1、Designation: D 6147 97 (Reapproved 2008)Standard Test Method forVulcanized Rubber and Thermoplastic ElastomerDetermination of Force Decay (Stress Relaxation) inCompression1This standard is issued under the fixed designation D 6147; the number immediately following the designation indicates the year
2、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 standard specifies two methods for determining thed
3、ecrease in counterforce exerted by a test specimen of vulca-nized rubber or thermoplastic elastomer which has beencompressed at a constant deformation under specified condi-tions of time and temperature.1.2 This document was developed based on testing in airand liquids.1.3 This standard does not pur
4、port to address all of thesafety concerns, if any, associated with its use. It is theresponsibility 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 Documents2.1 ASTM Standards:2D
5、573 Test Method for RubberDeterioration in an AirOvenD 1349 Practice for RubberStandard Temperatures forTestingD 3182 Practice for RubberMaterials, Equipment, andProcedures for Mixing Standard Compounds and Prepar-ing Standard Vulcanized SheetsD 3767 Practice for RubberMeasurement of DimensionsD 448
6、3 Practice for Evaluating Precision for Test MethodStandards in the Rubber and Carbon Black ManufacturingIndustriesE 145 Specification for Gravity-Convection and Forced-Ventilation Ovens3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 Compressive Stress, nthe time-dependent for
7、ce nec-essary to maintain a constant compressive strain, divided bythe original cross-sectional area over which the force isapplied.3.1.2 Force Decay, nthe decrease in stress which hasoccurred after a specified time-interval, during application of aconstant deformation, expressed as a percentage of
8、the stress atthe commencement of that time-interval. (Stress relaxation is asynonym.)4. Summary of Test Method4.1 Method AThe test specimen is compressed at the testtemperature, and it is maintained at this temperature through-out the test period, all force measurements being made at thetest tempera
9、ture.4.2 Method BThe test specimen compression and theinitial measurement of the counterforce take place at 23 6 2C(73 6 4F); the test specimen is then stored in a chambercontrolled at the test temperature, but it is removed from thechamber for each of the subsequent force measurements, whichare mad
10、e at 23 6 2C (73 6 4F).4.3 The two methods, A and B, of carrying out the measure-ment do not give the same values of force decay and compari-son of values obtained from the two methods must be avoided.The method selected for use depends on the purpose of the test.Thus, for fundamental studies and in
11、 applications wheresealing at elevated temperatures is a problem, method A maybe preferred, and in applications where temperature cyclingfrom normal to an elevated temperature is a problem, methodB may be preferred.5. Significance and Use5.1 When a constant strain is imposed on rubber, the forcenece
12、ssary to maintain that strain is not constant but decreaseswith time; this phenomenon is called force decay (stressrelaxation). Conversely, when rubber is subjected to a constantstress, an increase in the deformation takes place in time; this1This test method is under the jurisdiction of ASTM Commit
13、tee D11 on Rubberand is the direct responsibility of Subcommittee D11.14 on Time and Temperature-Dependent Physical Properties.Current edition approved Jan. 1, 2008. Published February 2008. Originallyapproved in 1997. Last previous edition approved in 2002 as D 6147 97 (2002).2For referenced ASTM s
14、tandards, 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 Drive, PO Box C700, West Conshohocke
15、n, PA 19428-2959, United States.behavior is called creep. These phenomena are of practicalsignificance in rubber articles, such as seals and gaskets.5.2 The processes responsible for force decay may be eitherphysical or chemical in nature, and under all normal conditionsboth processes will occur sim
16、ultaneously. However, at ambientor low temperatures and/or short times, force decay is domi-nated by physical processes, while at elevated temperaturesand/or long times, chemical processes are dominant. Hence, itis neither safe to extrapolate time/force decay curves in orderto predict force decay af
17、ter periods considerably longer thanthose covered by the test, nor to use tests at higher tempera-tures as accelerated tests to give information on force decay atlower temperatures.5.3 In addition to the need to specify the temperature andtime-intervals in a force decay test, it is also necessary to
18、specify the initial stress and the previous mechanical history ofthe test specimen, since these may also influence the measuredforce decay, particularly in rubbers containing fillers.6. Apparatus6.1 Compression Device, consisting of two parallel, flatplates, between the faces of which the test speci
