C1322

Designation: C 1322 96aStandard Practice forFractography and Characterization of Fracture Origins inAdvanced Ceramics1This standard is issued under the xed designation C 1322; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, 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. Scope1.1The objective of this practice is to provide an efficientand consistent methodology to locate and characterize fractureorigins inadvancedceramics. It is applicable toadvancedceramics whichare brittle; that is, the material adheres toHookes Lawup to fracture. In such materials, fracturecommences from a single location which is termed the fractureorigin. The fracture origin in brittle ceramics normally consistsof some irregularity or singularity in the material which acts asa stress concentrator. In the parlance of the engineer orscientist, these irregularities are termed aws or defects. Thelattershouldnot beconstruedtomeanthat thematerial hasbeen prepared improperly or is somehow faulty.1.2Thisstandarddoesnot purport toaddressall 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 Documents2.1ASTM Standards:C 162Terminology of Glass and Glass Products2C 242Terminologyof Ceramic Whitewares andRelatedProducts2C 1145Terminology of Advanced Ceramics3C 1211Test Method for Flexural Strength of AdvancedCeramics at Elevated Temperatures3C 1239Practice for Reporting Uniaxial Strength Data andEstimating Weibull Distribution Parameters for AdvancedCeramics3C 1256Practice for Interpreting Glass Fracture SurfaceFeatures2F 109TerminologyRelatingtoSurfaceImperfections onCeramics32.2 Military Standard:Military Handbook 790, Fractography and Characteriza-tion of Fracture Origins in Advanced Structural Ceramics,199243. Terminology3.1GeneralThe following terms are given as a basis foridentifying fracture origins that are common to advancedceramics. It shouldberecognizedthat originscanmanifestthemselves differently in various materials. The photographs inAppendix X1 show examples of the origins dened in 3.8 and3.9. Termsthat arecontainedinotherASTMstandardsarenoted at the end of the each denition.3.2advanced ceramic, na highly engineered, high-performance, predominatelynonmetallic, inorganic, ceramicmaterial having specic functional attributes. C 11453.3aw, nastructural discontinuityinanadvancedce-ramic body that acts as a highly localized stress raiser.NOTE 1The presence of suchdiscontinuities does not necessarilyimply that the ceramic has been prepared improperly or is faulty.3.4fractureorigin,nthe source from which brittle frac-ture commences. C 11453.5hackle, nas used in fractography, a line or lines on thecrack surface running in the local direction of cracking,separating parallel but noncoplanar portions of the cracksurface.3.6mirror, nas used in fractography of brittle materials, avery smooth region in the immediate vicinity of and surround-ing the fracture origin.3.7mist, nasusedinfractographyof brittlematerials,markingsonthesurfaceofanacceleratingcrackclosetoitseffective terminal velocity, observable rst as a misty appear-anceandwithincreasingvelocityrevealsabroustexture,elongated in the direction of crack propagation.3.8Inherently Volume-Distributed Origins:3.8.1agglomerate, n, (A(V))asusedinfractography, acluster of grains, particles, platelets, or whiskers, or a combi-nation thereof, present in a larger solid mass.NOTE 2The codes in parentheses after each term are provided for usein statistical analysis. Asuperscript Vstands for inherently volume-distributedoriginsandasuperscriptSforinherentlysurface-distributedorigins. C 11451This practice is under the jurisdiction of ASTM Committee C-28 on AdvancedCeramicsand is the direct responsibility of Subcommittee C28.05 on Processing.CurrenteditionapprovedDec. 10, 1996. PublishedFebruary1997. Originallypublished as C 1322 96. Last previous edition C 1322 96e1.2Annual Book of ASTM Standards, Vol 15.02.3Annual Book of ASTM Standards, Vol 15.01.4AvailablefromArmyResearchLaboratory-Materials Directorate, AberdeenProving Ground, MD 21005.1AMERICAN SOCIETY FOR TESTING AND MATERIALS100 Barr Harbor Dr., West Conshohocken, PA 19428Reprinted from the Annual Book of ASTM Standards. Copyright ASTM3.8.2compositional inhomogeneity, n, (CI(V))as used infractography, a microstructural irregularity related to thenonuniform distribution of an additive, a different crystalline orglass phase or in a multiphase material, the nonuniformdistribution of a second