ASTM D6992-03
(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 1.7 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 (T ULT). 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 appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:D6992–03
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 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 2. Referenced Documents
1.1 This test method covers accelerated testing for tensile 2.1 ASTM Standards:
creep, and tensile creep-rupture properties using the Stepped D 2990 Test Methods for Tensile, Compressive, and Flex-
Isothermal Method (SIM). ural Creep and Creep-Rupture of Plastics
1.2 The test method is focused on geosynthetic reinforce- D 4439 Terminology for Geosynthetics
ment materials such as yarns, ribs of geogrids, or narrow D 4595 Test Method for Tensile Properties of Geotextiles
geotextile specimens. by the Wide-Width Strip Method
1.3 The SIM tests are laterally unconfined tests based on D 5262 Test Method for Evaluating the Unconfined Creep
time-temperature superposition procedures. Behavior of Geosynthetics
1.4 Tensile tests are to be completed before SIM tests and
3. Terminology
the results are used to determine the stress levels for subse-
quent SIM tests defined in terms of the percentage of Ultimate 3.1 For definitions related to geosynthetics seeTerminology
D 4439.
Tensile Strength (T ). Additionally, the tensile test can be
ULT
designed to provide estimates of the initial elastic strain 3.2 FordefinitionsrelatedtocreepseeTestMethodsD 2990
and D 5262.
distributions appropriate for the SIM results.
1.5 Ramp and Hold (R+H) tests may be completed in 3.3 Definitions of Terms Specific to This Standard:
3.3.1 viscoelastic response—refers to polymeric creep,
conjunction with SIM tests. They are designed to provide
additional estimates of the initial elastic and initial rapid creep strain, stress relaxation or a combination thereof.
strain levels appropriate for the SIM results. 3.3.2 tensile creep—time-dependent deformation that oc-
curs when a specimen is subjected to a constant tensile load.
1.6 Values stated in SI units are to be regarded as standard.
The common units given in parentheses are for information 3.3.3 tensile creep-rupture—time dependent rupture that
terminates a creep test at high stress levels.
only.
1.7 This method can be used to establish the sustained load 3.3.4 time-temperature superposition—the practice of shift-
ing viscoelastic response curves obtained at different tempera-
creep and creep-rupture characteristics of a geosynthetic.
ResultsofthismethodaretobeusedtoaugmentresultsofTest tures along a horizontal log time axis so as to achieve a master
curve covering an extended range of time.
Method D 5262 and may not be used as the sole basis for
determinationoflongtermcreepandcreep-rupturebehaviorof 3.3.5 shift factor—the displacement along the log time axis
by which a section of the creep or creep modulus curve is
geosynthetic material.
1.8 This standard does not purport to address all of the moved to create the master curve at the reference temperature.
Shift factors are denoted by the symbolAT when the displace-
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- mentsaregenerallytoshortertimes(attenuation)orthesymbol
AT when the displacements are generally to longer times
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. (acceleration).
1 2
This test method is under the jurisdiction of ASTM Committee D35 on For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Geosynthetics and is the direct responsibility of Subcommittee D35.02 on Endur- contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
ance Properties. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Dec. 1, 2003. Published January 2004. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6992–03
polyaramid, poly-vinyl alcohol (PVA) and polypropylene yarns and
3.3.6 stepped isothermal method (SIM)—a method of expo-
narrowstrips.Additionalcorrelationstudiesonothermaterialsareneeded.
sure that uses temperature steps and dwell times to accelerate
creep response of a material being tested under load.
5.4 Creep rupture test data are used to develop a regression
3.3.7 mean test temperature—the arithmetic average of all
line relating creep stress to rupture time. These results predict
temperature readings of the atmosphere surrounding the test
the long term rupture strength expected for geosynthetics in
specimen for a particular temperature step, starting at a time
reinforcement applications.
not later than established temperature ramp time, and finishing
5.5 Tensile testing is used to establish the ultimate tensile
at a time just prior to the subsequent temperature reset.
strength (T ) of a material and to determine elastic stress,
ULT
3.3.8 ultimate tensile strength (T )—short term strength
strain and variations thereof for SIM tests.
ULT
value used to normalize creep rupture strengths.
5.6 Ramp and Hold (R+H) testing is done to establish the
3.3.9 offset modulus method or pointing—data analysis
range of creep strains experienced in the brief period of very
method used to normalize any prestrain in the samples by
rapid response following the peak of the load ramp.
shifting the origin of a stress versus strain curve to an axis
origin of coordinates; that is, to coordinates (0,0). 6. Apparatus
3.3.10 ramp and hold (R+H) test—a creep test of very short
6.1 Grips—GripsforSIMandR+Htestsshouldbethesame
duration; for example, 100 to 1000 s.
as the grips for ultimate strength tensile tests. Neither slippage
3.3.11 dwell time—time during which conditions (particular
nor excessive stress causing premature rupture should be
load) are held constant between temperature steps.
allowed to occur.
