ASTM E251-92(2009)
(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 gauges are the most widely used devices for the determination of materials, properties and for analyzing stresses in structures. However, performance parameters of strain gauges 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 gauges if they are to be used with acceptable accuracy of measurement.
Most performance parameters of strain gauges require mechanical testing that is destructive. Since test gauges 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 gauge.
Properly designed and manufactured strain gauges, 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 gauges cannot be calibrated. If calibration and traceability to a standard are required, strain gauges should not be employed.
To be used, strain gauges must be bonded to a structure. Good results depend heavily on the materials used to clean the bonding surface, to bond the gauge, 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 gauges. 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 gauge systems far exceed those from the structural deformations to be measured....
SCOPE
1.1 The purpose of this standard is to provide uniform test methods for the determination of strain gauge performance characteristics. Suggested testing equipment designs are included.
1.2 Test Methods E 251 describes methods and procedures for determining five strain gauge parameters:
Section Part I—General Requirements 7 Part II—Resistance at a Reference Temperature 8 Part III—Gauge Factor at a Reference Temperature 9 Part IV—Temperature Coefficient of Gauge Factor10 Part V—Transverse Sensitivity11 Part VI—Thermal Output12
1.3 Strain gauges 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 gauge 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 gauge resistance 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 it is subjected. Accuracy of strain gauge data can be no better than the precision of the gauge factor.
1.3.3 Changes in gauge 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 gauge's response to strains perpendicular to its measurement axis. Although transverse sensitivity is usually much less than 10 % of the gauge 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 gauge to temperature changes. Thermal output is an additive (not multiplicative) error. Therefore, it can often be much larger than the gauge output from stru...
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Designation: E251 − 92(Reapproved 2009)
Standard Test Methods for
Performance Characteristics of Metallic Bonded Resistance
Strain Gauges
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 (´) 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, 'Performance
Characteristics of Metallic Resistance Strain Gauges.’ Test Methods E251 has been modified and
expanded to be the United States ofAmerica’s compliant test specification. Throughout this standard
the terms “strain gauge” and “gauge” are to be understood to represent the longer, but more accurate,
“metallic bonded resistance strain gauges.”
1. Scope is subjected. Accuracy of strain gauge data can be no better
than the precision of the gauge factor.
1.1 The purpose of this standard is to provide uniform test
1.3.3 Changes in gauge factor as temperature varies also
methods for the determination of strain gauge performance
affect accuracy although to a much lesser degree since varia-
characteristics. Suggested testing equipment designs are in-
cluded. tions are usually small.
1.3.4 Transverse sensitivity is a measure of the strain
1.2 Test Methods E251 describes methods and procedures
gauge’s response to strains perpendicular to its measurement
for determining five strain gauge parameters:
axis.Although transverse sensitivity is usually much less than
Section
Part I—General Requirements 7 10% of the gauge factor, large errors can occur if the value is
Part II—Resistance at a Reference Temperature 8
not known with reasonable precision.
Part III—Gauge Factor at a Reference Temperature 9
1.3.5 Thermal output is the response of a strain gauge to
Part IV—Temperature Coefficient of Gauge Factor 10
Part V—Transverse Sensitivity 11
temperature changes. Thermal output is an additive (not
Part VI—Thermal Output 12
multiplicative) error. Therefore, it can often be much larger
1.3 Strain gauges are very sensitive devices with essentially
than the gauge output from structural loading. To correct for
infinite resolution. Their response to strain, however, is low
these effects, thermal output must be determined from gauges
and great care must be exercised in their use.The performance
bonded to specimens of the same material on which the tests
characteristics identified by these test methods must be known
are to run; often to the test structure itself.
to an acceptable accuracy to obtain meaningful results in field
1.4 Bonded resistance strain gauges differ from extensom-
applications.
eters in that they measure average unit elongation (∆L/L) over
1.3.1 Strain gauge resistance is used to balance instrumen-
tation circuits and to provide a reference value for measure- a nominal gauge length rather than total elongation between
definite gauge points. Practice E83 is not applicable to these
ments since all data are related to a change in the gauge
resistance from a known reference value. gauges.
1.3.2 Gauge factor is the transfer function of a strain gauge.
1.5 These test methods do not apply to transducers, such as
It relates resistance change in the gauge and strain to which it
load cells and extensometers, that use bonded resistance strain
gauges as sensing elements.
1.6 Straingaugesarepartofacomplexsystemthatincludes
These test methods are under the jurisdiction of ASTM Committee E28 on
Mechanical Testing and are the direct responsibility of Subcommittee E28.01 on
structure, adhesive, gauge, leadwires, instrumentation, and
Calibration of Mechanical Testing Machines and Apparatus.
