ASTM D6992-16(2023)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D6992 − 16 (Reapproved 2023)
Standard Test Method for
Accelerated Tensile Creep and Creep-Rupture of
Geosynthetic Materials Based on Time-Temperature
Superposition Using the Stepped Isothermal Method
This standard is issued under the fixed designation D6992; 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 priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
1.1 This test method covers accelerated testing for tensile
1.9 This international standard was developed in accor-
creep, and tensile creep-rupture properties using the Stepped
dance with internationally recognized principles on standard-
Isothermal Method (SIM).
ization established in the Decision on Principles for the
1.2 The test method is focused on geosynthetic reinforce-
Development of International Standards, Guides and Recom-
ment materials such as yarns, ribs of geogrids, or narrow
mendations issued by the World Trade Organization Technical
geotextile specimens.
Barriers to Trade (TBT) Committee.
1.3 The SIM tests are laterally unconfined tests based on
2. Referenced Documents
time-temperature superposition procedures.
2.1 ASTM Standards:
1.4 Tensile tests are to be completed before SIM tests and
D2990 Test Methods for Tensile, Compressive, and Flexural
the results are used to determine the stress levels for subse-
Creep and Creep-Rupture of Plastics
quent SIM tests defined in terms of the percentage of Ultimate
D4439 Terminology for Geosynthetics
Tensile Strength (T ). Additionally, the tensile test can be
ULT
D4595 Test Method for Tensile Properties of Geotextiles by
designed to provide estimates of the initial elastic strain
the Wide-Width Method
distributions appropriate for the SIM results.
D5262 Test Method for Determining the Unconfined Ten-
1.5 Ramp and Hold (R+H) tests may be completed in
sion Creep and Creep Rupture Behavior of Planar Geo-
conjunction with SIM tests. They are designed to provide
synthetics Used for Reinforcement Purposes
additional estimates of the initial elastic and initial rapid creep
3. Terminology
strain levels appropriate for the SIM results.
3.1 For definitions related to geosynthetics, see Terminol-
1.6 This method can be used to establish the sustained load
ogy D4439.
creep and creep-rupture characteristics of a geosynthetic.
Results of this method are to be used to augment results of Test
3.2 For definitions related to creep, see Test Methods D2990
Method D5262 and may not be used as the sole basis for
and D5262.
determination of long-term creep and creep-rupture behavior of
3.3 Definitions of Terms Specific to This Standard:
geosynthetic material.
3.3.1 creep modulus—in SIM analysis, the load divided by
1.7 The values stated in SI units are to be regarded as
the percent strain at any given point in time.
standard. No other units of measurement are included in this
3.3.2 dwell time—time during which conditions (particular
standard.
load) are held constant between temperature steps.
1.8 This standard does not purport to address all of the
3.3.3 mean test temperature—the arithmetic average of all
safety concerns, if any, associated with its use. It is the
temperature readings of the atmosphere surrounding the test
responsibility of the user of this standard to establish appro-
specimen for a particular temperature step, starting at a time
not later than established temperature ramp time, and finishing
at a time just prior to the subsequent temperature reset.
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. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Sept. 1, 2023. Published September 2023. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 2003. Last previous edition approved in 2016 as D6992 – 16. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/D6992-16R23. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6992 − 16 (2023)
3.3.4 offset modulus method or pointing—data analysis 5.2 The statements set forth in 1.6 are very important in the
method used to normalize any prestrain in the samples by context of significance and use, as well as scope of the
shifting the origin of a stress-versus-strain curve to an axis standard.
origin of coordinates; that is, to coordinates (0,0).
5.3 Creep test data are used to calculate the creep modulus
3.3.5 ramp and hold (R+H) test—a creep test of very short
of materials as a function of time. These data are then used to
duration; for example, 100 to 1000 s.
predict the long-term creep deformation expected of geosyn-
thetics used in reinforcement applications.
