ASTM E2208-02(2010)e1
(Guide)Standard Guide for Evaluating Non-Contacting Optical Strain Measurement Systems
Standard Guide for Evaluating Non-Contacting Optical Strain Measurement Systems
ABSTRACT
This guide assists potential users in understanding the issues related to the accuracy of non-contacting strain measurement systems and to provide a common framework for quantitative comparison of optical systems. The output from a non-contacting optical strain and deformation measurement system is generally divided into optical data and image analysis data. Each non-contacting optical strain measurement system must be evaluated to determine reliable estimates for the accuracy of the resulting Derived Data.
SCOPE
1.1 The purpose of this document is to assist potential users in understanding the issues related to the accuracy of non-contacting strain measurement systems and to provide a common framework for quantitative comparison of optical systems. The output from a non-contacting optical strain and deformation measurement system is generally divided into optical data and image analysis data. Optical data contains information related to specimen strains and the image analysis process converts the encoded optical information into strain data. The enclosed document describes potential sources of error in the strain data and describes general methods for quantifying the error and estimating the accuracy of the measurements when applying non-contacting methods to the study of events for which the optical integration time is much smaller than the inverse of the maximum temporal frequency in the encoded data (that is, events that can be regarded as static during the integration time). A brief application of the approach, along with specific examples defining the various terms, is given in the Appendix.
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Standards Content (Sample)
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Designation: E2208 − 02 (Reapproved 2010)
Standard Guide for
Evaluating Non-Contacting Optical Strain Measurement
Systems
This standard is issued under the fixed designation E2208; 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.
ε NOTE—3.1.1, 3.1.2 and 3.2.4 were editorially revised in December 2011.
1. Scope 3. Terminology
1.1 The purpose of this document is to assist potential users 3.1 Definitions:
in understanding the issues related to the accuracy of non-
3.1.1 accuracy—the quantitative difference between a test
contacting strain measurement systems and to provide a
measurement and a reference value.
common framework for quantitative comparison of optical
3.1.2 raw data—The sampled values of a sensor output.
systems. The output from a non-contacting optical strain and
3.2 Definitions of Terms Specific to This Standard:
deformation measurement system is generally divided into
optical data and image analysis data. Optical data contains 3.2.1 coherent illumination—light source where the differ-
ence in phase is solely a function of optical path differences;
information related to specimen strains and the image analysis
process converts the encoded optical information into strain interference is a direct consequence.
data. The enclosed document describes potential sources of
3.2.2 decoded data—measurement information related to
error in the strain data and describes general methods for
the displacement or displacement gradient field.
quantifying the error and estimating the accuracy of the
3.2.3 decoded data bandwidth—spatial frequency range of
measurements when applying non-contacting methods to the
the information after decoding of the optical data.
study of events for which the optical integration time is much
smallerthantheinverseofthemaximumtemporalfrequencyin
3.2.4 derived data—data obtained through processing of the
the encoded data (that is, events that can be regarded as static
raw data.
during the integration time). A brief application of the
3.2.5 dynamic range—the range of physical parameter val-
approach, along with specific examples defining the various
ues for which measurements can be acquired with the mea-
terms, is given in the Appendix.
surement system.
2. Referenced Documents
3.2.6 illumination wavelength—wavelength of illumination,
ζ.
2.1 ASTM Standards:
E8 Test Methods for Tension Testing of Metallic Materials
3.2.7 incoherent illumination—light source with random
E83 Practice for Verification and Classification of Exten-
variations in optical path differences; constructive or destruc-
someter Systems
tive interference of waves is not possible.
E251 Test Methods for Performance Characteristics of Me-
3.2.8 maximum temporal frequency of encoded data—
tallic Bonded Resistance Strain Gages
reciprocal of the shortest event time contained in the encoded
E399 Test Method for Linear-Elastic Plane-Strain Fracture
data (for example, time variations in displacement field).
Toughness K of Metallic Materials
Ic
3.2.9 measurement noise—variations in the measurements
E1823 TerminologyRelatingtoFatigueandFractureTesting
that are not related to actual changes in the physical property
being measured. May be quantified by statistical properties
This guide is under the jurisdiction of ASTM Committee E08 on Fatigue and
such as standard deviation.
Fracture and is the direct responsibility of Subcommittee E08.03 on Advanced
Apparatus and Techniques.
3.2.10 measurement resolution—smallest change in the
Current edition approved Nov. 1, 2010. Published January 2011. Originally
physical property that can be reliably measured.
approved in 2002. Last previous edition approved in 2002 as E2208–02. DOI:
10.1520/E2208-02R10E01.
