ASTM_D_2991_-_84_scan.pdf

-,ASTM D2993 84 R 0759530 0028437 8 I Designation:D 2991-84 4cni,AXERICAN SOCIETY FOR TESTING ANb.MATERIALS 1916 Rsce Ct.,Philadelphia,Pa.19103 Reprinted from the Annual Book of ASTM Standards,Copyright ASTM If net listed in the current combined index,will appear in the next editio!.Standard Practice for Testing STRESS-RELAXATION OF PLASTICS This standard is issued under the fixed designation D 2991;the number immediately following the designation indicates the year of original adoption or,in the case of revision,the year of last revkion.A number in parentheses indicates the year of last reapproval.A superscript epsilon(e)indicates an editorial change since the lasf revision or reapproval.1.Scope 1,I This practice covers the determination of the time-dependence of stress(stress relaxation)of plastics resisting long-duration constant strains at conditions of constant temperature and rela-tive humidity and negligible vibration.1.2 This standard may involve hazardoits ma-terials,operations,and equipment.This standard does not purport to address all o f the safity prob-lems associated with its use.It i s the responsibil-ity o f whoever uses this standard to consult and establish appropriate safety and health practices and determine the applicability o f regulatoiy limi-tations prior io use.2.Applicable Documents 2.1 ASTM Standards:D638 Test Method for Tensile Properties of D695 Test Method for Compressive Proper-Plastics ties of Rigid Plastics 3.Defdtions 3.1 stress relaxation-the time-dependent de-crease in stress in a solid under given constraint conditions.3.2 initial stress-the stress introduced into a specimen by imposing the given constraint con-ditions before stress relaxation takes place.4.Significance and Use 4.1 Data from stress-relaxation tests are useful.in predicting the reduction of stress in materials subjected to constant deformation for long times,such as the decrease in tightness of bolted or riveted joints.4.2 This test is highly sensitive to small changes in material composition and environ-mental conditions and hence is useful in evalu-ating the effect of such changes.5.Grips 5.1 The grips and gripping technique shall be designed to minimize eccentric loading of the specimen.Swivel or universal joints shall be used beyond each end of the specimen.The grips shall be so designed that no slippage of the specimen occurs.NOTE I-It is recommended that grips permit the final centering of the specimen prior to applying the strain or load.Grips that permit a displacement of the specimen within the grips during load application are not suitable.6.Temperature Control and Measurement 6.1 The temperature of the test space,espe-cially close to the gage length of the specimen,shall be maintained within+_TC by a suitable automatic device and shall be stated in reporting the results.NOTE 2-The thermal contraction and expansion associated with small temperature changes during the test may produce changes in the apparent relaxation rate especially near transition temperatures.Small changes in temperature near transitions also produce significant changes in relaxation rate.6.2 Care must be taken to ensure accurate temperature measurements over the gage length of the specimen throughout the test.The tem-perature-measuring devices shall be checked reg-This practice is under the jurisdiction of ASTM Committee D-20 on Plastics and is the direct responsibility ofSubcommittee D20.10 on Mechanical Properiies.Current edition approved Nov.30,1984.hbshed January 1985.OriginallypubIishedasD2991-71.Lastpreviousedition D 2991-71(1978).T h i s practice and Methods D 2990 replace Recommended Practices D 674,which have been discontinued.Annual Book ofASTMStandards.Vol 08.01.NOTICE:This standard has either been superseded and replaced by a new version or discontinued.Contact ASTM International(www.astm.org)for the latest information.ASTM D2991 84 m 0759510 00284LB T m ularly against temperature standards and shall indicate the temperature of the specimen gage area.6.3 Temperature measurements shall be made at frequent intervals,or continuously recorded to ensure an accurate determination of the aver-age test temperature and compliance with 5.1.7.Environmental Control and Measurement 7.1 When the test environment is air,the rel-ative humidity shall be controlled to within f5%during the test unless otherwise specified,or unless the relaxation behavior of the material under test has been shown to be unaffected by humidity.The controlling and measuring instru-ments shall be stable for long time intervals and accurate with-i-1%.(The control of relative hu-midity is known to be difficult at temperatures much outside the range from 10 to 40C(50 to 1 OOF).)7.2 When the environment is other than air,the composition of the test environment shall be maintained constant throughout the test.7.3 The specimens shall be preconditioned in the test environment for at least 48 h prior to being tested.Those materials whose relaxation properties a