1、Designation: C 1161 02c (Reapproved 2008)1Standard Test Method forFlexural Strength of Advanced Ceramics at AmbientTemperature1This standard is issued under the fixed designation C 1161; 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 () indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1NOTEAdded research report footno
3、te to Sections 11.3, 11.4, and 11.5 editorially in September 2008.1. Scope1.1 This test method covers the determination of flexuralstrength of advanced ceramic materials at ambient temperature.Four-point14 point and three-point loadings with prescribedspans are the standard. Rectangular specimens of
4、 prescribedcross-section sizes are used with specified features in pre-scribed specimen-fixture combinations.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 con
5、cerns, 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 Standards:2E4 Practices for Force Verification of
6、 Testing MachinesC 1239 Practice for Reporting Uniaxial Strength Data andEstimating Weibull Distribution Parameters for AdvancedCeramicsC 1322 Practice for Fractography and Characterization ofFracture Origins in Advanced CeramicsC 1368 Test Method for Determination of Slow CrackGrowth Parameters of
7、Advanced Ceramics by ConstantStress-Rate Flexural Testing at Ambient TemperatureE 337 Test Method for Measuring Humidity with a Psy-chrometer (the Measurement of Wet- and Dry-Bulb Tem-peratures)2.2 Military Standard:MIL-STD-1942 (MR) Flexural Strength of High Perfor-mance Ceramics at Ambient Tempera
8、ture33. Terminology3.1 Definitions:3.1.1 complete gage section, nthe portion of the specimenbetween the 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 st
9、atistical analysis onlyincludes portions of the specimen volume or surface which experiencetensile stresses.3.1.2 flexural strengtha measure of the ultimate strengthof a specified beam in bending.3.1.3 four-point14 point flexureconfiguration of flexuralstrength testing where a specimen is symmetrica
10、lly loaded attwo locations that are situated one quarter of the overall span,away from the outer two support bearings (see Fig. 1).3.1.4 Fully-articulating fixture, na flexure fixture de-signed to be used either with flat and parallel specimens or withuneven or nonparallel specimens. The fixture all
11、ows fullindependent articulation, or pivoting, of all rollers about thespecimen 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 A1 for schematic illustrations of the requ
12、iredpivoting movements.NOTE 3A three-point fixture has the inner pair of bearing cylindersreplaced by a single bearing cylinder.3.1.5 inert flexural strength, na measure of the strength ofspecified beam in bending as determined in an appropriate inertcondition whereby no slow crack growth occurs.NOT
13、E 4An inert condition may be obtained by using vacuum, low1This 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 200
14、8. Originallyapproved in 1990. Last previous edition approved in 2002 as C 116102ce1.2For 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
15、page onthe ASTM website.3Available from Standardization Documents Order Desk, DODSSP, Bldg. 4,Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http:/www.dodssp.daps.mil.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.tempera
16、tures, very fast test rates, or any inert media.3.1.6 inherent flexural strength, nthe flexural strength of amaterial in the absence of any effect of surface grinding orother surface finishing process, or of extraneous damage thatmay be present. The measured inherent strength is in general afunction
17、 of the flexure test method, test conditions, andspecimen size.3.1.7 inner gage section, nthe portion of the specimenbetween the inner two 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 all
18、owssome articulation, or pivoting, to ensure the top pair (or bottompair) of bearing cylinders pivot together about an axis parallelto the 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 cyli
19、nders on eitherside.NOTE 5See Annex A1 for schematic illustrations of the requiredpivoting movements.NOTE 6A three-point fixture has the inner pair of bearing cylindersreplaced by a single bearing cylinder.3.1.9 slow crack growth (SCG), nsubcritical crack growth(extension) which may result from, but
20、 is not restricted to, suchmechanisms as environmentally-assisted stress corrosion ordiffusive crack growth.3.1.10 three-point flexureconfiguration of flexuralstrength testing where a specimen is loaded at a locationmidway between two support bearings (see Fig. 1).4. Significance and Use4.1 This tes
21、t 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 (7 ksi) or greater.4.2 The flexure stress is computed based on simple beamtheory with assumptions that th
22、e material is isotropic andhomogeneous, the moduli of elasticity in tension and compres-sion are identical, and the material is linearly elastic. Theaverage grain size should be no greater than one fiftieth of thebeam thickness. The homogeneity and isotropy assumption inthe standard rule out the use
23、 of this test for continuousfiber-reinforced ceramics.4.3 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.Specimen siz
24、es and fixtures were chosen to provide a balancebetween practical configurations and resulting errors, as dis-cussed in MIL-STD 1942 (MR) and Refs (1) and (2).4Specificfixture and specimen configurations were designated in order topermit ready comparison of data without the need for Weibull-size sca
25、ling.4.4 The flexural strength of a ceramic material is dependenton 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 s
26、cope of thisstandard, is highly recommended for all purposes, especially ifthe data will be used for design as discussed in MIL-STD-1942(MR) and Refs (25) and Practices C 1322 and C 1239.4.5 The three-point test configuration exposes only a verysmall portion of the specimen to the maximum stress. Th
27、ere-fore, three-point flexural strengths are likely 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 fracture toughness testing, and it issometimes helpful in Weibull statistical
