1、Designation: E976 15Standard 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. Scope*1.1 This guide defines simple economical procedures fortesting or comparing the performance of acoustic emissionsensors. These p
3、rocedures 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 sta
4、ted 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 an
5、d 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 f
6、or 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 e
7、mission 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 o
10、f 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
11、are 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
12、sensor 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 p
14、rocedures 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 si
15、milar 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 Dec. 1, 2015. Published December 2015. Originallyapprov
16、ed in 1984. Last previous edition approved in 2010 as E976 - 10. DOI:10.1520/E0976-15.2For 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
17、 page onthe ASTM website.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1be taken when using these devices to minimize resonances sothat the characteristics of the senso
18、r are not masked by theseresonances.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 orseparate laboratories.4. Apparatus4.1 The essential elements of the apparatus for these pro
19、ce-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 Fig. 1.4.2 BlocksThe design of the block is not critical.However, the use of a “nonr
20、esonant” block is recommended foruse with an ultrasonic transducer and is required when thetransducer 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 a
21、luminum and low-alloysteel. After the bottom is faced and the taper 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
22、 the bottom face.The conic geometry and lack of any parallel surfaces 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
23、 themwith a layer of metal-filled epoxy.4.2.2 Gas-Jet Test BlockTwo gas-jet test blocks are shownin Fig. 3. The block shown in Fig. 3(a) is used for oppositesurface comparisons, which produce primarily compressionalwaves. That shown in Fig. 3(b) is for same surface compari-sons which produce primari
24、ly surface waves. The “nonreso-nant” block described in 4.2.1 can also be used with a gas jetin order to avoid exciting many resonant modes. The blocks inFig. 3 have been used successfully, but their design is notcritical. However it is suggested that the relative positions ofthe sensor and the jet
25、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, a consistent distance from the sensor end of the rod. SeePractice E2075 for additional details on this
26、technique.FIG. 1 Block Diagrams of Possible Experimental SetupsE976 1524.3 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 Ultrasonic TransducerRepeatable acoustic wavescan b
27、e 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 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 sho
28、uld be at least 1.25 cm to provide measur-able signal strength at 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 b
29、y 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 strength of the mechanical resonances excitedin a structure. Therefore, an ultrasonic transducer driven by awhite-n
30、oise 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 have a white-noise spectrum covering at leastthe frequency range from 10 kHz to 2 MHz and be capable ofan outpu
31、t 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, someresonances will be produced that may partially mask theresponse of the sensor under test. The sweep genera
32、tor 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 analyze the resulting response of the sensor under test.4.3.1.3 Pulse GeneratorThe ultrasonic transducer may beexci
33、ted 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 the damping time of the sensor, block, and transducer(typically 10 ms). The pulse repetition rate should be low(1
34、00 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 acquisition system (suchas a waveform basedAE system) or, in single-pulse mode, withthe counter in an acoustic emissi
35、on system.FIG. 2 The Beattie BlockE976 1534.3.2 Gas JetSuitable gases for this apparatus are extradry air, helium, etc. A pressure between 150 and 200 kPa isrecommended for helium or extra dry air. Once a pressure anda gas has been chosen, all further tests with the apparatusshould use that gas and
36、pressure. The gas jet should bepermanently attached to the test block (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 surfac
37、e of the block where the lead istouching. This stress release generates an acoustic wave. TheHsu pencil source uses a mechanical pencil. Care should betaken to always break the same length of the same type of lead.The normal lead diameter is 0.3 mm. The normal lead length is2.5 6 0.5 mm. The lead ha
38、rdness is 2H. Space permitting, thepencil is held at an angle of 30 degrees to the surface of the testblock or rod. The lead should always be broken at the same(a) Opposite Surface Comparison Setup(b) Same Surface Comparison TestFIG. 3 Gas-Jet Test BlocksFIG. 4 Acrylic Polymer RodE976 154spot on the
39、 block or rod with the same angle and orientation ofthe pencil. Spacing between the lead break and the center of thesensor should be at least 100 mm and must be documented.With distances shorter than 100 mm, it is harder to getconsistent results. The spacing between the lead break andsensor must be
40、large enough to avoid saturating the electronics,which would invalidate comparisons of amplitude response.The most desirable permanent record of a pencil lead break isthe waveform captured by a waveform-based data acquisitionsystem (such as an AE waveform-based instrument) withfrequency analysis (FF
41、T) capabilities.4.3.3.1 The Nielsen shoe, shown in Fig. 5, is a polytetra-fluoroethylene guide ring that can aid in breaking lead consis-tently. When the guide ring is constructed to the dimensions ofFig. 5, an angle of 30 degrees between pencil and structure willbe obtained for a 2.5 mm lead extens
42、ion. The dimension GT isrepresentative of the diameter of the pencil ferrule; typically itis 0.84 mm for a 0.3 mm pencil, but it depends on the brandand model of pencil. The length of the guide ring should bematched to the length of the pencil ferrule.4.3.3.2 The pencil lead break technique has othe
43、r uses inaddition to determining the reproducibility of sensor response,and it is not limited to use on blocks or rods. For some of theseother uses, larger lead diameters (which produce larger signals)may be appropriate and simple measurements of signal ampli-tude may suffice in lieu of full wavefor
44、m capture.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 Pream
45、plifierThe 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 pr
46、event this from occurring, it is recommended that shortsensor cables (2 m) be used and the resistive component ofthe preamplifier input impedance be 20 k or greater. Thepreamplifier gain should be fixed. Either 40 to 60-dB gains aresuitable for most sensors. The bandpass of the preamplifiershould be
47、 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 waveform g
48、enerated 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 printer.
49、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. Small 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 responseanaly
