1、Designation: D7972 14Standard Test Method forFlexural Strength of Manufactured Carbon and GraphiteArticles Using Three-Point Loading at Room Temperature1This standard is issued under the fixed designation D7972; the number immediately following the designation indicates the year oforiginal adoption
2、or, in the case of revision, the year of last revision. A number in 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 determination of the flexuralstrength of manufactured
3、 carbon and graphite articles using asquare, rectangular or cylindrical beam in three-point loadingat room temperature.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.3 This standard does not purport to address all of thes
4、afety 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 regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2C78 Test Method for Flexural
5、Strength of Concrete (UsingSimple Beam with Third-Point Loading)C559 Test Method for Bulk Density by Physical Measure-ments of Manufactured Carbon and Graphite ArticlesC1161 Test Method for Flexural Strength of AdvancedCeramics at Ambient TemperatureC1239 Practice for Reporting Uniaxial Strength Dat
6、a andEstimating Weibull Distribution Parameters for AdvancedCeramicsC1322 Practice for Fractography and Characterization ofFracture Origins in Advanced CeramicsD7775 Guide for Measurements on Small Graphite Speci-mensE4 Practices for Force Verification of Testing MachinesE691 Practice for Conducting
7、 an Interlaboratory Study toDetermine the Precision of a Test Method3. Terminology3.1 Definitions:3.1.1 flexural strengtha measure of the ultimate loadcarrying capacity of a specified beam in bending.3.1.2 grainin manufactured (synthetic) carbon andgraphite, particle of filler material (usually coke
8、 or graphite) inthe starting mix formulation. Also referred to as granularmaterial, filler particle, or aggregate material.4. Significance and Use4.1 This test method provides a framework for materialdevelopment, quality control, characterization, and design datageneration purposes. The user needs t
9、o assess the applicabilityof the method on the specific material and for the intended use,as shown by the interlaboratory study.4.2 This test method determines the maximum loading on agraphite specimen with simple beam geometry in threepointbending, and it provides a means for the calculation of fle
10、xuralstrength at ambient temperature and environmental conditions.4.3 The flexure stress is computed based on simple beamtheory with assumptions that the material is isotropic andhomogeneous, the moduli of elasticity in tension and compres-sion are identical, and the material is linearly elastic. Fo
11、rmaterials with large grains, the minimum specimen dimensionshould be significantly larger than the maximum grain size (seeGuide D7775).4.4 Flexural strength of a group of test specimens isinfluenced by several parameters associated with the testprocedure. Such factors include the loading rate, test
12、environment, specimen size, specimen preparation, and testfixtures. Specimen sizes and fixtures should be chosen toreduce errors due to material variability or testing parameters,such as friction and non-parallelism of specimen surfaces.4.5 The flexural strength of a manufactured graphite orcarbon m
13、aterial is dependent on both its inherent resistance tofracture and the size and severity of flaws. Variations in thesecause a natural scatter in test results for a sample of testspecimens. Fractographic analysis of fracture surfaces, al-though beyond the scope of this standard, is highly recom-mend
14、ed for all purposes, especially if the data will be used fordesign as discussed in Practices C1239 and C1322.1This test method is under the jurisdiction of ASTM Committee D02 onPetroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility ofSubcommittee D02.F0 on Manufactured Car
15、bon and Graphite Products.Current edition approved Dec. 1, 2014. Published February 2015. DOI: 10.1520/D7972-14.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the
16、standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States14.6 The three-point test configuration exposes only a verysmall portion of the specimen to the maximum stress.Therefore, three-point fl
17、exural strengths are likely to be muchgreater than four-point flexural strengths. Three-point flexurehas some advantages. It uses simpler test fixtures, allowingsmall specimen testing and fracture toughness measurements.However, four-point flexure is preferred and recommended formost characterizatio
18、n purposes.5. Apparatus5.1 LoadingSpecimens may be loaded in any suitabletesting machine provided that uniform rates of loading can bemaintained. The testing machine shall be equipped with ameans for retaining read-out of the maximum force applied tothe specimen. The accuracy of the testing machine
