ASTM E2491-23

Standard Guide for Evaluating Performance Characteristics of Phased-Array Ultrasonic Testing Instruments and Systems

SIGNIFICANCE AND USE
5.1 This guide is intended to evaluate performance assessment of combinations of phased-array probes and instruments. It is not intended to define performance and acceptance criteria, but rather to provide data from which such criteria may be established.  
5.2 Recommended procedures described in this guide are intended to provide performance-related measurements that can be reproduced under the specified test conditions using simple targets and the phased-array test system itself. It is intended for phased-array flaw detection instruments operating in the nominal frequency range of 1 MHz to 20 MHz, but the procedures are applicable to measurements on instruments utilizing significantly higher frequency components.  
5.3 This guide is not intended for service calibration, or maintenance of circuitry for which the manufacturer’s instructions are available.  
5.4 Implementation of specific assessments may require more detailed procedural instructions in a format of the using facility.  
5.5 The measurement data obtained may be employed by users of this guide to specify, describe, or provide performance criteria for procurement and quality assurance, or service evaluation of the operating characteristics of phased-array systems.  
5.6 Not all assessments described in this guide are applicable to all systems. All or portions of the guide may be used as determined by the user.
SCOPE
1.1 This guide covers procedures for evaluating some performance characteristics of phased-array ultrasonic examination instruments and systems.  
1.2 Evaluation of these characteristics is intended to be used for either comparing instruments and systems or, by periodic repetition, for detecting long-term changes in the characteristics of a given instrument or system. Significant changes may be indicative of impending failure, and, if beyond certain limits, will require corrective maintenance. Some electronic instrument characteristics in phased-array units are similar to non-phased-array units and may be measured as described in Practice E1065 or Guide E1324.  
1.3 Ultrasonic examination systems using pulsed-wave trains and A-scan presentation (rf or video) may be evaluated.  
1.4 This guide establishes no performance limits for examination systems; if such acceptance criteria are required, these shall be specified by the using parties. Where acceptance criteria are implied herein, they are for example only and are subject to more or less restrictive limits imposed by customer’s and end user’s controlling documents.  
1.5 The specific parameters to be evaluated, conditions, frequency of test, and report data required shall be determined by the user.  
1.6 This guide may be used for the evaluation of a complete examination system, including search unit, instrument, interconnections, scanner fixtures, connected alarm, and auxiliary devices. This guide is not intended to be used as a substitute for calibration or standardization of an instrument or system to inspect any given material.  
1.7 Required test apparatus includes selected test blocks and position encoders in addition to the instrument or system to be evaluated.  
1.8 Alternate procedures, such as examples described in this document, or others, may only be used with customer approval.  
1.9 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.10 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.  
1.11 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 Recommendatio...

General Information

• Status: Published
• Publication Date: 31-May-2023
• ICS: 19.100 - Non-destructive testing
• 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
• 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
• Refers: ASTM E1316-13b - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jun-2013

Overview

ASTM E2491-23, Standard Guide for Evaluating Performance Characteristics of Phased-Array Ultrasonic Testing Instruments and Systems provides practical guidance on how to evaluate and assess key performance parameters of phased-array ultrasonic testing (PAUT) instruments and systems. Developed by ASTM International, this document enables users and quality professionals in nondestructive testing (NDT) fields to generate reliable performance data for procurement, quality assurance, and ongoing system evaluation. This standard does not set specific acceptance criteria but outlines procedures that facilitate repeatable, benchmarked measurements for PAUT equipment.

Key Topics

• Evaluation Procedures
  The guide details procedures for evaluating performance characteristics such as:

  • Beam profile assessment
  • Beam steering capability
  • Element activity and uniformity
  • Focus capability and focal spot size
  • Computer parameter control and data display accuracy
  • Compensation for wedge attenuation and time delays
  • Receiver gain linearity

• System Scope
  Applies to full phased-array ultrasonic testing systems operating in the nominal frequency range of 1 MHz to 20 MHz, including higher-frequency devices. Evaluations can cover complete examination chains: probe, instrument, connections, scanners, alarms, and auxiliary equipment.

• Use of Simple Targets
  Emphasizes the use of simple targets such as side-drilled holes and calibrated test blocks to achieve reproducible, meaningful measurements under specified test conditions.

• Modular Application
  Recognizes that not every assessment is relevant to every PAUT system. Sections or individual procedures of the guide can be used as needed, depending on user requirements.

• Guidance, Not Calibration
  This standard is intended for performance evaluation only and does not replace manufacturer-recommended calibration or maintenance processes.

Applications

ASTM E2491-23 is valuable for:

• Quality Assurance and Procurement
  Organizations specifying phased-array ultrasonic instruments for purchase or contract compliance can use these procedures to define or verify performance expectations, ensuring that test equipment meets operational needs.

• Comparative Evaluation
  Facilities can periodically repeat standard measurements to monitor long-term instrument drift or degradation, facilitating the early detection of changes indicative of failure and the timely scheduling of maintenance.

• System Development and Validation
  PAUT system manufacturers and integrators may use this guide to standardize testing of new probes, components, or software.

• Service Evaluation and Documentation
  Service labs and technical teams can document instrument performance for audits, client reporting, or internal equipment certifications.

Related Standards

• ASTM E317 - Evaluating Performance Characteristics of Ultrasonic Pulse-Echo Testing Instruments and Systems
• ASTM E494 - Measuring Ultrasonic Velocity in Materials by Comparative Pulse-Echo Method
• ASTM E1065 - Evaluating Characteristics of Ultrasonic Search Units
• ASTM E1316 - Terminology for Nondestructive Examinations
• ASTM E1324 - Measuring Some Electronic Characteristics of Ultrasonic Testing Instruments

These related ASTM standards provide complementary procedures for evaluating ultrasonic testing instruments, supporting comprehensive NDT equipment validation.

