ASTM_E83.pdf

Designation: E83 − 10a

Standard Practice for Verification and Classification of Extensometer Systems1

This standard is issued under the fixed designation E83; 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.

This standard has been approved for use by agencies of the U.S. Department of Defense.

1. Scope

1.1 This practice covers procedures for the verification and

classification of extensometer systems, but it is not intended to be a complete purchase specification. The practice is applicable only to instruments that indicate or record values that are proportional to changes in length corresponding to either tensile or compressive strain. Extensometer systems are clas-sified on the basis of the magnitude of their errors.

1.2 Because strain is a dimensionless quantity, this docu-

ment can be used for extensometers based on either SI or US customary units of displacement.

NOTE 1—Bonded resistance strain gauges directly bonded to a speci-men cannot be calibrated or verified with the apparatus described in this practice for the verification of extensometers having definite gauge points.

(See procedures as described in Test Methods E251.)

1.3 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:2

E6 Terminology Relating to Methods of Mechanical Testing E21 Test Methods for Elevated Temperature Tension Tests of

Metallic Materials E251 Test Methods for Performance Characteristics of Me-tallic Bonded Resistance Strain Gages

3. Terminology

3.1 Definitions:

3.1.1 In addition to the terms listed, see Terminology E6.

3.1.2 calibration—a determination of the calibration factor for a system using established procedures.

3.1.3 calibration factor—the factor by which the change in extensometer reading must be multiplied to obtain the equiva-lent strain.

3.1.3.1 Discussion—For any extensometer, the calibration

factor is equal to the ratio of change in length to the product of the gauge length and the change in the extensometer reading.

For direct-reading extensometers the calibration factor is unity.

3.1.4 compressometer—a specialized extensometer used for sensing negative or compressive strain.

3.1.5 deflectometer—a specialized extensometer used for sensing of extension or motion, usually without reference to a specific gauge length.

3.1.6 error, in extensometer systems—the value obtained by subtracting the correct value of the strain from the indicated value given by the extensometer system.

3.1.7 extensometer, n—a device for sensing strain.

3.1.8 extensometer systems—a system for sensing and indi-cating strain.

3.1.8.1 Discussion—The system will normally include an

extensometer, conditioning electronics and auxiliary device (recorder, digital readout, computer, etc.). However, com-pletely self-contained mechanical devices are permitted. An extensometer system may be one of three types.

3.1.9 Type 1 extensometer system, n—an extensometer sys-

tem which both defines gauge length and senses extension, for example, a clip-on strain gauge type with conditioning elec-tronics.

3.1.10 Type 2 extensometer system, n—an extensometer

which senses extension and the gauge length is defined by specimen geometry or specimen features such as ridges or notches.

3.1.10.1 Discussion—A Type 2 extensometer is used where

the extensometer gauge length is determined by features on the specimen, for example, ridges, notches, or overall height (in case of compression test piece). The precision associated with gauge length setting for a Type 2 extensometer should be specified in relevant test method or product standard. The position readout on a testing machine is not recommended for use in a Type 2 extensometer system.

1 This practice 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 June 1, 2010. Published July 2010. Originally approved in 1950. Last previous edition approved in 2010 as E83 – 10. DOI: 10.1520/E0083- 10A.

2 For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards volume information, refer to the standard’s Document Summary page on the ASTM website.

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3.1.11 Type 3 extensometer system, n—an extensometer

system which intrinsically senses strain (ratiometric principle), for example, video camera system.

3.1.12 gauge length (L), n—the original length of that portion of the specimen over which strain or change of length is determined.

3.1.12.1 Discussion—If the device is used for sensing ex-

tension or motion, and gauge length is predetermined by the specimen geometry or specific test method, then only resolu-tion and strain error for a specified gauge length should determine the class of extensometer system.

3.1.13 resolution of the strain indicator—the smallest change in strain that can be estimated or ascertained on the strain indicating apparatus of the testing system, at any applied strain.

3.1.14 resolution of the digital type strain indicators (nu-meric displays, printouts, and so forth)—the resolution is the smallest change in strain that can be displayed on the strain indicator (may be a single digit or a combination of digits) at any applied strain.

3.1.14.1 Discussion—If the strain indication, for either type of strain indicator, fluctuates more than twice the resolution, as described in 3.1.13 or 3.1.14, the resolution expressed as a strain shall be equal to one-half the range of fluctuation.

3.1.15 verification—a determination that a system meets the requirements of a given classification after calibration accord-ing to established procedures.

3.1.16 verification apparatus—a device for verifying exten-someter systems.

