1、Designation: E976 10Standard Guide forDetermining the Reproducibility of Acoustic EmissionSensor Response1This standard is issued under the fixed designation E976; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last re
2、vision. A 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 guide defines simple economical procedures fortesting or comparing the performance of acoustic emissionsensors. These pr
3、ocedures allow the user to check for degra-dation of a sensor or to select sets of sensors with nearlyidentical performances. The procedures are not capable ofproviding an absolute calibration of the sensor nor do theyassure transferability of data sets between organizations.1.2 UnitsThe values stat
4、ed in SI units are to be regardedas standard. No other units of measurement are included in thisstandard.1.3 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 establish appro-priate safety and
5、 health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E750 Practice for Characterizing Acoustic Emission Instru-mentationE2075 Practice for Verifying the Consistency of AE-SensorResponse Using an Acrylic RodE2374 Guide fo
6、r Acoustic Emission System PerformanceVerification3. Significance and Use3.1 Acoustic emission data is affected by several character-istics of the instrumentation. The most obvious of these is thesystem sensitivity. Of all the parameters and componentscontributing to the sensitivity, the acoustic em
7、ission sensor isthe one most subject to variation. This variation can be a resultof damage or aging, or there can be variations betweennominally identical sensors. To detect such variations, it isdesirable to have a method for measuring the response of asensor to an acoustic wave. Specific purposes
8、for checkingsensors include: (1) checking the stability of its response withtime; (2) checking the sensor for possible damage afteraccident or abuse; (3) comparing a number of sensors for usein a multichannel system to ensure that their responses areadequately matched; and (4) checking the response
9、afterthermal cycling or exposure to a hostile environment. It is veryimportant that the sensor characteristics be always measuredwith the same sensor cable length and impedance as well as thesame preamplifier or equivalent. This guide presents severalprocedures for measuring sensor response. Some of
10、 theseprocedures require a minimum of special equipment.3.2 It is not the intent of this guide to evaluate AE systemperformance. Refer to Practice E750 for characterizing acous-tic instrumentation and refer to Guide E2374 for AE systemperformance verification.3.3 The procedures given in this guide a
11、re designed tomeasure the response of an acoustic emission sensor to anarbitrary but repeatable acoustic wave. These procedures in noway constitute a calibration of the sensor. The absolutecalibration of a sensor requires a complete knowledge of thecharacteristics of the acoustic wave exciting the s
12、ensor or apreviously calibrated reference sensor. In either case, such acalibration is beyond the scope of this guide.3.4 The fundamental requirement for comparing sensorresponses is a source of repeatable acoustic waves. Thecharacteristics of the wave do not need to be known as long asthe wave can
13、be reproduced at will. The sources and geom-etries given in this guide will produce primarily compressionalwaves. While the sensors will respond differently to differenttypes of waves, changes in the response to one type of wavewill imply changes in the responses to other types of waves.3.5 These pr
14、ocedures use a test block or rod. Such a deviceprovides a convenient mounting surface for the sensor andwhen appropriately marked, can ensure that the source and thesensor are always positioned identically with respect to eachother. The device or rod also provides mechanical loading ofthe sensor sim
15、ilar to that experienced in actual use. Care must1This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-tive Testing and is the direct responsibility of Subcommittee E07.04 on AcousticEmission Method.Current edition approved June 1, 2010. Published July 2010. Originally approvedin
16、 1984. Last previous edition approved in 2005 as E976 - 05. DOI: 10.1520/E0976-10.2For 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 pag
17、e onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.be taken when using these devices to minimize resonances sothat the characteristics of the sensor are not masked by theseresonances.3.6 These procedures allow compa
18、rison of responses onlyon the same test setup. No attempt should be made to compareresponses on different test setups, whether in the same orseparate laboratories.4. Apparatus4.1 The essential elements of the apparatus for these proce-dures are: (1) the acoustic emission sensor under test; (2)ablock