19、shall be inaccordance with Practice E4.5.2 FixtureThe three-point loading fixture shall consist ofbearing blocks or cylindrical bearings spaced in a three-pointloading configuration (see Test Method C1161). A hardenedsteel bearing block or its equivalent is necessary to preventdistortion of the load
20、ing member.5.2.1 The fixture shall ensure that forces applied to the beamare normal only and without eccentricity through the use ofspherical bearing blocks (see Test Method C78) or articulatingroller bearing assemblies (see 5.3 and Test Method C1161).5.2.2 The cylindrical bearing length shall be su
21、ch that thetest specimen width is fully supported, and the cylindricalbearing diameter shall be 0.75 to 1.5 times the specimenthickness/diameter.5.2.3 The lower support bearings shall be free to rotate inorder to relieve frictional constraints. The middle load bearingof the three-point fixture need
22、not rotate. The three bearingsshall be parallel over their length. The load application bearing(upper bearing) shall be centered with respect to the two lowersupport bearings within 60.10 mm.5.3 The directions of loads and reactions may be maintainedparallel by judicious use of linkages, rocker bear
23、ings, andflexure plates. Eccentricity of loading can be avoided by theuse of spherical bearing blocks or articulating roller bearings.5.3.1 Semi-articulated Three-point FixtureSpecimens pre-pared in accordance with the parallelism requirements of 6.1may be tested in a semi-articulated fixture. The m
24、iddle bearingshall be fixed and not free to roll. The two outer bearings shallbe parallel to each other over their length. The two outerbearings shall articulate together as a pair to match thespecimen surface, or the middle bearing shall articulate tomatch the specimen surface. All three bearings s
25、hall restuniformly and evenly across the specimen surface. The fixtureshall be designed to apply equal load to the two outer bearings.5.3.2 Fully-articulated Three-point FixtureSpecimensthat do not meet the parallelism requirements of 6.1 shall betested in a fully-articulated fixture. Well-machined
26、specimensmay also be tested in a fully-articulating fixture. The twosupport (outer) bearings shall be free to roll outwards. Themiddle bearing shall not roll. Any two of the bearings shall becapable of articulating to match the specimen surface.All threebearings shall rest uniformly and evenly acros
27、s the specimensurface. The fixture shall be designed to apply equal load to thetwo outer bearings.6. Test Specimen6.1 Specimen SizeThe size and geometry of the testspecimens used in this interlaboratory study are shown in Table1. It is recommended that the size of the test specimen isselected such t
28、hat the minimum dimension of the specimen isgreater than 5 times the largest particle dimension. It isrecommended that the test specimen has a length to thickness/diameter ratio of at least 6, and a width to thickness ratio notgreater than 2.6.1.1 For test specimens that do not meet this ratio forst
29、rength testing, see Ref (1)3and Guide D7775.6.2 PreparationThe test specimen shall be prepared toyield a parallelepiped of square or rectangular cross section ora cylinder. The faces of the parallelepiped specimens shall beparallel and flat within 0.025 mm/mm. In addition, the sampleshaving a maximu
30、m particle size less than 0.15 mm in diametermust be finished so that the surface roughness is less than 3 mRa. Sample edges should be free from visible flaws and chips.NOTE 1For ease of machining to conventional standards, 3 m Ra isequivalent to 125 in. AA. For finishing of specimens with maximumpa
31、rticle sizes of greater than 0.150 mm, grain structure and porosity canlimit the accurate measurement of roughness. In these cases, the surfaceroughness should be visually equivalent to 3 m Ra as estimated based onthe visible surface of the graphite.NOTE 2Surface preparation of test specimens can in
32、troduce machin-ing microcracks which may have a pronounced effect on flexural strength.Machining damage imposed during specimen preparation can be either arandom interfering factor, or an inherent part of the strength characteristicto be measured. With proper care and good machining practice, it isp
33、ossible to obtain fractures during strength testing from the materialsnatural flaws. Surface preparation can also lead to residual stresses.Universal or standardized test methods of surface preparation do not exist.It should be understood that final machining steps may or may not negatemachining dam
34、age introduced during the early course or intermediatemachining.6.3 MeasurementsAll dimensions shall be measured to anaccuracy of 0.5 % (see Test Method C559).6.4 OrientationThe specimen shall be marked or other-wise identified to denote its orientation with respect to theparent stock.3The boldface
