ASTM E1106-86(1997)
(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 problems, 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.
General Information
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Standards Content (Sample)
Designation: E 1106 – 86 (Reapproved 1997)
Standard Method for
Primary Calibration of Acoustic Emission Sensors
This standard is issued under the fixed designation E 1106; 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.
returns to quiescence, following the transient input, before any wave
1. Scope
reflected from the boundary of the calibration block returns to the transfer
1.1 This method covers the requirements for the absolute
standard (;100 μs). For low frequencies with periods on the order of the
calibration of acoustic emission (AE) sensors. The calibration
time window, this condition is problematical to prove.
yields the frequency response of a transducer to waves, at a
4.2 Applications Sensors—This method may also be used
surface, of the type normally encountered in acoustic emission
for the calibration of AE sensors for use in nondestructive
work. The transducer voltage response is determined at dis-
evaluation. Some of these sensors are less well behaved than
crete frequency intervals of approximately 10 kHz up to 1
devices suitable for a transfer standard. The stated accuracy for
MHz. The input is a given well-established dynamic displace-
such devices applies in the range of 100 kHz to 1 MHz and
ment normal to the mounting surface. The units of the
with less accuracy below 100 kHz.
calibration are output voltage per unit mechanical input (dis-
placement, velocity, or acceleration).
5. General Requirements
1.2 This standard does not purport to address all of the
5.1 A primary difficulty in any calibration of a mechanical/
safety concerns, if any, associated with its use. It is the
electrical transduction device is the determination of the
responsibility of the user of this standard to establish appro-
mechanical-motion input to the device. Using this calibration
priate safety and health practices and determine the applica-
procedure, the motional input may be determined by two
bility of regulatory limitations prior to use.
different means: theoretical calculation and measurement with
an absolute displacement transducer.
2. Referenced Documents
5.2 Theoretical Calculation—Elasticity theory has been
2.1 ASTM Standards:
used to calculate the dynamic displacement of the surface of an
E 114 Practice for Ultrasonic Pulse-Echo Straight-Beam
infinite half-space due to a normal point-force step function in
Testing by the Contact Method
time. The solutions give the displacement of any point on the
E 494 Practice for Measuring Ultrasonic Velocity in Mate-
surface as a function of time, yielding a waveform for the
rials
displacement called the seismic surface pulse.
E 650 Guide for Mounting Piezoelectric Acoustic Emission
5.2.1 This calibration method uses an approximation to this
Sensors
theoretical solution. See also Breckenridge and Hsu and
E 1316 Terminology for Nondestructive Examinations
Breckenridge . The half-space is approximated by a large
metal block in the form of a circular cylinder and the pointforce
3. Terminology
step function is closely approximated by the breaking of a glass
3.1 Refer to Terminology E 1316 for terminology used in
capillary against the plane surface of the block. The displace-
this method.
ment as a function of time should be calculated for the location
of the device under test (on the same surface of the block as the
4. Significance and Use
input). This calculation should be performed using a measured
4.1 Transfer Standards—One purpose of this method is for
value of the step function force and the elastic constants that
the direct calibration of displacement transducers for use as
are determined by speed of sound measurements on the block.
secondary standards for the calibration of AE sensors for use in
5.3 Absolute Displacement Measurement—An absolute
nondestructive evaluation. For this purpose, the transfer stan-
measurement of the dynamic normal surface displacement of
dard should be high-fidelity and very well behaved and
the block is required for this calibration method. The trans-
understood. If this can be established, the stated accuracy
ducer used for this measurement is a standard transducer
should apply over the full frequency range up to 1 MHz.
against which the device under test is compared. The standard
NOTE 1—The stated accuracy applies only if the transfer standard
transducer should meet or exceed the performance of the
This method is under the jurisdiction of ASTM Committee E-7 on Nondestruc-
tive Testing and is the direct responsibility of Subcommittee E07.04 on Acoustic Breckenridge, F. R., “Acoustic Emission Transducer Calibration by Means of
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, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 1106
capacitive transducer described by Breckenridge and where k 5 2pf/c, and f 5 frequency, c 5 Rayleigh speed, and
Greenspan . The important characteristics of the standard a 5 radius of the sensor face. Hence, the frequencies at which
transducer include high fidelity, high sensitivity, and operating the nulls occur are dependent upon the Rayleigh speed.
characteristics amenable to theoretical calculation. It should
6. Description of Typical Apparatus
also present no appreciable dynamic loading to the surface it is
6.1 A typical basic scheme for the calibration is shown in
measuring.
