1、Designation: C 1684 08Standard Test Method forFlexural Strength of Advanced Ceramics at AmbientTemperatureCylindrical Rod Strength1This standard is issued under the fixed designation C 1684; the number immediately following the designation indicates the year oforiginal adoption or, in the case of re
2、vision, 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 is for the determination of flexuralstrength of rod shape specimens of advanced
3、ceramic materialsat ambient temperature. In many instances it is preferable totest round specimens rather than rectangular bend specimens,especially if the material is fabricated in rod form. This methodpermits testing of machined, drawn, or as-fired rod shapedspecimens. It allows some latitude in t
4、he rod sizes and crosssection shape uniformity. Rod diameters between 1.5 and 8 mmand lengths from 25 to 85 mm are recommended, but othersizes are permitted. Four-point-14 point as shown in Fig. 1 isthe preferred testing configuration. Three-point loading ispermitted. This method describes the appar
5、atus, specimenrequirements, test procedure, calculations, and reporting re-quirements. The method is applicable to monolithic orparticulate- or whisker-reinforced ceramics. It may also beused for glasses. It is not applicable to continuous fiber-reinforced ceramic composites.1.2 This standard does n
6、ot 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 prior to use.2. Referenced Documents2.1 ASTM Standar
7、ds:2C 158 Test Methods for Strength of Glass by Flexure(Determination of Modulus of Rupture)C 1145 Terminology of Advanced CeramicsC 1161 Test Method for Flexural Strength of AdvancedCeramics at Ambient TemperatureC 1239 Practice for Reporting Uniaxial Strength Data andEstimating Weibull Distributio
8、n Parameters for AdvancedCeramicsC 1322 Practice for Fractography and Characterization ofFracture Origins in Advanced CeramicsC 1368 Test Method for Determination of Slow CrackGrowth Parameters of Advanced Ceramics by ConstantStress-Rate Flexural Testing at Ambient TemperatureE4 Practices for Force
9、Verification of Testing MachinesE 337 Test Method for Measuring Humidity with a Psy-chrometer (the Measurement of Wet- and Dry-Bulb Tem-peratures)3. Terminology3.1 Definitions:3.1.1 complete gage section, nthe portion of the specimenbetween the two outer loading points in four-point flexure andthree
10、-point flexure fixtures. C 11613.1.2 flaw, na structural discontinuity in an advancedceramic body that acts as a highly localized stress raiser.3.1.2.1 DiscussionThe presence of such discontinuitiesdoes not necessarily imply that the ceramic has been preparedimproperly or is faulty. C 13223.1.3 flex
11、ural strength, na measure of the ultimatestrength of a specified beam in bending. C 1145, C 11613.1.4 four-point-14 point flexure, nconfiguration of flex-ural strength testing where a specimen is symmetrically loadedat two locations that are situated one quarter of the overall spanaway from the oute
12、r two support loading points (see Fig. 1).C 1145, C 11613.1.5 fracture origin, nthe source from which brittlefracture commences. C 1145, C 13223.1.6 inert flexural strength, na measure of the strength ofspecified beam in bending as determined in an appropriate inertcondition whereby no slow crack gr
13、owth occurs.3.1.6.1 DiscussionAn inert condition may be obtained byusing vacuum, low temperatures, very fast test rates, or anyinert media. C 11613.1.7 inherent flexural strength, nthe flexural strength of amaterial in the absence of any effect of surface grinding orother surface finishing process,
14、or of extraneous damage thatmay be present. The measured inherent strength is in general a1This 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
15、. 1, 2008. Published January 2008.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM Internation
16、al, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.function of the flexure test method, test conditions, andspecimen size. C 11613.1.8 inner gage section, nthe portion of the specimenbetween the inner two loading points in a four-point flexurefixture. C 11613.1.9
17、 slow crack growth (SCG), nsubcritical crack growth(extension) which may result from, but is not restricted to, suchmechanisms as environmentally-assisted stress corrosion ordiffusive crack growth. C 1145, C 11613.1.10 three-point flexure, nconfiguration of flexuralstrength testing where a specimen
18、is loaded at a locationmidway between two support loading points (see Fig. 2).C 1145, C 11614. Significance and Use4.1 This test method may be used for material development,quality control, characterization, and design data generationpurposes. This test method is intended to be used with ceramicswho
