1、Designation: D7791 12Standard Test Method forUniaxial Fatigue Properties of Plastics1This standard is issued under the fixed designation D7791; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in
2、parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the determination of dynamicfatigue properties of plastics in uniaxial loading. This methodis applicable to rigid and sem
3、i-rigid plastics. Uniaxial loadingsystems with tension and compression capabilities are used todetermine these properties. Stress and strain levels are belowthe proportional limits of the material where the strains andstresses are relatively elastic.1.2 This test method can be used with two procedur
4、es:1.2.1 Procedure A, fatigue testing in tension.1.2.2 Procedure B, fatigue testing in compression, only forrigid plastics.1.3 Comparative tests can be run in accordance with eitherprocedure, provided that the procedure is found satisfactory forthe material being tested.1.4 The values stated in SI u
5、nits are to be regarded as thestandard. The values provided in parentheses are for informa-tion only.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 this standard to establish appro-priate safety and hea
6、lth practices and determine the applica-bility of regulatory limitations prior to use.NOTE 1There is no known ISO equivalent to this standard.2. Referenced Documents2.1 ASTM Standards:2D618 Practice for Conditioning Plastics for TestingD638 Test Method for Tensile Properties of PlasticsD695 Test Met
7、hod for Compressive Properties of RigidPlasticsD792 Test Methods for Density and Specific Gravity (Rela-tive Density) of Plastics by DisplacementD883 Terminology Relating to PlasticsD1505 Test Method for Density of Plastics by the Density-Gradient TechniqueD2839 Practice for Use of a Melt Index Stra
8、nd for Deter-mining Density of PolyethyleneD3479/D3479M Test Method for Tension-Tension Fatigueof Polymer Matrix Composite MaterialsD4883 Test Method for Density of Polyethylene by theUltrasound TechniqueD5947 Test Methods for Physical Dimensions of SolidPlastics SpecimensE4 Practices for Force Veri
9、fication of Testing MachinesE83 Practice for Verification and Classification of Exten-someter SystemsE466 Practice for Conducting Force Controlled ConstantAmplitude Axial Fatigue Tests of Metallic MaterialsE1942 Guide for Evaluating Data Acquisition SystemsUsed in Cyclic Fatigue and Fracture Mechani
10、cs Testing3. Terminology3.1 DefinitionsDefinitions of terms applying to this testmethod appear in Terminology D883.3.2 Definitions:3.2.1 compressive proportional limitmaximum elasticstress or strain exhibited by a material in compression asobserved in Test Method D695.3.2.2 mean strainalgebraic aver
11、age of the maximum andminimum strains in one cycle.3.2.3 mean stressalgebraic average of the maximum andminimum stresses in one cycle.3.2.4 R ratioratio of the minimum stress or strain to themaximum stress or strain that the specimen is loaded.3.2.5 tensile proportional limitmaximum elastic stress o
12、rstrain exhibited by a material in tension as observed in TestMethod D638.4. Summary of Test Method4.1 Procedure AA specimen of rectangular or circularcross section is gripped by the ends of the specimen, pulledequally in opposite directions, and released back to its originalposition or load. The sp
13、ecimen is cyclically loaded in tensionto a specific stress or strain level at a uniform frequency untilthe specimen ruptures or yields. From these tests, fatiguestrengths can be determined at specified numbers of cycles. Atleast, four different stress or strain levels are tested to construct1This te
14、st method is under the jurisdiction of ASTM Committee D20 on Plasticsand is the direct responsibility of Subcommittee D20.10.24 on Engineering andDesign Properties.Current edition approved April 1, 2012. Published May 2012. DOI:10.1520/D7791-12.2For referenced ASTM standards, visit the ASTM website,
15、 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 Conshohocken, PA 19428-2959, United States.a
16、 stress versus number of cycles to failure (S-N) curve or astrain versus number of cycles to failure (r-N) to determine theuniaxial endurance limit of the material in tension.4.2 Procedure BA specimen of rectangular or circularcross section is compressed equally from opposite directionsand released
17、back to its original position or load. The specimenis cyclically loaded in compression to a specific stress or strainlevel at a uniform frequency until the specimen ruptures oryields. From these tests, fatigue strengths can be determined atspecified numbers of cycles. At least, four different stress
18、 orstrain levels are tested to construct a stress versus number ofcycles to failure (S-N) curve or a strain versus number ofcycles to failure (r-N) to determine the uniaxial endurance limitof the material in compression.5. Significance and Use5.1 These fatigue tests are used to determine the effect
19、ofprocessing, surface condition, stress, and so forth, on thefatigue resistance of plastic material subjected to uniaxial stressfor relatively large numbers of cycles. The results can also beused as a guide for the selection of plastic materials for serviceunder conditions of repeated flexural stres
20、s.5.2 Properties can vary with specimen depth and test fre-quency. Test frequency can be 1-25 Hz but it is recommendedthat a frequency of 5 Hz or less be used.5.3 Material response in fatigue is not identical for allplastics. If a plastic does not exhibit an elastic region, wherestrain is reversible
21、, plastic deformation will occur duringfatigue testing, causing the amplitude of the programmed loador deformation to change during testing. In this situation,caution shall be taken when using the results for design as theyare generally not indicative of the true fatigue properties of thematerial.5.
