ASTM E494-20

Standard Practice for Measuring Ultrasonic Velocity in Materials by Comparative Pulse-Echo Method

ABSTRACT
This practice covers a test procedure for measuring ultrasonic velocities in materials with conventional ultrasonic pulse echo flaw detection equipment in which results are displayed in an A-scan display, and describes a method whereby unknown ultrasonic velocities in a material sample are determined by comparative measurements using a reference material whose ultrasonic velocities are accurately known. The ultrasonic testing system that shall be used shall consist of the test instrument, search unit, couplant, and the standard reference blocks. The test procedure shall include both longitudinal and transverse wave velocity measurements, which should conform to the theoretical values of the parameters.
SIGNIFICANCE AND USE
5.1 This practice describes a test procedure for the application of conventional ultrasonic methods to determine unknown ultrasonic velocities in a material sample by comparative measurements using a reference material whose ultrasonic velocities are accurately known.  
5.2 Although not all methods described in this practice are applied equally or universally to all velocity measurements in different materials, it does provide flexibility and a basis for establishing contractual criteria between users, and may be used as a general guideline for preparing a detailed procedure or specification for a particular application.  
5.3 This practice is directed towards the determination of longitudinal and shear wave velocities using the appropriate sound wave form. This practice also outlines methods to determine elastic modulus and can be applied in both contact and immersion mode.
SCOPE
1.1 This practice covers a test procedure for measuring ultrasonic velocities in materials with conventional ultrasonic pulse echo flaw detection equipment in which results are displayed in an A-scan display. This practice describes a method whereby unknown ultrasonic velocities in a material sample are determined by comparative measurements using a reference material whose ultrasonic velocities are accurately known.  
1.2 This procedure is intended for solid materials 5 mm (0.2 in.) thick or greater. The surfaces normal to the direction of energy propagation shall be parallel to at least ±3°. Surface finish for velocity measurements shall be 3.2 μm (125 μin.) root-mean-square (rms) or smoother.
Note 1: Sound wave velocities are cited in this practice using the fundamental units of meters per second, with inches per second supplied for reference in many cases. For some calculations, it is convenient to think of velocities in units of millimeters per microsecond. While these units work nicely in the calculations, the more natural units were chosen for use in the tables in this practice. The values can be simply converted from m/s to mm/μs by moving the decimal point three places to the left, that is, 3500 m/s becomes 3.5 mm/μs.  
1.3 Ultrasonic velocity measurements are useful for determining several important material properties. Young's modulus of elasticity, Poisson's ratio, acoustic impedance, and several other useful properties and coefficients can be calculated for solid materials with the ultrasonic velocities if the density is known (see Appendix X1).  
1.4 More accurate results than those obtained using this method can be obtained with more specialized ultrasonic equipment, auxiliary equipment, and specialized techniques. Some of the supplemental techniques are described in Appendix X2. (Material contained in Appendix X2 is for informational purposes only.)  
Note 2: Factors including techniques, equipment, types of material, and operator variables will result in variations in absolute velocity readings, sometimes by as much as ±5 %. Relative results with a single combination of the above factors can be expected to be much more accurate (probably within a 1 % tolerance).  
1.5 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses after SI uni...

General Information

• Status: Published
• Publication Date: 30-Nov-2020
• ICS: 77.040.20 - Non-destructive testing of metals
• Technical Committee: E07 - Nondestructive Testing
• Drafting Committee: E07.06 - Ultrasonic Method

Relations

• 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
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• 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
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• Effective Date: 01-Feb-2017
• Refers: ASTM E1316-16a - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Aug-2016
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• 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 E1316-14e1 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jun-2014
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• 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
• Refers: ASTM E1316-13b - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jun-2013

Overview

ASTM E494-20: Standard Practice for Measuring Ultrasonic Velocity in Materials by Comparative Pulse-Echo Method is an internationally recognized guideline for assessing ultrasonic velocities in solid materials. This standard provides a comparative methodology using conventional ultrasonic pulse-echo flaw detection equipment with results displayed in an A-scan format. The primary aim is to determine unknown ultrasonic velocities of a test material by comparison with a reference block of known ultrasonic velocity. This procedure enables accurate, repeatable, and non-destructive assessment of material properties important for quality assurance, characterization, and monitoring of structural integrity.

Ultrasonic velocity measurements, as outlined in this standard, are vital in calculating significant material properties such as Young’s modulus, Poisson’s ratio, and acoustic impedance, provided the material’s density is known. ASTM E494-20 applies to solid materials of 5 mm thickness or greater and emphasizes best practices for specimen preparation, equipment setup, and data reporting.

Key Topics

• Ultrasonic Pulse-Echo Method: Utilizes standard ultrasonic testing (UT) equipment to transmit pulses and analyze their echoes within the test material, comparing the time of flight with a calibrated reference specimen.
• Measurement Modes: Covers both longitudinal and transverse (shear) wave velocities, accommodating contact and immersion testing approaches.
• Reference Block Comparison: Ensures accuracy by comparing measured velocities in the test sample to those in a well-characterized reference block.
• Sample Preparation: Describes requirements for parallel, smooth surfaces (3.2 μm rms finish or better), and specimen thickness to secure reliable ultrasonic signal propagation and reflection.
• Material Characterization: Allows estimation of material properties (e.g., elastic modulus, Poisson’s ratio) using velocity results, supporting engineering evaluations.
• Equipment and Couplant Requirements: Recommends appropriate selection of transducers, couplants (depending on wave type), and instrument configuration to optimize data precision.
• Reporting and Accuracy: Provides a standardized checklist for reporting results, including instrument settings, couplant details, specimen geometry, and measurement frequencies. Notes possible sources of measurement variation and practical accuracy expectations.

Applications

ASTM E494-20 serves several industrial and scientific applications by delivering a reliable and non-destructive method for ultrasonic velocity measurement. Some notable applications include:

• Quality Control and Inspection: Verification of material homogeneity and integrity in metals, composites, ceramics, and polymers.
• Material Characterization: Calculation of elastic constants and other physical properties for research, engineering design, or acceptance criteria.
• Manufacturing and Process Monitoring: Assessment of process-induced variations in properties or the detection of residual stresses and texturing in processed products.
• Comparative Testing: Establishing consistent evaluation protocols across different test laboratories or facilities, suitable for contract specifications and audits.
• Safety and Structural Assessment: Monitoring the ongoing condition of structures where non-destructive techniques are required to avoid damaging operational assets.
• Education and Research: Supporting fundamental research in materials science and the development of new ultrasonic testing techniques.

Related Standards

For comprehensive ultrasonic evaluation and personnel qualification, ASTM E494-20 references and is complemented by several additional standards:

• ASTM C597: Pulse Velocity Through Concrete – addresses ultrasonic testing specific to concrete structures.
• ASTM E317: Performance Characteristics of Ultrasonic Testing Instruments.
• ASTM E797: Ultrasonic Thickness Measurement using Manual Pulse-Echo.
• ASTM E543: Qualification of Nondestructive Testing Agencies.
• ASTM E1316: Terminology for Nondestructive Examinations.
• ASNT SNT-TC-1A: Recommended Practice for NDT Personnel Qualification.
• ISO 9712: International standard for qualification and certification of nondestructive testing personnel.

These standards provide a framework for ensuring consistency, reliability, and technical competence in ultrasonic testing practices across various industries.

