1、Designation: C 1465 08Standard Test Method forDetermination of Slow Crack Growth Parameters ofAdvanced Ceramics by Constant Stress-Rate FlexuralTesting at Elevated Temperatures1This standard is issued under the fixed designation C 1465; the number immediately following the designation indicates the
2、year 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 c
3、rackgrowth (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 elevated temperatures. The strength degrada-tion exhibited with decreasing applied stress rate in a spe
4、cifiedenvironment 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 (3-5)2in which the term “fatigue” is usedinterchangeably with the term “slow crack growth.”
5、To avoid possibleconfusion with the “fatigue” phenomenon of a material which occursexclusively under cyclic loading, as defined in Terminology E 1823, thistest method uses the term “constant stress-rate testing” rather than“dynamic fatigue” testing.NOTE 2In glass and ceramics technology, static test
6、s of considerableduration are called “static fatigue” tests, a type of test designated asstress-rupture (Terminology E 1823).1.2 This test method is intended primarily to be used fornegligible creep of test specimens, with specific limits on creepimposed in this test method.1.3 This test method appl
7、ies primarily to advanced ceramicsthat are macroscopically homogeneous and isotropic. This testmethod may also be applied to certain whisker- or particle-reinforced ceramics that exhibit macroscopically homogeneousbehavior.1.4 This test method is intended for use with various testenvironments such a
8、s air, vacuum, inert, and any other gaseousenvironments.1.5 Values expressed in this standard test are in accordancewith the International System of Units (SI) and IEEE/ASTM SI 10.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is therespo
9、nsibility 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 Advanced CeramicsC 1211 Test Method for Flexural Strength of AdvancedC
10、eramics at Elevated TemperaturesC 1239 Practice for Reporting Uniaxial Strength Data andEstimating Weibull Distribution Parameters for AdvancedCeramicsC 1322 Practice for Fractography and Characterization ofFracture Origins in Advanced CeramicsC 1368 Test Method for Determination of Slow CrackGrowth
11、 Parameters of Advanced Ceramics by ConstantStress-Rate Flexural Testing at Ambient TemperatureD 1239 Test Method for Resistance of Plastic Films toExtraction by ChemicalsE4 Practices for Force Verification of Testing MachinesE6 Terminology Relating to Methods of Mechanical Test-ingE 220 Test Method
12、 for Calibration of Thermocouples ByComparison TechniquesE 230 Specification and Temperature-Electromotive Force(EMF) Tables for Standardized ThermocouplesE 337 Test Method for Measuring Humidity with a Psy-chrometer (the Measurement of Wet- and Dry-Bulb Tem-peratures)E 616 Terminology Relating to F
13、racture Testing (Discontin-ued 1996)4E 1150 Definitions of Terms Relating to Fatigue1This test method is under the jurisdiction of ASTM Committee C28 onAdvanced Ceramics and is the direct responsibility of Subcommittee C28.01 onMechanical Properties and Performance.Current edition approved Jan. 1, 2
14、008. Published January 2008. Originallyapproved in 2000. Last previous edition approved in 2006 as C 146500 (2006).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 Cust
15、omer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4Withdrawn.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.IEEE/ASTM SI 10 American
16、 National Standard for Use ofthe International System of Units (SI): The Modern MetricSystemE 1823 Terminology Relating to Fatigue and Fracture Test-ing3. Terminology3.1 DefinitionsThe terms described in TerminologiesC 1145, E6, and E 1823 are applicable to this test method.Specific terms relevant t
17、o 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 FL2t1, na constant rate ofincrease of maximum flexural stress applied
18、to a specifiedbeam by using either a constant load or constant displacementrate of a testing machine.3.1.3 environment, nthe aggregate of chemical speciesand energy that surrounds a test specimen. (E 1150)3.1.4 environmental chamber, na container surroundingthe test specimen and capable of providing
19、 controlled localenvironmental condition.3.1.5 flexural strength, sfFL2, na measure of theultimate strength of a specified beam specimen in bendingdetermined at a given stress rate in a particular environment.3.1.6 flexural strength-stress rate diagrama plot of flex-ural strength as a function of st
