1、Designation: C1684 131C1684 18Standard 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 cas
2、e of revision, 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.1 NOTEUnits statement was added to the scope editorially in April 2014.1. Scope1.1 This test method is
3、for the determination of flexural strength of rod shape rod-shaped specimens of advanced ceramicmaterials at ambient temperature. In many instances it is preferable to test round specimens rather than rectangular bend specimens,especially if the material is fabricated in rod form. This method permit
4、s testing of machined, drawn, or as-fired rod shapedrod-shaped specimens. It allows some latitude in the rod sizes and cross section shape uniformity. Rod diameters between 1.5 and8 mm and lengths from 25 to 85 mm are recommended, but other sizes are permitted. Four-point-14 point -point as shown in
5、 Fig.1 is the preferred testing configuration. Three-point loading is permitted. This method describes the apparatus, specimenrequirements, test procedure, calculations, and reporting requirements. The method is applicable to monolithic or particulate- orwhisker-reinforced ceramics. It may also be u
6、sed for glasses. It is not applicable to continuous fiber-reinforced ceramic composites.1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.1.3 This standard does not purport to address all of the safety concerns, if any, ass
7、ociated with its use. It is the responsibilityof the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine theapplicability of regulatory limitations prior to use.1.4 This international standard was developed in accordance with internat
8、ionally recognized principles on standardizationestablished in the Decision on Principles for the Development of International Standards, Guides and Recommendations issuedby the World Trade Organization Technical Barriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2C158 Test
9、Methods for Strength of Glass by Flexure (Determination of Modulus of Rupture)C1145 Terminology of Advanced CeramicsC1161 Test Method for Flexural Strength of Advanced Ceramics at Ambient TemperatureC1239 Practice for Reporting Uniaxial Strength Data and Estimating Weibull Distribution Parameters fo
10、r Advanced CeramicsC1322 Practice for Fractography and Characterization of Fracture Origins in Advanced CeramicsC1368 Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-RateStrength Testing at Ambient TemperatureE4 Practices for Force Verification o
11、f Testing MachinesE337 Test Method for Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperatures)3. Terminology3.1 Definitions:3.1.1 complete gage section, nthe portion of the specimen between the two outer loading points in four-point flexure andthree-point flexure f
12、ixtures. C11613.1.2 flaw, na structural discontinuity in an advanced ceramic body that acts as a highly localized stress raiser.1 This test method is under the jurisdiction of ASTM Committee C28 on Advanced Ceramics and is the direct responsibility of Subcommittee C28.01 on MechanicalProperties and
13、Performance.Current edition approved Aug. 1, 2013Jan. 1, 2018. Published September 2013January 2018. Originally approved in 2008. Last previous edition approved in 20082014as C1684 08.131. DOI: 10.1520/C1684-13E01.10.1520/C1684-18.2 For referencedASTM standards, visit theASTM website, www.astm.org,
14、or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes h
15、ave been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official d
16、ocument.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.1.2.1 DiscussionThe presence of such discontinuities does not necessarily imply that the ceramic has been prepared improperly or is faulty. C13223.1.3 flexural strength, FL2, n
17、a measure of the ultimate strength of a specified beam in bending. C1145, C11613.1.4 four-point-14 point -point flexure,nconfiguration of flexural strength testing where a specimen is symmetrically loadedat two locations that are situated one quarter one-quarter of the overall span away from the out
18、er two support loading points (seeFig. 1). C1145, C11613.1.5 fracture origin, nthe source from which brittle fracture commences. C1145, C13223.1.6 inert flexural strength, FL2, na measure of the strength of specified beam in bending as determined in an appropriateinert condition whereby no slow crac
19、k growth occurs.3.1.6.1 DiscussionAn inert condition may be obtained by using vacuum, low temperatures, very fast test rates, or any inert media. C11613.1.7 inherent flexural strength, FL2, nthe flexural strength of a material in the absence of any effect of surface grindingor other surface finishin
20、g process, or of extraneous damage that may be present. The measured inherent strength is in general afunction of the flexure test method, test conditions, and test specimen size. C11613.1.8 inner gage section, nthe portion of the specimen between the inner two loading points in a four-point flexure
21、 fixture.C11613.1.9 slow crack growth (SCG), nsubcritical crack growth (extension) which may result from, but is not restricted to, suchmechanisms as environmentally-assisted environmentally assisted stress corrosion or diffusive crack growth. C1145, C11613.1.10 three-point flexure, nconfiguration o
22、f flexural strength testing where a specimen is loaded at a location midwaybetween 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 m
23、ethod is intended to be used with ceramics whose strength is 50 MPa (7 ksi) or greater. The test methodmay also be used with glass test specimens, although Test Methods C158 is specifically designed to be used for glasses. This testmethod may be used with machined, drawn, extruded, and as-fired roun
24、d specimens. This test method may be used with specimensthat have elliptical cross section geometries.4.2 The flexure strength is computed based on simple beam theory with assumptions that the material is isotropic andhomogeneous, the moduli of elasticity in tension and compression are identical, an
25、d the material is linearly elastic. The averagegrain size should be no greater than one fiftieth one-fiftieth of the rod diameter. The homogeneity and isotropy assumptions in thestandard rule out the use of this test for continuous fiber-reinforced ceramics.FIG. 1 Four-Point-14 Point -Point Flexure
26、Loading ConfigurationC1684 1824.3 Flexural strength of a group of test specimens is influenced by several parameters associated with the test procedure. Suchfactors include the loading rate, test environment, specimen size, specimen preparation, and test fixtures (1-3).3 This methodincludes specific
27、 specimen-fixture size combinations, but permits alternative configurations within specified limits. Thesecombinations were chosen to be practical, to minimize experimental error, and permit easy comparison of cylindrical rod strengthswith data for other configurations. Equations for the Weibull eff
