1、Designation: E 251 92 (Reapproved 2009)Standard Test Methods forPerformance Characteristics of Metallic Bonded ResistanceStrain Gauges1This 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 () 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 Gauges. Test Methods E 251 has been modified andexpanded to be the United States of Am
4、ericas compliant test specification. Throughout this standardthe terms “strain gauge” and “gauge” are to be understood to represent the longer, but more accurate,“metallic bonded resistance strain gauges.”1. Scope1.1 The purpose of this standard is to provide uniform testmethods for the determinatio
5、n of strain gauge performancecharacteristics. Suggested testing equipment designs are in-cluded.1.2 Test Methods E 251 describes methods and proceduresfor determining five strain gauge parameters:SectionPart IGeneral Requirements 7Part IIResistance at a Reference Temperature 8Part IIIGauge Factor at
6、 a Reference Temperature 9Part IVTemperature Coefficient of Gauge Factor 10Part VTransverse Sensitivity 11Part VIThermal Output 121.3 Strain gauges are very sensitive devices with essentiallyinfinite resolution. Their response to strain, however, is lowand great care must be exercised in their use.
7、The performancecharacteristics identified by these test methods must be knownto an acceptable accuracy to obtain meaningful results in fieldapplications.1.3.1 Strain gauge resistance is used to balance instrumen-tation circuits and to provide a reference value for measure-ments since all data are re
8、lated to a change in the gaugeresistance from a known reference value.1.3.2 Gauge factor is the transfer function of a strain gauge.It relates resistance change in the gauge and strain to which itis subjected. Accuracy of strain gauge data can be no betterthan the precision of the gauge factor.1.3.3
9、 Changes in gauge 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 straingauges response to strains perpendicular to its measurementaxis. Although transverse sensitivity is usually m
10、uch less than10 % of the gauge factor, large errors can occur if the value isnot known with reasonable precision.1.3.5 Thermal output is the response of a strain gauge totemperature changes. Thermal output is an additive (notmultiplicative) error. Therefore, it can often be much largerthan the gauge
11、 output from structural loading. To correct forthese effects, thermal output must be determined from gaugesbonded to specimens of the same material on which the testsare to run; often to the test structure itself.1.4 Bonded resistance strain gauges differ from extensom-eters in that they measure ave
12、rage unit elongation (DL/L) overa nominal gauge length rather than total elongation betweendefinite gauge points. Practice E83is not applicable to thesegauges.1.5 These test methods do not apply to transducers, such asload cells and extensometers, that use bonded resistance straingauges as sensing e
13、lements.1.6 Strain gauges are part of a complex system that includesstructure, adhesive, gauge, leadwires, instrumentation, and(often) environmental protection. As a result, many thingsaffect the performance of strain gauges, including user tech-nique. A further complication is that strain gauges on
14、ce1These test methods 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 1, 2009. Published September 2009. Originallyapproved in 1964
15、. Last previous edition approved in 2003 as E 251 92 (2003).1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.installed normally cannot be reinstalled in another location.Therefore, gauge characteristics can be stated only on astatisti
16、cal basis.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. Re
17、ferenced Documents2.1 ASTM Standards:2E83 Practice for Verification and Classification of Exten-someter SystemsE 228 Test Method for Linear Thermal Expansion of SolidMaterials With a Push-Rod DilatometerE 289 Test Method for Linear Thermal Expansion of RigidSolids with InterferometryE 1237 Guide for
18、 Installing Bonded Resistance StrainGages2.2 OIML International Recommendation No. 62:8 Perfor-mance Characteristics of Metallic Resistance Strain Gauges3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 The vocabulary included herein has been chosen sothat specialized terms in t
19、he strain gauge field will be clearlydefined. A typical strain gauge nomenclature is provided inAppendix X1.3.1.1.1 batcha group of strain gauges of the same typeand lot, manufactured as a set (made at the same time andunder the same conditions).3.1.1.2 calibration apparatusequipment for determining
