ASTM E1949-03(2009)

This document is not anASTM standard and is intended only to provide the user of anASTM 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.
An American National Standard Designation: E 1949 – 03 (Reapproved 2009)
Designation:E1949–02
Standard Test Method for
Ambient Temperature Fatigue Life of Metallic Bonded
Resistance Strain Gages
This standard is issued under the fixed designation E 1949; 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, 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:
E 1237 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).
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.
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 E 1237. It is important
This test method is under the jurisdiction of ASTM Committee E28 on Mechanical Testing and is the direct responsibility of Subcommittee E28.14 on Strain Gages.
Current edition approved April 10, 2002. Published July 2002. Originally published as E1949–98. Last previous edition E1949–98.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, 2009. Published September 2009. Originally approved in 1998. Last previous edition approved in 2003 as E 1949 – 03.
For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at service@astm.org. For Annual Book ofASTM Standards
, Vol 03.01.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 (2009)
to note that good performance in cyclic applications requires the best installations possible.
6. Hazards
6.1 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 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 65µV/V
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
usevarioustypesofexcitationandread-outsystems.Suchindicatorsmaybeusedonlyaftertheirresolution,accuracy,andstability
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 Asuggested 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
epoxycompositematerial,withallfibersalignedwiththelongaxisofthebeam.Surfacespallingofmetallictestbeamsandcrazing
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.Acantilever
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.
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.Aloading 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 Ashut 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 machine,
so the machine can be programmed to shut off or take data at preselected intervals.
8. Conditioning
8.1 Ambient (Room Temperature) Conditions—The nominal temperature and relative humidity shall be 23°C (73°F) and 50 %,
respectively. In no case shall the temperature be less than 18°C (64°F) or greater than 25°C (77°F) and the relative humidity less
than 35 % or more than 60 %.
The sole source of supply of this material known to the committee at this time is
...