(often) environmental protection. As a result, many things
Current edition approved April 1, 2009. Published September 2009. Originally
affect the performance of strain gauges, including user tech-
approved in 1964. Last previous edition approved in 2003 as E251–92 (2003).
DOI: 10.1520/E0251-92R09. nique. A further complication is that strain gauges once
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E251 − 92 (2009)
installed normally cannot be reinstalled in another location.
Therefore, gauge characteristics can be stated only on a
statistical basis.
1.7 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
responsibility 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. Referenced Documents
2.1 ASTM Standards:
E83Practice for Verification and Classification of Exten-
someter Systems
E228Test Method for Linear Thermal Expansion of Solid
Materials With a Push-Rod Dilatometer
E289Test Method for Linear Thermal Expansion of Rigid
Solids with Interferometry
E1237Guide for Installing Bonded Resistance Strain Gages
2.2 OIML International Recommendation No. 62:' Perfor-
mance Characteristics of Metallic Resistance Strain Gauges
3. Terminology
FIG. 1 Typical Strain Gauge
3.1 Definitions of Terms Specific to This Standard:
3.1.1 The vocabulary included herein has been chosen so
that specialized terms in the strain gauge field will be clearly
L = the test structure length under the strain gauge at test
defined. A typical strain gauge nomenclature is provided in
strain,
Appendix X1.
L = the test structure length under the strain gauge at zero
o
3.1.1.1 batch—agroupofstraingaugesofthesametypeand
or reference strain,
lot,manufacturedasaset(madeatthesametimeandunderthe
∆R = the change in strain gauge resistance when strain is
same conditions).
changed from zero (or reference strain to test strain),
3.1.1.2 calibration apparatus—equipmentfordetermininga
characteristicofabondedresistancestraingaugebyaccurately
ε = the mechanical strain L2L L .
o/ o
producing the necessary strains, temperatures, and other con-
3.1.1.5 gauge length (see Fig. 1)—the length of the strain
ditions; and, by accurately measuring the resulting change of
sensitive section of a strain gauge in the measurement axis
gauge resistance.
direction. An approximation of this length is the distance
3.1.1.3 error-strain gauge—the value obtained by subtract-
betweentheinsideofthestraingaugeendloops.Sincethetrue
ing the actual value of the strain, determined from the
gauge length is not known, gauge length may be measured by
calibration apparatus, from the indicated value of the strain
other geometries (such as the outside of the end loops)
given by the strain gauge output. Errors attributable to mea-
providing that the deviation is defined.
suring systems are excluded.
3.1.1.6 grid (see Fig. 1)—that portion of the strain-sensing
3.1.1.4 gauge factor—the ratio between the unit change in
material of the strain gauge that is primarily responsible for
straingaugeresistanceduetostrainandthecausingstrain.The
resistance change due to strain.
gauge factor is dimensionless and is expressed as follows:
3.1.1.7 lot—a group of strain gauges with grid elements
R 2 R L 2 L ∆R
o o
from a common melt, subjected to the same mechanical and
K 5 / 5 /ε (1)
R L R
o o o
thermal processes during manufacturing.
3.1.1.8 matrix (see Fig. 1)—an electrically nonconductive
where:
layer of material used to support a strain gauge grid. The two
K = the gauge factor,
main functions of a matrix are to act as an aid for bonding the
R = the strain gauge resistance at test strain,
straingaugetoastructureandasanelectricallyinsulatinglayer
R = the strain gauge resistance at zero or reference strain,
o
in cases where the structure is electrically conductive.
3.1.1.9 measurement axis (grid) (see Fig. 1)—that axis that
is parallel with the grid lines.
3.1.1.10 strain gauge, metallic, resistive, bonded (see Fig.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
1)—a resistive element, with or without a matrix that is
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
attached to a solid body by cementing, welding, or other
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. suitable techniques so that the resistance of the element will
E251 − 92 (2009)
vary as the surface to which it is attached is deformed. These have serious repercussions. Resistance measurement is non-
test methods apply to gauges where the instantaneous gauge destructive and can be made for each gauge.
resistance, R, is given by the equation:
4.3 Properly designed and manufactured strain gauges,
R 5 R 11εK (2)
~ ! whose properties have been accurately determined and with
o
appropriate uncertainties applied, represent powerful measure-
where:
ment tools. They can determine small dimensional changes in
R = element resistance at reference strain and temperature
o
structures with excellent accuracy, far beyond that of other
levels (frequently initial test or balanced circuit
known devices. It is important to recognize, however, that
conditions),
individualstraingaugescannotbecalibrated.Ifcalibrationand
ε = linear strain of the surface in the direction of the
traceability to a standard are required, strain gauges should not
strain-sensitive axis of the gauge, and
be employed.
