ASTM C1332-96

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Designation: C 1332 – 96
Standard Test Method for
Measurement of Ultrasonic Attenuation Coefficients of
Advanced Ceramics by Pulse-Echo Contact Technique
This standard is issued under the fixed designation C 1332; 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 tive, or otherwise flaw-ridden samples that are unrepresentative
of the nature or inherent characteristics of the material under
1.1 This test method describes a procedure for measurement
examination.
of ultrasonic attenuation coefficients for advanced structural
1.7 This standard does not purport to address all of the
ceramic materials. The procedure is based on a broadband
safety concerns, if any, associated with its use. It is the
buffered piezoelectric probe used in the pulse-echo contact
responsibility of the user of this standard to establish appro-
mode and emitting either longitudinal or shear waves. The
priate safety and health practices and determine the applica-
primary objective of this test method is materials characteriza-
bility of regulatory limitations prior to use.
tion.
1.2 The procedure requires coupling an ultrasonic probe to
2. Referenced Documents
the surface of a plate-like sample and the recovery of succes-
2.1 ASTM Standards:
sive front surface and back surface echoes. Power spectra of
C 1331 Test Method for Measuring Ultrasonic Velocity in
the echoes are used to calculate the attenuation spectrum
Advanced Ceramics with the Broadband Pulse-Echo
(attenuation coefficient as a function of ultrasonic frequency)
Cross-Correlation Method
for the sample material. The transducer bandwidth and spectral
E 664 Practice for Measurement of Apparent Attenuation of
response are selected to cover a range of frequencies and
Longitudinal Ultrasonic Waves by Immersion Method
corresponding wavelengths that interact with microstructural
E 1316 Terminology for Nondestructive Examinations
features of interest in solid test samples.
E 1495 Guide for Acousto-Ultrasonic Assessment of Com-
1.3 The purpose of this test method is to establish funda-
posites, Laminates, and Bonded Joints
mental procedures for measurement of ultrasonic attenuation
2.2 ASNT Document:
coefficients. These measurements should distinguish and quan-
Recommended Practice SNT-TC-1A for Nondestructive
tify microstructural differences among solid samples and
Testing Personnel Qualification and Certification
therefore help establish a reference database for comparing
2.3 Military Standard:
materials and calibrating ultrasonic attenuation measurement
MIL-STD-410 Nondestructive Testing Personnel Qualifica-
equipment.
tion and Certification
1.4 This test method applies to monolithic ceramics and also
2.4 Additional references are cited in the text and at end of
polycrystalline metals. This test method may be applied to
this test method.
whisker reinforced ceramics, particulate toughened ceramics,
and ceramic composites provided that similar constraints on
sample size, shape, and finish are met as described herein for
3. Terminology
monolithic ceramics.
3.1 Definitions of Terms Specific to This Standard:
1.5 This test method sets forth the constraints on sample
3.1.1 acoustic impedance (Z)—a property (1) defined by a
size, shape, and finish that will assure valid attenuation
material’s density, p, and the velocity of sound within it, v,
coefficient measurements. This test method also describes the
where Z = rv.
instrumentation, methods, and data processing procedures for
3.1.2 attenuation coeffıcient (a)—decrease in ultrasound
accomplishing the measurements.
intensity with distance expressed in nepers (Np) per unit
1.6 This test method is not recommended for highly attenu-
ating materials such as very thick, very porous, rough-surfaced
monolithics or composites. This test method is not recom-
Annual Book of ASTM Standards, Vol 15.01.
mended for highly nonuniform, heterogeneous, cracked, defec-
Annual Book of ASTM Standards, Vol 03.03.
Available from the American Society for Nondestructive Testing, 1711 Arlin-
gate Ln., Columbus, OH 43228.