20、ress rate. Flexural strength andstress rate are both plotted on logarithmic scales.3.1.7 flexural strength-stress rate curvea curve fitted tothe values of flexural strength at each of several stress rates,based on the relationship between flexural strength and stressrate:log sf= 1/(n + 1) log s + lo
21、g D (see Appendix X1)3.1.7.1 DiscussionIn the ceramics literature, this is oftencalled a “dynamic fatigue” curve.3.1.8 fracture toughness, KICFL3/2, na generic term formeasures of resistance to extension of a crack. (E 616)3.1.9 inert flexural strength FL2, na measure of thestrength of a specified b
22、eam specimen in bending as deter-mined in an appropriate inert condition whereby no slow crackgrowth occurs.3.1.9.1 DiscussionAn inert condition at near room tem-perature may be obtained by using vacuum, low temperatures,very fast test rates, or any inert media. However, at elevatedtemperatures, the
23、 definition or concept of an inert condition isunclear since temperature itself acts as a degrading environ-ment. It has been shown that for some ceramics one approachto obtain an inert condition (thus, inert strength) at elevatedtemperatures is to use very fast (ultra-fast) test rates$3 3 104MPa/s,
24、 where the time for slow crack growth would beminimized or eliminated (6).3.1.10 slow crack growth (SCG), 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 intensi
25、ty factor, KIFL3/2, nthe magnitudeof the ideal-crack-tip stress field (stress-field singularly) sub-jected to Mode I loading in a homogeneous, linear elastic body.(E 616)3.1.12 R-curve, na plot of crack-extension resistance as afunction of stable crack extension. (E 616)3.2 Definitions of Terms Spec
26、ific to This Standard:3.2.1 slow crack growth parameters, n and D, ntheparameters estimated as constants in the flexural strength (inmegapascals)-stress rate (in megapascals per second) equation,which represent a measure of susceptibility to slow crackgrowth of a material (seeAppendix X1). For the u
27、nits of D, see9.3.1.4. Significance and Use4.1 For many structural ceramic components in service,their use is often limited by lifetimes that are controlled by aprocess of slow crack growth. This test method provides theempirical parameters for appraising the relative slow crackgrowth susceptibility
28、 of ceramic materials under specifiedenvironments at elevated temperatures. This test method issimilar to Test Method C 1368 with the exception that provi-sions for testing at elevated temperatures are given. Further-more, this test method may establish the influences of process-ing variables and co
29、mposition on slow crack growth as well ason strength behavior of newly developed or existing materials,thus allowing tailoring and optimizing material processing forfurther modification. In summary, this test method may be usedfor material development, quality control, characterization, andlimited d
30、esign data generation purposes.NOTE 3Data generated by this test method do not necessarily corre-spond to crack velocities that may be encountered in service conditions.The use of data generated by this test method for design purposes mayentail considerable extrapolation and loss of accuracy.4.2 In
31、this test method, the flexural stress computation isbased on simple beam theory, with the assumptions that thematerial is isotropic and homogeneous, the moduli of elasticityin tension and compression are identical, and the material islinearly elastic. The average grain size should be no greaterthan
32、one fiftieth (1/50) of the beam thickness.4.3 In this test method, the test specimen sizes and testfixtures were chosen in accordance with Test Method C 1211,which provides a balance between practical configurations andresulting errors, as discussed in Refs (7, 8). Only the four-pointtest configurat
33、ion is used in this test method.4.4 In this test method, the slow crack growth parameters (nand D) are determined based on the mathematical relationshipbetween flexural strength and applied stress rate, log sf= 1/(n+ 1) log s + log D, together with the measured experimentaldata. The basic underlying
34、 assumption on the derivation of thisrelationship is that slow crack growth is governed by anempirical power-law crack velocity, v=AKI/KICn(see Ap-pendix X1).NOTE 4There are various other forms of crack velocity laws whichare usually more complex or less convenient mathematically, or both, butmay be