Keywords: ASTM E494-20, ultrasonic velocity measurement, pulse-echo method, non-destructive testing, A-scan display, longitudinal wave velocity, shear wave velocity, reference block, material characterization, ultrasonic testing standard

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: E494 − 20
Standard Practice for
Measuring Ultrasonic Velocity in Materials by Comparative
Pulse-Echo Method
This standard is issued under the fixed designation E494; 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 (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
NOTE 2—Factors including techniques, equipment, types of material,
1. Scope*
and operator variables will result in variations in absolute velocity
1.1 This practice covers a test procedure for measuring
readings, sometimes by as much as 65%. Relative results with a single
ultrasonic velocities in materials with conventional ultrasonic combination of the above factors can be expected to be much more
accurate (probably within a 1% tolerance).
pulse echo flaw detection equipment in which results are
displayed in an A-scan display. This practice describes a
1.5 Units—The values stated in SI units are to be regarded
method whereby unknown ultrasonic velocities in a material
as standard. The values given in parentheses after SI units are
sample are determined by comparative measurements using a
providedforinformationonlyandarenotconsideredstandard.
reference material whose ultrasonic velocities are accurately
1.6 This standard does not purport to address all of the
known.
safety concerns, if any, associated with its use. It is the
1.2 This procedure is intended for solid materials 5 mm
responsibility of the user of this standard to establish appro-
(0.2in.) thick or greater. The surfaces normal to the direction priate safety, health, and environmental practices and deter-
of energy propagation shall be parallel to at least 63°. Surface
mine the applicability of regulatory limitations prior to use.
finish for velocity measurements shall be 3.2 µm (125 µin.) 1.7 This international standard was developed in accor-
root-mean-square (rms) or smoother.
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
NOTE 1—Sound wave velocities are cited in this practice using the
Development of International Standards, Guides and Recom-
fundamental units of meters per second, with inches per second supplied
mendations issued by the World Trade Organization Technical
for reference in many cases. For some calculations, it is convenient to
think of velocities in units of millimeters per microsecond. While these
Barriers to Trade (TBT) Committee.
units work nicely in the calculations, the more natural units were chosen
for use in the tables in this practice. The values can be simply converted
2. Referenced Documents
from m/s to mm/µs by moving the decimal point three places to the left,
that is, 3500 m/s becomes 3.5 mm/µs.
2.1 ASTM Standards:
1.3 Ultrasonic velocity measurements are useful for deter- C597Test Method for Pulse Velocity Through Concrete
E317PracticeforEvaluatingPerformanceCharacteristicsof
miningseveralimportantmaterialproperties.Young’smodulus
Ultrasonic Pulse-Echo Testing Instruments and Systems
of elasticity, Poisson’s ratio, acoustic impedance, and several
without the Use of Electronic Measurement Instruments
other useful properties and coefficients can be calculated for
E543Specification forAgencies Performing Nondestructive
solid materials with the ultrasonic velocities if the density is
Testing
known (see Appendix X1).
E797Practice for Measuring Thickness by Manual Ultra-
1.4 More accurate results than those obtained using this
sonic Pulse-Echo Contact Method
method can be obtained with more specialized ultrasonic
E1316Terminology for Nondestructive Examinations
equipment, auxiliary equipment, and specialized techniques.
2.2 ASNT Documents:
Some of the supplemental techniques are described in Appen-
SNT-TC-1A Recommended Practice for Nondestructive
dix X2. (Material contained in Appendix X2 is for informa-
Testing Personnel Qualification and Certification
tional purposes only.)
1 2
This practice is under the jurisdiction of ASTM Committee E07 on Nonde- For referenced ASTM standards, visit the ASTM website, www.astm.org, or
structive Testing and is the direct responsibility of Subcommittee E07.06 on contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Ultrasonic Method. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Dec. 1, 2020. Published January 2021. Originally the ASTM website.
approved in 1973. Last previous edition approved in 2015 as E494–15. DOI: AvailablefromAmericanSocietyforNondestructiveTesting(ASNT),P.O.Box
10.1520/E0494-20. 28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http://www.asnt.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
E494 − 20
ASNI/ASNT-CP-189 Standard for Qualification and Certi- or certifying agency, as applicable. The practice or standard
fication of Nondestructive Testing Personnel used and its applicable revision shall be identified in the
2.3 AIA Document:
contractual agreement between the using parties.
NAS-410Certification and Qualification of Nondestructive
6.3 Qualification of Nondestructive Agencies—If specified
Testing Personnel
in the contractual agreement, NDT agencies shall be qualified
2.4 ISO Standard:
and evaluated as described in Practice E543. The applicable
ISO 9712Non-Destructive Testing—Qualification and Cer-
edition of Practice E543 shall be specified in the contractual
tification of NDT Personnel
agreement.
3. Terminology
6.4 Reporting Criteria—Reporting criteria for the examina-
3.1 Definitions—For definitions of terms used in this
tion results shall be in accordance with 9.1 unless otherwise
practice, see Terminology E1316.
specified.
4. Summary of Practice
7. Apparatus
4.1 Several possible wave modes can propagate in solids.
7.1 The ultrasonic testing system to be used in this practice
This procedure is concerned with two velocities of
shall include the following:
propagation,namelythoseassociatedwithlongitudinal(v)and
l
7.1.1 Test Instrument—Any ultrasonic instrument compris-
transverse (v) waves.The longitudinal velocity is independent
t
ing a time base, transmitter (pulser), receiver (echo amplifier),
of sample geometry when the dimensions at right angles to the
andanA-scanindicatorcircuittogenerate,receive,anddisplay
beam are very large compared with beam width and wave
electrical signals related to ultrasonic waves. Equipment shall
length. The transverse velocity is little affected by physical
allow reading the on-screen positions of A and A (defined in
dimensions of the sample. The procedure described in Section
k l/s
8 is, as noted in the Scope, for use with conventional pulse 8.1.4), along theA-scan base line within 60.5 mm (0.020 in.).
echo flaw detection equipment only. For maximum accuracy, the highest possible frequency that
will present at least two easily distinguishable back wall
5. Significance and Use
reflections, and preferably five, in both materials shall be used.
5.1 This practice describes a test procedure for the applica-
7.1.2 Search Unit—The search unit shall generate and
tion of conventional ultrasonic methods to determine unknown
receive ultrasonic waves of an appropriate size, type, and
ultrasonic velocities in a material sample by comparative
frequency, designed for tests by the contact method. Contact
measurements using a reference material whose ultrasonic