3.1.16.1 Discussion—This device is used to simulate the

change in length experienced by a test specimen as a result of the applied force. The extensometer may either be attached directly to the mechanism or interfaced with it in a manner similar to normal operation (that is, possibly without contact for some optical extensometers).

4. Verification Apparatus

4.1 The apparatus for verifying extensometer systems shall

provide a means for applying controlled displacements to a simulated specimen and for measuring these displacements accurately. It may consist of a rigid frame, suitable coaxial spindles, or other fixtures to accommodate the extensometer being verified, a mechanism for moving one spindle or fixture axially with respect to the other, and a means for measuring accurately the change in length so produced,3 or any other device or mechanism that will accomplish the purpose equally well. The mechanism provided for moving one spindle relative to the other shall permit sensitive adjustments. The changes in length shall be measured, for example, by means of an interferometer, calibrated standard gauge blocks and an indicator, a calibrated micrometer screw, or a calibrated laser measurement system. If standard gauge blocks and an indicator, or a micrometer screw, are used, they shall be calibrated and their limits of accuracy and sensitivity stated.

The errors of the verification apparatus shall not exceed one third of the permissible error of the extensometer.

4.2 The verification apparatus shall be calibrated at intervals not to exceed two years.

NOTE 2—He-Ne laser interferometer measurement systems based on the 0.633 µm wavelength line are considered to be primary-based displacement standards and do not require recalibration.4

4.3 If the verification apparatus is to be used to verify

extensometers used for bidirectional tests, the errors of the verification apparatus should be measured in both directions of travel so as to include any backlash present.

5. Verification Procedure for Extensometer Systems

5.1 General Requirements—The verification of an exten-

someter system should not be done unless the components of the system are in good working condition. Thoroughly inspect all parts associated with smooth operation of the instrument to ensure there are no excessively worn components. Repair or replace parts as necessary. Remove any dirt particles which may have accumulated through normal use of the instrument.

Verification of the system shall be performed whenever parts are interchanged or replaced.

5.1.1 The verification of an extensometer system refers to a

specific extensometer used with a specific readout device.

Unless it can be demonstrated that autographic extensometers and recorders of a given type may be used interchangeably without introducing errors that would affect the classification of the extensometer, the extensometer shall be calibrated with the readout device with which it is to be used.

5.1.2 Prior to the initial verification, the extensometer

should be calibrated according to the manufacturer’s instruc-tions or established procedures. The calibration procedure may include adjustment of span or determination of calibration factor, or both.

5.2 Gauge Length Measurement Method—Measure the

gauge length of self-setting instruments by either the direct or indirect method.

NOTE 3—The following is an example of an indirect method. Set the extensometer to its starting position and mount it on a soft rod of the typical specimen size or diameter. After the extensometer is removed, measure the distance between the marks left by the gauge points (or knife edges). If there are four or more gauge points, take the average of the individual lengths as the gauge length. The differences between individual measurements shall not exceed the tolerance given for the class of extensometer. If there are two gauge points (or knife edges), but on opposite sides of the specimen, attach the extensometer twice rotating it 180° with respect to the rod. Take the average of the lengths thus established on each side of the rod as the gauge length.

5.2.1 Make two measurements of the gauge length. Deter-

mine and record the error from each measurement, which is the difference between the measured gauge length and the speci-fied gauge length, expressed as a percent of the specified gauge length.

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 Techniques for Extensometry: Possible Standards of Strain Measurement,” Journal of Testing and Evaluation, JTEVA, Vol. 21, No. 6, November 1993, pp. 515–521.

4 A letter from NIST (National Institute of Standards and Technology) has been filed at ASTM International Headquarters and may be obtained by requesting Research Report RR: E28-1013.

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5.2.2 For extensometer devices that do not have a self-

setting gauge length during use, such as deflectometers and some high-temperature tensile or creep extensometers, verifi-cation run errors should be calculated using the gauge length for which the device is used. Separate classifications should be established for each gauge length or range used.

5.2.3 Some extensometers have the capability to measure

the gauge length set by or chosen by the user. If this measurement is used in the calculation of strain, then it is the inherent measurement accuracy that is the important factor rather than the error between the chosen length and the actual.

NOTE 4—An example of an extensometer that is described by 5.2.3 is an optical extensometer that measures the position of “flags” attached to the test specimen. The flags are positioned at the approximate required gauge length and the instrument measures the position of the flags (the actual gauge length) before and after the specimen is stressed. Although this kind of device usually has a stated accuracy of gauge length, it must be verified by either direct or indirect methods at the appropriate gauge lengths.

5.3 Position of Extensometer—Carefully position the exten-

someter on or interface it to the verification device in the same manner as it is normally used for typical specimens. For extensometers that attach directly to the specimen, the verifi-cation device should allow attachment to pieces that are similar to the specimen on which the extensometer will be attached.

