1、Designation: D7779 11Standard 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 adoption or, in the case of revision, the year
2、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 a graphite to crack extension at ambient tempera
3、-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 determines the stress intensity factor,K, from
4、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 of the deflection gauge is sensitive to fracture
5、behavior 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 different types of resistance to crackgrowth, such as
6、 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 the inhomogeneous microstructure of graphite,the
7、 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 Method E399, which measure fracture tough-ness,
8、KIc, by one set of specific operational procedures, is that Test MethodE399 focuses on the start of crack extension from a fatigue precrack formetallic materials. This test method for graphite makes use of a machinednotch with sharp cracking at the root of the notch because of the nature ofgraphite.
9、 Therefore, fracture toughness values determined with thismethod may not be interchanged with KIcas defined in Test Method E399.1.4 This test method gives fracture toughness values, KIcand critical strain energy release rate, GIcfor specific condi-tions of environment, deformation rate, and temperat
10、ure. 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 information on the test method, the applica-bility
11、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 SectionScope 1Referenced Documents 2Terminology 3Summ
12、ary 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 units of measurement are included in thisstandard
13、.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 to establish appro-priate safety and health pr
14、actices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2C709 Terminology Relating to Manufactured Carbon andGraphiteC1161 Test Method for Flexural Strength of Advanced1This test method is under the jurisdiction of ASTM Committee D02
15、onPetroleum Products and Lubricants and is the direct responsibility of SubcommitteeD02.F0 on Manufactured Carbon and Graphite Products.Current edition approved Dec. 1, 2011. Published March 2012. DOI:10.1520/D7779-11.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact AS
16、TM 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.Ceramics at Ambient Temperat
17、ureC1421 Test Methods for Determination of Fracture Tough-ness of Advanced Ceramics at Ambient TemperatureE4 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
18、Measurement of Wet- and Dry-Bulb Tem-peratures)E399 Test Method for Linear-Elastic Plane-Strain FractureToughness KIcof Metallic MaterialsE561 Test Method for K-R Curve DeterminationE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test MethodE1823 Terminology Rela
19、ting to Fatigue and Fracture Test-ingE2309 Practices for Verification of Displacement Measur-ing Systems and Devices Used in Material Testing Ma-chinesIEEE/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 des
20、cribed in Terminology C709 and E1823are applicable to the test methods prescribed herein. Appropri-ate 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 crackextensi
21、on expressed in terms of the stress-intensity factor, K,strain energy release rate, G, or values of J derived 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 tre
22、nd in the material. E5613.1.4 slow crack growth, (SCG), nsub-critical crackgrowth (extension) which may 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, nmag
23、nitude of theideal-crack-tip stress field (stress field singularity) for a par-ticular mode in a homogeneous, 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 lengt
24、hand the crack length which the crack has traveled duringtesting. Any contributions from crack branching 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. Th
25、is identification may be designated by a letter orletters indicating the plane and direction of crack extension.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 mi
26、crostructural grain orientation of the testspecimen.3.2.2.2 DiscussionThe crack plane can be defined byletter(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 thecharacteri
27、stic grain orientation of the test specimen. As illus-trated in Annex A1, the tensile stress direction 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
28、, or longitudinal axis (if M or EX are notapplicable)R = radial directionC = circumferential directionR/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, corresp
29、onding to two adjacent sidesof the test specimen.3.2.2.4 DiscussionDepending on how test specimens arecut 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
30、the test specimen the plane anddirection of crack extension with respect to the applied tensilestress should 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 direc-tion, reference ax
31、es may be arbitrarily assigned but must beclearly identified.3.2.3 critical crack depth, L, ncrack depth 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 n