---

Keywords: ASTM E2491-23, phased-array ultrasonic testing, PAUT, nondestructive testing, performance evaluation, beam profile, beam steering, focusing capability, NDT standards, ultrasonic testing instruments, quality assurance.

For more information and the official standard, visit ASTM International.

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: E2491 − 23
Standard Guide for
Evaluating Performance Characteristics of Phased-Array
Ultrasonic Testing Instruments and Systems
This standard is issued under the fixed designation E2491; 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* 1.8 Alternate procedures, such as examples described in this
document, or others, may only be used with customer approval.
1.1 This guide covers procedures for evaluating some per-
formance characteristics of phased-array ultrasonic examina- 1.9 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this
tion instruments and systems.
standard.
1.2 Evaluation of these characteristics is intended to be used
1.10 This standard does not purport to address all of the
for either comparing instruments and systems or, by periodic
safety concerns, if any, associated with its use. It is the
repetition, for detecting long-term changes in the characteris-
responsibility of the user of this standard to establish appro-
tics of a given instrument or system. Significant changes may
priate safety, health, and environmental practices and deter-
be indicative of impending failure, and, if beyond certain
mine the applicability of regulatory limitations prior to use.
limits, will require corrective maintenance. Some electronic
1.11 This international standard was developed in accor-
instrument characteristics in phased-array units are similar to
dance with internationally recognized principles on standard-
non-phased-array units and may be measured as described in
ization established in the Decision on Principles for the
Practice E1065 or Guide E1324.
Development of International Standards, Guides and Recom-
1.3 Ultrasonic examination systems using pulsed-wave
mendations issued by the World Trade Organization Technical
trains and A-scan presentation (rf or video) may be evaluated.
Barriers to Trade (TBT) Committee.
1.4 This guide establishes no performance limits for exami-
2. Referenced Documents
nation systems; if such acceptance criteria are required, these
shall be specified by the using parties. Where acceptance
2.1 ASTM Standards:
criteria are implied herein, they are for example only and are
E317 Practice for Evaluating Performance Characteristics of
subject to more or less restrictive limits imposed by customer’s
Ultrasonic Pulse-Echo Testing Instruments and Systems
and end user’s controlling documents.
without the Use of Electronic Measurement Instruments
E494 Practice for Measuring Ultrasonic Velocity in Materi-
1.5 The specific parameters to be evaluated, conditions,
als by Comparative Pulse-Echo Method
frequency of test, and report data required shall be determined
E1065 Practice for Evaluating Characteristics of Ultrasonic
by the user.
Search Units
1.6 This guide may be used for the evaluation of a complete
E1316 Terminology for Nondestructive Examinations
examination system, including search unit, instrument,
E1324 Guide for Measuring Some Electronic Characteristics
interconnections, scanner fixtures, connected alarm, and aux-
of Ultrasonic Testing Instruments
iliary devices. This guide is not intended to be used as a
substitute for calibration or standardization of an instrument or
3. Terminology
system to inspect any given material.
3.1 Refer to Terminology E1316 for definitions of terms in
1.7 Required test apparatus includes selected test blocks and
this guide.
position encoders in addition to the instrument or system to be
evaluated. 4. Summary of Guide
4.1 Phased-array instruments and systems have similar in-
dividual components as traditional ultrasonic systems. These
This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-
tive Testing and is the direct responsibility of Subcommittee E07.06 on Ultrasonic
Method. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved June 1, 2023. Published August 2023. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 2006. Last previous edition approved in 2018 as E2491 – 13 (2018). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/E2491-23. the ASTM website.
*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
E2491 − 23
include pulsers, receivers, probes, and interconnecting cables. 4.8 Procedures for assessment of several parameters in
The most significant difference is that phased-array systems phased-array systems are described in Annex A1 – Annex A7.
form the transmitted ultrasonic pulse by constructive phase 4.8.1 These include; determination of beam profile (Annex
interference from the wavelets formed off the individually A1), beam steering capability (Annex A2), element activity
pulsed elements of the phased-array probes. (Annex A3), focusing capability (Annex A4), software calcu-
lations (Annex A5), compensation for wedge attenuation and
4.2 Each phased-array probe consists of a series of individu-
delay (Annex A6), and receiver linearity (Annex A7).
ally wired elements that are activated separately using a
programmable time delay pattern. Varying the number of
5. Significance and Use
elements used and the delay time between the pulses to each
5.1 This guide is intended to evaluate performance assess-
element allows control of the beam. Depending on the probe
ment of combinations of phased-array probes and instruments.
design, it is possible to electronically vary the angle (incident
It is not intended to define performance and acceptance criteria,
or skew), the focal distance, the beam dimensions, or a
but rather to provide data from which such criteria may be
combination of the three. In the receiving mode, acoustic
established.
energy is received by the elements and the signals undergo a
5.2 Recommended procedures described in this guide are
summation process utilizing the same type of time delay
intended to provide performance-related measurements that
process as was used to generate the pulse.
can be reproduced under the specified test conditions using
4.3 The degree of beam steering available is dependent on
simple targets and the phased-array test system itself. It is
several parameters including; number of elements, element
intended for phased-array flaw detection instruments operating
pitch, element dimensions, element array shape, resonant
in the nominal frequency range of 1 MHz to 20 MHz, but the
frequency of the elements, the material into which the beam is
procedures are applicable to measurements on instruments
directed, the minimum delay possible between firing of adja-
utilizing significantly higher frequency components.
cent pulsers and receivers, and the pulser voltage characteris-
5.3 This guide is not intended for service calibration, or
tics.
maintenance of circuitry for which the manufacturer’s instruc-
4.4 Pulser and receiver parameters in phased-array systems
tions are available.
are generally computer controlled and the received signals are
5.4 Implementation of specific assessments may require
typically displayed on computer monitors via computer data
more detailed procedural instructions in a format of the using
acquisition systems and may be stored to computer files.
facility.
4.5 Although most systems use piezo-electric materials for
5.5 The measurement data obtained may be employed by
the elements, electro-magnetic acoustic transducer (EMAT)
users of this guide to specify, describe, or provide performance
devices have also been designed and built using phased-array