5.4 Temperature Control—Verify the extensometer at ap-

proximately the same temperature at which it will be used.

Allow sufficient time for the verification device and extensom-eter to reach satisfactory temperature stability. Maintain tem-perature stability by excluding drafts throughout the subse-quent verification. Record the temperature during each verification run.

NOTE 5—Extensometers used for high-temperature testing may be verified at ambient temperature to insure proper operation, but fixtures should be designed to verify performance at the actual test temperature.

This is especially true with optical extensometers which may be adversely affected by air density changes associated with thermal gradients and turbulence, environmental chamber windows, or specimen changes due to the environment. See Appendix X2.

5.5 Method of Reading—Read the instrument or, in the case

of an autographic extensometer, measure the record in the same manner as during use.

5.5.1 For extensometer with dial micrometers or digital

readouts, the readings shall be recorded. Extensometers that use autographic methods shall have their charts read and recorded using a suitable measuring device, such as a vernier or dial caliper. The use of an optical magnifying device is recommended when reading and measuring autographic re-cords.

NOTE 6—When autographic extensometer systems are used, care should be taken to minimize errors introduced by variances in the graph paper. These errors can be due to dimensional changes from reproduction or humidity changes. Direct measurement of the trace soon after it was made eliminates the graph paper errors and is desirable for systems verification.

NOTE 7—If an extensometer is equipped with a dial micrometer, it may be necessary to lightly tap the dial micrometer to minimize the effects of friction and to ensure that the most stable and reproducible readings are obtained. If the dial micrometer is tapped during the verification procedure, include this information in the report.

5.6 Zero Adjustment—After temperature stability has been

achieved, displace the verification device (with extensometer in the test position) to a slightly negative value and return to zero. If the reading does not return to zero, adjust and repeat the procedure until the reading does return to zero.

5.7 Number of Readings—For any strain range, verify the

extensometer system by applying at least five displacement values, not including zero, at least two times, with the difference between any two successive displacement applica-tions being no greater than one-third the difference between the selected maximum and minimum displacements.

5.7.1 Extensometers need not be verified beyond the range

over which they will be used. Multi-range (multi-magnification) extensometers shall be verified for each range to be used.

NOTE 8—If the connection between the gauge points attached to the specimen and the indicating device is made through geared wheels or micrometer screws, relatively large periodic errors may exist which might not be disclosed by this overall procedure. For such extensometers it may be necessary to take additional readings within one turn of any geared wheel, micrometer screw, or the travel of one tooth of any meshing gear.

5.7.2 When it is desired to establish the range of an

extensometer system designed to automatically select or extend ranges below 10 % of full scale without the influence of the operator, the number of readings shall depend on how many overlapping decades are in the range. Extensometer readings should be chosen starting with the minimum reading and are grouped in overlapping decades such that the maximum reading on one decade is the minimum on the next decade.

There are to be at least five strain applications per decade, unless the maximum, or the minimum strain on the range is reached before completing the decade. Strain (displacements) in each decade are to be approximately 1:1, 2:1, 4:1, 7:1, and 10:1, starting with the minimum strain in each decade.

5.7.2.1 In no case should the distance between two succes-

sive strains (displacements) within a decade differ by more than one-third the difference between the minimum and maxi-mum strains in that decade. Strains in the second successive run are to be approximately the same as those of the first run.

Report all percent values of accuracy, and report the indicator resolution at least once per decade.

5.7.3 Lower Limit Criteria—as indicated in Table 1, all

verified strain readings must have a resolution at least one-half the allowable error, that is, the resolution is a limiting factor to determine a lower limit of the range. The lowest verified strain reading must be at least 100 times the indicator resolution.

Extensometer results used below the lowest verified strain reading may not comply with the error limit specified by this standard practice.

NOTE 9—Example: For an extensometer with a gauge length of 1 in.

and 50 % strain, the full scale displacement value is 0.5 in. If the machine (system) resolution is 0.00005 in., which meets the criteria for the B1 class, the lower limit (verification range) would be 0.00005 in. x 100 =

0.005 in., or 0.5 % strain. The suitable verification points for a single range extensometer system would be in percent strain 0.5, 1.0, 2.0, 3.5, 5, 10, 20, 35, and 50. (See for single range system Fig. X1.1 and Fig. X1.2 for multirange.)

5.8 Number of Runs—Take at least two complete sets of

extensometer readings for the same changes of length. After

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5.8.1 If the initial verification run (the “as found” run)

produces satisfactory results which classify an extensometer system according to Table 1 specifications, then the data may be used as run—one of the two required for the verification report.

