1、Designation: E2208 02 (Reapproved 2018)1Standard Guide forEvaluating Non-Contacting Optical Strain MeasurementSystems1This standard is issued under the fixed designation E2208; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the ye
2、ar of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1NOTE3.1.2 was editorially revised in June 2018.1. Scope1.1 The purpose of this document is to assist potential usersin unde
3、rstanding the issues related to the accuracy of non-contacting strain measurement systems and to provide acommon framework for quantitative comparison of opticalsystems. The output from a non-contacting optical strain anddeformation measurement system is generally divided intooptical data and image
4、analysis data. Optical data containsinformation related to specimen strains and the image analysisprocess converts the encoded optical information into straindata. The enclosed document describes potential sources oferror in the strain data and describes general methods forquantifying the error and
5、estimating the accuracy of themeasurements when applying non-contacting methods to thestudy of events for which the optical integration time is muchsmaller than the inverse of the maximum temporal frequency inthe encoded data (that is, events that can be regarded as staticduring the integration time
6、). A brief application of theapproach, along with specific examples defining the variousterms, is given in the Appendix.1.2 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelop
7、ment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2E8 Test Methods for Tension Testing of Metallic MaterialsE83 Practice for Verification and Classification of Exten
8、-someter SystemsE251 Test Methods for Performance Characteristics of Me-tallic Bonded Resistance Strain GagesE399 Test Method for Linear-Elastic Plane-Strain FractureToughness KIcof Metallic MaterialsE1823 Terminology Relating to Fatigue and Fracture Testing3. Terminology3.1 Definitions:3.1.1 accura
9、cythe quantitative difference between a testmeasurement and a reference value.3.1.2 raw datathe sampled values of a sensor output.3.2 Definitions of Terms Specific to This Standard:3.2.1 coherent illuminationlight source where the differ-ence in phase is solely a function of optical path differences
10、;interference is a direct consequence.3.2.2 decoded datameasurement information related tothe displacement or displacement gradient field.3.2.3 decoded data bandwidthspatial frequency range ofthe information after decoding of the optical data.3.2.4 derived datadata obtained through processing of the
11、raw data.3.2.5 dynamic rangethe range of physical parameter val-ues for which measurements can be acquired with the mea-surement system.3.2.6 illumination wavelengthwavelength of illumination,.3.2.7 incoherent illuminationlight source with randomvariations in optical path differences; constructive o
12、r destruc-tive interference of waves is not possible.3.2.8 maximum temporal frequency of encoded datareciprocal of the shortest event time contained in the encodeddata (for example, time variations in displacement field).3.2.9 measurement noisevariations in the measurementsthat are not related to ac
13、tual changes in the physical propertybeing measured. May be quantified by statistical propertiessuch as standard deviation.1This guide is under the jurisdiction of ASTM Committee E08 on Fatigue andFracture and is the direct responsibility of Subcommittee E08.03 on AdvancedApparatus and Techniques.Cu
14、rrent edition approved May 1, 2018. Published June 2018. Originallyapproved in 2002. Last previous edition approved in 2010 as E220802(2010)1.DOI: 10.1520/E2208-02R18E01.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annu
15、al Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally r
16、ecognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.13.2.10 measurement resolutionsmallest change in thephysical
17、property that can be reliably measured.3.2.11 numerical aperture, (N.A.)non-dimensional mea-sure of diffraction-limitation for imaging system; N.A. = D/ffor a simple lens system, where D is lens diameter and f is lensfocal length.3.2.12 optical datarecorded images of specimen, contain-ing encoded in
18、formation related to the displacement or dis-placement gradient field, or both.3.2.13 optical data bandwidthspatial frequency range ofthe optical pattern (for example, fringes, speckle pattern, etc.)that can be recorded in the images without aliasing or loss ofinformation.3.2.14 optical integration
19、timetime over which digitalimage data is averaged to obtain a discretely sampled repre-sentation of the object.3.2.15 optical resolution, (OR)distance, d = / (2 N.A.),between a pair of lines that can be quantatively determined.3.2.16 quantization levelnumber of bits used in the digitalrecording of o
20、ptical data by each sensor for image analysis.The quantization level is one of the parameters determining thefidelity of the recorded optical images. It is determined by thecamera selected for imaging and typically is 8 bits for mostcameras.3.2.17 recording resolution (pixels/length), number ofoptic
21、al sensor elements (pixels) used to record an image of aregion of length L on object.3.2.18 spatial resolution for encoded dataone-half of theperiod of the highest frequency component contained in thefrequency band of the encoded data.3.2.19 spatial resolution for optical dataone-half of theperiod o
22、f the highest frequency component contained in thefrequency band of the optical data. Note that decoded data mayhave a lower spatial resolution due to the decoding process.3.2.20 systematic errorsbiased variations in the measure-ments due to the effects of test environment, hardware and/orsoftware.