32、ecessary to initiate acrack for subsequent propagation on further loading.3.2.5 small crack, nbeing small 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 sca
33、le. The specificphysical dimensions that define “small” vary with the particu-lar material, geometric configuration, and loadings of interest.E18233.2.6 stable crack extension, ncrack propagation whichprovides measurable data of the dependence of stress intensityfactor on crack extension and which o
34、ccurs over some mea-surable time duration.3.2.7 three-point flexure, nflexure configuration where abeam test specimen is loaded at a location midway betweentwo support bearings. C11613.2.8 unstable crack extensionuncontrollable crackpropagation which yields no measurable data of the depen-dence of s
35、tress intensity factor on crack extension.D7779 1123.3 Symbols:3.3.1 acrack depth, including the machined notch (seeFig. 1).3.3.2 a/Wnormalized notch depth.3.3.3 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
36、.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 application of force to abeam test specimen in three-point flexure. The test specimencontains a straight-through notch in the center. The equation
37、sfor calculating the fracture toughness have been established onthe basis of linear-elastic stress analyses.4.2 Notched Beam MethodA straight-through notch ismachined in a beam test specimen. The applied force on thenotched test specimen as a function of time and actuatordisplacement or specimen def
38、lection in three-point flexure, or acombination thereof, are recorded for analysis. The fracturetoughness, KIc, is calculated from the maximum (fracture)force, the test specimen dimensions, the measured notch depth,and the support span of the test fixture. Calculation of strainenergy release rate, G
39、, requires a determination of specimencompliance, and crack length at each load point of the loadversus displacement curve. The maximum G derived from thestrain energy release rate versus crack growth curve is re-corded.5. Significance and Use5.1 This test method may be used for guidance for materia
40、ldevelopment to improve toughness, material comparison, qual-ity assessment, and characterization.5.2 The fracture toughness value provides information onthe initiation of fracture in graphite containing a straight-through notch; the information on stress intensity factorbeyond fracture toughness as
41、 a function of crack extensionprovides information on the crack propagation resistance oncea fracture crack has been initiated to propagate through the testspecimen.6. Apparatus6.1 TestingTest the specimens in a testing machine thathas provisions for autographic recording of force applied to thetest
42、 specimen versus time and actuator displacement or deflec-tion of the specimen, or both, in the notch plane. The testingmachine shall conform to the requirements of Practice E4.6.2 Deflection MeasurementThe deflection gauge shouldbe capable of resolving 0.001 mm. Practices E2309 coverprocedures and
43、requirements for the calibration and verifica-tion of displacement measuring systems.6.3 Recording EquipmentProvide digital data acquisitionfor automatically recording the applied force versus displace-ment.6.4 FixturesUse a three-point test fixture constructed withhigh stiffness materials (see Fig.
44、 A1.2). Choose the outersupport span, S, such that 5 # (S/W) # 10. The outer tworollers shall be free to roll outwards from support locations tominimize friction effects. The middle flexure roller shall befixed. The specimen should overhang each of the outer rollersby a minimum distance equal to the
45、 specimen dimension, W.6.5 Dimension-Measuring DevicesMeasure and report allapplicable specimen dimensions to an accuracy of 0.013 mm.Flat, anvil-type micrometers shall be used for measuring testspecimen dimensions. Ball-tipped or sharp-anvil micrometersare not recommended as they may damage the tes
46、t specimensurface by inducing localized cracking. Non-contacting (forexample, optical comparator, light microscopy, etc.) measure-ments are recommended for notch depth measurements. Mea-sure and report the notch depth to an accuracy of 0.0025 mm.7. Test Specimen7.1 Test Specimen ConfigurationThe spe
47、cimen 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 similar device across thenotch tip to encourage stable crack extension from the as-machined notch tip.7.1.1 The included angle of the razo
48、r 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, measurable manner across the width of the notch.Manual sharpening introduces uncertainty in the initial notchdepth and may also cause prema
49、ture 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 to 20 percent porosity, and the graphite grains haveMorowski microcracks within them, it is expected that theinitiation of a “natural” crack with a narrow width is likely,thus avoiding the need to initiate a fatigue crack in graphite forfracture toughness determination.7.2 Test Spec