3.3.12 creep modulus—in SIM analysis, the load divided by
6.2 Testing Machine—A universal testing machine or a
the percent strain at any given point in time.
dead-weightloadingsystemwiththefollowingcapabilitiesand
accessories shall be used for testing.
4. Summary of Test Method
6.2.1 Load measurement and control,
4.1 SIM—Aprocedure 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
7.3 Remove one (1) test specimen from the sample for each
Section 8). Tensile tests to support creep and creep-rupture
SIM test.
tests are performed under the same control of loading or strain
7.4 Remove one (1) test specimen from the sample for each
rate as used to load or strain the test specimens during creep or
R+H test.
creep rupture tests.
4.3 R+H—Test specimens are ramp loaded at a predeter-
8. Test Specimens
mined loading rate to a predetermined load and held under
8.1 Geogrid specimens should be single ribs, unless other-
constant load (short term creep test).
wise agreed upon.
8.2 Yarn specimens of geogrids or geotextiles should be
5. Significance and Use
single ply or multiple ply strands, unless otherwise agreed
5.1 Use of the Stepped Isothermal Method decreases the
upon.
time required for creep to occur and the obtaining of the
8.3 Geotextile specimens should be 50 mm wide strips,
associated data.
unless otherwise agreed upon.
5.2 The statements set forth in 1.7 are very important in the
NOTE 2—Single geogrid ribs and narrow strip specimens are preferred
context of significance and use, as well as scope of the
todeterminetheeffectofappliedloadonthetensilecreeppropertiesofthe
standard.
material separate from the effect of sample width on the tensile properties
5.3 Creep test data are used to calculate the creep modulus
of the material. However, correlation between narrow geotextile strips or
of materials as a function of time. These data are then used to
single geogrid ribs to wider representative specimens should be estab-
predict the long-term creep deformation expected of geosyn- lished.
thetics used in reinforcement applications.
8.4 The length of the test specimen is determined by the
type of grip used. Refer to specific tensile test procedure for
NOTE 1—Currently, SIM testing has focused mainly on woven and
knitted geogrids and woven geotextiles made from polyester, aramid, guidance.
D6992–03
8.5 Number of Tests: so different from that of the previous that it cannot be
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
only a single SIM test is to be performed, the location of the 11. Procedures
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-rupture curve representing multiple rupture times. 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
range of strain rates expressed in percent per minute, as agreed
curve, for example the percent T at 1 3 10 h (= 110 year)
ULT upon.Generally10 63 %perminute(or20 63 %perminute
rupture life.
for European practice) will be satisfactory.
NOTE 4—Alinear ramp of load versus time will not generally result in
9. Conditioning
a linear strain versus time relationship because stress versus strain curves
9.1 Tensile and SIM testing shall be conducted using 20 6 are not linear for most geosynthetic materials.
1°C as the reference or temperature standard. If the laboratory
11.2 Achieve the test loads for R+H and SIM tests within 6
is not within this range, perform tensile tests in a suitable
2 % of the target loads, and maintain any achieved load within
environmental chamber capable of controlled cooling and
6 0.5 % 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 6 2 % of the target
mable or set-point controller so as to maintain temperature to
load and limited toa1to2 second time duration is acceptable
20 6 1°C. When agreed to, a reference temperature other than
for 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 6 0.5 % 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
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
11.7 Inspectthespecimeninstallationtobesurethematerial
ambient air without special controls.
is properly aligned with the grips and with the loading axis.
10.2 The standard reference temperature is 20°C unless
11.8 Insurethattheloadcellusediscalibratedproperlysuch
otherwise agreed to. The individual reference temperature for
that it will accurately measure the range of tensile loads
each SIM test is the average achieved temperature of the first
anticipated.
isothermal dwell.
11.9 Insure that the extensometer used (if any) is calibrated
10.3 Testing temperatures are to be within 6 2°C of the
properly such that it will accurately measure the range of
target test temperatures. It is critically important that the test
tensile strains anticipated. If rupture is anticipated, take pre-
specimen has equilibrated throughout its thickness so as to
cautions to insure that the rupture event will not damage the
avoid nonisothermal conditions. Initial trials are necessary to
extensometer or create a hazard for the machine operator.
establish this minimum equilibrium time.
11.10 Unless otherwise agreed upon, a 100 mm gage length
NOTE 3—Laboratoryexperiencehassuggestedthattheuseofcalibrated
shall be used for geosynthetic products and a 250 to 300 mm
thermocouples located near, affixed to or embedded within the test
gage length shall be used for precursor yam products.
specimen may facilitate a successful temperature compliance test for the
11.11 Time, load and extension data shall be collected at a
specimen material. It is suggested that the laboratory perf
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