ASTM E251-92(2003)
(Test Method)Standard Test Methods for Performance Characteristics of Metallic Bonded Resistance Strain Gages
Standard Test Methods for Performance Characteristics of Metallic Bonded Resistance Strain Gages
SIGNIFICANCE AND USE
Strain gages are the most widely used devices for the determination of materials, properties and for analyzing stresses in structures. However, performance parameters of strain gages are affected by both the materials from which they are made and their geometric design. These test methods detail the minimum information that must accompany strain gages if they are to be used with acceptable accuracy of measurement.
Most performance parameters of strain gages require mechanical testing that is destructive. Since test gages cannot be used again, it is necessary to treat data statistically and then apply values to the remaining population from the same lot or batch. Failure to acknowledge the resulting uncertainties can have serious repercussions. Resistance measurement is non-destructive and can be made for each gage.
Properly designed and manufactured strain gages, whose properties have been accurately determined and with appropriate uncertainties applied, represent powerful measurement tools. They can determine small dimensional changes in structures with excellent accuracy, far beyond that of other known devices. It is important to recognize, however, that individual strain gages cannot be calibrated. If calibration and traceability to a standard are required, strain gages should not be employed.
To be used, strain gages must be bonded to a structure. Good results depend heavily on the materials used to clean the bonding surface, to bond the gage, and to provide a protective coating. Skill of the installer is another major factor in success. Finally, instrumentation systems must be carefully designed to assure that they do not unduly degrade the performance of the gages. 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 error signals from strain gage systems far exceed those from the structural deformations to be measured. Great care m...
SCOPE
1.1 The purpose of this standard is to provide uniform test methods for the determination of strain gage performance characteristics. Suggested testing equipment designs are included.
1.2 Test Methods E 251 describes methods and procedures for determining five strain gage parameters:SectionPart I-General Requirements7Part II-Resistance at a Reference Temperature8Part III-Gage Factor at a Reference Temperature9Part IV-Temperature Coefficient of Gage Factor10Part V-Transverse Sensitivity11Part VI-Thermal Output12
1.3 Strain gages are very sensitive devices with essentially infinite resolution. Their response to strain, however, is low and great care must be exercised in their use. The performance characteristics identified by these test methods must be known to an acceptable accuracy to obtain meaningful results in field applications.
1.3.1 Strain gage resistance is used to balance instrumentation circuits and to provide a reference value for measurements since all data are related to a change in the gage resistance from a known reference value.
1.3.2 Gage factor is the transfer function of a strain gage. It relates resistance change in the gage and strain to which it is subjected. Accuracy of strain gage data can be no better than the precision of the gage factor.
1.3.3 Changes in gage factor as temperature varies also affect accuracy although to a much lesser degree since variations are usually small.
1.3.4 Transverse sensitivity is a measure of the strain gage's response to strains perpendicular to its measurement axis. Although transverse sensitivity is usually much less than 10 % of the gage factor, large errors can occur if the value is not known with reasonable precision.
1.3.5 Thermal output is the response of a strain gage to temperature changes. Thermal output is an additive (not multiplicative) error. Therefore, it can often be much larger than the gage output from structural loading. To correct for these...
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Designation: E 251 – 92 (Reapproved 2003)
Standard Test Methods for
Performance Characteristics of Metallic Bonded Resistance
Strain Gages
This standard is issued under the fixed designation E251; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript 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.
INTRODUCTION
The Organization of International Legal Metrology is a treaty organization with approximately 75
member nations. In 1984, OIML issued International Recommendation No. 62, 8Performance
Characteristics of Metallic Resistance Strain Gages.’ Test Methods E251 has been modified and
expanded to be the United States ofAmerica’s compliant test specification. Throughout this standard
the terms “strain gage” and “gage” are to be understood to represent the longer, but more accurate,
“metallic bonded resistance strain gages.”
1. Scope subjected. Accuracy of strain gage data can be no better than
the precision of the gage factor.
1.1 The purpose of this standard is to provide uniform test
1.3.3 Changes in gage factor as temperature varies also
methods for the determination of strain gage performance
affect accuracy although to a much lesser degree since varia-
characteristics. Suggested testing equipment designs are in-
tions are usually small.
cluded.
