1、Designation: E251 92 (Reapproved 2014)Standard Test Methods forPerformance Characteristics of Metallic Bonded ResistanceStrain Gages1This standard is issued under the fixed designation E251; the number immediately following the designation indicates the year oforiginal adoption or, in the case of re
2、vision, 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.This standard has been approved for use by agencies of the U.S. Department of Defense.INTRODUCTIONThe Organiza
3、tion of International Legal Metrology is a treaty organization with approximately 75member nations. In 1984, OIML issued International Recommendation No. 62, “PerformanceCharacteristics of Metallic Resistance Strain Gages.” Test Methods E251 has been modified andexpanded to be the United States of A
4、mericas compliant test specification. Throughout this standardthe terms “strain gage” and “gage” are to be understood to represent the longer, but more accurate,“metallic bonded resistance strain gages.”1. Scope1.1 The purpose of these test methods are to provideuniform test methods for the determin
5、ation of strain gageperformance characteristics. Suggested testing equipment de-signs are included.1.2 Test Methods E251 describes methods and proceduresfor determining five strain gage parameters:SectionPart IGeneral Requirements 7Part IIResistance at a Reference Temperature 8Part IIIGage Factor at
6、 a Reference Temperature 9Part IVTemperature Coefficient of Gage Factor 10Part VTransverse Sensitivity 11Part VIThermal Output 121.3 Strain gages are very sensitive devices with essentiallyinfinite resolution. Their response to strain, however, is lowand great care must be exercised in their use. Th
7、e performancecharacteristics identified by these test methods must be knownto an acceptable accuracy to obtain meaningful results in fieldapplications.1.3.1 Strain gage resistance is used to balance instrumenta-tion circuits and to provide a reference value for measurementssince all data are related
8、 to a change in the gage resistance froma known reference value.1.3.2 Gage factor is the transfer function of a strain gage. Itrelates resistance change in the gage and strain to which it issubjected. Accuracy of strain gage data can be no better thanthe precision of the gage factor.1.3.3 Changes in
9、 gage factor as temperature varies alsoaffect accuracy although to a much lesser degree since varia-tions are usually small.1.3.4 Transverse sensitivity is a measure of the strain gagesresponse to strains perpendicular to its measurement axis.Although transverse sensitivity is usually much less than
10、 10 %of the gage factor, large errors can occur if the value is notknown with reasonable precision.1.3.5 Thermal output is the response of a strain gage totemperature changes. Thermal output is an additive (notmultiplicative) error. Therefore, it can often be much largerthan the gage output from str
11、uctural loading. To correct forthese effects, thermal output must be determined from gagesbonded to specimens of the same material on which the testsare to run, often to the test structure itself.1.4 Bonded resistance strain gages differ from extensom-eters in that they measure average unit elongati
12、on (L/L) overa nominal gage length rather than total elongation betweendefinite gauge points. Practice E83 is not applicable to thesegages.1.5 These test methods do not apply to transducers, such asload cells and extensometers, that use bonded resistance straingages as sensing elements.1.6 strain ga
13、ges are part of a complex system that includesstructure, adhesive, gage, lead wires, instrumentation, and(often) environmental protection. As a result, many thingsaffect the performance of strain gages, including user tech-nique.Afurther complication is that strain gages once installed1These test me
14、thods are under the jurisdiction of ASTM Committee E28 onMechanical Testing and are the direct responsibility of Subcommittee E28.01 onCalibration of Mechanical Testing Machines and Apparatus.Current edition approved April 15, 2014. Published August 2014. Originallyapproved in 1964. Last previous ed
15、ition approved in 2009 as E251 92 (2009).DOI: 10.1520/E0251-92R14.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1normally cannot be reinstalled in another location. Therefore,gage characteristics can be stated only on a statistical b
16、asis.1.7 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referen
17、ced Documents2.1 ASTM Standards:2E83 Practice for Verification and Classification of Exten-someter SystemsE228 Test Method for Linear Thermal Expansion of SolidMaterials With a Push-Rod DilatometerE289 Test Method for Linear Thermal Expansion of RigidSolids with InterferometryE1237 Guide for Install