criteria for procurement and quality assurance, or service
instrumentation.
evaluation of the operating characteristics of phased-array
4.6 Most phased array systems can use encoders for auto-
systems.
mated and semi-automated scanning.
5.6 Not all assessments described in this guide are appli-
4.7 Side Drilled Holes used as targets in this document
cable to all systems. All or portions of the guide may be used
should have diameters less than the wavelength of the pulse as determined by the user.
being assessed and long enough to avoid end effects from
6. Keywords
causing interfering signals. This will typically be accomplished
when the hole diameter is between about 1.5 mm and 2.5 mm 6.1 characterization; focal point; phased-array; phased-array
and 20 mm to 25 mm in length.
probe; sound beam profile; ultrasound
E2491 − 23
ANNEXES
(Mandatory Information)
A1. DETERMINATION OF PHASED-ARRAY BEAM PROFILE
A1.1 Introduction available elements to form the beam the number of remaining
elements for the electronic raster may be too small to allow the
A1.1.1 This annex describes procedures to determine beam
beam to pass over the target. In this case, it will be necessary
profiles of phased-array probes. Either immersion or contact
to have encoded mechanical motion and assess each focal law
probe applications can be addressed using these procedures.
along the active plane separately.
However, it should be cautioned that assessments of contact
probes may suffer from variability greater than imposed A1.2.3 Side drilled holes should be arranged at various
tolerances if proper precautions are not taken to ensure depths in a flaw-free sample of the test material in which focal
constant coupling conditions. laws have been programmed for. Using the linear scan feature
of the phased-array system the beam is passed over the targets
A1.2 Test Setup
at the various depths of interest. The electronic scan is
A1.2.1 For single focal laws where the beam is fixed (that illustrated schematically in Fig. A1.1.
is, not used in an electronic or sectorial scan mode) and the
A1.2.4 Data collection of the entire waveform over the
probe is used in an immersion setup, the ball-target or
range of interest shall be made. The display shall represent
hydrophone options described in Practice E1065 may be used.
amplitude as a color or grayscale. Time or equivalent distance
For phased array probes used in a dynamic fashion where
in the test material shall be presented along one axis and
several focal laws are used to produce sectorial or electronic
distance displaced along the other axis. This is a typical B-scan
scanning it may be possible to make beam-profile assessments
as illustrated in Fig. A1.2.
with no or little mechanical motion. Where mechanical motion
A1.2.5 Data display for an electronic scan using a phased-
is used it shall be encoded to relate signal time and amplitude
array probe mounted on a wedge can be similarly made using
to distance moved. Encoder accuracy shall be verified to be
simple orthogonal representation of time versus displacement
within tolerances appropriate for the measurements made.
or it can be angle-corrected as illustrated in Fig. A1.3.
Descriptions made for electronic scan and sectorial scan beam
profile assessments will be made for contact probes; however, A1.2.6 Resolution along the displacement axis will be a
when assessment in water is required the machined targets may function of the scan increment of the electronic scan or, if the
be replaced with rods or balls as appropriate. scan uses an encoded mechanical fixture the resolution, will be
dependent on the encoder step-size used for sampling.
A1.2.2 Linear-Array Probes—Assessment of the beam in
the active plane should be made by use of an electronic scan A1.2.7 Resolution along the beam axis will be a function of
sequence. For phased array probes using a large portion of the the intervals between the target paths. For highly focused
FIG. A1.1 Electronic Scan of Side Drilled Holes
E2491 − 23
FIG. A1.2 B-Scan Display of Electronic Scan Represented in Fig. A1.1 (Depth is in the vertical axis and electronic-scan distance is rep-
resented along the horizontal axis.)
FIG. A1.3 Angle-Corrected B-Scan of a Phased-Array Beam (in Shear Wave Mode) from a Side Drilled Hole (Off-axis lobe effects can be
seen in the display.)
beams it may be desirable to have small differences between A1.2.10 Fig. A1.5 illustrates an alternative to the stepped
the sound paths to the target paths (for example, 1 mm or
intervals shown in Fig. A1.4. A through hole may be arranged
2 mm).
perpendicular to the required refracted angle to provide a
continuous transition of path length to the target.
A1.2.8 Beam profiling in the passive plane should also be
made, which will require mechanical scanning.
A1.2.11 A projected C-scan can be used to size the beam
A1.2.9 Waveform collection of signals using a combination based on either color or grayscale indicating amplitude drop or
of electronic scanning in the active plane and encoded me-
a computer display that plots amplitude with respect to
chanical motion in the passive plane provides data that can be
displacement. The projected C-scan option is schematically
projection-corrected to provide beam dimensions in the passive
represented in Fig. A1.6.
plane. Fig. A1.4 illustrates a method for beam assessment in
the passive plane. This technique uses a corner reflection from
an end-drilled hole at depths established by a series of steps.
E2491 − 23
FIG. A1.4 Scanning End-Drilled Holes to Obtain Beam Dimensions in Passive Plane
FIG. A1.5 Representation of an Inclined Hole for Beam Characterization in the Passive Plane
FIG. A1.6 Representation of Projected C-Scan of Corner Effect Scan Seen in Fig. A1.4
A2. DETERMINATION OF PHASED-ARRAY BEAM STEERING LIMITS
A2.1 Introduction A2.1.2 Recommended limits to establish the working range
of angular sweep of a phased-array probe relate to the
A2.1.1 This annex describes procedures to determine prac-
divergence of the beam of each element in the probe array.
tical limits for beam steering capabilities of a phased-array
When used in pulse-echo mode the steering limit is considered
probe and as such applies to the active plane(s) only. Either
to be within the 6 dB divergence envelope of the individual
immersion or contact probe applications can be addressed
elements. It is therefore possible to calculate a theoretical limit
using these procedures. However, it should be cautioned that
based on nominal frequency and manufacturer provided infor-
assessments of contact probes may suffer from variability
mation on the element dimensions.
greater than imposed tolerances if proper precautions are not
taken to ensure constant coupling conditions.
E2491 − 23
A2.1.3 Several parameters can affect the theoretical calcu- the nominal exit point. Side drilled holes may be arranged in
lations. These are primarily related to the nominal frequency of other planes (angles) of interest.
the probe. Some parameters affecting actual frequency include;
A2.2.3 Assessments are made placing the probe such that
pulse length, damping, use of a delay-line or refracting wedge,
the center of beam ray (as determined by measuring the beam
and variations in manufacturing processes on thickness lapping
exit point) enters the block at the indicated centerline. For
and matching layers. Beam steering capability will also be
analysis of a probe where all the elements in a single plane are
affected by project requirements of the beam. Applications
used without a delay line or refracting wedge, the midpoint of
where focusing is necessary may not achieve the same limits as
