ASTM E2373-04
(Practice)Standard Practice for Use of the Ultrasonic Time of Flight Diffraction (TOFD) Technique
Standard Practice for Use of the Ultrasonic Time of Flight Diffraction (TOFD) Technique
SIGNIFICANCE AND USE
This practice provides general principles for the application of the Time-of-Flight Diffraction Technique as a tool for detection and sizing of discontinuities.
TOFD is a nondestructive ultrasonic examination technique that is not based on amplitude response. However, sufficient sensitivity is required to identify indications for evaluation.
Techniques used are typically applied to welded joints in carbon steel but the principles may be applicable to other applications including other materials with suitable validation procedures agreeable to the contracting parties.
In addition to a stand-alone ultrasonic detection technique TOFD may be used in conjunction with weld examinations such as those described in Practices E 164 and E 1961 where it may be used to improve sizing estimates of flaws detected by the manual or mechanized pulse-echo techniques and help discriminate between flaws and geometric reflectors.
The technique has proven effective on thicknesses from 9 to 300 mm [0.375 to 12 in.]. TOFD has been used on thicknesses outside of this range but special considerations are necessary. Techniques developed outside of this range of thickness shall be demonstrated as capable of meeting the required detection and sizing requirements of the specification used.
SCOPE
1.1 This practice establishes the requirements for developing ultrasonic examination procedures using the ultrasonic technique known as Time-of-Flight Diffraction (TOFD).
1.2 The values stated in SI units are to be regarded as standard. Inch-pound units are provided for information.
1.3 Consistent with ASTM Policy, TOFD may be regarded as an ultrasonic test method whereby the qualities and characteristics of the item tested are evaluated, measured and in some cases identified. Measurements may be subject to precision and bias that may be determined statistically or as a function of some parameter(s) such as wavelength. This practice may be used for applications that would be qualitative and properly addressed as examinations as well as quantitative and more properly addressed as tests.
1.4 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:E2373–04
Standard Practice for
Use of the Ultrasonic Time of Flight Diffraction (TOFD)
Technique
This standard is issued under the fixed designation E2373; 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.
1. Scope for Detection , Location and Sizing of Flaws, British
Standards Institute, 1993
1.1 This practice establishes the requirements for develop-
Code Case 2235 ASME Boiler and Pressure Vessel Code
ing ultrasonic examination procedures using the ultrasonic
technique known as Time-of-Flight Diffraction (TOFD).
3. Terminology
1.2 The values stated in SI units are to be regarded as
3.1 Definitions—Related terminology is defined in Termi-
standard. Inch-pound units are provided for information.
nology E1316.
1.3 Consistent with ASTM Policy, TOFD may be regarded
3.2 Definitions of Terms Specific to This Standard:
as an ultrasonic test method whereby the qualities and charac-
3.2.1 B-scan display—a sectional view of the plotted in-
teristicsoftheitemtestedareevaluated,measuredandinsome
spection data formed by the stacking of A-scans. (Some users
casesidentified.Measurementsmaybesubjecttoprecisionand
refer to stacked A-scans from non-parallel scans as D-scans
bias that may be determined statistically or as a function of
and reserve those used with parallel scans as B-scans.)
some parameter(s) such as wavelength. This practice may be
3.2.2 back-wall echo—a specular reflection from the back-
used for applications that would be qualitative and properly
wall of the component being examined (usually assumed to be
addressed as examinations as well as quantitative and more
a plate).
properly addressed as tests.
3.2.3 lateral wave—a compression wave that travels by the
1.4 This standard does not purport to address all of the
most direct route from the transmitting probe to the receiving
safety concerns, if any, associated with its use. It is the
probe in a TOFD configuration.
responsibility of the user of this standard to establish appro-
3.2.4 parallel scan—a scan whereby the probe pair motion
priate safety and health practices and determine the applica-
is parallel to the ultrasonic beam axis.Also called a B-scan by
bility of regulatory limitations prior to use.
some users.
2. Referenced Documents 3.2.5 PCS—abbreviationforprobecenterspacing.Refersto
the distance between the marked exit points of a pair ofTOFD
2.1 ASTM Standards:
probes for a specific application.
E164 PracticeforUltrasonicContactExaminationofWeld-
3.2.6 non-parallel or longitudinal scan—a scan whereby
ments
the probe pair motion is perpendicular to the ultrasonic beam
E1065 Guide for Evaluating Characteristics of Ultrasonic
axis.
Search Units
3.2.7 RF waveforms—the non-rectified A-scan.
E1316 Terminology for Nondestructive Examinations
E1324 Guide for Measuring Some Electronic Characteris-
4. Significance and Use
tics of Ultrasonic Examination Instruments
4.1 This practice provides general principles for the appli-
2.2 Other Documents:
cationoftheTime-of-FlightDiffractionTechniqueasatoolfor
BS 7706 (1993) Guide to Calibration and Setting-up of the
detection and sizing of discontinuities.
