ASTM E1949-03(2014)

Standard Test Method for Ambient Temperature Fatigue Life of Metallic Bonded Resistance Strain Gages

SIGNIFICANCE AND USE
4.1 Strain gages are the most widely used devices for measuring strains and for evaluating stresses in structures. In many applications there are often cyclic loads which can cause strain gage failure. Performance parameters of strain gages are affected by both the materials from which they are made and their geometric design.  
4.2 The determination of most strain gage parameters requires mechanical testing that is destructive. Since gages tested for fatigue life cannot be used again, it is necessary to treat data statistically. In general, longer and wider gages with lower resistances will have greater fatigue life. Optional additions to gages (integral leads are an example) will often reduce fatigue life.  
4.3 To be used, strain gages must be bonded to a structure. Good results, particularly in a fatigue environment, 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 selected and calibrated to ensure that they do not unduly degrade the performance of the gages.  
4.4 This test method encompasses only fully reversed strain cycles.  
4.5 Fatigue failure of a strain gage may not involve visible cracking or fracture of the gage, but merely sufficient zero shift to compromise the accuracy of the gage output for static strain components.
SCOPE
1.1 This test method covers a uniform procedure for the determination of strain gage fatigue life at ambient temperature. A suggested testing equipment design is included.  
1.2 This test method does not apply to force transducers or extensometers that use bonded resistance strain gages as sensing elements.  
1.3 Strain gages are part of a complex system that includes structure, adhesive, gage, lead wires, instrumentation, and (often) environmental protection. As a result, many things affect the performance of strain gages, including user technique. A further complication is that strain gages, once installed, normally cannot be reinstalled in another location. Therefore, it is not possible to calibrate individual strain gages; performance characteristics are normally presented on a statistical basis.  
1.4 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 appropriate safety and health practices and determine the applicability of regulatory limitations prior to its use.

General Information

• Status: Historical
• Publication Date: 14-Apr-2014
• ICS: 19.060 - Mechanical testing
• Technical Committee: E28 - Mechanical Testing
• Drafting Committee: E28.01 - Calibration of Mechanical Testing Machines and Apparatus

Relations

• Replaces: ASTM E1949-03(2009) - Standard Test Method for Ambient Temperature Fatigue Life of Metallic Bonded Resistance Strain Gages
• Effective Date: 15-Apr-2014