the element array shall be aligned with the centerline. For focal
applications where the beam is not focused as well as steered.
laws using only a portion of the total available elements, the
A2.1.4 Steering capability may be specific to a sound path
midpoint of the element aperture shall be aligned with the
distance, aperture, and material. For the purposes of this
centerline. When delay lines, refracting wedges, or immersion
procedure, assessment of beam steering capability will be
methods are used, corrections will be required to compensate
based on a comparison of signal to noise ratios at varying
for movement of the “apparent” exit point along the block
angular displacements.
entry surface. When a probe is used in direct contact with a
verification block as illustrated in Fig. A2.2, the lack of
A2.2 Test Set-Up—Configure the probe focal laws for the
symmetry either side of the centerline prevents both positive
conditions of the test. This will include immersion or contact,
and negative sweep angles being assessed simultaneously. To
refracting wedge or delay-line, unfocused or a defined focal
assess the sweep limit in the two directions when using this
distance, and the test material to be used.
style of block requires that the probe be assessed in one
A2.2.1 Prepare a series of side drilled holes in the material
direction first and then rotated 180° and the opposite sweep
to be used for the application at the distance or distances to be
assessed.
used in the application. The side-drilled-hole pattern should be
A2.2.4 Angular steps between A-scan samples will have an
as illustrated in Fig. A2.1. Holes indicated in Fig. A2.1 are at
effect on the perceived sweep limits. A maximum of 1°
5° intervals at a 25 mm and 50 mm distance from a center
between S-scan samples is recommended for steering assess-
where the probe is located.
ment. Angular steps are limited by the system timing-delay
A2.2.2 Similar assessments are possible for different appli-
capabilities between pulses and element pitch characteristics.
cations. When a set of focal laws is arranged to provide
Most of the targets illustrated in Fig. A2.1 and Fig. A2.2 are
resolution in a plane instead of a sound path distance, the plane
separated by 5°; however, greater or lesser intervals may be
of interest may be used to assess the steering limits of the
used depending on the required resolution.
beam. The block used for assessment would be arranged with
side drilled holes in the plane of interest. Such a plane-specific A2.2.5 Assessment of steering limits shall be made using
the dB difference between the maximum and minimum signal
block is illustrated in Fig. A2.2 where a series of holes is made
in a vertical and horizontal plane at a specified distance from amplitudes between two adjacent side drilled holes. For
NOTE 1—Block dimensions 150 mm by 75 mm by 25 mm (typical)
FIG. A2.1 Beam Steering Assessment Block—Constant Sound Path
E2491 − 23
FIG. A2.2 Beam Steering Assessment Block—Single Plane
example, when a phased-array probe is configured to sweep mid-angle. Conversely, a system may be limited to S-scans not
+45° from the beam centerline on a block such as illustrated in
exceeding the angles assessed to achieve a specified signal
Fig. A2.1, the highest of the SDHs which achieves a 6 dB
separation, for example, –20 dB between 2 mm diameter SDHs
amplitude may be considered the maximum steering capability
separated by 5°.
of the probe configuration.
A2.3 An alternative assessment may use a single SDH at a
A2.2.6 Acceptable limits of steering may be indicated by
specified depth or sound path distance. Displaying the A-scan
the maximum and minimum angles that can achieve a pre-
for the maximum and minimum angles used would assess the
specified separation between adjacent holes. Depending on the
steering capability by observing the S/N ratio at the peaked
application a 6 dB or 20 dB (or some other value) may be
response. Steering limit would be a pre-defined S/N ratio being
specified as the required separation.
achieved. Caution must be taken when using this method so as
A2.2.7 Steering capabilities may be used as a prerequisite;
to not peak on grating lobe signals. This method will also
for example, a phased-array system is required to achieve a
require confirmation that the SDH is positioned at the calcu-
minimum steering capability for 5° resolution of 2 mm diam-
lated refracted angle.
eter side drilled holes of plus and minus 20° from a nominal
E2491 − 23
A3. DETERMINATION OF PHASED-ARRAY ELEMENT ACTIVITY
A3.1 Introduction should be within a range of 62 dB of any previous assessments
and within 62 dB of each other.
A3.1.1 This assessment is used to determine whether all
elements of the phased-array probe are active and of uniform
A3.2.7 The total number of inactive elements and number
acoustic energy. Therefore, during normal operation in a timed
of adjacent inactive elements in a probe should be agreed upon
sequence, each of the elements is addressed by a separate
and identified in a written procedure. This number may be
pulser and receiver, a method should be used that ensures
different for baseline and in-service verifications. Some
uniform electronic performance of the phased-array elements
phased-array probes may have several hundred elements and
and any differences are attributable to the probe itself. To
even new phased-array probes may be found to have inactive
ensure that any variation of element performance is due only to
elements as a result of manufacturing difficulties ensuring the
probe construction, a single pulser-receiver channel is selected
electrical connections to elements with dimensions on the order
to address each element.
of a fraction of a millimetre.
A3.2 Test Set-Up
A3.2.8 The number of inactive elements allowed should be
based on performance of other capabilities such as focusing
A3.2.1 Acoustically couple the probe directly to the 25 mm
and steering limits of the focal laws being used. No simple rule
thickness of an IIW (International Institute of Welding) block
for the number of inactive elements can be made for all
with a uniform layer of couplant. This may be accomplished by
phased-array probes. Typically, if more than 25 % of the
a contact-gap technique such that the probe-to-block interface
elements in a probe are inactive, then sensitivity and steering
is under water (to ensure uniform coupling). Alternatively an
immersion method using a fixed water path may be used and capabilities may be compromised. Similarly, the number of
adjacent elements allowed to be inactive should be determined
the water-steel interface signal monitored instead of the steel
wall thickness. by the steering and electronic raster resolution required by the
application.
A3.2.2 Configure an electronic scan consisting of one ele-
ment that is stepped along one element at a time for the total
A3.2.9 Stability of coupling is essential for the comparison
number of elements in the array. (This should ensure that the
assessment. If using a contact method and the assessment of
pulser-receiver number one is used in each focal law or if the
elements produces signals outside the 62 dB range, then the
channel is selectable it should be the same channel used for
coupling should be checked and the test run again. If still
each element). Set the pulser parameters to optimize the
outside the acceptable range, then the probe should be removed
response for the nominal frequency of the probe array and
from service and corrected prior to further use. The test using
establish a pulse-echo response from the block backwall or
a fixed water path to a water/steel interface will red
...

