ASTM E2373/E2373M-19

Standard Practice for Use of the Ultrasonic Time of Flight Diffraction (TOFD) Technique

SIGNIFICANCE AND USE
4.1 This practice provides general principles for the application of the Time-of-Flight Diffraction Technique as a tool for detection and sizing of discontinuities.  
4.2 TOFD is a nondestructive ultrasonic examination technique that is not based on amplitude response. However, sufficient sensitivity is required to identify indications for evaluation.  
4.3 TOFD techniques 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.  
4.4 In addition to a stand-alone ultrasonic detection technique, TOFD may be used in conjunction with weld examinations 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.
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 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 quantitative examinations as well as quantitative tests.  
1.3 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used independently of the other, and values from the two systems shall not be combined.  
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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Nov-2019
• ICS: 25.160.40 - Welded joints and welds
• Technical Committee: E07 - Nondestructive Testing
• Drafting Committee: E07.06 - Ultrasonic Method

Relations

• Replaces: ASTM E2373/E2373M-14 - Standard Practice for Use of the Ultrasonic Time of Flight Diffraction (TOFD) Technique
• Effective Date: 01-Dec-2019
• Refers: ASTM E1316-24 - Standard Terminology for <?Pub Dtl?>Nondestructive Examinations
• Effective Date: 01-Feb-2024
• Refers: ASTM E1316-19b - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Dec-2019
• Refers: ASTM E1316-19 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Mar-2019
• Refers: ASTM E1316-18 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jan-2018
• Refers: ASTM E1316-17a - Standard Terminology for Nondestructive Examinations
• Effective Date: 15-Jun-2017
• Refers: ASTM E1316-17 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Feb-2017
• Refers: ASTM E1316-16a - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Aug-2016
• Refers: ASTM E1316-16 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Feb-2016
• Refers: ASTM E1316-15a - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Dec-2015
• Refers: ASTM E1316-15 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Sep-2015
• Refers: ASTM E1316-14 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jun-2014
• Refers: ASTM E1316-14e1 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jun-2014
• Refers: ASTM E1316-13d - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Dec-2013
• Refers: ASTM E1316-13c - Standard Terminology for Nondestructive Examinations
• Effective Date: 15-Jun-2013

Overview

ASTM E2373/E2373M-19 sets forth best practices for using the Ultrasonic Time of Flight Diffraction (TOFD) technique. Developed by ASTM International, this standard is central to nondestructive ultrasonic examination, enabling reliable detection and sizing of discontinuities in various materials. While TOFD is most commonly applied to welded joints in carbon steel, its general principles can extend to other materials, provided suitable validation procedures are established. The standard guides users in applying TOFD as either a primary or supplemental tool in ultrasonic weld inspection, with specific focus on repeatability, data integrity, and accuracy in flaw assessment.

Keywords: Time-of-Flight Diffraction, TOFD, ultrasonic testing, nondestructive testing, ASTM E2373/E2373M-19, flaw sizing, weld inspection.

Key Topics

• Application Scope:

  • TOFD is primarily designed for the detection and sizing of internal discontinuities such as cracks, voids, and inclusions in welded joints, especially in carbon steel.
  • Can be applied to various other materials with proper validation, as agreed by involved parties.

• Technique Principles:

  • Utilizes ultrasonic pulses and measures the time-of-flight of both diffracted and reflected waves to identify flaws.
  • Not based on amplitude response, but it is essential to apply sufficient sensitivity to confidently detect indications for evaluation.
  • Suitable for thicknesses from 9 to 300 mm, with proven performance, and adaptable for a wider range with demonstration of capability.

• Procedural Elements:

  • Requires documented ultrasonic examination procedures specific to each application.
  • Measurements may involve both qualitative assessments and quantitative sizing, with statistical evaluations for precision and bias.
  • Each measurement system (SI and inch-pound) must be applied independently-mixing units is not permitted.

• Data Handling and Reporting:

  • Mandates storage and analysis of raw A-scan and B-scan waveform data.
  • Reports must detail scan methods, procedure parameters, material description, scan coverage, and operator information.
  • Results are evaluated against acceptance criteria determined by contracting parties.

• Personnel Qualification:

  • Operators must be qualified according to accredited NDT personnel standards (e.g., ANSI/ASNT CP-189, SNT-TC-1A, ISO 9712).

Applications

• Weld Inspection:

  • Widely used for non-destructive testing of welds in pipelines, pressure vessels, and structural steelwork in industries such as oil & gas, power generation, and construction.
  • Can complement other ultrasonic inspection methods (e.g., pulse-echo) for improved flaw discrimination and sizing.

• Broader Material Testing:

  • With appropriate validation, TOFD can be extended to alloys, nonferrous metals, and other materials where ultrasonic wave propagation is feasible.
  • Applicable for detecting and sizing flaws in thick components, composite structures, and critical hardware where component integrity is essential.

• Compliance and QA/QC:

  • Supports regulatory and safety compliance by providing reliable, quantifiable inspection results.
  • Useful for quality assurance and control in manufacturing and field repair processes.

Related Standards

Professionals using ASTM E2373/E2373M-19 should be aware of related standards for comprehensive nondestructive testing practices:

• ASTM E164 - Practice for Contact Ultrasonic Testing of Weldments
• ASTM E1065 - Practice for Evaluating Characteristics of Ultrasonic Search Units
• ASTM E1961 - Practice for Mechanized Ultrasonic Testing of Girth Welds Using Zonal Discrimination
• ASTM E1316 - Terminology for Nondestructive Examinations
• ISO 16828 - Ultrasonic Testing-Time-of-Flight Diffraction Technique as a Method for Detection and Sizing of Discontinuities
• EN 583-6 - Non-destructive Testing: Ultrasonic Examination, TOFD
• ASNT SNT-TC-1A / ANSI/ASNT CP-189 - Standards for Qualification and Certification of NDT Personnel