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E1949 − 03(Reapproved 2014)
Standard Test Method for
Ambient Temperature Fatigue Life of Metallic Bonded
Resistance Strain Gages
This standard is issued under the fixed designation E1949; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 4. Significance and Use
1.1 This test method covers a uniform procedure for the 4.1 Strain gages are the most widely used devices for
determination of strain gage fatigue life at ambient tempera- measuring strains and for evaluating stresses in structures. In
ture. A suggested testing equipment design is included. many applications there are often cyclic loads which can cause
strain gage failure. Performance parameters of strain gages are
1.2 This test method does not apply to force transducers or
affected by both the materials from which they are made and
extensometers that use bonded resistance strain gages as
their geometric design.
sensing elements.
4.2 The determination of most strain gage parameters re-
1.3 Strain gages are part of a complex system that includes
quires mechanical testing that is destructive. Since gages tested
structure, adhesive, gage, lead wires, instrumentation, and
forfatiguelifecannotbeusedagain,itisnecessarytotreatdata
(often) environmental protection. As a result, many things
statistically. In general, longer and wider gages with lower
affect the performance of strain gages, including user tech-
resistances will have greater fatigue life. Optional additions to
nique. A further complication is that strain gages, once
gages (integral leads are an example) will often reduce fatigue
installed, normally cannot be reinstalled in another location.
life.
Therefore, it is not possible to calibrate individual strain gages;
performance characteristics are normally presented on a statis- 4.3 To be used, strain gages must be bonded to a structure.
tical basis. Good results, particularly in a fatigue environment, depend
heavily on the materials used to clean the bonding surface, to
1.4 This standard does not purport to address all of the
bond the gage, and to provide a protective coating. Skill of the
safety concerns, if any, associated with its use. It is the
installer is another major factor in success. Finally, instrumen-
responsibility of the user of this standard to establish appro-
tation systems must be carefully selected and calibrated to
priate safety and health practices and determine the applica-
ensure that they do not unduly degrade the performance of the
bility of regulatory limitations prior to its use.
gages.
2. Referenced Documents
4.4 This test method encompasses only fully reversed strain
2.1 ASTM Standards: cycles.
E1237 Guide for Installing Bonded Resistance Strain Gages
4.5 Fatigue failure of a strain gage may not involve visible
cracking or fracture of the gage, but merely sufficient zero shift
3. Terminology
to compromise the accuracy of the gage output for static strain
3.1 Definitions of Terms Specific to This Standard:
components.
3.1.1 strain gage fatigue life, n—the number of fully re-
versed strain cycles corresponding to the onset of degraded
5. Interferences
gage performance, whether due to excessive zero shift or other
5.1 In order to ensure that strain gage test data are within a
detectable failure mode (see 9.6).
defined accuracy, the gages must be properly bonded and
protected with acceptable materials. Aids in the strain gage
This test method is under the jurisdiction of ASTM Committee E28 on installation and verification thereof can be found in Guide
Mechanical Testing and is the direct responsibility of Subcommittee E28.01 on
E1237. It is important to note that good performance in cyclic
Calibration of Mechanical Testing Machines and Apparatus.
applications requires the best installations possible.
Current edition approved April 15, 2014. Published August 2014. Originally
approved in 1998. Last previous edition approved in 2009 as E1949 – 03(2009).
6. Hazards
DOI: 10.1520/E1949-03R14.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
6.1 Warning—In the specimen surface cleaning, gage
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
bonding, and protection steps of strain gage installations,
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. hazardous chemicals are often employed. Users of these test
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1949 − 03 (2014)
methods are responsible for contacting manufactures of such 7.2.1.1 Beam specimens must be cut such that the glass
chemicals for applicable Material Safety Data Sheets, and to fibers are aligned with the long dimension of the specimen. A
adhere to the required precautions. cantilever specimen is recommended for this testing because it
provides a range of strain levels in a single test. (A conse-
7. Apparatus quenceisthatthespecimen’sstrainlevelneartheclampisvery
high. Normal structural materials will not survive such high
7.1 Test Measurement Requirements:
levels and may fail in ways that imply strain gage failure when
7.1.1 For fatigue life determination the uncertainty of the
such is not the case.) A test beam should be used for one test
relative resistance change measurement shall not exceed 65
only.
µΩ/Ω or 60.1 % of the actual value, whichever is greater.
7.2.2 A suggested fatigue testing machine is illustrated in
7.1.2 Several methods are available for measuring the
Fig. 2. For a specimen with overall dimensions as shown in
change of gage resistance with sufficient resolution and accu-
Fig. 1 and a thickness of 9.5 mm (0.375 in.), the crank should
racy. In general, any method that is convenient may be used
deflect the beam approximately 15 mm (0.6 in.) to produce a
after it has been shown that the particular combination of
suitable strain range from 6500 µm/m to 63500 µm/m. A