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: E2491 − 13 (Reapproved 2018) E2491 − 23
Standard Guide for
Evaluating Performance Characteristics of Phased-Array
Ultrasonic Testing Instruments and Systems
This standard is issued under the fixed designation E2491; 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 Scope*
1.1 This guide describescovers procedures for evaluating some performance characteristics of phased-array ultrasonic examination
instruments and systems.
1.2 Evaluation of these characteristics is intended to be used for either comparing instruments and systems or, by periodic
repetition, for detecting long-term changes in the characteristics of a given instrument or system that system. Significant changes
may be indicative of impending failure, and which, and, if beyond certain limits, will require corrective maintenance. Instrument
characteristics measured in accordance with this guide are expressed in terms that relate to their potential usefulness for ultrasonic
examinations. Other Some electronic instrument characteristics in phased-array units are similar to non-phased-array units and may
be measured as described in Practice E1065 or Guide E1324.
1.3 Ultrasonic examination systems using pulsed-wave trains and A-scan presentation (rf or video) may be evaluated.
1.4 This guide establishes no performance limits for examination systems; if such acceptance criteria are required, these mustshall
be specified by the using parties. Where acceptance criteria are implied herein, they are for example only and are subject to more
or less restrictive limits imposed by customer’s and end user’s controlling documents.
1.5 The specific parameters to be evaluated, conditions and conditions, frequency of test, and report data required, must also
required shall be determined by the user.
1.6 This guide may be used for the evaluation of a complete examination system, including search unit, instrument,
interconnections, scanner fixtures and connected alarm and auxiliary devices, primarily in cases where such a system is used
repetitively without change or substitution. fixtures, connected alarm, and auxiliary devices. This guide is not intended to be used
as a substitute for calibration or standardization of an instrument or system to inspect any given material.
1.7 Required test apparatus includes selected test blocks and position encoders in addition to the instrument or system to be
evaluated.
1.8 Precautions relating to the applicability of the procedures and interpretation of the results are included.
1.8 Alternate procedures, such as examples described in this document, or others, may only be used with customer approval.
This guide 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 Nov. 1, 2018June 1, 2023. Published December 2018August 2023. Originally approved in 2006. Last previous edition approved in 20132018
as E2491 – 13.E2491 – 13 (2018). DOI: 10.1520/E2491-13R18.10.1520/E2491-23.
*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
E2491 − 23
1.9 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.10 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.
1.11 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:
E317 Practice for Evaluating Performance Characteristics of Ultrasonic Pulse-Echo Testing Instruments and Systems without the
Use of Electronic Measurement Instruments
E494 Practice for Measuring Ultrasonic Velocity in Materials by Comparative Pulse-Echo Method
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
3. Terminology
3.1 Refer to Terminology E1316 for definitions of terms in this guide.
4. Summary of Guide
4.1 Phased-array instruments and systems have similar individual components as are found in traditional ultrasonic systems that
are based on single channel or multiplexed pulse-echo units. traditional ultrasonic systems. These include pulsers, receivers,
probes, and interconnecting cables. The most significant difference is that phased-array systems form the transmitted ultrasonic
pulse by constructive phase interference from the wavelets formed off the individually pulsed elements of the phased-array probes.
4.2 Each phased-array probe consists of a series of individually wired elements that are activated separately using a programmable
time delay pattern. Varying the number of elements used and the delay time between the pulses to each element allows control of
the beam. Depending on the probe design, it is possible to electronically vary the angle (incident or skew), or the focal distance,
or the beam dimensions, or a combination of the three. In the receiving mode, acoustic energy is received by the elements and the
signals undergo a summation process utilizing the same type of time delay process as was used during transmission.to generate
the pulse.
4.3 The degree of beam steering available is dependent on several parameters including; number of elements, pitch of the element
spacing,element pitch, element dimensions, element array shape, resonant frequency of the elements, the material into which the
beam is directed, the minimum delay possible between firing of adjacent pulsers and receivers, and the pulser voltage
characteristics.
4.4 Pulser and receiver parameters in phased-array systems are generally computer controlled and the received signals are
typically displayed on computer monitors via computer data acquisition systems and may be stored to computer files.
4.5 Although most systems use piezo-electric materials for the elements, electro-magnetic acoustic transducer (EMAT) devices
have also been designed and built using phased-array instrumentation.
4.6 Most phased array systems can use encoders for automated and semi-automated scanning.
4.7 Side Drilled Holes used as targets in this document should have diameters less than the wavelength of the pulse being assessed
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.
