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TitleD 6992 – 03
Tags Deformation (Engineering) Fracture Viscoelasticity Creep (Deformation)
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Designation: D 6992 ± 03

Standard Test Method for
Accelerated Tensile Creep and Creep-Rupture of
Geosynthetic Materials Based on Time-Temperature
Superposition Using the Stepped Isothermal Method 1

This standard is issued under the ®xed designation D 6992; 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.

1. Scope

1.1 This test method covers accelerated testing for tensile
creep, and tensile creep-rupture properties using the Stepped
Isothermal Method (SIM).

1.2 The test method is focused on geosynthetic reinforce-
ment materials such as yarns, ribs of geogrids, or narrow
geotextile specimens.

1.3 The SIM tests are laterally uncon®ned tests based on
time-temperature superposition procedures.

1.4 Tensile tests are to be completed before SIM tests and
the results are used to determine the stress levels for subse-
quent SIM tests de®ned in terms of the percentage of Ultimate
Tensile Strength (TULT). Additionally, the tensile test can be
designed to provide estimates of the initial elastic strain
distributions appropriate for the SIM results.

1.5 Ramp and Hold (R+H) tests may be completed in
conjunction with SIM tests. They are designed to provide
additional estimates of the initial elastic and initial rapid creep
strain levels appropriate for the SIM results.

1.6 Values stated in SI units are to be regarded as standard.
The common units given in parentheses are for information
only.

1.7 This method can be used to establish the sustained load
creep and creep-rupture characteristics of a geosynthetic.
Results of this method are to be used to augment results of Test
Method D 5262 and may not be used as the sole basis for
determination of long term creep and creep-rupture behavior of
geosynthetic material.

1.8 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

D 2990 Test Methods for Tensile, Compressive, and Flex-
ural Creep and Creep-Rupture of Plastics

D 4439 Terminology for Geosynthetics
D 4595 Test Method for Tensile Properties of Geotextiles

by the Wide-Width Strip Method
D 5262 Test Method for Evaluating the Uncon®ned Creep

Behavior of Geosynthetics

3. Terminology

3.1 For de®nitions related to geosynthetics see Terminology
D 4439.

3.2 For de®nitions related to creep see Test Methods D 2990
and D 5262.

3.3 De®nitions of Terms Speci®c to This Standard:
3.3.1 viscoelastic responseÐrefers to polymeric creep,

strain, stress relaxation or a combination thereof.
3.3.2 tensile creepÐtime-dependent deformation that oc-

curs when a specimen is subjected to a constant tensile load.
3.3.3 tensile creep-ruptureÐtime dependent rupture that

terminates a creep test at high stress levels.
3.3.4 time-temperature superpositionÐthe practice of shift-

ing viscoelastic response curves obtained at different tempera-
tures along a horizontal log time axis so as to achieve a master
curve covering an extended range of time.

3.3.5 shift factorÐthe displacement along the log time axis
by which a section of the creep or creep modulus curve is
moved to create the master curve at the reference temperature.
Shift factors are denoted by the symbol AT when the displace-
ments are generally to shorter times (attenuation) or the symbol
AT when the displacements are generally to longer times
(acceleration).

1 This test method is under the jurisdiction of ASTM Committee D35 on
Geosynthetics and is the direct responsibility of Subcommittee D35.02 on Endur-
ance Properties.

Current edition approved Dec. 1, 2003. Published January 2004.

2 For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [email protected] ForAnnual Book of ASTM
Standardsvolume information, refer to the standard's Document Summary page on
the ASTM website.

1

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

Page 3

8.5 Number of Tests:
8.5.1 A single specimen is usually sufficient to de®ne a

master creep or relaxation curve using the SIM. However, if
only a single SIM test is to be performed, the location of the
onset of creep strain or modulus curve should be con®rmed
using at least two short term creep (R+H) tests.

8.5.2 Generally 12 to 18 specimens are needed to de®ne a
stress-rupture curve representing multiple rupture times. Fewer
specimens would be needed to de®ne a speci®c region of the
curve, for example the percent TULT at 13 10

6 h (= 110 year)
rupture life.

9. Conditioning

9.1 Tensile and SIM testing shall be conducted using 206
1ÉC as the reference or temperature standard. If the laboratory
is not within this range, perform tensile tests in a suitable
environmental chamber capable of controlled cooling and
heating. The environmental chamber should have a program-
mable or set-point controller so as to maintain temperature to
20 6 1ÉC. When agreed to, a reference temperature other than
20ÉC can be utilized. Also, when agreed to, the results of
testing under this standard can be shifted from one reference
temperature to another.

9.2 Allow the specimen adequate time to come to tempera-
ture equilibrium in the laboratory or environmental chamber.
Generally this can be accomplished within a few hours (see
Note 3).

9.3 Record the relative humidity in the laboratory or envi-
ronmental chamber for all tests.

10. Selection of Test Conditions

10.1 The standard environment for testing is dry, since the
effect of elevated temperature is to reduce the humidity of
ambient air without special controls.

