1、Designation: E328 13Standard Test Methods forStress Relaxation for Materials and Structures1This standard is issued under the fixed designation E328; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A numb
2、er in parentheses indicates the year of last reapproval. Asuperscript epsilon () 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 stan
3、dard 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, uniform c
4、ompression, bending or torsion. To choose from among these types ofstress, three 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 component. For ex
5、ample,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 beaccurately
6、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 buckling.Therefore
7、, 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-sectional area.Str
8、ains 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 bendingtests.
9、 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. Scope*NOTE 1The method of testing for the stress relaxation of plastics hasbeen w
10、ithdrawn from this standard, and the responsibility has beentransferred to Practice D2991.1.1 These test methods cover the determination of the timedependence of stress (stress relaxation) in materials andstructures under conditions of approximately constantconstraint, constant environment, and negl
11、igible vibration.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. 1, 2013. Published May 2014. Originallyapproved in 1967. Last previous approved in 2008
12、as E32802(2008). DOI:10.1520/E0328-13.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1In the procedures recommended, the material or structure isinitially constrained by
13、 externally applied forces, and thechange in the external force necessary to maintain this con-straint is determined as a function of time.1.2 Specific methods for conducting stress relaxation testson materials subjected to tension, compression, bending andtorsion stresses are described in Parts A,
14、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 periods required forthese types of tests are often unsuited for routine testing or forspecification in the purchas
15、e of material. However, these testsare valuable tools in obtaining practical design information onthe stress relaxation of materials subjected to the conditionsenumerated, and in investigations of the fundamental behaviorof materials.1.4 UnitsThe values stated in inch-pound units are to beregarded a
16、s standard. The values given in parentheses aremathematical conversions to SI units that are provided forinformation only and are not considered standard.1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of t
17、his 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:2D2991 Test Method for Stress-Relaxation of Plastics (With-drawn 1990)3E4 Practices for Force Verification of Testing M
18、achinesE6 Terminology Relating to Methods of Mechanical TestingE8/E8M Test Methods for Tension Testing of Metallic Ma-terialsE9 Test Methods of Compression Testing of Metallic Mate-rials at Room TemperatureE83 Practice for Verification and Classification of Exten-someter SystemsE139 Test Methods for
19、 Conducting Creep, Creep-Rupture,and Stress-Rupture Tests of Metallic MaterialsE1012 Practice for Verification of Testing Frame and Speci-men Alignment Under Tensile and Compressive AxialForce Application3. Terminology3.1 Definitions:Terms common to mechanical testing.3.1.1 stress relaxation, nthe t
20、ime-dependent decrease instress in a solid under given constraint conditions.3.1.2 initial stress, 0, FL2,nthe stress introduced intoa specimen by imposing the given constraint conditions beforestress relaxation begins.3.1.2.1 DiscussionThis is sometimes call instantaneousstress.3.1.3 zero time, t0,
21、nthe time when the given stress orconstraint conditions are initially obtained in a stress relaxationtest.3.1.4 relaxation rate FL2T1,nthe absolute value ofthe slope of the relaxation curve at a given time.3.1.4.1 DiscussionArelaxation curve is a plot of either theremaining time or relaxed stress as
22、 a function of time.3.1.5 relaxed stress FL2,nthe initial stress minus theremaining stress at a given time during a stress-relaxation test.3.1.6 remaining stress FL2,nthe stress remaining at agiven time during a stress-relaxation test.3.2 Definitions of Terms Specific to This Standard:3.2.1 spherome
23、teran instrument used to measure circularor spherical curvature.3.2.2 indicated nominal temperature or indicatedtemperaturethe temperature that is indicated by thetemperature-measuring device.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at ser
24、viceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3The last approved version of this historical standard is referenced onwww.astm.org.E328 1324. Summary of Test Methods4.1 In each of the various methods of stress applic
25、ationdescribed in the applicable specific sections, the specimen issubjected to an increasing force 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
26、the initial force(moment, torque) as measured at zero time, the specimengeometry, and the appropriate 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), period
