1、Designation: E 1942 98 (Reapproved 2004)Standard Guide forEvaluating Data Acquisition Systems Used in Cyclic Fatigueand Fracture Mechanics Testing1This standard is issued under the fixed designation E 1942; the number immediately following the designation indicates the year oforiginal adoption or, i
2、n the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers how to understand and minimize theerrors associated with data a
3、cquisition 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 is required. Itdoes not cover static load verification, for which the user isreferred to t
4、he 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 E 467.1.2 The output of the fatigue and fracture mechanics dataacquisition systems described in this guide is essential
5、ly 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 transducers connected to the specimen under test,and Derived Data, which are digital values obtain
6、ed 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 material adequately. It does this by settingminimum or maximum targets for key system para
7、meters,suggesting how to measure these parameters if their actualvalues are not known.2. Referenced Documents2.1 ASTM Standards:2E4 Practices for Force Verification of Testing MachinesE83 Practice for Verification and Classification of Exten-someterE 467 Practice for Verification of Constant Amplitu
8、de Dy-namic Forces in an Axial Fatigue Testing SystemE 1823 Terminology Relating to Fatigue and Fracture Test-ing3. Terminology3.1 Definitions:3.1.1 bandwidth T1the frequency at which the ampli-tude response of the channel has fallen to 1/=2 of its valueat low frequency.3.1.1.1 DiscussionThis defini
9、tion 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 sensorwaveform taken at fixed time intervals. Each sample representsthe actual sensor value at that instant of time.3.1.2.1
10、DiscussionFig. 2 shows examples of Basic Datasamples.3.1.3 data rate T1the date rate is1td Hertz where thetime intervals between samples is tdin seconds.3.1.3.1 DiscussionThe data rate is the number of datasamples per second made available to the user, assuming therate is constant.3.1.4 Derived Data
11、any waveform parameter which isderived from one or several of the Basic Data samples.3.1.4.1 DiscussionFig. 2 illustrates examples of DerivedData.3.1.5 noise levelthe standard deviation of the datasamples of noise in the transducer channel, expressed in theunits appropriate to that channel.3.1.6 pea
12、kthe point of maximum load in constant ampli-tude loading (see Terminology E 1823).3.1.7 phase difference 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
13、meaning in reference to a second cyclicwaveform of the same frequency.3.1.8 sampling rate T1the rate at which the analog-to-digital converter samples a waveform. This rate may not bevisible to the user of the data acquisition system.1This guide is under the jurisdiction of ASTM Committee E08 on Fati
14、gue andFracture and is the direct responsibility of Subcommittee E08.03 on AdvancedApparatus and Techniques.Current edition approved May 1, 2004. Published June 2004. Originallyapproved in 1998. Last previous edition approved in 1998 as E 1942 - 98e1.2For referenced ASTM standards, visit the ASTM we
15、bsite, 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 Conshohocken, PA 19428-2959, United St
16、ates.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 muchhigher rate than the rate at which data are made available to theuser. (Such a technique is commonly known as over-sampling).3.1.
