ASTM E328-2002 Standard Test Methods for Stress Relaxation Tests for Materials and Structures《材料和结构件的应力松弛试验》.pdf

1、Designation: E 328 02Standard Test Methods forStress Relaxation for Materials and Structures1This standard is issued under the fixed designation E 328; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A nu

2、mber in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.INTRODUCTIONThese test methods cover a broad range of testing activities. To aid in locating the subject matterpertinent to a particular test, the s

3、tandard is divided into a general section, which applies to all stressrelaxation tests for materials and structures. This general section is followed by letter-designated partsthat apply to tests for material characteristics when subjected to specific, simple stresses, such asuniform tension, unifor

4、m compression, bending or torsion. To choose from among these types ofstress, the following factors should be considered:(1) When the material data are to be applied to the design of a particular class of component, thestress during the relaxation test should be similar to that imposed on the compon

5、ent. For example,tension tests are suitable for bolting applications and bending tests for leaf springs.(2) Tension and compression relaxation tests have the advantage that the stress can be reportedsimply and unequivocally. During bending relaxation tests, the state of stress is complex, but can be

6、accurately determined when the initial strains are elastic. If plastic strains occur on application offorce, stresses can usually be determined within a bounded range only. Tension relaxation tests, whencompared to compression tests, have the advantage that it is unnecessary to guard against bucklin

7、g.Therefore, when the test method is not restricted by the type of stress in the component, tension testingis recommended.(3) Bending tests for relaxation, when compared to tension and compression tests, have theadvantage of using lighter and simpler apparatus for specimens of the same cross-section

8、al area.Strains are usually calculated from deflection or curvature measurements. Since the specimens canusually be designed so that these quantities are much greater than the axial deformation in a directstress test, strain is more easily measured and more readily used for machine control in the be

9、ndingtests. Due to the small forces normally required and the simplicity of the apparatus when static fixturesare sufficient, many specimens can be placed in a single oven or furnace when tests are made atelevated temperatures.1. ScopeNOTE 1The method of testing for the stress relaxation of plastics

10、 hasbeen withdrawn from this standard, and the responsibility has beentransferred to Practice D 2991.1.1 These test methods cover the determination of the timedependence of stress (stress relaxation) in materials andstructures under conditions of approximately constant con-straint, constant environm

11、ent, and negligible vibration. In theprocedures recommended, the material or structure is initiallyconstrained by externally applied forces, and the change in theexternal force necessary to maintain this constraint is deter-mined as a function of time.1.2 Specific methods for conducting stress relax

12、ation testson materials subjected to tension, compression, bending andtorsion stresses are described in Parts A, B, C, and D,respectively. These test methods also include recommendationsfor the necessary testing equipment and for the analysis of thetest data.1.3 It is recognized that the long time p

13、eriods required forthese types of tests are often unsuited for routine testing or forspecification in the purchase of material. However, these testsare valuable tools in obtaining practical design information onthe stress relaxation of materials subjected to the conditionsenumerated, and in investig

14、ations of the fundamental behaviorof materials.1.4 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 standard to establish appro-priate safety and health practices and determine the applica-bility of regu

15、latory limitations prior to use.1These test methods are under the jurisdiction of ASTM Committee E28 onMechanical Testing and is the direct responsibility of Subcommittee E28.04 onUniaxial Testing.Current edition approved Nov 10, 2002. Published April 2003. Originallyapproved in 1967. Last previous

16、approved 1986 as E32886(96)e1.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.2. Referenced Documents2.1 ASTM Standards:D 2991 Practice for Testing Stress-Relaxation of Plastics2E4 Practices for Force Verification of Testing Machines

17、3E8 Test Methods for Tension Testing of Metallic Materials3E9 Test Methods of Compression Testing of Metallic Ma-terials at Room Temperature3E 83 Practice for Verification and Classification of Exten-someters3E 139 Practice for Conducting Creep, Creep-Rupture, andStress-Rupture Tests of Metallic Mat

18、erials3E 1012 Practice for Verification of Specimen AlignmentUnder Tensile Loading33. Terminology3.1 Definitions:3.1.1 stress relaxationthe time-dependent decrease instress in a solid under given constraint conditions.3.1.1.1 DiscussionThe general stress relaxation test isperformed by isothermally a