3.2.11 numerical aperture, (N.A.)—non-dimensional mea-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
sure of diffraction-limitation for imaging system; N.A. = D/f
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
for a simple lens system, where D is lens diameter and f is lens
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. focal length.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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E2208 − 02 (2010)
3.2.12 optical data—recorded images of specimen, contain- temperature, humidity, lighting, out-of-plane displacements
ing encoded information related to the displacement or dis- (for 2-D systems) etc. Hardware effects include lens
placement gradient field, or both.
aberrations, thermal drift in recording media, variations in
sensing elements, interlacing of lines, phase lag due to refresh
3.2.13 optical data bandwidth—spatial frequency range of
rates, depth of field for recording system, etc. Software effects
the optical pattern (for example, fringes, speckle pattern, etc.)
include interpolation errors, search algorithm processes, image
that can be recorded in the images without aliasing or loss of
boundary effects, etc.
information.
3.2.14 optical integration time—time over which digital
4. Description of General Optical Non-Contacting Strain
image data is averaged to obtain a discretely sampled repre-
Measurement Systems
sentation of the object.
4.1 Figs. 1 and 2 show schematics of typical moiré and
3.2.15 optical resolution, (OR)—distance, d = ζ / (2 N.A.),
digital image correlation setups used to make displacement
between a pair of lines that can be quantatively determined.
field measurements. In its most basic form, an optical non-
3.2.16 quantization level—number of bits used in the digital
contacting strain measurement system such as shown in Figs. 1
recording of optical data by each sensor for image analysis.
and 2, consists of five components. The five components are
The quantization level is one of the parameters determining the
(a) an illumination source, (b ) a test specimen, (c) a method to
fidelity of the recorded optical images. It is determined by the
apply forces to the specimen, (d) a recording media to obtain
camera selected for imaging and typically is 8 bits for most
images of the object at each load level of interest and (e)an
cameras.
image analysis procedure to convert the encoded deformation
3.2.17 recording resolution (pixels/length), κ—number of
information into strain data. Since the encoded information in
optical sensor elements (pixels) used to record an image of a
the optical images may be related either to displacement field
region of length L on object.
components or to the displacement gradient field components,
3.2.18 spatial resolution for encoded data—one-half of the
image analysis procedures will be somewhat different for each
period of the highest frequency component contained in the
case. However, regardless of which form is encoded in the
frequency band of the encoded data.
images, the images are the Basic Data and the displacement
3.2.19 spatial resolution for optical data—one-half of the
fieldsandthestrainfieldswillbepartoftheDerivedData.This
period of the highest frequency component contained in the
guide is primarily concerned with general features of (a) the
frequency band of the optical data. Note that decoded data may
illumination source, (d) image recording components, and (e)
have a lower spatial resolution due to the decoding process.
image analysis procedures. ASTM standards for specimen
design and loading, such asTest Methods E8 for tensile testing
3.2.20 systematic errors—biased variations in the measure-
ments due to the effects of test environment, hardware and/or of metals or Test Method E399 for plane strain fracture
software. Test environment effects include changes in toughness provide the basis for (b, c).
FIG. 1 Typical Optical Moiré Systems for In-Plane Displacement Measurement
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E2208 − 02 (2010)
FIG. 2 Typical Digital Image Correlation Setups For (a) In-Plane Displacement Measurement and (b) Three-Dimensional Displacement
Measurement
5. Error Sources may severely impact the ability to extract encoded information
from the image data. For example, if the translation is large
5.1 At each stage of the flow of data in the measurement
compared to the measurement resolution and the optical
system, errors can be introduced. These are considered in the
resolution of the recording media is low, then the high
sequence in which they occur in this guide.
frequency encoded information may be lost.