E2491 − 23
and long enough to avoid end effects from causing interfering signals. This will typically be accomplished when the hole diameter
is between about 1.5 mm and 2.5 mm and 20 mm to 25 mm in length.
4.8 Procedures for assessment of several parameters in phased-array systems are described in Annex A1 – Annex A7.
4.8.1 These include; determination of beam profile (Annex A1), beam steering capability (Annex A2), element activity (Annex
A3), focusing capability (Annex A4), software calculations (Annex A5), compensation for wedge attenuation and delay (Annex
A6), and receiver linearity (Annex A7).
5. Significance and Use
5.1 This guide is intended to evaluate performance assessment of combinations of phased-array probes and instruments. It is not
intended to define performance and acceptance criteria, but rather to provide data from which such criteria may be established.
5.2 Recommended procedures described in this guide are intended to provide performance-related measurements that can be
reproduced under the specified test conditions using simple targets and the phased-array test system itself. It is intended for
phased-array flaw detection instruments operating in the nominal frequency range of 1 MHz to 20 MHz, but the procedures are
applicable to measurements on instruments utilizing significantly higher frequency components.
5.3 This guide is not intended for service calibration, or maintenance of circuitry for which the manufacturer’s instructions are
available.
5.4 Implementation of specific assessments may require more detailed procedural instructions in a format of the using facility.
5.5 The measurement data obtained may be employed by users of this guide to specify, describe, or provide a performance criteria
for procurement and quality assurance, or service evaluation of the operating characteristics of phased-array systems.
5.6 Not all assessments described in this guide are applicable to all systems. All or portions of the guide may be used as determined
by the user.
6. Procedure
6.1 Procedures for assessment of several parameters in phased-array systems are described in Annex A1 – Annex A7.
6.1.1 These include; determination of beam profile, beam steering capability, element activity, focusing capability, software
calculations (controls and display of received signals), compensation for wedge attenuation, receiver gain linearity.
6. Keywords
6.1 characterization; focal point; phased-array; phased-array probe; sound beam profile; ultrasound
E2491 − 23
ANNEXES
(Mandatory Information)
A1. DETERMINATION OF PHASED-ARRAY BEAM PROFILE
A1.1 Introduction
A1.1.1 This annex describes procedures to determine beam profiles of phased-array probes. Either immersion or contact probe
applications can be addressed using these procedures. However, it should be cautioned that assessments of contact probes may
suffer from variability greater than imposed tolerances if proper precautions are not taken to ensure constant coupling conditions.
A1.2 Test Setup
A1.2.1 For single focal laws where the beam is fixed (that is, not used in an electronic or sectorial scan mode) and the probe is
used in an immersion setup, the ball-target or hydrophone options described in Practice E1065 may be used. For phased array
probes used in a dynamic fashion where several focal laws are used to produce sectorial or electronic scanning it may be possible
to make beam-profile assessments with no or little mechanical motion. Where mechanical motion is used it shall be encoded to
relate signal time and amplitude to distance moved. Encoder accuracy shall be verified to be within tolerances appropriate for the
measurements made. Descriptions made for electronic scan and sectorial scan beam profile assessments will be made for contact
probes; however, when assessment in water is required the machined targets may be replaced with rods or balls as appropriate.
A1.2.2 Linear-Array Probes—Linear-array probes have an active plane and an inactive or passive plane. Assessment of the beam
in the active plane should be made by use of an electronic scan sequence for probes with sufficient number of elements to
electronically advance the beam past the targets of interest. sequence. For phased array probes using a large portion of the available
elements to form the beam the number of remaining elements for the electronic raster may be too small to allow the beam to pass
over the target. In this case, it will be necessary to have encoded mechanical motion and assess each focal law along the active
plane separately.
A1.2.3 Side-drilled Side drilled holes should be arranged at various depths in a flaw-free sample of the test material in which focal
laws have been programmed for. Using the linear scan feature of the phased-array system the beam is passed over the targets at
the various depths of interest. The electronic scan is illustrated schematically in Fig. A1.1.
A1.2.4 Data collection of the entire waveform over the range of interest shall be made. The display shall represent amplitude as
a color or grayscale. Time or equivalent distance in the test material shall be presented along one axis and distance displaced along
the other axis. This is a typical B-scan as illustrated in Fig. A1.2.
A1.2.5 Data display for an electronic scan using a phased-array probe mounted on a wedge can be similarly made using simple
orthogonal representation of time versus displacement or it can be angle corrected angle-corrected as illustrated in Fig. A1.3.
A1.2.6 Resolution along the displacement axis will be a function of the step sizescan increment of the electronic scan or, if the
scan uses an encoded mechanical fixture the resolution, will be dependent on the encoder step-size used for sampling.
E2491 − 23
FIG. A1.1 Electronic Scan of Side Drilled Holes
FIG. A1.2 B-Scan Display of Electronic Scan Represented in Fig. A1.1 (Depth is in the vertical axis and electronic-scan distance is rep-