3.3.6 shift factor—the displacement along the log time axis
NOTE 1—Currently, SIM testing has focused mainly on woven and
by which a section of the creep or creep modulus curve is
knitted geogrids and woven geotextiles made from polyester, aramid,
moved to create the master curve at the reference temperature.
polyaramid, poly-vinyl alcohol (PVA), and polypropylene yarns and
Shift factors are denoted by the symbol AT when the displace-
narrow strips. Additional correlation studies on other materials are needed.
ments are generally to shorter times (attenuation) or the symbol
5.4 Creep-rupture test data are used to develop a regression
AT when the displacements are generally to longer times
line relating creep stress to rupture time. These results predict
(acceleration).
the long-term rupture strength expected for geosynthetics in
3.3.7 stepped isothermal method (SIM)—a method of expo-
reinforcement applications.
sure that uses temperature steps and dwell times to accelerate
5.5 Tensile testing is used to establish the ultimate tensile
creep response of a material being tested under load.
strength (T ) of a material and to determine elastic stress,
ULT
3.3.8 tensile creep—time-dependent deformation that oc-
strain, and variations thereof for SIM tests.
curs when a specimen is subjected to a constant tensile load.
5.6 Ramp and Hold (R+H) testing is done to establish the
3.3.9 tensile creep rupture—time dependent rupture that
range of creep strains experienced in the brief period of very
terminates a creep test at high stress levels.
rapid response following the peak of the load ramp.
3.3.10 time-temperature superposition—the practice of
shifting viscoelastic response curves obtained at different
6. Apparatus
temperatures along a horizontal log time axis so as to achieve
6.1 Grips—Grips for SIM and R+H tests should be the same
a master curve covering an extended range of time.
as the grips for ultimate strength tensile tests. Neither slippage
3.3.11 ultimate tensile strength (T )—short-term strength
ULT
nor excessive stress causing premature rupture should be
value used to normalize creep rupture strengths.
allowed to occur.
3.3.12 viscoelastic response—refers to polymeric creep,
6.2 Testing Machine—A universal testing machine or a
strain, stress relaxation, or a combination thereof.
dead-weight loading system with the following capabilities and
4. Summary of Test Method accessories shall be used for testing.
6.2.1 Load measurement and control,
4.1 SIM—A procedure whereby specified temperature steps
6.2.2 Strain measurement and control,
and dwell times are used to accelerate viscoelastic creep
6.2.3 Time measurement,
characteristics during which strain and load are monitored as a
6.2.4 Environmental temperature chamber to facilitate con-
function of time.
trol of test conditions,
4.1.1 Tensile Creep—Constant tensile load in conjunction
6.2.4.1 Temperature measurement and control facilities,
with specified temperature steps and dwell times are used to
6.2.5 Other environmental measurement and control, and
accelerate creep strain response.
6.2.6 Computer data acquisition and control.
4.1.2 Tensile Creep-Rupture—A tensile creep test where
high stress levels are used during testing to ensure rupture,
7. Sampling
while specified temperature steps and dwell times are used to
7.1 The specimens used for tensile, R+H, and SIM tests
accelerate creep strain response characteristics. Strain is moni-
should all be taken from the same sample.
tored as a function of time.
7.2 Remove sufficient test specimens for tensile testing in
4.2 Tensile Tests—Test specimens are rapidly loaded over a
accordance with the selected tensile testing procedure (see
short period to achieve rupture. The selection of a suitable
Section 8).
tensile test is dependent upon the type of material tested (see
Section 8). Tensile tests to support creep and creep-rupture
7.3 Remove one test specimen from the sample for each
tests are performed under the same control of loading or strain
SIM test.
rate as used to load or strain the test specimens during creep or
7.4 Remove one test specimen from the sample for each
creep rupture tests.
R+H test.
4.3 R+H—Test specimens are ramp loaded at a predeter-
mined loading rate to a predetermined load and held under
8. Test Specimens
constant load (short-term creep test).
8.1 Geogrid specimens should be single ribs, unless other-
wise agreed upon.
5. Significance and Use
5.1 Use of the Stepped Isothermal Method decreases the 8.2 Yarn specimens of geogrids or geotextiles should be
time required for creep to occur and the obtaining of the single ply or multiple ply strands, unless otherwise agreed
associated data. upon.