20、a characteristic of a bonded resistance strain gauge by accu-rately producing the necessary strains, temperatures, and otherconditions; and, by accurately measuring the resulting changeof gauge resistance.3.1.1.3 error-strain gaugethe value obtained by subtract-ing the actual value of the strain, de
21、termined from thecalibration apparatus, from the indicated value of the straingiven by the strain gauge output. Errors attributable to mea-suring systems are excluded.3.1.1.4 gauge factorthe ratio between the unit change instrain gauge resistance due to strain and the causing strain. Thegauge factor
22、 is dimensionless and is expressed as follows:K 5R 2 RoRo/L 2 LoLo5DRRo/ (1)where:K = the gauge factor,R = the strain gauge resistance at test strain,Ro= the strain gauge resistance at zero or reference strain,L = the test structure length under the strain gauge at teststrain,Lo= the test structure
23、length under the strain gauge atzero or reference strain,DR = the change in strain gauge resistance when strain ischanged from zero (or reference strain to test strain), = the mechanical strain L 2 Lo /Lo.3.1.1.5 gauge length (see Fig. 1)the length of the strainsensitive section of a strain gauge in
24、 the measurement axisdirection. An approximation of this length is the distancebetween the inside of the strain gauge end loops. Since the truegauge length is not known, gauge length may be measured byother geometries (such as the outside of the end loops)providing that the deviation is defined.3.1.
25、1.6 grid (see Fig. 1)that portion of the strain-sensingmaterial of the strain gauge that is primarily responsible forresistance change due to strain.3.1.1.7 lota group of strain gauges with grid elementsfrom a common melt, subjected to the same mechanical andthermal processes during manufacturing.3.
26、1.1.8 matrix (see Fig. 1)an electrically nonconductivelayer of material used to support a strain gauge grid. The twomain functions of a matrix are to act as an aid for bonding thestrain gauge to a structure and as an electrically insulating layerin cases where the structure is electrically conductiv
27、e.3.1.1.9 measurement axis (grid) (see Fig. 1)that axis thatis parallel with the grid lines.3.1.1.10 strain gauge, 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 that the resis
28、tance of the element willvary as the surface to which it is attached is deformed. Thesetest methods apply to gauges where the instantaneous gaugeresistance, R, is given by the equation:R 5 Ro1 1K! (2)where:Ro= element resistance at reference strain and temperaturelevels (frequently initial test or b
29、alanced circuit con-ditions), = linear strain of the surface in the direction of thestrain-sensitive axis of the gauge, andK = a proportionality factor (see gauge factor).3.1.1.11 strain, linearthe unit elongation induced in aspecimen either by a stress field (mechanical strain) or by atemperature c
30、hange (thermal expansion).3.1.1.12 temperature coeffcient of gauge factorthe ratioof the unit variation of gauge factor to the temperaturevariation, expressed as follows:SKt12 Kt0Kt0DS1T12 T0D(3)where:T1= the test temperature,T0= the reference temperature,Kt1= the gauge factor at test temperature, a
31、ndKt0= the gauge factor at reference temperature.3.1.1.13 thermal expansionthe dimensional change of anunconstrained specimen subject to a change in temperature thatis uniform throughout the material.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Servic
32、e at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.E 251 92 (2009)23.1.1.14 thermal outputthe reversible part of the tempera-ture induced indicated strain of a strain gauge installed on anunrestrained test specim
33、en when exposed to a change intemperature.3.1.1.15 transverse axis (see Fig. 1)the strain gauge axisat 90 to the measurement axis.3.1.1.16 transverse sensitivitythe ratio, expressed as apercentage, of the unit change of resistance of a strain gaugemounted perpendicular to a uniaxial strain field (tr
34、ansversegauge) to the unit resistance change of a similar gauge mountedparallel to the same strain field (longitudinal gauge).3.1.1.17 typea group of strain gauges that are nominallyidentical with respect to physical and manufacturing charac-teristics.4. Significance and Use4.1 Strain gauges are the