5.8.2 If the initial verification run produces results which are outside of expectations, for example, Class C instead of B1, and adjustments are necessary, then this first verification run might be reported “as found” data and used in accordance with applicable quality control programs. Calibration adjustments may then be made to the extensometer system after which two required verification runs shall be conducted and reported on the verification report and certificate.

5.8.3 The algebraic difference between errors of the two

verification runs shall not exceed the required Classification criteria listed in Table 1.

5.9 Direction of Verification Displacement:

5.9.1 Extensometers Used for Unidirectional Tests—

Extensometers used for unidirectional tests (for example, tension tests) shall be verified by applying displacement in the direction of testing normally used. If start-up backlash is evident, the verification device (with extensometer in place) may be displaced to a slightly negative value and returned to zero before each run.

NOTE 10—This verification procedure does not measure the initial backlash in the extensometer that may appear after it is first attached to the specimen. If the extensometer is used with open or closed loop-type test equipment in load control, the users should disregard readings taken during the initial part of the loading curve. If the extensometer is used with closed loop test equipment in strain control, the backlash could result in large tension or compression loads during the initial part of the loading curve.

5.9.2 Extensometers Used for Bidirectional Tests—

Extensometers used for bidirectional tests (for example, hys-teresis tests, fatigue tests, and so forth) (See Appendix X3) shall be verified by applying both increasing and decreasing values of displacement over the total range of intended use.

Displace the verification device (with extensometer in place) to a slightly negative value and return to zero before each run.

During each run, displace the extensometer to the maximum positive value, then to the maximum negative value, and then back to zero, stopping at each verification point along the way in each direction.

5.10 Determination of Errors—Calculate the error of the

extensometer system for each change in length of the verifi-cation apparatus. Errors are based on net values from the zero point to each successive verification point, not on increments between verification points.

6. Classification of Extensometer Systems

6.1 Classify extensometer systems in accordance with the

requirements as to maximum error of strain indicated by the extensometer system shown in Table 1. The maximum allow-able error in each class is the fixed error or the variable error, whichever is greater. The fixed error will establish the maxi-mum allowable error for readings near zero, but the variable error may establish the maximum allowable error for readings near full scale. Two examples of this procedure are presented in Appendix X1. In addition, the gauge length error for Type 1 extensometers shall not exceed the greater of the values shown in Table 1.

6.1.1 Type 2 extensometer systems shall be classified using

the smallest gauge length for which they are used. They may be verified at additional gauge lengths if desired.

6.1.2 Type 3 extensometer systems, operating over a range

of gauge lengths, shall be verified at the minimum and maximum gauge lengths used. They may be verified at additional gauge lengths if desired.

NOTE 11—For Type 3 systems, precision marked, divided test pieces may be used to establish known gauge lengths on the calibration device.

Known extensions enable the applied strains to be set. These applied strains are compared with the indicated strains from the Type 3 exten-someter systems, in order to establish its classification in accordance with the requirements for resolution and strain error in Table 1.

6.2 Separate classifications may be established for different ranges of multi-range (multiple-magnification) extensometer systems.

TABLE 1 Classification of Extensometer Systems

ClassificationA Relative Error of gauge Length (max %)

(See 5.2)

Resolution not to Exceed the Greater of: Error of StrainB not to Exceed the Greater of:

Fixed Value (in./in., m/m)

% of Reading Fixed Error (in./in., m/m)

Relative Error (% of strain)

Class A ±0.1 0.00001 0.05 ±0.00002 ±0.1 Class B-1 ±0.25 0.00005 0.25 ±0.0001 ±0.5 Class B-2 ±0.5 0.0001 0.25 ±0.0002 ±0.5 Class C ±1 0.0005 0.5 ±0.001 ±1 Class D ±1 0.005 0.5 ±0.01 ±1 Class E ±1 0.05 0.5 ±0.1 ±1

A Class A classification is very difficult to achieve at short (1 in. (25 mm) or less) gauge lengths, so the commercial availability of an extensometer system that meets this requirement may be very limited or nonexistent.

B The strain of an Extensometer System is the ratio of applied extension to the gauge length.

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7. Verification of Multiple Strain Readouts

7.1 When an extensometer is to be used with two or more

readout devices (for example, a graphic recorder and a digital readout), steps must be taken to assure that errors are not introduced by interactions (mechanical or electrical) between the readout devices or between the readouts and the extensometer, and that values from each readout device satisfy appropriate performance criteria. (Different accuracy classifi-cations could be given to the systems using different readout devices.) This can best be accomplished by verifying each system (extensometer and readout device) individually and also in combinations that would be used simultaneously. As an alternative, after individual verifications have been made, the combination can be checked at three points (about 20, 50, and 90 % of full scale range are recommended); and, if values for each system do not differ from the individual verification values by more than 20 % of the class tolerance, the combined system shall be considered to meet the same requirements as the individual systems. If readout devices are always used in combination, individual verifications are not required when the combined system is verified as a unit.