instruments or components used produces a system with the
loading rate of 1800 cycles/min has proven efficient, but not so
required accuracy.
fast as to cause higher mode bending. While not absolutely
7.1.3 Many types of instruments are available for obtaining
essential, there are several features that provide for a safer and
strain data directly from a resistance strain gage. These
more accurate machine, as follows:
instruments use various types of excitation and read-out
7.2.2.1 A thick plastic shield to prevent injury in case of
systems. Such indicators may be used only after their
specimen or machine failure.
resolution, accuracy, and stability have been verified by con-
7.2.2.2 A shut off device consisting of micro switches
necting a resistor that can be varied in accurately known
positioned above and below the specimen (near the crank) and
increments in place of the gage and calibrating the strain
wired in the motor power circuit to shut off power in case of
indicator over the entire range for which it will be used. The
specimen rupture; and
calibrating resistor steps shall be accurate to 0.1 % of the
7.2.2.3 An electric counter geared to the drive system, or
resistance change or 2 ppm of the total resistance, whichever is
some other counting device appropriately connected to the
greater. Effects from the following influences on measurement
machine,sothemachinecanbeprogrammedtoshutoffortake
accuracy must be quantified and found within limits that
data at preselected intervals.
preserve the required overall system accuracy: thermal emfs
within the bridge circuit and within the gage leadwire, reactive
8. Conditioning
changes within the bridge and lead circuits, initial bridge
8.1 Ambient (Room Temperature) Conditions—The nominal
unbalance, and battery conditions or power line fluctuations.
temperature and relative humidity shall be 23°C (73°F) and
7.2 Mechanical Equipment Requirements:
50 %,respectively.Innocaseshallthetemperaturebelessthan
7.2.1 A suggested cantilever test beam is shown in Fig. 1.
18°C (64°F) or greater than 25°C (77°F) and the relative
The beam must have a fatigue life exceeding that of the strain
humidity less than 35 % or more than 60 %.
gages to be tested. One material which meets this requirement
is 3M’s , which is a unidirectional glass-reinforced epoxy
9. Procedure
composite material, with all fibers aligned with the long axis of
9.1 Strain levels for the test should be selected based on the
the beam. Surface spalling of metallic test beams and crazing
expected fatigue life for the test gages.Typical values might be
of plastic specimens are examples of beam failures that will
62000 µm/m, 62400 µm/m, and 62800 µm/m. (It may be
produce faulty, misleadingly low, strain gage fatigue life.
necessary to select at least one substantially lower strain level
if it is desirable to indicate a no-failure strain level; see 9.6.2)
Normally six or more strain gages are tested at each strain
level.
3 9.2 Strain Gage Attachment Requirements:
The sole source of supply of this material known to the
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E1949 − 03 (Reapproved 2009) E1949 − 03 (Reapproved 2014)
Standard Test Method for
Ambient Temperature Fatigue Life of Metallic Bonded
Resistance Strain Gages
This standard is issued under the fixed designation E1949; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers a uniform procedure for the determination of strain gage fatigue life at ambient temperature. A
suggested testing equipment design is included.
1.2 This test method does not apply to force transducers or extensometers that use bonded resistance strain gages as sensing
elements.
1.3 Strain gages are part of a complex system that includes structure, adhesive, gage, leadwires, lead wires, instrumentation, and
(often) environmental protection. As a result, many things affect the performance of strain gages, including user technique. A
further complication is that strain gages, once installed, normally cannot be reinstalled in another location. Therefore, it is not
possible to calibrate individual strain gages; performance characteristics are normally presented on a statistical basis.
1.4 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 appropriate safety and health practices and determine the applicability of regulatory
limitations prior to its use.
2. Referenced Documents
2.1 ASTM Standards:
E1237 Guide for Installing Bonded Resistance Strain Gages
3. Terminology
3.1 strain gage fatigue life, n—the number of fully reversed strain cycles corresponding to the onset of degraded gage
performance, whether due to excessive zero shift or other detectable failure mode (see Section 9.6).
3.1 Definitions of Terms Specific to This Standard:
3.1.1 strain gage fatigue life, n—the number of fully reversed strain cycles corresponding to the onset of degraded gage
performance, whether due to excessive zero shift or other detectable failure mode (see 9.6).
4. Significance and Use
4.1 Strain gages are the most widely used devices for measuring strains and for evaluating stresses in structures. In many
applications there are often cyclic loads which can cause strain gage failure. Performance parameters of strain gages are affected
by both the materials from which they are made and their geometric design.
4.2 The determination of most strain gage parameters requires mechanical testing that is destructive. Since gages tested for
fatigue life cannot be used again, it is necessary to treat data statistically. In general, longer and wider gages with lower resistances
will have greater fatigue life. Optional additions to gages (integral leads are an example) will often reduce fatigue life.
4.3 To be used, strain gages must be bonded to a structure. Good results, particularly in a fatigue environment, 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 selected and calibrated to ensure that they do
