ASTM E976-00
(Guide)Standard Guide for Determining the Reproducibility of Acoustic Emission Sensor Response
Standard Guide for Determining the Reproducibility of Acoustic Emission Sensor Response
SCOPE
1.1 This guide defines simple economical procedures for testing or comparing the performance of acoustic emission sensors. These procedures allow the user to check for degradation of a sensor or to select sets of sensors with nearly identical performances. The procedures are not capable of providing an absolute calibration of the sensor nor do they assure transferability of data sets between organizations.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E 976 – 00
Standard Guide for
Determining the Reproducibility of Acoustic Emission
Sensor Response
This standard is issued under the fixed designation E 976; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Principles of Application
1.1 This guide defines simple economical procedures for 3.1 The procedures given in this guide are designed to
testing or comparing the performance of acoustic emission measure the response of an acoustic emission sensor to an
sensors. These procedures allow the user to check for degra- arbitrary but repeatable acoustic wave. These procedures in no
dation of a sensor or to select sets of sensors with nearly way constitute a calibration of the sensor. The absolute
identical performances. The procedures are not capable of calibration of a sensor requires a complete knowledge of the
providing an absolute calibration of the sensor nor do they characteristics of the acoustic wave exciting the sensor or a
assure transferability of data sets between organizations. previously calibrated reference sensor. In either case, such a
1.2 This standard does not purport to address all of the calibration is beyond the scope of this guide.
safety concerns, if any, associated with its use. It is the 3.2 The fundamental requirement for comparing sensor
responsibility of the user of this standard to establish appro- responses is a source of repeatable acoustic waves. The
priate safety and health practices and determine the applica- characteristics of the wave do not need to be known as long as
bility of regulatory limitations prior to use. the wave can be reproduced at will. The sources and geom-
etries given in this guide will produce primarily compressional
2. Significance and Use
waves. While the sensors will respond differently to different
2.1 Acoustic emission data is affected by several character- types of waves, changes in the response to one type of wave
istics of the instrumentation. The most obvious of these is the
will imply changes in the responses to other types of waves.
system sensitivity. Of all the parameters and components 3.3 These procedures all use a test block or rod. Such a
contributing to the sensitivity, the acoustic emission sensor is
device provides a convenient mounting surface for the sensor
the one most subject to variation. This variation can be a result
and when appropriately marked, can ensure that the source and
of damage or aging, or there can be variations between the sensor are always positioned identically with respect to
nominally identical sensors. To detect such variations, it is
each other.The device or rod also provides mechanical loading
desirable to have a method for measuring the response of a of the sensor similar to that experienced in actual use. Care
sensor to an acoustic wave. Specific purposes for checking
must be taken when using these devices to minimize reso-
sensors include: (1) checking the stability of its response with nances so that the characteristics of the sensor are not masked
time; (2) checking the sensor for possible damage after
by these resonances.
accident or abuse; (3) comparing a number of sensors for use 3.4 These procedures allow comparison of responses only
in a multichannel system to ensure that their responses are
on the same test setup. No attempt should be made to compare
adequately matched; and (4) checking the response after responses on different test setups, whether in the same or
thermal cycling or exposure to a hostile environment. It is very
separate laboratories.
important that the sensor characteristics be always measured
4. Apparatus
withthesamesensorcablelengthandimpedanceaswellasthe
same preamplifier or equivalent. This guide presents several 4.1 The essential elements of the apparatus for these proce-
procedures for measuring sensor response. Some of these dures are: (1) the acoustic emission sensor under test; (2)a
procedures require a minimum of special equipment. block or rod; (3) a signal source; and (4) measuring and
recording equipment.
4.1.1 Block diagrams of some of the possible experimental
This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-
setups are shown in Fig. 1.
tive Testing and is the direct responsibility of Subcommittee E07.04 on Acoustic
Emission.