Fig. 1. A glass capillary, B, of diameter about 0.2 mm, is
5.3.1 For a calibration, the standard transducer and the
squeezed between the tip of the loading screw, C, and the upper
device to be calibrated are both placed on the same surface of
face of the large steel transfer block, A. When the capillary
the block as the mechanical input and equidistant in opposite
breaks, the sudden release of force is a step function whose
directions from it. This guarantees that both experience the
risetime is of the order of 0.1 μs. The magnitude of the force
same displacement-time history. Comparison of the output of
step is measured by the combination of the PZT disc, D,inthe
the transfer standard or AE sensor with the output of the
loading screw and a charge amplifier, E, connected to a storage
standard transducer yields a calibration of the device under
oscilloscope, F. The standard capacitive transducer, G, and the
test.
device under test, H, are placed equally distant (usually 100
5.3.2 Other relative geometries for the input and transducers
mm) from the source and in opposite directions from it. It is
are possible, but results from other geometries should only be
obvious from the symmetry that the surface displacements
used to supplement results from the “same surface” geometry.
would be the same at the two transducer locations if it were not
AE waves in structures are most frequently dominated by
for the loading effects of the transducers. The loading effect of
surface wave phenomena, and the calibration should be based
the standard capacitive transducer is negligible and the loading
on the transducer’s response to such waves.
effect of the unknown sensor is part of its calibration.
5.4 Units for the Calibration—An AE sensor may be
6.1.1 Voltage transients from the two transducers are re-
considered to respond to either stress or strain at its front face.
corded simultaneously by digital recorders, I, and the informa-
The actual stress and strain at the front face of a mounted
tion is stored for processing by the computer, J.
sensor depend on the interaction between the mechanical
6.1.2 With such a system, it is possible to do the necessary
impedance of the sensor (load) and that of the mounting block
comparison between the signal from the unknown sensor and
(driver). Neither the stress nor the strain is amenable to direct
that from the standard transducer or with the displacement
measurement at this location. However, the free displacement
waveform calculated by elasticity theory. A similar result
that would occur at the surface of the block in the absence of
should be obtained either way.
the sensor can be inferred from either elasticity theory calcu-
6.2 The Transfer Block—The transfer block must be made
lations or from measurements made elsewhere on the surface.
from specially chosen material. It should be as defect-free as
Since AE sensors are used to monitor motion at a free surface
possible and should undergo an ultrasonic longitudinal inspec-
of a structure and interactive effects between sensor and
tion at 2 ⁄4 MHz. The method described in Practice E 114
structure are generally of no interest, the free surface motion is
should be used. The block should contain no flaws which give
the appropriate input variable. It is, therefore, recommended
a reflection larger than 10 % of the first back wall reflection.
that the units of calibration should be voltage per unit of free
The material should also be highly uniform as determined by
motion; for example, volts per meter.
pulse-echo time of flight measurements through the block at a
5.5 Block Material:
minimum of 15 locations regularly spaced over the surface (see
5.5.1 Since the calibration depends on the interaction of the
Practice E 494). The individual values of the longitudinal and
mechanical impedance of the block and that of the AE sensor,
shear wave speed should differ from the average by no more
a calibration procedure must specify the material of the block.
than 61 part and 63 parts in 10 , respectively. A transfer block
Calibrations performed on blocks of different materials will
and calibration apparatus is shown in Fig. 2.