not unduly degrade the performance of the gages.
This test method is under the jurisdiction of ASTM Committee E28 on Mechanical Testing and is the direct responsibility of Subcommittee E28.01 on Calibration of
Mechanical Testing Machines and Apparatus.
Current edition approved April 1, 2009April 15, 2014. Published September 2009August 2014. Originally approved in 1998. Last previous edition approved in 20032009
as E1949 – 03.E1949 – 03(2009). DOI: 10.1520/E1949-03R09.10.1520/E1949-03R14.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1949 − 03 (2014)
4.4 This test method encompasses only fully reversed strain cycles.
4.5 Fatigue failure of a strain gage may not involve visible cracking or fracture of the gage, but merely sufficient zero shift to
compromise the accuracy of the gage output for static strain components.
5. Interferences
5.1 In order to ensure that strain gage test data are within a defined accuracy, the gages must be properly bonded and protected
with acceptable materials. Aids in the strain gage installation and verification thereof can be found in Guide E1237. It is important
to note that good performance in cyclic applications requires the best installations possible.
6. Hazards
6.1 Warning—Warning—In the specimen surface cleaning, gage bonding, and protection steps of strain gage installations,
hazardous chemicals are often employed. Users of these test methods are responsible for contacting manufactures of such
chemicals for applicable Material Safety Data Sheets, and to adhere to the required precautionsIn the specimen surface cleaning,
gage bonding, and protection steps of strain gage installations, hazardous chemicals are often employed. Users of these test
methods are responsible for contacting manufactures of such chemicals for applicable Material Safety Data Sheets, and to adhere
to the required precautions.
7. Apparatus
7.1 Test Measurement Requirements:
7.1.1 For fatigue life determination the uncertainty of the relative resistance change measurement shall not exceed 65 μΩ/Ω
or 60.1 % of the actual value, whichever is greater.
7.1.2 Several methods are available for measuring the change of gage resistance with sufficient resolution and accuracy. In
general, any method that is convenient may be used after it has been shown that the particular combination of instruments or
components used produces a system with the required accuracy.
7.1.3 Many types of instruments are available for obtaining strain data directly from a resistance strain gage. These instruments
use various types of excitation and read-out systems. Such indicators may be used only after their resolution, accuracy, and stability
have been verified by connecting a resistor that can be varied in accurately known increments in place of the gage and calibrating
the strain indicator over the entire range for which it will be used. The calibrating resistor steps shall be accurate to 0.1 % of the
resistance change or 2 ppm of the total resistance, whichever is greater. Effects from the following influences on measurement
accuracy must be quantified and found within limits that preserve the required overall system accuracy: thermal emfs within the
bridge circuit and within the gage leadwire, reactive changes within the bridge and lead circuits, initial bridge unbalance, and
battery conditions or power line fluctuations.
7.2 Mechanical Equipment Requirements:
7.2.1 A suggested cantilever test beam is shown in Fig. 1. The beam must have a fatigue life exceeding that of the strain gages
to be tested. One material which meets this requirement is 3M’s Aerospace FP 525, , which is a unidirectional glass-reinforced
epoxy composite material, with all fibers aligned with the long axis of the beam. Surface spalling of metallic test beams and crazing
of plastic specimens are examples of beam failures that will produce faulty, misleadingly low, strain gage fatigue life.
7.2.1.1 Beam specimens must be cut such that the glass fibers are aligned with the long dimension of the specimen. A cantilever
specimen is recommended for this testing because it provides a range of strain levels in a single test. (A consequence is that the
specimen’s strain level near the clamp is very high. Normal structural materials will not survive such high levels and may fail in
ways that imply strain gage failure when such is not the case.) A test beam should be used for one test only.
The sole source of supply of this material known to the committee at this time is 3M, Product Information Ctr., at Bldg. 515-3N-06, St. Paul, MN 55144-1000. If you
aware of alternative suppliers, please provide this information to ASTM Headquarters. Your comments will receive careful consideration at a meeting of the responsible
technical committee, which you may attend.
FIG. 1 Cantilever Test Beam
E1949 − 03 (2014)
7.2.2 A suggested fatigue testing machine is illustrated in Fig. 2. For a specimen with overall dimensions as shown in Fig. 1
and a thickness of 9.5 mm (0.375 in.), the crank should deflect the beam approximately 15 mm (0.6 in.) to produce a suitable strain
range from 6500 μm/m to 63500 μm/m. A loading rate of 1800 cycles/min has proven efficient, but not so fast as to cause higher
mode bending. While not absolutely essential, there are several features that provide for a safer and more accurate machine, as
follows:
7.2.2.1 A thick plastic shield to prevent injury in case of specimen or machine failure.
7.2.2.2 A shut off device consisting of micro switches positioned above and below the specimen (near the crank) and wired in
the motor power circuit to shut off power in case of specimen rupture; and
7.2.2.3 An electric counter geared to the drive system, or some other counting device appropriately connected to the m
...