23、Test environment effects include changes intemperature, humidity, lighting, out-of-plane displacements(for 2-D systems) etc. Hardware effects include lensaberrations, thermal drift in recording media, variations insensing elements, interlacing of lines, phase lag due to refreshrates, depth of field
24、for recording system, etc. Software effectsinclude interpolation errors, search algorithm processes, imageboundary effects, etc.4. Description of General Optical Non-Contacting StrainMeasurement Systems4.1 Figs. 1 and 2 show schematics of typical moir anddigital image correlation setups used to make
25、 displacementfield measurements. In its most basic form, an optical non-contacting strain measurement system such as shown in Figs. 1and 2, consists of five components. The five components are(a) an illumination source, (b ) a test specimen, (c) a method toapply forces to the specimen, (d) a recordi
26、ng media to obtainimages of the object at each load level of interest and (e)animage analysis procedure to convert the encoded deformationinformation into strain data. Since the encoded information inthe optical images may be related either to displacement fieldcomponents or to the displacement grad
27、ient field components,image analysis procedures will be somewhat different for eachcase. However, regardless of which form is encoded in theimages, the images are the Basic Data and the displacementfields and the strain fields will be part of the Derived Data. Thisguide is primarily concerned with g
28、eneral features of (a) theillumination source, (d) image recording components, and (e)image analysis procedures. ASTM standards for specimenFIG. 1 Typical Optical Moir Systems for In-Plane Displacement MeasurementE2208 02 (2018)12design and loading, such as Test Methods E8 for tensile testingof meta
29、ls or Test Method E399 for plane strain fracturetoughness provide the basis for (b, c).5. Error Sources5.1 At each stage of the flow of data in the measurementsystem, errors can be introduced. These are considered in thesequence in which they occur in this guide.5.2 Errors Introduced in Recording Pr
30、ocessSince themedia used to record Basic Data can introduce additional errorsin the Derived Data, each set of experimental data must includea detailed description of the recording media used. If a digitalcamera system is used to record images, data to be includedshould be the camera manufacturer, ca
31、mera output form (forexample, analog or digital), camera spatial resolution, dataacquisition board type, pixel quantization level (for example, 8bits), ratio of pixel dimensions, lens type and manufacturer.When photographic film is used to record images, the filmcharacteristics and method of process
32、ing, as well as lens typeand manufacturer used in imaging should be documented.5.3 Errors Due to Extraneous VibrationsDepending uponthe measurement resolution, system vibrations can increaseerrors in the encoded information which may result in addi-tional extraction errors. Provided that the period
33、of vibration issufficiently small relative to the integration time, and theamplitude of the disturbance is small relative to the quantitybeing measured, sensor averaging may reduce the effect ofvibrations on the displacement fields and the strain fields.5.4 Errors Due to Lighting VariationsSince the
34、 BasicData is image data, lighting variations during the experimentmay affect (1) the actual encoded information (for example,phase shift in coherent methods) and (2) extraction of theencoded information. For incoherent methods, light variationsof several quantization levels may degrade the Derived
35、Dataextracted from the images. Similar effects are possible forcoherent methods if there are, for instance, slight changes inthe wavelength of the illumination. In both cases, use of imageprocessing methods that are insensitive to lighting variations(for example, normalized cross correlation) will i
36、ncrease theaccuracy of the extracted data.5.5 Errors Due to Rigid Body MotionDepending upon themeasurement resolution, rigid body translation and/or rotationmay severely impact the ability to extract encoded informationfrom the image data. For example, if the translation is largecompared to the meas
