1、Designation: D 6992 03Standard Test Method forAccelerated Tensile Creep and Creep-Rupture ofGeosynthetic Materials Based on Time-TemperatureSuperposition Using the Stepped Isothermal Method1This standard is issued under the fixed designation D 6992; the number immediately following the designation i
2、ndicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers accelerated te
3、sting for tensilecreep, and tensile creep-rupture properties using the SteppedIsothermal Method (SIM).1.2 The test method is focused on geosynthetic reinforce-ment materials such as yarns, ribs of geogrids, or narrowgeotextile specimens.1.3 The SIM tests are laterally unconfined tests based ontime-t
4、emperature superposition procedures.1.4 Tensile tests are to be completed before SIM tests andthe results are used to determine the stress levels for subse-quent SIM tests defined in terms of the percentage of UltimateTensile Strength (TULT). Additionally, the tensile test can bedesigned to provide
5、estimates of the initial elastic straindistributions appropriate for the SIM results.1.5 Ramp and Hold (R+H) tests may be completed inconjunction with SIM tests. They are designed to provideadditional estimates of the initial elastic and initial rapid creepstrain levels appropriate for the SIM resul
6、ts.1.6 Values stated in SI units are to be regarded as standard.The common units given in parentheses are for informationonly.1.7 This method can be used to establish the sustained loadcreep and creep-rupture characteristics of a geosynthetic.Results of this method are to be used to augment results
7、of TestMethod D 5262 and may not be used as the sole basis fordetermination of long term creep and creep-rupture behavior ofgeosynthetic material.1.8 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this stan
8、dard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D 2990 Test Methods for Tensile, Compressive, and Flex-ural Creep and Creep-Rupture of PlasticsD 4439 Terminology for Geosynth
9、eticsD 4595 Test Method for Tensile Properties of Geotextilesby the Wide-Width Strip MethodD 5262 Test Method for Evaluating the Unconfined CreepBehavior of Geosynthetics3. Terminology3.1 For definitions related to geosynthetics see TerminologyD 4439.3.2 For definitions related to creep see Test Met
10、hods D 2990and D 5262.3.3 Definitions of Terms Specific to This Standard:3.3.1 viscoelastic responserefers to polymeric creep,strain, stress relaxation or a combination thereof.3.3.2 tensile creeptime-dependent deformation that oc-curs when a specimen is subjected to a constant tensile load.3.3.3 te
11、nsile creep-rupturetime dependent rupture thatterminates a creep test at high stress levels.3.3.4 time-temperature superpositionthe practice of shift-ing viscoelastic response curves obtained at different tempera-tures along a horizontal log time axis so as to achieve a mastercurve covering an exten
12、ded range of time.3.3.5 shift factorthe displacement along the log time axisby which a section of the creep or creep modulus curve ismoved 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 (attenu
13、ation) or the symbolAT when the displacements are generally to longer times(acceleration).1This test method is under the jurisdiction of ASTM Committee D35 onGeosynthetics and is the direct responsibility of Subcommittee D35.02 on Endur-ance Properties.Current edition approved Dec. 1, 2003. Publishe
14、d January 2004.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor
15、 Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.3.6 stepped isothermal method (SIM)a method of expo-sure that uses temperature steps and dwell times to acceleratecreep response of a material being tested under load.3.3.7 mean test temperaturethe arithmetic average of alltemper
16、ature readings of the atmosphere surrounding the testspecimen for a particular temperature step, starting at a timenot later than established temperature ramp time, and finishingat a time just prior to the subsequent temperature reset.3.3.8 ultimate tensile strength (TULT)short term strengthvalue us
17、ed to normalize creep rupture strengths.3.3.9 offset modulus method or pointingdata analysismethod used to normalize any prestrain in the samples byshifting the origin of a stress versus strain curve to an axisorigin of coordinates; that is, to coordinates (0,0).3.3.10 ramp and hold (R+H) testa cree
18、p test of very shortduration; for example, 100 to 1000 s.3.3.11 dwell timetime during which conditions (particularload) are held constant between temperature steps.3.3.12 creep modulusin SIM analysis, the load divided bythe percent strain at any given point in time.4. Summary of Test Method4.1 SIMA
19、procedure whereby specified temperature stepsand dwell times are used to accelerate viscoelastic creepcharacteristics during which strain and load are monitored as afunction of time.4.1.1 Tensile CreepConstant tensile load in conjunctionwith specified temperature steps and dwell times are used toacc
20、elerate creep strain response.4.1.2 Tensile Creep-RuptureA tensile creep test wherehigh stress levels are used during testing to ensure rupture,while specified temperature steps and dwell times are used toaccelerate creep strain response characteristics. Strain is moni-tored as a function of time.4.