Practical Value

Applying ASTM E2373/E2373M-19 ensures that ultrasonic testing using TOFD is performed consistently, with clear documentation and adherence to validated procedures. This boosts confidence in structural integrity assessments, minimizes risk of undetected defects, and facilitates compliance with contractual and regulatory requirements-ensuring safety, reliability, and quality in critical infrastructure and manufacturing projects.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E2373/E2373M − 19
Standard Practice for
Use of the Ultrasonic Time of Flight Diffraction (TOFD)
Technique
ThisstandardisissuedunderthefixeddesignationE2373/E2373M;thenumberimmediatelyfollowingthedesignationindicatestheyear
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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope* E543Specification forAgencies Performing Nondestructive
Testing
1.1 This practice establishes the requirements for develop-
E1065Practice for Evaluating Characteristics of Ultrasonic
ing ultrasonic examination procedures using the ultrasonic
Search Units
technique known as Time-of-Flight Diffraction (TOFD).
E1316Terminology for Nondestructive Examinations
1.2 Consistent with ASTM Policy, TOFD may be regarded
E1324GuideforMeasuringSomeElectronicCharacteristics
as an ultrasonic test method whereby the qualities and charac-
of Ultrasonic Testing Instruments
teristics of the item tested are evaluated, measured, and, in
E1961Practice for Mechanized Ultrasonic Testing of Girth
some cases, identified. Measurements may be subject to
Welds Using Zonal Discrimination with Focused Search
precision and bias that may be determined statistically or as a
Units
function of some parameter(s) such as wavelength. This
2.2 ASNT Documents:
practice may be used for applications that would be quantita-
SNT-TC-1A Recommended Practice for Nondestructive
tive examinations as well as quantitative tests.
Testing Personnel Qualification and Certification
1.3 The values stated in either SI units or inch-pound units
ANSI/ASNT CP-189Standard for Qualification and Certifi-
are to be regarded separately as standard. The values stated in
cation of Nondestructive Testing Personnel
each system are not necessarily exact equivalents; therefore, to
2.3 AIA Document:
ensure conformance with the standard, each system shall be
NAS-410Certification and Qualification of Nondestructive
used independently of the other, and values from the two
Testing Personnel
systems shall not be combined.
2.4 ISO Standards:
1.4 This standard does not purport to address all of the
ISO 9712Non-Destructive Testing—Qualification and Cer-
safety concerns, if any, associated with its use. It is the
tification of NDT Personnel
responsibility of the user of this standard to establish appro-
ISO 5577Non-Destructive Testing—Ultrasonic Testing—
priate safety, health, and environmental practices and deter-
Vocabulary
mine the applicability of regulatory limitations prior to use.
ISO 16828Non-Destructive Testing—Ultrasonic Testing—
1.5 This international standard was developed in accor-
Time-of-Flight Diffraction Technique as A Method for
dance with internationally recognized principles on standard-
Detection and Sizing of Discontinuities
ization established in the Decision on Principles for the
2.5 Other Document:
Development of International Standards, Guides and Recom-
Code Case 2235ASME Boiler and Pressure Vessel Code
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
3. Terminology
2. Referenced Documents
3.1 Definitions—Related terminology is defined in Termi-
2.1 ASTM Standards:
nology E1316.
E164Practice for Contact Ultrasonic Testing of Weldments
3.2 Definitions of Terms Specific to This Standard:
This practice is under the jurisdiction of ASTM Committee E07 on Nonde-
structive Testing and is the direct responsibility of Subcommittee E07.06 on
Ultrasonic Method. AvailablefromAmericanSocietyforNondestructiveTesting(ASNT),P.O.Box
Current edition approved Dec. 1, 2019. Published January 2020. Originally 28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http://www.asnt.org.
approved in 2004. Last previous edition approved in 2014 as E2373/E2373M–14. Available fromAerospace IndustriesAssociation ofAmerica, Inc. (AIA), 1000
DOI: 10.1520/E2373_E2373M-19. WilsonBlvd.,Suite1700,Arlington,VA22209-3928,http://www.aia-aerospace.org.
2 5
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Available from International Organization for Standardization (ISO), 1, ch. de
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM la Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http://www.iso.org.
Standards volume information, refer to the standard’s Document Summary page on Available from the American Society of Mechanical Engineers, ASME
the ASTM website. International, 22 Law Drive, Box 2900, Fairfield, NJ 07007-2900.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2373/E2373M − 19
3.2.1 B-scan display, n—plot of Time-of-Flight vs. probe 5.2.1 If specified in the contractual agreement, personnel
position formed by the stacking of A-scans. performing examinations to this standard shall be qualified in
accordance with a nationally or internationally recognized
3.2.1.1 Discussion—Some users refer to stacked A-scans
NDT personnel qualification practice or standard such as
from non-parallel scans as D-scans and parallel scans as
ANSI/ASNT CP-189, SNT-TC-1A, ISO 9712, NAS-410, or a
B-scans. D-scans are currently not defined in Terminology
similar document and certified by the employer or certifying
E1316 and are given a different definition in ISO 5577.
agency, as applicable. The practice or standard used and its
3.2.2 back-wall echo, n—a specular reflection from the
applicable revision shall be identified in the contractual agree-
back-wall of the component being examined (usually assumed
ment between the using parties.
to be a plate).
5.3 Qualification of Nondestructive Agencies—If specified
3.2.3 lateral wave, n—a surface-skimming longitudinal
in the contractual agreement, NDT agencies shall be qualified
wave traveling from the transmitting probe to the receiving
and evaluated as described in Specification E543. The appli-
probe in aTOFD configuration (also called a creeping wave in
cable edition of Specification E543 shall be specified in the
some applications).
contractual agreement.
3.2.4 non-parallel or longitudinal scan, n—a scan whereby
5.4 Procedures and Techniques—The procedures and tech-
the probe pair motion is perpendicular to the ultrasonic beam