8. Verification of Data Acquisition Systems

8.1 Extensometer systems in which strain values are indi-

cated on displays or printouts of data acquisition systems, be they instantaneous, delayed, stored or retransmitted, which are verified in accordance with the provisions of Section 5 and classified in accordance with the provisions of Section 6, shall be deemed to comply with this practice.

9. Time Interval Between Verifications

9.1 It is recommended that extensometer systems be verified

annually unless more frequent verification is required to comply with product or customer specifications. In no case shall the time interval between verifications exceed 18 months unless an extensometer is being used on a long-time test running beyond the 18-month period. In such cases, the extensometer system shall be verified immediately after completion of the test. (See Note 12.)

9.1.1 An extensometer system shall not be used after an

adjustment or repair that could affect its accuracy without first verifying its accuracy utilizing the procedure described in this practice.

NOTE 12—If a test is expected to last more than 18 months, it is recommended that the extensometer system be verified immediately before as well as upon completion of the test.

10. Accuracy Assurance Between Verifications

10.1 Some product-testing procedures may require daily, weekly, or monthly spot checks to ascertain that an extensometer, recorder, or display, and so forth, or combina-tions thereof etc., are capable of producing accurate strain values between the verifications specified in Section 9. Spot checks may be performed on ranges of interest or at strain levels of interest utilizing a verification device that complies with Section 4 for the strain level(s) at which the spot checks are made.

10.2 Check the extensometer gauge length (see 5.1).

10.3 Make spot checks of extensometer readings at approxi-

mately 10 and 50 % of a range unless otherwise agreed upon or stipulated by the material supplier or user.

10.4 The extensometer gauge length and strain measure-

ment errors shall not exceed the allowable errors at the spot check points for the specified class of extensometer. Should errors be greater than allowable at any of the spot check points, the extensometer system is to be completely verified immedi-ately.

10.5 When spot checks are made, a clear, concise record

must be maintained as agreed upon between the supplier and the user. The record shall contain gauge length and spot check test data; the name, serial number, verification date, verification agency of the verification device(s) used to make spot checks;

the name of person making the spot check; and documentation of the regular verification data and schedule.

10.6 The extensometer system shall be considered verified

up to the date of the last successful spot check verification provided that the extensometer system is verified in accordance with Section 5 on a regular schedule in accordance with Section 9. Otherwise, spot checks are not valid.

11. Report

11.1 The report shall include the following:

11.1.1 Method of gauge length verification used.

11.1.2 Serial numbers and names of the manufacturers of all

apparatus used in verifying the extensometer system.

11.1.3 Serial number and name of the manufacturer of the

extensometer verified, or if it is an extensometer system composed of separable components, the serial number and manufacturer of each component of the systems verified.

11.1.4 gauge length of the extensometer. For variable gauge length extensometers, state the gauge lengths verified.

11.1.5 Temperature of the extensometer during verification.

11.1.6 Complete record of the readings of the extensometer

and of the verification apparatus.

11.1.7 Calibration factor, if applicable.

11.1.8 Error in gauge length for each measurement of gauge

length.

11.1.9 Error and relative error of the extensometer for each

extensometer reading, the maximum algebraic error difference (repeatability) and the associated resolution for each range (decade).

11.1.10 Class of the extensometer system. If separate clas-

sifications are established for various ranges, report the range (or magnification) and strain values associated with each classification.

11.1.11 If the classification applies to bidirectional testing, it shall be so stated. Otherwise, the classification shall be considered to be unidirectional in the direction of normal use (that is, opening for tension testing, closing for compression testing, and so forth).

11.1.12 The name of the person performing the classifica-

tion and the date it was performed.

11.2 Information to be available upon request shall include

the following:

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11.2.1 A statement indicating how, by whom, and when the

most recent calibration of the apparatus used in verifying the extensometer system was made.

11.2.2 A statement of the errors of the verification appara-

tus.

11.2.3 Position of the extensometer during verification.

11.2.4 Method of interfacing or attaching the extensometer

to the verification device.

APPENDIXES

(Nonmandatory Information)

X1. EXAMPLE OF PROCEDURE FOR VERIFICATION AND CLASSIFICATION OF EXTENSOMETERS

X1.1 An example of a verification report for a 25mm and 100 % extensometer used on a single range testing machine is given in Fig. X1.1.

X1.1.1 The first two columns represent actual (applied) strains through calibration apparatus.