Current edition approved Dec. 10, 2000. Published February 2001. Originally
published as E 976 – 84. Last previous edition E 976 – 99.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E976–00
FIG. 1 Block Diagrams of Possible Experimental Setups
4.2 Blocks—The design of the block is not critical. How- not critical. However it is suggested that the relative positions
ever, the use of a “nonresonant” block is recommended for use of the sensor and the jet be retained.
with an ultrasonic transducer and is required when the trans- 4.2.3 Acrylic Polymer Rod—Apolymethylmethacrylate rod
ducer drive uses any form of coherent electrical signal. is shown in Fig. 4.The sensor is mounted on the end of the rod
4.2.1 Conical “Nonresonant” Block—The Beattie block, and the acoustic excitation is applied by means of pencil lead
shown in Fig. 2, can be machined from a 10-cm (4-in.) break, a consistent distance from the sensor end of the rod. See
diameter metal billet. The preferred materials are aluminum Appendix X1 for additional details on this technique.
and low-alloy steel.After the bottom is faced and the taper cut, 4.3 Signal Sources—Three signal sources are recom-
the block is clamped at a 10° angle and the top face is milled. mended: an electrically driven ultrasonic transducer, a gas jet,
The dimensions given will provide an approximate circle just and an impulsive source produced by breaking a pencil lead.
over 2.5 cm (1 in.) in diameter for mounting the sensor. The 4.3.1 Ultrasonic Transducer—Repeatable acoustic waves
acoustic excitation should be applied at the center of the can be produced by an ultrasonic transducer permanently
bottom face. The conic geometry and lack of any parallel bonded to a test block. The transducer should be heavily
surfaces reduce the number of mechanical resonances that the damped to provide a broad frequency response and have a
block can support. A further reduction in possible resonances centerfrequencyinthe2.25to5.0-MHzrange.Thediameterof
of the block can be achieved by roughly machining all surfaces the active element should be at least 1.25 cm (0.5 in.) to
except where the sensor and exciter are mounted and coating provide measurable signal strength at the position of the sensor
them with a layer of metal-filled epoxy. under test. The ultrasonic transducer should be checked for
4.2.2 Gas-Jet Test Block—Two gas-jet test blocks are adequate response in the 50 to 200-kHz region before perma-
shown in Fig. 3. The block shown in Fig. 3(a) is used for nent bonding to the test block.
opposite surface comparisons, which produce primarily com- 4.3.1.1 White Noise Generator—An ultrasonic transducer
pressional waves. That shown in Fig. 3(b) is for same surface driven by a white noise generator produces an acoustic wave
comparisons which produce primarily surface waves. The that lacks coherent wave trains of many wave lengths at one
“nonresonant” block described in 4.2.1 can also be used with a frequency. This lack of coherent wave trains greatly reduces
gas jet in order to avoid exciting many resonant modes. The the number and strength of the mechanical resonances excited
blocks in Fig. 3 have been used successfully but their design is in a structure. Therefore, an ultrasonic transducer driven by a
E976–00
FIG. 2 The Beattie Block
white-noisegeneratorcanbeusedwitharesonantblockhaving (<100 pulses/s) so that each acoustic wave train is damped out
parallel sides. However, the use of a “nonresonant” block such before the next one is excited.
as that described in 4.2.1 is strongly recommended. The
4.3.1.4 The pulse generator should be used with an oscillo-
generator should have a white-noise spectrum covering at least
scope and camera or, in single-pulse mode, with the counter in
the frequency range from 10 kHz to 2 MHz and be capable of
an acoustic emission system. Not enough energy is generated
an output level of 1 V rms.
above 200 kHz for effective use with a spectrum analyzer.
4.3.1.2 Sweep Generator—The ultrasonic transducer can be
4.3.2 Gas Jet—Suitable gases for this apparatus are extra
driven by a sweep generator in conjunction with a “nonreso-
dry air, helium, etc. A pressure between 150 and 200 kPa (20
nant” block. Even with this block, some resonances will be
to 30 psi) is recommended for helium or extra dry air. Once a
produced that may partially mask the response of the sensor
pressure and a gas has been chosen, all further tests with the
under test. The sweep generator should have a maximum
apparatus should use that gas and pressure. The gas jet should
frequency of at least 2 MHz and the sweep speed should be be permanently attached to the test block (see Fig. 3(a) and 3
compatible with the XY recorder used. It is recommended that
(b)).
a sweep generator be used with an a-c voltmeter with a
4.3.3 Pencil Lead Break—A repeatable acoustic wave can
logarithmic output.