21、2 Tensile TestsTest specimens are rapidly loaded over ashort period to achieve rupture. The selection of a suitabletensile test is dependent upon the type of material tested (seeSection 8). Tensile tests to support creep and creep-rupturetests are performed under the same control of loading or strai
22、nrate as used to load or strain the test specimens during creep orcreep rupture tests.4.3 R+HTest specimens are ramp loaded at a predeter-mined loading rate to a predetermined load and held underconstant load (short term creep test).5. Significance and Use5.1 Use of the Stepped Isothermal Method dec
23、reases thetime required for creep to occur and the obtaining of theassociated data.5.2 The statements set forth in 1.7 are very important in thecontext of significance and use, as well as scope of thestandard.5.3 Creep test data are used to calculate the creep modulusof materials as a function of ti
24、me. These data are then used topredict the long-term creep deformation expected of geosyn-thetics used in reinforcement applications.NOTE 1Currently, SIM testing has focused mainly on woven andknitted geogrids and woven geotextiles made from polyester, aramid,polyaramid, poly-vinyl alcohol (PVA) and
25、 polypropylene yarns andnarrow strips. Additional correlation studies on other materials are needed.5.4 Creep rupture test data are used to develop a regressionline relating creep stress to rupture time. These results predictthe long term rupture strength expected for geosynthetics inreinforcement a
26、pplications.5.5 Tensile testing is used to establish the ultimate tensilestrength (TULT) of a material and to determine elastic stress,strain and variations thereof for SIM tests.5.6 Ramp and Hold (R+H) testing is done to establish therange of creep strains experienced in the brief period of veryrap
27、id response following the peak of the load ramp.6. Apparatus6.1 GripsGrips for SIM and R+H tests should be the sameas the grips for ultimate strength tensile tests. Neither slippagenor excessive stress causing premature rupture should beallowed to occur.6.2 Testing MachineA universal testing machine
28、 or adead-weight loading system with the following capabilities andaccessories shall be used for testing.6.2.1 Load measurement and control,6.2.2 Strain measurement and control,6.2.3 Time measurement,6.2.4 Environmental temperature chamber to facilitate con-trol of test conditions,6.2.4.1 Temperatur
29、e measurement and control facilities,6.2.5 Other environmental measurement and control, and6.2.6 Computer data acquisition and control.7. Sampling7.1 The specimens used for tensile, R+H and SIM testsshould all be taken from the same sample.7.2 Remove sufficient test specimens for tensile testing ina
30、ccordance with the selected tensile testing procedure (seeSection 8).7.3 Remove one (1) test specimen from the sample for eachSIM test.7.4 Remove one (1) test specimen from the sample for eachR+H test.8. Test Specimens8.1 Geogrid specimens should be single ribs, unless other-wise agreed upon.8.2 Yar
31、n specimens of geogrids or geotextiles should besingle ply or multiple ply strands, unless otherwise agreedupon.8.3 Geotextile specimens should be 50 mm wide strips,unless otherwise agreed upon.NOTE 2Single geogrid ribs and narrow strip specimens are preferredto determine the effect of applied load
32、on the tensile creep properties of thematerial separate from the effect of sample width on the tensile propertiesof the material. However, correlation between narrow geotextile strips orsingle geogrid ribs to wider representative specimens should be estab-lished.8.4 The length of the test specimen i
33、s determined by thetype of grip used. Refer to specific tensile test procedure forguidance.D69920328.5 Number of Tests:8.5.1 A single specimen is usually sufficient to define amaster creep or relaxation curve using the SIM. However, ifonly a single SIM test is to be performed, the location of theons
34、et of creep strain or modulus curve should be confirmedusing at least two short term creep (R+H) tests.8.5.2 Generally 12 to 18 specimens are needed to define astress-rupture curve representing multiple rupture times. Fewerspecimens would be needed to define a specific region of thecurve, for exampl
35、e the percent TULTat 1 3 106h (= 110 year)rupture life.9. Conditioning9.1 Tensile and SIM testing shall be conducted using 20 61C as the reference or temperature standard. If the laboratoryis not within this range, perform tensile tests in a suitableenvironmental chamber capable of controlled coolin
36、g andheating. The environmental chamber should have a program-mable or set-point controller so as to maintain temperature to20 6 1C. When agreed to, a reference temperature other than20C can be utilized. Also, when agreed to, the results oftesting under this standard can be shifted from one referenc
37、etemperature 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 (seeNote 3).9.3 Record the relative humidity in the laboratory or envi-ronmental chamber for all tests.10.
