ASTM E1495-02(2007)
(Guide)Standard Guide for Acousto-Ultrasonic Assessment of Composites, Laminates, and Bonded Joints
Standard Guide for Acousto-Ultrasonic Assessment of Composites, Laminates, and Bonded Joints
SIGNIFICANCE AND USE
General—Conventional ultrasonics should be considered first for the detection of overt flaws such as delaminations in composites. Thereafter, AU should be considered for composites that are proved to be free of major flaws or discontinuities. The AU method is intended almost exclusively for assessing the collective effects of dispersed defects and subcritical flaw populations. These are material aberrations that influence AU measurements and also underlie mechanical property variations, dynamic load response, and impact and fracture resistance.
Specific Advantages—The AU method can be used to evaluate composite laminate and bond quality using access to only one surface as, for example, the exterior surface of pressure vessels. It is unnecessary to utilize angle beam fixtures because the method can always be applied with probes at normal incidence. The method can be applied using dry coupling with elastomer pads attached to the probes, and there is no need to immerse the examination object in water.
General Applications—The AU method was devised to assess diffuse discontinuity populations and any associated changes of the mechanical properties of composites and composite-like materials. The AU method has been used to evaluate fiber-reinforced composites (6), composite laminates (7), filament-wound pressure vessels (8), adhesive bonds (9), paper and wood products (10), and cable and rope (11). The method has been shown to be particularly practical for assessing the strength of adhesively bonded joints. It has also been shown to be useful for assessing microporosity (12), micro-cracking (13), hydrothermal aging (14), and damage produced by impacts (15) and fatigue (16).
SCOPE
1.1 This guide explains the rationale and basic technology for the acousto-ultrasonic (AU) method. Guidelines are given for nondestructive evaluation (NDE) of flaws and physical characteristics that influence the mechanical properties and relative strength of composite structures (for example, filament-wound pressure vessels), adhesive bonds (for example, joints between metal plates), and interlaminar and fiber/matrix bonds in man-made composites and natural composites (for example, wood products).
1.2 This guide covers technical details and rules that must be observed to ensure reliable and reproducible quantitative AU assessments of laminates, composites, and bonded structures. The underlying principles, prototype apparatus, instrumentation, standardization, examination methods, and data analysis for such assessments are covered. Limitations of the AU method and guidelines for taking advantage of its capabilities are cited.
1.3 The objective of AU is to assess subtle flaws and associated strength variations in composite structures and bonded joints. Discontinuities such as large voids, disbonds, or extended lack of contact at interfaces can be assessed by other NDE methods such as conventional ultrasonics.
1.4 Additional information may be found in the publications cited in the list of references at the end of this guide. The referenced works provide background on research, applications, and various aspects of signal acquisition, processing, and interpretation.
1.5 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
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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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: E1495 – 02 (Reapproved 2007)
Standard Guide for
Acousto-Ultrasonic Assessment of Composites, Laminates,
and Bonded Joints
This standard is issued under the fixed designation E1495; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 This guide explains the rationale and basic technology 2.1 ASTM Standards:
for the acousto-ultrasonic (AU) method. Guidelines are given E543 Specification for Agencies Performing Nondestruc-
for nondestructive evaluation (NDE) of flaws and physical tive Testing
characteristics that influence the mechanical properties and E1316 Terminology for Nondestructive Examinations
relative strength of composite structures (for example, 2.2 ASNT Standard:
filament-wound pressure vessels), adhesive bonds (for ex- ANSI/ASNT CP-189 Standard for Qualification and Certi-
ample, joints between metal plates), and interlaminar and fication of Nondestructive Testing Personnel
fiber/matrix bonds in man-made composites and natural com- SNT-TC-1A Recommended Practicem for Personnel Quali-
posites (for example, wood products). fications and Certification in Nondestructive Testing
1.2 This guide covers technical details and rules that must 2.3 AIA Document:
be observed to ensure reliable and reproducible quantitative NAS-410 Certification and Qualification of Nondestructive
AU assessments of laminates, composites, and bonded struc- Testing Personnel
tures. The underlying principles, prototype apparatus, instru-
3. Terminology
mentation, standardization, examination methods, and data
analysis for such assessments are covered. Limitations of the 3.1 Definitions:
3.1.1 acousto-ultrasonics (AU)—a nondestructive examina-
AU method and guidelines for taking advantage of its capa-
bilities are cited. tion method that uses induced stress waves to detect and assess
the diffuse defect states, damage conditions, and variations of
1.3 The objective of AU is to assess subtle flaws and
associated strength variations in composite structures and mechanical properties of an examination structure. The AU
method combines aspects of acoustic emission (AE) signal
bonded joints. Discontinuities such as large voids, disbonds, or
analysis with ultrasonic materials characterization methods
extended lack of contact at interfaces can be assessed by other
NDE methods such as conventional ultrasonics. (Terminology E1316).
3.1.2 Additional related definitions may be found in Termi-
1.4 Additionalinformationmaybefoundinthepublications
cited in the list of references at the end of this guide. The nology E1316.