straight beam longitudinal mode shall be used for longitudinal
velocities are accurately known.
velocity measurements, and contact straight beam shear mode
for transverse velocity measurements.
5.2 Although not all methods described in this practice are
applied equally or universally to all velocity measurements in 7.1.3 Couplant—For longitudinal velocity measurements,
different materials, it does provide flexibility and a basis for
the couplant should be the material used in practice, for
establishing contractual criteria between users, and may be example, clean light-grade oil. For transverse velocity
used as a general guideline for preparing a detailed procedure
measurements, a high viscosity material such as resin or solid
or specification for a particular application. bond should be used. In some materials isopolybutene, honey,
or other high-viscosity materials have been used effectively.
5.3 This practice is directed towards the determination of
Most liquids will not support transverse waves. In porous
longitudinal and shear wave velocities using the appropriate
materials special nonliquid couplants are required. The cou-
sound wave form. This practice also outlines methods to
plant must not be deleterious to the material.
determine elastic modulus and can be applied in both contact
7.1.4 Standard Reference Blocks:
and immersion mode.
7.1.4.1 Velocity Standard—Any material of known velocity,
6. Basis of Application
that can be penetrated by the acoustical wave, and that has an
6.1 The following items are subject to contractual agree-
appropriate surface roughness, shape, thickness, and parallel-
ment between the parties using or referencing this practice: ism. The velocity of the standard should be determined by
some other technique of higher accuracy, or by comparison
6.2 Personnel Qualification—If specified in the contractual
with water velocity that is known (see Appendix X2.5 and
agreement, personnel performing to this practice shall be
AppendixX4).Thereferenceblockshouldhaveanattenuation
qualified in accordance with a nationally or internationally
similar to that of the test material.
recognized NDT personnel qualification practice or standard
7.1.4.2 For horizontal linearity check, see Practice E317.
such as ASNI/ASNT-CP-189, SNT-TC-1A, NAS-410,
ISO9712, or a similar document and certified by the employer
8. Procedure
Available fromAerospace IndustriesAssociation ofAmerica, Inc. (AIA), 1250
8.1 Wave Velocity—Determine bulk, longitudinal or shear
Eye St., NW, Washington, DC 20005.
wave velocity (v ) by comparing the transit time of the wave
5 l/s
Available from International Organization for Standardization (ISO), ISO
mode in the unknown material to the transit time of the same
Central Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier,
Geneva, Switzerland, http://www.iso.org. mode in a known velocity standard (v ).
k
E494 − 20
8.1.1 Select samples of unknown and known materials with 9. Report
flat parallel surfaces and measure the thickness of each to an
9.1 The following are data which should be included in a
accuracy of 60.02 mm (0.001 in.) or 0.1%, whichever is
report on velocity measurements:
greater.
9.1.1 Longitudinal Wave:
8.1.2 Align the search unit over each sample and obtain a
9.1.1.1 A =_________m (in.)
k
nominalsignalpattern(seeFig.1)ofasmanybackwallechoes
9.1.1.2 n =_________
k
as are clearly defined. The time base (sweep control) must be
9.1.1.3 t =_________m (in.)
k
set the same for both measurements.
9.1.1.4 v =_________m⁄s (in.⁄s)
k
8.1.3 Using a scale or caliper, measure the distance at the
9.1.1.5 A =_________m (in.)
u
base line between the leading edge of the first back wall echo
9.1.1.6 n =_________
u
andtheleadingedgeofthelastbackechothatisclearlydefined
9.1.1.7 t =_________m (in.)
u
on the known and unknown samples. For better accuracy,
9.1.1.8 v(using Eq 1)=___m⁄s (in./s)
l
adjusttheamplitudeofthelastbackechobymeansofthegain
9.1.2 Transverse Wave:
controltoapproximatelythesameheightasthefirstbackecho,
9.1.2.1 A =_________m (in.)
k
after the position of the leading edge of the first back echo has
9.1.2.2 n =_________
k
been fixed. This allows more accurate time or distance mea-
9.1.2.3 t =_________m (in.)
k
surements. The position of the leading edge of the last back
9.1.2.4 v =_________m⁄s (in.⁄s)
k
echo is then determined. The signal has traversed a distance
9.1.2.5 A =_________m (in.)
u
twice the thickness of the specimen between each back echo.
9.1.2.6 n =_________
u
The signal traversing the specimen and returning is called a
9.1.2.7 t =_________m (in.)
u
round trip. In Fig. 1, the signal has made six round trips
9.1.2.8 v (using Eq 1)=___m⁄s (in./s)
s
between Echo 1 and Echo 7. Count the number of round trips
9.1.2.9 Displacement orientation = _________°
fromfirstechousedtothelastechomeasuredonbothsamples.
9.1.3 Timebase (sweep control)
This number will be one less than the number of echoes used.
9.1.4 Horizontal linearity
Note that the sample thickness, number of round trips, and
9.1.5 Test frequency
distance from front to last back echo measured need not be the
9.1.6 Couplant
same between the measurements in the unknown and known
9.1.7 Search unit:
materials.
9.1.7.1 Frequency
8.1.4 Calculate the value of the unknown velocity as fol-
9.1.7.2 Size
lows:
9.1.7.3 Shape
A n t v
~ ! 9.1.7.4 Type
k u u k
v 5 (1)
u
~A n t ! 9.1.7.5 Serial number
u k k
9.1.8 Sample geometry
where:
9.1.9 Instrument:
A = distance from first to Nth back echo on the known
k
9.1.9.1 Name
material, m (in.), measured along the baseline of the
9.1.9.2 Model number
A-scan display,
9.1.9.3 Serial number
n = number of round trips, known material,
k
9.1.9.4 Pertinent control settings
t = thickness of known material, m (in.),
k
v = velocity in known material, m/s (in./s),
k
10. Technical Hazards
A = distance from the first to the Nth back echo on the
u
10.1 Material Properties—Both the known and unknown
unknown material, m (in.), measured along the base-
samplesshouldbehomogeneous,isotropicmaterials.Measure-
line of the A-scan display,
ments should be taken at multiple locations in multiple
n = number of round trips, unknown material, and
u
orientations to ensure consistency. Best results are obtained
t = thickness, unknown material, m (in.).
u
NOTE 3—The units used in measurement are not significant as long as from samples that are low textured and fine grained.
the system is consistent.
10.1.1 Processed Materials—Rolled or otherwise processed
materials may have texture and residual stresses that may limit
the accuracy of this method.
10.1.2 Composite Materials—Fibrous composite materials
should never be considered isotropic.This method may still be
useful if both the known and unknown samples have the exact
same lay-up. The fiber orientation will not affect the through-
thickness longitudinal velocity, but it will affect the shear
velocity. Therefore, the displacement of the propagating shear
wave should be noted.
10.2 Shear Measurements—Shear measurements should
generally be performed using a normal incidence shear-wave
FIG. 1 Initial Pulse and 7 Back Echoes search unit.
E494 − 20
10.3 Dispersion—Dispersion will increase the transit time 11. Keywords
for each successive back wall echo and will spread out the
11.1 measure of ultrasonic velocity; nondestructive testing;
received wave pulse. Although dispersion is more obvious in
ultrasonic properties of materials; ultrasonic thickness gauges;