28、ective volume and Weibull effective surface are included.4.4 The flexural strength of a ceramic material is dependent on both its inherent resistance to fracture and the size and severityof flaws in the material. Flaws in rods may be intrinsically volume-distributed throughout the bulk. Some of thes
29、e flaws by chancemay be located at or near the outer surface. Flaws may alternatively be intrinsically surface-distributed with all flaws located onthe outer specimen surface. Grinding cracks fit the latter category. Variations in the flaws cause a natural scatter in strengths fora set of test speci
30、mens. Fractographic analysis of fracture surfaces, although beyond the scope of this standard, is highlyrecommended for all purposes, especially if the data will be used for design as discussed in Refs (3-5) and Practices C1322 andC1239.4.5 The three-point test configuration exposes only a very smal
31、l portion of the specimen to the maximum stress. Therefore,three-point flexural strengths are likely to be greater than four-point flexural strengths. Three-point flexure has some advantages.It uses simpler test fixtures, it is easier to adapt to high temperature and fracture toughness testing, and
32、it is sometimes helpful inWeibull statistical studies. It also uses smaller force to break a specimen. It is also convenient for very short, stubby specimenswhich would be difficult to test in four-point loading. Nevertheless, four-point flexure is preferred and recommended for mostcharacterization
33、purposes.5. Interferences5.1 The effects of time-dependent phenomena, such as stress corrosion or slow crack growth on strength tests conducted atambient temperature, can be meaningful even for the relatively short times involved during testing. Such influences must beconsidered if flexure tests are
34、 to be used to generate design data. Slow crack growth can lead to a rate dependency of flexuralstrength. The testing rate specified in this standard may or may not produce the inert flexural strength whereby negligible slowcrack growth occurs. See Test Method C1368.5.2 Surface preparation of test s
35、pecimens can introduce machining microcracks which may have a pronounced effect on flexuralstrength (6). Machining damage imposed during specimen preparation can be either a random interfering factor, or an inherent partof the strength characteristic to be measured. With proper care and good machini
36、ng practice, it is possible to obtain fractures fromthe materials natural flaws. Surface preparation can also lead to residual stresses. It should be understood that final machiningsteps may or may not negate machining damage introduced during the early coarse or intermediate machining.5.3 This test
37、 method allows several options for the preparation of specimens. The method allows testing of as-fabricated (e.g.,(for example, 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
38、for many (but certainly not all) ceramics. Centerless or transverse grindingaligns the severest machining microcracks perpendicular to the rod tension stress axis. The specimen may fracture from the3 The boldface numbers in parentheses refer to the list of references at the end of this standard.FIG.
39、 2 Three-Point Flexure Loading ConfigurationC1684 183machining microcracks. Transverse-ground specimens in many instances may provide a more “practical strength” that is relevantto machined ceramic components whereby it may not be possible to favorably align the machining direction. Therefore, this
40、testmethod allows transverse grinding for normal specimen preparation purposes. Longitudinal grinding, which is commonly used toorient grinding damage cracks in rectangular bend bars, is less commonly used for rod specimens, but is also permitted by thistest method.6. Apparatus6.1 LoadingSpecimens m
41、ay be loaded in any suitable testing machine provided that uniform rates of direct loading can bemaintained. The force measuring system shall be free of initial lag at the loading rates used and shall be equipped with a meansfor retaining read-out of the maximum force applied to the specimen. The ac
42、curacy of the testing machine shall be in accordancewith Practices E4.6.2 Four-Point FlexureFour-point-14 point -point fixtures are the preferred configuration. When possible, use one of the outersupport and inner loading span combinations listed in Table 1. Other span sizes may be used if these siz
43、es are not suitable for aspecific round part. The ratio of the fixture outer span length to the specimen diameter shall not be less than 3.0.6.3 Three-Point FlexureThree-point flexure may be used if four-point is not satisfactory, such as if the specimens are veryshort and stubby and consequently re
44、quire very large breaking forces in four-point loading. When possible, use one of the supportspans listed in Table 1 for three-point loading. Other span sizes may be used if these sizes are not suitable for a specific round part.The outer fixture span length to specimen diameter ratio shall not be l
45、ess than 3.0.6.4 Loading RollersForce shall be applied to the test pieces directly by rollers as described in this section (6.4) or alternativelyby rollers with cradles as described in 6.5.6.4.1 This test method permits direct contact of rod specimens with loading and support rollers. Direct contact
46、 may cause twoproblems, however. The crossed cylinder arrangement creates intense contact stresses in both the loading roller and the testspecimen due to the very small contact footprint. The magnitude of the contact stresses depends upon the applied forces, the rollerand test specimen diameters, an
47、d their elastic properties.6.4.2 SectionParagraph 6.4.5 provides guidance on how to minimize or eliminate permanent deformation that may occur in theloading rollers due to contact stresses.6.4.3 Direct loading by rollers onto the rod test specimens may cause premature test specimen fracture invalida
48、ting the test.Examples are shown in Annex A1. Contact stresses may generate shallow Hertzian cone cracks in the test specimen. Minorcracking at an inner loading point (on the compression-loaded side of the test rod) usually is harmless since it does not causespecimen breakage and forces are transmit
49、ted through the crack faces. In extreme conditions, however, such as loading of short,stubby specimens in 3-pointthree-point or 4-pointfour-point loading, the magnitude of the forces and contact stresses may be greatenough to drive a Hertzian crack deep into the test specimen cross section. Contact cracks at the outer support rollers may bedeleterious and cause an undesirable fracture of the specimen, even though these locations are far away from the inner span in4-pointfour-point loading or the middle in 3-pointthree-point loading. Examples o
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