1、Designation: C 1211 02 (Reapproved 2008)Standard Test Method forFlexural Strength of Advanced Ceramics at ElevatedTemperatures1This standard is issued under the fixed designation C 1211; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revisi
2、on, 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 determination of the flexuralstrength of advanced ceramics at elevated temper
3、atures.2Four-point-14 point and three-point loadings with prescribedspans are the standard. Rectangular specimens of prescribedcross-section are used with specified features in prescribedspecimen-fixture combinations.1.2 The values stated in SI units are to be regarded as thestandard. The values giv
4、en in parentheses are for informationonly.1.3 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 practices and determine the applica-bility of regulator
5、y limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3C 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 Fractogr
6、aphy and Characterization ofFracture Origins in Advanced CeramicsC 1341 Test Method for Flexural Properties of ContinuousFiber-Reinforced Advanced Ceramic CompositesC 1368 Test Method for Determination of Slow CrackGrowth Parameters of Advanced Ceramics by ConstantStress-Rate Flexural Testing at Amb
7、ient TemperatureC 1465 Test Method for Determination of Slow CrackGrowth Parameters of Advanced Ceramics by ConstantStress-Rate Flexural Testing at Elevated TemperaturesE4 Practices for Force Verification of Testing MachinesE 220 Test Method for Calibration of Thermocouples ByComparison TechniquesE
8、230 Specification and Temperature-Electromotive Force(EMF) Tables for Standardized Thermocouples2.2 Military Standard:MIL-STD 1942(A) Flexural Strength of High PerformanceCeramics at Ambient Temperature43. Terminology3.1 Definitions:3.1.1 complete gage section, nthe portion of the specimenbetween th
9、e two outer bearings in four-point flexure andthree-point flexure fixtures.NOTE 1In this standard, the complete four-point flexure gage sectionis twice the size of the inner gage section. Weibull statistical analyses, inthis instance, only include portions of the specimen volume or surfacewhich expe
10、rience tensile stresses.3.1.2 flexural strengtha measure of the ultimate strengthof a specified beam in bending.3.1.3 four-point-1/4 point flexurea configuration of flex-ural strength testing in which a specimen is symmetricallyloaded at two locations that are situated at one-quarter of theoverall s
11、pan, away from the outer two support bearings (seeFig. 1).3.1.4 fully-articulating fixture, na flexure fixture designedto be used either with flat and parallel specimens or withuneven or nonparallel specimens. The fixture allows fullindependent articulation, or pivoting, of all rollers about thespec
12、imen long axis to match the specimen surface. In addition,the upper or lower pairs are free to pivot to distribute forceevenly to the bearing cylinders on either side.NOTE 2See Annex A2 for schematic illustrations of the requiredpivoting movements.NOTE 3A three-point fixture has the inner pair of be
13、aring cylinders1This 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, 2008. Published January 2008. Originallyapproved in 1992. Last previou
14、s edition approved in 1998 as C 1211-98a.2Elevated temperatures typically denote, but are not restricted to 200 to 1600C.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refe
15、r to the standards Document Summary page onthe ASTM website.4Available from Standardization Documents Order Desk, DODSSP, Bldg. 4,Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http:/www.dodssp.daps.mil. This document is a 1990 update of the original MIL-STD1942(MR), dated November 1983.1
16、Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.Copyright by ASTM Intl (all rights reserved); Mon Sep 15 23:16:50 EDT 2008Downloaded/printed byGuo Dehua (CNIS) pursuant to License Agreement. No further reproductions authorized.replace
17、d by a single bearing cylinder.3.1.5 inert flexural strength, na measure of the strength ofa specified beam specimen in bending as determined in anappropriate inert condition whereby no slow crack growthoccurs.3.1.6 inherent flexural strength, nthe flexural strength of amaterial in the absence of an
18、y effect of surface grinding orother surface finishing process, or of extraneous damage thatmay be present. The measured inherent strength is in general afunction of the flexure test method, test conditions, andspecimen size.3.1.7 inner gage section, nthe portion of the specimenbetween the inner two