some materials, most materials (including metals) exhibit
ultrasonic velocity
dispersion. It is therefore recommended that the procedure
listed in Section 8 be repeated for different combinations of
echoes (for example, 1-7, 2-3, 1-4).
APPENDIXES
(Nonmandatory Information)
X1. FORMULAS
X1.1 Useofthetechniqueinthispracticewillgiveresultsin where:
2 2
some instances which are only approximate calculations. The
z = acoustic impedance (kg/m · s (or lb/in. · s)).
determination of longitudinal and transverse velocity of sound
X1.1.4 Shear Modulus (see Note X1.3):
in a material makes it possible to approximately calculate the
G 5 ρv
elastic constants, Poisson’s ratio, elastic moduli, acoustic
s
impedance, reflection coefficient, and transmission coefficient.
X1.1.5 Bulk Modulus (see Note X1.3):
In this Appendix, the formulas for calculating some of these
2 2
K 5 ρ v 2 4/3 v
@ ~ ! #
l s
factors are as follows (see Note X1.1):
X1.1.6 Reflection Coeffıcient for Energy (R):
X1.1.1 Poisson’s Ratio:
2 2
2 2
R 5 Z 2 Z / Z 1Z
~ ! ~ !
σ 5 1 22 v /v /2 1 2 v /v
@ ~ ! # @ ~ ! # 2 1 2 1
s l s l
where:
where:
Z = acoustic impedance in Medium 1, and
σ = Poisson’s ratio, 1
Z = acoustic impedance in Medium 2.
v = ultrasonic transverse velocity, m/s (or in./s), and
s
v = ultrasonic longitudinal velocity, m/s (or in./s).
l
X1.1.7 Transmission Coeffıcient for Energy (T):
X1.1.2 Young’s Modulus of Elasticity:
T 5 ~4Z Z !/~Z 1Z !
2 1 2 1
2 2 2 2 2
E 5 @ρv 3v 24v #/ v 2 v
~ ! ~ !
s l s l s
NOTE X1.1—The dynamic elastic constants may differ from those
determined by static tensile measurements. In the case of metals,
where:
ceramics, and glasses, the differences are of the order of 1%, and may be
3 3
ρ = density, kg/m (or lb/in. ),
corrected by known theoretical formulas. For plastics the differences may
v = longitudinal velocity, m/s (or in./s),
be larger, but can be corrected by correlation.
l
2 −4 2
v = transverse velocity, m/s (or in./s), and
s NOTE X1.2—Conversion factor: 1 N/m =1.4504×10 lb/in. .
2 2
E = Young’s modulus of elasticity, N/m (or lb/in. ) (see
NOTE X1.3—When using pounds per cubic inch for density and inches
Notes X1.2 and X1.3).
per second for velocity, results must be divided by g (acceleration due to
gravity)toobtainresultsinpoundspersquareinchfor E, G,or Kandalso
X1.1.3 Acoustic Impedance (see Note X1.3):
to obtain results for Z in pounds per square inch per second.Acceleration
z 5 ρ v due to gravity (g)=386.4 in./s · s.
l
X2. IMPORTANT TECHNIQUES FOR MEASURING ULTRASONIC VELOCITY IN MATERIALS
X2.1 Introduction which can be used for these measurements even though
designed primarily for other measurements (for example,
X2.1.1 Several techniques are available for precise mea-
thickness gauges).
surement of ultrasonic velocity in materials. Most of these
techniques require specialized or auxiliary equipment.
X2.1.3 Various methods have been introduced to solve the
X2.1.2 Instruments are available commercially which auto- problem of the accurate measurement of time interval or
matically measure sound velocity or time interval or both. number of waves in the specimen. It would be beyond the
There is a growing list of manufacturers who make ultrasonic scope of this Appendix to attempt to include all these tech-
instruments, including pulser, receiver, and display designed niques. However, it is considered of value to those using this
specifically for making these measurements automatically or practicetoknowsomeofthesetechniques.ThisAppendixwill
E494 − 20
be useful to those who have more refined equipment or shouldbeequaltothatwhichisrequiredforthespecimen.The
auxiliary equipment available and to those who wish more attenuation member must be inserted between the interferom-
accurate results. eter probe and the cable. It serves to change the height of the
interferometer echo independently of other conditions of test.
X2.1.4 ThisAppendixwillincludesometechniquesthatare
only suitable for the laboratory. It is only under strictly X2.5.4 A reflector dips into the tank containing the liquid
controlled conditions such as are available in the laboratory and is held on an adjustable mechanism so that it cannot be
thatthegreatestaccuracycanbeachieved.Suchmeasurements tilted. This mechanism can be moved to and fro rapidly by
may be slow and require very carefully prepared specimens.A disengagement.The fine adjustment is carried out by means of
list of references (1-28) is provided for more detailed infor- a spindle. One complete revolution of the spindle changes the
mation. travel path by 1 mm. One scale division of the spindle knob
represents ⁄100 mm (0.0004 in.).
X2.2 Special Features Built Into the Ultrasonic Equip-
X2.5.5 The tank must be filled with liquid in which the
ment
ultrasonic velocity is known. In the case of water at 20°C,
X2.2.1 Ultrasonic equipment is available that provides
velocity=1483.1 m/s. The temperature coefficient is ∆v/∆t
means adequate for the measurement of acoustic wave propa-
=+2.5 m/s ·°C. A check of the temperature in the case of
gation with respect to time.
water is therefore absolutely necessary (see Appendix X4).
X2.5.6 Mixtures can also be used, for example, water
X2.3 Precision Oscilloscope
alcohol (18% weight percentage), whose temperature coeffi-
X2.3.1 An auxiliary precision cathode ray oscilloscope can
cient is zero at room temperature.
be used to observe the echo pattern. Using the precision
X2.5.7 Calculate velocity as follows:
standardized horizontal display of the oscilloscope, the transit
time between successive multiple back reflections is deter-
Velocity ~m/s! 3Thickness ~m!
water x
Velocity m/s 5
~ !
x
mined. Calculate velocity as follows: Distance m
~ !
inwater
Velocity ~m/s ~orin./s!! 5 @2 thickness ~m orin.!#/@Time ~s!#
or
Velocity in./s 3Thickness in.
~ ! ~ !
X2.4 Electronic Time Marker water x
Velocity in./s 5
~ !
x
Distance ~in.!
inwater
X2.4.1 An accessory is frequently available that displays
one or more visual marks, usually a step, on the display of the
X2.6 Pulse Velocity Through Concrete (see Test Method
basic instrument. It is usually superimposed on the standard
C597)
echo pattern. The mark is moved using a standardized control.
X2.6.1 Frequency of pulse generator 10 to 50 kHz—
The control reads time directly in microseconds.
Repetitive pulses at rate not less than 50/s.
X2.4.2 Thetechniqueistoalignthesteponthedisplay,first
X2.6.2 Press the faces of the search units against the faces
with the first back reflection, and then, using the second
of the concrete after establishing contact through a coupling
marker, if available, with the second back reflection. Based on
medium. Wetting the concrete with water, oil, or other viscous
control readings at both instances, the elapsed time for a round
materials may be used to exclude entrapped air from between
trip through the specimen is determined. (Calculation is the
the contact surfaces of the diaphragms of the search unit and
same as in X2.3).
the surface of the concrete. Measure the length of the shortest
direct path between the centers of the diaphragms and the time
X2.5 Ultrasonic Interferometer (Velocity Comparator)
of travel on the A-scan display by aligning the strobe marker
X2.5.1 The measurement of ultrasonic velocity is carried
pulse opposite the received wave front and reading the stan-
out by comparing transmission times of a pulse in a specimen
dardized dial, or by counting the number of cycles of the