22、4 The results of these fatigue tests are suitable for appli-cation in design only when the specimen test conditionsrealistically simulate service conditions or some methodologyof accounting for service conditions is available and clearlydefined.5.5 This procedure accommodates various specimen prepa-
23、ration techniques. Comparison of results obtained from speci-mens prepared in different manners shall not be consideredcomparable unless equivalency has been demonstrated.6. Apparatus6.1 Testing MachineThe testing machine shall essentiallymeet the specifications set forth by Test Method D638 forProc
24、edure A and Test Method D695 for Procedure B except asdescribed below. The error in the deflection measuring systemshall not exceed 60.5 % of the maximum deflection. Themachine shall be able to execute sinusoidal or square/trapezoidal load or deflection programs at the specified testfrequency and ma
25、intain an error of 61 % or less of themaximum programmed load or deflection.6.2 Recording EquipmentCalibrated equipment must beused to record the following information during testing at adata acquisition rate and filter in accordance with GuideE1942:6.2.1 Load versus time6.2.2 Change in length versu
26、s time6.2.3 Number of cycles6.3 MicrometersApplicable apparatus in accordance withTest Methods D5947 shall be used to measure the width andthickness or diameter of the test specimen.6.4 Extensometers/CompressometerA suitable instrumentshall be used for determining the distance between twodesignated
27、points within the gage length of the test specimen asthe specimen is stretched or compressed. This instrument shallbe essentially free of inertia at the specified speed of testing.Extensometers shall be classified and their calibration periodi-cally verified in accordance with Practice E83.Anextenso
28、meter/compressometer with a maximum strain error of0.0002 mm/mm (in./in.) that automatically and continuouslyrecords shall be used. Crosshead stroke of the testing machinecan also be used to record deflection as long as it meets thesame error requirements listed above.6.5 Supporting Jig (Procedure B
29、 Only)A supporting jigcan be used for thin specimens or high compressive loads andshall be in accordance with Test Method D695.7. Sampling, Test Specimens, and Test Units7.1 Specimens shall be taken from samples that accuratelyrepresent the material or design that is being tested.7.2 The specimens c
30、an be cut from sheets, plates, or moldedshapes, or can be molded to the desired finished dimensions.The actual dimensions used for calculations shall be measuredin accordance with Test Methods D5947.7.3 Procedure ASpecimen dimensions, shape, surfaceconditions, and limitations shall be in compliance
31、with TestMethod D638.7.4 Procedure BSpecimen dimensions, shape, surfaceconditions, and limitations shall be in compliance with TestMethod D695.7.5 Specimens cut from non-uniform thick molded partsections shall be machined equally and minimally on both sidesto create a uniform thickness in the gage.