37、urement resolution and the opticalresolution of the recording media is low, then the highfrequency encoded information may be lost.5.6 Errors in Extraction ProcessThe encoded informationextracted from the recorded images is degraded by errorsintroduced by the image processing method used. Errorsintr
38、oduced by the extraction process can be a combination ofrandom errors as well as systematic errors (for example,peak-estimator bias or drift in Fourier correlation methods).Improved methods for image processing may significantlyreduce extraction errors and special care should be taken toreduce syste
39、matic errors.5.6.1 For example, one can define an engineering measureof normal strain along the “n” direction as:nn5Lnfinal2 Lninitial!LninitialHere, nnis defined by Lnfinal=(Ln+ un)2+(ut1)2+(ut2)1/2and (un, ut1, ut2) are finite changes in displace-ment along the perpendicular directions n, t1and t2
40、for pointsat either end of line Ln. Thus, errors in strain nncan be due to(1) errors in the initial length of the line element and (2 ) errorsin the displacement components (un, ut1, ut2). In bothcases, extraction of Derived Data from the Basic Data is thesource of error.5.7 Errors in Processing Ext
41、racted DataErrors are intro-duced when the form of extracted Derived Data in 5.6 isprocessed to obtain strain data. This process can involve a widerange of mathematical operations including (1) numericaldifferentiation of derived displacement data and (2) smoothingof displacement or displacement gra
42、dient data. Errors intro-duced by the choice of post-processing method can include, butFIG. 2 Typical Digital Image Correlation Setups For (a) In-Plane Displacement Measurement and (b) Three-Dimensional DisplacementMeasurementE2208 02 (2018)13are not limited to, (1) reduction of spatial resolution,
43、(2)systematic under-prediction of strain in areas of high straingradients, (3) phase errors in the signal due to non-symmetricoperators etc.6. System Evaluation Process6.1 Each non-contacting optical strain measurement systemmust be evaluated to determine reliable estimates for theaccuracy of the re
44、sulting Derived Data. Given the wide rangeof methods that have been developed, this guide will notaddress specific details involving the application of any tech-nique. Rather, the guidelines are provided as a general frame-work for evaluation of non-contacting optical strain measure-ment systems.6.2
45、 In the following sections, a direct comparison betweenestablished measurement methods and non-contacting methodsis recommended. However, it must be noted that, even thoughthis approach does provide a direct, quantitative measure ofagreement between two, independent measurement data sets.Practice E8
46、3 and Test Methods E251 provide only averagevalues for strain over a specific area on the specimen. Thus,good agreement with the average value obtained from theDerived Data in the same area does not verify (1) the accuracyof local variations observed in the Derived Data or (2) theaccuracy of the Der
47、ived Data in regions outside the area wherecomparisons were made.6.2.1 For example, if a finite difference in displacementcomponents is used to determine strain components such asnn, then errors in relative displacement components can bedirectly related to strain errors using equations such as:nnLnf
48、inalLninitialWhere Ennis an estimate for the error in the normal strainnn. Here, Lnfinalis the error in final length due to errors in themeasured displacement components. Through calibration, thecontribution of inaccuracies in the relative displacement com-ponents (un, ut1, ut2) to strain error can
49、be determined.However, the accuracy of the Derived Data outside of thisregion, which may be a region of importance, cannot beverified without additional comparisons.6.3 Comparison to Standard Measurement Methods forSimilar Test ConditionsNon-contacting measurements can bemade under diverse conditions (for example, high temperature,in-situ structures, laboratory test frames, vacuum). Due to thediversity of conditions, calibration tests are recommended onsimilar components under similar conditions to assess accu-racy.
