ASTM E1942-1998(2018)e1 Standard Guide for Evaluating Data Acquisition Systems Used in Cyclic Fatigue and Fracture Mechanics Testing.pdf

1、Designation: E1942 98 (Reapproved 2018)1Standard Guide forEvaluating Data Acquisition Systems Used in Cyclic Fatigueand Fracture Mechanics Testing1This standard is issued under the fixed designation E1942; the number immediately following the designation indicates the year oforiginal adoption or, in

2、 the case of revision, the year 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.1NOTESections 3.1.3, A1.2.2.1, A1.2.3, and A1.2.4 were editorially corrected in August 2018.1.

3、Scope1.1 This guide covers how to understand and minimize theerrors associated with data acquisition in fatigue and fracturemechanics testing equipment. This guide is not intended to beused instead of certified traceable calibration or verification ofdata acquisition systems when such certification

4、is required. Itdoes not cover static load verification, for which the user isreferred to the current revision of Practices E4, or staticextensometer verification, for which the user is referred to thecurrent revision of Practice E83. The user is also referred toPractice E467.1.2 The output of the fa

5、tigue and fracture mechanics dataacquisition systems described in this guide is essentially astream of digital data. Such digital data may be considered tobe divided into two types Basic Data, which are a sequence ofdigital samples of an equivalent analog waveform representingthe output of transduce

6、rs connected to the specimen under test,and Derived Data, which are digital values obtained from theBasic Data by application of appropriate computational algo-rithms. The purpose of this guide is to provide methods thatgive confidence that such Basic and Derived Data describe theproperties of the m

7、aterial adequately. It does this by settingminimum or maximum targets for key system parameters,suggesting how to measure these parameters if their actualvalues are not known.1.3 This international standard was developed in accor-dance with internationally recognized principles on standard-ization e

8、stablished in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2E4 Practices for Force Verification of Testing MachinesE83

9、Practice for Verification and Classification of Exten-someter SystemsE467 Practice for Verification of Constant Amplitude Dy-namic Forces in an Axial Fatigue Testing SystemE1823 Terminology Relating to Fatigue and Fracture Testing3. Terminology3.1 Definitions:3.1.1 bandwidth T1the frequency at which

10、 the amplituderesponse of the channel has fallen to 1/=2 of its value at lowfrequency.3.1.1.1 DiscussionThis definition assumes the sensorchannel response is low-pass, as in most materials testing. Anillustration of bandwidth is shown in Fig. 1.3.1.2 Basic Data samplethe sampled value of a sensorwav

11、eform taken at fixed time intervals. Each sample representsthe actual sensor value at that instant of time.3.1.2.1 DiscussionFig. 2 shows examples of Basic Datasamples.3.1.3 data rate T1the data rate is1td Hertz where thetime intervals between samples is1td in seconds.3.1.3.1 DiscussionThe data rate

12、 is the number of datasamples per second made available to the user, assuming therate is constant.3.1.4 derived datadata obtained through processing of theraw data.3.1.4.1 DiscussionFig. 2 illustrates examples of DerivedData.3.1.5 noise levelthe standard deviation of the data samplesof noise in the

13、transducer channel, expressed in the unitsappropriate to that channel.1This guide is under the jurisdiction of ASTM Committee E08 on Fatigue andFracture and is the direct responsibility of SubcommitteeE08.03 on AdvancedApparatus and Techniques.Current edition approved June 1, 2018. Published August

14、2018. Originallyapproved in 1998. Last previous edition approved in 2010 as E1942 - 98(2010)1.DOI: 10.1520/E1942-98R18E012For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refe

15、r 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 recognized principles on standardization establish

16、ed 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.13.1.6 peakthe point of maximum load in constant ampli-tude loading (see Terminology E1823).3.1.7 phase differ

17、ence the angle in degrees separatingcorresponding parts of two waveforms (such as peaks), whereone complete cycle represents 360.3.1.7.1 DiscussionThe phase difference of a cyclic wave-form only has meaning in reference to a second cyclicwaveform of the same frequency.3.1.8 sampling rate T1the rate

18、at which the analog-to-digital converter samples a waveform. This rate may not bevisible to the user of the data acquisition system.3.1.8.1 DiscussionA distinction is made here betweensampling rate and data rate, because in some data acquisitionsystems, the analog waveform may be sampled at a muchhi