35、numbers in parentheses refer to the list of references at the end ofthis standard.TABLE 1 Specimen Sizes and Testing Configurations in theInterlaboratory StudyConfigurationNominalSpecimenSize(mm)SpecimenThickness, d(mm)SupportSpan, L(mm)CrossheadSpeed,mm/s(mm/m)I 10106410 50.00 0.0042 (0.25)II 9.5 4
36、.8 644.8 50.00 0.0087 (0.52)III 10 64 10 50.00 0.0042 (0.25)IV 252515025 100.00 0.0067 (0.40)V 25 150 25 100.00 0.0067 (0.40)D7972 1426.5 DryingEach specimen must be dried in a vented ovenat 110C for a period of 2h (see Test Method C559). Thesample must then be stored in a dry environment or adesicc
37、ator and held there prior to testing.NOTE 3Water, either in the form of liquid or as humidity in air, canhave an effect on flexural mechanical behavior. Excessive adsorbed watercan result in a reduced failure stress due to a decrease in fracture surfaceenergy.7. Procedure7.1 Place test specimens on
38、their appropriate fixtures inspecific testing configurations, as shown in Fig. 1. A fullyarticulating fixture is required if the specimen parallelismrequirements cannot be met.7.2 Position the specimen on the support bearings on thethree-point test fixture so that there is an approximately equalamou
39、nt of overhang of the specimen beyond the supportbearings.7.3 Position the specimen front to back so that the specimenis directly centered below the axis of the applied load.7.4 The load bearing shall make contact with the uppersurface of the test specimen. The support bearing blocks mustbe parallel
40、 to each other and perpendicular to the test surfaces.7.5 Load the specimen at a uniform rate such that breakageoccurs from flexure rather than impact. As guideline, breakingshould not occur in less than 10 s.7.6 Preserve the fractured specimens until released by theresponsible engineer.8. Test Data
41、 Record8.1 All dimensions measured according to Test MethodC559 shall be recorded.8.2 Record the test duration, the break force and fracturelocation.8.3 The load at failure must be recorded to an accuracy ofbetter than 62 % of the full-scale value. A full-scale value of5 kN would require recording t
42、o an accuracy of at least6100 N.9. Calculation9.1 If the fracture occurs directly underneath the loadbearing, calculate the flexural strength as follows:9.1.1 For square cross-section specimens: 5 3PL! 2d3! (1)where:P = break force,L = support span, andd = specimen thickness.9.1.2 For rectangular cr
43、oss-section specimens: 5 3PL! 2bd2! (2)where:b = specimen width.9.1.3 For circular cross-section specimens: 5 8PL! D3! (3)where:D = specimen diameter.9.2 If the fracture does not occur directly underneath theload bearing block, the location of the fracture shall berecorded as such, and the results o
44、f the test shall be reported.9.3 If fracture occurs in less than 10 s, the results shall bediscarded but reported.NOTE 4It should be recognized that the above equations do notnecessarily give the stress that was acting directly on the origin that causedfailure. The equations do not account for subsu
45、rface origins or breaksaway from the area under maximum flexure stress (directly below the loadbearing), nor do they correct for the potential tension/compressioninequality in modulus (behavior that is not linear elastic) commonlyaccepted in graphite. For conventional Weibull analysis, use the calcu
46、latedmaximum stress in the specimen at failure from the equations as shown.10. Report10.1 The report of each test shall include the following:10.1.1 Sample identification,10.1.2 Average width and thickness or diameter to betterthan 0.025 mm,10.1.3 Average weight, g, and density, g/cm3, to within0.5
47、%,10.1.4 Support span length, mm,10.1.5 Rate of loading, mm/min, and test duration, s,10.1.6 Maximum applied load, N,10.1.7 Flexural strength calculated to the nearest 10 kPa,10.1.8 Defects in specimen,10.1.9 Orientation and location of specimen,10.1.10 Failure location, and10.1.11 Environmental con
48、ditions, that is, humidity andtemperature.10.2 Description of test machine and three-point test fixture,including pictures or schematics.FIG. 1 The Three-Point Fixture ConfigurationD7972 14310.3 Description of the machining and specimen surfacepreparation technique or the estimated surface roughness
49、.11. Precision and Bias411.1 The flexure strength of graphite is not a deterministicquantity, but will vary from one specimen to another. Therewill be an inherent statistical scatter in the results for finitespecimen populations (for example, 30 specimens). Weibullstatistics can model this variability as discussed in PracticeC1322 and Ref (2).11.2 Experimental Errors:11.2.1 The experimental errors in the flexure test have beenthoroughly analyzed and documented in Refs (1, 3). Thespecifications and tolerances in this test method have b