and in the comparison travel path. The ultrasonic velocities in
timing wave between the transmitted and received pulse.
liquids (for example, water) are well known and consequently
the velocity in the specimen can be determined with an
X2.7 Pulse Echo Twin-Probe Method
accuracy of about 0.1%.
X2.7.1 This method uses a single-probe housing containing
X2.5.2 In practice, the echo in the specimen is made to
two dedicated elements: one a sender, the other a receiver. If
coincide with the echo from the interferometer travel path
the unit is so equipped, the dual mode is selected to ensure the
which is obtained by altering the latter to the point of
proper pulser and display mode are enabled.
interference. The ultrasonic velocities of the specimen and
X2.7.2 Since ultrasonic velocity measurements are princi-
interferometer liquid are in the ratio of their lengths and these
pally measurements of time, based on the thickness of a
two quantities must be exactly measurable.
specimen, and since many thickness measuring instruments
X2.5.3 A normal probe is clamped to the open tank by
successfully measure thickness to a high degree of accuracy
means of a clasp on one side. The frequency of the probe
using this method it seems appropriate to include this method
of velocity measurement in the practice.
The boldface numbers in parentheses refer to the list of references appended to NOTE X2.1—With the twin-probe method the pulse-echo transit time is
this practice. a non-linear function of specimen thickness, which may introduce
E494 − 20
significant errors when that technique is used for velocity measurements.
instrument. Check the actual thickness of the test area with
The non-linearity is discussed in Practice E797. Errors in velocity
calipers or a micrometer.
measurement can be minimized by use of a reference block having both
X2.7.6.3 Calculate unknown velocity as follows:
velocityandthicknessnearlyequaltothatofthespecimentobemeasured.
Single search unit systems are generally more suitable for precision
actualthickness
V 5 V steel 3
velocity measurements.
x
indicatedthickness
X2.7.3 All instruments where the twin-probe method of
where:
thickness measurement is recommended, including A-scan
V = unknown velocity.
display units as well as meter read-out units, have precisely x
standardized scales. The parallax problem is removed from
X2.8 Harmonic Wave Method (Zero Method)
manyoftheA-scandisplayunitssincethescaleisengravedon
the inside face of the display or is integral with the output
X2.8.1 Wallthicknessmeasurementbymeansofultrasonic,
signal. Parallax is not a major problem with meter read-out
echo-sounding instruments will become inaccurate if only a
units or digital read-out units.
few echoes can be utilized because of either high absorption,
corrosion,orunfavorableradiationgeometry.Insuchcases,the
X2.7.4 Most twin-probe thickness measuring instruments
accuracy of the results can be improved by the tuning of the
use the first echo for measurement read-out. Thus the test
wall thickness meter to the harmonic waves of the echo
rangesareusuallyfixedandpreciselystandardized.Thereisno
frequency (harmonic wave method).
need to produce several back echoes to obtain an average
transit time.
X2.8.2 Up to now the interferometer method has been used
X2.7.5 Specimens with curved surfaces present less mea- for the precision measurement of sound propagation. Further
development of the harmonic wave method can replace the
suringproblemsasthefirstbackechoismorerepresentativeof
depth or time than a later back echo, say the fifth from a tube rather complicated and time-consuming interferometer method
inallthosecaseswheretheultimateaccuracyofthelatterisnot
wall.Insmalldiametertubingtheerrormaybegreaterthanfor
required. Under normal conditions, a measuring accuracy of
equivalent flat specimens.
0.5% or better can be obtained with the so-called “zero-
X2.7.6 Procedure:
method.”
X2.7.6.1 Standardize the instrument and probe on a step
block of known velocity. By adjustment of sweep delay and X2.8.3 Amodification of the method utilizes bursts of radio
range controls, ensure that thickness readings for two or more frequency(rf)radiatedfromasearchunitintoalongbufferrod
thicknesses(highandlow)occurattheirproperdistances(Fig. and then into the sample, which is a few wavelengths thick.
X2.1). The instrument and probe are properly standardized for The buffer rod is long enough to contain the entire rf bursts,
(1020or1095)steelat5900m/s(2.32×10 in./s)(see7.1.4.1). while the burst is long enough to occupy the three round-trips
X2.7.6.2 Measure the thickness of part with unknown ve- in the specimen. Thus the burst interferes with itself as it
locity without changing sweep or range controls on the reverberates within the specimen. One characteristic echo
FIG. X2.1 Instrument Setup to Avoid Errors Due to Parallax
E494 − 20
pattern occurs when the round-trip distance in the specimen is
γ = a phase angle associated with wave reflected at the
equal to an odd number of half-wavelengths; an even number
search unit end, and
gives a different pattern. The two patterns alternate as the rf
p = an integer (1, 2, 3. . .).
frequency is changed. One plots phase versus frequency in
Since T is approximately some multiple of the round-trip
unitsofcyclesversusMHz.Onecycleofphaseoccursforeach
delaytimeδ,theappliedpulseoccursonceforeveryroundtrip
repetitionofoneofthecharacteristicpatterns;betweenthetwo
delay for p=1. Usually, a number of measurements of T at
patterns there is ⁄2 cycle of phase. The slope of the phase
different frequencies between f, the resonance frequency of the
versus frequency line is the delay time t in microseconds, and
search unit, and 0.9 f are made to obtain the difference in T
r
for a specimen of thickness L, the velocity is
between f and another frequency f. The negative value of ∆T
r
v 52L/t
that is smallest in magnitude corresponds to n=0; except for
specimens of very low mechanical impedance, the delay time
X2.9 Phase Comparison Method
is then given by δ= T+(γ⁄2πf). The velocity in the sample is
X2.9.1 Thismethodconsistsofsuperimposingtheechoesof V=2L/δ, where L is the sample length.
two pulses which have made different numbers of round trips.
X2.10.2 The advantage of this particular method is that the
Iftheechoesaremadeexactlyinphasebyacriticaladjustment
coupling to the search unit is taken into account so that this
of frequency, the expression for phase angles may be written
method is well suited to measurements aimed at pressure and
as:
temperaturevariations.Withthismethod,theeffectofcoupling
γ 2 @ 2LW /v#522 πn between search unit and specimen can be made negligibly
~ !
n
small. So far as the accuracy of this method is concerned, it is
where:
within a few parts in 10 in ideal conditions, while that of the
v = velocity of propagation, 4
phase comparison in X2.9 is within one part in 10 . In this
W =2 π times resonant frequency (f ),
n n
method, however, it is possible to send a strong signal into the
L = thickness,
specimen, so that the velocity measurement can be made even
n = number of waves, and
if the attenuation is high.
γ = phaseangleduetothesealbetweenthesearchunitand
X2.10.3 The limitation of both techniques is expected to
the specimen.
depend on various factors besides porosity, such as grain size
Consequently, the velocity is expressed by:
and grain boundary conditions.
v 5 ~2Lf !/@n1~γ/2π!#
n
X2.11 Phase Velocity by Pulse-Echo-Overlap Method
...

