1、Designation: C 1368 06Standard Test Method forDetermination of Slow Crack Growth Parameters ofAdvanced Ceramics by Constant Stress-Rate FlexuralTesting at Ambient Temperature1This standard is issued under the fixed designation C 1368; the number immediately following the designation indicates the ye
2、ar oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the determination of slow cra
3、ckgrowth (SCG) parameters of advanced ceramics by usingconstant stress-rate flexural testing in which flexural strength isdetermined as a function of applied stress rate in a givenenvironment at ambient temperature. The strength degradationexhibited with decreasing applied stress rate in a specified
4、environment is the basis of this test method which enables theevaluation of slow crack growth parameters of a material.NOTE 1This test method is frequently referred to as “dynamicfatigue” testing (Refs (1-3)2) in which the term “fatigue” is usedinterchangeably with the term “slow crack growth.” To a
5、void possibleconfusion with the “fatigue” phenomenon of a material which occursexclusively under cyclic loading, as defined in Definitions E 1823, this testmethod uses the term “constant stress-rate testing” rather than “dynamicfatigue” testing.NOTE 2In glass and ceramics technology, static tests of
6、 considerableduration are called “static fatigue” tests, a type of test designated asstress-rupture (See Definitions E 1823).1.2 Values expressed in this test method are in accordancewith the International System of Units (SI) and IEEE/ASTM SI 10.1.3 This standard does not purport to address all of
7、thesafety 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:3C 1145 Terminology of Adv
8、anced CeramicsC 1161 Test Method for Flexural Strength of AdvancedCeramics at Ambient TemperatureC 1239 Practice for Reporting Uniaxial Strength Data andEstimating Weibull Distribution Parameters for AdvancedCeramicsC 1322 Practice for Fractography and Characterization ofFracture Origins in Advanced
9、 CeramicsE4 Practices for Force Verification of Testing MachinesE6 Terminology Relating to Methods of Mechanical Test-ingE 337 Test Method for Measuring Humidity with a Psy-chrometer (the Measurement of Wet- and Dry-Bulb Tem-peratures)IEEE/ASTM SI 10 American National Standard for Use ofthe Internat
10、ional System of Units (SI): The Modern MetricSystemE 1823 Terminology Relating to Fatigue and Fracture Test-ing3. Terminology3.1 DefinitionsThe terms described in TerminologyC 1145, Terminology E6, Terminology E 616, and DefinitionsE 1823 are applicable to this test method. Specific termsrelevant to
11、 this test method are as follows:3.1.1 advanced ceramic, na highly engineered, high-performance, predominately nonmetallic, inorganic, ceramicmaterial having specific functional attributes. (C 1145)3.1.2 constant stress rate,s, na constant rate of maximumstress applied to a specified beam by using e
12、ither a constantloading or constant displacement rate of a testing machine.3.1.3 environment, nthe aggregate of chemical speciesand energy that surrounds a test specimen. (E 1823)3.1.4 environmental chamber, nthe container of bulkvolume surrounding a test specimen. (E 1823)3.1.5 flexural strength, s
13、f, na measure of the strength of aspecified beam specimen in bending determined at a givenstress rate in a particular environment.3.1.6 flexural strength-stress rate curve, na curve fitted tothe values of flexural strength at each of several stress rates,based on the relationship between flexural st
14、rength and stressrate: log sf= 1/(n +1)logs + log D. (See Appendix X1.)1This test method is under the jurisdiction of ASTM Committee C28 onAdvanced Ceramics and is the direct responsibility of Subcommittee C28.01 onProperties and Performance.Current edition approved Jan. 1, 2006. Published January 2
15、006. Originallyapproved in 1997. Last previous edition approved in 2001 as C 1368 01.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.or
16、g. 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.NOTE 3In the ceramics literature, this is often called a dynamicf
17、atigue curve.3.1.7 flexural strength-stress rate diagram, na plot offlexural strength against stress rate. Both flexural strength andstress rate are plotted on log-log scales.3.1.8 fracture toughness, na generic term for measures ofresistance to extension of a crack. (E 616)3.1.9 inert flexural stre
18、ngth, na measure of the strength ofa specified beam specimen in bending as determined in anappropriate inert condition whereby no slow crack growthoccurs.NOTE 4An inert condition may be obtained by using vacuum, lowtemperatures, very fast test rates, or any inert mediums.3.1.10 slow crack growth (SC
19、G), nsubcritical crackgrowth (extension) which may result from, but is not restrictedto, such mechanisms as environmentally-assisted stress corro-sion or diffusive crack growth.3.1.11 stress intensity factor, KI, nthe magnitude of theideal-crack-tip stress field (stress-field singularity) subjected
20、tomode I loading in a homogeneous, linear elastic body.(E 616)3.2 Definition of Term Specific to This Standard:3.2.1 slow crack growth parameters, n and D, ntheparameters estimated as constants in the flexural strength-stressrate equation, which represent the degree of slow crack growthsusceptibilit
21、y of a material. (See Appendix X1.)4. Significance and Use4.1 For many structural ceramic components in service,their use is often limited by lifetimes that are controlled by aprocess of SCG. This test method provides the empiricalparameters for appraising the relative SCG susceptibility ofceramic m
22、aterials under specified environments. Furthermore,this test method may establish the influences of processingvariables and composition on SCG as well as on strengthbehavior of newly developed or existing materials, thus allow-ing tailoring and optimizing material processing for furthermodification.