niques to be used shall be as specified in the contractual
axis.
agreement.
3.2.5 parallel scan, n—a scan whereby the probe pair
5.5 Surface Preparation—The pre-examination surface
motion is parallel to the ultrasonic beam axis.
preparation criteria shall be in accordance with 6.3 unless
3.2.6 PCS, n—abbreviation for probe center spacing; refers
otherwise specified.
to the distance between the marked exit points of a pair of
5.6 Timing of Examination—The timing of examination
TOFD probes for a specific application.
shall be in accordance with 6.3 unless otherwise specified.
3.2.7 RF waveforms, n—the non-rectified A-scan.
5.7 Extent of Examination—Theextentofexaminationshall
be in accordance with 6.3 unless otherwise specified.
4. Significance and Use
5.8 Reporting Criteria/Acceptance Criteria—Reporting cri-
4.1 This practice provides general principles for the appli-
teria for the examination results shall be in accordance with
cationoftheTime-of-FlightDiffractionTechniqueasatoolfor
Section 8 unless otherwise specified. Since acceptance criteria
detection and sizing of discontinuities.
(for example, for reference radiographs) are not specified in
4.2 TOFD is a nondestructive ultrasonic examination tech-
this practice, they shall be specified in the contractual agree-
nique that is not based on amplitude response. However,
ment.
sufficient sensitivity is required to identify indications for
5.9 Re-examination of Repaired/Reworked Items—
evaluation.
Reexamination of repaired/reworked items is not addressed in
4.3 TOFD techniques are typically applied to welded joints
this standard and if required shall be specified in the contrac-
in carbon steel, but the principles may be applicable to other
tual agreement.
applications including other materials with suitable validation
procedures agreeable to the contracting parties.
6. Procedures
4.4 In addition to a stand-alone ultrasonic detection
6.1 Introduction:
technique, TOFD may be used in conjunction with weld
6.1.1 TOFD is an ultrasonic examination technique that can
examinations such as those described in Practices E164 and
provide improved detection and sizing capabilities of discon-
E1961 where it may be used to improve sizing estimates of
tinuities compared to standard ultrasonic pulse-echo tech-
flaws detected by the manual or mechanized pulse-echo
niques.Itusesforwardscatteredtipdiffractionandreflectionof
techniques and help discriminate between flaws and geometric
transmitted ultrasonic pulses. This document describes the
reflectors.
requirementsforTOFDequipmentandproceduresonflatplate
surfaces.Guidanceformorecomplexgeometriesisprovidedin
4.5 The technique has proven effective on thicknesses from
theAppendix.GeneralguidanceonTOFDcanalsobefoundin
9 to 300 mm [0.375 to 12 in.]. TOFD has been used on
ISO 16828. Typical acceptance criteria and performance dem-
thicknesses outside of this range but special considerations are
onstration requirements that may be used with TOFD tech-
necessary. Techniques developed outside of this range of
niques are found in ASME Code Case 2235.
thickness shall be demonstrated as capable of meeting the
required detection and sizing requirements of the specification
used.
Reference to ASME CC2235 is made only as an example of an existing code
wherethemutuallyagreeduponacceptancecriteriaallowsTOFDtobeapplied.This
5. Basis of Application
does not suggest that application of ASME CC2235 would be appropriate in all
5.1 The following items are subject to contractual agree- cases.ItshouldberecognizedthatthehighsensitivityoftheTOFDtechniquecould
result in indications from reflectors in plate materials that meet all plate ultrasonic
ment between the parties using or referencing this standard.
specification requirements. Such indications should not be considered unacceptable
5.2 Personnel Qualification unless they fail to meet the acceptance criteria agreed upon in 8.1.
E2373/E2373M − 19
6.1.2 Because phase inversions of signals play an important Thescanplanshallalsoincludeultrasonicbeamangle(s)used,
role in the evaluation of TOFD results, all procedures devel- beam directions with respect to some reference such as a weld
oped using this practice shall require that the equipment centerline, and volume examined.
presentation use and store RF waveforms.
6.3 Examination Materials and Surface Preparation:
6.1.3 Whether motorized or manually-operated, probe mo-
6.3.1 TOFD technique can be applied to both metals and
tion must be encoded for position and probes held in a fixture
nonmetals. Best results are had on fine-grained isotropic
that maintains correct PCS during scanning. Time based
materials with low attenuation including some finer grained
sampling of data collection is not acceptable.
austenitic alloys and aluminum. With suitable validation
6.1.4 Fig. 1 illustrates the typical probe configuration for a
procedures, agreeable to the contracting parties, coarser-
TOFD examination.The figure uses a weld for convenience of
grained and anisotropic materials may also be examined using
references; however, TOFD need not be restricted to just weld
TOFD. These usually require additional modifications to fre-
examinations.
quencies and digital signal processing.
6.1.5 The lateral wave and back-wall echo signals provide
6.3.2 The scanning area shall be clear of weld spatter and
convenient references. For most applications, mode converted
otherconditionswhichmayinterferewiththemovementofthe
signals from flaws are not used and therefore flaw indications
probes, the coupling liquid, or the transmission of acoustic
are usually recognized as occurring between the lateral wave
energy into the material. Any surface condition such as
and back-wall echo signals.Although it is more often the case
geometry, coating, and so forth, impeding the ultrasonic
to use the refracted longitudinal mode in the examination
examination shall be noted for corrective action prior to
piece, some applications may produce better results when the
scanning.
incident angle is greater than the first critical angle, thereby
6.3.3 The TOFD technique may be used with immersion,
providing a refracted shear mode in the examination piece.
contact, or gap techniques. Single element or phased array
When using a refracted longitudinal mode in the examination
piezoelectricprobesmaybeused.EMATorothernon-standard
piece, the direct shear and head waves also are generated;
probes may also be used with suitable validation procedures
however, due to their lower acoustic velocities, shear waves