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: E494 − 15 E494 − 20
Standard Practice for
Measuring Ultrasonic Velocity in Materials by Comparative
Pulse-Echo Method
This standard is issued under the fixed designation E494; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope*
1.1 This practice covers a test procedure for measuring ultrasonic velocities in materials with conventional ultrasonic pulse echo
flaw detection equipment in which results are displayed in an A-scan display. This practice describes a method whereby unknown
ultrasonic velocities in a material sample are determined by comparative measurements using a reference material whose ultrasonic
velocities are accurately known.
1.2 This procedure is intended for solid materials 5 mm (0.2 in.) thick or greater. The surfaces normal to the direction of energy
propagation shall be parallel to at least 63°. Surface finish for velocity measurements shall be 3.2 μm (125 μin.) root-mean-square
(rms) or smoother.
NOTE 1—Sound wave velocities are cited in this practice using the fundamental units of metresmeters per second, with inches per second supplied for
reference in many cases. For some calculations, it is convenient to think of velocities in units of millimetresmillimeters per microsecond. While these
units work nicely in the calculations, the more natural units were chosen for use in the tables in this practice. The values can be simply converted from
m/s to mm/μs by moving the decimal point three places to the left, that is, 3500 m/s becomes 3.5 mm/μs.
1.3 Ultrasonic velocity measurements are useful for determining several important material properties. Young’s modulus of
elasticity, Poisson’s ratio, acoustic impedance, and several other useful properties and coefficients can be calculated for solid
materials with the ultrasonic velocities if the density is known (see Appendix X1).
1.4 More accurate results than those obtained using this method can be obtained with more specialized ultrasonic equipment,
auxiliary equipment, and specialized techniques. Some of the supplemental techniques are described in Appendix X2. (Material
contained in Appendix X2 is for informational purposes only.)
NOTE 2—Factors including techniques, equipment, types of material, and operator variables will result in variations in absolute velocity readings,
sometimes by as much as 5 %.65 %. Relative results with a single combination of the above factors can be expected to be much more accurate (probably
within a 1 % tolerance).
1.5 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided
for information only and are not considered standard.
1.6 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.
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 Dec. 1, 2015Dec. 1, 2020. Published December 2015January 2021. Originally approved in 1973. Last previous edition approved in 20102015
as E494 - 10.E494 – 15. DOI: 10.1520/E0494-15.10.1520/E0494-20.
*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
E494 − 20
1.7 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:
C597 Test Method for Pulse Velocity Through Concrete
E317 Practice for Evaluating Performance Characteristics of Ultrasonic Pulse-Echo Testing Instruments and Systems without the
Use of Electronic Measurement Instruments
E543 Specification for Agencies Performing Nondestructive Testing
E797 Practice for Measuring Thickness by Manual Ultrasonic Pulse-Echo Contact Method
E1316 Terminology for Nondestructive Examinations
2.2 ASNT Documents:
SNT-TC-1A Recommended Practice for Nondestructive Testing Personnel Qualification and Certification
ASNI/ASNT-CP-189 Standard for Qualification and Certification of Nondestructive Testing Personnel
2.3 AIA Document:
NAS-410 Certification and Qualification of Nondestructive Testing Personnel
2.4 ISO Standard:
ISO 9712 Non-Destructive Testing—Qualification and Certification of NDT Personnel
3. Terminology
3.1 Definitions—For definitions of terms used in this practice, see Terminology E1316.
3.1 Definitions—For definitions of terms used in this practice, see Terminology E1316.
4. Summary of Practice
4.1 Several possible wave modes of vibration can propagate in solids. This procedure is concerned with two velocities of
propagation, namely those associated with longitudinal (v ) and transverse (v ) waves. The longitudinal velocity is independent of
l t
sample geometry when the dimensions at right angles to the beam are very large compared with beam areawidth and wave length.
The transverse velocity is little affected by physical dimensions of the sample. The procedure described in Section 8 is, as noted
in the scope,Scope, for use with conventional pulse echo flaw detection equipment only.
5. Significance and Use
5.1 This practice describes a test procedure for the application of conventional ultrasonic methods to determine velocity in
materials wherein unknown ultrasonic velocities in a material sample are determined by comparative measurements using a
reference material whose ultrasonic velocities are accurately known.
5.2 Although not all methods described in this practice are applied equally or universally to all velocity measurements in different
materials, it does provide flexibility and a basis for establishing contractual criteria between users, and may be used as a general
guideline for preparing a detailed procedure or specification for a particular application.
5.3 This practice is directed towards the determination of longitudinal and shear wave velocities using the appropriate sound wave
form. This practice also outlines methods to determine elastic modulus and can be applied in both contact and immersion mode.
6. Basis of Application
6.1 The following items are subject to contractual agreement between the parties using or referencing this practice:
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), 1250 Eye St., NW, Washington, DC 20005.
Available from International Organization for Standardization (ISO), ISO Central Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier, Geneva,
Switzerland, http://www.iso.org.
E494 − 20
6.2 Personnel Qualification—If specified in the contractual agreement, personnel performing to this practice shall be qualified in
accordance with a nationally or internationally recognized NDT personnel qualification practice or standard such as ASNI/ASNT-
CP-189, SNT-TC-1A, NAS-410, ISO 9712, 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.
6.3 Qualification of Nondestructive Agencies—If specified in the contractual agreement, NDT agencies shall be qualified and
evaluated as described in Practice E543. The applicable edition of Practice E543 shall be specified in the contractual agreement.
6.4 Reporting Criteria—Reporting criteria for the examination results shall be in accordance with 9.1 unless otherwise specified.
7. Apparatus
7.1 The ultrasonic testing system to be used in this practice shall include the following:
7.1.1 Test Instrument—Any ultrasonic instrument comprising a time base, transmitter (pulser), receiver (echo amplifier), and an
A-scan indicator circuit to generate, receive, and display electrical signals related to ultrasonic waves. Equipment shall allow
reading the on-screen positions of A , and A , A , A (defined in 8.1.4 and 8.2.4), along the A-scan base line within 60.5 mm
k sl/s t l
(0.020 in.). For maximum accuracy, the highest possible frequency that will present at least two easily distinguishable back
echowall reflections, and preferably five, in both materials shall be used.
7.1.2 Search Unit—The search unit shall generate and receive ultrasonic waves of an appropriate size, type, and frequency,
designed for tests by the contact method. Contact straight beam longitudinal mode shall be used for longitudinal velocity
measurements, and contact straight beam shear mode for transverse velocity measurements.
7.1.3 Couplant—For longitudinal velocity measurements, the couplant should be the material used in practice, for example, clean
light-grade oil. For transverse velocity measurements, a high viscosity material such as resin or solid bond should be used. In some
materials isopolybutene, honey, or other high-viscosity materials have been used effectively. Most liquids will not support
transverse waves. In porous materials special nonliquid couplants are required. The couplant must not be deleterious to the
material.
7.1.4 Standard Reference Blocks:
7.1.4.1 Velocity Standard—Any material of known velocity, that can be penetrated by the acoustical wave, and that has an
appropriate surface roughness, shape, thickness, and parallelism. The velocity of the standard should be determined by some other
technique of higher accuracy, or by comparison with water velocity that is known (see Appendix X2.5 and Appendix X4). The
reference block should have an attenuation similar to that of the test material.
7.1.4.2 For horizontal linearity check, see Practice E317.
8. Procedure
8.1 Longitudinal Wave Velocity—Determine bulk, longitudinal or shear wave velocity (v ) by comparing the transit time of a
ll/s
longitudinal the wave mode in the unknown material to the transit time of ultrasound the same mode in a known velocity standard
(v ).
k
8.1.1 Select samples of each unknown and known materials with flat parallel surfaces and measure the thickness of each to an
accuracy of 60.02 mm (0.001 in.) or 0.1 %, whichever is greater.
8.1.2 Align the search unit over each sample and obtain a nominal signal pattern (see Fig. 1) of as many back wall echoes as are
clearly defined. The time base (sweep control) must be set the same for both measurements.
8.1.3 Using a scale or caliper, measure the distance at the base line between the leading edge of the first back wall echo and the
leading edge of the last back echo that is clearly defined on the known and unknown sample.samples. For better accuracy, adjust
the amplitude of the last back echo by means of the gain control to approximately the same height as the first back echo, after the
position of the leading edge of the first back echo has been fixed. This allows more accurate time or distance measurements. The
