1、Designation: D7791 12D7791 17Standard 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 nu
2、mber in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope Scope*1.1 This test method covers the determination of dynamic fatigue properties of plastics in uniaxial loading. This method isapplicable
3、to rigid and semi-rigid plastics. Uniaxial loading systems with tension and compression capabilities are used todetermine these properties. Stress and strain levels are below the proportional limits of the material where the strains and stressesare relatively elastic.1.2 This test method can be used
4、 with two procedures:1.2.1 Procedure A, fatigue testing in tension.1.2.2 Procedure B, fatigue testing in compression, only for rigid plastics.1.3 Comparative tests can be run in accordance with either procedure, provided that the procedure is found satisfactory for thematerial being tested.1.4 The v
5、alues stated in SI units are to be regarded as the standard. The values provided in parentheses are for information only.1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish approp
6、riate safety and health practices and determine the applicability of regulatorylimitations prior to use.NOTE 1There is no known ISO equivalent to this standard.1.6 This international standard was developed in accordance with internationally recognized principles on standardizationestablished in the
7、Decision on Principles for the Development of International Standards, Guides and Recommendations issuedby the World Trade Organization Technical Barriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2D618 Practice for Conditioning Plastics for TestingD638 Test Method for Tensi
8、le Properties of PlasticsD695 Test Method for Compressive Properties of Rigid PlasticsD792 Test Methods for Density and Specific Gravity (Relative Density) of Plastics by DisplacementD883 Terminology Relating to PlasticsD1505 Test Method for Density of Plastics by the Density-Gradient TechniqueD2839
9、 Practice for Use of a Melt Index Strand for Determining Density of PolyethyleneD3479/D3479M Test Method for Tension-Tension Fatigue of Polymer Matrix Composite MaterialsD4883 Test Method for Density of Polyethylene by the Ultrasound Technique (Withdrawn 2017)3D5947 Test Methods for Physical Dimensi
10、ons of Solid Plastics SpecimensE4 Practices for Force Verification of Testing MachinesE83 Practice for Verification and Classification of Extensometer SystemsE466 Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic MaterialsE1942 Guide for Evaluating Data Acqu
11、isition Systems Used in Cyclic Fatigue and Fracture Mechanics Testing1 This test method is under the jurisdiction of ASTM Committee D20 on Plastics and is the direct responsibility of Subcommittee D20.10 on Mechanical Properties.Current edition approved April 1, 2012May 1, 2017. Published May 2012Ma
12、y 2017. DOI:10.1520/D7791-12.Originally approved in 2012. Last previous edition approvedin 2012 as D7791 - 12. DOI:10.1520 D7791-17.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume informa
13、tion, refer to the standards Document Summary page on the ASTM website.3 The last approved version of this historical standard is referenced on www.astm.org.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made
14、 to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.*A Su
15、mmary 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 States13. Terminology3.1 DefinitionsDefinitions of terms applying to this test method appear in Terminology D883.3.2 Definitions:3.2.
