Designation: E 83 – 02

Standard Practice for

Verification and Classification of Extensometer System1
This standard is issued under the fixed designation E 83; 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.

1. Scope 3.1.1 calibration—a determination of the calibration factor

1.1 This practice covers procedures for the verification and for a system using established procedures.
classification of extensometer systems, but it is not intended to 3.1.2 calibration factor—the factor by which the change in
be a complete purchase specification. The practice is applicable extensometer reading must be multiplied to obtain the equiva-
only to instruments that indicate or record values that are lent strain.
proportional to changes in length corresponding to either 3.1.2.1 Discussion—For any extensometer, the calibration
tensile or compressive strain. Extensometer systems are clas- factor is equal to the ratio of change in length to the product of
sified on the basis of the magnitude of their errors. the gage length and the change in the extensometer reading.
1.2 Because strain is a dimensionless quantity, this docu- For direct-reading extensometers the calibration factor is unity.
ment can be used for extensometers based on either SI or US 3.1.3 compressometer—a specialized extensometer used for
customary units of displacement. sensing negative or compressive strain.
3.1.4 deflectometer—a specialized extensometer used for
NOTE 1—Bonded resistance strain gages directly bonded to a specimen sensing of extension or motion, usually without reference to a
cannot be calibrated or verified with the apparatus described in this
specific gage length.
practice for the verification of extensometers having definite gage points.
(See procedures as described in Test Methods E 251.) 3.1.5 error, in extensometer systems—the value obtained by
subtracting the correct value of the strain from the indicated
1.3 This standard does not purport to address all of the value given by the extensometer system.
safety concerns, if any, associated with its use. It is the 3.1.6 extensometer, n—a device for sensing strain.
responsibility of the user of this standard to establish appro- 3.1.7 extensometer systems—a system for sensing and indi-
priate safety and health practices and determine the applica- cating strain.
bility of regulatory limitations prior to use. 3.1.7.1 Discussion—The system will normally include an
2. Referenced Documents extensometer, conditioning electronics and auxiliary device
(recorder, digital readout, computer, etc.). However, com-
2.1 ASTM Standards: pletely self-contained mechanical devices are permitted. An
E 6 Terminology Relating to Methods of Mechanical Test- extensometer system may be one of three types.
ing2 3.1.8 Type 1 extensometer system, n—an extensometer sys-
E 21 Test Methods for Elevated Temperature Tension Tests tem which both defines gage length and senses extension, for
of Metallic Materials2 example, a clip-on strain gage type with conditioning electron-
E 251 Test Methods for Performance Characteristics of
ics.
Metallic Bonded Resistance Strain Gages2
3.1.9 Type 2 extensometer system, n—an extensometer
3. Terminology which senses extension and the gage length is defined by
specimen geometry or specimen features such as ridges or
3.1 Definitions: In addition to the terms listed, see Termi-
notches.
nology E 6.
3.1.9.1 Discussion—A Type 2 extensometer is used where
the extensometer gage length is determined by features on the
specimen, for example, ridges, notches, or overall height (in
1
This practice is under the jurisdiction of ASTM Committee E28 on Mechanical case of compression test piece). The precision associated with
Testing and is the direct responsibility of Subcommittee E28.01 on Calibration of gage length setting for a Type 2 extensometer should be
Mechanical Testing Machines and Apparatus.
Current edition approved June 10, 2002. Published August 2002. Originally
specified in relevant test method or product standard. The
published as E 83 – 50. Last previous edition E 83 – 00e1. position readout on a testing machine is not recommended for
2
Annual Book of ASTM Standards, Vol 03.01. use in a Type 2 extensometer system.

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.