38、 Selection of Test Conditions10.1 The standard environment for testing is dry, since theeffect of elevated temperature is to reduce the humidity ofambient air without special controls.10.2 The standard reference temperature is 20C unlessotherwise agreed to. The individual reference temperature forea
39、ch SIM test is the average achieved temperature of the firstisothermal dwell.10.3 Testing temperatures are to be within 6 2C of thetarget test temperatures. It is critically important that the testspecimen has equilibrated throughout its thickness so as toavoid nonisothermal conditions. Initial tria
40、ls are necessary toestablish this minimum equilibrium time.NOTE 3Laboratory experience has suggested that the use of calibratedthermocouples located near, affixed to or embedded within the testspecimen may facilitate a successful temperature compliance test for thespecimen material. It is suggested
41、that the laboratory perform the plannedSIM temperature steps using an unloaded sacrificial test specimen and,with the use of these thermocouples, measure the temperature change ofthe specimen at its thickest or most mass-dense region. The time requiredfor the specimen to reach the target temperature
42、 is recorded and used asthe minimum dwell time. The upper limit of the temperature ramp time isnot known. Successful tests with some materials have been run withtemperature ramp times of up to four minutes.10.4 Test temperatures are to be maintained within 6 1.0Cof the mean achieved temperature.10.4
43、.1 Temperature steps and dwell times must be such thatthe steady state creep rate at the beginning of a new step is notso different from that of the previous that it cannot beestablished within the identified ramp time.11. Procedures11.1 The same or similar load or strain control shall beapplied to
44、the tensile tests and the load ramp portion of R+Hand SIM (creep and creep-rupture) tests. The load rate control(in units of kN per min) that is applied shall achieve a narrowrange of strain rates expressed in percent per minute, as agreedupon. Generally 10 6 3 % per minute (or 20 6 3 % per minutefo
45、r European practice) will be satisfactory.NOTE 4A linear ramp of load versus time will not generally result ina linear strain versus time relationship because stress versus strain curvesare not linear for most geosynthetic materials.11.2 Achieve the test loads for R+H and SIM tests within 62 % of th
46、e target loads, and maintain any achieved load within6 0.5 % of its values for the duration of the test. A briefovershoot of the target load that is within 6 2 % of the targetload and limited toa1to2second time duration is acceptablefor load control systems.11.3 Replicate test loads for R+H and SIM
47、tests should bewithin 6 0.5 % of the average of the achieved loads for a testset.11.4 Pretensioning up in accordance with the governingtensile test is acceptable. The method used to define zero strainis to be identified and reported.11.5 The same or similar grips shall be used for tensile,R+H and SI
48、M tests. Care should be taken to use grips that donot initiate failure or incur slippage at stress levels which mayproduce specimen rupture (for example, at loads greater than55 % of TULTfor polyester).11.6 Inspect grips to insure loading surfaces are clean andthat padding, if used, is free of defec
49、ts and is secured properly.11.7 Inspect the specimen installation to be sure the materialis properly aligned with the grips and with the loading axis.11.8 Insure that the load cell used is calibrated properly suchthat it will accurately measure the range of tensile loadsanticipated.11.9 Insure that the extensometer used (if any) is calibratedproperly such that it will accurately measure the range oftensile strains anticipated. If rupture is anticipated, take pre-cautions to insure that the rupture event will not damage theextensometer or create a hazard for