agreeable to the contracting parties.
arrive later in time than the back-wall signal.
6.3.4 The acoustic coupling shall be obtained by using a
6.1.6 Fig. 2 is a sketch of a typical presentation for a
medium suitable for the purpose and compatible with the
non-parallel scan of a butt weld in a plate with an imbedded
material being examined. Water, coupling gels or pastes,
flaw. The right side of Fig. 2 illustrates a waveform extracted
greases, and oils are typically used. Water additives such as
from a B-scan display showing the lateral wave, upper tip
environmentally-safe wetting agent and corrosion inhibitors
diffracted, lower tip diffracted, and the back-wall echo signals.
may be used to enhance acoustic coupling and protect the
The left side of Fig. 2 indicates the probe placement with
examination piece. For examination where ambient tempera-
respect to the weld (upper left) and relative motion of the
tures are below 0°C [32°F] methyl alcohol or similar media
probes (a non-parallel scan is indicated).
may be used. For examination at elevated temperatures, the
6.1.7 Fig. 3 illustrates an actual TOFD scan with five
examination surface or probes may require cool-down or
indications(identifiedontheleft)andanextractedA-scanfrom
specially designed high-temperature couplants. The coupling
one of the indications.
medium selected shall provide uniform and reliable examina-
6.2 Written Procedure—A documented examination strat-
tion in the temperature range of intended use. Couplant and
egy or scan plan shall be provided showing probe placement,
scanning conditions, including temperature, used for standard-
movement, and component coverage that provides a standard-
ization shall be the same as that used in the examination.
ized and repeatable methodology for component acceptance.
6.3.5 Examination should occur after welding when the
surface temperature is cooled to less than 40°C [100°F].
Surfacepreparationshallbeadequatetoprovidesurfaceaccess
to examine the entire weld volume and heat affected zones.
6.4 Qualification and Certification of Personnel—If speci-
fiedinthecontractualagreement,personnelperformingexami-
nations to this practice shall be qualified in accordance with a
nationally-recognized NDT personnel qualification standard
and certified by the employer or certifying agency as appli-
cable. The practice or standard used and its applicable version
shall be identified in the contractual agreement between the
using parties and should include a requirement for training
specific to TOFD.
6.5 Equipment Requirements—An ultrasonic system for
TOFD shall provide a means of transmitting, receiving,
storing, displaying, and analyzing ultrasonic signals.
Additionally, it shall provide a fixed spacing between the
FIG. 1 TOFD Configuration and Signal Origins transmittingandreceivingprobesandensurethatprobemotion
E2373/E2373M − 19
FIG. 2 Schematic Representation of TOFD Scan (Left) and Data
Display (Right)
FIG. 3 Sample Display (Real Data) of a Nonparallel Scan of a
Weld with Flaws Identified
is encoded and its position maintained within prescribed 6.5.1.3 The bandwidth of the ultrasonic receiver shall be at
toleranceswithrespecttoareferencepositionsuchastheweld leastequaltothatofthenominalprobefrequencyandsuchthat
centerline. the −6 dB bandwidth of the probe does not fall outside of the
6.5.1 Electronics: −6 dB bandwidth of the receiver.
6.5.1.1 The instrument shall provide a linear “A” scan 6.5.1.4 Receiver gain control shall be available to adjust
presentation for both setting up scan parameters and for signal signalamplitudeinincrementsof1dBorless.Sincediffracted
analysis.Instrumentlinearitymaybedeterminedinaccordance signal amplitudes may be significantly lower than for pulse-
with the procedures detailed in Guide E1324, within six echo techniques, it may be necessary to incorporate a pre-
months of the intended end use date. For digital-based amplifier in the system.
instruments, alternative calibration methods may be used to 6.5.1.5 Analogue to digital conversion of waveforms shall
verify amplitude and time-base output linearity.Acopy of the have sampling rates at least four times that of the nominal
calibration certificate shall be kept on file by the user of the frequency of the probe.When digital signal processing is to be
equipment. Instrument linearity shall be such that the accuracy carried out on the raw data, this shall be increased to eight
of indicated amplitude or time is within 65% of the actual times the nominal frequency of the probe.
full-scale amplitude or time. 6.5.2 Data Display and Recording:
6.5.1.2 Theultrasonicpulsermayprovideexcitationvoltage 6.5.2.1 The data display used for TOFD shall allow the
by tone burst, uni-polar, or bi-polar square wave. Pulse width operator to view the un-rectified A-scan and position the start
shall be tunable to allow optimization of pulse amplitude and and length of a gate that determines the extent of the A-scan
duration. time-base that is collected.
E2373/E2373M − 19
6.5.2.2 Datacollectionequipmentshallpermitstorageofall intended scan axis on the examination piece is maintained to a
gated A-scans to a magnetic or optical storage medium. tolerance agreed upon between contracting parties.
EquipmentusedforTOFDshallrequirecomputersoftwarethat 6.5.4.2 Probe motion may be achieved using motorized or
provides a B-scan display of the collected waveforms (as manual means, but in all cases, the mechanical holder for the
illustratedinFig.2).TheB-scandisplayshallhaveaminimum probes shall be equipped with a positional encoder that is
of64gray-scaleorcolorlevels.(StorageofjustB-scanimages synchronized with the sampling of A-scans.
without the underlyingA-scan waveforms is not an acceptable
6.6 Apparatus Set-up—Fig. 4 provides a schematic of the
form of data recording.)
minimum equipment requirements for a TOFD examination.
6.5.2.3 Computer software for TOFD displays shall include
6.7 Probe Selection:
algorithms to linearize cursors or the waveform time-base to
6.7.1 Probe selection shall be based on the application
permit depth and vertical extent estimations.
requirements. The following tables provide initial recom-
6.5.2.4 In addition to storage of waveform data, including
mended probe parameters for specified thickness ranges in
amplitude and time-base details, the TOFD equipment shall
ferritic steels. For austenitic or other attenuative materials,
alsostorepositionalinformationindicatingtherelativeposition
nominal frequencies normally need to be reduced and element