position of the leading edge of the last back echo is then determined. The signal has traversed a distance twice the thickness of
the specimen between each back echo. The signal traversing the specimen and returning is called a round trip. In Fig. 1, the signal
E494 − 20
FIG. 1 Initial Pulse and 7 Back Echoes
has made six round trips between Echo 1 and Echo 7. Count the number of round trips from first echo used to the last echo
measured on both samples. This number will be one less than the number of echoes used. Note that the sample thickness, number
of round trips, and distance from front to last back echo measured need not be the same.same between the measurements in the
unknown and known materials.
8.1.4 Calculate the value of the unknown velocity as follows:
v 5 A n t v / A n t (1)
~ ! ~ !
1 k l l k l k k
~A n t v !
k u u k
v 5 (1)
u
A n t
~ !
u k k
where:
A = distance from first to Nth back echo on the known material, m (in.), measured along the baseline of the A-scan display,
k
n = number of round trips, unknown material,
l
n = number of round trips, known material,
k
t = thickness of unknown material, m (in.),
l
t = thickness of known material, m (in.),
k
v = velocity in known material, m/s (in./s),
k
A = distance from the first to the Nth back echo on the unknown material, m (in.), measured along the baseline of the A-scan
l
display,
A = distance from the first to the Nth back echo on the unknown material, m (in.), measured along the baseline of the A-scan
u
display,
n = number of round trips, known material, and
k
n = number of round trips, unknown material, and
u
t = thickness, known material, m (in.).
k
t = thickness, unknown material, m (in.).
u
NOTE 3—The units used in measurement are not significant as long as the system is consistent.
8.2 Transverse Velocity—Determine transverse velocity (v ) by comparing the transit time of a transverse wave in an unknown
s
material to the transit time of a transverse wave in a material of known velocity (v ).
t
8.2.1 Select samples of each with flat parallel surfaces and measure the thickness of each to an accuracy of 60.02 mm (0.001 in.)
or 0.1 %, whichever is greater.
8.2.2 Align the search unit (see Fig. 1) over each sample and obtain an optimum signal pattern of as many back echoes as are
clearly defined. The time base (sweep control) must be the same for both measurements.
8.2.3 Using a scale or caliper measure the distance at the base line between the leading edge of the first back echo and the leading
edge of the last back echo that is clearly defined on the known and unknown sample. For better accuracy, adjust the amplitude of
the last back echo by means of the gain control to approximately the same height as the first back echo, after the position of the
leading edge of the first back echo has been fixed. This adds high-frequency components of the signal which have been attenuated.
Then determine the position of the leading edge of the last back echo. Count the number of round trips from first echo used to the
last echo measured on both samples. This number will be one less than the number of echoes used. Note that the sample thickness,
number of round trips, and distance from first to last back echo measured need not be the same.
8.2.4 Calculate the value of the unknown velocity as follows:
v 5 A n t v / A n t (2)
~ ! ~ !
s t s s t s t t
E494 − 20
where:
A = distance from first to Nth back echo on the known material, m (in.), measured along the baseline of the A-scan display,
t
n = number of round trips, unknown material,
s
t = thickness of unknown material, m (in.),
s
v = velocity of transverse wave in known material, m/s (in./s),
t
A = distance from the first to the Nth back echo on the unknown material, m (in.), measured along the baseline of the A-scan
s
display,
n = number of round trips, known material, and
t
t = thickness, known material, m (in.). (See Note 3).
t
9. Report
9.1 The following are data which should be included in a report on velocity measurements:
9.1.1 Longitudinal Wave:
9.1.1.1 A = _________m (in.)
k
9.1.1.2 n = _________
lk
9.1.1.3 t = _________m (in.)
lk
9.1.1.4 v = _________m ⁄s (in. ⁄s)
k
9.1.1.5 A = _________m (in.)
lu
9.1.1.6 n = _________
ku
9.1.1.7 t = _________m (in.)
ku
9.1.1.8 v (using Eq 1) = ___m ⁄s (in./s)
l
9.1.2 Transverse Wave:
9.1.2.1 A = _________m (in.)
tk
9.1.2.2 n = _________
sk
9.1.2.3 t = _________m (in.)
sk
9.1.2.4 v = _________m ⁄s (in. ⁄s)
tk
9.1.2.5 A = _________m (in.)
su
9.1.2.6 n = _________
tu
9.1.2.7 t = _________m (in.)
tu
9.1.2.8 v (using Eq 21) = ___m ⁄s (in./s)
s
9.1.2.9 Displacement orientation = _________°
9.1.3 Timebase (sweep control)
9.1.4 Horizontal linearity
9.1.5 Test frequency
E494 − 20
9.1.6 Couplant
9.1.7 Search unit:
9.1.7.1 Frequency
9.1.7.2 Size
9.1.7.3 Shape
9.1.7.4 Type
9.1.7.5 Serial number
9.1.8 Sample geometry
9.1.9 Instrument:
9.1.9.1 Name
9.1.9.2 Model number
9.1.9.3 Serial number
9.1.9.4 Pertinent control settings
10. Technical Hazards
10.1 Material Properties—Both the known and unknown samples should be homogeneous, isotropic materials. Measurements
should be taken at multiple locations in multiple orientations to ensure consistency. Best results are obtained from samples that
are low textured and fine grained.
10.1.1 Processed Materials—Rolled or otherwise processed materials may have texture and residual stresses that may limit the
accuracy of this method.
10.1.2 Composite Materials—Fibrous composite materials should never be considered isotropic. This method may still be useful
if both the known and unknown samples have the exact same lay-up. The fiber orientation will not affect the through-thickness
longitudinal velocity, but it will affect the shear velocity. Therefore, the displacement of the propagating shear wave should be
noted.
10.2 Shear Measurements—Shear measurements should generally be performed using a normal incidence shear-wave search unit.
10.3 Dispersion—Dispersion will increase the transit time for each successive back wall echo and will spread out the received
wave pulse. Although dispersion is more obvious in some materials, most materials (including metals) exhibit dispersion. It is
therefore recommended that the procedure listed in Section 8 be repeated for different combinations of echoes (for example, 1-7,
2-3, 1-4).
11. Keywords
11.1 measure of ultrasonic velocity; nondestructive testing; ultrasonic properties of materials; ultrasonic thickness gages;gauges;
ultrasonic velocity
E494 − 20
APPENDIXES
(Nonmandatory Information)
X1. FORMULAS
X1.1 Use of the technique in this practice will give results in some instances which are only approximate calculations. The
determination of longitudinal and transverse velocity of sound in a material makes it possible to approximately calculate the elastic
constants, Poisson’s ratio, elastic moduli, acoustic impedance, reflection coefficient, and transmission coefficient. In this Appendix,
the formulas for calculating some of these factors are as follows (see Note X1.1):
X1.1.1 Poisson’s Ratio:
2 2
σ5 12 2 v /v /2 12 v /v
@ ~ ! # @ ~ ! #
s l s l
where:
σ = Poisson’s ratio,
v = ultrasonic transverse velocity, m/s (or in./s), and
s
v = ultrasonic longitudinal velocity, m/s (or in./s).
l
X1.1.2 Young’s Modulus of Elasticity:
2 2 2 2 2
E 5 @ρv ~3v 2 4v !#/~v 2 v !
s l s l s
where:
3 3
ρ = density, kg/m (or lb/in. ),
v = longitudinal velocity, m/s (or in./s),
l
v = transverse velocity, m/s (or in./s), and
s
2 2
E = Young’s modulus of elasticity, N/m (or lb/in. ) (see Notes X1.2 and X1.3).
X1.1.3 Acoustic Impedance (see Note X1.3):
z 5 ρ v
l
where:
2 2
z = acoustic impedance (kg/m · s (or lb/in. · s)).
X1.1.4 Shear Modulus (see Note X1.3):
G 5 ρv
s
X1.1.5 Bulk Modulus (see Note X1.3):
2 2
K 5 ρ v 2 4/3 v
@ ~ ! #
l s
X1.1.6 Reflection Coeffıcient for Energy (R):
2 2
R 5 Z 2 Z / Z 1Z
~ ! ~ !
2 1 2 1
E494 − 20
where:
Z = acoustic impedance in Medium 1, and
Z = acoustic impedance in Medium 2.
X1.1.7 Transmission Coeffıcient for Energy (T):
T 5 4Z Z / Z 1Z
~ ! ~ !
2 1 2 1
NOTE X1.1—The dynamic elastic constants may differ from those determined by static tensile measurements. In the case of metals, ceramics, and glasses,
the differences are of the order of 1 %, and may be corrected by known theoretical formulas. For plastics the differences may be larger, but can be
corrected by correlation.
2 −4 2
NOTE X1.2—Conversion factor: 1 N/m = 1.4504 × 10 lb/in. .
NOTE X1.3—When using pounds per cubic inch for density and inches per second for velocity, results must be divided by g (acceleration due to gravity)
to obtain results in pounds per square inch for E, G, or K and also to obtain results for Z in pounds per square inch per second. Acceleration due to gravity
(g) = 386.4 in./s · s.
X2. IMPORTANT TECHNIQUES FOR MEASURING ULTRASONIC VELOCITY IN MATERIALS
X2.1 Introduction
X2.1.1 Several techniques are available for precise measurement of ultrasonic velocity in materials. Most of these techniques
require specialized or auxiliary equipment.
X2.1.2 Instruments are available commercially which automatically measure sound velocity or time interval or both. There is a
growing list of manufacturers who make ultrasonic instruments, including pulser, receiver, and display designed specifically for
making these measurements automatically or which can be used for these measurements even though designed primarily for other
measurements (for example, thickness gauges).
X2.1.3 Various methods have been introduced to solve the problem of the accurate measurement of time interval or number of
waves in the specimen. It would be beyond the scope of this Appendix to attempt to include all these techniques. However, it is
considered of value to those using this practice to know some of these techniques. This Appendix will be useful to those who have
more refined equipment or auxiliary equipment available and to those who wish more accurate results.
X2.1.4 This Appendix will include some techniques that are only suitable for the laboratory. It is only under strictly controlled
conditions such as are available in the laboratory that the greatest accuracy can be achieved. Such measurements may be slow and