phase. C 11453.8.3crack, n, (CK(V))as used in fractography, a plane offracture without complete separation. C 11453.8.4inclusion, n, (I(V))asusedinfractography, afor-eign body from other than the normal composition of the bulkadvanced ceramic. C 11453.8.5large grain(s), n, (LG(V))as used in fractography, asingle (or cluster of) grain(s) having a size signicantly greaterthanthatencompassedbythenormalgrainsizedistribution.C 11453.8.6pore, n, (P(V))asusedinfractography, adiscretecavity or void in a solid material. C 11453.8.7porous region, n, (PR(V))as used in fractography, a3-dimensional zone of porosity or microporosity. C 11453.8.8porous seam, n, (PS(V))as used in fractography, a2-dimensional area of porosity or microporosity. C 11453.9Inherently Surface-Distributed Origins:3.9.1handling damage, n, (HD(S))as used in fractogra-phy, scratches, chips, cracks, etc., due to the handling of thespecimen/component. C 11453.9.2machiningdamage,n,(MD(S))asusedinfractog-raphy, surface/subsurface microcracks or chips created duringthe machining process, for example, striations, scratches, andimpact cracks.NOTE 3Machiningmayresult insurfaceor subsurfacecracks, orboth.3.9.3pit, n, (PT(S))as used in fractography, a cavitycreated on the specimen/component surface during thereaction/interaction between the material and the environment,for example, corrosion or oxidation. C 11453.9.4surfacevoid, n, (SV(S))asusedinfractography, acavity created at the surface/exterior as a consequence of thereaction/interactionbetweenthematerial andtheprocessingenvironment, for example, surface reaction layer or bubble thatis trapped during processing.3.10Miscellaneous Origins:3.10.1unidentied origin, n, (?)as used in this practice,an uncertain or undetermined fracture origin.3.11Other terms or fracture origin types may be devised bythe user if those listed in 3.8 and 3.9 are inadequate. In suchinstances the user shall explicitlydene the nature of thefracture origin (aw) and whether it is inherently volume- orsurface-distributed. Additional terms for surface imperfectionscan be found in Terminology F 109 and supplementary fractureorigintypesfor ceramicsandglassesmaybefoundinTheCeramic Glossary5and Terminologies C 162 and C 242. Ex-amples of additional terms are hard agglomerate, glassyinclusion, chip, or closed chip.3.12The word surface may also apply to the exterior of atest specimen cut froma bulk ceramic or component, oralternatively, the original surface of the component in theas-red state. It is recommended that the terms original-surfaceoras-processedsurfacebeusedifappropriate, forexample,as-processed, surface-distributed origin.4. Summary of Practices4.1Wheneverpossible, test thespecimen(s)/component(s)to failure in a fashion that preserves the primary fracturesurface(s) and all associated fragments for further fracto-graphic analysis.4.2 Carefully handle and store the specimen(s)/component(s) to minimize additional damage or contaminationof the fracture surface(s), or both.4.3Visually inspect the fractured specimen(s)/component(s)(1 to 103) in order to determine crack branching patterns, anyevidenceof abnormal failurepatterns (indicativeof testingmisalignments), the primary fracture surfaces, the location ofthe mirror and, if possible, the fracture origin. Specimen/component reconstruction may be helpful in this step.4.4Useanoptical microscope(10to2003) toexamineboth mating halves of the primary fracture surface in order tolocate and, if possible, characterize the origin. If the fractureorigin cannot be characterized, then conduct the optical exami-nation with the purpose of expediting subsequent examinationwith the scanning electron microscope (SEM).4.5Inspect the external surfaces of the specimen(s)/component(s) near the origin for evidence of handling ormachining damage or any interactions that may have occurredbetween these surfaces and the environment.4.6Clean and prepare the specimen(s)/component(s) forSEM examination, if necessary.4.7Carryout SEMexamination(10to20003) of bothmating halves of the primary fracture surface.4.8Characterize the strength-limiting origin by its identity,location, and size. When appropriate, use the chemical analysiscapability of the SEM to help characterize the origin.4.9If necessary, repeat 4.5 using the SEM.4.10Keep appropriate records and photographs at each stepinordertocharacterizetheorigin, showitslocationandthegeneral features of the fractured specimen/component, as wellas for future reference.4.11Compare the measured origin size to that estimated byfracture mechanics. If these sizes are not in general agreementthen an explanation shall be given to account for the discrep-ancy.4.12For anewmaterial, or anewset of processingorexposure conditions, it is highly recommended that a represen-tative polished section of the microstructure be photographedto show the normal microstructural features such as grain sizeand porosity.5. Signicance and Use5.1This practice is suitable for monolithic and some com-posite ceramics, for example, particulate- and whisker-reinforced and continuous-grain-boundary phase ceramics.