1、Designation: E 251 92 (Reapproved 2003)Standard Test Methods forPerformance Characteristics of Metallic Bonded ResistanceStrain Gages1This standard is issued under the fixed designation E 251; the number immediately following the designation indicates the year oforiginal adoption or, in the case of
2、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.This standard has been approved for use by agencies of the Department of Defense.INTRODUCTIONThe Organizati
3、on of International Legal Metrology is a treaty organization with approximately 75member nations. In 1984, OIML issued International Recommendation No. 62, 8PerformanceCharacteristics of Metallic Resistance Strain Gages. Test Methods E 251 has been modified andexpanded to be the United States of Ame
4、ricas 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 this standard is to provide uniform testmethods for the determination of
5、 strain gage performancecharacteristics. Suggested testing equipment designs are in-cluded.1.2 Test Methods E 251 describes methods and proceduresfor determining five strain gage parameters:SectionPart IGeneral Requirements 7Part IIResistance at a Reference Temperature 8Part IIIGage Factor at a Refe
6、rence 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. The perfo
7、rmancecharacteristics 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 to a c
8、hange 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 gage f
9、actor 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 %of
10、 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 structural
11、 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 elongation (DL/
12、L) overa nominal gage length rather than total elongation betweendefinite gage points. Practice E83is 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 gages are
13、part of a complex system that includesstructure, adhesive, gage, leadwires, instrumentation, and (of-ten) environmental protection. As a result, many things affect1These test methods are under the jurisdiction of ASTM Committee E28 onMechanical Testing and are the direct responsibility of Subcommitt
14、ee E28.01 onCalibration of Mechanical Testing Machines and Apparatus.Current edition approved June 10, 2003. Published January 2004. Originallyapproved in 1964. Last previous edition approved in 1998 as E 251 86 (1998).1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohoc
15、ken, PA 19428-2959, United States.the performance of strain gages, including user technique. Afurther complication is that strain gages once installed nor-mally cannot be reinstalled in another location. Therefore, gagecharacteristics can be stated only on a statistical basis.1.7 This standard does
16、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.1.8 The values stated in SI units are
17、to be regarded as thestandard.2. Referenced Documents2.1 ASTM Standards:E83 Practice for Verification and Classification of Exten-someters2E 228 Test Method for Linear Thermal Expansion of SolidMaterials with a Vitreous Silica Dilatometer3E 289 Test Method for Linear Thermal Expansion of RigidSolids
18、 with Interferometry3E 1237 Guide for Installing Bonded Resistance StrainGages22.2 OIML International Recommendation No. 62:8 Perfor-mance Characteristics of Metallic Resistance Strain Gages3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 The vocabulary included herein has been
19、 chosen sothat specialized terms in the strain gage field will be clearlydefined. A typical strain gage nomenclature is provided inAppendix X1.3.1.1.1 batcha group of strain gages of the same type andlot, manufactured as a set (made at the same time and under thesame conditions).3.1.1.2 calibration
20、apparatusequipment for determininga characteristic 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.3.1.1.3 error-strain gagethe value obtained by subtract-ing the
21、 actual value of the strain, determined from thecalibration apparatus, from the indicated value of the straingiven by the strain gage output. Errors attributable to measur-ing systems are excluded.3.1.1.4 gage factorthe ratio between the unit change instrain gage resistance due to strain and the cau
22、sing strain. Thegage factor is dimensionless and is expressed as follows:K 5R 2 RoRo/L 2 LoLo5DRRo/e (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
23、,Lo= the test structure length under the strain gage at zeroor reference strain,DR = the change in strain gage resistance when strain ischanged from zero (or reference strain to test strain),e = the mechanical strain L 2 Lo /Lo.3.1.1.5 gage length (see Fig. 1)the length of the strainsensitive sectio
24、n of a strain gage in the measurement axisdirection. An approximation of this length is the distancebetween the inside of the strain gage end loops. Since the truegage length is not known, gage length may be measured byother geometries (such as the outside of the end loops)providing that the deviati
25、on is defined.3.1.1.6 grid (see Fig. 1)that portion of the strain-sensingmaterial of the strain gage that is primarily responsible forresistance change due to strain.3.1.1.7 lota group of strain gages with grid elements froma common melt, subjected to the same mechanical and thermalprocesses during
26、manufacturing.3.1.1.8 matrix (see Fig. 1)an electrically nonconductivelayer of material used to support a strain gage grid. The twomain functions of a matrix are to act as an aid for bonding thestrain gage to a structure and as an electrically insulating layerin cases where the structure is electric
27、ally conductive.3.1.1.9 measurement axis (grid) (see Fig. 1)that axis thatis parallel with the grid lines.3.1.1.10 strain gage, metallic, resistive, bonded (see Fig.1)a resistive element, with or without a matrix that isattached to a solid body by cementing, welding, or othersuitable techniques so t
28、hat the resistance of the element willvary as the surface to which it is attached is deformed. Thesetest methods apply to gages where the instantaneous gageresistance, R, is given by the equation:R 5 Ro1 1eK! (2)where:Ro= element resistance at reference strain and temperaturelevels (frequently initi
29、al test or balanced circuit con-ditions),e = linear strain of the surface in the direction of thestrain-sensitive axis of the gage, andK = a proportionality factor (see gage factor).3.1.1.11 strain, linearthe unit elongation induced in aspecimen either by a stress field (mechanical strain) or by ate
30、mperature change (thermal expansion).3.1.1.12 temperature coeffcient of gage factorthe ratio ofthe unit variation of gage factor to the temperature variation,expressed as follows:SKt12 Kt0Kt0DS1T12 T0D(3)where:T1= the test temperature,T0= the reference temperature,Kt1= the gage factor at test temper
31、ature, andKt0= the gage factor at reference temperature.2Annual Book of ASTM Standards, Vol 03.01.3Annual Book of ASTM Standards, Vol 14.02.E 251 92 (2003)23.1.1.13 thermal expansionthe dimensional change of anunconstrained specimen subject to a change in temperature thatis uniform throughout the ma
32、terial.3.1.1.14 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.3.1.1.15 transverse axis (see Fig. 1)the strain gage axis at90 to the measurement axis.3.1.1.16 transverse
33、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).3.1.1.17 typea group of stra
34、in 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 ofstrain ga
35、ges 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 requiremechanical
36、 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 measurement is non-
37、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 excellent ac
38、curacy, 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.Good result
39、s 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 performance of
40、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. Great care m
41、ust beexercised in documenting magnitudes of error signals so thatrealistic values can be placed on associated uncertainties.5. Interferences5.1 In order to assure that strain gage test data are within adefined accuracy, the gages must be properly bonded andprotected with acceptable materials. It is
42、 normally simple toascertain that strain gages are not performing properly. Themost common 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 install
43、ation is suspect. Aids in straingage installation and verification thereof can be found in GuideE 1237.6. 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 conta
44、cting 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 %, re
45、spectively. In no case shall the temperaturebe 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 Elevated and LowerTemper
46、aturesThe 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. AtFIG. 1 Typical Strain
47、 GageE 251 92 (2003)3elevated 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 tempera
48、ture, can be real-ized by a 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
49、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 CircuitIn this circuit, a change ingage resistance is matched by an equal unit resistance changein a calibrated arm of the bridge circuit so as to produce abalanced condition with zero electrical output. This circuit isnot sensitive to excitation voltage changes except for self-heating effects.
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