、 thecalculations tractable, the cylinder is treated as a section of aninfinite cylinder. Electrical guards are attached to each end toeliminate end effects that would otherwise be severe.6.4.2 The sensing area of the transducer is 12.4 mm longand effectively less than 1 mm wide. The long axis of this areais tangent to an advancing wavefront from the capillary source.6.4.3 The sensitivity of the transducer is approximately12 3 106V/m and the minimum detectable rms displacement is4 3 1012m. The calculated frequency response of the trans-ducer based on its effec
