ASTM E1106-86(2002)e1
(Test Method)Standard Method for Primary Calibration of Acoustic Emission Sensors
Standard Method for Primary Calibration of Acoustic Emission Sensors
SCOPE
1.1 This method covers the requirements for the absolute calibration of acoustic emission (AE) sensors. The calibration yields the frequency response of a transducer to waves, at a surface, of the type normally encountered in acoustic emission work. The transducer voltage response is determined at discrete frequency intervals of approximately 10 kHz up to 1 MHz. The input is a given well-established dynamic displacement normal to the mounting surface. The units of the calibration are output voltage per unit mechanical input (displacement, velocity, or acceleration).
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 1106 – 86 (Reapproved 2002)
Standard Method for
Primary Calibration of Acoustic Emission Sensors
This standard is issued under the fixed designation E1106; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Editorially replaced the term “inspection” with “examination” in 6.2 in June 2002.
1. Scope understood. If this can be established, the stated accuracy
should apply over the full frequency range up to 1 MHz.
1.1 This method covers the requirements for the absolute
calibration of acoustic emission (AE) sensors. The calibration
NOTE 1—The stated accuracy applies only if the transfer standard
yields the frequency response of a transducer to waves, at a
returns to quiescence, following the transient input, before any wave
reflected from the boundary of the calibration block returns to the transfer
surface, of the type normally encountered in acoustic emission
standard (;100 µs). For low frequencies with periods on the order of the
work. The transducer voltage response is determined at dis-
time window, this condition is problematical to prove.
crete frequency intervals of approximately 10 kHz up to 1
MHz. The input is a given well-established dynamic displace- 4.2 Applications Sensors—This method may also be used
for the calibration of AE sensors for use in nondestructive
ment normal to the mounting surface. The units of the
calibration are output voltage per unit mechanical input (dis- evaluation. Some of these sensors are less well behaved than
devicessuitableforatransferstandard.Thestatedaccuracyfor
placement, velocity, or acceleration).
1.2 This standard does not purport to address all of the such devices applies in the range of 100 kHz to 1 MHz and
with less accuracy below 100 kHz.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
5. General Requirements
priate safety and health practices and determine the applica-
5.1 Aprimary difficulty in any calibration of a mechanical/
bility of regulatory limitations prior to use.
electrical transduction device is the determination of the
2. Referenced Documents
mechanical-motion input to the device. Using this calibration
2.1 ASTM Standards: procedure, the motional input may be determined by two
different means: theoretical calculation and measurement with
E114 Practice for Ultrasonic Pulse-Echo Straight-Beam
Examination by the Contact Method an absolute displacement transducer.
E494 Practice for Measuring Ultrasonic Velocity in Mate- 5.2 Theoretical Calculation—Elasticity theory has been
usedtocalculatethedynamicdisplacementofthesurfaceofan
rials
E650 Guide for Mounting PiezoelectricAcoustic Emission infinite half-space due to a normal point-force step function in
time. The solutions give the displacement of any point on the
Sensors
E1316 Terminology for Nondestructive Examinations surface as a function of time, yielding a waveform for the
displacement called the seismic surface pulse.
3. Terminology
5.2.1 This calibration method uses an approximation to this
3.1 Refer to Terminology E1316 for terminology used in theoretical solution. See also Breckenridge and Hsu and
this method. Breckenridge . The half-space is approximated by a large
metalblockintheformofacircularcylinderandthepointforce
4. Significance and Use
stepfunctioniscloselyapproximatedbythebreakingofaglass
4.1 Transfer Standards—One purpose of this method is for capillary against the plane surface of the block. The displace-
the direct calibration of displacement transducers for use as
mentasafunctionoftimeshouldbecalculatedforthelocation
secondarystandardsforthecalibrationofAEsensorsforusein ofthedeviceundertest(onthesamesurfaceoftheblockasthe
nondestructive evaluation. For this purpose, the transfer stan-
input). This calculation should be performed using a measured
dard should be high fidelity and very well behaved and value of the step function force and the elastic constants that
are determined by speed of sound measurements on the block.
This method is under the jurisdiction of ASTM Committee E07 on Nonde-
structive Testing and is the direct responsibility of Subcommittee E07.04 on Breckenridge, F. R., “Acoustic Emission Transducer Calibration by Means of
Acoustic Emission. the Seismic Surface Pulse,” Journal of Acoustic Emission Vol 1, pp. 87–94.