1 5
This test method is under the jurisdiction of ASTM Committee C-28 on Available from Standardization Documents Order Desk, Bldg. 4 Section D, 700
Advanced Ceramics and is the direct responsibility of Subcommittee C28.02 on Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
Design and Evaluation.Current edition approved March 10,1996. Published May The boldface numbers in parentheses refer to a list of references at the end of
1996. this standard.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
C 1332
length, herein, a = [ln(I / I)]/d, where a is attenuation cropores. Measured ultrasound energy loss which, if not
coefficient, d is path length or distance, I is original intensity corrected, may include losses due to diffraction, individual
and I is attenuated intensity (2). macroflaws, surface roughness, couplant variations, and trans-
3.1.3 attenuation spectrum—the attenuation coefficient, a, ducer defects.
expressed as a function of ultrasonic frequency, f, or plotted as 3.1.13 reflection coeffıcient (R)—measure of relative inten-
a versus f, over a range of ultrasonic frequencies within the sity of sound waves reflected back into a material at an
bandwidth of the transducer and associated pulser-receiver interface, defined in terms of the acoustic impedance of the
instrumentation. material in which the sound wave originates (Z ) and the
3.1.4 back surface—the surface of a test sample which is acoustic impedance of the material interfaced with it (Z ),
i
opposite to the front surface and from which back surface where R =[(Z − Z )/(Z + Z )] .
i 0 i 0
echoes are returned at normal incidence directly to the trans- 3.1.14 test sample— a solid coupon or material part that
ducer. meets the constraints needed to make the attenuation coeffi-
3.1.5 bandwidth—the frequency range of an ultrasonic cient measurements described herein, that is, a test sample or
probe, defined by convention as the difference between the part having flat, parallel, smooth, preferably ground/polished
lower and upper frequencies at which the signal amplitude is 6 opposing (front and back) surfaces and having no discrete
dB down from the frequency at which maximum signal flaws or anomalies that are unrepresentative of the inherent
amplitude occurs. The frequency at which the maximum properties of the material.
occurs is termed the center frequency of the probe or trans- 3.1.15 transmission coeffıcient (T)—measure of relative in-
ducer. tensity of sound waves transmitted through an interface,
3.1.6 broadband transducer—an ultrasonic transducer ca- defined in terms of the acoustic impedance of the material in
pable of sending and receiving undistorted signals over a broad which the sound wave originates (Z ) and the acoustic imped-
bandwidth, consisting of thin damped piezoelectric crystal in a ance of the material interfaced with it ( Z ), where T =
i
buffered probe (search unit). (4Z Z )/(Z + Z ) so that R + T =1.
0 i 0
i
3.1.7 buffered probe—an ultrasonic search unit as defined in 3.1.16 wavelength (l)—distance that sound (of a particular
Terminology E 1316 but containing a delay line or buffer rod to frequency) travels during one period (during one oscillation), l
which the piezoelement, that is, transducer consisting of a = v/f, where v is the velocity of sound in the material and
piezoelectric crystal, is affixed. The buffer rod separates the where velocity is measured in cm/μs, and wavelength in cm,
piezoelement from the test sample (see Fig. 1). herein.
3.1.8 buffer rod—an integral part of a buffered probe or 3.2 Other terms used in this test method are defined in
search unit, usually a quartz or fused silica cylinder that Terminology E 1316.
provides a time delay between the excitation pulse from the
4. Summary of Test Method
piezoelement and echoes returning from a sample coupled to
4.1 This test method describes a procedure for determining
the free end of the buffer rod.
a material’s inherent attenuation coefficient and attenuation
3.1.9 free surface—the back surface of a solid test sample
spectrum by means of a buffered broadband probe operating in
interfaced with a very low density medium, usually air or other
the pulse-echo contact mode on a solid sample that has smooth,
gas, to assure that the back surface reflection coefficient equals
flat, parallel surfaces.
1 to a high degree of precision.
4.2 The procedure described in this test method involves
3.1.10 frequency (f)—number of oscillations per second of
digital acquisition and computer processing of ultrasonic echo
ultrasonic waves, measured in megahertz, MHz, herein.
waveforms returned by the test sample. Test sample con-
3.1.11 front surface—the surface of a test sample to which
the buffer rod is coupled at normal incidence (designated as test straints, probing methods, data validity criteria, and measure-
ment corrections are prescribed herein.
surface in Terminology E 1316).
3.1.12 inherent attenuation—ultrasound energy loss in a
5. Significance and Use
solid as a result of scattering, diffusion, and absorption. This
5.1 This test method is useful for characterizing material
standard assumes that the dominant inherent losses are due to
microstructure or measuring variations in microstructure that
Rayleigh and stochastic scattering (2) by the material micro-
occur because of material processing conditions and thermal,
structure, for example, by grains, grain boundaries, and mi-
mechanical, or chemical exposure (3). When applied to mono-
lithic or composite ceramics, the procedure should reveal
microstructural gradients due to density, porosity, and grain
variations. This test method may also be applied to polycrys-
talline metals to assess variations in grain size, porosity, and
multiphase constituents.
