1、Designation: C 1211 02Standard 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 revision, the year of la
2、st 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. Scope*1.1 This test method covers determination of the flexuralstrength of advanced ceramics at elevated temperatures.2Four-poin
3、t-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 given in parentheses
4、 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 regulatory limitations pri
5、or to use.2. Referenced Documents2.1 ASTM Standards:C 1161 Test Method for Flexural Strength of AdvancedCeramics at Ambient Temperature3C 1239 Practice for Reporting Uniaxial Strength Data andEstimating Weibull Distribution Parameters for AdvancedCeramics3C 1322 Practice for Fractography and Charact
6、erization ofFracture Origins in Advanced Ceramics3C 1341 Test Method for Flexural Properties of ContinuousFiber Reinforced Advanced Ceramic Composites3C 1368 Test Method for Determination of Slow CrackGrowth Parameters of Advanced Ceramics by ConstantStress-Rate Flexural Testing at Ambient Temperatu
7、re3C 1465 Test Method for Determination of Slow CrackGrowth Parameters of Advanced Ceramics by ConstantStress-Rate Flexural Testing at Elevated Temperatures3E 4 Practices for Force Verification of Testing Machines4E 220 Test Method for Calibration of Thermocouples byComparison Techniques5E 230 Tempe
8、rature Electromotive Force (EMF) Tables forStandardized Thermocouples52.2 Military Standard:MIL-STD 1942(A) Flexural Strength of High PerformanceCeramics at Ambient Temperature63. Terminology3.1 Definitions:3.1.1 complete gage section, nthe portion of the specimenbetween the two outer bearings in fo
9、ur-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 experience tensile stresses.3.
10、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 span, away from the outer t
11、wo 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 thespecimen long axis to match th
12、e 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 bearing cylindersreplaced by
13、 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.1This test method is under the jurisdiction of ASTM Committee C28 onAdvanced Ceramics and is t
14、he direct responsibility of Subcommittee C28.01 onProperties and Performance.Current edition approved Dec. 10, 2002. Published June 2003. Originallyapproved in 1992. Last previous edition approved in 1998 as C 1211-98a.2Elevated temperatures typically denote, but are not restricted to 200 to 1600C.3
15、Annual Book of ASTM Standards, Vol 15.01.4Annual Book of ASTM Standards, Vol 03.01.5Annual Book of ASTM Standards, Vol 14.03.6Available from Standardization Documents Order Desk, Bldg. 4 Section D, 700Robbins Ave., Philadelphia, PA 19111-5094. This document is a 1990 update of theoriginal MIL-STD 19
16、42(MR), dated November 1983.1*A Summary of Changes section appears at the end of this standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.6 inherent flexural strength, nthe flexural strength of amaterial in the absence of any
17、 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
18、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
19、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 in
20、ner 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 c
21、onfiguration 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 metho
22、d 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 ma
23、terial 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
24、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 configu
25、rations and resulting errors asdiscussed in MIL-STD 1942(A), Test Method C 1161, andRefs (13).7Specific 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 depen
26、denton 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 purpos
27、es, 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 stren
28、gth 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, t
29、hen 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 par
30、ticularly 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 standard
31、 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, many
32、 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 th
33、e inert flexural strength.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 to be much greaterthan four-point flexural strengths. Three-point flexure has some7The boldface numbers in pa
34、rentheses 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 022advantages. It uses simpler test fixtures, it is easier to adapt tohigh temperature, and it is sometimes
35、helpful in Weibullstatistical studies. However, four-point flexure is preferred andrecommended for most characterization 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
36、 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 characterizati
37、on purposes.5. Interferences5.1 Time-dependent phenomena, such as stress corrosionand slow crack growth, can interfere with determination of theflexural strength at room and elevated temperatures. Creepphenomena also become significant at elevated temperatures.Creep deformation can cause stress rela
38、xation in a flexurespecimen during a strength test, thereby causing the elasticformulation that is used to compute 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. Machin
39、ing damageimposed during specimen preparation can be either a randominterfering factor or an inherent part of the 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 lea
40、d to residualstresses. Universal or standardized test methods of surfacepreparation do not exist. It should be understood 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 o
41、fflexural strength. The testing rate specified in this standard mayor may not produce the inert flexural strength whereby negli-gible slow crack growth occurs. See Test Method C 1368, C1465, and Ref (5) for more information about possible ratedependencies of flexural strength and methodologies for q
42、uan-tifying the rate sensitivity6. Apparatus6.1 LoadingSpecimens may be force in any suitabletesting machine provided 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 retainin
43、g readout of the maximum force as well asa force-time or force-deflection record. The accuracy of thetesting machine shall be in accordance with Practices E 4.86.2 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 Fi
44、xtures (Fig. 1), hav-ing a support span as given in Table 1.6.4 Bearings, three- and four-point flexure.6.4.1 Cylindrical 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
45、106psi) and a flexural strength no lessthan 275 MPa (40 ksi). The loading cylinders must remainelastic (and have no 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
46、that is resistant to permanentdeformation.6.4.2 The bearing cylinder diameter shall be approximately1.5 times the 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 b
47、earing for the three-point configurations shall bepositioned midway between the support bearings within 60.10mm. The 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 f
48、ree 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 b
49、earings roll outward and the inner-loading bearings roll inward.96.5 Semiarticulating Four-Point FixtureSpecimens pre-pared in accordance with the parallelism requirements of 7.1may be tested in a semiarticulating fixture as illustrated in Fig.2 and in Fig. A2.1(a). All four bearings shall be free to roll. Thetwo inner bearings shall be parallel to each other to within0.015 mm over their length. The two outer bearings shall beparallel to each other to within 0.015 mm over their length. Theinner bearings shall be supported independently of the outerbearings. All
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