1、Designation: E 976 05Standard Guide forDetermining the Reproducibility of Acoustic EmissionSensor Response1This standard is issued under the fixed designation E 976; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last
2、revision. A 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 guide defines simple economical procedures fortesting or comparing the performance of acoustic emissionsensors. These
3、 procedures 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 This standard doe
4、s 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 health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Stan
5、dards:2E 750 Practice for Characterizing Acoustic Emission In-strumentationE 2075 Practice for Verifying the Consistency ofAE-SensorResponse Using an Acrylic RodE 2374 Guide for Acoustic Emission System PerformanceVerification3. Significance and Use3.1 Acoustic emission data is affected by several c
6、haracter-istics of the instrumentation. The most obvious of these is thesystem sensitivity. Of all the parameters and componentscontributing to the sensitivity, the acoustic emission sensor isthe one most subject to variation. This variation can be a resultof damage or aging, or there can be variati
7、ons 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 for checkingsensors include: (1) checking the stability of its response withtime; (2) checking the sensor for possible damag
8、e 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 afterthermal cycling or exposure to a hostile environment. It is veryimportant that the sensor characteristics be always mea
9、suredwith the same sensor cable length and impedance as well as thesame preamplifier or equivalent. This guide presents severalprocedures for measuring sensor response. Some of theseprocedures require a minimum of special equipment.3.2 It is not the intent of this guide to evaluate AE systemperforma
10、nce. Refer to Practice E 750 for characterizing acous-tic instrumentation and refer to Guide E 2374 for AE systemperformance verification.3.3 The procedures given in this guide are designed tomeasure the response of an acoustic emission sensor to anarbitrary but repeatable acoustic wave. These proce
11、dures 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 sensor or apreviously calibrated reference sensor. In either case, such acalibration is beyond the scope of this guide.3.4
12、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 be reproduced at will. The sources and geom-etries given in this guide will produce primarily compressionalwaves. While th
13、e 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 procedures use a test block or rod. Such a deviceprovides a convenient mounting surface for the sensor andwhen appropriately
14、 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 similar to that experienced in actual use. Care mustbe taken when using these devices to minimize resonances sothat the chara
15、cteristics of the sensor are not masked by theseresonances.1This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-tive Testing and is the direct responsibility of Subcommittee E07.04 on AcousticEmission.Current edition approved Dec. 1, 2005. Published December 2005. Originallyappr
16、oved in 1984. Last previous edition approved in 2000 as E 976 - 00.2For 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 we
17、bsite.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.6 These procedures allow comparison of responses onlyon the same test setup. No attempt should be made to compareresponses on different test setups, whether in the same orsepara
18、te laboratories.4. Apparatus4.1 The essential elements of the apparatus for these proce-dures are: (1) the acoustic emission sensor under test; (2)ablock 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
19、 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 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
20、 Fig. 2, can be machined from a 10-cm (4-in.)diameter metal billet. The preferred materials are aluminumand low-alloy steel. After the bottom is faced and the taper cut,the block is clamped at a 10 angle and the top face is milled.The dimensions given will provide an approximate circle justover 2.5
21、cm (1 in.) in diameter for mounting the sensor. Theacoustic excitation should be applied at the center of thebottom face. The conic geometry and lack of any parallelsurfaces reduce the number of mechanical resonances that theblock can support. A further reduction in possible resonancesof the block c
22、an be achieved by roughly machining all surfacesexcept where the sensor and exciter are mounted and coatingthem with a layer of metal-filled epoxy.4.2.2 Gas-Jet Test BlockTwo gas-jet test blocks areshown in Fig. 3. The block shown in Fig. 3(a) is used foropposite surface comparisons, which produce p
23、rimarily 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 also be used with agas jet in order to avoid exciting many resonant modes. Theblocks in Fig. 3 have been used successfully, but t
24、heir 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 rodis shown in Fig. 4. The sensor is mounted on the end of the rodand the acoustic excitation is applied by means of pencil leadbreak
25、, a consistent distance from the sensor end of the rod. SeePractice E 2075 for additional details on this technique.4.3 Signal SourcesThree signal sources are recom-mended: an electrically driven ultrasonic transducer, a gas jet,and an impulsive source produced by breaking a pencil lead.4.3.1 Ultras