require very carefully prepared specimens. A list of references (1-28) is provided for more detailed information.
X2.2 Special Features Built Into the Ultrasonic Equipment
X2.2.1 Ultrasonic equipment is available that provides means adequate for the measurement of acoustic wave propagation with
respect to time.
The boldface numbers in parentheses refer to the list of references appended to this practice.
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X2.3 Precision Oscilloscope
X2.3.1 An auxiliary precision cathode ray oscilloscope can be used to observe the echo pattern. Using the precision standardized
horizontal display of the oscilloscope, the transit time between successive multiple back reflections is determined. Calculate
velocity as follows:
Velocity m/s or in./s 5 2 thickness m or in. / Time s
~ ~ !! @ ~ !# @ ~ !#
X2.4 Electronic Time Marker
X2.4.1 An accessory is frequently available that displays one or more visual marks, usually a step, on the display of the basic
instrument. It is usually superimposed on the standard echo pattern. The mark is moved using a standardized control. The control
reads time directly in microseconds.
X2.4.2 The technique is to align the step on the display, first with the first back reflection, and then, using the second marker, if
available, with the second back reflection. Based on control readings at both instances, the elapsed time for a round trip through
the specimen is determined. (Calculation is the same as in X2.3).
X2.5 Ultrasonic Interferometer (Velocity Comparator)
X2.5.1 The measurement of ultrasonic velocity is carried out by comparing transmission times of a pulse in a specimen and in
the comparison travel path. The ultrasonic velocities in liquids (for example, water) are well known and consequently the velocity
in the specimen can be determined with an accuracy of about 0.1 %.
X2.5.2 In practice, the echo in the specimen is made to coincide with the echo from the interferometer travel path which is
obtained by altering the latter to the point of interference. The ultrasonic velocities of the specimen and interferometer liquid are
in the ratio of their lengths and these two quantities must be exactly measurable.
X2.5.3 A normal probe is clamped to the open tank by means of a clasp on one side. The frequency of the probe should be equal
to that which is required for the specimen. The attenuation member must be inserted between the interferometer probe and the
cable. It serves to change the height of the interferometer echo independently of other conditions of test.
X2.5.4 A reflector dips into the tank containing the liquid and is held on an adjustable mechanism so that it cannot be tilted. This
mechanism can be moved to and fro rapidly by disengagement. The fine adjustment is carried out by means of a spindle. One
complete revolution of the spindle changes the travel path by 1 mm. One scale division of the spindle knob represents ⁄100 mm
(0.0004 in.).
X2.5.5 The tank must be filled with liquid in which the ultrasonic velocity is known. In the case of water at 20°C,
velocity = 1483.1 m/s. The temperature coefficient is Δv/Δt = + 2.5 m/s ·°C. A check of the temperature in the case of water is
therefore absolutely necessary (see Appendix X4).
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X2.5.6 Mixtures can also be used, for example, water alcohol (18 % weight percentage), whose temperature coefficient is zero at
room temperature.
X2.5.7 Calculate velocity as follows:
Velocity m/s 3Thickness m
~ ! ~ !
water x
Velocity m/s 5
~ !
x
Distance m
~ !
in water
or
Velocity in./s 3Thickness in.
~ ! ~ !
water x
Velocity in./s 5
~ !
x
Distance ~in.!
in water
X2.6 Pulse Velocity Through Concrete (see Test Method C597)
X2.6.1 Frequency of pulse generator 10 to 50 kHz—Repetitive pulses at rate not less than 50/s.
X2.6.2 Press the faces of the search units against the faces of the concrete after establishing contact through a coupling medium.
Wetting the concrete with water, oil, or other viscous materials may be used to exclude entrapped air from between the contact
surfaces of the diaphragms of the search unit and the surface of the concrete. Measure the length of the shortest direct path between
the centers of the diaphragms and the time of travel on the A-scan display by aligning the strobe marker pulse opposite the received
wave front and reading the standardized dial, or by counting the number of cycles of the timing wave between the transmitted and
received pulse.
X2.7 Pulse Echo Twin-Probe Method
X2.7.1 This method uses a single-probe housing containing two dedicated elements: one a sender, the other a receiver. If the unit
is so equipped, the dual mode is selected to ensure the proper pulser and display mode are enabled.
X2.7.2 Since ultrasonic velocity measurements are principally measurements of time, based on the thickness of a specimen, and
since many thickness measuring instruments successfully measure thickness to a high degree of accuracy using this method it
seems appropriate to include this method of velocity measurement in the practice.
NOTE X2.1—With the twin-probe method the pulse-echo transit time is a non-linear function of specimen thickness, which may introduce significant errors
when that technique is used for velocity measurements. The non-linearity is discussed in Practice E797. Errors in velocity measurement can be minimized
by use of a reference block having both velocity and thickness nearly equal to that of the specimen to be measured. Single search unit systems are
generally more suitable for precision velocity measurements.
X2.7.3 All instruments where the twin-probe method of thickness measurement is recommended, including A-scan display units
as well as meter read-out units, have precisely standardized scales. The parallax problem is removed from many of the A-scan
display units since the scale is engraved on the inside face of the display or is integral with the output signal. Parallax is not a major
problem with meter read-out units or digital read-out units.
X2.7.4 Most twin-probe thickness measuring instruments use the first echo for measurement read-out. Thus the test ranges are
usually fixed and precisely standardized. There is no need to produce several back echoes to obtain an average transit time.
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X2.7.5 Specimens with curved surfaces present less measuring problems as the first back echo is more representative of depth or
time than a later back echo, say the fifth from a tube wall. In small diameter tubing the error may be greater than for equivalent
flat specimens.
X2.7.6 Procedure:
X2.7.6.1 Standardize the instrument and probe on a step block of known velocity. By adjustment of sweep delay and range
controls, ensure that thickness readings for two or more thicknesses (high and low) occur at their proper distances (Fig. X2.1). The
instrument and probe are properly standardized for (1020 or 1095) steel at 5900 m/s (2.32 × 10 in./s) (see 7.1.4.1).
X2.7.6.2 Measure the thickness of part with unknown velocity without changing sweep or range controls on the instrument. Check
the actual thickness of the test area with calipers or a micrometer.
X2.7.6.3 Calculate unknown velocity as follows:
actual thickness
V 5 V steel 3
x
indicated thickness
where:
V = unknown velocity.
x
X2.8 Harmonic Wave Method (Zero Method)
X2.8.1 Wall thickness measurement by means of ultrasonic, echo-sounding instruments will become inaccurate if only a few
FIG. X2.1 Instrument Setup to Avoid Errors Due to Parallax
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echoes can be utilized because of either high absorption, corrosion, or unfavorable radiation geometry. In such cases, the accuracy
of the results can be improved by the tuning of the wall thickness meter to the harmonic waves of the echo frequency (harmonic
wave method).
X2.8.2 Up to now the interferometer method has been used for the precision measurement of sound propagation. Further
development of the harmonic wave method can replace the rather complicated and time-consuming interferometer method in all
those cases where the ultimate accuracy of the latter is not required. Under normal conditions, a measuring accuracy of 0.5 % or
better can be obtained with the so-called “zero-method.”
X2.8.3 A modification of the method utilizes bursts of radio frequency (rf) radiated from a search unit into a long buffer rod and
then into the sample, which is a few wavelengths thick. The buffer rod is long enough to contain the entire rf bursts, while the burst
is long enough to occupy the three round-trips in the specimen. Thus the burst interferes with itself as it reverberates within the
specimen. One characteristic echo pattern occurs when the round-trip distance in the specimen is equal to an odd number of
half-wavelengths; an even number gives a different pattern. The two patterns alternate as the rf frequency is changed. One plots
phase versus frequency in units of cycles versus MHz. One cycle of phase occurs for each repetition of one of the characteristic
patterns; between the two patterns there is ⁄2 cycle of phase. The slope of the phase versus frequency line is the delay time t in
microseconds, and for a specimen of thickness L, the velocity is
v 5 2L/t
X2.9 Phase Comparison Method
X2.9.1 This method consists of superimposing the echoes of two pulses which have made different numbers of round trips. If the
echoes are made exactly in phase by a critical adjustment of frequency, the expression for phase angles may be written as:
γ2 2L W /v 522 πn
@~ ! #
n
where:
v = velocity of propagation,
W = 2 π times resonant frequency (f ),
n n
L = thickness,
n = number of waves, and
γ = phase angle due to the seal between the search unit and the specimen.
Consequently, the velocity is expressed by:
v 5 2L f / n1 γ/2π
~ ! @ ~ !#
n
X2.9.2 It has been experimentally proven that size and shape effects are reduced to effectively zero whenever there are at least
100 wave lengths of sound in the specimen thickness. High frequencies (10 to 20 MHz) are generally used to m
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