1.3.4 Transversesensitivityisameasureofthestraingage’s
1.2 Test Methods E251 describes methods and procedures
response to strains perpendicular to its measurement axis.
for determining five strain gage parameters:
Although transverse sensitivity is usually much less than 10%
Section
of the gage factor, large errors can occur if the value is not
Part I—General Requirements 7
Part II—Resistance at a Reference Temperature 8
known with reasonable precision.
Part III—Gage Factor at a Reference Temperature 9
1.3.5 Thermal output is the response of a strain gage to
Part IV—Temperature Coefficient of Gage Factor 10
temperature changes. Thermal output is an additive (not
Part V—Transverse Sensitivity 11
Part VI—Thermal Output 12
multiplicative) error. Therefore, it can often be much larger
than the gage output from structural loading. To correct for
1.3 Strain gages are very sensitive devices with essentially
these effects, thermal output must be determined from gages
infinite resolution. Their response to strain, however, is low
bonded to specimens of the same material on which the tests
and great care must be exercised in their use.The performance
are to run; often to the test structure itself.
characteristics identified by these test methods must be known
1.4 Bonded resistance strain gages differ from extensom-
to an acceptable accuracy to obtain meaningful results in field
eters in that they measure average unit elongation (DL/L) over
applications.
a nominal gage length rather than total elongation between
1.3.1 Strain gage resistance is used to balance instrumenta-
definite gage points. Practice E83 is not applicable to these
tioncircuitsandtoprovideareferencevalueformeasurements
gages.
sincealldataarerelatedtoachangeinthegageresistancefrom
1.5 These test methods do not apply to transducers, such as
a known reference value.
load cells and extensometers, that use bonded resistance strain
1.3.2 Gage factor is the transfer function of a strain gage. It
gages as sensing elements.
relates resistance change in the gage and strain to which it is
1.6 Strain gages are part of a complex system that includes
structure, adhesive, gage, leadwires, instrumentation, and (of-
These test methods are under the jurisdiction of ASTM Committee E28 on
ten) environmental protection. As a result, many things affect
Mechanical Testing and are the direct responsibility of Subcommittee E28.01 on
Calibration of Mechanical Testing Machines and Apparatus.
Current edition approved Nov. 15, 2005. Published January 2004. Originally
approved in 1964. Last previous edition approved in 1998 as E251–86 (1998).
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 251 – 92 (2003)
the performance of strain gages, including user technique. A
L = the test structure length under the strain gage at test
further complication is that strain gages once installed nor-
strain,
mallycannotbereinstalledinanotherlocation.Therefore,gage
L = the test structure length under the strain gage at zero
o
characteristics can be stated only on a statistical basis.
or reference strain,
1.7 This standard does not purport to address all of the
DR = the change in strain gage resistance when strain is
safety concerns, if any, associated with its use. It is the changedfromzero(orreferencestraintoteststrain),
responsibility of the user of this standard to establish appro- e = the mechanical strain L 2 L L .
o o
/
priate safety and health practices and determine the applica-
3.1.1.5 gage length (see Fig. 1)—the length of the strain
bility of regulatory limitations prior to use.
sensitive section of a strain gage in the measurement axis
1.8 The values stated in SI units are to be regarded as the
direction. An approximation of this length is the distance
standard.
between the inside of the strain gage end loops. Since the true
gage length is not known, gage length may be measured by
2. Referenced Documents
other geometries (such as the outside of the end loops)
2.1 ASTM Standards:
providing that the deviation is defined.
E83 Practice for Verification and Classification of Exten-
3.1.1.6 grid (see Fig. 1)—that portion of the strain-sensing
someters
material of the strain gage that is primarily responsible for
E228 Test Method for Linear Thermal Expansion of Solid
resistance change due to strain.
Materials with a Vitreous Silica Dilatometer
3.1.1.7 lot—agroupofstraingageswithgridelementsfrom
E289 Test Method for Linear Thermal Expansion of Rigid
a common melt, subjected to the same mechanical and thermal
Solids with Interferometry
processes during manufacturing.