ofthewaveformwithrespecttotheadjacentwaveform(s);that
sizes increased.
is, encoded position.
6.5.3 Probes—Ultrasonic probes used for TOFD techniques
TABLE 1 For Steel Thickness Ranges up to 75 mm [3 in.]
shall conform to the following minimum requirements:
6.5.3.1 Two probes shall be used in a pitch-catch arrange- Nominal Wall Nominal
Element Size Recommended
Thickness Frequency
ment (TOFD pair).
mm [in.] Angles
mm [in.] (MHz)
6.5.3.2 Each probe in the TOFD pair shall have the same
<12 [0.5] 10 to 15 2 to 6 [0.08 to 0.25] 60 to 70°
nominal frequency.
12 to <35 [0.5 to 1.4] 5 to 10 2 to 6 [0.25 to 0.5] 50 to 70°
35 to <75 [1.4 to 3] 2 to 5 6 to 12 [0.25 to 0.5] 45 to 65°
6.5.3.3 The TOFD pair shall have the same element dimen-
sions.
6.5.3.4 Thepulsedurationoftheprobeshallnotexceedtwo
cycles as measured to the 20 dB level below the peak response 6.7.2 For thickness ranges in steel 75 to 300 mm, the beam
(Practice E1065 may be used to evaluate characteristics of divergence from a single element is not likely to provide
probes). sufficientintensityforgooddetectionovertheentirethickness.
6.5.3.5 PracticeE1065,AnnexA1maybeusedtodetermine For thickness 75 mm [3 in.] and greater (in steel), and when
theprobebandwidth.Thisshouldbeusedtoassessthereceiver required in smaller thicknesses, the examination piece shall be
bandwidth requirements as stated in 6.5.1. divided into multiple zones and sensitivity targets shall be
6.5.3.6 Probes may be focused or unfocused. Unfocused placed in a reference block at least at 25% and 75% through
probes are recommended for detection and focused probes are thickness in each zone to verify that there is adequate beam
recommended for improved resolution for sizing. coverage for the multiple zone technique used.
6.5.3.7 Probes may be single element or phased array. 6.7.3 On thick sections requiring more than oneTOFD pair,
EMATprobesmaybeusedwithsuitablevalidationprocedures thelateralwaveorback-wallsignalmaynotalwaysbevisible.
agreeable to the contracting parties. Therefore, provision in the linearizing algorithms must be
6.5.4 Mechanics: madetopermitinputsofotherparametersinsteadofthelateral
6.5.4.1 Mechanical holders shall be used to ensure that and back-wall signal positions. For wall thickness less than
probe spacing is maintained at a fixed distance (typically 75mm[3in.],techniquequalificationsmayrequiretheytoobe
termed the “probe center spacing” or PCS in other literature). divided into smaller ranges with each range addressed by a
The mechanical holders shall also ensure that alignment to the dedicated TOFD pair.
FIG. 4 Schematic of Minimum TOFD Apparatus
E2373/E2373M − 19
TABLE 2 For Multiple Zones in Steel with Thicknesses Up to
thickness so the sensitivity may be set using the response from
300 mm [12 in.]
the target at 75% wall thickness.
Nominal Nominal
6.8.4 When using more than one TOFD pair for thicker
Element Size Recommended
Wall Frequency
mm [in.] Angle
sections, multiple channel equipment may be used. If only
mm [in.] (MHz)
single channel capability is available, multiple scans may be
<35 5 to 15 3 to 6 [0.125 to 0.25] 50 to 70°
[<1.4]
used with a separate scan for each TOFD pair.
$35 to 300 1 to 5 6 to 12.5 [0.25 to 0.5] 45 to 65°
6.8.5 When setting sensitivity by using side drilled holes in
[$1.4 to 12]
thereferenceblock,theamplituderesponsefromtheweakerof
the two reference side-drilled holes shall be set to 80% full
screen height. This shall be considered “reference sensitivity.”
6.8 Sensitivity:
ScanninglevelsensitivityindBaboveorbelowthislevelshall
6.8.1 TOFD is a non-amplitude based detection and sizing
be established between the contracting parties. When multiple
technique; however, sufficient sensitivity must be used to
zones are used for thicker sections, evidence of the volume
ensure flaw indications can be seen on the B-scan display. In
coverage should be established by detection of the nearest
most cases where a single TOF
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E2373/E2373M − 14 E2373/E2373M − 19
Standard Practice for
Use of the Ultrasonic Time of Flight Diffraction (TOFD)
Technique
This standard is issued under the fixed designation E2373/E2373M; 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*
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 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 andmeasured, 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 quantitative examinations as well as quantitative and more
properly addressed as tests.
1.3 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each
system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used
independently of the other, and values from the two systems shall not be combined.
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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
E164 Practice for Contact Ultrasonic Testing of Weldments
E543 Specification for Agencies Performing Nondestructive Testing
E1065 Practice for Evaluating Characteristics of Ultrasonic Search Units
E1316 Terminology for Nondestructive Examinations
E1324 Guide for Measuring Some Electronic Characteristics of Ultrasonic Testing Instruments
E1961 Practice for Mechanized Ultrasonic Testing of Girth Welds Using Zonal Discrimination with Focused Search Units
2.2 ASNT Documents:
SNT-TC-1A Recommended Practice for Nondestructive Testing Personnel Qualification and Certification
ANSI/ASNT CP-189 Standard for Qualification and Certification of Nondestructive Testing Personnel
2.3 Aerospace Industries Association AIA Document:
NAS-410 Certification and Qualification of Nondestructive Testing Personnel
2.4 ISO Standard:Standards:
ISO 9712 Non-destructiveNon-Destructive Testing—Qualification and Certification of NDT Personnel
ISO 5577 Non-Destructive Testing—Ultrasonic Testing—Vocabulary
This practice is under the jurisdiction of ASTM Committee E07 on Nondestructive Testing and is the direct responsibility of Subcommittee E07.06 on Ultrasonic Method.
Current edition approved June 1, 2014Dec. 1, 2019. Published June 2014January 2020. Originally approved in 2004. Last previous edition approved in 20092014 as
E2373 - 09.E2373/E2373M – 14. DOI: 10.1520/E2373_E2373M-14.10.1520/E2373_E2373M-19.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from American Society for Nondestructive Testing (ASNT), P.O. Box 28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http://www.asnt.org.
Available from Aerospace Industries Association of America, Inc. (AIA), 1000 Wilson Blvd., Suite 1700, Arlington, VA 22209-3928, http://www.aia-aerospace.org.