23、 In summary this test method may be used formaterial development, quality control, characterization, andlimited design data generation purposes.4.2 The flexural stress computation is based on simple beamtheory, with the assumptions that the material is isotropic andhomogeneous, the moduli of elastic
24、ity in tension and compres-sion are identical, and the material is linearly elastic. Theaverage grain size should be no greater than one fiftieth of thebeam thickness.4.3 The specimen sizes and fixtures were chosen in accor-dance with Test Method C 1161, which provides a balancebetween practical con
25、figurations and resulting errors, as dis-cussed in Refs (4, 5). Only the four-point test configuration isused in this test method.4.4 The SCG parameters (n and D) are determined by fittingthe measured experimental data to a mathematical relationshipbetween flexural strength and applied stress rate,
26、log sf=1/(n+1) log s + log D. The basic underlying assumption on thederivation of this relationship is that SCG is governed by anempirical power-law crack velocity, v=AKI/KICn(seeAppen-dix X1).NOTE 5There are various other forms of crack velocity laws whichare usually more complex or less convenient
27、 mathematically, or both, butmay be physically more realistic (Ref (6). It is generally accepted thatactual data cannot reliably distinguish between the various formulations.Therefore, the mathematical analysis in this test method does not coversuch alternative crack velocity formulations.4.5 The ma
28、thematical relationship between flexural strengthand stress rate was derived based on the assumption that theslow crack growth parameter is at least n $ 5 (Refs (1, 7, 8).Therefore, if a material exhibits a very high susceptibility toSCG, that is, n 2000MPa/s) may remain unchanged so that a plateau
29、is observed inthe plot of strength versus stress rate (Ref (7). If the strengthdata determined in this plateau region are included in theanalysis, a misleading estimate of the SCG parameters will beobtained. Therefore, the strength data in the plateau shall beexcluded as data points in estimating th
30、e SCG parameters ofthe material. This test method addresses for this factor byrecommending that the highest stress rate by #2000 MPa/s.NOTE 7The strength plateau of a material can be checked bymeasuring an inert flexural strength in an appropriate inert medium.NOTE 8When testing in environments with
31、 less than 100% concen-tration of the corrosive medium (for example, air), the use of stress ratesgreater than 1 MPa/s can result in significant errors in the slow crackgrowth parameters due to averaging of the regions of the slow crackgrowth curve (16). Such errors can be avoided by testing in 100%
32、concentration of the corrosive medium (for example, in water instead ofhumid air). For the case of 100% concentration of the corrosive medium,stress rates as large as 2000 MPa/s may be acceptable.5.3 Surface preparation of test specimens can introducefabrication flaws which may have pronounced effec
33、ts on SCGbehavior. Machining damage imposed during specimen prepa-ration can be either a random interfering factor or an inherentpart of the strength characteristics to be measured. Surfacepreparation can also lead to residual stress. Universal orstandardized test methods of surface preparation do n
34、ot exist. Itshould be understood that the final machining steps may ormay not negate machining damage introduced during the earlycoarse or intermediate machining steps. In some cases, speci-mens need to be tested in the as-processed condition tosimulate a specific service condition. Therefore, speci
35、menfabrication history may play an important role in slow crackgrowth as well as in strength behavior.6. Apparatus6.1 Testing MachineTesting machines used for this testmethod shall conform to the requirements of Practices E4.Specimens may be loaded in any suitable testing machineprovided that unifor
36、m test rates, either using load-controlled ordisplacement-controlled mode, can be maintained. The loadsused in determining flexural strength shall be accurate within61.0 % at any load within the selected load rate and load rangeof the testing machine as defined in Practices E4. The testingmachine sh