Ultrasonic Time-of-Flight Diffraction (TOFD) Technique
4.2 TOFD is a nondestructive ultrasonic examination tech-
nique that is not based on amplitude response. However,
sufficient sensitivity is required to identify indications for
This practice is under the jurisdiction of ASTM Committee E07 on Nonde-
structive Testing and is the direct responsibility of Subcommittee E07.06 on
evaluation.
Ultrasonic Method.
Current edition approved July 1, 2004. Published July 2004.
2 3
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E2373–04
4.3 Techniquesusedaretypicallyappliedtoweldedjointsin
carbon steel but the principles may be applicable to other
applications including other materials with suitable validation
procedures agreeable to the contracting parties.
4.4 In addition to a stand-alone ultrasonic detection tech-
nique TOFD may be used in conjunction with weld examina-
tions such as those described in Practices E164 and E1961
where it may be used to improve sizing estimates of flaws
detected by the manual or mechanized pulse-echo techniques
and help discriminate between flaws and geometric reflectors.
4.5 The technique has proven effective on thicknesses from
9 to 300 mm [0.375 to 12 in.]. TOFD has been used on
thicknesses outside of this range but special considerations are
necessary. Techniques developed outside of this range of
thickness shall be demonstrated as capable of meeting the
required detection and sizing requirements of the specification
used.
FIG. 1
5. Procedures
5.1 Introduction:
refracted transverse shear mode in the examination piece.
5.1.1 TOFD is an ultrasonic examination technique that can
When using a refracted compression mode in the examination
provide improved detection and sizing capabilities of discon-
piece the direct shear and head waves also are generated;
tinuities compared to standard ultrasonic pulse-echo tech-
however, due to their lower acoustic velocities, shear waves
niques.Itusesforwardscatteredtipdiffractionandreflectionof
arrive later in time than the back-wall signal.
transmitted ultrasonic pulses. This document describes the
5.1.6 Fig. 2 is a sketch of a typical presentation for a
requirementsforTOFDequipmentandproceduresonflatplate
non-parallel scan of a butt weld in a plate with an imbedded
surfaces.Guidanceformorecomplexgeometriesisprovidedin
flaw. The right side of Fig. 2 illustrates a waveform extracted
theAppendix.GeneralguidanceonTOFDcanalsobefoundin
from a B-scan display showing the lateral wave, upper tip
British Standards BS7706. Acceptance criteria typical and
diffracted, lower tip diffracted and the back-wall echo signals.
performance demonstration requirements that may be used
The left side of Fig. 2 indicates the probe placement with
with TOFD techniques are found in ASME Code Case 2235 .
respect to the weld (upper left) and relative motion of the
5.1.2 Because phase inversions of signals play an important
probes (a non-parallel scan is indicated).
role in the evaluation of TOFD results, all procedures devel-
5.1.7 Fig. 3 illustrates an actual TOFD scan with five
oped using this practice shall require that the equipment
indications(identifiedontheleft)andanextractedA-scanfrom
presentation use and store RF waveforms.
one of the indications.
5.1.3 Whether motorized or manually-operated, probe mo-
5.2 Written Procedure—A documented examination strat-
tion must be encoded for position and probes held in a fixture
egy or scan plan shall be provided showing probe placement,
that maintains correct PCS during scanning. Time based
movement, and component coverage that provides a standard-
sampling of data collection is not acceptable.
ized and repeatable methodology for component acceptance.
5.1.4 Fig. 1 illustrates the typical probe configuration for a
Thescanplanshallalsoincludeultrasonicbeamangle(s)used,
TOFD examination.The figure uses a weld for convenience of
beam directions with respect to some reference such as a weld
references; however, TOFD need not be restricted to just weld
centerline, and volume examined.
examinations.
5.3 Examination Materials and Surface Preparation:
5.1.5 The lateral wave and back-wall echo signals provide
5.3.1 TOFD technique can be applied to both metals and
convenient references. For most applications mode converted
nonmetals. Best results are had on fine-grained isotropic
signals from flaws are not used and therefore flaw indications
materials with low attenuation including some finer grained
are usually recognized as occurring between the lateral wave
austenitic alloys and aluminum. With suitable validation pro-
and back-wall echo signals.Although it is more often the case
cedures, agreeable to the contracting parties, coarser-grained
to use refracted compression mode in the examination piece,
and anisotropic materials may also be examined using TOFD.
someapplicationsmayproducebetterresultswhentheincident
These usually require additional modifications to frequencies
angleisgreaterthanthefirstcriticalangle,therebyprovidinga
and digital signal processing.
5.3.2 The scanning area shall be clear of weld spatter and
5 otherconditionswhichmayinterferewiththemovementofthe
Reference to ASME CC2235 is made only as an example of an existing code
probes, the coupling liquid, or the transmission of acoustic
wherethemutuallyagreeduponacceptancecriteriaallowsTOFDtobeapplied.This
does not suggest that application of ASME CC2235 would be appropriate in all
energy into the material. Any surface condition such as
cases.ItshouldberecognizedthatthehighsensitivityoftheTOFDtechniquecould
geometry, coating, and so forth, impeding the ultrasonic
result in indications from reflectors in plate materials that meet all plate ultrasonic
examination shall be noted for corrective action prior to
specification requirements. Such indications should not be considered unacceptable
unless they fail to meet the acceptance criteria agreed upon in 7.1. scanning.