E 1237 Guide for Installing Bonded Resistance Strain
3.1.1.8 matrix (see Fig. 1)—an electrically nonconductive
Gages
2.2 OIML International Recommendation No. 62:8 Perfor- layer of material used to support a strain gage grid. The two
mance Characteristics of Metallic Resistance Strain Gages main functions of a matrix are to act as an aid for bonding the
strain gage to a structure and as an electrically insulating layer
in cases where the structure is electrically conductive.
3. Terminology
3.1.1.9 measurement axis (grid) (see Fig. 1)—that axis that
3.1 Definitions of Terms Specific to This Standard:
is parallel with the grid lines.
3.1.1 The vocabulary included herein has been chosen so
3.1.1.10 strain gage, metallic, resistive, bonded (see Fig.
that specialized terms in the strain gage field will be clearly
1)—a resistive element, with or without a matrix that is
defined. A typical strain gage nomenclature is provided in
attached to a solid body by cementing, welding, or other
Appendix X1.
suitable techniques so that the resistance of the element will
3.1.1.1 batch—a group of strain gages of the same type and
vary as the surface to which it is attached is deformed. These
lot,manufacturedasaset(madeatthesametimeandunderthe
test methods apply to gages where the instantaneous gage
same conditions).
resistance, R, is given by the equation:
3.1.1.2 calibration apparatus—equipment for determining
R 5 R ~11eK! (2)
acharacteristicofabondedresistancestraingagebyaccurately o
producing the necessary strains, temperatures, and other con-
where:
ditions; and, by accurately measuring the resulting change of
R = elementresistanceatreferencestrainandtemperature
o
gage resistance.
levels (frequently initial test or balanced circuit con-
3.1.1.3 error-strain gage—the value obtained by subtract-
ditions),
ing the actual value of the strain, determined from the
e = linear strain of the surface in the direction of the
calibration apparatus, from the indicated value of the strain
strain-sensitive axis of the gage, and
given by the strain gage output. Errors attributable to measur-
K = a proportionality factor (see gage factor).
ing systems are excluded.
3.1.1.11 strain, linear—the unit elongation induced in a
3.1.1.4 gage factor—the ratio between the unit change in
specimen either by a stress field (mechanical strain) or by a
strain gage resistance due to strain and the causing strain. The
temperature change (thermal expansion).
gage factor is dimensionless and is expressed as follows:
3.1.1.12 temperature coeffıcient of gage factor—the ratio of
R 2 R L 2 L DR
o o
the unit variation of gage factor to the temperature variation,
K 5 / 5 /e (1)
R L R
o o o
expressed as follows:
where:
K 2 K 1
t1 t0
· (3)
S DS D
K = the gage factor,
K T 2 T
t0 1 0
R = the strain gage resistance at test strain,
where:
R = the strain gage resistance at zero or reference strain,
o
T = the test temperature,
T = the reference temperature,
K = the gage factor at test temperature, and
t1
Annual Book of ASTM Standards, Vol 03.01.
K = the gage factor at reference temperature.
3 t0
Annual Book of ASTM Standards, Vol 14.02.
E 251 – 92 (2003)
4.3 Properly designed and manufactured strain gages,
whose properties have been accurately determined and with
appropriate uncertainties applied, represent powerful measure-
ment tools. They can determine small dimensional changes in
structures with excellent accuracy, far beyond that of other
known devices. It is important to recognize, however, that
individual strain gages cannot be calibrated. If calibration and
traceability to a standard are required, strain gages should not
be employed.
4.4 To be used, strain gages must be bonded to a structure.
Good results depend heavily on the materials used to clean the
bonding surface, to bond the gage, and to provide a protective
coating.Skilloftheinstallerisanothermajorfactorinsuccess.
Finally, instrumentation systems must be carefully designed to
assure that they do not unduly degrade the performance of the
gages.Inmanycases,itisimpossibletoachievethisgoal.Ifso,
allowance must be made when considering accuracy of data.
