1、Designation: E2208 02 (Reapproved 2010)Standard 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 yea
2、r 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.1. Scope1.1 The purpose of this document is to assist potential usersin understanding the issues related to the accuracy of n
3、on-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 analysis data. Optical data containsinformation r
4、elated 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 estimating the accuracy of themeasurements when a
5、pplying 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). A brief application of the ap-proach, along wi
6、th specific examples defining the variousterms, is given in the Appendix.2. Referenced Documents2.1 ASTM Standards:2E8 Test Methods for Tension Testing of Metallic MaterialsE83 Practice for Verification and Classification of Exten-someter SystemsE251 Test Methods for Performance Characteristics of M
7、e-tallic Bonded Resistance Strain GaugesE399 Test Method for Linear-Elastic Plane-Strain FractureToughness KIcof Metallic MaterialsE1823 Terminology Relating to Fatigue and Fracture Test-ing3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 accuracyquantitative relationship of th
8、e measure-ments to the value obtained by standard measurement tech-niques3.1.2 basic datadata obtained directly by the measure-ment system. For optical, non-contacting methods, a two-dimensional array of image intensity data is generally the basicdata.3.1.3 coherent illuminationlight source where th
9、e differ-ence in phase is solely a function of optical path differences;interference is a direct consequence.3.1.4 decoded datameasurement information related tothe displacement or displacement gradient field.3.1.5 decoded data bandwidthspatial frequency range ofthe information after decoding of the
10、 optical data.3.1.6 derived datadata obtained through processing of thebasic data. Typically, this is displacement field data.3.1.7 dynamic rangethe range of physical parameter val-ues for which measurements can be acquired with the mea-surement system.3.1.8 illumination wavelengthwavelength of illu
11、mination,z.3.1.9 incoherent illuminationlight source with randomvariations in optical path differences; constructive or destruc-tive interference of waves is not possible.3.1.10 maximum temporal frequency of encoded datareciprocal of the shortest event time contained in the encodeddata (for example,
12、 time variations in displacement field).3.1.11 measurement noisevariations in the measurementsthat are not related to actual changes in the physical propertybeing measured. May be quantified by statistical propertiessuch as standard deviation.1This guide is under the jurisdiction of ASTM Committee E
13、08 on Fatigue andFracture and is the direct responsibility of Subcommittee E08.03 on AdvancedApparatus and Techniques.Current edition approved Nov. 1, 2010. Published January 2011. Originallyapproved in 2002. Last previous edition approved in 2002 as 02. DOI: 10.1520/E2208-02R10.2For referenced ASTM
14、 standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohoc
15、ken, PA 19428-2959, United States.3.1.12 measurement resolutionsmallest change in thephysical property that can be reliably measured.3.1.13 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 an
16、d f is lensfocal length.3.1.14 optical datarecorded images of specimen, contain-ing encoded information related to the displacement or dis-placement gradient field, or both.3.1.15 optical data bandwidthspatial frequency range ofthe optical pattern (for example, fringes, speckle pattern, etc.)that ca
17、n be recorded in the images without aliasing or loss ofinformation.3.1.16 optical integration timetime over which digitalimage data is averaged to obtain a discretely sampled repre-sentation of the object.3.1.17 optical resolution, (OR)distance, d = z / (2 N.A.),between a pair of lines that can be q
18、uantatively determined.3.1.18 quantization levelnumber of bits used in the digitalrecording of optical 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
19、 typically is 8 bits for mostcameras.3.1.19 recording resolution (pixels/length), knumber ofoptical sensor elements (pixels) used to record an image of aregion of length L on object.3.1.20 spatial resolution for encoded dataone-half of theperiod of the highest frequency component contained in thefre
20、quency band of the encoded data.3.1.21 spatial resolution for optical dataone-half of theperiod of 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.3.1.22 systematic errorsbiased
21、variations in the measure-ments due to the effects of test environment, hardware and/orsoftware. Test environment effects include changes in tempera-ture, humidity, lighting, out-of-plane displacements (for 2-Dsystems) etc. Hardware effects include lens aberrations, ther-mal drift in recording media
22、, variations in sensing elements,interlacing of lines, phase lag due to refresh rates, depth of fieldfor recording system, etc. Software effects include interpola-tion errors, search algorithm processes, image boundary ef-fects, etc.4. Description of General Optical Non-Contacting StrainMeasurement
23、Systems4.1 Figs. 1 and 2 show schematics of typical moir anddigital image correlation setups used to make 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)