17、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 determinedby the analog-digital converter used, and typically may be 12or 16 bits. If the word size is w, then the smallest step
18、 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 Terminology E 1823).4. Description of a Basic Data Acquisition System4.1 In its most basic form, a mechanical testing systemconsists of
19、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 applied to the specimen (see Fig. 3). The output fromthese transducers may be in digital or analog form, but if theyare analog,
20、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 digitallyfiltered and manipulated to result in a stream of output BasicData which is presented to the user in the form of a displayedor printe
21、d 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 forces and displacements applied to the specimen, or thestresses and strains applied to the specimen and so forth. Suchparameters a
22、re 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 electrical measurementsystem, and the computer processing of data. This guide isspecifically concerned only with the electrical m
23、easurementsystem commencing at the output of the transducers. Beforethe mechanical system is investigated for dynamic errors by themethods given in Practice E 467, this guide can be used toascertain that the electrical measurement system has adequateperformance for the measurements required for Prac
24、tice E 467.If the requirements of Practice E 467 for the mechanicalsystem and the recommendations of this guide are met, then theuser has confidence that the Basic Data produced by the testingsystem are adequate for processing by subsequent computeralgorithms to produce further Derived Data.4.1.2 At
25、 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 guide. The sequence includes:4.2 Errors Due to Bandwidth Limitations in the SignalConditioningWhere there is analog signal con
26、ditioning 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 of demodulation. Afterdigital conversion, additional digital filtering may be applied toreduce noise components. These bandw
27、idth restrictions resultin cyclic signals at higher frequencies having an apparentamplitude 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 compar
28、ing phase in twochannels with different bandwidths.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
29、 example, when the peak value of a waveform 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.FIG. 1 3-dB Bandwidth of Sensor ChannelFIG. 2 Basic and Derived DataFIG. 3 Sources of Error in
30、 Data Acquisition SystemsE 1942 98 (2004)24.4 Errors Due to Noise and DriftNoise added to thesignal being measured causes measurement uncertainty. Shortterm noise causes variability or random error, and includesanalog noise at the transducer output due to electrical ormechanical pick up, and analog
31、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 transduceroutput or its analog signal conditioning due to temperature oraging effects, are indistinguishable from slow chan
32、ges 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 Requirements5.1 How This Section is OrganizedThis section gives thesteps that must be taken to ensure the errors are controlled.The
33、re 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 onoverall accuracy of a system with 1 % accuracy, recommenda-tions are given in this guide, which result in a 0.2 % errorb
34、ound 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 withappropriate adjustments to bandwidth, data rate, and so forth.5.1.1 In this section, which is intended to be used in th
35、eorder 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 eachcase either:Maximum value $ actual valueorMinimum value # actual value.However, if the actual value is not known, then help i
36、s 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 employed (for example,sinusoidal, triangular, square).5.3 Minimum BandwidthIf the waveform is sinusoidal orsquare, then
37、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 minimum bandwidth is 100 Hz. For a discussionof minimum bandwidth, see A1.2.1 and A1.2.2.5.4 Actual BandwidthThe actual bandw
38、idth must be equalto or greater than the minimum bandwidth. If this conditioncannot be met, then the errors will increase as shown in A1.2.1andA1.2.2. If the actual bandwidth is not known, then it can beascertained using one of the suggested methods in A1.2.3,orotherwise.5.5 Minimum Data RateFor mea
39、surement 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 minimum data rate is400 points/cycle, or 400f points/s. If the data acquisitionsystem produces the peak value as an out
40、put, 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 which samples arepresented is less than this minimum value. For a discussion ofdata rate, see A1.3.1.5.6 Actual Data Ra
41、teThe 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 Permitted Noise LevelThe noise level is thestandard deviation of the noise in the transducer channel,expressed in the un
42、its 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 load channel is 100 kN, then the standarddeviation of the noise in that channel must not exceed 0.2 kN.5.8 Actual Noise Le
43、velThe 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 that 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
44、 the maximum permit-ted noise level, it can usually be reduced by reducing band-width, but this will require beginning again at 5.3 to verify thatthe bandwidth reduction is permissible.5.9 Maximum Permissible Phase Difference and MaximumPermissible Data SkewThese terms are discussed in A1.5.1and A1.
45、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 shifts between displacement and force due to thematerial under test are 10 to 20, then an acceptable value forthe maximum p
46、hase 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 SkewMethods forestimating the combined effect of phase shift and data skew ina data acquisition system are given in A1.5.3.6
47、. Report6.1 The purpose of the report is to record that due consid-eration was given to essential performance parameters of thedata 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
48、should besufficient but succinct. The report should contain the followinginformation, preferably in a tabular format.6.2 Measurement Equipment DescriptionThis should in-clude the manufacturers name, model number, and serialnumber for the test hardware used.6.3 Waveshape and Highest Frequency Used Du
49、ring theTest6.4 Minimum Bandwidth, Actual Bandwidth, and a NoteAbout its SourceThe source is a note describing how actualbandwidth was ascertained, for example, from a manufactur-ers data sheet or by a measurement.6.5 Minimum Data Rate, Actual Data Rate, and a NoteAbout SourceThe source is a note describing how actual dataE 1942 98 (2004)3rate was ascertained, for example, from a manufacturersdatasheet or by a measurement.6.6 Maximum Permissible Noise Level, Actual Noise Level,and a Note About SourceThe source is a note describi