19、 bearings in a four-point flexure fixture.3.1.8 semi-articulating fixture, na flexure fixture designedto be used with flat and parallel specimens. The fixture allowssome articulation, or pivoting, to ensure the top pair (or bottompair) of bearing cylinders pivot together about an axis parallelto the
20、 specimen long axis, in order to match the specimensurfaces. In addition, the upper or lower pairs are free to pivotto distribute force evenly to the bearing cylinders on eitherside.NOTE 4See Annex A2 for schematic illustrations of the requiredpivoting movements.NOTE 5A three-point fixture has the i
21、nner pair of bearing cylindersreplaced by a single bearing cylinder.3.1.9 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.3.1.10 three-point flexurea
22、configuration of flexuralstrength testing in which a specimen is loaded at a positionmidway between two support bearings (see Fig. 1).4. Significance and Use4.1 This test method may be used for material development,quality control, characterization, and design data generationpurposes. This test meth
23、od is intended to be used with ceramicswhose flexural strength is ; 50 MPa (; 7 ksi) or greater.4.2 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 m
24、aterial is linearly elastic. Theaverage grain size should be no greater than150 of the beamthickness. The homogeneity and isotropy assumptions in thetest method rule out the use of it for continuous fiber-reinforcedcomposites for which Test Method C 1341 is more appropriate.4.3 The flexural strength
25、 of a group of test specimens isinfluenced by several parameters associated with the testprocedure. Such factors include the testing rate, test environ-ment, specimen size, specimen preparation, and test fixtures.Specimen and fixture sizes were chosen to provide a balancebetween the practical config
26、urations and resulting errors asdiscussed in MIL-STD 1942(A), Test Method C 1161, andRefs (13).5Specific fixture and specimen configurations weredesignated in order to permit the ready comparison of datawithout the need for Weibull size scaling.4.4 The flexural strength of a ceramic material is depe
27、ndenton both its inherent resistance to fracture and the size andseverity of flaws. Variations in these cause a natural scatter intest results for a sample of test specimens. Fractographicanalysis of fracture surfaces, although beyond the scope of thistest method, is highly recommended for all purpo
28、ses, espe-cially if the data will be used for design as discussed in MILSTD 1942 (A) and Ref (4) and Practices C 1322 and C 1239.4.5 This method determines the flexural strength at elevatedtemperature and ambient environmental conditions at a nomi-nal, moderately fast testing rate. The flexural stre
29、ngth underthese conditions may or may not necessarily be the inertflexural strength. Flexure strength at elevated temperature maybe strongly dependent on testing rate, a consequence of creep,stress corrosion, or slow crack growth. If the purpose of the testis to measure the inert flexural strength,
30、then extra precautionsare required and faster testing rates may be necessary.NOTE 6Many ceramics are susceptible to either environmentally-assisted slow crack growth or thermally activated slow crack growth.Oxide ceramics, glasses, glass ceramics, and ceramics containing bound-ary phase glass are pa
31、rticularly susceptible to slow crack growth. Timedependent effects that are caused by environmental factors (e.g. water ashumidity in air) may be minimized through the use of inert testingatmosphere such as dry nitrogen gas or vacuum. Alternatively, testingrates faster than specified in this standar
32、d may be used if the goal is tomeasure the inert strength. Thermally activated slow crack growth mayoccur at elevated temperature even in inert atmospheres. Testing ratesfaster than specified in this standard should be used if the goal is tomeasure the inert flexural strength. On the other hand, man
33、y ceramics suchas boron carbide, silicon carbide, aluminum nitride and many siliconnitrides have no sensitivity to slow crack growth at room or moderatelyelevated temperatures and for such materials, the flexural strengthmeasured under in laboratory ambient conditions at the nominal testingrate is t
34、he inert flexural strength.5The boldface numbers in parentheses refer to the list of references at the end ofthe text.NOTE 1Configuration:A: L = 20 mmB: L = 40 mmC: L = 80 mmFIG. 1 Four-Point-14 Point and Three-Point Fixture ConfigurationsC 1211 02 (2008)2Copyright by ASTM Intl (all rights reserved)
35、; Mon Sep 15 23:16:50 EDT 2008Downloaded/printed byGuo Dehua (CNIS) pursuant to License Agreement. No further reproductions authorized.4.6 The three-point test configuration exposes only a verysmall portion of the specimen to the maximum stress. There-fore, three-point flexural strengths are likely