19、pplying a force to a specimen withfixed value of constraint. The constraint is maintained constantand the constraining force is determined as a function of time.The major problem in the stress relaxation test is that constantconstraint can be very difficult to maintain. The effects on testresults ar

20、e very significant and considerable attention must begiven to minimize the constraint variation.Also, experimentersshould determine and report the extent of variation in eachstress relaxation test so that this factor can be taken intoconsideration.3.1.2 initial stress FL2the stress introduced into a

21、specimen by imposing the given constraint conditions beforestress relaxation begins.3.1.2.1 DiscussionThere are many methods of performingthe stress relaxation test, each with a different starting proce-dure. However, the constraint is usually obtained initially bythe application of an external forc

22、e at either a specific forceapplication rate or a specific strain rate. The two methods willproduce the characteristic behavior shown in Fig. 1 when theinitial stress, s0, exceeds the proportional limit. Some testingmachines, while reaching the constraint value, do not produceeither a constant force

23、 application rate or constant strain rate,but something in between. However, the general characteristicsof the data will be similar to those indicated. The stressapplication rate in either case should be reasonably rapid, butwithout impact or vibration, so that any relaxation during thestress applic

24、ation period will be small.3.1.3 zero time, t0the time when the given stress orconstraint conditions are initially obtained in a stress relaxationtest.3.1.3.1 DiscussionThe stress relaxation test is consideredto have started at zero time, t0in Fig. 1. This is the referencetime from which the observe

25、d reduction in force to maintainconstant constraint is based. Selection of this time does notimply that the force application procedure or period, or both,are not significant test parameters. These must always beconsidered in the application of the data.2Annual Book of ASTM Standards, Vol 08.02.3Ann

26、ual Book of ASTM Standards, Vol 03.01.FIG. 1 Characteristic Behavior During Force Application Period in a Relaxation TestE3280223.1.4 relaxation ratethe absolute value of the slope of therelaxation curve at a given time.3.1.5 spherometeran instrument used to measure circularor spherical curvature.3.

27、1.6 indicated nominal temperature or indicatedtemperaturethe temperature that is indicated by thetemperature-measuring device.4. Summary of Test Methods4.1 In each of the various methods of stress applicationdescribed in the applicable specific sections, the specimen issubjected to an increasing for

28、ce until the specified initial strainis attained (see zero time in 3.1.3 and in Fig. 1). For theduration of the test, the specimen constraint is maintainedconstant. The initial stress is calculated from the initial force(moment, torque) as measured at zero time, the specimengeometry, and the appropr

29、iate elastic constants, often usingsimple elastic theory. The remaining stress may be calculatedfrom the force (moment or torque) determined under constraintconditions either continuously (4.1.1), periodically (4.1.2), orby elastic springback at the end of the test period 4.1.3 (seeFig. 3).4.1.1 Rea

30、dings are taken continuously from a force indica-tor while the apparatus adjusts the force to maintain constraintwithin specified bounds.NOTE 2Most force, moment, or torque measuring devices depend onthe devices elasticity to measure the quantities involved. Therefore, it isnecessary that when using

31、 such devices, to maintain the total strainconstant within an upper and lower bound as shown in Fig. 4.4.1.2 The force required to lift the specimen just free of oneor more constraints during the test period is measured.4.1.3 The elastic springback is measured after removing thetest stress at the en

32、d of the test period.4.2 With 4.1.1 and 4.1.2, a single specimen can be used toobtain data for a curve of stress versus time. With 4.1.3, thesame specimens may be used to determine the remaining orrelaxed stress after various time intervals, if it can be demon-strated for a given material that ident

33、ical results are obtained ineither using virgin or reloaded specimens. Otherwise, indi-vidual specimens must be used for each point on the curve.5. Significance and Use5.1 Relaxation test data are necessary when designing mostmechanically fastened joints to assure the permanent tightnessof bolted or

34、 riveted assemblies, press or shrink-fit components,FIG. 2 Typical Relaxation CurvesFIG. 3 Stress-Strain Diagram for Determining Relaxation inStressFIG. 4 Derivation of Stress-Relaxation Curve from ContinuousRelaxation TechniqueE328023rolled-in tubes, etc. Other applications include predicting thede