Test conditions can, in some instances, be so severe that error
signals from strain gage systems far exceed those from the
structural deformations to be measured. Great care must be
exercised in documenting magnitudes of error signals so that
realistic values can be placed on associated uncertainties.
5. Interferences
FIG. 1 Typical Strain Gage
5.1 In order to assure that strain gage test data are within a
defined accuracy, the gages must be properly bonded and
3.1.1.13 thermal expansion—the dimensional change of an
protected with acceptable materials. It is normally simple to
unconstrainedspecimensubjecttoachangeintemperaturethat
ascertain that strain gages are not performing properly. The
is uniform throughout the material.
most common symptom is instability with time or temperature
3.1.1.14 thermal output—thereversiblepartofthetempera-
change.Ifstraingagesdonotreturntotheirzeroreadingwhen
ture induced indicated strain of a strain gage installed on an
theoriginalconditionsarerepeated,orthereisloworchanging
unrestrained test specimen when exposed to a change in
resistance to ground, the installation is suspect. Aids in strain
temperature.
gageinstallationandverificationthereofcanbefoundinGuide
3.1.1.15 transverse axis(seeFig.1)—thestraingageaxisat
E1237.
90° to the measurement axis.
3.1.1.16 transverse sensitivity—the ratio, expressed as a
6. Hazards
percentage, of the unit change of resistance of a strain gage
6.1 In the specimen surface cleaning, gage bonding, and
mounted perpendicular to a uniaxial strain field (transverse
protection steps of strain gage installation, hazardous chemi-
gage) to the unit resistance change of a similar gage mounted
cals may be used. Users of these test methods are responsible
parallel to the same strain field (longitudinal gage).
for contacting manufacturers of these chemicals for applicable
3.1.1.17 type—a group of strain gages that are nominally
Material Safety Data Sheets and to adhere to the required
identical with respect to physical and manufacturing charac-
precautions.
teristics.
4. Significance and Use 7. Test Requirements
4.1 Strain gages are the most widely used devices for the 7.1 General Environmental Requirements:
determination of materials, properties and for analyzing 7.1.1 Ambient Conditions at Room Temperature—The
stresses in structures. However, performance parameters of nominal temperature and relative humidity shall be 23°C
straingagesareaffectedbyboththematerialsfromwhichthey (73°F) and 50%, respectively. In no case shall the temperature
aremadeandtheirgeometricdesign.Thesetestmethodsdetail be less that 18°C (64°F) nor greater than 25°C (77°F) and the
the minimum information that must accompany strain gages if relative humidity less than 35% nor more than 60%. The
they are to be used with acceptable accuracy of measurement. fluctuationsduringanyroomtemperaturetestofanygageshall
4.2 Most performance parameters of strain gages require not exceed6 2°C and 6 5% RH.
mechanical testing that is destructive. Since test gages cannot 7.1.2 Ambient Conditions at Elevated and Lower
be used again, it is necessary to treat data statistically and then Temperatures—The temperature adjustment error shall not
apply values to the remaining population from the same lot or exceed 6 2°C (6 3.6°F) or 6 2% of the deviation from room
batch. Failure to acknowledge the resulting uncertainties can temperature, whichever is greater. The total uncertainty of
have serious repercussions. Resistance measurement is non- temperatureshallnotexceed 62°C(63.6°F),or 61%ofthe
destructive and can be made for each gage. deviation from room temperature, whichever is greater. At
E 251 – 92 (2003)
FIG. 3 Unbalanced-Bridge Circuit
FIG. 2 Wheatstone-Bridge Circuit
resistor that can be varied so that the total arm resistance
elevated temperatures the mixing ratio shall be constant, that
changes in accurately known steps. This resistor should be in
meansindependentoftemperature,atanominalvalueof0.009
the opposite arm of the bridge circuit from the gage. This
g of water per1gofairata pressure of 1 bar. This value
circuit is shown in Fig. 3.
corresponds to a relative humidity of 50% at 23°C (73°F).
7.2.2.3 Several types of instruments are available for ob-
taining strain data directly from a resistance strain gage.These
NOTE 1—This mixing ratio, independent of tem
...
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