18、ing Bonded Resistance Strain Gages2.2 Other Standards:3OIML International Recommendation No. 62 PerformanceCharacteristics of Metallic Resistance Strain Gages3. Terminology3.1 The vocabulary included herein has been chosen so thatspecialized terms in the strain gage field are clearly defined. Atypic
19、al strain gage nomenclature is provided in Appendix X1.3.2 Definitions of Terms Specific to This Standard:3.2.1 batcha group of strain gages of the same type andlot, manufactured as a set (made at the same time and under thesame conditions).3.2.2 calibration apparatus equipment for determining achar
20、acteristic of a bonded resistance strain gage by accuratelyproducing the necessary strains, temperatures, and other con-ditions; and, by accurately measuring the resulting change ofgage resistance.3.2.3 error-strain gage the value obtained by subtractingthe actual value of the strain, determined fro
21、m the calibrationapparatus, from the indicated value of the strain given by thestrain gage output.3.2.3.1 DiscussionErrors attributable to measuring sys-tems are excluded.3.2.4 gage factor the ratio between the unit change instrain gage resistance due to strain and the causing strain.3.2.4.1 Discuss
22、ionThe gage factor is dimensionless and isexpressed as follows:K 5R 2 RoRoL 2 LoLo5RRo(1)where:K = the gage factor,R = the strain gage resistance at test strain,Ro= the strain gage resistance at zero or reference strain,L = the test structure length under the strain gage at teststrain,Lo= the test s
23、tructure length under the strain gage at zeroor reference strain,R = the change in strain gage resistance when strain ischanged from zero (or reference strain to test strain), =the mechanical strainL2LoLo.3.2.5 gage length (see Fig. 1)the length of the strainsensitive section of a strain gage in the
24、 measurement axisdirection.3.2.5.1 DiscussionAn approximation of this length is thedistance between the inside of the strain gage end loops. Sincethe true gage length is not known, gage length may bemeasured by other geometries (such as the outside of the endloops) providing that the deviation is de
25、fined.3.2.6 grid (see Fig. 1)that portion of the strain-sensingmaterial of the strain gage that is primarily responsible forresistance change due to strain.3.2.7 lota group of strain gages with grid elements from acommon melt, subjected to the same mechanical and thermalprocesses during manufacturin
26、g.3.2.8 matrix(see Fig. 1)an electrically nonconductivelayer of material used to support a strain gage grid.3.2.8.1 DiscussionThe two main functions of a matrix areto act as an aid for bonding the strain gage to a structure andas an electrically insulating layer in cases where the structureis electr
27、ically conductive.2For 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.3Available from OIML International Organiza
28、tion of Legal Metrology, BIML,11, rue Turgot, F-75009 Paris, France, http:/www.oiml.org/enFIG. 1 Typical Strain GageE251 92 (2014)23.2.9 measurement axis (grid) (see Fig. 1)that axis that isparallel with the grid lines.3.2.10 strain gage, metallic, resistive, bonded (see Fig.1)a resistive element, w
29、ith or without a matrix that isattached to a solid body by cementing, welding, or othersuitable techniques so that the resistance of the element willvary as the surface to which it is attached is deformed.3.2.10.1 DiscussionThese test methods apply to gageswhere the instantaneous gage resistance, R,
30、 is given by theequation:R 5 Ro11K! (2)where:Ro= element resistance at reference strain and temperaturelevels (frequently initial test or balanced circuitconditions), = linear strain of the surface in the direction of thestrain-sensitive axis of the gage, andK = a proportionality factor (see gage fa
31、ctor).3.2.11 strain, linearthe unit elongation induced in a speci-men either by a stress field (mechanical strain) or by atemperature change (thermal expansion).3.2.12 temperature coeffcient of gage factorthe ratio ofthe unit variation of gage factor to the temperature variation,expressed as follows
32、:SKt12 Kt0Kt0DS1T12 T0D(3)where:T1= the test temperature,T0= the reference temperature,Kt1= the gage factor at test temperature, andKt0= the gage factor at reference temperature.3.2.13 thermal expansionthe dimensional change of anunconstrained specimen subject to a change in temperature thatis unifo
33、rm throughout the material.3.2.14 thermal outputthe reversible part of the tempera-ture induced indicated strain of a strain gage installed on anunrestrained test specimen when exposed to a change intemperature.3.2.15 transverse axis (see Fig. 1)the strain gage axis at90 to the measurement axis.3.2.