Current edition approved April 25, 1986. Published June 1986. Hsu, N. N., and Breckenridge, F. R., “Characterization and Calibration of
Annual Book of ASTM Standards, Vol 03.03. Acoustic Emission Sensors,” Materials Evaluation, Vol 39, 1981, pp. 60–68.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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E 1106 – 86 (2002)
5.3 Absolute Displacement Measurement—An absolute polymethyl methacrylate block. In general, the sensitivity will
measurement of the dynamic normal surface displacement of be less if the block is made of a less rigid or less dense
the block is required for this calibration method. The trans- material.
ducer used for this measurement is a standard transducer 5.5.2 TheRayleighspeedinthematerialoftheblockaffects
against which the device under test is compared. The standard surface wave calibrations. For a sensor having a circular
transducer should meet or exceed the performance of the aperture (mounting face) with uniform sensitivity over the
capacitive transducer described by Breckenridge and face, the aperture effect predicts nulls at the zeroes of J (ka),
Greenspan . The important characteristics of the standard wherek=2pf/c, and f =frequency, c =Rayleigh speed, and
transducer include high fidelity, high sensitivity, and operating a =radius of the sensor face. Hence, the frequencies at which
characteristics amenable to theoretical calculation. It should the nulls occur are dependent upon the Rayleigh speed.
also present no appreciable dynamic loading to the surface it is
6. Description of Typical Apparatus
measuring.
6.1 A typical basic scheme for the calibration is shown in
5.3.1 For a calibration, the standard transducer and the
Fig. 1. A glass capillary, B, of diameter about 0.2 mm, is
device to be calibrated are both placed on the same surface of
squeezedbetweenthetipoftheloadingscrew,C,andtheupper
the block as the mechanical input and equidistant in opposite
face of the large steel transfer block, A. When the capillary
directions from it. This guarantees that both experience the
breaks, the sudden release of force is a step function whose
same displacement-time history. Comparison of the output of
risetime is of the order of 0.1 µs. The magnitude of the force
the transfer standard or AE sensor with the output of the
step is measured by the combination of the PZTdisc, D, in the
standard transducer yields a calibration of the device under
loadingscrewandachargeamplifier, E,connectedtoastorage
test.
oscilloscope, F.The standard capacitive transducer, G, and the
5.3.2 Otherrelativegeometriesfortheinputandtransducers
device under test, H, are placed equally distant (usually 100
are possible, but results from other geometries should only be
mm) from the source and in opposite directions from it. It is
used to supplement results from the “same surface” geometry.
obvious from the symmetry that the surface displacements
AE waves in structures are most frequently dominated by
wouldbethesameatthetwotransducerlocationsifitwerenot
surface wave phenomena, and the calibration should be based
for the loading effects of the transducers. The loading effect of
on the transducer’s response to such waves.
the standard capacitive transducer is negligible and the loading
5.4 Units for the Calibration—An AE sensor may be
effect of the unknown sensor is part of its calibration.
considered to respond to either stress or strain at its front face.
6.1.1 Voltage transients from the two transducers are re-
The actual stress and strain at the front face of a mounted
corded simultaneously by digital recorders, I, and the informa-
sensor depend on the interaction between the mechanical
tion is stored for processing by the computer, J.
impedance of the sensor (load) and that of the mounting block
6.1.2 With such a system, it is possible to do the necessary
(driver). Neither the stress nor the strain is amenable to direct
comparison between the signal from the unknown sensor and
measurement at this location. However, the free displacement
that from the standard transducer or with the displacement
that would occur at the surface of the block in the absence of
waveform calculated by elasticity theory. A similar result
the sensor can be inferred from either elasticity theory calcu-
should be obtained either way.
lations or from measurements made elsewhere on the surface.
6.2 The Transfer Block—The transfer block must be made
SinceAE sensors are used to monitor motion at a free surface
from specially chosen material. It should be as defect-free as
of a structure and interactive effects between sensor and
possible and should undergo an ultrasonic longitudinal exami-
structure are generally of no interest, the free surface motion is
nation at 2.25 MHz. The method described in PracticeE114
the appropriate input variable. It is, therefore, recommended
should be used. The block should contain no flaws which give
that the units of calibration should be voltage per unit of free
a reflection larger than 10% of the first back wall reflection.
motion; for example, volts per metre.