19、se strength is 50 MPa (7 ksi) or greater. The test methodmay also be used with glass test specimens, although TestMethods C 158 is specifically designed to be used for glasses.This test method may be used with machined, drawn, extruded,and as-fired round specimens. This test method may be usedwith s
20、pecimens that have elliptical cross section geometries.4.2 The flexure strength 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 material is linearly elastic. Theaverage
21、grain size should be no greater than one fiftieth of therod diameter. The homogeneity and isotropy assumptions inthe standard rule out the use of this test for continuousfiber-reinforced ceramics.FIG. 1 Four-Point-14 Point Flexure Loading ConfigurationFIG. 2 Three-Point Flexure Loading Configuration
22、C16840824.3 Flexural strength of a group of test specimens isinfluenced by several parameters associated with the testprocedure. Such factors include the loading rate, test environ-ment, specimen size, specimen preparation, and test fixtures(1-3).3This method includes specific specimen-fixture sizec
23、ombinations, but permits alternative configurations withinspecified limits. These combinations were chosen to be prac-tical, to minimize experimental error, and permit easy com-parison of cylindrical rod strengths with data for other con-figurations. Equations for the Weibull effective volume andWei
24、bull effective surface are included.4.4 The flexural strength of a ceramic material is dependenton both its inherent resistance to fracture and the size andseverity of flaws in the material. Flaws in rods may beintrinsically volume-distributed throughout the bulk. Some ofthese flaws by chance may be
25、 located at or near the outersurface. Flaws may alternatively be intrinsically surface-distributed with all flaws located on the outer specimensurface. Grinding cracks fit the latter category. Variations in theflaws cause a natural scatter in strengths for a set of testspecimens. Fractographic analy
26、sis of fracture surfaces, al-though beyond the scope of this standard, is highly recom-mended for all purposes, especially if the data will be used fordesign as discussed in Refs (3-5) and Practices C 1322 andC 1239.4.5 The three-point test configuration exposes only a verysmall portion of the speci
27、men to the maximum stress. There-fore, three-point flexural strengths are likely to be greater thanfour-point flexural strengths. Three-point flexure has someadvantages. It uses simpler test fixtures, it is easier to adapt tohigh temperature and fracture toughness testing, and it issometimes helpful
28、 in Weibull statistical studies. It also usessmaller force to break a specimen. It is also convenient for veryshort, stubby specimens which would be difficult to test infour-point loading. Nevertheless, four-point flexure is preferredand recommended for most characterization purposes.5. Interference
29、s5.1 The effects of time-dependent phenomena, such as stresscorrosion or slow crack growth on strength tests conducted atambient temperature, can be meaningful even for the relativelyshort times involved during testing. Such influences must beconsidered if flexure tests are to be used to generate de
30、signdata. 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 whereby negli-gible slow crack growth occurs. See Test Method C 1368.5.2 Surface preparation of test specimens can introducemac
31、hining microcracks which may have a pronounced effecton flexural strength (6). Machining damage imposed duringspecimen preparation can be either a random interfering factor,or an inherent part of the strength characteristic to be mea-sured. With proper care and good machining practice, it ispossible
32、 to obtain fractures from the materials natural flaws.Surface preparation can also lead to residual stresses. It shouldbe understood that final machining steps may or may notnegate machining damage introduced during the early coarse orintermediate machining.5.3 This test method allows several option
33、s for the prepa-ration of specimens. The method allows testing of as-fabricated(e.g., as-fired or as-drawn), application-matched machining,customary, or one of three specific grinding procedures. Thelatter “standard procedures” (see 7.2.4) are satisfactory formany (but certainly not all) ceramics. C
34、enterless or transversegrinding aligns the severest machining microcracks perpen-dicular to the rod tension stress axis. The specimen mayfracture from the machining microcracks. Transverse-groundspecimens in many instances may provide a more “practicalstrength” that is relevant to machined ceramic c