19、mens arecompressed. The two devices shown schematically in Fig. 1and Fig. 2 were found to be acceptable through interlaboratorytesting. Material of construction of the jig shall not be subjectto attack from the test environment (for example acidic oil).6.2 The finish of the surface of the compressio
20、n plates shallnot exceed 0.6 m (24 micro inches) arithmetical meandeviation from the mean line of the profile. The plates shall besufficiently rigid to withstand the stress without bending, andof sufficient size to ensure that the whole of the compressed testspecimen is within the area of the plates
21、.6.3 The compression device shall be connected with suit-able equipment for compressing the test specimen to thespecified compression within 30 s. It shall be capable of settingand maintaining the compression during the whole duration oftest and shall be such that it can be kept in an oven at thespe
22、cified test temperature. Care shall be taken to ensure thatthere is no loss of heat from the test specimen, for example byconduction through metal parts which are connected with theoutside of the oven.6.4 Counterforce measuring device, capable of measuringcompression forces in the desired range with
23、 an accuracy of61 %. The device may be such as to contain the test specimensduring the whole duration of the test, in which case continuousmeasurements are possible. Alternatively, a testing machinemay be used in which the counterforce is measured afterprescribed time-intervals on test specimens, co
24、mpressed in asuitable jig, by applying a slight increase in the compression ofthe test specimen. This additional compression shall be assmall as possible and in no case more than a force of 1N or halfpercent (12 %) of original load for balance type machine and0.1 mm (0.004 in) or 2 % of original def
25、lection for stress-straintype machines, in a time not greater than 30 s after commenc-ing the additional compression.6.5 Oven, with temperature control to maintain the specifiedtemperature within the prescribed tolerances. Satisfactory cir-culation of the air shall be maintained by means of a fan (s
26、eeTest Method D 573 or Specification E 145).6.6 Temperature reading device (optional), with for ex-ample a thermocouple as the sensing element, mounted in acentral position in the oven.7. Test Specimen7.1 Type of Test SpecimenThe test specimen shall be oneof the following:7.1.1 A cylindrical disc of
27、 diameter 13.0 6 0.5 mm (0.5126 0.020 in) and thickness 6.3 6 0.3 mm (0.248 6 0.012 in).7.1.2 Cylindrical disc of diameter 29.0 6 0.5 mm (1.142 60.020 in) and thickness 12.5 6 0.5 mm (0.492 6 0.020 in).7.1.3 Washer of 12.5 6 0.5 mm (0.492 6 0.020 in) insidediameter and 19.0 6 0.5 mm (0.748 6 0.020 i
28、n) outsidediameter with 2.0 6 0.2 mm (0.080 6 0.008 in) thickness.Within the sample test specimens, thickness variation shall bewithin 0.025 mm (0.001 in).7.1.4 Other test specimen sizes may be used as dictated byend use.7.2 The first three test specimens described above wereevaluated in interlabora
29、tory testing. The washer type testspecimen (7.1.3) is preferred.7.3 Preparation of Test SpecimensTest specimens may beprepared by taking a sample of the compound or material ofinterest. If this is a compound it shall be cured and molded inaccordance with Practice D 3182, or prepared by cutting fromm
30、olded sheets. Cutting is carried out by means of a sharp,rotating, circular die or revolving knife, lubricated with soapywater, and brought carefully into contact with the rubber.Alternatively, the die or knife is kept stationary and the rubberrotated against it. The rubber is mounted on suitable ba
31、ckingmaterial and the cutting pressure is kept small enough to avoid“cupping” of the cut surface.7.4 Due to shape factor effect, equivalent results may not beobtained for the different test specimen shapes describedabove.7.5 Number of Test SpecimensAt least three test speci-mens shall be used for ea
32、ch test.7.6 Time-Interval Between Vulcanization and TestingThetime-interval between vulcanization and testing shall be at least24 h at ambient conditions.8. Procedure8.1 Carefully clean the operating surfaces of the compres-sion device. The test specimen surface shall be free from moldrelease agent
33、or dusting powder.8.2 Measure the thickness of each test specimen at thecentral portion with an accuracy of 0.01 mm (0.0004 in). Thethickness shall be determined according to Practice D 3767.8.3 Method A8.3.1 Preheat the compression device to the test tempera-ture.8.3.2 Preheat the test specimen to
34、the test temperature inaccordance with Practice D 1349. A preheating period of 30012min. is recommended.8.3.3 Compress the preheated test piece by 25 6 1 % in thecompression device (6.1) at the test temperature; use a com-pression of 15 6 1 % if a compression of 25 % cannot beobtained. Apply the com