19、 or rod; (3) a signal source; and (4) measuring andrecording equipment.4.1.1 Block diagrams of some of the possible experimentalsetups are shown in Fig. 1.4.2 BlocksThe design of the block is not critical. How-ever, the use of a “nonresonant” block is recommended for usewith an ultrasonic transducer
20、 and is required when the trans-ducer drive uses any form of coherent electrical signal.4.2.1 Conical “Nonresonant” BlockThe Beattie block,shown in Fig. 2, can be machined from a 10-cm diameter metalbillet. The preferred materials are aluminum and low-alloysteel. After the bottom is faced and the ta
21、per cut, the block isclamped at a 10 angle and the top face is milled. Thedimensions given will provide an approximate circle just over2.5 cm in diameter for mounting the sensor. The acousticexcitation should be applied at the center of the bottom face.The conic geometry and lack of any parallel sur
22、faces reducethe number of mechanical resonances that the block cansupport.Afurther reduction in possible resonances of the blockcan be achieved by roughly machining all surfaces exceptwhere the sensor and exciter are mounted and coating themwith a layer of metal-filled epoxy.4.2.2 Gas-Jet Test Block
23、Two gas-jet test blocks areshown in Fig. 3. The block shown in Fig. 3(a) is used foropposite surface comparisons, which produce primarily com-pressional waves. That shown in Fig. 3(b) is for same surfacecomparisons which produce primarily surface waves. The“nonresonant” block described in 4.2.1 can
24、also be used with agas jet in order to avoid exciting many resonant modes. Theblocks in Fig. 3 have been used successfully, but their designis not critical. However it is suggested that the relativepositions of the sensor and the jet be retained.4.2.3 Acrylic Polymer RodA polymethylmethacrylate rodi
25、s shown in Fig. 4. The sensor is mounted on the end of the rodand the acoustic excitation is applied by means of pencil leadbreak, a consistent distance from the sensor end of the rod. SeePractice E2075 for additional details on this technique.4.3 Signal SourcesThree signal sources are recom-mended:
26、 an electrically driven ultrasonic transducer, a gas jet,and an impulsive source produced by breaking a pencil lead.FIG. 1 Block Diagrams of Possible Experimental SetupsE976 1024.3.1 Ultrasonic TransducerRepeatable acoustic wavescan be produced by an ultrasonic transducer permanentlybonded to a test
27、 block, or attached face-to-face to the AEsensor under test. The transducer should be heavily damped toprovide a broad frequency response and have a center fre-quency in the 2.25 to 5.0-MHz range. The diameter of theactive element should be at least 1.25 cm to provide measur-able signal strength at
28、the position of the sensor under test. Theultrasonic transducer should be checked for adequate responsein the 50 to 200-kHz region before permanent bonding to thetest block.4.3.1.1 White Noise GeneratorAn ultrasonic transducerdriven by a white noise generator produces an acoustic wavethat lacks cohe
29、rent wave trains of many wave lengths at onefrequency. This lack of coherent wave trains greatly reducesthe number and strength of the mechanical resonances excitedin a structure. Therefore, an ultrasonic transducer driven by awhite-noise generator can be used with a resonant block havingparallel si
30、des. However, the use of a “nonresonant” block suchas that described in 4.2.1 is strongly recommended. Thegenerator should have a white-noise spectrum covering at leastthe frequency range from 10 kHz to 2 MHz and be capable ofan output level of 1 V rms.4.3.1.2 Sweep GeneratorThe ultrasonic transduce
31、r can bedriven by a sweep generator (or swept wave burst) in conjunc-tion with a “nonresonant” block. Even with this block, someresonances will be produced that may partially mask theresponse of the sensor under test. The sweep generator shouldhave a maximum frequency of at least 2 MHz and should be
32、used with a digital oscilloscope or waveform based dataacquisition system with frequency analysis (FFT) capabilitiesto analyze the resulting response of the sensor under test.4.3.1.3 Pulse GeneratorThe ultrasonic transducer may beexcited by a pulse generator. The pulse width should be eitherslightly
33、 less than one-half the period of the center frequency ofthe transducer (#0.22 s for a 2.25 MHz transducer) or longerthan the damping time of the sensor, block, and transducer(typically 10 ms). The pulse repetition rate should be low(100 pulses/s) so that each acoustic wave train is damped outbefore
34、 the next one is excited.4.3.1.4 The pulse generator should be used with a digitaloscilloscope or waveform based data acquisition system (suchas a waveform basedAE system) or, in single-pulse mode, withthe counter in an acoustic emission system.4.3.2 Gas JetSuitable gases for this apparatus are extr