37、all have a minimum capability of applying at leastfour test rates with at least three orders of magnitude, rangingfrom 101to 102N/s for load-controlled mode and from 107to104m/s for displacement-controlled mode.6.2 Test FixturesThe configurations and mechanical prop-erties of test fixtures should be
38、 in accordance with Test MethodC 1161. The materials from which the test fixtures includingbearing cylinders are fabricated shall be effectively inert to thetest environment so that they do not react with or contaminatethe environment.NOTE 9For testing in water, for example, it is recommended that t
39、hetest fixture be fabricated from stainless steel which is effectively inert towater. The bearing cylinders may be machined from hardenable stainlesssteel (for example, 440C grade) or a ceramic material such as siliconnitride, silicon carbide, or alumina.6.2.1 Four-Point FlexureThe four-point-14 poi
40、nt fixtureconfiguration as described in 6.2 of Test Method C 1161 shallbe used in this test method.6.2.2 Bearing CylindersThe requirements of dimensionsand mechanical properties of bearing cylinders as described in6.4 of Test Method C 1161 shall be used in this test method. Itshould be noted that th
41、e bearing cylinders shall be free to rotatein order to relieve frictional constraints, as described in 6.4.4 ofTest Method C 1161.6.2.3 Semiarticulating Four-Point FixtureThe semiar-ticulating four-point fixture as described in 6.5 of Test MethodC 1161 may be used in this test method. This fixture s
42、hall beused when the parallelism requirements of test specimens aremet in accordance with 7.1 of Test Method C 1161.6.2.4 Fully Articulating Four-Point FixtureThe fully ar-ticulating four-point fixture as described in 6.6 of Test MethodC 1161 may be used in this test method. Specimens which donot me
43、et the parallelism requirements of 7.1 of Test MethodC 1161, due to the nature of fabrication process (as-fired,heat-treated, or oxidized), shall be tested in this fully articu-lating fixture.6.2.5 Compliance of Test FixtureThe test fixtures shall bestiffer than the specimen, so that most of the cro
44、sshead oractuator travel is imposed onto the specimen.6.3 Data AcquisitionAccurate determination of both frac-ture load and test time is important since it affects not onlyfracture strength but applied stress rate. At the minimum, anautographic record of applied load versus time should bedetermined
45、during testing. Either analog chart recorders ordigital data acquisition systems can be used for this purpose.Ideally, an analog chart recorder should be used in conjunctionwith the digital data acquisition system to provide an immedi-ate record of the test as a supplement to the digital record.Reco
46、rding devices should be accurate to 1.0 % of the recordingrange and should have a minimum data acquisition rate of 1000Hz (or 1 KHz) with a response of 5000 Hz (or 5 KHz) deemedmore than sufficient. The appropriate data acquisition ratedepends on the test rate; the higher the test rate the higher th
47、eacquisition rate, and vise versa.6.4 Environmental FacilityIf testing is conducted in anyenvironment other than ambient air, an appropriate environ-mental chamber shall be constructed to facilitate handling andmonitoring of the test environment so that constant testconditions can be maintained. The
48、 chamber shall be effectivelycorrosion-resistant to the test environment so that it does notreact with or change the environment. The chamber should belarge enough to fully immerse the test specimens in theenvironment, particularly for liquid environments. A circula-tion system to replenishment the
49、test environment may bedesirable. It should provide continuous filtration of the testmedium in order to remove foreign debris and corrosiveproduct.Additionally, the facility shall be able to safely containthe test environment.C13680637. Test Specimen7.1 Specimen SizeThe types and dimensions of rectangu-lar beam specimens as described in 7.1 of Test Method C 1161shall be used in this test method.7.2 Specimen PreparationSpecimen fabrication andpreparation methods as described in 7.2 of Test Method C 1161shall be used in this test method.7.3
