1、Designation: D7779 11 (Reapproved 2015) An American National StandardStandard Test Method forDetermination of Fracture Toughness of Graphite atAmbient Temperature1This standard is issued under the fixed designation D7779; the number immediately following the designation indicates the year oforiginal
2、 adoption 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 and provides a measure of theresistance of
3、a graphite to crack extension at ambient tempera-ture and atmosphere expressed in terms of stress-intensityfactor, K, and strain energy release rate, G. These crack growthresistance properties are determined using beam test specimenswith a straight-through sharp machined V-notch.1.2 This test method
4、 determines the stress intensity factor,K, from applied force and gross specimen deflection measuredaway from the crack tip. The stress intensity factor calculatedat the maximum applied load is denoted as fracture toughness,KIc, and is known as the critical stress intensity factor. If theresolution
5、of the deflection gauge is sensitive to fracturebehavior in the test specimen and can provide a measure of thespecimen compliance, strain energy release rate, G, can bedetermined as a function of crack extension.1.3 This test method is applicable to a variety of grades ofgraphite which exhibit diffe
6、rent types of resistance to crackgrowth, such as growth at constant stress intensity (strainenergy release rate), or growth with increasing stress intensity(strain energy release rate), or growth with decreasing stressintensity (strain energy release rate). It is generally recognizedthat because of
7、the inhomogeneous microstructure of graphite,the general behavior will exhibit a mixture of all three duringthe test. The crack resistance behavior exhibited in the test isusually referred to as an “R-curve.”NOTE 1One difference between the procedure in this test method andtest methods such as Test
8、Method E399, which measure fracturetoughness, KIc, by one set of specific operational procedures, is that TestMethod E399 focuses on the start of crack extension from a fatigueprecrack for metallic materials. This test method for graphite makes use ofa machined notch with sharp cracking at the root
9、of the notch because ofthe nature of graphite. Therefore, fracture toughness values determinedwith this method may not be interchanged with KIcas defined in TestMethod E399.1.4 This test method gives fracture toughness values, KIcand critical strain energy release rate, GIcfor specific condi-tions o
10、f environment, deformation rate, and temperature. Frac-ture toughness values for a graphite grade can be functions ofenvironment, deformation rate, and temperature.1.5 This test method is divided into two major parts. Thefirst major part is the main body of the standard, whichprovides general inform
11、ation on the test method, the applica-bility to materials comparison and qualification, and require-ments and recommendations for fracture toughness testing.The second major part is composed of annexes, which provideinformation related to test apparatus and test specimen geom-etry.Main Body SectionS
12、cope 1Referenced Documents 2Terminology 3Summary of Test Method 4Significance and Use 5Apparatus 6Test Specimen 7Procedure 8Specimen Dryness 9Calculation of Results 10Report 11Precision and Bias 12Keywords 13Annex Annex A11.6 The values stated in SI units are to be regarded asstandard. No other unit
13、s of measurement are included in thisstandard.1.6.1 Measurement units expressed in these test methods arein accordance with IEEE/ASTM SI 10.1.7 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 t
14、o establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.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
15、Manufactured Carbon and Graphite Products.Current edition approved June 1, 2015. Published July 2015. Originally approvedin 2011. Last previous edition approved in 2011 as D7779 11. DOI:10.1520/D7779-11R15.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-
16、2959. United States12. Referenced Documents2.1 ASTM Standards:2C709 Terminology Relating to Manufactured Carbon andGraphiteC1161 Test Method for Flexural Strength of AdvancedCeramics at Ambient TemperatureC1421 Test Methods for Determination of Fracture Tough-ness of Advanced Ceramics at Ambient Tem
17、peratureE4 Practices for Force Verification of Testing MachinesE177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE337 Test Method for Measuring Humidity with a Psy-chrometer (the Measurement of Wet- and Dry-Bulb Tem-peratures)E399 Test Method for Linear-Elastic Plane-Strain Fr
18、actureToughness KIcof Metallic MaterialsE561 Test Method forK-R Curve DeterminationE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test MethodE1823 Terminology Relating to Fatigue and Fracture TestingE2309 Practices for Verification of Displacement MeasuringSyste