1
 E 83 – 02
3.1.10 Type 3 extensometer system, n—an extensometer their limits of accuracy and sensitivity stated. The errors of the
system which intrinsically senses strain (ratiometric principle), verification apparatus shall not exceed one third of the permis-
for example, video camera system. sible error of the extensometer.
3.1.11 gage length (L), n—the original length of that portion 4.2 The verification apparatus shall be calibrated at intervals
of the specimen over which strain or change of length is not to exceed two years.
determined. NOTE 2—He-Ne laser interferometer measurement systems based on
3.1.11.1 Discussion—If the device is used for sensing the 0.633 µm wavelength line are considered to be primary-based
extension or motion, and gage length is predetermined by the displacement standards and do not require recalibration.4
specimen geometry or specific test method, then only resolu-
4.3 If the verification apparatus is to be used to verify
tion and strain error for a specified gage length should
extensometers used for bidirectional tests, the errors of the
determine the class of extensometer system.
verification apparatus should be measured in both directions of
3.1.12 resolution of the strain indicator—the smallest
travel so as to include any backlash present.
change in strain that can be estimated or ascertained on the
strain indicating apparatus of the testing system, at any applied 5. Verification Procedure for Extensometer Systems
strain. 5.1 General Requirements—The verification of an exten-
3.1.13 resolution of the digital type strain indicators (nu- someter system should not be done unless the components of
meric displays, printouts, and so forth)—the resolution is the the system are in good working condition. Thoroughly inspect
smallest change in strain that can be displayed on the strain all parts associated with smooth operation of the instrument to
indicator (may be a single digit or a combination of digits) at ensure there are no excessively worn components. Repair or
any applied strain. replace parts as necessary. Remove any dirt particles which
3.1.13.1 Discussion—If the strain indication, for either type may have accumulated through normal use of the instrument.
of strain indicator, fluctuates more than twice the resolution, as Verification of the system shall be performed whenever parts
described in 3.1.11 or 3.1.12, the resolution expressed as a are interchanged or replaced.
strain shall be equal to one-half the range of fluctuation. 5.1.1 The verification of an extensometer system refers to a
3.1.14 verification—a determination that a system meets the specific extensometer used with a specific readout device.
requirements of a given classification after calibration accord- Unless it can be demonstrated that autographic extensometers
ing to established procedures. and recorders of a given type may be used interchangeably
3.1.15 verification apparatus—a device for verifying exten- without introducing errors that would affect the classification
someter systems. of the extensometer, the extensometer shall be calibrated with
3.1.15.1 Discussion—This device is used to simulate the the readout device with which it is to be used.
change in length experienced by a test specimen as a result of 5.1.2 Prior to the initial verification, the extensometer
the applied force. The extensometer may either be attached should be calibrated according to the manufacturer’s instruc-
directly to the mechanism or interfaced with it in a manner tions or established procedures. The calibration procedure may
similar to normal operation (that is, possibly without contact include adjustment of span or determination of calibration
for some optical extensometers). factor, or both.
5.2 Gage Length Measurement Method— Measure the gage
4. Verification Apparatus length of self-setting instruments by either the direct or indirect
4.1 The apparatus for verifying extensometer systems shall method.
provide a means for applying controlled displacements to a
NOTE 3—The following is an example of an indirect method. Set the
simulated specimen and for measuring these displacements extensometer to its starting position and mount it on a soft rod of the
accurately. It may consist of a rigid frame, suitable coaxial typical specimen size or diameter. After the extensometer is removed,
spindles, or other fixtures to accommodate the extensometer measure the distance between the marks left by the gage points (or knife
being verified, a mechanism for moving one spindle or fixture edges). If there are four or more gage points, take the average of the
axially with respect to the other, and a means for measuring individual lengths as the gage length. The differences between individual
accurately the change in length so produced,3 or any other measurements shall not exceed the tolerance given for the class of
extensometer. If there are two gage points (or knife edges), but on opposite
device or mechanism that will accomplish the purpose equally
sides of the specimen, attach the extensometer twice rotating it 180° with
well. The mechanism provided for moving one spindle relative respect to the rod. Take the average of the lengths thus established on each
to the other shall permit sensitive adjustments. The changes in side of the rod as the gage length.
length shall be measured, for example, by means of an
5.2.1 Make two measurements of the gage length. Deter-
interferometer, calibrated standard gage blocks and an indica-
mine and record the error from each measurement, which is the
tor, a calibrated micrometer screw, or a calibrated laser
difference between the measured gage length and the specified
measurement system. If standard gage blocks and an indicator,
gage length, expressed as a percent of the specified gage
or a micrometer screw, are used, they shall be calibrated and
length.
5.2.2 For extensometer devices that do not have a self-
setting gage length during use, such as deflectometers and
3
A review of some past, current, and possible future methods for calibrating
strain measuring devices is given in the paper by Watson, R. B., “Calibration
4
Techniques for Extensometry: Possible Standards of Strain Measurement,” Journal A letter from NIST (National Institute of Standards and Technology) is
of Testing and Evaluation, JTEVA, Vol. 21, No. 6, November 1993, pp. 515–521. available for reference. Request RR:E 28-1013 from ASTM Headquarters.