34、16 transverse sensitivitythe ratio, expressed as apercentage, of the unit change of resistance of a strain gagemounted perpendicular to a uniaxial strain field (transversegage) to the unit resistance change of a similar gage mountedparallel to the same strain field (longitudinal gage).3.2.17 typea g
35、roup of strain gages that are nominallyidentical with respect to physical and manufacturing charac-teristics.4. Significance and Use4.1 Strain gages are the most widely used devices for thedetermination of materials, properties and for analyzingstresses in structures. However, performance parameters
36、 ofstrain gages are affected by both the materials from which theyare made and their geometric design. These test methods detailthe minimum information that must accompany strain gages ifthey are to be used with acceptable accuracy of measurement.4.2 Most performance parameters of strain gages requi
37、remechanical testing that is destructive. Since test gages cannotbe used again, it is necessary to treat data statistically and thenapply values to the remaining population from the same lot orbatch. Failure to acknowledge the resulting uncertainties canhave serious repercussions. Resistance measure
38、ment is non-destructive and can be made for each gage.4.3 Properly designed and manufactured strain gages,whose properties have been accurately determined and withappropriate uncertainties applied, represent powerful measure-ment tools. They can determine small dimensional changes instructures with
39、excellent accuracy, far beyond that of otherknown devices. It is important to recognize, however, thatindividual strain gages cannot be calibrated. If calibration andtraceability to a standard are required, strain gages should notbe employed.4.4 To be used, strain gages must be bonded to a structure
40、.Good results depend heavily on the materials used to clean thebonding surface, to bond the gage, and to provide a protectivecoating. Skill of the installer is another major factor in success.Finally, instrumentation systems must be carefully designed toassure that they do not unduly degrade the per
41、formance of thegages. In many cases, it is impossible to achieve this goal. If so,allowance must be made when considering accuracy of data.Test conditions can, in some instances, be so severe that errorsignals from strain gage systems far exceed those from thestructural deformations to be measured.
42、Great care must beexercised in documenting magnitudes of error signals so thatrealistic values can be placed on associated uncertainties.5. Interferences5.1 To assure that strain gage test data are within a definedaccuracy, the gages must be properly bonded and protectedwith acceptable materials. It
43、 is normally simple to ascertainthat strain gages are not performing properly. The mostcommon symptom is instability with time or temperaturechange. If strain gages do not return to their zero reading whenthe original conditions are repeated, or there is low or changingresistance to ground, the inst
44、allation is suspect. Aids in straingage installation and verification thereof can be found in GuideE1237.6. Hazards6.1 In the specimen surface cleaning, gage bonding, andprotection steps of strain gage installation, hazardous chemi-cals may be used. Users of these test methods are responsiblefor con
45、tacting manufacturers of these chemicals for applicableMaterial Safety Data Sheets and to adhere to the requiredprecautions.7. Test Requirements7.1 General Environmental Requirements:7.1.1 Ambient Conditions at Room TemperatureThenominal temperature and relative humidity shall be 23C(73F) and 50 %,
46、respectively. In no case shall the temperatureE251 92 (2014)3be less that 18C (64F) nor greater than 25C (77F) and therelative humidity less than 35 % nor more than 60 %. Thefluctuations during any room temperature test of any gage shallnot exceed6 2C and 6 5 % RH.7.1.2 Ambient Conditions at Elevate
47、d and LowerTemperaturesThe temperature adjustment error shall notexceed 6 2C (6 3.6F) or 6 2 % of the deviation from roomtemperature, whichever is greater. The total uncertainty oftemperature shall not exceed 6 2C (6 3.6F), or 6 1 % of thedeviation from room temperature, whichever is greater. Atelev
48、ated temperatures the mixing ratio shall be constant, thatmeans independent of temperature, at a nominal value of 0.009g of water per1gofairatapressure of 1 bar. This valuecorresponds to a relative humidity of 50 % at 23C (73F).NOTE 1This mixing ratio, independent of temperature, can be realizedby a
49、 furnace that is well connected to an atmosphere meeting theconditions of 7.1.1.7.2 Test Measurement Requirements:7.2.1 Several methods are available for measuring thechange of gage resistance with sufficient resolution and accu-racy. In general, any of these methods that are convenient maybe used after it has been shown that the particular combinationof instruments or components used produce a system with therequired accuracy.7.2.2 Examples of potentially satisfactory methods are asfollows:7.2.2.1 Balanced Bridge Circuit