K = a proportionality factor (see gauge factor).
4.4 To be used, strain gauges must be bonded to a structure.
3.1.1.11 strain, linear—the unit elongation induced in a
Good results depend heavily on the materials used to clean the
specimen either by a stress field (mechanical strain) or by a
bondingsurface,tobondthegauge,andtoprovideaprotective
temperature change (thermal expansion).
coating.Skilloftheinstallerisanothermajorfactorinsuccess.
3.1.1.12 temperature coeffıcient of gauge factor—the ratio
Finally, instrumentation systems must be carefully designed to
of the unit variation of gauge factor to the temperature
assure that they do not unduly degrade the performance of the
variation, expressed as follows:
gauges. In many cases, it is impossible to achieve this goal. If
K 2 K 1
t1 t0 so, allowance must be made when considering accuracy of
· (3)
S D S D
K T 2 T
data. Test conditions can, in some instances, be so severe that
t0 1 0
error signals from strain gauge systems far exceed those from
where:
the structural deformations to be measured. Great care must be
T = the test temperature,
exercised in documenting magnitudes of error signals so that
T = the reference temperature,
realistic values can be placed on associated uncertainties.
K = the gauge factor at test temperature, and
t1
K = the gauge factor at reference temperature.
t0
5. Interferences
3.1.1.13 thermal expansion—the dimensional change of an
5.1 In order to assure that strain gauge test data are within a
unconstrainedspecimensubjecttoachangeintemperaturethat
defined accuracy, the gauges must be properly bonded and
is uniform throughout the material.
protected with acceptable materials. It is normally simple to
3.1.1.14 thermal output—the reversible part of the tempera-
ascertain that strain gauges are not performing properly. The
ture induced indicated strain of a strain gauge installed on an
most common symptom is instability with time or temperature
unrestrained test specimen when exposed to a change in
change. If strain gauges do not return to their zero reading
temperature.
when the original conditions are repeated, or there is low or
3.1.1.15 transverse axis (see Fig. 1)—the strain gauge axis
changing resistance to ground, the installation is suspect.Aids
at 90° to the measurement axis.
in strain gauge installation and verification thereof can be
3.1.1.16 transverse sensitivity—the ratio, expressed as a
found in Guide E1237.
percentage, of the unit change of resistance of a strain gauge
mounted perpendicular to a uniaxial strain field (transverse
6. Hazards
gauge)totheunitresistancechangeofasimilargaugemounted
6.1 In the specimen surface cleaning, gauge bonding, and
parallel to the same strain field (longitudinal gauge).
protection steps of strain gauge installation, hazardous chemi-
3.1.1.17 type—a group of strain gauges that are nominally
cals may be used. Users of these test methods are responsible
identical with respect to physical and manufacturing charac-
for contacting manufacturers of these chemicals for applicable
teristics.
Material Safety Data Sheets and to adhere to the required
precautions.
4. Significance and Use
4.1 Strain gauges are the most widely used devices for the 7. Test Requirements
determination of materials, properties and for analyzing
7.1 General Environmental Requirements:
stresses in structures. However, performance parameters of
7.1.1 Ambient Conditions at Room Temperature—The
strain gauges are affected by both the materials from which
nominal temperature and relative humidity shall be 23°C
they are made and their geometric design. These test methods
(73°F) and 50%, respectively. In no case shall the temperature
detail the minimum information that must accompany strain
be less that 18°C (64°F) nor greater than 25°C (77°F) and the
gauges if they are to be used with acceptable accuracy of
relative humidity less than 35% nor more than 60%. The
measurement.
fluctuations during any room temperature test of any gauge
4.2 Most performance parameters of strain gauges require shall not exceed6 2°C and 6 5% RH.
mechanical testing that is destructive. Since test gauges 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
E251 − 92 (2009)
FIG. 3 Unbalanced-Bridge Circuit
FIG. 2 Wheatstone-Bridge Circuit
temperatureshallnotexceed 62°C(63.6°F),or 61%ofthe voltages, the readout (recording) system may be calibrated in
terms of unit resistance change of a bridge arm by use of a
deviation from room temperature, whichever is greater. At
elevated temperatures the mixing ratio shall be constant, that calibrating resistor that can be varied so that the total arm
resistance changes in accurately known steps. This resistor
meansindependentoftemperature,atanominalvalueo
...
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