16、1 compressive proportional limitmaximum elastic stress or strain exhibited by a material in compression as observed inTest Method D695.3.2.2 mean strainalgebraic average of the maximum and minimum strains in one cycle.3.2.3 mean stressalgebraic average of the maximum and minimum stresses in one cycl
17、e.3.2.4 R ratioratio of the minimum stress or strain to the maximum stress or strain that the specimen is loaded.3.2.5 tensile proportional limitmaximum elastic stress or strain exhibited by a material in tension as observed in Test MethodD638.4. Summary of Test Method4.1 Procedure AA specimen of re
18、ctangular or circular cross section is gripped by the ends of the specimen, pulled equally inopposite directions, and released back to its original position or load. The specimen is cyclically loaded in tension to a specificstress or strain level at a uniform frequency until the specimen ruptures or
19、 yields. From these tests, fatigue strengths can bedetermined at specified numbers of cycles. At least, four different stress or strain levels are tested to construct a stress versusnumber of cycles to failure (S-N) curve or a strain versus number of cycles to failure (r-N) to determine the uniaxial
20、 endurancelimit of the material in tension.4.2 Procedure BA specimen of rectangular or circular cross section is compressed equally from opposite directions andreleased back to its original position or load. The specimen is cyclically loaded in compression to a specific stress or strain levelat a un
21、iform frequency until the specimen ruptures or yields. From these tests, fatigue strengths can be determined at specifiednumbers of cycles. At least, four different stress or strain levels are tested to construct a stress versus number of cycles to failure(S-N) curve or a strain versus number of cyc
22、les to failure (r-N) to determine the uniaxial endurance limit of the material incompression.5. Significance and Use5.1 These fatigue tests are used to determine the effect of processing, surface condition, stress, and so forth, on the fatigueresistance of plastic material subjected to uniaxial stre
23、ss for relatively large numbers of cycles. The results can also be used as aguide for the selection of plastic materials for service under conditions of repeated flexural stress.5.2 Properties can vary with specimen depth and test frequency. Test frequency can be 1-25 Hz but it is recommended that a
24、frequency of 5 Hz or less be used.5.3 Material response in fatigue is not identical for all plastics. If a plastic does not exhibit an elastic region, where strain isreversible, plastic deformation will occur during fatigue testing, causing the amplitude of the programmed load or deformation tochang
25、e during testing. In this situation, caution shall be taken when using the results for design as they are generally not indicativeof the true fatigue properties of the material.5.4 The results of these fatigue tests are suitable for application in design only when the specimen test conditions realis
26、ticallysimulate service conditions or some methodology of accounting for service conditions is available and clearly defined.5.5 This procedure accommodates various specimen preparation techniques. Comparison of results obtained from specimensprepared in different manners shall not be considered com
27、parable unless equivalency has been demonstrated.6. Apparatus6.1 Testing MachineThe testing machine shall essentially meet the specifications set forth by Test Method D638 for ProcedureAand Test Method D695 for Procedure B except as described below. The error in the deflection measuring system shall
28、 not exceed60.5 % of the maximum deflection. The machine shall be able to execute sinusoidal or square/trapezoidal load or deflectionprograms at the specified test frequency and maintain an error of 61 % or less of the maximum programmed load or deflection.6.2 Recording EquipmentCalibrated equipment
29、 must be used to record the following information during testing at a dataacquisition rate and filter in accordance with Guide E1942:6.2.1 Load versus time6.2.2 Change in length versus time6.2.3 Number of cycles6.3 MicrometersApplicable apparatus in accordance with Test Methods D5947 shall be used t
30、o measure the width andthickness or diameter of the test specimen.6.4 Extensometers/CompressometerA suitable instrument shall be used for determining the distance between two designatedpoints within the gagegauge length of the test specimen as the specimen is stretched or compressed. This instrument
31、 shall beD7791 172essentially free of inertia at the specified speed of testing. Extensometers shall be classified and their calibration periodicallyverified in accordance with Practice E83. An extensometer/compressometer with a maximum strain error of 0.0002 mm/mm(in./in.) that automatically and co
32、ntinuously records shall be used. Crosshead stroke of the testing machine can also be used torecord deflection as long as it meets the same error requirements listed above.6.5 Supporting Jig (Procedure B Only)A supporting jig can be used for thin specimens or high compressive loads and shallbe in ac
