ASTM D6992-03(2009)
(Test Method)Standard Test Method for Accelerated Tensile Creep and Creep-Rupture of Geosynthetic Materials Based on Time-Temperature Superposition Using the Stepped Isothermal Method
Standard Test Method for Accelerated Tensile Creep and Creep-Rupture of Geosynthetic Materials Based on Time-Temperature Superposition Using the Stepped Isothermal Method
SIGNIFICANCE AND USE
Use of the Stepped Isothermal Method decreases the time required for creep to occur and the obtaining of the associated data.
The statements set forth in 1.6 are very important in the context of significance and use, as well as scope of the standard.
Creep test data are used to calculate the creep modulus of materials as a function of time. These data are then used to predict the long-term creep deformation expected of geosynthetics used in reinforcement applications.
Note 1—Currently, SIM testing has focused mainly on woven and knitted geogrids and woven geotextiles made from polyester, aramid, polyaramid, poly-vinyl alcohol (PVA) and polypropylene yarns and narrow strips. Additional correlation studies on other materials are needed.
Creep rupture test data are used to develop a regression line relating creep stress to rupture time. These results predict the long term rupture strength expected for geosynthetics in reinforcement applications.
Tensile testing is used to establish the ultimate tensile strength (TULT) of a material and to determine elastic stress, strain and variations thereof for SIM tests.
Ramp and Hold (R+H) testing is done to establish the range of creep strains experienced in the brief period of very rapid response following the peak of the load ramp.
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 reinforcement materials such as yarns, ribs of geogrids, or narrow geotextile specimens.
1.3 The SIM tests are laterally unconfined 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 subsequent SIM tests defined 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 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.
1.7 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
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 appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D6992 − 03(Reapproved 2009)
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.Anumber 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 and health practices and determine the applica-
bility of regulatory limitations prior to use.
1.1 This test method covers accelerated testing for tensile
creep, and tensile creep-rupture properties using the Stepped
2. Referenced Documents
Isothermal Method (SIM).
2.1 ASTM Standards:
1.2 The test method is focused on geosynthetic reinforce-
D2990Test Methods forTensile, Compressive, and Flexural
ment materials such as yarns, ribs of geogrids, or narrow
Creep and Creep-Rupture of Plastics
geotextile specimens.
D4439Terminology for Geosynthetics
1.3 The SIM tests are laterally unconfined tests based on
D4595Test Method for Tensile Properties of Geotextiles by
time-temperature superposition procedures.
the Wide-Width Strip Method
D5262Test Method for Evaluating the Unconfined Tension
1.4 Tensile tests are to be completed before SIM tests and
Creep and Creep Rupture Behavior of Geosynthetics
the results are used to determine the stress levels for subse-
quent SIM tests defined in terms of the percentage of Ultimate
3. Terminology
Tensile Strength (T ). Additionally, the tensile test can be
ULT
designed to provide estimates of the initial elastic strain
3.1 FordefinitionsrelatedtogeosyntheticsseeTerminology
distributions appropriate for the SIM results.
D4439.
1.5 Ramp and Hold (R+H) tests may be completed in
3.2 FordefinitionsrelatedtocreepseeTestMethodsD2990
conjunction with SIM tests. They are designed to provide
and D5262.
additional estimates of the initial elastic and initial rapid creep
3.3 Definitions of Terms Specific to This Standard:
strain levels appropriate for the SIM results.
3.3.1 viscoelastic response—refers to polymeric creep,
1.6 This method can be used to establish the sustained load
strain, stress relaxation or a combination thereof.
creep and creep-rupture characteristics of a geosynthetic.
ResultsofthismethodaretobeusedtoaugmentresultsofTest 3.3.2 tensile creep—time-dependent deformation that oc-
Method D5262 and may not be used as the sole basis for curs when a specimen is subjected to a constant tensile load.
determinationoflongtermcreepandcreep-rupturebehaviorof
3.3.3 tensile creep-rupture—time dependent rupture that
geosynthetic material.
terminates a creep test at high stress levels.
1.7 The values stated in SI units are to be regarded as
3.3.4 time-temperature superposition—the practice of shift-
standard. No other units of measurement are included in this
ing viscoelastic response curves obtained at different tempera-
standard.
tures along a horizontal log time axis so as to achieve a master
1.8 This standard does not purport to address all of the
curve covering an extended range of time.
safety concerns, if any, associated with its use. It is the
3.3.5 shift factor—the displacement along the log time axis
responsibility of the user of this standard to establish appro-
by which a section of the creep or creep modulus curve is
moved to create the master curve at the reference temperature.