35、 most widely used devices for thedetermination of materials, properties and for analyzingstresses in structures. However, performance parameters ofstrain gauges are affected by both the materials from whichthey are made and their geometric design. These test methodsdetail the minimum information tha
36、t must accompany straingauges if they are to be used with acceptable accuracy ofmeasurement.4.2 Most performance parameters of strain gauges requiremechanical testing that is destructive. Since test gauges cannotbe used again, it is necessary to treat data statistically and thenapply values to the r
37、emaining population from the same lot orbatch. Failure to acknowledge the resulting uncertainties canhave serious repercussions. Resistance measurement is non-destructive and can be made for each gauge.4.3 Properly designed and manufactured strain gauges,whose properties have been accurately determi
38、ned and withappropriate uncertainties applied, represent powerful measure-ment tools. They can determine small dimensional changes instructures with excellent accuracy, far beyond that of otherknown devices. It is important to recognize, however, thatindividual strain gauges cannot be calibrated. If
39、 calibration andtraceability to a standard are required, strain gauges should notbe employed.4.4 To be used, strain gauges must be bonded to a structure.Good results depend heavily on the materials used to clean thebonding surface, to bond the gauge, and to provide a protectivecoating. Skill of the
40、installer is another major factor in success.Finally, instrumentation systems must be carefully designed toassure that they do not unduly degrade the performance of thegauges. In many cases, it is impossible to achieve this goal. Ifso, allowance must be made when considering accuracy ofdata. Test co
41、nditions can, in some instances, be so severe thaterror signals from strain gauge systems far exceed those fromthe structural deformations to be measured. Great care must beexercised in documenting magnitudes of error signals so thatrealistic values can be placed on associated uncertainties.5. Inter
42、ferences5.1 In order to assure that strain gauge test data are within adefined accuracy, the gauges must be properly bonded andprotected with acceptable materials. It is normally simple toascertain that strain gauges are not performing properly. Themost common symptom is instability with time or tem
43、peraturechange. If strain gauges do not return to their zero readingwhen the original conditions are repeated, or there is low orchanging resistance to ground, the installation is suspect. Aidsin strain gauge installation and verification thereof can befound in Guide E 1237.6. Hazards6.1 In the spec
44、imen surface cleaning, gauge bonding, andprotection steps of strain gauge installation, hazardous chemi-cals may be used. Users of these test methods are responsiblefor contacting manufacturers of these chemicals for applicableMaterial Safety Data Sheets and to adhere to the requiredprecautions.7. T
45、est Requirements7.1 General Environmental Requirements:7.1.1 Ambient Conditions at Room TemperatureThenominal temperature and relative humidity shall be 23C(73F) and 50 %, respectively. In no case shall the temperaturebe less that 18C (64F) nor greater than 25C (77F) and therelative humidity less th
46、an 35 % nor more than 60 %. Thefluctuations during any room temperature test of any gaugeshall not exceed6 2C and 6 5 % RH.7.1.2 Ambient Conditions at Elevated and LowerTemperaturesThe temperature adjustment error shall notexceed 6 2C (6 3.6F) or 6 2 % of the deviation from roomtemperature, whicheve
47、r 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. Atelevated temperatures the mixing ratio shall be constant, thatmeans independent of temperature, at a nominal value of 0.009FIG. 1 Typical Strain G
48、augeE 251 92 (2009)3g 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 real-ized by a furnace that is well connected to an atmosphere meeting theconditions of 7.1.1.7.2 Test Measurem
49、ent Requirements:7.2.1 Several methods are available for measuring thechange of gauge resistance with sufficient resolution andaccuracy. In general, any of these methods that are convenientmay be used after it has been shown that the particularcombination of instruments or components used produce asystem with the required accuracy.7.2.2 Examples of potentially satisfactory methods are asfollows:7.2.2.1 Balanced Bridge CircuitIn this circuit, a change ingauge resistance is matched by an equal unit resistance changein a calibrated a