E2373–04
FIG. 2
FIG. 3
5.3.3 The TOFD technique may be used with immersion, 5.4 Qualification and Certification of Personnel—If speci-
contact or gap techniques. Single element or phased array fiedinthecontractualagreement,personnelperformingexami-
piezoelectricprobesmaybeused.EMATorothernon-standard nations to this practice shall be qualified in accordance with a
probes may also be used with suitable validation procedures nationally-recognized NDT personnel qualification standard
agreeable to the contracting parties. and certified by the employer or certifying agency as appli-
5.3.4 The acoustic coupling shall be obtained by using a cable. The practice or standard used and its applicable version
medium suitable for the purpose and compatible with the shall be identified in the contractual agreement between the
material being examined. Water, coupling gels or pastes, using parties and should include a requirement for training
greases and oils are typically used. Water additives such as specific to TOFD.
environmentally-safe wetting agent and corrosion inhibitors 5.5 Equipment Requirements—An ultrasonic system for
may be used to enhance acoustic coupling and protect the TOFD shall provide a means of transmitting, receiving, stor-
examination piece. For examination where ambient tempera- ing, displaying and analyzing ultrasonic signals. As well, it
tures are below 0°C [32°F] methyl alcohol or similar media shall provide a fixed spacing between the transmitting and
may be used. For examination at elevated temperatures the receiving probes and ensure that probe motion is encoded and
examination surface or probes may require cool-down or its position maintained within prescribed tolerances with re-
specially designed high-temperature couplants. The coupling spect to a reference position such as the weld centerline.
medium selected shall provide uniform and reliable examina- 5.5.1 Electronics:
tion in the temperature range of intended use. Couplant and 5.5.1.1 The instrument shall provide a linear “A” scan
scanning conditions, including temperature, used for standard- presentation for both setting up scan parameters and for signal
ization shall be the same as that used in the examination. analysis.Instrumentlinearitymaybedeterminedinaccordance
E2373–04
with the procedures detailed in Guide E1324, within six 5.5.2.3 Computer software for TOFD displays shall include
months of the intended end use date. For digital-based instru- algorithms to linearize cursors or the waveform time-base to
ments alternative calibration methods may be used to verify permit depth and vertical extent estimations.
amplitude and time-base output linearity. A copy of the 5.5.2.4 In addition to storage of waveform data including
amplitude and time-base details, the TOFD equipment shall
calibration certificate shall be kept on file by the user of the
equipment. Instrument linearity shall be such that the accuracy alsostorepositionalinformationindicatingtherelativeposition
ofthewaveformwithrespecttotheadjacentwaveform(s);that
of indicated amplitude or time is within 65% of the actual
full-scale amplitude or time. is, encoded position.
5.5.3 Probes—UltrasonicprobesusedforTOFDtechniques
5.5.1.2 Theultrasonicpulsermayprovideexcitationvoltage
shall conform to the following minimum requirements:
by tone burst, uni-polar or bi-polar square wave. Pulse width
5.5.3.1 Two probes shall be used in a pitch-catch arrange-
shall be tunable to allow optimization of pulse amplitude and
ment (TOFD pair).
duration.
5.5.3.2 Each probe in the TOFD pair shall have the same
5.5.1.3 The bandwidth of the ultrasonic receiver shall be at
nominal frequency.
leastequaltothatofthenominalprobefrequencyandsuchthat
5.5.3.3 The TOFD pair shall have the same element dimen-
the −6 dB bandwidth of the probe does not fall outside of the
sions.
−6 dB bandwidth of the receiver.
5.5.3.4 Thepulsedurationoftheprobeshallnotexceedtwo
5.5.1.4 Receiver gain control shall be available to adjust
cycles as measured to the 20 dB level below the peak response
signalamplitudeinincrementsof1dBorless.Sincediffracted
(Guide E1065 may be used to evaluate characteristics of
signal amplitudes may be significantly lower than for pulse-
probes).
echo techniques it may be necessary to incorporate a pre-
5.5.3.5 Guide E1065,AnnexA1 may be used to determine
amplifier in the system.
theprobebandwidth.Thisshouldbeusedtoassessthereceiver
5.5.1.5 Analogue to digital conversion of waveforms shall
bandwidth requirements as stated in 5.5.1.
have sampling rates at least four times that of the nominal
5.5.3.6 Probes may be focused or unfocused. Unfocused
frequency of the probe.When digital signal processing is to be
probes are recommended for detection and focused probes are
carriedoutontherawdatathisshallbeincreasedtoeighttimes
recommended for improved resolution for sizing.
the nominal frequency
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