ASTM_E_1942_-_98_2010e1.pdf

Designation:E194298(Reapproved 2010)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 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.1NOTE3.1.4 was editorially revised in December 2011.1.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 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 fatigue 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 transducers 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 material 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.2.Referenced Documents2.1 ASTM Standards:2E4 Practices for Force Verification of Testing MachinesE83 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 the amplituderesponse of the channel has fallen to1/=2of 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 sensorwaveform 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 date rate is1tdHertz 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 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 transducer channel,expressed in the unitsappropriate to that channel.3.1.6 peakthe point of maximum load in constant ampli-tude loading(see Terminology E1823).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 meaning in reference to a second cyclicwaveform of the same frequency.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 Nov.1,2010.Published January 2011.Originallyapproved in 1998.Last previous edition approved in 2004 as E1942-98(2004).DOI:10.1520/E1942-98R10E012For referenced ASTM 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.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 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.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).3.1.9 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