24、 an illumination source, (b) a test specimen, (c) a method toapply forces to the specimen, (d) a recording 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 informatio
25、n inthe optical images may be related either to displacement fieldcomponents or to the displacement gradient 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 displaceme
26、ntfields and the strain fields will be part of the Derived Data. Thisguide is primarily concerned with general features of (a) theillumination source, (d) image recording components, and (e)image analysis procedures. ASTM standards for specimendesign and loading, such as Test Methods E8 for tensile
27、testingFIG. 1 Typical Optical Moir Systems for In-Plane Displacement MeasurementE2208 02 (2010)2of metals 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. Thes
28、e are considered in thesequence in which they occur in this guide.5.2 Errors Introduced in Recording ProcessSince 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
29、digitalcamera system is used to record images, data to be includedshould be the camera manufacturer, camera 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 manu
30、facturer.When photographic film is used to record images, the filmcharacteristics and method of processing, 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
31、in the encoded information which may result in addi-tional extraction errors. Provided that the period 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 ofvib
32、rations on the displacement fields and the strain fields.5.4 Errors Due to Lighting VariationsSince the 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 infor
33、mation. For incoherent methods, light variationsof several quantization levels may degrade the Derived 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 imageprocessin
34、g methods that are insensitive to lighting variations(for example, normalized cross correlation) will increase 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 extra
35、ct encoded informationfrom the image data. For example, if the translation is largecompared to the measurement 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 f
36、rom the recorded images is degraded by errorsintroduced by the image processing method used. Errorsintroduced 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 ima
37、ge processing may significantlyreduce extraction errors and special care should be taken toreduce systematic 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+ Dun)2+(Dut1)2+(Dut
38、2)1/2and (Dun, Dut1, Dut2) are finite changes in displace-ment along the perpendicular directions n, t1and t2for 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 (Dun, Dut1, Dut2).
39、In bothcases, extraction of Derived Data from the Basic Data is thesource of error.5.7 Errors in Processing Extracted 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
40、 (1) numericaldifferentiation of derived displacement data and (2) smoothingof displacement or displacement gradient data. Errors intro-duced by the choice of post-processing method can include, butare not limited to, (1) reduction of spatial resolution, (2)FIG. 2 Typical Digital Image Correlation S
41、etups For (a) In-Plane Displacement Measurement and (b) Three-Dimensional DisplacementMeasurementE2208 02 (2010)3systematic 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-contactin
42、g optical strain measurement systemmust be evaluated to determine reliable estimates for theaccuracy of the resulting 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
43、 are provided as a general frame-work for evaluation of non-contacting optical strain measure-ment systems.6.2 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
44、 provide a direct, quantitative measure ofagreement between two, independent measurement data sets.Practice E83 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
45、does not verify (1) the accuracyof local variations observed in the Derived Data or (2) theaccuracy of the Derived 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 er
46、rors in relative displacement components can bedirectly related to strain errors using equations such as:EnnDLnfinalLninitialWhere Ennis an estimate for the error in the normal strainnn. Here, DLnfinalis the error in final length due to errors in themeasured displacement components. Through calibrat
47、ion, thecontribution of inaccuracies in the relative displacement com-ponents (Dun, Dut1, Dut2) to strain error can 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 S
48、tandard Measurement Methods forSimilar Test ConditionsNon-contacting measurements canbe made under diverse conditions (for example, high tempera-ture, in-situ structures, laboratory test frames, vacuum). Due tothe diversity of conditions, calibration tests are recommendedon similar components under
49、similar conditions to assessaccuracy. In this approach, the effects of those phenomenapresent in the test condition but not accounted for in laboratorytests or computer simulations are directly included in the errorassessment.6.3.1 For these tests, direct comparison of the non-contacting measurements to independent measurements byestablished methods using documented ASTM procedures (forexample, extensometers (Practice E83), strain gages (TestMethods E251) whenever possible is the most reliable way toobtain quantitative estimates for the acc
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