Available from International Organization for Standardization (ISO), 1, ch. de la Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http://www.iso.org.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2373/E2373M − 19
ISO 16828 Non-Destructive Testing—Ultrasonic Testing—Time-of-Flight Diffraction Technique as A Method for Detection and
Sizing of Discontinuities
2.5 Other Documents:Document:
Code Case 2235 ASME Boiler and Pressure Vessel Code
EN 583-6 Non-destructive Testing: Ultrasonic Examination. Time-of-flight Diffraction Technique as a Method for Detection and
Sizing of Discontinuities
3. Terminology
3.1 Definitions—Related terminology is defined in Terminology E1316.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 B-scan display—display, n—a sectional view of the plotted inspection dataplot of Time-of-Flight vs. probe position formed
by the stacking of A-scans. (Some users refer to stacked A-scans from non-parallel scans as D-scans and reserve those used with
parallel scans as B-scans.)
3.2.1.1 Discussion—
Some users refer to stacked A-scans from non-parallel scans as D-scans and parallel scans as B-scans. D-scans are currently not
defined in Terminology E1316 and are given a different definition in ISO 5577.
3.2.2 back-wall echo—echo, n—a specular reflection from the back-wall of the component being examined (usually assumed
to be a plate).
3.2.3 lateral wave—wave, n—a compression wave that travels by the most direct route surface-skimming longitudinal wave
traveling from the transmitting probe to the receiving probe in a TOFD configuration.configuration (also called a creeping wave
in some applications).
3.2.4 non-parallel or longitudinal scan, n—a scan whereby the probe pair motion is perpendicular to the ultrasonic beam axis.
3.2.5 parallel scan—scan, n—a scan whereby the probe pair motion is parallel to the ultrasonic beam axis. Also called a B-scan
by some users.
3.2.6 PCS—PCS, n—abbreviation for probe center spacing. Refersspacing; refers to the distance between the marked exit points
of a pair of TOFD probes for a specific application.
3.2.6 non-parallel or longitudinal scan—a scan whereby the probe pair motion is perpendicular to the ultrasonic beam axis.
3.2.7 RF waveforms—waveforms, n—the non-rectified A-scan.
4. Significance and Use
4.1 This practice provides general principles for the application of the Time-of-Flight Diffraction Technique as a tool for
detection and sizing of discontinuities.
4.2 TOFD is a nondestructive ultrasonic examination technique that is not based on amplitude response. However, sufficient
sensitivity is required to identify indications for evaluation.
4.3 Techniques usedTOFD techniques 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.
4.4 In addition to a stand-alone ultrasonic detection technique, TOFD may be used in conjunction with weld examinations 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.
5. Basis of Application
5.1 The following items are subject to contractual agreement between the parties using or referencing this standard.
5.2 Personnel Qualification
5.2.1 If specified in the contractual agreement, personnel performing examinations to this standard shall be qualified in
accordance with a nationally or internationally recognized NDT personnel qualification practice or standard such as ANSI/ASNT-
CP-189, ANSI/ASNT CP-189, SNT-TC-1A, ISO 9712, NAS-410, or a similar document and certified by the employer or certifying
agency, as applicable. The practice or standard used and its applicable revision shall be identified in the contractual agreement
between the using parties.
Available from the American Society of Mechanical Engineers, ASME International, 22 Law Drive, Box 2900, Fairfield, NJ 07007-2900.
E2373/E2373M − 19
5.3 Qualification of Nondestructive Agencies—If specified in the contractual agreement, NDT agencies shall be qualified and
evaluated as described in Specification E543. The applicable edition of Specification E543 shall be specified in the contractual
agreement.
5.4 Procedures and Techniques—The procedures and techniques to be used shall be as specified in the contractual agreement.
5.5 Surface Preparation—The pre-examination surface preparation criteria shall be in accordance with 6.3 unless otherwise
specified.
5.6 Timing of Examination—The timing of examination shall be in accordance with 6.3 unless otherwise specified.
5.7 Extent of Examination—The extent of examination shall be in accordance with 6.3 unless otherwise specified.
5.8 Reporting Criteria/Acceptance Criteria—Reporting criteria for the examination results shall be in accordance with Section
8 unless otherwise specified. Since acceptance criteria (for example, for reference radiographs) are not specified in this practice,
they shall be specified in the contractual agreement.
5.9 Re-examination of Repaired/Reworked Items—Reexamination of repaired/reworked items is not addressed in this standard
and if required shall be specified in the contractual agreement.
6. Procedures
6.1 Introduction:
6.1.1 TOFD is an ultrasonic examination technique that can provide improved detection and sizing capabilities of
discontinuities compared to standard ultrasonic pulse-echo techniques. It uses forward scattered tip diffraction and reflection of
transmitted ultrasonic pulses. This document describes the requirements for TOFD equipment and procedures on flat plate surfaces.
Guidance for more complex geometries is provided in the Appendix. General guidance on TOFD can also be found in EN 583–6.
Acceptance criteria typical ISO 16828. Typical acceptance criteria and performance demonstration requirements that may be used
with TOFD techniques are found in ASME Code Case 22352235. .
6.1.2 Because phase inversions of signals play an important role in the evaluation of TOFD results, all procedures developed
using this practice shall require that the equipment presentation use and store RF waveforms.
6.1.3 Whether motorized or manually-operated, probe motion must be encoded for position and probes held in a fixture that
maintains correct PCS during scanning. Time based sampling of data collection is not acceptable.
6.1.4 Fig. 1 illustrates the typical probe configuration for a TOFD examination. The figure uses a weld for convenience of
references; however, TOFD need not be restricted to just weld examinations.
6.1.5 The lateral wave and back-wall echo signals provide convenient references. For most applications, mode converted signals