yield transducer sensitivity versus frequency curves that are
6.3 The Step Function Source—The step function force
different in shape and in average magnitude. The amount by
events are to be made by breaking glass capillary tubing (Fig.
which such averages differ may be very large. A transducer
3). The capillaries are drawn down from ordinary laboratory
calibrated on a glass or aluminum block will have an average
glass tubing made of borosilicate glass. Sizes of the capillary
sensitivity that may be from 50 to 100% of the value obtained
may range from about 0.1 mm to 0.3 mm outside diameter,
on steel, and will have an average sensitivity that may be as
with 0.2 mm being typical. A bore size equal to the wall
little as 3 % of the value obtained on steel if calibrated on a
thickness gives the best results. The force obtained is usually
polymethyl methacrylate block. In general, the sensitivity will
between 10 N and 30 N, with 20 N being typical.
be less if the block is made of a less rigid or less dense
6.3.1 The capillary is to be laid horizontally on a piece of
material.
microscope cover glass (0.08 by 1.5 by 1.5 mm) which has
5.5.2 The Rayleigh speed in the material of the block affects
been cemented to the top face of the steel block with salol
surface wave calibrations. For a sensor having a circular
(phenyl salicylate) or cyanoacrylate cement. The force is
aperture (mounting face) with uniform sensitivity over the
applied to the capillary by a solid glass rod (2 mm in diameter)
face, the aperture effect predicts nulls at the zeroes of J (ka),
which has been laid horizontally on top of the capillary and at
right angles to it. The rod is forced downward by the loading
Breckenridge, F. R., and Greenspan, M., “Surface-Wave Displacement: Abso-
screw until the capillary breaks. The loading screw is to be
lute Measurements Using a Capacitive Transducer,” Journal, Acoustic Society of
America, Vol 69, pp 1177–1185. threaded through a yoke above the calibration surface. The
E 1106
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
E 1106
FIG. 3 Glass Capillary Source
loading screw should contain a ceramic force transducer which boundaries. The ringing contains only frequencies above 2
has been calibrated by dead weights. Thus, although the size of MHz. Furthermore, the effects on both standard transducer and
a source event cannot be predicted in advance, its magnitude unknown sensor are the same; therefore, the calibration is not
may be measured and used for the elasticity theory calculation affected.
of the surface displacement. 6.4 The Standard Transducer—The standard transducer to
6.3.2 Ideally, the capillary should rest directly on the steel be used for the absolute measurement of displacement in the
with no cover glass interposed. It may be found necessary to calibration is to have characteristics at least as good as the
use the cover slide to prevent damage to the block surface. The capacitive transducer described by Breckenridge and
presence of the cover glass does alter the waveform very Greenspan. This device, shown in Figs. 4 and 5, essentially
slightly; a slight ringing occurs due to reflections at its consists of an inertial mass (about 40 g) mounted on compliant
FIG. 4 Photograph of the Capacitive Transducer and its Reflection in the Steel Block
E 1106
NOTE 1—All dimensions are given in millimetres. Here l is the length of the active electrode, 2a is its diameter, and g is the width of the guard gap.
FIG. 5 Longitudinal Section Through the Transducer
supports and separated from the top surface of the steel block
by an air gap of about 4 μm. This gap is determined by
measuring the capacitance between the transducer and the
transfer block using a three-terminal ratio arm bridge as
described by Breckenridge and Greenspan. The inertial mass
is a brass cylinder with its axis horizontal. When the block
surface moves at frequencies above the natural resonance of
the mass on its compliant supports (approximately 1 kHz), the
brass cylinder remains approximately stationary. The brass
cylinder is polarized to 100 Vdc through a large valued resistor.
The large resistance causes the capacitor to operate essentially
in a fixed charge condition so that the voltage varies inversely
with capacitance for the frequencies of interest.
6.4.1 For use as a primary standard, it is essential that the
sensitivity of the transducer be calculable. To make the
calculations tractable, the cylinder is trea
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