26、onic TransducerRepeatable acoustic wavescan be produced by an ultrasonic transducer permanentlybonded to a test block, or attached face-to-face to the AEsensor under test. The transducer should be heavily damped toFIG. 1 Block Diagrams of Possible Experimental SetupsE976052provide a broad frequency
27、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 0.5 in. to providemeasurable signal strength at the position of the sensor undertest. The ultrasonic transducer should be checked for adequateresponse in the 50 to 200-kHz
28、 region before permanentbonding to the test block.4.3.1.1 White Noise GeneratorAn ultrasonic transducerdriven by a white noise generator produces an acoustic wavethat lacks coherent wave trains of many wave lengths at onefrequency. This lack of coherent wave trains greatly reducesthe number and stre
29、ngth of the mechanical resonances excitedin a structure. Therefore, an ultrasonic transducer driven by awhite-noise generator can be used with a resonant block havingparallel sides. However, the use of a “nonresonant” block suchas that described in 4.2.1 is strongly recommended. Thegenerator should
30、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 transducer can bedriven by a sweep generator (or swept wave burst) in conjunc-tion with a “nonresonant” block. Even with this block,
31、 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 beused with a digital oscilloscope or waveform based dataacquisition system with frequency analysis (FFT) capabilitiesto anal
32、yze 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 less than one-half the period of the center frequency ofthe transducer (#0.22 s for a 2.25 MHz transducer) or longerthan t
33、he 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 the next one is excited.4.3.1.4 The pulse generator should be used with a digitaloscilloscope or waveform based data acqui
34、sition 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 extradry air, helium, etc. A pressure between 150 and 200 kPa (20to 30 psi) is recommended for helium or extra dry air. Once ap
35、ressure and a gas has been chosen, all further tests with theapparatus should use that gas and pressure. The gas jet shouldbe permanently attached to the test block (see Fig. 3(a) and3(b).FIG. 2 The Beattie BlockE9760534.3.3 Pencil Lead BreakA repeatable acoustic wave canbe generated by carefully br
36、eaking 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 generates an acoustic wave. TheHsu pencil source uses a mechanical pencil with a 0.3-mmdiameter lead (0.5-mm lead
37、 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 type of lead (lengthsbetween 2 and 3 mm are preferred).3The lead should always bebroken at the same spot on th
38、e block or rod with the same angleand orientation of the pencil. Spacing between the lead breakand sensor should be at least 10 cm (4 in.). With distancesshorter than that, it is harder to get consistent results. The mostdesirable permanent record of a pencil lead break is the waveform captured by a
39、 waveform based data acquisition system3Pentel 2H lead has been found satisfactory for this purpose.(a) Opposite Surface Comparison Setup(b) Same Surface Comparison TestFIG. 3 Gas-Jet Test BlocksFIG. 4 Acrylic Polymer RodE976054(such as an AE waveform based instrument) with frequencyanalysis (FFT) c
40、apabilities.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 allows an easy comparison, either with anothersensor or with the same sensor at a different time.4.4.1 Pre
41、amplifierThe 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 load on the sensor can distort the low-frequency response of a sensor with low inherent capacitance.To
42、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 be fixed. Either 40 to 60-dB gains aresuitable for most sensors. The bandpass of the preamplifiershould
43、 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 preamplifier assigned to it in an experiment.4.4.2 Waveform Based Instruments and StorageOscilloscopesThe wavefor
44、m 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. This waveform canbe recorded on computer media, displayed on a computerscreen or printed out on a printe
45、r. 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 comparingwaveforms, emphasis should be placed on the initial few cyclesand on the large amplitude features. Sma
46、ll variations late in thewaveform are often produced by slight changes in the couplingor position of the sensor under test. The waveform can also beconverted into the frequency domain by means of a fast fouriertransform (FFT) for amplitude versus frequency responseanalysis.4.4.3 Spectrum AnalyzersSp
47、ectrum analyzers can be usedwith acoustic signals generated by ultrasonic transducers thatare driven by either white-noise generators 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
48、 analyzer. A suitable spectrumanalyzer should be capable of displaying a spectrum coveringthe frequency range from 20 kHz to 1.2 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 ca
49、n 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 the impulsive sourcethat is built into many AE systems with automated pulsingcapabilities. One or more of several significant AE signalfeatures (such as amplitude, counts or en
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