35、pression within 30 s. When reached,the final compression shall be fixed and maintained during theD 6147 97 (2008)2FIG.1Shawbury-WallaceStressRelaxationFixtureD 6147 97 (2008)3entire test period (apart from the further small compressionwhich may be used for measurement of counterforce, men-tioned in
36、6.4).8.3.4 After the compression has been applied for 30012min.,measure the counterforce with an accuracy of 61 %, still at thetest temperature.8.3.5 Repeat the measurement of the counterforce afterdifferent times, according to 10.1. Take all measurements at thetest temperature.8.3.6 Due to operator
37、 variability, all counterforce measure-ments shall be performed three (3) times for each specimen andthe median value used.8.4 Method B8.4.1 Compress the test piece by 25 6 1 % at 23 6 2C (736 4F); a compression of 15 6 1 % may be used when acompression of 25 % cannot be obtained. Apply the compres-
38、sion within 30 s. When reached, the final compression shall befixed and maintained during the entire test period (apart fromthe further small compression which may be used for measure-ment of counterforce, mentioned in 6.4).8.4.2 After the compression has been applied for 30012min.measure the counte
39、rforce with an accuracy of 61 %, still at thestandard laboratory temperature.8.4.3 Store the compressed test specimen in the oven (6.5)at the specified elevated temperature.8.4.4 When making measurements of counterforce after thetimes specified, remove the apparatus from the oven, maintainit at the
40、standard laboratory temperature for 2 0.210h, determinethe counterforce and then return to the test environment for afurther time. It is important that the apparatus and testspecimen reach thermal equilibrium within 2 h, and forcecooling may be necessary. In method B, the 2 h required aftereach time
41、-interval, to attain standard laboratory temperature,shall be included in the duration of test.8.4.5 Repeat the measurement of the counterforce afterdifferent times, according to 10.1. Take all measurements atambient temperature.8.4.6 Due to operator variability, all counterforce measure-ments shall
42、 be performed three (3) times for each test specimenand the median value used.9. Liquid Immersions Testing Procedure9.1 In some applications force decay testing under liquidimmersion conditions is necessary. Under these conditions thefollowing additional preparation of the jig is necessary.9.2 Apply
43、 a lubricant to the top and bottom surface of thejig. It shall consist of a thin coating of the test fluid havingsubstantially no action on the rubber.9.3 The test specimen/test fixture shall be completely im-mersed in liquid under consideration. The immersed testFIG. 2 Wykeham Farrance Stress Relax
44、ation FixtureD 6147 97 (2008)4specimen shall be in the compressed condition, similar totesting in air as discussed in Section 8. Care shall be takenwhile performing force decay under liquid immersions (espe-cially at elevated temperatures) to avoid hazardous test condi-tions. Environmentally safe pr
45、actices shall be followed whileperforming liquid immersion testing. If the effect of liquid oninside and outside surfaces of rubber is desired, the washertype test specimen (7.1.3), is recommended. In this case thefixture used shall have an inlet and vent hole of at least 3.2 60.3 mm (0.125 6 0.012
46、in) diameter to allow flow of liquidtowards inside diameter of the sample.9.4 Follow test procedures in 8.3 for Method A or 8.4 forMethod B.10. Duration and Temperature of Test10.1 Duration of TestThe duration of the test shall bemutually agreed upon by the customer and the supplier. A testperiod of
47、 16820h has been found suitable for many elastomers.The test period begins after the initial compression. If longertesting times are needed, a logarithmic time-scale may be used.In method B, the 2 h required after each time-interval, to attainstandard laboratory temperature, shall be included in the
48、duration of test.10.2 Temperature of TestThe temperature of test shall bechosen from Practice D 1349. Unless for technical reasonsanother temperature is required, the ambient temperature shallbe 23 6 2C (73 6 4F).11. Calculation of Test Results11.1 The compression force decay (stress relaxation), R(
49、t),after a specified duration of test, t, expressed as a percentage ofthe initial counterforce, is calculated by Eq 1.Rt! 5Fo FtFo3 100 (1)where:Fo = initial counterforce measured 30 min. after compres-sion of the test specimen, andFt = counterforce measured after the specified duration oftest.11.2 The median value of the calculated results for com-pression force decay shall be used.11.3 For some applications, it may be more useful tocalculate compressive force retention values, that is, FR(t) atvarious times after the compression is applied, rath