resented along the horizontal axis.)
A1.2.7 Resolution along the beam axis will be a function of the intervals between the target paths. For highly focused beams it
may be desirable to have small differences between the sound paths to the target paths (for example, 1 mm or 2 mm).
A1.2.8 Beam profiling in the passive plane can also be made. The passive plane in a linear-array probe is perpendicular to the
active plane and refers to the plane in which no beam steering is possible by phasing effects. Beam profiling in the passive direction
should also be made, which will require mechanical scanning.
A1.2.9 Waveform collection of signals using a combination of electronic scanning in the active plane and encoded mechanical
motion in the passive plane provides data that can be projection-corrected to provide beam dimensions in the passive plane. Fig.
A1.4 illustrates a method for beam assessment in the passive plane. This technique uses a corner reflection from an end-drilled hole
at depths established by a series of steps.
E2491 − 23
FIG. A1.3 Angle-Corrected B-Scan of a Phased-Array Beam (in Shear Wave Mode) from a Side Drilled Hole (Off-axis lobe effects can be
seen in the display.)
FIG. A1.4 Scanning End-Drilled Holes to Obtain Beam Dimensions in Passive Plane
A1.2.10 Fig. A1.5 illustrates an alternative to the stepped intervals shown in Fig. A1.4. A through hole may be arranged
perpendicular to the required refracted angle to provide a continuous transition of path length to the target.
A1.2.11 A projected C-scan can be used to size the beam based on either color or grayscale indicating amplitude drop or a
computer display that plots amplitude with respect to displacement. The projected C-scan option is schematically represented in
Fig. A1.6.
FIG. A1.5 Representation of an Inclined Hole for Beam Characterization in the Passive Plane
E2491 − 23
FIG. A1.6 Representation of Projected C-Scan of Corner Effect Scan Seen in Fig. A1.4
A2. DETERMINATION OF PHASED-ARRAY BEAM STEERING LIMITS
A2.1 Introduction
A2.1.1 This annex describes procedures to determine practical limits for beam steering capabilities of a phased-array probe and
as such applies to the active plane(s) only. Either immersion or contact probe applications can be addressed using these procedures.
However, it should be cautioned that assessments of contact probes may suffer from variability greater than imposed tolerances
if proper precautions are not taken to ensure constant coupling conditions.
A2.1.2 Recommended limits to establish the working range of angular sweep of a phased-array probe relate to the divergence of
the beam of each element in the probe array. When used in pulse-echo mode the steering limit is considered to be within the
6-dB6 dB divergence envelope of the individual elements. It is therefore possible to calculate a theoretical limit based on nominal
frequency and manufacturer provided information on the element dimensions. However, several parameters can affect the
theoretical calculations. These are primarily related to the nominal frequency of the probe. Some parameters affecting actual
frequency include; pulse length, damping, use of a delay-line or refracting wedge and variations in manufacturing processes on
thickness lapping and matching layers.
A2.1.3 For the purposes of this procedure, assessment of beam steering capability will be based on a comparison of signal to noise
ratios at varying angular displacements. Several parameters can affect the theoretical calculations. These are primarily related to
the nominal frequency of the probe. Some parameters affecting actual frequency include; pulse length, damping, use of a delay-line
or refracting wedge, and variations in manufacturing processes on thickness lapping and matching layers. Beam steering capability
will also be affected by project requirements of the beam. Applications where focusing is necessary may not achieve the same
limits as applications where the beam is not focused as well as steered.
A2.1.4 Steering capability may be specific to a sound path distance, aperture, and material. For the purposes of this procedure,
assessment of beam steering capability will be based on a comparison of signal to noise ratios at varying angular displacements.
A2.2 Test Set-Up—Configure the probe focal laws for the conditions of the test. This will include immersion or contact, refracting
wedge or delay-line, unfocused or a defined focal distance, and the test material to be used.
E2491 − 23
A2.2.1 Prepare a series of side drilled holes in the material to be used for the application at the distance or distances to be used
in the application. The side-drilled-hole pattern should be as illustrated in Fig. A2.1. Holes indicated in Fig. A2.1 are at 5° intervals
at a 25-mm and 50-mm25 mm and 50 mm distance from a center where the probe is located.
A2.2.2 Similar assessments are possible for different applications. When a set of focal laws is arranged to provide resolution in
a plane instead of a sound path distance, the plane of interest may be used to assess the steering limits of the beam. The block used
for assessment would be arranged with side drilled holes in the plane of interest. Such a plane-specific block is illustrated in Fig.
A2.2 where a series of holes is made in a vertical and horizontal plane at a specified distance from the nominal exit point. Side
drilled holes may be arranged in other planes (angles) of interest.
A2.2.3 Assessments are made placing the probe such that the center of beam ray (as determined by measuring the beam exit point)
enters the block at the indicated centerline. For analysis of a probe where all the elements in a single plane are used without a delay
line or refracting wedge, the midpoint of the element array shall be aligned with the centerline. For focal laws using only a portion