5.2 This test method is useful for measuring and comparing
microstructural variations among different samples of the same
material or for sensing and measuring subtle microstructural
variations within a given sample.
5.3 This test method is useful for mapping variations in the
FIG. 1 Cross Section of Buffered Broadband Ultrasonic Probe attenuation coefficient and the attenuation spectrum as they
C 1332
pertain to variations in the microstructure and associated include the following (see Fig. 2). Appropriate equipment can
properties of monolithic ceramics, ceramic composites and be assembled from any of several suppliers.
metals.
7.1.1 Buffered Probe, meeting the following requirements:
5.4 This test method is useful for establishing a reference
7.1.1.1 The probe should have a center frequency that
database for comparing materials and for calibrating ultrasonic
corresponds to an ultrasonic wavelength that is less than one
attenuation measurement equipment.
fifth of the thickness, d, of the test sample.
5.5 This test method is not recommended for highly attenu-
7.1.1.2 The probe bandwidth should match the bandwidth of
ating monolithics or composites that are thick, highly porous,
received echoes. This may require transducer bandwidths of
or that have rough or highly textured surfaces. For these
from 50 to 200 MHz.
materials Practice E 664 may be appropriate. Guide E 1495 is
7.1.1.3 The probe should be well constructed, carefully
recommended for assessing attenuation differences among
selected, and shown to be free of internal defects and structural
composite plates and laminates that may exhibit, for example,
anomalies that distort received echoes.
pervasive matrix porosity or matrix crazing in addition to
7.1.1.4 The frequency spectra of the first two echoes re-
having complex fiber architectures or thermomechanical deg-
turned by the free end of the buffer should be essentially
radation (3). The proposed ASTM Standard Test Method for
gaussian (bell shaped).
Measuring Ultrasonic Velocity in Advanced Ceramics
7.1.2 Buffer Rod, with length that results in a time delay $3
(C 1331) is recommended for characterizing monolithic ceram-
times the interval between two successive echoes from the
ics with significant porosity or porosity variations (4).
back surface of the test sample. This imposes a limit on the test
sample thickness if the buffer rod length is fixed or predeter-
6. Personnel Qualifications
mined by design.
6.1 It is recommended that nondestructive evaluation/
7.1.3 Couplant, meeting the following requirements:
examination personnel applying this test method be qualified in
7.1.3.1 The couplant should be a fluid such as glycerine or
accordance with a nationally recognized personnel qualifica-
an ultrasonic gel that will not corrode, damage, or be absorbed
tion practice or standard such as ASNT SNT-TC-1A, MIL STD
by the test sample or part being examined.
410, or a similar document. The qualification practice or
7.1.3.2 The couplant film or couplant layer thickness should
standard used and its applicable revision(s) should be specified
be much less than the ultrasound wavelength in the couplant at
in a contractual agreement.
the probe’s center frequency.
6.2 Knowledge of the principles of ultrasonic testing is
7.1.3.3 Ideally, to avoid echo distortions, the acoustic im-
required. Personnel applying this test method shall be experi-
pedance of the couplant should be between that of the buffer
enced practitioners of ultrasonic examinations and associated
rod material and test sample (5). With fluid couplants, just
methods for signal acquisition, processing, and interpretation.
reducing the couplant layer thickness is usually more practical
6.3 Personnel shall have proficiency in computer program-
than impedance matching by changing the fluid. For example,
ming and signal processing using digital methods for time and
if glycerine is used between a fused quartz buffer and a steel
frequency domain signal analysis. Familiarity with the Fourier
sample, the couplant layer thickness should be less than 1 μm.
transform and associated spectrum analysis methods for ultra-
7.1.3.4 Dry coupling, for example, with an elastomer or thin
sonic signals is required.
deformable polymer film, may be used provided that echo
7. Apparatus
distortions or phase inversions are avoided by acoustic imped-
7.1 The instrumentation and apparatus for pulse-echo con- ance matching (5) and by substantially reducing the couplant
tact ultrasonic attenuation coefficient measurement should layer thickness.
FIG. 2 Block Diagram of Computer System for Ultrasonic Signal Acquis
...