3.2 Definitions of Terms Specific to This Standard:
referenced works provide background on research, applica-
tions,andvariousaspectsofsignalacquisition,processing,and 3.2.1 stress wave factor (SWF)—a generic measure of the
relative energy loss (attenuation) or propagation efficiency of
interpretation.
1.5 The values stated in SI units are to be regarded as the stress waves generated by the AU method. There are many
ways to define and calculate the SWF. Several of these are
standard. The values given in parentheses are for information
only. described in Section 11 of this guide.
1.6 This standard does not purport to address all of the
4. Summary of Guide
safety concerns, if any, associated with its use. It is the
4.1 General—Two probes are attached to a sample in a
responsibility of the user of this standard to establish appro-
send-receive configuration. One (a pulsed sending probe) is
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc- Standards volume information, refer to the standard’s Document Summary page on
tive Testing and is the direct responsibility of Subcommittee E07.04 on Acoustic the ASTM website.
Emission Method. AvailablefromAmericanSocietyforNondestructiveTesting(ASNT),P.O.Box
Current edition approved July 1, 2007. Published July 2007. Originally approved 28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http://www.asnt.org.
in 1992. Last previous edition approved in 2002 as E1495 - 02. DOI: 10.1520/ Available fromAerospace IndustriesAssociation ofAmerica, Inc. (AIA), 1000
E1495-02R07. WilsonBlvd.,Suite1700,Arlington,VA22209-3928,http://www.aia-aerospace.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1495 – 02 (2007)
optimized for wave generation, while the other (a receiving in composites. Thereafter, AU should be considered for com-
probe) is optimized for signal sensing. The probes are attached posites that are proved to be free of major flaws or disconti-
to the sample surface at normal incidence.The usual, and often nuities. The AU method is intended almost exclusively for
most practical, configuration has piezoelectric probes, a sender assessing the collective effects of dispersed defects and sub-
and receiver, on the same side of the examination part (1). critical flaw populations. These are material aberrations that
Measurements are performed by allowing ultrasonic stress influence AU measurements and also underlie mechanical
waves to interact with a volume of material between the property variations, dynamic load response, and impact and
probes. The waves are modified by the material microstructure fracture resistance.
and morphology (2).
5.2 Specific Advantages—The AU method can be used to
4.2 Principle—The AU method measures the relative effi- evaluate composite laminate and bond quality using access to
ciency of stress wave propagation in a material. The dominant
only one surface as, for example, the exterior surface of
attribute measured is stress wave attenuation. Lower attenua- pressurevessels.Itisunnecessarytoutilizeanglebeamfixtures
tion, a high SWF value, means better stress wave energy
because the method can always be applied with probes at
transmission for many composites and, therefore, better trans- normal incidence. The method can be applied using dry
mission and redistribution of dynamic strain energy. More
coupling with elastomer pads attached to the probes, and there
efficient strain energy transfer and strain redistribution during
is no need to immerse the examination object in water.
loading or impact corresponds to increased strength and
5.3 General Applications—The AU method was devised to
fracture resistance in composite structures and adhesive bonds.
assess diffuse discontinuity populations and any associated
A lower SWF usually indicates regions in which strain energy
changes of the mechanical properties of composites and
is likely to concentrate and result in crack growth and fracture
composite-like materials. The AU method has been used to
(3).
evaluate fiber-reinforced composites (6), composite laminates
4.3 Structure Configuration Effects—In monolithic plates
(7), filament-wound pressure vessels (8), adhesive bonds (9),
andhomogeneouscompositeslabs,theSWFwillexhibitsignal
paper and wood products (10), and cable and rope (11). The
attenuation effects due to variations in microstructure, mor-
method has been shown to be particularly practical for assess-
phology, porosity, cure state, microcrack populations, etc. (4).
ing the strength of adhesively bonded joints. It has also been
A lower SWF typically corresponds to regions of higher
shown to be useful for assessing microporosity (12), micro-
attenuation. In laminated structures or bonded joints, however,
cracking (13), hydrothermal aging (14), and damage produced
interfaces and bondlines can produce either lower or higher
by impacts (15) and fatigue (16).
SWF values, depending on the bond quality (5). Delaminated
regions can produce higher SWF values because more energy
6. Basis of Application
is reflected or channeled to the receiving probe.
