ASTM E251-92(1998)

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: E 251 – 92 (Reapproved 1998)
Standard Test Methods for
Performance Characteristics of Metallic Bonded Resistance
Strain Gages
This standard is issued under the fixed designation E 251; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number 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 E 251 has been modified and
expanded to be the United States of America’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 the precision of the gage factor.
1.3.3 Changes in gage factor as temperature varies also
1.1 The purpose of this standard is to provide uniform test
affect accuracy although to a much lesser degree since varia-
methods for the determination of strain gage performance
tions are usually small.
characteristics. Suggested testing equipment designs are in-
1.3.4 Transverse sensitivity is a measure of the strain gage’s
cluded.
response to strains perpendicular to its measurement axis.
1.2 Test Methods E 251 describes methods and procedures
Although transverse sensitivity is usually much less than 10 %
for determining five strain gage parameters:
of the gage factor, large errors can occur if the value is not
Section
known with reasonable precision.
Part I—General Requirements 7
Part II—Resistance at a Reference Temperature 8
1.3.5 Thermal output is the response of a strain gage to
Part III—Gage Factor at a Reference Temperature 9
temperature changes. Thermal output is an additive (not
Part IV—Temperature Coefficient of Gage Factor 10
multiplicative) error. Therefore, it can often be much larger
Part V—Transverse Sensitivity 11
Part VI—Thermal Output 12
than the gage output from structural loading. To correct for
these effects, thermal output must be determined from gages
1.3 Strain gages are very sensitive devices with essentially
bonded to specimens of the same material on which the tests
infinite resolution. Their response to strain, however, is low
are to run; often to the test structure itself.
and great care must be exercised in their use. The performance
1.4 Bonded resistance strain gages differ from extensom-
characteristics identified by these test methods must be known
eters in that they measure average unit elongation (DL/L) over
to an acceptable accuracy to obtain meaningful results in field
a nominal gage length rather than total elongation between
applications.
definite gage points. Practice E 83 is not applicable to these
1.3.1 Strain gage resistance is used to balance instrumenta-
gages.
tion circuits and to provide a reference value for measurements
1.5 These test methods do not apply to transducers, such as
since all data are related to a change in the gage resistance from
load cells and extensometers, that use bonded resistance strain
a known reference value.
gages as sensing elements.
1.3.2 Gage factor is the transfer function of a strain gage. It
1.6 Strain gages are part of a complex system that includes
relates resistance change in the gage and strain to which it is
structure, adhesive, gage, leadwires, instrumentation, and (of-
subjected. Accuracy of strain gage data can be no better than
ten) environmental protection. As a result, many things affect
the performance of strain gages, including user technique. A
1 further complication is that strain gages once installed nor-
These test methods are under the jurisdiction of ASTM Committee E-28 on
Mechanical Testing and are the direct responsibility of Subcommittee E28.14 on mally cannot be reinstalled in another location. Therefore, gage
Strain Gages.
characteristics can be stated only on a statistical basis.
Current edition approved Jan. 15, 1992. Published March 1992. Originally
1.7 This standard does not purport to address all of the
published as E 251 – 64 T. Last previous edition E 251 – 86.
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NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 251
safety concerns, if any, associated with its use. It is the
DR = the change in strain gage resistance when strain is
responsibility of the user of this standard to establish appro-
changed from zero (or reference strain to test strain),
priate safety and health practices and determine the applica-
e = the mechanical strain L 2 L L .
o/ o
bility of regulatory limitations prior to use.
3.1.1.5 gage length (see Fig. 1)—the length of the strain
1.8 The values stated in SI units are to be regarded as the
sensitive section of a strain gage in the measurement axis
standard.
direction. An approximation of this length is the distance
between the inside of the strain gage end loops. Since the true
2. Referenced Documents
gage length is not known, gage length may be measured by
2.1 ASTM Standards:
other geometries (such as the outside of the end loops)
E 83 Practice for Verification and Classification of Exten- providing that the deviation is defined.
someters
3.1.1.6 grid (see Fig. 1)—that portion of the strain-sensing
E 228 Test Method for Linear Thermal Expansion of Solid
material of the strain gage that is primarily responsible for
Materials with a Vitreous Silica Dilatometer
resistance change due to strain.