of the total available elements, the midpoint of the element aperture shall be aligned with the centerline. When delay lines,
refracting wedges, or immersion methods are used, corrections will be required to compensate for movement of the “apparent” exit
point along the block entry surface. When a probe is used in direct contact with a verification block as illustrated in Fig. A2.2, the
lack of symmetry either side of the centerline prevents both positive and negative sweep angles being assessed simultaneously. To
assess the sweep limit in the two directions when using this style of block requires that the probe be assessed in one direction first
and then rotated 180° and the opposite sweep assessed.
A2.2.4 Angular steps between A-scan samples will have an effect on the perceived sweep limits. A maximum of 1° between S-scan
samples is recommended for steering assessment. Angular steps are limited by the system timing-delay capabilities between pulses
and element pitch characteristics. Most of the targets illustrated in Fig. A2.1 and Fig. A2.2 are separated by 5°; however, greater
or lesser intervals may be used depending on the required resolution.
A2.2.5 Assessment of steering limits shall be made using the dB difference between the maximum and minimum signal
amplitudes between two adjacent side drilled holes. For example, when a phased array phased-array probe is configured to sweep
NOTE 1—Block dimensions 150 by 75150 mm by 75 mm by 25 mm (typical)
FIG. A2.1 Beam Steering Assessment Block—Constant Sound Path
E2491 − 23
FIG. A2.2 Beam Steering Assessment Block—Single Plane
+45° from the beam centerline on a block such as illustrated in Fig. A2.1, the higherhighest of the pair of the SDHs which achieves
a 6-dB separation shall6 dB amplitude may be considered the maximum steering capability of the probe configuration.
A2.2.6 Acceptable limits of steering may be indicated by the maximum and minimum angles that can achieve a pre-specified
separation between adjacent holes. Depending on the application a 6-dB or 20-dB6 dB or 20 dB (or some other value) may be
specified as the required separation.
A2.2.7 Steering capabilities may be used as a prerequisite; for example, a phased array phased-array system is required to achieve
a minimum steering capability for 5° resolution of 2-mm2 mm diameter side drilled holes of plus and minus 20° from a nominal
mid-angle. Conversely, a system may be limited to S-scans not exceeding the angles assessed to achieve a specified signal
separation, for example, –20 dB between 2-mm2 mm diameter SDHs separated by 5°.
A2.3 An alternative assessment may use a single SDH at a specified depth or sound path distance. Displaying the A-scan for the
maximum and minimum angles used would assess the steering capability by observing the S/N ratio at the peaked response.
Steering limit would be a pre-defined S/N ratio being achieved. Caution must be taken when using this method so as to not peak
on grating lobe signals. This method will also require confirmation that the SDH is positioned at the calculated refracted angle.
E2491 − 23
A3. DETERMINATION OF PHASED-ARRAY ELEMENT ACTIVITY
A3.1 Introduction
A3.1.1 This assessment is used to determine thatwhether all elements of the phased array phased-array probe are active and of
uniform acoustic energy. Because,Therefore, during normal operation in a timed sequence, each of the elements is addressed by
a separate pulser and receiver, a method mustshould be used that ensures theuniform electronic performance of the phased-array
instrument is identical from element to element elements and any differences are attributable to the probe itself. To ensure that any
variation of element performance is due only to probe construction, a single pulser-receiver channel is selected to address each
element.
A3.2 Test Set-Up
A3.2.1 Connect the phased array probe to be tested to the phased-array ultrasonic instrument and remove any delay line or
refracting wedge from the probe.
A3.2.1 Acoustically couple the probe directly to the 25-mm25 mm thickness of an IIW (International Institute of Welding) block
with a uniform layer of couplant. This may be accomplished by a contact-gap technique such that the probe-to-block interface is
under water (to ensure uniform coupling). Alternatively an immersion method using a fixed water path may be used and the
water-steel interface signal monitored instead of the steel wall thickness.
A3.2.2 Configure an electronic scan consisting of one element that is stepped along one element at a time for the total number
of elements in the array. (This should ensure that the pulser-receiver number 1one is used in each focal law or if the channel is
selectable it should be the same channel used for each element). Set the pulser parameters to optimize the response for the nominal
frequency of the probe array and establish a pulse-echo response from the block backwall or waterpath to 80 % display height for
each element in the probe.
A3.2.3 Observe the A-scan display for each element in the array and record the receiver gain required to achieve the 80 % signal
amplitude for each element. Results may be recorded on a table similar to that in Table A3.1.
A3.2.4 Note and record any elements that do not provide a backwall or waterpath signal (inactive elements). Results may be
recorded on a table similar to that in Table A3.1.
A3.2.5 If a prepackaged program is available for checking element activity, this can be used as an alternative.
A3.2.6 Data collected is used to assess probe uniformity and functionality. Comparison to previous assessments is made using the
TABLE A3.1 Probe Element Activity Chart: Enter Receiver Gain for 80 % FSH
Element 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Gain
Active (S)
Inactive (x)
E2491 − 23
same instrument settings (including gain) that were save
...