19、ms and Devices Used in Material Testing MachinesIEEE/ASTM SI 10 Standard for Use of the InternationalSystem of Units (SI) (The Modern Metric System)3. Terminology3.1 Definitions:3.1.1 The terms described in Terminology C709 and E1823are applicable to the test methods prescribed herein. Appropri-ate
20、sources for each definition are provided after each defini-tion in parentheses.3.1.2 crack extension resistance, KRFL-3/2, GRFL-1, orJRFL-1, nmeasure of the resistance of a material to crackextension expressed in terms of the stress-intensity factor, K,strain energy release rate, G, or values of J d
21、erived using theJ-integral concept. E18233.1.3 R-curve, nplot of stress intensity or strain energyrelease rate as a function of stable crack extension and providesa measure of crack propagation trend in the material. E5613.1.4 slow crack growth, (SCG), nsub-critical crackgrowth (extension) which may
22、 result from, but is not restrictedto, such mechanisms as environmentally-assisted stress corro-sion or diffusive crack growth, usually at constant load.3.1.5 stress-intensity factor, KFL-3/2, nmagnitude of theideal-crack-tip stress field (stress field singularity) for a par-ticular mode in a homoge
23、neous, linear-elastic body. E18233.2 Definitions of Terms Specific to This Standard:3.2.1 crack depth, a L, nlength of the crack in a notchedbeam specimen, which includes the machined notched lengthand the crack length which the crack has traveled duringtesting. Any contributions from crack branchin
24、g or othersecondary cracking are not included in this measurement.3.2.2 crack extension orientation, ndirection of propaga-tion in relation to a characteristic direction of the graphitespecimen. This identification may be designated by a letter orletters indicating the plane and direction of crack e
25、xtension.The letter or letters represent the direction normal to the crackplane and the direction of crack propagation.3.2.2.1 DiscussionThe characteristic direction should beassociated with the microstructural grain orientation of the testspecimen.3.2.2.2 DiscussionThe crack plane can be defined by
26、letter(s) representing the direction of tensile stress normal tothe crack plane. And the direction of crack extension can bedefined by letter(s) representing the direction parallel to thecharacteristic grain orientation of the test specimen. As illus-trated in Annex A1, the tensile stress direction
27、is notated first,followed by a hyphen, and then the crack extension direction.The legend given in Test Methods C1421 includes the follow-ing:M = molding directionEX = extrusion directionAXL = axial, or longitudinal axis (if M or EX are notapplicable)R = radial directionC = circumferential directionR
28、/C = mixed radial and circumferential directions3.2.2.3 DiscussionFor a graphite test specimen of rectan-gular cross section, R and C may be replaced by rectilinearcoordinate axes, x and y, corresponding to two adjacent sidesof the test specimen.3.2.2.4 DiscussionDepending on how test specimens arec
29、ut from a graphite product, the crack plane may be longitu-dinal to the forming direction, or circumferential, or radial, ora mixture of these directions as shown in Annex A1.3.2.2.5 DiscussionFor the test specimen the plane anddirection of crack extension with respect to the applied tensilestress s
30、hould be recorded. Report the orientation of thespecimen and crack propagation direction with respect to thegrain direction.3.2.2.6 DiscussionIf there is no primary product direction,reference axes may be arbitrarily assigned but must be clearlyidentified.3.2.3 critical crack depth, L, ncrack depth
31、at whichcatastrophic fracture initiation occurs, corresponding to themaximum in the applied load.3.2.4 fracture toughness, KFL-3/2, nproperty which de-fines the critical stress intensity factor necessary to initiate acrack for subsequent propagation on further loading.3.2.5 small crack, nbeing small
32、 when all physical dimen-sions (in particular, with length and depth of a surface crack)are small in comparison to a relevant microstructural scale,continuum mechanics scale, or physical size scale. The specificphysical dimensions that define “small” vary with the particu-lar material, geometric con
33、figuration, and loadings of interest.E18233.2.6 stable crack extension, ncrack propagation whichprovides measurable data of the dependence of stress intensity2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of A
34、STMStandards volume information, refer to the standards Document Summary page onthe ASTM website.D7779 11 (2015)2factor on crack extension and which occurs over some mea-surable time duration.3.2.7 three-point flexure, nflexure configuration where abeam test specimen is loaded at a location midway b