33、cordance with Test Method D695.7. Sampling, Test Specimens, and Test Units7.1 Specimens shall be taken from samples that accurately represent the material or design that is being tested.7.2 The specimens can be cut from sheets, plates, or molded shapes, or can be molded to the desired finished dimen
34、sions. Theactual dimensions used for calculations shall be measured in accordance with Test Methods D5947.7.3 Procedure ASpecimen dimensions, shape, surface conditions, and limitations shall be in compliance with Test MethodD638.7.4 Procedure BSpecimen dimensions, shape, surface conditions, and limi
35、tations shall be in compliance with Test MethodD695.7.5 Specimens cut from non-uniform thick molded part sections shall be machined equally and minimally on both sides to createa uniform thickness in the gage.gauge. It must be noted that machining the thickness of plastic can change the mechanicalpr
36、operties and caution shall be taken when applying the results to design.7.6 It is recommended that density measurements be taken from each sample in the gagegauge in accordance with Test MethodsD792, Test Method D1505, Practice D2839, or Test Method D4883 to ensure that the process used to fabricate
37、 the specimenscreates consistent and uniform material.8. Number of Test Specimens8.1 At least three test specimens shall be tested at each of the four stress or strain levels (minimum of twelve specimens for eachtest). For additional sample sizes consult Table 1 of Test Method D3479/D3479M.8.2 In th
38、e case of anisotropic materials, a single direction shall be chosen and maintained for all stress or strain levels.9. Calibration and Standardization9.1 All equipment shall be calibrated in accordance with the manufacturers specifications and Practice E4 or Practice E83,Class B-2.9.2 Specimens for e
39、ach test shall be identically processed and prepared for accurate results.10. Conditioning10.1 ConditioningCondition the test specimens in accordance with Procedure A of Practice D618 unless otherwise specifiedby contract or the relevant ASTM material specification. Condition time is specified as a
40、minimum. Temperature and humiditytolerances shall be in accordance with Practice D618 unless specified differently by contract or material specification.10.2 Test ConditionsConduct the tests at the same temperature and humidity used for conditioning with tolerances inaccordance with Practice D618 un
41、less otherwise specified by contract 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 diameter of the specimento the nearest 0.025 mm (0.001 in) at several points along
42、 its length. The minimum measurement shall be used for calculations.These measurements shall be made in accordance with Test Methods D5947 and Test Method D638.11.1.2 Procedure BTake measurements of all specimens. Measure and record the depth and width, or diameter of thespecimen to the nearest 0.02
43、5 mm (0.001 in) at several points along its length. The minimum measurement shall be used forcalculations. These measurements shall be made in accordance with Test Methods D5947 and Test Method D695.11.2 Apparatus Setup:11.2.1 Place the test specimen in the test machine, taking care to align the cen
44、ter 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 (for Procedure B only).11.2.3 Place the extensometer/compressometer on the specimen.11.3 Test Program Setup:11.3.1 Program the testing machine to follow a dynami
45、c load (sinusoidal, square, or trapezoidal) or deflection waveform withthe amplitude set to the selected load or deflection level. Unless specified, the default waveform shall be sinusoidal. The selectedD7791 173level shall be below the proportional limit of the material as determined byTest Methods
46、 D638 (ProcedureA) orTest Method D695(Procedure B). Set the test frequency (maximum 25 Hz).11.3.2 Stress Level SelectionAll stress levels shall not exceed the proportional limit of the material. The associated load ofa selected stress level can be determined by the following equation:P 5A (1)where:P
47、 5 load in N, 5 stress level in MPa, andA 5 gage cross sectional area mm2!.P 5A (1)where:P = load in N, = stress level in MPa, andA = gauge cross sectional area (mm2).11.3.3 Strain Level SelectionAll strain levels shall not exceed the proportional limit of the material. The associated extensionor co
48、mpression of a selected strain level can be determined by the following equation:D 5rL (2)where:D 5 extension or compression in mm,r 5 strain mm/mm!,andL 5 gage length in mm.D 5rL (2)where:D = extension or compression in mm,r = strain (mm/mm), andL = gauge length in mm.11.3.4 Set the end of test to
49、107 cycles.11.4 Testing:11.4.1 The test is recorded continuously using a computerized data retrieval system which records the force and deflectionfeedback.11.4.2 Testing continues until the specimen fails by one of the following criteria:11.4.2.1 Ruptured in the gagegauge area,11.4.2.2 Yielded in the gagegauge area. For load controlled tests, the specimen has yielded if the maximum deflection increases.For strain controlled tests, the specimen has yielded if the maximum load decreases.11.4.2.3 Reached end of test by completing the number of programmed