10.2 The standard reference temperature is 20ÉC unless
otherwise agreed to. The individual reference temperature for
each SIM test is the average achieved temperature of the ®rst
isothermal dwell.

10.3 Testing temperatures are to be within6 2ÉC of the
target test temperatures. It is critically important that the test
specimen has equilibrated throughout its thickness so as to
avoid nonisothermal conditions. Initial trials are necessary to
establish this minimum equilibrium time.

NOTE 3ÐLaboratory experience has suggested that the use of calibrated
thermocouples located near, affixed to or embedded within the test
specimen may facilitate a successful temperature compliance test for the
specimen material. It is suggested that the laboratory perform the planned
SIM temperature steps using an unloaded sacri®cial test specimen and,
with the use of these thermocouples, measure the temperature change of
the specimen at its thickest or most mass-dense region. The time required
for the specimen to reach the target temperature is recorded and used as
the minimum dwell time. The upper limit of the temperature ramp time is
not known. Successful tests with some materials have been run with
temperature ramp times of up to four minutes.

10.4 Test temperatures are to be maintained within6 1.0ÉC
of the mean achieved temperature.

10.4.1 Temperature steps and dwell times must be such that
the steady state creep rate at the beginning of a new step is not

so different from that of the previous that it cannot be
established within the identi®ed ramp time.

11. Procedures

11.1 The same or similar load or strain control shall be
applied to the tensile tests and the load ramp portion of R+H
and SIM (creep and creep-rupture) tests. The load rate control
(in units of kN per min) that is applied shall achieve a narrow
range of strain rates expressed in percent per minute, as agreed
upon. Generally 106 3 % per minute (or 206 3 % per minute
for European practice) will be satisfactory.

NOTE 4ÐA linear ramp of load versus time will not generally result in
a linear strain versus time relationship because stress versus strain curves
are not linear for most geosynthetic materials.

11.2 Achieve the test loads for R+H and SIM tests within6
2 % of the target loads, and maintain any achieved load within
6 0.5 % of its values for the duration of the test. A brief
overshoot of the target load that is within6 2 % of the target
load and limited to a 1 to 2second time duration is acceptable
for load control systems.

11.3 Replicate test loads for R+H and SIM tests should be
within 6 0.5 % of the average of the achieved loads for a test
set.

11.4 Pretensioning up in accordance with the governing
tensile test is acceptable. The method used to de®ne zero strain
is to be identi®ed and reported.

11.5 The same or similar grips shall be used for tensile,
R+H and SIM tests. Care should be taken to use grips that do
not initiate failure or incur slippage at stress levels which may
produce specimen rupture (for example, at loads greater than
55 % of TULT for polyester).

11.6 Inspect grips to insure loading surfaces are clean and
that padding, if used, is free of defects and is secured properly.

11.7 Inspect the specimen installation to be sure the material
is properly aligned with the grips and with the loading axis.

11.8 Insure that the load cell used is calibrated properly such
that it will accurately measure the range of tensile loads
anticipated.

11.9 Insure that the extensometer used (if any) is calibrated
properly such that it will accurately measure the range of
tensile strains anticipated. If rupture is anticipated, take pre-
cautions to insure that the rupture event will not damage the
extensometer or create a hazard for the machine operator.

11.10 Unless otherwise agreed upon, a 100 mm gage length
shall be used for geosynthetic products and a 250 to 300 mm
gage length shall be used for precursor yam products.

11.11 Time, load and extension data shall be collected at a
minimum rate of two readings per second during the initial
loading ramp portions of tests and a minimum rate of two
readings per minute during constant load portions of tests. If
load is applied by means of dead weights, with or without a
lever, regular measurement of load after the ramp is not
necessary.

11.12 The environmental chamber and temperature cooler
shall be capable of maintaining the specimen temperature
within 6 1ÉC in range of 0 to 100ÉC, and of changing the
specimen temperature by up to 15ÉC, within the identi®ed
ramp time (see Note 3).

D 6992 ± 03

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APPENDIX

(Nonmandatory Information)

X1.

X1.1 The following table and graphs are typical of those
used in the report section of the SIM test procedure. Figs.

X1.1-X1.8 show the results for a polyester yam before and
after scaling and shifting.

FIG. X1.1 Stress and Creep Strain versus Linear Time

D 6992 ± 03

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FIG. X1.2 Stress and Secant Modulus versus Strain

FIG. X1.3 Stress and Creep Modulus versus Linear Time

D 6992 ± 03

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FIG. X1.4 Creep Modulus versus Log Time After Rescaling

FIG. X1.5 Master Creep Modulus versus Log Time Curve at the Step One Reference Temperature

D 6992 ± 03

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FIG. X1.6 Master Creep Strain versus Log Time at the Step One Reference Temperature

FIG. X1.7 Figure X.6 with Re-scaled Y-axis

D 6992 – 03

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