1、Designation: C1684 131Standard Test Method forFlexural Strength of Advanced Ceramics at AmbientTemperatureCylindrical Rod Strength1This standard is issued under the fixed designation C1684; the number immediately following the designation indicates the year oforiginal adoption or, in the case of rev
2、ision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1NOTEUnits statement was added to the scope editorially in April 2014.1. Scope1.1 This test method is for the d
3、etermination of flexuralstrength of rod shape specimens of advanced 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,
4、drawn, or as-fired rod shapedspecimens. It allows some latitude in the 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 configurati
5、on. Three-point loading ispermitted. This method describes the apparatus, 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 con
6、tinuous fiber-reinforced ceramic composites.1.2 The values stated in SI units are to be regarded as thestandard. The values given 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 o
7、f 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:2C158 Test Methods for Strength of Glass by Flexure (De-termination of Modulus of Rupture)C1145 Terminol
8、ogy of Advanced CeramicsC1161 Test Method for Flexural Strength of AdvancedCeramics at Ambient TemperatureC1239 Practice for Reporting Uniaxial Strength Data andEstimating Weibull Distribution Parameters for AdvancedCeramicsC1322 Practice for Fractography and Characterization ofFracture Origins in A
9、dvanced CeramicsC1368 Test Method for Determination of Slow CrackGrowth Parameters of Advanced Ceramics by ConstantStress-Rate Strength Testing at Ambient TemperatureE4 Practices for Force Verification of Testing MachinesE337 Test Method for Measuring Humidity with a Psy-chrometer (the Measurement o
10、f 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-point flexure fixtures. C11613.1.2 flaw, na structural discontinuity in an advancedceramic body that acts as a
11、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. C13223.1.3 flexural strength, na measure of the ultimatestrength of a specified beam in bending. C1145, C11613.1.4 four-point-14
12、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 outer two support loading points (see Fig. 1).C1145, C11613.1.5 fracture origin, nthe source from which brittlefracture
13、commences. C1145, C13223.1.6 inert flexural strength, na measure of the strength ofspecified beam in bending as determined in an appropriate inertcondition whereby no slow crack growth occurs.3.1.6.1 DiscussionAn inert condition may be obtained by1This test method is under the jurisdiction of ASTM C
14、ommittee C28 onAdvanced Ceramics and is the direct responsibility of Subcommittee C28.01 onMechanical Properties and Performance.Current edition approved Aug. 1, 2013. Published September 2013. Originallyapproved in 2008. Last previous edition approved in 2008 as C1684 08. DOI:10.1520/C1684-13E01.2F
15、or 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.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C70
16、0, West Conshohocken, PA 19428-2959. United States1using vacuum, low temperatures, very fast test rates, or anyinert media. C11613.1.7 inherent flexural strength, nthe flexural strength of amaterial in the absence of any effect of surface grinding orother surface finishing process, or of extraneous
17、damage thatmay be present. The measured inherent strength is in general afunction of the flexure test method, test conditions, andspecimen size. C11613.1.8 inner gage section, nthe portion of the specimenbetween the inner two loading points in a four-point flexurefixture. C11613.1.9 slow crack growt
18、h (SCG), nsubcritical crack growth(extension) which may result from, but is not restricted to, suchmechanisms as environmentally-assisted stress corrosion ordiffusive crack growth. C1145, C11613.1.10 three-point flexure, nconfiguration of flexuralstrength testing where a specimen is loaded at a loca
19、tionmidway between two support loading points (see Fig. 2).C1145, C11614. 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 ceramicswhose strength is 50 MPa
20、 (7 ksi) or greater. The test methodmay also be used with glass test specimens, although TestMethods C158 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 specimens that have ell
21、iptical cross section geometries.FIG. 1 Four-Point-14 Point Flexure Loading ConfigurationFIG. 2 Three-Point Flexure Loading ConfigurationC1684 13124.2 The flexure strength is computed based on simple beamtheory with assumptions that the material is isotropic andhomogeneous, the moduli of elasticity
22、in tension and compres-sion are identical, and the material is linearly elastic. Theaverage 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.4.3 Flexural
23、 strength of a group of test specimens isinfluenced by several parameters associated with the testprocedure. Such factors include the loading rate, testenvironment, specimen size, specimen preparation, and testfixtures (1-3).3This method includes specific specimen-fixturesize combinations, but permi
24、ts alternative configurationswithin specified limits. These combinations were chosen to bepractical, to minimize experimental error, and permit easycomparison of cylindrical rod strengths with data for otherconfigurations. Equations for the Weibull effective volume andWeibull effective surface are i
25、ncluded.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 located at or near the oute
26、rsurface. 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 analysis of fracture surfaces, al
27、-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 C1322 andC1239.4.5 The three-point test configuration exposes only a verysmall portion of the specimen to the maximum stress.Ther
28、efore, three-point flexural strengths are likely to be greaterthan four-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 in Weibull statistical studies.
29、 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. Interferences5.1 The effects of time-depende
30、nt 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 designdata. Slow crack growth can
31、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 C1368.5.2 Surface preparation of test specimens can introducemachining microcracks which may have
32、 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 to obtain fractures from the mat
33、erials 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 options for the prepa-ration of specime
34、ns. 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. Centerless or transversegrinding a
35、ligns 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 componentswhereby it may not be po
36、ssible 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 commonly usedfor rod specimens, but is
37、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 be equipped witha means for reta
38、ining 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 are thepreferred configuration. When possible, use one of the outersupport and inner loading span combinations li
39、sted in Table 1.Other span sizes may be used if these sizes are not suitable fora specific round part. The ratio of the fixture outer span lengthto the specimen diameter shall not be less than 3.0.6.3 Three-Point FlexureThree-point flexure may be usedif four-point is not satisfactory, such as if the
40、 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 for3The boldface numbers in parentheses refe
41、r 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 4C1684 1313a specific round part. The outer fixture span length to specimendiameter ratio shall not be less than 3.0.6.4 Loading Rollers
42、Force 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 permits direct contact of rod speci-mens with loading and support rollers. Direct contact maycause two problems, however.
43、The crossed cylinder arrange-ment creates intense contact stresses in both the loading rollerand the test specimen due to the very small contact footprint.The magnitude of the contact stresses depends upon the appliedforces, the roller and test specimen diameters, and their elasticproperties.6.4.2 S
44、ection 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. Examples are shown in Annex A1.
45、 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 crack faces. In extreme conditi
46、ons, 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 be deleterious and cause an unde
47、sirable 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 upon the test material properties
48、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 fracture will occur since larger force
49、s 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 contact 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