35、 physically more realistic (9).The mathematical analysis in this testmethod does not cover such alternative crack velocity formulations.4.5 In this test method, the mathematical relationship be-tween flexural strength and stress rate was derived based on theassumption that the slow crack growth para
36、meter is at least n$ 5 (3, 10). Therefore, if a material exhibits a very highC1465082susceptibility to slow crack growth, that is, n 1000MPa/s) may remain unchanged so that a plateau is observed inthe plot of strength versus stress rate, see Fig. 1a (6).Ifthestrength data determined in this plateau
37、region are included inthe analysis, a misleading estimate of the SCG parameters willNOTE 1The arrows indicate unacceptable data points. The data pointmarked with 8N, in which a significant nonlinearity occurs, indicates astrength value estimated by extrapolation of the linear regression linerepresen
38、ted by the rest of the strength data.FIG. 1 Schematic Diagrams Showing Unacceptable Data Points inConstant Stress-Rate Testing at Elevated TemperaturesC1465083be obtained. Therefore, the strength data in the plateau shall beexcluded as data points in estimating the SCG parameters ofthe material. Thi
39、s test method addresses this issue by recom-mending that the highest stress rate be #1000 MPa/s.5.4 Aconsiderable strength degradation may be observed atlower stress rates and higher temperatures for some materials.In these cases, excessive creep damage in the form of creepcavities, micro- or macro-
40、cracks, or both, develop in the tensilesurface (12-15). This results in a nonlinearity in the relation-ship between log (flexural strength) and log (applied stressrate), see Fig. 1b. It has been reported that the strengthdegradation with respect to the expected normal strength (atPoint N in Fig. 1b)
41、 ranged from 15 to 50 % (12-14). If thesedata points are used in the analysis, then an underestimate ofthe SCG parameters will be obtained. Hence, the strength dataexhibiting such a significant strength degradation occurring atlower stress rates shall be excluded as data points in obtainingthe SCG p
42、arameters of the material.5.5 Contrary to the case of significant strength degradation,an appreciable strength increase may occur for some ceramicsat lower stress rates (see Fig. 1c), due to crack healing or cracktip blunting which dominates slow crack growth (12, 16).Ithasbeen reported that the str
43、ength increase with respect to theexpected normal strength (at point N in Fig. 1c) ranged from 15to 60 % (12, 16). Since the phenomenon results in a deviationfrom the linear relationship between log (flexural strength) andlog (applied stress rate), an overestimate of SCG parametersmay be obtained if
44、 such strength data are included in theanalysis.Therefore, any data exhibiting a significant or obviousincrease in strength at lower stress rates shall be excluded asdata points in estimating the SCG parameters of the material.NOTE 6It has been shown that some preloading (up to 80 % offracture load)
45、 prior to testing may be used to minimize or eliminate thestrength-increase phenomenon by minimizing or eliminating a chance forcrack healing (or blunting) through shortening test time, as verified onsome advanced ceramics such as alumina and silicon nitride (12, 17).Ingeneral, preloading may be eff
46、ective to reduce overall creep deformationof test specimens due to reduced test time. Refer to 8.10 for moreinformation regarding preloading and its application.5.6 Surface preparation of test specimens can introducefabrication flaws that may have pronounced effects on flexuralstrength. Machining da
47、mage 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 not exist. Itshould be understo
48、od 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, specimenfabrication history may pla
49、y an important role in strengthbehavior, which consequently may affect the values of the SCGparameters to be determined.6. Apparatus6.1 Test MachineTest machines used for this test methodshall conform to the requirements of Practices E4. Testspecimens may be loaded in any suitable test machine providedthat uniform test rates, either using load-control ordisplacement-control mode, can be maintained. The loads usedin determining flexural strength shall be accurate within61.0 % at any load within the selected test rate and load rangeof th