35、adry air, helium, etc. A pressure between 150 and 200 kPa isFIG. 2 The Beattie BlockE976 103recommended for helium or extra dry air. Once a pressure anda gas has been chosen, all further tests with the apparatusshould use that gas and pressure. The gas jet should bepermanently attached to the test b
36、lock (see Fig. 3(a) and 3(b).4.3.3 Pencil Lead BreakA repeatable acoustic wave canbe generated by carefully breaking a pencil lead against the testblock or rod. When the lead breaks, there is a sudden release ofthe stress on the surface of the block where the lead istouching. This stress release gen
37、erates an acoustic wave. TheHsu pencil source uses a mechanical pencil with a 0.3-mmdiameter lead (0.5-mm lead is also acceptable but produces alarger signal). The Nielsen shoe, shown in Fig. 5 can aid inbreaking the lead consistently. Care should be taken to alwaysbreak the same length of the same
38、type of lead (lengthsbetween 2 and 3 mm are preferred).3The lead should always bebroken at the same spot on the block or rod with the same angleand orientation of the pencil. Spacing between the lead breakand sensor should be at least 10 cm. With distances shorter than3Pentel 2H lead has been found
39、satisfactory for this purpose.(a) Opposite Surface Comparison Setup(b) Same Surface Comparison TestFIG. 3 Gas-Jet Test BlocksFIG. 4 Acrylic Polymer RodE976 104that, it is harder to get consistent results. The most desirablepermanent record of a pencil lead break is the wave formcaptured by a wavefor
40、m based data acquisition system (such asan AE waveform based instrument) with frequency analysis(FFT) capabilities.4.4 Measuring and Recording EquipmentThe output ofthe sensor under test must be amplified before it can bemeasured. After the measurement, the results should be storedin a form that all
41、ows an easy comparison, either with anothersensor or with the same sensor at a different time.4.4.1 PreamplifierThe preamplifier, together with the sen-sor to preamp coaxial cable, provides an electrical load for thesensor, amplifies the output, and filters out unwanted frequen-cies. The electrical
42、load on the sensor can distort the low-frequency response of a sensor with low inherent capacitance.To prevent this from occurring, it is recommended that shortsensor cables (2 m) be used and the resistive component ofthe preamplifier input impedance be 20 kV or greater. Thepreamplifier gain should
43、be fixed. Either 40 to 60-dB gains aresuitable for most sensors. The bandpass of the preamplifiershould be at least 20 to 1200 kHz. It is recommended that onepreamplifier be set aside to be used exclusively in the testsetup. However, it may be appropriate at times to test a sensorwith the preamplifi
44、er assigned to it in an experiment.4.4.2 Waveform Based Instruments and StorageOscilloscopesThe waveform generated by a sensor in re-sponse to a single pulse or a pencil lead break can be measuredand stored by a transient recorder, digital oscilloscope, or awaveform-based acoustic emission system. T
45、his waveform canbe recorded on computer media, displayed on a computerscreen or printed out on a printer. Digitization rates should beat least ten samples per highest frequency period in thewaveform. Lower rates might result in distortion or loss ofamplitude accuracy of the wave shape. When comparin
46、gwaveforms, emphasis should be placed on the initial few cyclesand on the large amplitude features. Small variations late in thewaveform are often produced by slight changes in the couplingor position of the sensor under test. The waveform can also beEditorially corrected.FIG. 5 Guide Ring for Impul
47、sive SourceE976 105converted into the frequency domain by means of a fast fouriertransform (FFT) for amplitude versus frequency responseanalysis.4.4.3 Spectrum AnalyzersSpectrum analyzers can be usedwith acoustic signals generated by ultrasonic transducers thatare driven by either white-noise genera
48、tors or tracking-sweepgenerators, by gas-jet sources or by acoustic signals, producedby any source, that are captured on a transient recorder andreplayed into the spectrum analyzer. A suitable spectrumanalyzer should be capable of displaying a spectrum coveringthe frequency range from 20 kHz to 1.2
49、MHz. The amplitudeshould be displayed on a logarithmic scale covering a rangefrom at least 50 dB in order to display the entire dynamic rangeof the sensor. The spectrum can be recorded photographicallyfrom an oscilloscope. However, the most useful output is anXY graph showing the sensor amplitude response or powerversus frequency as shown in Fig. 6.4.4.4 Acoustic Emission SystemA sensor can be charac-terized by using an acoustic emission system and an impulsivesource such as a pencil lead break, an ultrasonic (or AE)transducer driven by a pulse generator, or