The material should also be highly uniform as determined by
5.5 Block Material:
pulse-echo time of flight measurements through the block at a
5.5.1 Since the calibration depends on the interaction of the
minimumof15locationsregularlyspacedoverthesurface(see
mechanical impedance of the block and that of theAE sensor,
Practice E494). The individual values of the longitudinal and
a calibration procedure must specify the material of the block.
shear wave speed should differ from the average by no more
Calibrations performed on blocks of different materials will
than 61partand 63partsin10 ,respectively.Atransferblock
yield transducer sensitivity versus frequency curves that are
and calibration apparatus is shown in Fig. 2.
different in shape and in average magnitude. The amount by
6.3 The Step Function Source—The step function force
which such averages differ may be very large. A transducer
events are to be made by breaking glass capillary tubing (Fig.
calibrated on a glass or aluminum block will have an average
3). The capillaries are drawn down from ordinary laboratory
sensitivity that may be from 50 to 100% of the value obtained
glass tubing made of borosilicate glass. Sizes of the capillary
on steel, and will have an average sensitivity that may be as
may range from about 0.1 mm to 0.3 mm outside diameter,
little as 3% of the value obtained on steel if calibrated on a
with 0.2 mm being typical. A bore size equal to the wall
thickness gives the best results. The force obtained is usually
between 10 N and 30 N, with 20 N being typical.
Breckenridge, F. R., and Greenspan, M., “Surface-Wave Displacement: Abso-
6.3.1 The capillary is to be laid horizontally on a piece of
lute Measurements Using a Capacitive Transducer,” Journal, Acoustic Society of
America, Vol 69, pp 1177–1185. microscope cover glass (0.08 by 1.5 by 1.5 mm) which has
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E 1106 – 86 (2002)
A—steel transfer block
B—capillary source
C—loading screw
D—PZT disc
E—charge amplifier
F—storage oscilloscope
G—standard transducer
H—transducer under test
I—transient recorders
J—computer
FIG. 1 Schematic Diagram of the Apparatus
FIG. 2 Photograph of the Steel Block with the Calibration Apparatus in Place
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E 1106 – 86 (2002)
FIG. 3 Glass Capillary Source
been cemented to the top face of the steel block with salol the mass on its compliant supports (approximately 1 kHz), the
(phenyl salicylate) or cyanoacrylate cement. The force is brass cylinder remains approximately stationary. The brass
appliedtothecapillarybyasolidglassrod(2mmindiameter) cylinderispolarizedto100Vdcthroughalargevaluedresistor.
which has been laid horizontally on top of the capillary and at The large resistance causes the capacitor to operate essentially
right angles to it. The rod is forced downward by the loading in a fixed charge condition so that the voltage varies inversely
screw until the capillary breaks. The loading screw is to be with capacitance for the frequencies of interest.
threaded through a yoke above the calibration surface. The 6.4.1 For use as a primary standard, it is essential that the
loadingscrewshouldcontainaceramicforcetransducerwhich sensitivity of the transducer be calculable. To make the
hasbeencalibratedbydeadweights.Thus,althoughthesizeof calculations tractable, the cylinder is treated as a section of an
a source event cannot be predicted in advance, its magnitude infinite cylinder. Electrical guards are attached to each end to
may be measured and used for the elasticity theory calculation eliminate end effects that would otherwise be severe.
of the surface displacement. 6.4.2 The sensing area of the transducer is 12.4 mm long
6.3.2 Ideally, the capillary should rest directly on the steel and effectively less than 1 mm wide.The long axis of this area
with no cover glass interposed. It may be found necessary to istangenttoanadvancingwavefrontfromthecapillarysource.
usethecoverslidetopreventdamagetotheblocksurface.The 6.4.3 The sensitivity of the transducer is approximately
presence of the cover glass does alter the waveform very 12 310 V/mandtheminimumdetectablermsdisplacementis
−12
slightly; a slight ringing occurs due to reflections at its 4 310 m. The calculated frequency response of the trans-
boundaries. The ringing contains only frequencies above 2 ducer based on its effective aperture width and its deviation
MHz. Furthermore, the effects on both standard transducer and from the curvature of the wavefronts is shown in Fig. 6.At1
unknown sensor are the same; therefore, the calibration is not MHz the amplitude is down by less than 10% and the phase
affected. lag is about 8°. Expressions in Breckenridge and Greenspan
6.4 The Standard Transducer—The standard transducer to can be used to calculate the response at frequencies of interest.
be used for the absolute measurement of displacement in the The total estimated uncertainty in the displacement measure-
calibration is to have characteristic
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