6.1 Personnel Qualification
4.4 In-Plane Measurements—Offsetting probes enables the
6.1.1 If specified in the contractual agreement, personnel
collection of stress wave reverberations that have traveled
performing examinations to this standard shall be qualified in
in-plane from sender to receiver. It is therefore possible to
accordance with a nationally recognized NDT personnel quali-
measure in-plane, mechanical property variations in principal
fication practice or standard such as ANSI/ASNT CP-189,
load directions in fiber-reinforced laminates or adhesively
SNT-TC-1A, NAS-410, or a similar document and certified by
bonded joints (that is, properties such as interlaminar shear
the employer or certifying agency, as applicable. The practice
strength and adhesive bond strength).
or standard used and its applicable revision shall be identified
4.5 Signal Collection Criterion—With the AU method,
in the contractual agreement between the using parties.
instead of singling out specific echoes, all of the multiple
6.2 Qualification of Nondestructive Agencies
reverberations, including signals from internal reflectors and
6.2.1 If specified in the contractual agreement, NDT agen-
scatterers, are collected and analyzed together. Even with
cies shall be qualified and evaluated as described in Practice
pulse-echo or through-transmission configurations, all stress
E543. The applicable edition of Practice E543 shall be speci-
wave reflections and reverberations in a local volume of
fied in the contractual agreement.
material are collected and evaluated, as in backscatter,
6.3 Proper application of the AU method requires the
forward-scatter, and diffuse field analysis.
involvement of an NDE specialist to plan and guide the
4.6 Wavelength Criterion—In composite panels or bonded
examination procedure. Knowledge of the principles of ultra-
plates, the sender should produce wavelengths that are com-
sonic examination is required. Personnel applying AU should
parable to or less than the panel or plate thickness. Suitable
be experienced practitioners of conventional ultrasonic and
wavelengths are those passed by the examination piece at
acoustic emission examination and associated methods for
frequencies equal to or greater than the sending probe center
signal acquisition, processing, and interpretation.
frequencies.
6.4 Particular emphasis should be placed on personnel
5. Significance and Use
having proficiency in computer signal processing and the use
5.1 General—Conventional ultrasonics should be consid- of digital methods for time and frequency domain signal
ered first for the detection of overt flaws such as delaminations
analysis. Familiarity with ultrasonic spectrum analysis using
digital Fourier transforms is mandatory. Spectral distribution,
multiple regression, and pattern recognition analyses and
The boldface numbers in parentheses refer to the list of references at the end of
this guide. adaptive learning procedures are important.
E1495 – 02 (2007)
6.5 Application of theAU method also requires proficiency some standard value, for example, the maximum value found
in developing and designing reference standards. The develop- for the optimum condition of a representative material sample
ment of reference standards is needed for each type of material or structure. This is appropriate where many nominally iden-
and configuration to be examined. Because AU measurements tical articles will be examined.
are relative and comparative, experimental examinations con- 8.3 Reference Standards—Normalization of the SWF is the
firmedbydestructivetestingareneededtoavoidambiguitiesin first step toward establishing a reference standard. The second
the interpretation of results. step is to fabricate a set of samples exhibiting the full range of
expected material conditions and flaw states. One of these
7. Limitations
samples should represent the optimum condition of the mate-
7.1 General—The AU method possesses the limitations rial. This procedure should be followed by the development of
common to all ultrasonic methods that attempt to measure benchmark structures that can be used as comparative stan-
either absolute or relative attenuation. When instrument set- dards.
tings and probe configurations are optimized for AU, they are
9. System Configuration
unsuitable for conventional ultrasonic flaw detection.
7.2 Signal Reproducibility Factors—TheAUresultsmaybe 9.1 Standard Configuration—Four possible AU probe con-
affected adversely by the following factors: (1) improper figurations are shown in Fig. 1. With the probes on the same
selection of type and amount of couplant, (2) couplant thick- side of a panel, examination proceeds by holding the probes in
ness variations and bubbles, (3) specimen surface roughness afixtureandmovingthemasaunittocovertheexaminedarea.
and texture, (4) probe misalignment and insufficient pressure, For zero offset between probes, the configuration reduces to
(5) probe resonances and insufficient damping, and (6) insuf-
either the pulse-echo or through-transmission mode, as shown
ficient instrument bandwidth. in Fig. 1 (b) and (d) respectively. The prototype apparatus
depicted in Fig. 2 illustrates the essential features of a standard
8. Standardization
configuration.
8.1 Self-Standardization—The sender and receiver probes 9.2 Probes—Two classes of piezoelectric probes are appro-
can be used to verify each other. Deficiencies in the instrumen- priate: (1) resonant and non-resonant AE sensors, and (2)
tation and probe response become evident by comparing the damped broadband ultrasonic probes. Resonant AE sensors
results with the standard waveforms established previously for have more sensitivity, but the signals transmitted by the test
areferenceitem.CommercialultrasonicprobesandAEsensors piece may be of sufficient strength such that sensitivity is not a
respond to deformation (stress) waves in a complex fashion problem. One reason for avoiding resonant sensors is that they
that involves both normal and in-plane displacements of the have ringdown characteristics that may be difficult to separate
examination sample surface. Although it is possible to stan- from the multiple reflections transmitted by the examination
dardize such probes in an absolute sense, even sensors of the sample.
same design and specification should be treated as unique and 9.2.1 Probe Bandwidth—Non-resonant AE sensors have a
definitely noninterchangeable. flatter frequency response curve than resonant sensors. This
8.2 Stress Wave Factor Normalization—
...
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