19、gher rate than the rate at which data are made available to theuser. (Such a technique is commonly known as over-sampling).3.1.9 word sizethe number of significant bits in a singledata sample.3.1.9.1 DiscussionThe word size is one parameter whichdetermines the system resolution. Usually it will be d

20、eterminedby the analog-digital converter used, and typically may be 12or 16 bits. If the word size is w, then the smallest step changein the data that can be seen is 1 part in 2w, that is thequantization step is d =2w.3.1.10 valleyThe point of minimum load in constantamplitude loading (see Terminolo

21、gy E1823).4. Description of a Basic Data Acquisition System4.1 In its most basic form, a mechanical testing systemconsists of a test frame with grips which attach to a testspecimen, a method of applying forces to the specimen, and anumber of transducers which measure the forces and displace-ments ap

22、plied to the specimen (see Fig. 3). The output fromthese transducers may be in digital or analog form, but if theyare analog, they are first amplified and filtered and thenconverted to digital form using analog-to-digital converters(ADCs). The resulting stream of digital data may be digitallyfiltere

23、d and manipulated to result in a stream of output BasicData which is presented to the user in the form of a displayedor printed output, or as a data file in a computer. Variousalgorithms may be applied to the Basic Data to deriveparameters representing, for example, the peaks and valleys ofthe force

24、s and displacements applied to the specimen, or thestresses and strains applied to the specimen and so forth. Suchparameters are the Derived Data.4.1.1 The whole measurement system may be divided intothree sections for the purpose of verification: the mechanicaltest frame and its components, the ele

25、ctrical measurementsystem, and the computer processing of data. This guide isspecifically concerned only with the electrical measurementsystem commencing at the output of the transducers. Beforethe mechanical system is investigated for dynamic errors by themethods given in Practice E467, this guide

26、can be used toascertain that the electrical measurement system has adequateperformance for the measurements required for Practice E467.If the requirements of Practice E467 for the mechanical systemand the recommendations of this guide are met, then the userhas confidence that the Basic Data produced

27、 by the testingsystem are adequate for processing by subsequent computeralgorithms to produce further Derived Data.4.1.2 At each stage of the flow of data in the electricalmeasurement system, errors can be introduced. These shouldbe considered in the sequence in which these are dealt with inthis gui

28、de. The sequence includes:4.2 Errors Due to Bandwidth Limitations in the SignalConditioningWhere there is analog signal conditioning priorto analog-to-digital conversion, there will usually be restric-tions on the analog bandwidth in order to minimize noise and,in some cases, to eliminate products o

29、f demodulation. Afterdigital conversion, additional digital filtering may be applied toreduce noise components. These bandwidth restrictions resultin cyclic signals at higher frequencies having an apparentFIG. 1 3-dB Bandwidth of Sensor ChannelFIG. 2 Basic and Derived DataFIG. 3 Sources of Error in

30、Data Acquisition SystemsE1942 98 (2018)12amplitude which is lower than the true value, and if thewaveform is not sinusoidal, also having waveform distortion.The bandwidth restrictions also cause phase shifts which resultin phase measurement errors when comparing phase in twochannels with different b

31、andwidths.4.3 Errors Due to Incorrect Data RateErrors can resultfrom an insufficient data rate, where the intervals between datasamples are too large and intervening events are not recordedin the Basic Data. These result also in errors in the DerivedData, for example, when the peak value of a wavefo

32、rm ismissed during sampling. Data skew, where the Basic Data arenot acquired at the same instant in time, can produce similarerrors to phase shifts between channels.4.4 Errors Due to Noise and DriftNoise added to thesignal being measured causes measurement uncertainty. Shortterm noise causes variabi

33、lity or random error, and includesanalog noise at the transducer output due to electrical ormechanical pick up, and analog noise added in the amplifier,together with digital noise, or quantization, due to the finitedigital word length of the ADC system.4.4.1 Long-term effects, such as drifts in the

34、transduceroutput or its analog signal conditioning due to temperature oraging effects, are indistinguishable from slow changes in theforces and displacements seen by the specimen, and cause amore systematic error.4.4.2 Further details of these sources of error are given inAnnex A1.5. System Requirem