X1.1.2 The next two columns represent the extensometer strain readings from the testing instrument (indicated strain).

X1.1.3 The last four columns represent errors in actual strain (in./in. or m/m) as a percent of reading.

X1.2 Fig. X1.3 and Fig. X1.4 the extensometer errors and specifications for 25mm, 100% estensometer, with the errors plotted from an actual verification.

X1.3 Fig. X1.3 is an example of a verification report for the same extensometer shown in Fig. X1.1 but used as a multi-range system (100 % and 10 % ranges).

X1.4 The data for a typical autographic extensometer are given in Fig. X1.4.

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FIG. X1.1 Extensometer Verification Report for a Single Range System

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FIG. X1.2 Extensometer Verification Report for a Multiple Range System

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NOTE 1—The verification reports shown in Fig. X1.1 and Fig. X1.2 have reported errors that may be mistakenly determined to be out of the B-1 classification indicated on the example reports. The allowable Relative error for a B-1 classification is +/-0.5%. The example reports list, +0.520% error for the RUN 1, at the +1.0% strain value. However this value has a Fixed error of 0.000052 mm/mm which is below the allowable 0.0001 mm/mm for the classification.

FIG. X1.3 Extensometer Errors and Specifications 25mm, 100% Extensometer (100% Range, Fig. X1.2)

FIG. X1.4 Extensometer Errors and Specifications 25mm, 100% Extensometer (10% Range, Fig. X1.2)

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X2. TEMPERATURE EFFECTS ON EXTENSOMETER CLASSIFICATION

X2.1 Using an extensometer on a test specimen at a tem-perature other than the temperature at which the verification was performed can cause errors in the strain reading. The source of these errors varies with the type of extensometer being used. It may be due to a shift in the null point of the device, a change in span, or an error in the gauge length. See Fig. X2.1.

X2.2 Some common typical sources of error are as follows:

X2.2.1 Clip-On Type Extensometers:

X2.2.1.1 Dimensional changes due to thermal effects giving rise to errors in zero, span, and gauge length, X2.2.1.2 Sensitivity changes as a function of temperature of the transducer being used (that is, strain gages, capacitive devices, LVDTs, etc.) giving span errors, X2.2.1.3 Shifts in the null point of the transducer being used as a function of temperature, X2.2.1.4 Sensitivity changes as a function of dielectric changes of the environment, and

X2.2.1.5 Sensitivity changes as a function of modulus change in extensometer arms or element.

X2.2.2 Non-Contact Type Extensometers:

X2.2.2.1 Refraction effects due to windows, temperature gradients, or turbulence can cause errors in some type exten-someters (for example, scanning beams measuring distance between flags), X2.2.2.2 Loss of transparency in windows can reduce accuracy, X2.2.2.3 Changes in speed of light, if not corrected, will cause errors in some laser techniques, X2.2.2.4 Changes in the surface of the specimen due to oxidation, frosting, corrosion, and so forth, will cause problems with some techniques, and

X2.2.2.5 Radiation emitted by high-temperature specimens or heaters may affect the performance of various optical extensometers.

X2.3 Since there are a wide variety of new extensometry instruments based on new technology, it is impossible to predict or correct, or both, for all possible sources of error when using these methods at high or low temperatures. The only prudent approach is to perform a Practice E83 type of verification within the environment to be used. It is recom-mended that the same environmental chamber (furnace or cryostat) be used for the verification that is being used for the material test. This will insure similar thermal and optical conditions exist and, hence, similar effects on the strain measurement should be observed.

X2.4 Examples of methods that could be employed for verification are as follows (also see Test Methods E21, para-graph 5.4.1).

X2.4.1 Environmental Effects on Span—The schematic be-low shows a clip-on extensometer in an environmental cham-ber mounted on a split specimen attached to a micrometer head. Although it may be difficult to establish effects on zero (null point) or gauge length accuracy, this setup can be used to measure any changes to the span of the device.

X2.4.2 Effects on the Null Point—A material of zero or known expansion coefficients can be used to measure the effects of temperature on the null or zero of the extensometer.

The device is either mounted on or focused toward (for optical non-contact extensometers) the “dummy” specimen. The known effects of temperature (in the absence of stress) on the material’s dimension can be compared against the measured effect from the extensometer to give a measurement of the change in zero.

X2.4.3 Effects on gauge Length Accuracy:

X2.4.3.1 This quantity is more difficult to determine since common methods (as described in this practice) for determin-ing accuracy of gauge length are difficult to apply at high or low temperatures.