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
CurrenteditionapprovedJune1,2009.PublishedJuly2009.Originallyapproved contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
in 2003. Last previous edition approved in 2003 as D6992 – 03. DOI: 10.1520/ Standards volume information, refer to the standard’s Document Summary page on
D6992-03R09. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6992 − 03 (2009)
Shift factors are denoted by the symbolAT when the displace- 5.3 Creep test data are used to calculate the creep modulus
mentsaregenerallytoshortertimes(attenuation)orthesymbol of materials as a function of time. These data are then used to
AT when the displacements are generally to longer times predict the long-term creep deformation expected of geosyn-
(acceleration). thetics used in reinforcement applications.
NOTE 1—Currently, SIM testing has focused mainly on woven and
3.3.6 stepped isothermal method (SIM)—a method of expo-
knitted geogrids and woven geotextiles made from polyester, aramid,
sure that uses temperature steps and dwell times to accelerate
polyaramid, poly-vinyl alcohol (PVA) and polypropylene yarns and
creep response of a material being tested under load.
narrowstrips.Additionalcorrelationstudiesonothermaterialsareneeded.
3.3.7 mean test temperature—the arithmetic average of all
5.4 Creep rupture test data are used to develop a regression
temperature readings of the atmosphere surrounding the test
line relating creep stress to rupture time. These results predict
specimen for a particular temperature step, starting at a time
the long term rupture strength expected for geosynthetics in
not later than established temperature ramp time, and finishing
reinforcement applications.
at a time just prior to the subsequent temperature reset.
5.5 Tensile testing is used to establish the ultimate tensile
3.3.8 ultimate tensile strength (T )—short term strength
ULT
strength (T ) of a material and to determine elastic stress,
ULT
value used to normalize creep rupture strengths.
strain and variations thereof for SIM tests.
3.3.9 offset modulus method or pointing—data analysis
5.6 Ramp and Hold (R+H) testing is done to establish the
method used to normalize any prestrain in the samples by
range of creep strains experienced in the brief period of very
shifting the origin of a stress versus strain curve to an axis
rapid response following the peak of the load ramp.
origin of coordinates; that is, to coordinates (0,0).
6. Apparatus
3.3.10 ramp and hold (R+H) test—acreeptestofveryshort
duration; for example, 100 to 1000 s.
6.1 Grips—GripsforSIMandR+Htestsshouldbethesame
as the grips for ultimate strength tensile tests. Neither slippage
3.3.11 dwell time—time during which conditions (particular
load) are held constant between temperature steps. nor excessive stress causing premature rupture should be
allowed to occur.
3.3.12 creep modulus—in SIM analysis, the load divided by
the percent strain at any given point in time.
6.2 Testing Machine—A universal testing machine or a
dead-weightloadingsystemwiththefollowingcapabilitiesand
4. Summary of Test Method
accessories shall be used for testing.
4.1 SIM—Aprocedure whereby specified temperature steps 6.2.1 Load measurement and control,
and dwell times are used to accelerate viscoelastic creep
6.2.2 Strain measurement and control,
characteristics during which strain and load are monitored as a
6.2.3 Time measurement,
function of time. 6.2.4 Environmental temperature chamber to facilitate con-
4.1.1 Tensile Creep—Constant tensile load in conjunction
trol of test conditions,
with specified temperature steps and dwell times are used to 6.2.4.1 Temperature measurement and control facilities,
accelerate creep strain response.
6.2.5 Other environmental measurement and control, and
4.1.2 Tensile Creep-Rupture—A tensile creep test where 6.2.6 Computer data acquisition and control.
high stress levels are used during testing to ensure rupture,
7. Sampling
while specified temperature steps and dwell times are used to
accelerate creep strain response characteristics. Strain is moni-
7.1 The specimens used for tensile, R+H and SIM tests
tored as a function of time.
should all be taken from the same sample.
4.2 Tensile Tests—Test specimens are rapidly loaded over a
7.2 Remove sufficient test specimens for tensile testing in
short period to achieve rupture. The selection of a suitable
accordance with the selected tensile testing procedure (see
tensile test is dependent upon the type of material tested (see
Section 8).