32、It must be noted thatmachining the thickness of plastic can change the mechanicalproperties and caution shall be taken when applying the resultsto design.7.6 It is recommended that density measurements be takenfrom each sample in the gage in accordance with Test MethodsD792, Test Method D1505, Pract
33、ice D2839, or Test MethodD4883 to ensure that the process used to fabricate the speci-mens creates consistent and uniform material.8. Number of Test Specimens8.1 At least three test specimens shall be tested at each ofthe four stress or strain levels (minimum of twelve specimensfor each test). For a
34、dditional sample sizes consult Table 1 ofTest Method D3479/D3479M.8.2 In the case of anisotropic materials, a single directionshall be chosen and maintained for all stress or strain levels.9. Calibration and Standardization9.1 All equipment shall be calibrated in accordance with themanufacturers spe
35、cifications and Practice E4 or Practice E83,Class B-2.9.2 Specimens for each test shall be identically processedand prepared for accurate results.D7791 12210. Conditioning10.1 ConditioningCondition the test specimens in accor-dance with Procedure A of Practice D618 unless otherwisespecified by contr
36、act or the relevant ASTM material specifica-tion. Condition time is specified as a minimum. Temperatureand humidity tolerances shall be in accordance with PracticeD618 unless specified differently by contract or materialspecification.10.2 Test ConditionsConduct the tests at the same tem-perature and
37、 humidity used for conditioning with tolerances inaccordance with Practice D618 unless otherwise specified bycontract or the relevant ASTM material specification.11. Procedure11.1 Specimen Measurements:11.1.1 Procedure ATake measurements of all specimens.Measure and record the depth and width, or di
38、ameter of thespecimen to the nearest 0.025 mm (0.001 in) at several pointsalong its length. The minimum measurement shall be used forcalculations. These measurements shall be made in accordancewith Test Methods D5947 and Test Method D638.11.1.2 Procedure BTake measurements of all specimens.Measure a
39、nd record the depth and width, or diameter of thespecimen to the nearest 0.025 mm (0.001 in) at several pointsalong its length. The minimum measurement shall be used forcalculations. These measurements shall be made in accordancewith Test Methods D5947 and Test Method D695.11.2 Apparatus Setup:11.2.
40、1 Place the test specimen in the test machine, takingcare to align the center line of its axis with the loading axis inaccordance with 8.2 of Practice E466.11.2.2 If needed, align the jig support to the load axis (forProcedure B only).11.2.3 Place the extensometer/compressometer on the speci-men.11.
41、3 Test Program Setup:11.3.1 Program the testing machine to follow a dynamicload (sinusoidal, square, or trapezoidal) or deflection waveformwith the amplitude set to the selected load or deflection level.Unless specified, the default waveform shall be sinusoidal. Theselected level shall be below the
42、proportional limit of thematerial as determined by Test Methods D638 (ProcedureA) orTest Method D695 (Procedure B). Set the test frequency(maximum 25 Hz).11.3.2 Stress Level SelectionAll stress levels shall notexceed the proportional limit of the material. The associatedload of a selected stress lev
43、el can be determined by thefollowing equation:P 5sA (1)where:P = load in N,s = stress level in MPa, andA = gage cross sectional area (mm2).11.3.3 Strain Level SelectionAll strain levels shall notexceed the proportional limit of the material. The associatedextension or compression of a selected strai
44、n level can bedetermined by the following equation:D 5 rL (2)where:D = extension or compression in mm,r = strain (mm/mm), andL = gage length in mm.11.3.4 Set the end of test to 107cycles.11.4 Testing:11.4.1 The test is recorded continuously using a computer-ized data retrieval system which records t
45、he force and deflec-tion feedback.11.4.2 Testing continues until the specimen fails by one ofthe following criteria:11.4.2.1 Ruptured in the gage area,11.4.2.2 Yielded in the gage area. For load controlled tests,the specimen has yielded if the maximum deflection increases.For strain controlled tests
46、, the specimen has yielded if themaximum load decreases.11.4.2.3 Reached end of test by completing the number ofprogrammed cycles (about 107cycles).11.4.3 Repeat the above steps, using untested specimens, atleast three times at each of the four stress or strain levels. Foradditional sample sizes con
47、sult Table 1 of Test MethodD3479/D3479M.11.5 RetestsThe test must be repeated if the specimenfails outside the gage area or if frequency or amplitude is notmaintained throughout the test.12. Plotting and Interpretation of Results12.1 Plotting Results:12.1.1 S-N CurvePlot the maximum stress level ver
48、susthe logarithm of the number of cycles to failure (Nf). Plot alltest data and define the S-N curve by creating a best-fit curve.Indicate on the curve specimens that did not fail by an arrowdirected away from the plotted point in the direction ofincreasing cycles.12.1.2 r-N CurvePlot the maximum st
49、rain level versus thelogarithm of the number of cycles to failure (Nf). Plot all testdata and define the r-N curve by creating a best-fit curve.Indicate on the curve specimens that did not fail by an arrowdirected away from the plotted point in the direction ofincreasing cycles.12.1.3 As the number of specimens tested at the given stressor strain levels increases, use probability plots and otherstatistical analysis to create mean S/r-N curves.12.2 Interpretation of ResultsIf the S-N or r-N curvebecomes horizontally asymptotic to a constant stress or strain,t