X2.4.3.2 For example, if an extensometer is “clipped” on a specimen that was immersed in a cryogenic liquid, the accu-racy of the gauge length setting could be determined at room temperature by conventional methods. The actual low-temperature gauge length would be different since the material would change in length according to its coefficient of expan-sion characteristics. If these are known and the temperature is known, then the low-temperature gauge length can be calcu-lated.

X2.4.3.3 If the null and span characteristics of the exten-someter are known, then the change in output of the extensom-eter (attached to the specimen) as the temperature is loweredFIG. X2.1 Schematic of Test Set-Up

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X2.5 Another possible approach for non-contact type sys-tems is the use of reference specimens. Some of these extensometers measure the distance between “flags” on the specimen at zero load as the gauge length. The accuracy of the gauge length is dependent on the accuracy of the instrument and does not rely on precise placement of the flags. A possible method to verify this at high temperature is the use of a precision reference specimen with known gauge length (place-ment of marks or flags) and known thermal mechanical properties. This specimen is then placed in the environmental chamber, and the environmental and optical conditions to be used in the test can be reproduced. The gauge length can be calculated from the known properties of the reference speci-men and compared with the measurement.

X2.6 This appendix is not intended to be a detailed test procedure for verification of the classification of extensometers as a function of all environments. It should be used as a guide to make the user aware of possible errors in strain that result from environmental effects. Manufacturers of extensometers should provide performance characteristics of their instruments under typical operating test conditions. Although this informa-tion is seldom adequate to allow the user to ignore errors due to their specific test conditions, it can be used as a starting point for further analysis.

X3. FREQUENCY EFFECTS ON THE CLASSIFICATION OF EXTENSOMETERS

X3.1 The usable bandwidth of extensometers is a function of both mechanical and electrical characteristics. It is naive to perform a static verification as described in Practice E83 and run dynamic (cyclic) testing and assume that the classification remains unchanged. In general, both the amplitude and phase of the instrument may change as a function of frequency effects. Some examples follow.

X3.1.1 Mechanical Effects—If the specimen extensometer system approaches a resonance, there will be shifts in the phase between the input and the resulting strain as well as errors in the amplitude reading. The error in strain amplitude approach-ing a resonance will be positive, will peak at the resonance (for underdamped systems) and will become a negative error beyond the resonant point. This effect depends not only on the resonant frequency but the Q of the resonance. Extensometers that contain large mechanical elements may have inertial effects which at high frequencies cause forces on the attach-ment points to the specimen. These forces can lead to slipping of the extensometer which will give errors in strain.

X3.1.2 Electronic Effects—Most extensometer systems in-clude electronics which have bandwidths determined by the detail of the design. These should be specified by the manu-facturer. They result from the following characteristics:

X3.1.2.1 Response of the readout device. For example, if a chart recorder or XY plotter were being used, then the pen response might be the limiting factor; or if a digital readout device is used, they are often filtered to avoid digit instability.

This filter will have a roll-off frequency, which may be the limiting factor of the bandwidth.

X3.1.2.2 For digital instruments the digital sampling rate will affect the bandwidth. Aliasing can also be a factor in digital systems.

X3.1.2.3 On the opposite end of the spectrum long-term drift characteristics must be considered for extensometers to be used in long-term creep testing.

X3.2 Qualitative Test of Dynamic Performance:

X3.2.1 There are a number of methods that can be used to check the dynamic performance of an extensometer. These methods, if carefully implemented, would give quantitative information on the dynamic effects on the accuracy of the extensometer. This appendix is not intended to provide specific procedures for a dynamic calibration of an extensometer. An extensometer of known dynamic properties can be used in parallel with the device in question, and a comparison of the results can be made as a function of frequency. A strain-gaged specimen can be used as a reference, or a specimen with known properties could be used to verify the extensometer results.

X3.2.2 It is recommended, however, that as an absolute minimum the test illustrated in Fig. X3.1 be used to check the extensometer over the frequency range of interest. The test conditions (frequency and amplitude) should be applied to the extensometer attached to a single anvil as shown. Ideally, this test should result in zero output over the performance range of interest. Any output under these test conditions will likely cause deviations in the accuracy of the extensometer system as determined by the static verification described in Practice E83.

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X4. IDENTIFYING AND DETERMINING MEASUREMENT UNCERTAINTY COMPONENTS DURING AN EXTENSOMETER

VERIFICATION

X4.1 The measurement uncertainty determined using this appendix is the measurement uncertainty of the errors reported during verification of an extensometer system. It is not the measurement uncertainty of the extensometer or the measure-ment uncertainty of test results determined using the exten-someter. The uncertainty in the errors in displacement and the uncertainty of the error in gauge length, determined during an extensometer verification, can be determined using this appen-dix.