35、etweentwo support bearings. C11613.2.8 unstable crack extensionuncontrollable crack propa-gation which yields no measurable data of the dependence ofstress intensity factor on crack extension.3.3 Symbols:3.3.1 acrack depth, including the machined notch (seeFig. 1).3.3.2 a/Wnormalized notch depth.3.3
36、.3 Bthe specimen width (see Fig. 1).3.3.4 g(a/W)geometric function of the ratio a/W.3.3.5 Ltest specimen length (see Fig. 1).3.3.6 Pforce.3.3.7 Pmaxmaximum force.3.3.8 Ssupport span (see Fig. A1.2).3.3.9 Wthe specimen depth (see Fig. 1).4. Summary of Test Method4.1 This test method involves an appli
37、cation of force to abeam test specimen in three-point flexure. The test specimencontains a straight-through notch in the center. The equationsfor calculating the fracture toughness have been established onthe basis of linear-elastic stress analyses.4.2 Notched Beam MethodA straight-through notch ism
38、achined in a beam test specimen. The applied force on thenotched test specimen as a function of time and actuatordisplacement or specimen deflection in three-point flexure, or acombination thereof, are recorded for analysis. The fracturetoughness, KIc, is calculated from the maximum (fracture)force,
39、 the test specimen dimensions, the measured notch depth,and the support span of the test fixture. Calculation of strainenergy release rate, G, requires a determination of specimencompliance, and crack length at each load point of the loadversus displacement curve. The maximum G derived from thestrai
40、n energy release rate versus crack growth curve is re-corded.5. Significance and Use5.1 This test method may be used for guidance for materialdevelopment to improve toughness, material comparison, qual-ity assessment, and characterization.5.2 The fracture toughness value provides information onthe i
41、nitiation of fracture in graphite containing a straight-through notch; the information on stress intensity factorbeyond fracture toughness as a function of crack extensionprovides information on the crack propagation resistance oncea fracture crack has been initiated to propagate through the testspe
42、cimen.6. Apparatus6.1 TestingTest the specimens in a testing machine thathas provisions for autographic recording of force applied to thetest specimen versus time and actuator displacement or deflec-tion of the specimen, or both, in the notch plane. The testingmachine shall conform to the requiremen
43、ts of Practice E4.6.2 Deflection MeasurementThe deflection gauge shouldbe capable of resolving 0.001 mm. Practices E2309 coverprocedures and requirements for the calibration and verifica-tion of displacement measuring systems.6.3 Recording EquipmentProvide digital data acquisitionfor automatically r
44、ecording the applied force versus displace-ment.6.4 FixturesUse a three-point test fixture constructed withhigh stiffness materials (see Fig. A1.2). Choose the outersupport span, S, such that 5 (S/W) 10. The outer two rollersshall be free to roll outwards from support locations tominimize friction e
45、ffects. The middle flexure roller shall befixed. The specimen should overhang each of the outer rollersby a minimum distance equal to the specimen dimension, W.6.5 Dimension-Measuring DevicesMeasure and report allapplicable specimen dimensions to an accuracy of 0.013 mm.Flat, anvil-type micrometers
46、shall be used for measuring testspecimen dimensions. Ball-tipped or sharp-anvil micrometersare not recommended as they may damage the test specimensurface by inducing localized cracking. Non-contacting (forexample, optical comparator, light microscopy, etc.) measure-ments are recommended for notch d
47、epth measurements. Mea-sure and report the notch depth to an accuracy of 0.0025 mm.7. Test Specimen7.1 Test Specimen ConfigurationThe specimen shall havea straight-through machined V-notch with a maximum notchroot radius of 0.10 mm. The notch may be sharpened bydrawing an industrial razor blade or s
48、imilar device across thenotch tip to encourage stable crack extension from the as-machined notch tip.7.1.1 The included angle of the razor blade edge should beless than the specimen notch angle. It is recommended that thesharpening process be controlled such that this step is made ina consistent, me
49、asurable manner across the width of the notch.Manual sharpening introduces uncertainty in the initial notchdepth and may also cause premature failure.7.1.2 The ease with which a crack initiates from a machinednotch depends on both the width of the notch, particularly thenotch tip asperity, and the average grain size of the materialunder consideration. Because typical graphite grades contain10 percent to 20 percent porosity, and the graphite grains haveMorowski microcracks within them, it is expected that theFIG. 1 Specimen Dimensi