(Long- or continuous-ber reinforced ceramics are excluded.)For some materials, the location and identication of fractureorigins may not be possible due to the specic microstructure.5.2This practice is principally oriented towards character-ization of fracture origins in specimens loaded in so-called fast5The American Ceramic Society, Westerville, OH 1984.C 13222fracture testing, but the approach can be extended to includeother modes of loading as well.5.3Theprocedures describedwithinareprimarilyappli-cable to mechanical test specimens, although the same proce-dures may be relevant to component failure analyses as well. Itiscustomarypracticetotestanumberofspecimens(consti-tuting a sample) to permit statistical analysis of the variabilityof the materials strength. It is usually not difficult to test thespecimensinamannerthatwillfacilitatesubsequentfracto-graphicanalysis. Thismaynot bethecasewithcomponentfailure analyses.5.4Optimum fractographic analysis requires examination ofasmanysimilarspecimensorcomponentsaspossible. Thiswill enhance the chances of successful interpretations. Exami-nation of only one or a few specimens can be misleading. Ofcourse, in some instances the fractographer may have access toonly one or a few fractured specimens or components.5.5Successful and complete fractography also requirescareful consideration of all ancillary information that may beavailable, such as microstructural characteristics, materialfabrication, properties and service histories, component orspecimen machining, or preparation techniques.NOTE 4A VAMAS round robin on fractographic analysis of ceramicorigins highlights the importance of suchadditional information. SeeARL-TR-656 (or VAMAS Report No. 19) for details.6,75.6Fractographicinspectionandanalysis canbeatime-consumingprocess. Experiencewill ingeneral enhancethechances of correct interpretation and characterization, but willnot obviate the need for time and patience.5.7This practice is applicable to quality control, materialsresearch and development, and design. It will also serve as abridge betweenmechanical testingstandards andstatisticalanalysispracticestopermit comprehensiveinterpretationofdatafordesign.Animportant featureofthispracticeistheadoption of a consistent manner of characterizing fractureorigins, including origin nomenclature. This will further enablethe construction of efficient computer databases.5.8Theirregularitieswhichact asfractureoriginsinad-vanced ceramics can develop during or after fabrication of thematerial. Large irregularities (relative to the average size of themicrostructural features) such as pores, agglomerates, andinclusions are typically introduced during processing and can(in one sense) be considered intrinsic to the manufacture. Otherorigins can be introduced after processing as a result ofmachining, handling, impact, wear, oxidation, andcorrosion.These can be considered extrinsic origins. However, machiningdamage may be considered intrinsic to the manufacture to theextent that machining is a natural consequence of producing anished specimen or component. It is beyond the scope of thispractice to discuss the development of origins or their behaviorfrom a fracture mechanics viewpoint.NOTE 5For additional information on fracture origins and theirbehavior from a fracture mechanics viewpoint see Appendix X2. Fracturemechanicsisusedinthispracticeasacheckonthesizeofthefeatureidentied as an origin (see 7.2.4.4).5.9Regardless of howorigins develop they are eitherinherentlyvolume-distributedthroughout thebulkofthece-ramicmaterial (for example, agglomerates, largegrains, orpores) or inherently surface-distributed on the ceramic material(for example, handling damage, pits from oxidation, or corro-sion). The distinction is a consequence of how the specimen