from flaws are not used and therefore flaw indications are usually recognized as occurring between the lateral wave and back-wall
echo signals. Although it is more often the case to use the refracted compressionlongitudinal mode in the examination piece, some
applications may produce better results when the incident angle is greater than the first critical angle, thereby providing a refracted
FIG. 1 TOFD Configuration and Signal Origins
Reference to ASME CC2235 is made only as an example of an existing code where the mutually agreed upon acceptance criteria allows TOFD to be applied. This does
not suggest that application of ASME CC2235 would be appropriate in all cases. It should be recognized that the high sensitivity of the TOFD technique could result in
indications from reflectors in plate materials that meet all plate ultrasonic specification requirements. Such indications should not be considered unacceptable unless they fail
to meet the acceptance criteria agreed upon in 8.1.
E2373/E2373M − 19
transverse shear mode in the examination piece. When using a refracted compressionlongitudinal mode in the examination piece,
the direct shear and head waves also are generated; however, due to their lower acoustic velocities, shear waves arrive later in time
than the back-wall signal.
6.1.6 Fig. 2 is a sketch of a typical presentation for a non-parallel scan of a butt weld in a plate with an imbedded flaw. The
right side of Fig. 2 illustrates a waveform extracted from a B-scan display showing the lateral wave, upper tip diffracted, lower
tip diffracted, and the back-wall echo signals. The left side of Fig. 2 indicates the probe placement with respect to the weld (upper
left) and relative motion of the probes (a non-parallel scan is indicated).
6.1.7 Fig. 3 illustrates an actual TOFD scan with five indications (identified on the left) and an extracted A-scan from one of
the indications.
6.2 Written Procedure—A documented examination strategy or scan plan shall be provided showing probe placement,
movement, and component coverage that provides a standardized and repeatable methodology for component acceptance. The scan
plan shall also include ultrasonic beam angle(s) used, beam directions with respect to some reference such as a weld centerline,
and volume examined.
6.3 Examination Materials and Surface Preparation:
6.3.1 TOFD technique can be applied to both metals and nonmetals. Best results are had on fine-grained isotropic materials with
low attenuation including some finer grained austenitic alloys and aluminum. With suitable validation procedures, agreeable to the
contracting parties, coarser-grained and anisotropic materials may also be examined using TOFD. These usually require additional
modifications to frequencies and digital signal processing.
6.3.2 The scanning area shall be clear of weld spatter and other conditions which may interfere with the movement of the
probes, the coupling liquid, or the transmission of acoustic energy into the material. Any surface condition such as geometry,
coating, and so forth, impeding the ultrasonic examination shall be noted for corrective action prior to scanning.
6.3.3 The TOFD technique may be used with immersion, contact, or gap techniques. Single element or phased array
piezoelectric probes may be used. EMAT or other non-standard probes may also be used with suitable validation procedures
agreeable to the contracting parties.
6.3.4 The acoustic coupling shall be obtained by using a medium suitable for the purpose and compatible with the material being
examined. Water, coupling gels or pastes, greases, and oils are typically used. Water additives such as environmentally-safe wetting
agent and corrosion inhibitors may be used to enhance acoustic coupling and protect the examination piece. For examination where
ambient temperatures are below 0°C [32°F]0 °C [32 °F] methyl alcohol or similar media may be used. For examination at elevated
temperatures, the examination surface or probes may require cool-down or specially designed high-temperature couplants. The
coupling medium selected shall provide uniform and reliable examination in the temperature range of intended use. Couplant and
scanning conditions, including temperature, used for standardization shall be the same as that used in the examination.
6.3.5 Examination should occur after welding when the surface temperature is cooled to less than 40°C [100°F].40 °C [100 °F].
Surface preparation shall be adequate to provide surface access to examine the entire weld volume and heat affected zones.
6.4 Qualification and Certification of Personnel—If specified in the contractual agreement, personnel performing examinations
to this practice shall be qualified in accordance with a nationally-recognized NDT personnel qualification standard and certified
by the employer or certifying agency as applicable. The practice or standard used and its applicable version shall be identified in
the contractual agreement between the using parties and should include a requirement for training specific to TOFD.
FIG. 2 Schematic Representation of TOFD Scan (Left) and Data
Display (Right)
E2373/E2373M − 19
FIG. 3 Sample Display (Real Data) of a Nonparallel Scan of a
Weld with Flaws Identified
6.5 Equipment Requirements—An ultrasonic system for TOFD shall provide a means of transmitting, receiving, storing,
displaying, and analyzing ultrasonic signals. As well, Additionally, it shall provide a fixed spacing between the transmitting and
receiving probes and ensure that probe motion is encoded and its position maintained within prescribed tolerances with respect to
a reference position such as the weld centerline.
6.5.1 Electronics:
6.5.1.1 The instrument shall provide a linear “A” scan presentation for both setting up scan parameters and for signal analysis.
Instrument linearity may be determined in accordance with the procedures detailed in Guide E1324, within six months of the
intended end use date. For digital-based instruments, alternative calibration methods may be used to verify amplitude and
time-base output linearity. A copy of the calibration certificate shall be kept on file by the user of the equipment. Instrument
linearity shall be such that the accuracy of indicated amplitude or time is within 65 % of the actual full-scale amplitude or time.
6.5.1.2 The ultrasonic pulser may provide excitation voltage by tone burst, uni-polaruni-polar, or bi-polar square wave. Pulse
width shall be tunable to allow optimization of pulse amplitude and duration.