35、omponentswhereby it may not be possible to favorably align the machin-ing direction. Therefore, this test method allows transversegrinding for normal specimen preparation purposes. Longitu-dinal grinding, which is commonly used to orient grindingdamage cracks in rectangular bend bars, is less common
36、ly usedfor rod specimens, but is also permitted by this test method.6. Apparatus6.1 LoadingSpecimens may be loaded 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
37、 be equipped witha means for retaining read-out of the maximum force applied tothe specimen. The accuracy of the testing machine shall be inaccordance with Practices E4.6.2 Four-Point FlexureFour-point-14 point fixtures arethe preferred configuration. When possible, use one of theouter support and i
38、nner loading span combinations listed inTable 1. Other span sizes may be used if these sizes are notsuitable for a specific round part. The ratio of the fixture outerspan length to the specimen diameter shall not be less than 3.0.6.3 Three-Point FlexureThree-point flexure may be usedif four-point is
39、 not satisfactory, such as if the specimens arevery short and stubby and consequently require very largebreaking forces in four-point loading. When possible, use oneof the support spans listed in Table 1 for three-point loading.Other span sizes may be used if these sizes are not suitable fora specif
40、ic round part. The outer fixture span length to specimendiameter ratio shall not be less than 3.0.6.4 Loading RollersForce shall be applied to the testpieces directly by rollers as described in this section (6.4)oralternatively by rollers with cradles as described in 6.5.6.4.1 This test method permi
41、ts direct contact of rod speci-mens with loading and support rollers. Direct contact maycause two problems, however. The crossed cylinder arrange-ment creates intense contact stresses in both the loading rollerand the test specimen due to the very small contact footprint.3The boldface numbers in par
42、entheses refer to the list of references at the end ofthis standard.TABLE 1 Preferred Fixture SpansConfigurationSupport Outer Span(Lo), mmLoading Inner Span(Li), mmA20 10B4 2C8 4C1684083The magnitude of the contact stresses depends upon the appliedforces, the roller and test specimen diameters, and
43、their elasticproperties.6.4.2 Section 6.4.5 provides guidance on how to minimizeor eliminate permanent deformation that may occur in theloading rollers due to contact stresses.6.4.3 Direct loading by rollers onto the rod test specimensmay cause premature test specimen fracture invalidating thetest.
44、Examples are shown in Annex A1. Contact stresses maygenerate shallow Hertzian cone cracks in the test specimen.Minor cracking at an inner loading point (on the compression-loaded side of the test rod) usually is harmless since it does notcause specimen breakage and forces are transmitted throughthe
45、crack faces. In extreme conditions, however, such asloading of short stubby specimens in 3-point or 4-pointloading, the magnitude of the forces and contact stresses maybe great enough to drive a Hertzian crack deep into the testspecimen cross section. Contact cracks at the outer supportrollers may b
46、e deleterious and cause an undesirable fracture ofthe specimen, even though these locations are far away fromthe inner span in 4-point loading or the middle in 3-pointloading. Examples of such deleterious contact cracks areshown in Annex A1. The propensity for fracture from contactcracks depends upo
47、n the test material properties and the testingconfiguration. The lower the materials fracture toughness andthe higher the elastic modulus, the more likely that contactcracks will cause premature fracture. The larger the testspecimen diameter for a given test span, the more likely thatcontact fractur
48、e will occur since larger forces are applied tobreak them. In other words, short stubby rod specimens aremore likely to have problems than long slender rods. Thisstandard allows considerable latitude in the selection of speci-men sizes and testing geometries. If specimens break prema-turely from con
49、tact cracks, the user shall either: reduce the testspecimen diameter, or use longer rod specimens with longerspan test fixtures, or use fixtures with cradles (see 6.5), or shiftto three-point loading.6.4.4 The rollers shall be free to rotate or roll to minimizefrictional constraint as the specimen stretches or contractsduring loading. The sole exception is the middle-load roller inthree-point flexure which need not rotate. Note that theouter-support rollers roll outward and the inner-loading rollersroll inward. The rollers may roll on a fixture base as shown inF
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