5.2 Errors Introduced in Recording Process—Since the
5.6 Errors in Extraction Process—The encoded information
mediausedtorecordBasicDatacanintroduceadditionalerrors
intheDerivedData,eachsetofexperimentaldatamustinclude extracted from the recorded images is degraded by errors
introduced by the image processing method used. Errors
a detailed description of the recording media used. If a digital
camera system is used to record images, data to be included introduced by the extraction process can be a combination of
should be the camera manufacturer, camera output form (for random errors as well as systematic errors (for example,
example, analog or digital), camera spatial resolution, data peak-estimator bias or drift in Fourier correlation methods).
acquisition board type, pixel quantization level (for example, 8 Improved methods for image processing may significantly
bits), ratio of pixel dimensions, lens type and manufacturer.
reduce extraction errors and special care should be taken to
When photographic film is used to record images, the film reduce systematic errors.
characteristics and method of processing, as well as lens type
5.6.1 For example, one can define an engineering measure
and manufacturer used in imaging should be documented.
of normal strain along the “n” direction as:
5.3 Errors Due to Extraneous Vibrations—Depending upon final initial
L 2 L
~ !
n n
ε 5
the measurement resolution, system vibrations can increase nn initial
L
n
errors in the encoded information which may result in addi-
final 2 2
Here, ε is defined by L =[(L + ∆u ) +(∆u ) +
tional extraction errors. Provided that the period of vibration is
nn n n n t1
1/2
sufficiently small relative to the integration time, and the (∆u )] and (∆u , ∆u , ∆u ) are finite changes in displace-
t2 n t1 t2
ment along the perpendicular directions n, t and t for points
amplitude of the disturbance is small relative to the quantity
1 2
being measured, sensor averaging may reduce the effect of at either end of line L . Thus, errors in strainε can be due to
n nn
vibrations on the displacement fields and the strain fields. (1) errors in the initial length of the line element and (2 ) errors
in the displacement components (∆u , ∆u , ∆u ). In both
n t1 t2
5.4 Errors Due to Lighting Variations—Since the Basic
cases, extraction of Derived Data from the Basic Data is the
Data is image data, lighting variations during the experiment
source of error.
may affect (1) the actual encoded information (for example,
phase shift in coherent methods) and (2) extraction of the
5.7 Errors in Processing Extracted Data—Errors are intro-
encoded information. For incoherent methods, light variations
duced when the form of extracted Derived Data in 5.6 is
of several quantization levels may degrade the Derived Data
processedtoobtainstraindata.Thisprocesscaninvolveawide
extracted from the images. Similar effects are possible for
range of mathematical operations including (1) numerical
coherent methods if there are, for instance, slight changes in
differentiation of derived displacement data and (2) smoothing
the wavelength of the illumination. In both cases, use of image
of displacement or displacement gradient data. Errors intro-
processing methods that are insensitive to lighting variations
ducedbythechoiceofpost-processingmethodcaninclude,but
(for example, normalized cross correlation) will increase the
are not limited to, (1) reduction of spatial resolution, (2)
accuracy of the extracted data.
systematic under-prediction of strain in areas of high strain
5.5 Errors Due to Rigid Body Motion—Depending upon the gradients, (3) phase errors in the signal due to non-symmetric
measurement resolution, rigid body translation and/or rotation operators etc.
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E2208 − 02 (2010)
6. System Evaluation Process obtained from the Derived Data. If this approach is used, all
data acquisition and analysis procedures must remain the same
6.1 Each non-contacting optical strain measurement system
as used in the actual tests, with clear documentation provided
must be evaluated to determine reliable estimates for the
to demonstrate that the same procedure has been used for both
accuracy of the resulting Derived Data. Given the wide range
tests.
of methods that have been developed, this guide will not
address specific details involving the application of any tech- 6.4 Comparison to Standard Measurement Methods for
nique. Rather, the guidelines are provided as a general frame-
Simulated Test Conditions in Laboratory Environments—For
work for evaluation of non-contacting optical strain measure- those cases where laboratory tests can be used to approximate
ment systems.
actual test conditions, calibration tests should be performed on
laboratory specimens for accuracy assessment. In this
6.2 In the following sections, a direct comparison between
approach, the effects of phenomena present in actual test
established measurement methods and non-contacting methods
conditions must be accounted for in laboratory tests so that
is recommended. However, it must be noted that, even though
potential errors associated with the testing environment are
this approach does provide a direct, quantitative measure of
included. For these tests, direct comparison of the non-
agreement between two, independent measurement data sets.
contacting measurements to independent measurements by
Practice E83 and Test Methods E251 provide only average
established methods using documentedASTM procedures (see
values for strain over a specific area on the specimen. Thus,
Practice E83 and Test Methods E251) whenever possible are
good agreement with the average value obtained from the
recommended to obtain quantitative estimates of the accuracy
Derived Data in the same area does not verify (1) the accuracy
of the Derived Data. If this approach is used, all data
of local variations observed in the Derived Data or (2) the
acquisition and analysis procedures must remain the same as
accuracy of the Derived Data in regions outside the area where
used for actu
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