6.5.1.3 The bandwidth of the ultrasonic receiver shall be at least equal to that of the nominal probe frequency and such that the
−6 dB bandwidth of the probe does not fall outside of the −6 dB bandwidth of the receiver.
6.5.1.4 Receiver gain control shall be available to adjust signal amplitude in increments of 1 dB or less. Since diffracted signal
amplitudes may be significantly lower than for pulse-echo techniques, it may be necessary to incorporate a pre-amplifier in the
system.
6.5.1.5 Analogue to digital conversion of waveforms shall have sampling rates at least four times that of the nominal frequency
of the probe. When digital signal processing is to be carried out on the raw data, this shall be increased to eight times the nominal
frequency of the probe.
6.5.2 Data Display and Recording:
6.5.2.1 The data display used for TOFD shall allow the operator to view the un-rectified A-scan and position the start and length
of a gate that determines the extent of the A-scan time-base that is collected.
6.5.2.2 Data collection equipment shall permit storage of all gated A-scans to a magnetic or optical storage medium. Equipment
used for TOFD shall require computer software that provides a B-scan display of the collected waveforms (as illustrated in Fig.
2). The B-scan display shall have a minimum of 64 gray-scale or color levels. (Storage of just B-scan images without the
underlying A-scan waveforms is not an acceptable form of data recording.)
6.5.2.3 Computer software for TOFD displays shall include algorithms to linearize cursors or the waveform time-base to permit
depth and vertical extent estimations.
6.5.2.4 In addition to storage of waveform data, including amplitude and time-base details, the TOFD equipment shall also store
positional information indicating the relative position of the waveform with respect to the adjacent waveform(s); that is, encoded
position.
6.5.3 Probes—Ultrasonic probes used for TOFD techniques shall conform to the following minimum requirements:
6.5.3.1 Two probes shall be used in a pitch-catch arrangement (TOFD pair).
6.5.3.2 Each probe in the TOFD pair shall have the same nominal frequency.
6.5.3.3 The TOFD pair shall have the same element dimensions.
6.5.3.4 The pulse duration of the probe shall not exceed two cycles as measured to the 20 dB level below the peak response
(Guide(Practice E1065 may be used to evaluate characteristics of probes).
6.5.3.5 GuidePractice E1065, Annex A1 may be used to determine the probe bandwidth. This should be used to assess the
receiver bandwidth requirements as stated in 6.5.1.
E2373/E2373M − 19
6.5.3.6 Probes may be focused or unfocused. Unfocused probes are recommended for detection and focused probes are
recommended for improved resolution for sizing.
6.5.3.7 Probes may be single element or phased array. EMAT probes may be used with suitable validation procedures agreeable
to the contracting parties.
6.5.4 Mechanics:
6.5.4.1 Mechanical holders shall be used to ensure that probe spacing is maintained at a fixed distance (typically termed the
“probe center spacing” or PCS in other literature). The mechanical holders shall also ensure that alignment to the intended scan
axis on the examination piece is maintained to a tolerance agreed upon between contracting parties.
6.5.4.2 Probe motion may be achieved using motorized or manual means, but in all cases, the mechanical holder for the probes
shall be equipped with a positional encoder that is synchronized with the sampling of A-scans.
6.5.5
6.6 Apparatus Set-up—Fig. 4 provides a schematic of the minimum equipment requirements for a TOFD examination.
6.7 Probe Selection:
6.7.1 Probe selection shall be based on the application requirements. The following tables provide initial recommended probe
parameters for specified thickness ranges in ferritic steels. For austenitic or other attenuative materials, nominal frequencies
normally need to be reduced and element sizes increased.
TABLE 1 For Steel Thickness Ranges up to 75 mm [3 in.]
Nominal Wall Nominal
Element Size Recommended
Thickness Frequency
mm [in.] Angles
mm [in.] (MHz)
<12 [0.5] 10 to 15 2 to 6 [0.08 to 0.25] 60 to 70°
12 to <35 [0.5 to 1.4] 5 to 10 2 to 6 [0.25 to 0.5] 50 to 70°
35 to <75 [1.4 to 3] 2 to 5 6 to 12 [0.25 to 0.5] 45 to 65°
6.7.2 For thickness ranges in steel 75 to 300 mm, the beam divergence from a single element is not likely to provide sufficient
intensity for good detection over the entire thickness. For thickness 75 mm [3 in.] and greater (in steel) steel), and when required
in smaller thicknesses, the examination piece shall be divided into multiple zones. For thickness 75 mm [3 in.] and greater (in steel)
and when required in smaller thickness, zones and sensitivity targets shall be placed in a reference block at least at 25 % and 75 %
through thickness in each zone to verify that there is adequate beam coverage for the multiple zone technique used.
TABLE 2 For Multiple Zones in Steel with Thicknesses Up to 300
mm [12 in.]300 mm [12 in.]
Nominal Nominal
Element Size Recommended
Wall Frequency
mm [in.] Angle
mm [in.] (MHz)
<35 5 to 15 3 to 6 [0.125 to 0.25] 50 to 70°
[<1.4]
$35 to 300 1 to 5 6 to 12.5 [0.25 to 0.5] 45 to 65°
[$1.4 to 12]
FIG. 4 Schematic of Minimum TOFD Apparatus
E2373/E2373M − 19
6.7.3 On thick sections requiring more than one TOFD pair, the lateral wave or back-wall signal may not always be visible.
Therefore, provision in the linearizing algorithms must be made to permit inputs of other parameters instead of the lateral and
back-wall signal positions. For wall thickness less than 75 mm [3 in.], 75 mm [3 in.], technique qualifications may require they
too be divided into smaller ranges with each range addressed by a dedicated TOFD pair.
6.8 Sensitivity:
6.8.1 TOFD is a non-amplitude based detection and sizing technique; however, sufficient sensitivity must be used to ensure flaw
indications can be seen on the B-scan display. In most cases where a single TOFD pair is used an adequate sensitivity can be
achieved by setting the lateral wave amplitude to 40 to 90 % of the full screen height. Alternatively sensitivity may be established
based on a noise level from grain scatter (typically 10-15 % 10 to 15 % screen height or 6 dB greater than the electrical noise prior
to the lateral wave signal) or from the response from reference targets.
6.8.2 Unless alternative sensitivity targets or techniques are agreed upon by the contracting parties, sensitivity shall be assessed
using the response from side-drilled holes. Examples of referenc
...