27、ically (4.1.2), orby elastic springback at the end of the test period 4.1.3.4.1.1 Readings 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 ela
28、sticity to measure the quantities involved. Therefore, it isnecessary that when using such devices, to maintain the total strainconstant within an upper and lower bound as shown in Fig. 2.4.1.2 The force required to lift the specimen just free of oneor more constraints during the test period is meas
29、ured.4.1.3 The elastic springback is measured after removing thetest stress at the end 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 aft
30、er various time intervals, if it can be demon-strated for a given material that identical results are obtained ineither using untested 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 d
31、esigning mostmechanically fastened joints to assure the permanent tightnessof bolted or riveted assemblies, press or shrink-fit components,rolled-in tubes, etc. Other applications include predicting thedecrease in the tightness of gaskets, in the hoop stress ofsolderless wrapped connections, in the
32、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, cracks,holes, fillets, etc., may be predicted from stress relaxation data.Such test data are also u
33、seful 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 these methods arelimited to conditions of approximately constant constraint andenvironment.5.3 The g
34、eneral stress relaxation test is performed byisothermally applying a force to a specimen with fixed value ofconstraint. The constraint is maintained constant, and theconstraining force is determined as a function of time. Themajor problem in the stress relaxation test is that constantconstraint can
35、be very difficult to maintain. The effects on testresults are very significant, and considerable attention must beFIG. 1 Characteristic Behavior During Force Application Periodin a Relaxation Test (a)FIG. 1 Characteristic Behavior During Force Application Periodin a Relaxation Test (b) (continued)E3
36、28 133given 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.5.4 There are many methods of performing the stressrelaxation test, each with a different starting
37、procedure.However, the constraint is usually obtained initially by theapplication of an external force 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, 0, exceeds the proportional lim
38、it. Some testingmachines, while reaching the constraint value, do not produceeither a constant force application rate or constant strain rate,but something in between. However, the general characteristicsof the data will be similar to those indicated. The stress-application rate in either case shoul
39、d be reasonably rapid, butwithout impact or vibration, so that any relaxation during thestress application period will be small.5.5 The stress relaxation test is considered to have started atzero time, t0in Fig. 1.This is the reference time from which theobserved reduction in force to maintain const
40、ant constraint isbased. Selection of this time does not imply that the forceapplication procedure or period, or both, are not significant testparameters. These must always be considered in the applica-tion of the data.6. Apparatus6.1 See the appropriate paragraph under each section.6.2 The equipment
41、 should be located in a draft-free,constant-temperature environment, 6 5F (3C).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 mostim
42、portant single factor in a stress relaxation test since the stressrelaxation rate, dimensions, and constraint conditions of thespecimen are dependent upon the test temperature.Any type ofheating or cooling that permits close temperature control of thetest space environment is satisfactory.7.2 The te
43、mperature should be recorded, preferably continu-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 gauge length, should not exceed 6 5F (3C) or61/2 %,
44、whichever is greater. These limits should applyinitially 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 cons
45、traint (such as elastic follow up)should not exceed 60.000025 in./in. (mm/mm).7.4 Temperature measurement should be made in accor-dance with Practice E139.8. Vibration Control8.1 Since stress relaxation tests are quite sensitive to shockand vibration, the test equipment and mounting should belocated
46、 so that the specimen is isolated from vibration.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. Metal plate, sheet, strip, bar, or rodmay be machined to the desired shape.9.2 Residua
47、l stresses may significantly 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 gauge length and meet the following tolerances
48、:FIG. 2 Derivation of Stress-Relaxation Curve from ContinuousRelaxation TechniqueE328 134Nominal Diameter or WidthTolerance, % of Diameteror Width0.100 in. (2.5 mm) 0.50.250 in. (6.4 mm) 0.40.375 in. (9.5 mm) 0.30.500 in. (12.7 mm) 0.210. Environment10.1 If the test temperature is different from amb
49、ient,specimens previously fitted with strain gages or extensometersshould be exposed to the test temperature 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 time or log time. Log stress versus logtime plots may also be employed.11.2 For convenience in comparing the relative relaxationcharacteristics