35、crease in the tightness of gaskets, in the hoop stress ofsolderless wrapped connections, in the constraining force ofsprings, and the stability of wire tendons in prestressedconcrete.5.2 The ability of a material to relax at high-stress concen-trations such as are present at notches, inclusions, cra

36、cks,holes, fillets, etc., may be predicted from stress relaxation data.Such test data are also useful to judge the heat-treatmentcondition necessary for the thermal relief of residual internalstresses in forgings, castings, weldments, machined or cold-worked surfaces, etc. The tests outlined in thes

37、e methods arelimited to conditions of approximately constant constraint andenvironment.5.3 The test results are highly sensitive to small changes inenvironmental conditions and thus require precise control oftest conditions and methods.5.4 The reproducibility of data will depend on the mannerwith wh

38、ich all test conditions are controlled. The effects ofaging or residual stress may significantly affect results, as mayvariations in material composition.6. Apparatus6.1 See the appropriate paragraph under each section.6.2 It is recommended that the equipment be located in adraft-free, constant-temp

39、erature environment, 63C (65F).7. Temperature Control and Measurement7.1 The test space (controlled temperature room, furnace, orcold box) should be capable of being maintained at a constanttemperature by a suitable automatic device. This is the mostimportant single factor in a stress relaxation tes

40、t since the stressrelaxation rate, dimensions, and constraint conditions of thespecimen are dependent upon the test temperature.Any type ofheating or cooling which permits close temperature control ofthe test space environment is satisfactory.7.2 The temperature should be recorded, preferably contin

41、u-ously or at least periodically. Temperature variations of thespecimens from the indicated nominal test temperature due toall causes, including cycling of the controller or position alongthe specimen gage length, should not exceed 6 3C (5F) or61/2 %, whichever is greater. These limits should applyi

42、nitially and for the duration of the test.7.3 The combined strain resulting from differential thermalexpansion (associated with normal temperature variation of theenvironment) between the test specimen and the constraint andother variations in the constraint (such as elastic follow up)should not exc

43、eed 60.000025 in./in. (mm/mm).7.4 Temperature measurement should be made in accor-dance with Practice E 139.8. Vibration Control8.1 Since stress relaxation tests are quite sensitive to shockand vibration, the test equipment and mounting should belocated so that the specimen is isolated from vibratio

44、n.9. Test Specimens9.1 The test specimens should be of a shape most appropri-ate for the testing method and end use. Wire may be tested inthe “as-received” condition and in the case of metal plate,sheet, strip, bar, or rod, they may be machined to the desiredshape.9.2 Residual stresses may significa

45、ntly alter the stress relax-ation characteristics of the material and care should be exer-cised in machining to prevent alteration of the residual stresses.9.3 Specimens for testing must have a uniform cross-sectionthroughout the gage length and meet the following tolerances:Nominal Diameter or Widt

46、hTolerance, % of Diameteror Width0.100 in. (2.5 mm) 60.50.250 in. (6.4 mm) 60.40.375 in. (9.5 mm) 60.30.500 in. (12.7 mm) 60.210. Environment10.1 If the test temperature is different from ambient,specimens previously fitted with strain gages or extensometersshould be exposed to the test temperature

47、for a period of timesufficient to obtain dimensional stability before starting thetests.10.2 The stress relaxation test may be started immediatelyupon achieving thermal equilibrium.11. Guide for Processing Test Data11.1 The remaining stress, relaxed stress, or applied forcemay be plotted against tim

48、e or log time. Log stress versus logtime plots may also be employed.11.2 For convenience in comparing the relative relaxationcharacteristics of materials, the ratio “Fraction Initial StressRelaxed” may be plotted against time. This ratio is thedifference between the initial stress and the remaining

49、stress atany time divided by the initial stress.12. Report12.1 It is recommended that the report include as much ofthe following information as is appropriate:12.1.1 Material Being Tested:12.1.1.1 Chemical composition,12.1.1.2 Microstructure,12.1.1.3 Mechanical properties,12.1.2 Specimen geometry,12.1.3 Testing machine or apparatus,12.1.4 Strain measurement method,12.1.5 Temperature measurement method,12.1.6 Atmosphere.12.1.7 Relaxation Test Data:12.1.7.1 Initial stress and strain data,12.1.7.2 Final stress and strain data,12.1.7.3 Plot of data.A. METHOD FOR CONDUCT