28、studies. However,four-point flexure is preferred and recommended for mostcharacterization purposes.4.6 This method determines the flexural strength at ambienttemperature and environmental conditions. The flexuralstrength under ambient conditions may or may not necessarilybe the inert flexural streng
29、th.NOTE 7time dependent effects may be minimized through the use ofinert testing atmosphere such as dry nitrogen gas, oil, or vacuum.Alternatively, testing rates faster than specified in this standard may beused. Oxide ceramics, glasses, and ceramics containing boundary phaseglass are susceptible to
30、 slow crack growth even at room temperature.Water, either in the form of liquid or as humidity in air, can have asignificant effect, even at the rates specified in this standard. On the otherhand, many ceramics such as boron carbide, silicon carbide, aluminumnitride and many silicon nitrides have no
31、 sensitivity to slow crack growthat room temperature and the flexural strength in laboratory ambientconditions is the inert flexural strength.4The boldface numbers in parentheses refer to the references at the end of thistest method.NOTE 1Configuration:A: L = 20 mmB: L = 40 mmC: L = 80 mmFIG. 1 The
32、Four-Point14 Point and Three-Point FixtureConfigurationC 1161 02c (2008)125. Interferences5.1 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
33、testing. Such influences must beconsidered if flexure tests are to be used to generate designdata. Slow crack growth can lead a rate dependency of flexuralstrength. The testing rate specifed in this standard may or maynot produce the inert flexural strength whereby negligible slowcrack growth occurs
34、. See Test Method C 1368.5.2 Surface preparation of test specimens can introducemachining microcracks which may have a pronounced effecton flexural strength. Machining damage imposed during speci-men preparation can be either a random interfering factor, or aninherent part of the strength characteri
35、stic to be measured.Withproper care and good machining practice, it is possible toobtain fractures from the materials natural flaws. Surfacepreparation can also lead to residual stresses. Universal orstandardized test methods of surface preparation do not exist. Itshould be understood that final mac
36、hining steps may or maynot negate machining damage introduced during the earlycourse or intermediate machining.5.3 This test method allows several options for the machin-ing of specimens, and includes a general procedure (“Stan-dard” procedure, 7.2.4), which is satisfactory for many (butcertainly no
37、t all) ceramics. The general procedure used pro-gressively finer longitudinal grinding steps that are designed tominimize subsurface microcracking. Longitudinal grindingaligns the most severe subsurface microcracks parallel to thespecimen tension stress axis. This allows a greater opportunityto meas
38、ure the inherent flexural strength or “potential strength”of the material as controlled by the materials natural flaws. Incontrast, transverse grinding aligns the severest subsurfacemachining microcracks perpendicular to the tension stress axisand the specimen is more likely to fracture from the mac
39、hiningmicrocracks. Transverse-ground specimens in many instancesmay provide a more “practical strength” that is relevant tomachined ceramic components whereby it may not be possibleto favorably align the machining direction. Transverse-groundspecimens may be tested in accordance with 7.2.2. Data fro
40、mtransverse-ground specimens may correlate better with datafrom biaxial disk or plate strength tests, wherein machiningdirection cannot be aligned.6. Apparatus6.1 LoadingSpecimens may be loaded in any suitabletesting machine provided that uniform rates of direct loadingcan be maintained. The force-m
41、easuring system shall be free ofinitial lag at the loading rates used and shall 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 E4but within 0.5 %.6.2 Four-Point FlexureFour-point14 p
42、oint fixtures (Fig.1) shall have support and loading spans as shown in Table 1.6.3 Three-Point FlexureThree-point fixtures (Fig. 1) shallhave a support span as shown in Table 1.6.4 BearingsThree- and four-point flexure:6.4.1 Cylindrical bearing edges shall be used for the supportof the test specimen
43、 and for the application of load. Thecylinders shall be made of hardened steel which has a hardnessno less than HRC 40 or which has a yield strength no less than1240 MPa (;180 ksi). Alternatively, the cylinders may bemade of a ceramic with an elastic modulus between 2.0 and 4.03 105MPa (3060 3 106ps
44、i) and a flexural strength no lessthan 275 MPa (;40 ksi). The portions of the test fixture thatsupport the bearings may need to be hardened to preventpermanent deformation. The cylindrical bearing length shall beat least three times the specimen width. The above require-ments are intended to ensure
45、that ceramics with strengths up to1400 MPa (;200 ksi) and elastic moduli as high as 4.8 3 105MPa (70 3 106psi) can be tested without fixture damage.Higher strength and stiffer ceramic specimens may requireharder bearings.6.4.2 The bearing cylinder diameter shall be approximately1.5 times the beam de
46、pth of the test specimen size employed.See Table 2.6.4.3 The bearing cylinders shall be carefully positionedsuch that the spans are accurate within 60.10 mm. The loadapplication bearing for the three-point configurations shall bepositioned midway between the support bearing within 60.10mm. The load
47、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 flex
48、ure which need not rotate). Thiscan be accomplished by mounting the cylinders in needlebearing assemblies, or more simply by mounting the cylindersas shown in Fig. 2 and Fig. 3. Annex A1 illustrates the actionrequired of the bearing cylinders. Note that the outer-supportbearings roll outward and the
49、 inner-loading bearings rollinward.6.5 SemiarticulatingFour-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. A1.1a. All four bearings shall be free to roll. Thetwo inner bearings shall be parallel to each other to within0.015 mm over their length and they shall articulate together asa pair. The two outer bearings shall be parallel to each other towithin 0.015 mm over their length and they shall articulatetogether as a pair. Th