E 289 Test Method for Linear Thermal Expansion of Rigid
3.1.1.7 lot—a group of strain gages with grid elements from
Solids with Interferometry
a common melt, subjected to the same mechanical and thermal
E 1237 Guide for Installing Bonded Resistance Strain
processes during manufacturing.
Gages
3.1.1.8 matrix (see Fig. 1)—an electrically nonconductive
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, manufactured as a set (made at the same time and under the
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)
o
a characteristic of a bonded resistance strain gage by accurately
producing the necessary strains, temperatures, and other con-
where:
ditions; and, by accurately measuring the resulting change of
R = element resistance at reference strain and temperature
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 the unit variation of gage factor to the temperature variation,
o o
K 5 / 5 /e (1)
R L R
o o o expressed as follows:
K 2 K 1
t1 t0
where:
· (3)
S DS D
K T 2 T
t0 1 0
K = the gage factor,
R = the strain gage resistance at test strain,
where:
R = the strain gage resistance at zero or reference strain,
o
T = the test temperature,
L = the test structure length under the strain gage at test
T = the reference temperature,
strain,
K = the gage factor at test temperature, and
t1
L = the test structure length under the strain gage at zero
o
K = the gage factor at reference temperature.
t0
or reference strain,
3.1.1.13 thermal expansion—the dimensional change of an
unconstrained specimen subject to a change in temperature that
is uniform throughout the material.
2 3.1.1.14 thermal output—the reversible part of the tempera-
Annual Book of ASTM Standards, Vol 03.01.
Annual Book of ASTM Standards, Vol 14.02. ture induced indicated strain of a strain gage installed on an
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 251
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. 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 must be
exercised in documenting magnitudes of error signals so that
realistic values can be placed on associated uncertainties.
5. Interferences
5.1 In order to assure that strain gage test data are within a
defined accuracy, the gages must be properly bonded and
protected with acceptable materials. It is normally simple to
ascertain that strain gages are not performing properly. The
FIG. 1 Typical Strain Gage
most common symptom is instability with time or temperature
change. If strain gages do not return to their zero reading when
the original conditions are repeated, or there is low or changing
unrestrained test specimen when exposed to a change in
resistance to ground, the installation is suspect. Aids in strain
temperature.
gage installation and verification thereof can be found in Guide
3.1.1.15 transverse axis (see Fig. 1)—the strain gage axis at
E 1237.
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
mounted perpendicular to a uniaxial strain field (transverse
6.1 In the specimen surface cleaning, gage bonding, and
gage) to the unit resistance change of a similar gage mounted
protection steps of strain gage installation, hazardous chemi-
parallel to the same strain field (longitudinal gage).
cals may be used. Users of these test methods are responsible
3.1.1.17 type—a group of strain gages that are nominally
for contacting manufacturers of these chemicals for applicable
identical with respect to physical and manufacturing charac-
Material Safety Data Sheets and to adhere to the required
teristics.
precautions.
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
strain gages are affected by both the materials from which they
(73°F) and 50 %, respectively. In no case shall the temperature
are made and their geometric design. These test methods detail
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.
fluctuations during any room temperature test of any gage shall
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
be used again, it is necessary to treat data statistically and then 7.1.2 Ambient Conditions at Elevated and Lower
apply values to the remaining population from the same lot or Temperatures—The temperature adjustment error shall not
batch. Failure to acknowledge the resulting uncertainties can exceed 6 2°C (6 3.6°F) or 6 2 % of the deviation from room
have serious repercussions. Resistance measurement is non- temperature, whichever is greater. The total uncertainty of
destructive and can be made for each gage. temperature shall not exceed 6 2°C (6 3.6°F), or 6 1 % of the
4.3 Properly designed and manufactured strain gages, deviation from room temperature, whichever is greater. At
whose properties have been accurately determined and with elevated temperatures the mixing ratio shall be constant, that
appropriate uncertainties applied, represent powerful measure- means independent of temperature, at a nominal value of 0.009
ment tools. They can determine small dimensional changes in g of water per1gofair ata pressure of 1 bar. This value
structures with excellent accuracy, far beyond that of other corresponds to a relative humidity of 50 % at 23°C (73°F).
...