36、to be much greaterthan four-point flexural strengths. Three-point flexure has someadvantages. It uses simpler test fixtures, it is easier to adapt tohigh temperature, and it is sometimes helpful in Weibullstatistical studies. However, four-point flexure is preferred andrecommended for most character
37、ization purposes.4.7 The three-point test configuration exposes only a verysmall portion of the specimen to the maximum stress. There-fore, three-point flexural strengths are likely to be much greaterthan four-point flexural strengths. Three-point flexure has someadvantages. It uses simpler test fix
38、tures, it is easier to adapt tohigh temperature, and it is sometimes helpful in Weibullstatistical studies. However, four-point flexure is preferred andrecommended for most characterization purposes.5. Interferences5.1 Time-dependent phenomena, such as stress corrosionand slow crack growth, can inte
39、rfere with determination of theflexural strength at room and elevated temperatures. Creepphenomena also become significant at elevated temperatures.Creep deformation can cause stress relaxation in a flexurespecimen during a strength test, thereby causing the elasticformulation that is used to comput
40、e the strength to be in error.5.2 Surface preparation of the test specimens can introducemachining damage such as microcracks that may have apronounced effect on flexural strength. Machining damageimposed during specimen preparation can be either a randominterfering factor or an inherent part of the
41、 strength character-istic to be measured. With proper care and good machiningpractice, it is possible to obtain fractures from the materialsnatural flaws. Surface preparation can also lead to residualstresses. Universal or standardized test methods of surfacepreparation do not exist. It should be un
42、derstood that finalmachining steps may or may not negate machining damageintroduced during the early coarse or intermediate machining.5.3 Slow crack growth can lead to a rate dependency offlexural strength. The testing rate specified in this standard mayor may not produce the inert flexural strength
43、 whereby negli-gible slow crack growth occurs. See Test Method C 1368,C 1465, and Ref (5) for more information about possible ratedependencies of flexural strength and methodologies for quan-tifying the rate sensitivity6. Apparatus6.1 LoadingSpecimens may be force in any suitabletesting machine prov
44、ided that uniform rates of direct loadingcan be maintained. The force measuring system shall be free ofinitial lag at the loading rates used and shall be equipped witha means for retaining readout of the maximum force as well asa force-time or force-deflection record. The accuracy of thetesting mach
45、ine shall be in accordance with Practices E4.66.2 Four-Point Flexure Four-Point-14 Point Fixtures (Fig.1), having support spans as given in Table 1.6.3 Three-Point Flexure Three-Point Fixtures (Fig. 1), hav-ing a support span as given in Table 1.6.4 Bearings, three- and four-point flexure.6.4.1 Cyli
46、ndrical bearings shall be used for support of thetest specimen and for load application. The cylinders may bemade of a ceramic with an elastic modulus between 200 and400 GPa (30 to 60 3 106psi) and a flexural strength no lessthan 275 MPa (40 ksi). The loading cylinders must remainelastic (and have n
47、o plastic deformation) over the load andtemperature ranges used, and they must not react chemicallywith or contaminate the test specimen. The test fixture shallalso be made of a ceramic that is resistant to permanentdeformation.6.4.2 The bearing cylinder diameter shall be approximately1.5 times the
48、beam depth of the test specimen size used (seeTable 2).6.4.3 The bearing cylinders shall be positioned carefullysuch that the spans are accurate to within 60.10 mm. The loadapplication bearing for the three-point configurations shall bepositioned midway between the support bearings within 60.10mm. T
49、he load application (inner) bearings for the four-pointconfigurations shall be centered with respect to the support(outer) bearings within 60.10 mm.6.4.4 The bearing cylinders shall be free to rotate in order torelieve frictional constraints (with the exception of the middle-load bearing in three-point flexure, which need not rotate). Thiscan be accomplished as shown in Fig. 2 and Fig. 3. Annex A2illustrates the action required of the bearing cylinders. Notethat the outer-support bearings roll outward and the inner-loading bearings roll inward.76.5 Semiarti