begeneratedbycarefullybreakingapencilleadagainstthetest
4.3.1.3 Pulse Generator—The ultrasonic transducer may be block or rod.When the lead breaks, there is a sudden release of
excited by a pulse generator. The pulse width should be either the stress on the surface of the block where the lead is
slightly less than one-half the period of the center frequency of touching. This stress release generates an acoustic wave. The
the transducer (#0.22 µs for a 2.25 MHz transducer) or longer Hsu pencil source uses a mechanical pencil with a 0.3-mm
than the damping time of the sensor, block, and transducer diameter lead (0.5-mm lead is also acceptable but produces a
(typically >10 ms). The pulse repetition rate should be low larger signal). The Nielsen shoe, shown in Fig. 5 can aid in
E976–00
(a) Opposite Surface Comparison Setup
(b) Same Surface Comparison Test
FIG. 3 Gas-Jet Test Blocks
between2and3mmarepreferred). Theleadshouldalwaysbe
brokenatthesamespotontheblockorrodwiththesameangle
and orientation of the pencil. Spacing between the lead break
and sensor should be at least 10 cm (4 in.). With distances
shorter than that, it is harder to get consistent results. The most
desirable permanent record of a pencil lead break is the wave
form captured by a transient recorder or oscilloscope.
4.4 Measuring and Recording Equipment— The output of
the sensor under test must be amplified before it can be
measured. After the measurement, the results should be stored
in a form that allows an easy comparison, either with another
FIG. 4 Acrylic Polymer Rod
sensor or with the same sensor at a different time.
breaking the lead consistently. Care should be taken to always
break the same length of the same type of lead (lengths Pentel 2H lead has been found satisfactory for this purpose.
E976–00
(a) Nielsen Shoe on Hsu Pencil Source
(b) Nielsen Shoe
____________
ôP
Editorially corrected.
FIG. 5 Guide Ring for Impulsive SourceôP
4.4.1 Preamplifier—Thepreamplifier,togetherwiththesen- recorded photographically from an oscilloscope. However, the
most useful output is an XY plot of the spectrum as shown in
sor to preamp coaxial cable, provides an electrical load for the
Fig. 6.
sensor, amplifies the output, and filters out unwanted frequen-
cies. The electrical load on the sensor can distort the low- 4.4.3 Voltmeters—An a-c voltmeter can be used to measure
sensor outputs produced by signals generated by an ultrasonic
frequency response of a sensor with low inherent capacitance.
transducer driven by a sweep generator. The response of the
To prevent this from occurring, it is recommended that short
voltmeter should be flat over the frequency range from 10 kHz
sensor cables (<2 m) be used and the resistive component of
to 2 MHz. It is desirable that the voltmeter either have a
the preamplifier input impedance be 20 kV or greater. The
logarithmic output or be capable of driving a logarithmic
preamplifier gain should be fixed. Either 40 to 60-dB gains are
converter. The output of the voltmeter or converter is recorded
suitable for most sensors. The bandpass of the preamplifier
on an XY recorder as a function of frequency.
should be at least 20 to 1200 kHz. It is recommended that one
4.4.3.1 The limited dynamic range of an rms voltmeter
preamplifier be set aside to be used exclusively in the test
makes it less desirable than an a-c averaging voltmeter when
setup. However, it may be appropriate at times to test a sensor
used with a sweep generator. However, a rough estimate of a
with the preamplifier assigned to it in an experiment.
sensor performance can be obtained by using an rms or a-c
4.4.2 Spectrum Analyzers—A very useful instrument for
voltmeter to measure the output of a sensor driven by a wide
testing sensor response is the spectrum analyzer. Spectrum
band source such as a white-noise generator or a gas jet.
analyzers can be used with acoustic signals generated by
4.4.4 Acoustic Emission System—A sensor can be charac-
ultrasonic transducers that are driven by either white-noise
terized by using an acoustic emission system and an impulsive
generators or tracking-sweep generators, by gas-jet sources or
source such as a pencil lead break, an ultrasonic (or AE)
by acoustic signals, produced by any source, that are captured
transducer driven by a pulse generator, or the impulsive source
onatransientrecorderandreplayedintothespectrumanalyzer.
that is built into many AE systems with automated pulsing
Asuitable spectrum analyzer should be capable of displaying a
capabilities. One or more of several significant AE signal
spectrum covering the frequency range from 20 kHz to 1.2
features (such as amplitude, counts or energy) can be used to
MHz. The amplitude should be displayed on a logarithmic
characterize the sensor response. The acoustic emission fea-
scale covering a range from at least 50 dB in order to display
tures from each signal pulse should be measured for multiple
the entire dynamic range of the sensor. The spectrum can be pulses (at least three). Data recorded should be the individual
E97
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