D6992 − 16 (2023)
SIM temperature steps using an unloaded sacrificial test specimen and,
8.3 Geotextile specimens should be 50 mm wide strips,
with the use of these thermocouples, measure the temperature change of
unless otherwise agreed upon.
the specimen at its thickest or most mass-dense region. The time required
NOTE 2—Single geogrid ribs and narrow strip specimens are preferred
for the specimen to reach the target temperature is recorded and used as
to determine the effect of applied load on the tensile creep properties of the
the minimum dwell time. The upper limit of the temperature ramp time is
material separate from the effect of sample width on the tensile properties
not known. Successful tests with some materials have been run with
of the material. However, correlation between narrow geotextile strips or
temperature ramp times of up to 4 min.
single geogrid ribs to wider representative specimens should be estab-
lished.
10.4 Test temperatures are to be maintained within 61.0 °C
of the mean achieved temperature.
8.4 The length of the test specimen is determined by the
type of grip used. Refer to specific tensile test procedure for 10.4.1 Temperature steps and dwell times must be such that
guidance. 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
8.5 Number of Tests:
established within the identified ramp time.
8.5.1 A single specimen is usually sufficient to define a
master creep or relaxation curve using the SIM. However, if
11. Procedures
only a single SIM test is to be performed, the location of the
onset of creep strain or modulus curve should be confirmed
11.1 The same or similar load or strain control shall be
using at least two short-term creep (R+H) tests.
applied to the tensile tests and the load ramp portion of R+H
8.5.2 Generally, twelve to 18 specimens are needed to
and SIM (creep and creep-rupture) tests. The load rate control
define a stress-rupture curve representing multiple rupture
(in units of kN per min) that is applied shall achieve a narrow
times. Fewer specimens would be needed to define a specific
range of strain rates expressed in percent per minute, as agreed
region of the curve, for example the percent T at 1 × 10 h
ULT upon. Generally, 10 6 3 % per min (or 20 6 3 % per min for
(= 110 year) rupture life.
European practice) will be satisfactory.
NOTE 4—A linear ramp of load versus time will not generally result in
9. Conditioning
a linear strain-versus-time relationship because stress-versus-strain curves
are not linear for most geosynthetic materials.
9.1 Tensile and SIM testing shall be conducted using 20 6
1 °C as the reference or temperature standard. If the laboratory 11.2 Achieve the test loads for R+H and SIM tests within
is not within this range, perform tensile tests in a suitable 62 % of the target loads, and maintain any achieved load
environmental chamber capable of controlled cooling and within 60.5 % of its values for the duration of the test. A brief
heating. The environmental chamber should have a program- overshoot of the target load that is within 62 % of the target
mable or set-point controller so as to maintain temperature to load and limited to a 1 to 2 s time duration is acceptable for
20 6 1 °C. When agreed to, a reference temperature other than load control systems.
20 °C can be utilized. Also, when agreed to, the results of
11.3 Replicate test loads for R+H and SIM tests should be
testing under this standard can be shifted from one reference
within 60.5 % of the average of the achieved loads for a test
temperature to another.
set.
9.2 Allow the specimen adequate time to come to tempera-
11.4 Pretensioning up in accordance with the governing
ture equilibrium in the laboratory or environmental chamber.
tensile test is acceptable. The method used to define zero strain
Generally this can be accomplished within a few hours (see
is to be identified and reported.
Note 3).
11.5 The same or similar grips shall be used for tensile,
9.3 Record the relative humidity in the laboratory or envi-
R+H, and SIM tests. Care should be taken to use grips that do
ronmental chamber for all tests.
not initiate failure or incur slippage at stress levels which may
produce specimen rupture (for example, at loads greater than
10. Selection of Test Conditions
55 % of T for polyester).
ULT
10.1 The standard environment for testing is dry, since the
11.6 Inspect grips to ensure loading surfaces are clean and
effect of elevated temperature is to reduce the humidity of
that padding, if used, is free of defects and is secured properly.
ambient air without special controls.
10.2 The standard reference temperature is 20 °C unle
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