orcomponent is prepared. For example, inclusions may bescattered throughout the bulk ceramic material (inherentlyvolume-distributed), but whena particular specimenis cutfrom the bulk ceramic material the strength-limiting inclusioncouldbe locatedat the specimensurface. Thus a volume-distributed origin in a ceramic material can be in any specimen,volume-located, surface-located, near surface-located, or edge-located.5.10As fabricators improvematerials bycareful processcontrol, thus eliminating large, abnormal microstructural fea-tures, advanced ceramics will become strength-limited byorigins that come from the large-sized end of the distribution ofthe normal microstructural features. Such origins can beconsideredmainstreammicrostructural features. Inother in-stances, regions of slightlydifferent microstructure (locallyhigher microporosity) or microcracks between grains (possiblyintroduced by thermoelastic strains) may act as failure origins.These origins will blend in well with the background micro-structure and will be extremely difficult or impossible todiscern even with careful scanning electron microscopy. Thispracticecanstillbeusedtoanalyzesuchfailureorigins,butspecic origin denitions may need to be devised.NOTE 6SeeX2.4.5 for examples.5.11Thispracticeisaderivativeof andanextensionofMilitary Handbook 790 and includes revisions prompted by afractographic round-robin exercise which was organized underthe auspices of VAMAS (Versailles Project on AdvancedMaterialsandStandards). Theresultsofthisexercisecanbefound in ARL-TR-6566and VAMAS Report No. 197(asynopsis of these results can be found inAppendix X3).Additional background information is available in MTL Tech-nical Report TR 90-57.86. Apparatus6.1GeneralExamples of the equipment described in 6.2through 6.6 are illustrated in Appendix X4.6.2Binocular Stereomicroscope, with adjustable magnica-tion between 10 to 2003 and directional light source (see Fig.X4.1.). Acamera or video monitor systemused with thismicroscope is a useful option (see Fig. X4.2.).6.3Cleaning and Preparation Equipment, such as an ultra-sonic bath and a diamond cut-off wheel.6.4ScanningElectronMicroscope(SEM), withenergyorwavelength dispersive spectroscopy (see Fig. X4.3).6Swab, J. J., and Quinn, G. D., Fractography of Advanced Structural Ceramics:Results fromthe VAMAS Fractography Round Robin Exercise, U.S. ArmyResearch Laboratory, Watertown, MA, 02172 ARL-TR-656, December 1994.7Swab, J. J., and Quinn, G. D., Fractography of Advanced Structural Ceramics:Results from the VAMAS Fractography Round Robin Exercise, VAMAS ReportNo. 19, National Institute of Standards and Technology, Gaithersburg, MD 20899,February 1995.8Quinn, G. D., Swab, J. J., and Slavin, M. J., A Proposed Standard Practice forFractographic AnalysisofMonolithic AdvancedCeramics,U.S. ArmyMaterialsTechnology Laboratory, Watertown, MA, 02172 MTL TR 90-57, November 1990.C 132236.5Peripheral Equipment, suchashandmagnifyinglens,tweezers, grips, and compressed air, as shown in Fig. X4.4.6.6Macrophotography Camera Stand(see Fig. X4.5), if acamera system is not available on the stereomicroscope.7. Detailed Procedures and Characterization7.1Procedure:7.1.1GeneralLocation, identication, andcharacteriza-tion of fracture origins in advanced ceramics can sometimes beaccomplished using simple optical microscopy techniquesthoughit moreoftenrequiresscanningelectronmicroscopy(SEM). It may not be feasible, practical, or even necessary toexamineall fracturesurfaces withtheSEM. Theextent offractographic analysis required will depend upon the purposeof the analysis andthe fractographic conduciveness of thematerial.7.1.1.1The nature of the fractographic analysis will dependon whether the results will be used for quality control,materials research and development, or design. Table 1 givessuggestedsamplingguidelines for medium-to-highstrengthadvanced ceramics.7.1.1.2The fractographic analysis will also depend on theconduciveness of the material to this analysis. Some ceramicsare easy to analyze; fracture origins are readily visible with anoptical microscope and the SEM is not needed. Alternatively,origins may be too small to discern with an optical microscope,difficult to differentiate from the normal microstructure, or toodifficult toseeinsometranslucent materials, thus, theSEMexamination is necessary. Coarse-grained or porous materialsmay have no fractographic markings that permit origin identi-cation, and optical