Section 8). Tensile tests to support creep and creep-rupture
7.3 Remove one (1) test specimen from the sample for each
tests are performed under the same control of loading or strain
SIM test.
rateasusedtoloadorstrainthetestspecimensduringcreepor
7.4 Remove one (1) 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-
5. Significance and Use
wise agreed upon.
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.
5.2 The statements set forth in 1.6 are very important in the
8.3 Geotextile specimens should be 50 mm wide strips,
context of significance and use, as well as scope of the
unless otherwise agreed upon.
standard. NOTE 2—Single geogrid ribs and narrow strip specimens are preferred
D6992 − 03 (2009)
todeterminetheeffectofappliedloadonthetensilecreeppropertiesofthe for the specimen to reach the target temperature is recorded and used as
material separate from the effect of sample width on the tensile properties the minimum dwell time.The upper limit of the temperature ramp time is
of the material. However, correlation between narrow geotextile strips or not known. Successful tests with some materials have been run with
single geogrid ribs to wider representative specimens should be estab- temperature ramp times of up to four minutes.
lished.
10.4 Testtemperaturesaretobemaintainedwithin 61.0°C
8.4 The length of the test specimen is determined by the
of the mean achieved temperature.
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 12 to 18 specimens are needed to define a
and SIM (creep and creep-rupture) tests. The load rate control
stress-rupturecurverepresentingmultiplerupturetimes.Fewer
(in units of kN per min) that is applied shall achieve a narrow
specimens would be needed to define a specific region of the
rangeofstrainratesexpressedinpercentperminute,asagreed
curve, for example the percent T at1×10 h (= 110 year)
upon.Generally10 63%perminute(or20 63%perminute
ULT
rupture life.
for European practice) will be satisfactory.
NOTE 4—Alinear ramp of load versus time will not generally result in
a linear strain versus time relationship because stress versus strain curves
9. Conditioning
are not linear for most geosynthetic materials.
9.1 Tensile and SIM testing shall be conducted using 20 6
11.2 AchievethetestloadsforR+HandSIMtestswithin 6
1°C as the reference or temperature standard. If the laboratory
2% of the target loads, and maintain any achieved load within
is not within this range, perform tensile tests in a suitable
6 0.5% of its values for the duration of the test. A brief
environmental chamber capable of controlled cooling and
overshoot of the target load that is within 6 2% of the target
heating. The environmental chamber should have a program-
load and limited toa1to2 second time duration is acceptable
mable or set-point controller so as to maintain temperature to
for load control systems.
20 6 1°C.When agreed to, a reference temperature other than
11.3 Replicate test loads for R+H and SIM tests should be
20°C can be utilized. Also, when agreed to, the results of
within 6 0.5% of the average of the achieved loads for a test
testing under this standard can be shifted from one reference
set.
temperature to another.
11.4 Pretensioning up in accordance with the governing
9.2 Allow the specimen adequate time to come to tempera-
tensiletestisacceptable.Themethodusedtodefinezerostrain
ture equilibrium in the laboratory or environmental chamber.
is to be identified and reported.
Generally this can be accomplished within a few hours (see
Note 3).
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
9.3 Record the relative humidity in the laboratory or envi-
not initiate failure or incur slippage at stress levels which may
ronmental chamber for all tests.
produce specimen rupture (for example, at loads greater than
55% of T for polyester).
10. Selection of Test Conditions ULT
11.6 Inspect grips to insure loading surfaces are clean and
10.1 The standard environment for testing is dry, since the
that padding, if used, is free of defects and is secured properly.
effect of elevated temperature is to reduce the humidity of
ambient air without special controls.
11.7 Inspectthespecimeninstallationtobesurethematerial
is properly aligned with the grips and with the loading axis.
10.2 The standard reference temperature is 20°C unless
otherwise agreed to. The individual reference temperature for
11.8 Insurethattheloadcellusediscalibratedproperlysuch
each SIM test is the average achieved temperature of the first
that it will accurately measure the range of tensile loads
isothermal dwell.
anticipated.
10.3 Testing temperatures are to be within 6 2°C of the
11.9 Insure that the extensometer used (if any) is calibrated
target test temperatures. It is critically important that the test
properly such that it will accurately measure the range of
specimen has equilibrated throughout its thickness so as to
tensile strains anticipated. If rupture is anticipated, take pre-
avoid nonisothermal conditions. Initial trials are necessary to
cautions to insure that the rupture event will not damage the
establish this minimum equilibrium time.
extenso
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