X4.2 Displacement Error Measurement Uncertainty— Under normal conditions, the measurement uncertainty of the reported errors in displacement of an extensometer system is a combination of three major components: the measurement uncertainty of the calibration laboratory performing the verification, the uncertainty component of the resolution of the extensometer system at the displacement the error is being determined, and the uncertainty component of the resolution of the extensometer system at zero displacement.

X4.3 The measurement uncertainty of the calibration labo-ratory performing the verification is a combination of factors such as, but not limited to:

X4.3.1 The measurement uncertainty of the laboratory’s displacement standards, X4.3.2 The measurement uncertainty of the gauge length measurement process, X4.3.3 Environmental effects such as temperature variations, X4.3.4 Drift in the displacement standard, X4.3.5 Measurement uncertainty of the verification of the displacement standard, X4.3.6 And repeatability of the displacement standard in actual use.

NOTE X4.1—A laboratory’s measurement uncertainty should be based on the maximum uncertainty of the displacement standards used and the worst environmental conditions allowed. It may be advantageous to evaluate the measurement uncertainty of the actual displacement standard used at the actual displacement for which the measurement uncertainty of the error of the extensometer is being determined.

NOTE X4.2—If there are circumstances in which verification is per-formed under conditions outside of the laboratory’s normal operating parameters, additional components may need to be considered. For example, a laboratory may permit a 5 °C temperature variation to occur during verification and has factored this into their measurement uncer-tainty. When greater temperature variations occur, the uncertainty due to this increased temperature variation should be included in the determina-tion of measurement uncertainty.

NOTE X4.3—A calibration laboratory’s measurement uncertainty is usually expressed as an expanded uncertainty using a coverage factor of two. If this is the case, prior to combining it with the other uncertainty components, divide it by two.

X4.4 The uncertainty component due to the resolution of the displacement or strain indicator being verified can be deter-mined by dividing the resolution of the displacement or strain indicator at the displacement or strain where uncertainty is being evaluated by the quantity of two times the square root of three.

X4.4.1 The uncertainty component due to the resolution of the displacement or strain indicator at zero can be determined by dividing the resolution of the displacement or strain indicator at zero by the quantity of two times the square root of three.

FIG. X3.1 Extensometer on a Single Moving Anvil

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X4.4.2 The three major components can be combined by squaring each component, adding them together, and then taking the square root of the sum to determine the combined measurement uncertainty of the error determined for the extensometer system.

X4.4.3 The expanded measurement uncertainty may then be determined by multiplying the combined uncertainty by two, for a confidence level of approximately 95%.

X4.4.4 The uncertainty in the errors of displacement may by expressed in strain units of mm/mm or inches/inch by multi-plying the expanded uncertainty by a sensitivity coefficient equal to:

aδl 5 l0

(X4.1) where:

aδ = the sensitivity coefficient for converting displacement uncertainties to strain uncertainties.

l0 = the nominal gauge length of the extensometer

X4.5 Gauge Length error Measurement Uncertainty:

X4.5.1 Under normal conditions, the measurement uncer-tainty of the reported errors in gauge length of an extensometer system is due to the measurement uncertainty of the calibration laboratory performing the verification and the environment in which the gauge length error is determined.

X4.5.2 The measurement uncertainty of the calibration laboratory performing the verification is a combination of factors such as, but not limited to:

X4.5.2.1 The measurement uncertainty of the gauge length measurement process, X4.5.2.2 Environmental effects such as temperature variations, X4.5.2.3 Drift in the gauge length standard, X4.5.2.4 Measurement uncertainty of the verification of the gauge length standard, X4.5.2.5 And repeatability of the gauge length standard in actual use.

NOTE X4.4—A laboratory’s measurement uncertainty should be based on the maximum uncertainty of the displacement standards used and the worst environmental conditions allowed.

NOTE X4.5—If there are circumstances in which verification is per-formed under conditions outside of the laboratory’s normal operating parameters, additional components may need to be considered. For example, a laboratory may permit a 5 °C temperature variation to occur during verification and has factored this into their measurement uncer-tainty. When greater temperature variations occur, the uncertainty due to this increased temperature variation should be included in the determina-tion of measurement uncertainty.

NOTE X4.6—A calibration laboratory’s measurement uncertainty is usually expressed as an expanded uncertainty using a coverage factor of two. If this is the case, since there is may be no need to combine it with other components, the laboratory’s measurement uncertainty can be used directly to describe the uncertainty of the reported error in gauge length.

X4.5.3 The expanded uncertainty in the gauge length may by expressed in strain units of mm/mm or inches/inch at a strain level by multiplying the expanded uncertainty by a sensitivity coefficient equal to:

agl 5 δ l0 2 (X4.2) where:

agl = the sensitivity coefficient for converting gauge…