35、ents5.1 How This Section is OrganizedThis section gives thesteps that must be taken to ensure the errors are controlled.There are several sources of error in the electrical system, andthese may add both randomly and deterministically. To givereasonable assurance that these errors have a minor effect

36、 onoverall accuracy of a system with 1 % accuracy, recommenda-tions are given in this guide, which result in a 0.2 % errorbound for each individual source of error. However, Annex A1also shows how the error varies with each parameter, so thatthe user may choose to use larger or smaller error bounds

37、withappropriate adjustments to bandwidth, data rate, and so forth.5.1.1 In this section, which is intended to be used in theorder written, a minimum value or a maximum value isrecommended for each parameter. If the actual value of eachparameter is known, then the system requirement is that in eachca

38、se either:Maximum value actual valueorMinimum value actual value.However, if the actual value is not known, then help is givenas to how to determine it.5.2 Frequency and WaveshapeThe first step is to deter-mine the highest cyclic frequency, f Hz, at which testing willoccur, and the waveshape to be e

39、mployed (for example,sinusoidal, triangular, square).5.3 Minimum BandwidthIf the waveform is sinusoidal orsquare, then the minimum bandwidth is 10f Hz to measure thepeak value. If the waveform is triangular, then the minimumbandwidth is 100f Hz. For example, for a 10Hz sinusoidalwaveform, the minimu

40、m bandwidth is 100 Hz. For a discussionof minimum bandwidth, see A1.2.1 and A1.2.2.5.4 Actual BandwidthThe actual bandwidth must be equalto or greater than the minimum bandwidth. If this conditioncannot be met, then the errors will increase as shown in A1.2.1and A1.2.2. If the actual bandwidth is no

41、t known, then it can beascertained using one of the suggested methods in A1.2.3,orotherwise.5.5 Minimum Data RateFor measurement of the peakvalue of sinusoidal or square waveforms, the minimum datarate is 50 points/cycle, or 50f points/s. For measurement of thepeak value of triangular waveforms, the

42、 minimum data rate is400 points/cycle, or 400f points/s. If the data acquisitionsystem produces the peak value as an output, then the internalBasic Data rate used should equal or exceed the appropriateminimum data rate (depending on waveform type). This shouldbe verified even if the external rate at

43、 which samples arepresented is less than this minimum value. For a discussion ofdata rate, see A1.3.1.5.6 Actual Data RateThe actual data rate must equal orexceed the minimum data rate. If the actual data rate is notknown, then it must be ascertained using a method such as thatin A1.3.2.5.7 Maximum

44、Permitted Noise LevelThe noise level is thestandard deviation of the noise in the transducer channel,expressed in the units appropriate to the channel. The maxi-mum permitted noise level is 0.2 % of the expected peak valueof the waveform being measured. For example, if the expectedpeak value in a lo

45、ad channel is 100 kN, then the standarddeviation of the noise in that channel must not exceed 0.2 kN.5.8 Actual Noise LevelThe actual noise level must beequal to or less than the maximum permitted noise level. If theactual noise level is not known, then it must be ascertainedusing a method such as t

46、hat in A1.4.6. Guidance on how toinvestigate sources of noise is given in A1.4.7.5.8.1 If the actual noise level exceeds the maximum permit-ted noise level, it can usually be reduced by reducingbandwidth, but this will require beginning again at 5.3 to verifythat the bandwidth reduction is permissib

47、le.5.9 Maximum Permissible Phase Difference and MaximumPermissible Data SkewThese terms are discussed in A1.5.1and A1.5.2. No value is recommended for the maximumpermissible phase difference and data skew between channels,since this is very dependent on the testing application. Iftypical phase shift

48、s between displacement and force due to thematerial under test are 10 to 20, then an acceptable value forthe maximum phase difference might be 1. However, if typicalphase shifts are 2 to 3, the acceptable value for the maximumphase difference might be only 0.1.5.10 Actual Phase Shift and Data SkewMe

49、thods for esti-mating the combined effect of phase shift and data skew in adata acquisition system are given in A1.5.3.6. Report6.1 The purpose of the report is to record that due consid-eration was given to essential performance parameters of theE1942 98 (2018)13data acquisition system when performing a particular fatigue orfracture mechanics test. Since the report should ideally be anattachment to each set of such test results, it should besufficient but succinct. The report should contain the followinginformation, prefer