and SEM microscopy will prove useless.7.1.2An origin type may not reveal itself clearly in somespecimens and may only be detected after a number ofexamples are viewed and a pattern begins to emerge. It is oftennecessary to reexamine many of the specimens and reevaluatethe initial appraisal. Fractographic interpretations basedononly one or a few specimens can be very misleading.NOTE 7The examination of all specimens shall include the examina-tion of both mating halves of the primary fracture surface irrespective ofthe purpose of the fractographic analysis.7.1.3To maximize the amount of information obtained froma fractographic exercise, care shall be taken in all steps startingwiththeinitial testingofthespecimenorcomponent. Notetaking and record keeping during every step of the procedurewill greatly assist the analyst in understanding the originpopulations in a material, comparing the populations betweenmaterials, and reviewing the data at some later date.7.1.4Specimens that fail duringmachining, handling, orwithout measurement of a failure stress, should be examined,when feasible, to determine the fracture origins. The fact thatthese types of fracture occurred should be noted and reported.7.1.5 Mechanical TestingAfew simple precautionsshould be taken prior to breaking the specimen. The test siteshouldbe kept cleantominimize pickupof contaminants.Markingsofsomesortshouldbeplacedonthespecimentomaintain a point of reference and to aid in the reconstruction ofthe specimen. The markings shall not damage the specimen orlead to contamination of the fracture surfaces. Ane pencil lineis often sufficient to mark the gage length (maximum stress) ina exure specimen or for a circular specimen, to be tested indirect tension, anaxial, zero-degree reference. Testingthatallowsthebrokenfragmentsofthespecimentohurtleaboutshall beavoided. Incidental impact damagetothefracturesurfaces can destroy the origin, alter its appearance, or causesecondary fractures. A compliant material that covers the hardsurfaces of the xture or prevents pieces from ying about, orboth, is sufficient to minimize this damage. All fragments fromthe broken specimen shall be retained for reconstruction,unlessit canbepositivelyestablishedthat somepiecesareincidental or trivial.7.1.6HandlingandStorageBrokenspecimens shall behandled and stored so as to minimize the possibility of damageor contamination of the fracture surfaces, or both. Avoidhandlingthespecimen, especiallythefracturesurface, withyour hands. Body oils and skin fragments can easily change orobscurethecharacter of thefracturesurface. Duringrecon-structionof thespecimen, minimizerubbingthefragmentstogethersincethismayabradeorchipthefracturesurfaces,and damage the fracture surface. Avoid picking or eventouchingthefracturesurfacewithsharpinstrumentsasthismayalterorcontaminatethefracturesurface.Thespecimenshall be stored in a clean and orderly fashion as much time canbe lost trying to sort out mixed-up specimens. Many containerTABLE 1 Suggested Sampling GuidelinesLevel 1 to 103 Visual 10 to 2003 Optical 10 to 20003 SEMLevel 1Quality control Specimens that fail to meet minimumstrength requirementsSpecimens that fail to meet minimumstrength requirementsOptionalLevel 2Quality controlMaterials developmentAll specimens All specimens, if possible, always bothfracture halves; see Note 7Representative specimens, for example:2 of each origin typethe 5 lowest strength specimensat least 2 optically unidentiableorigins, if presentLevel 3Materials developmentDesignAll specimens All specimens, if possible, always bothfracture halves; see Note 7All specimens, or as many specimensas necessary such that combinedoptical and SEM characterize 90 %(100 % for design) of all identiableoriginsC 13224types are readily available for storage, see Fig. X4.4 and Table2.NOTE 8Thelaboratoryenvironment containsamyriadofmaterialssuch as clays, waxes, adhesives, and resins that should be avoidedwherever possible. Many of these materials, once they are affixed to thespecimen, are very tenacious and often impossible to remove. AppendixX5 shows some contaminants on ceramic fracture surfaces as viewed withan SEM.7.1.7Visual Inspection (1 to 103) Visually examine thefragmentedspecimen/component piecesinorder tondtheprimaryfracturesurfaces, thegeneral regionof thefractureorigin, and if possible the fracture mirror. Hand magniers canbe helpful. Reconstruction is valuable in observing the crack(s)and crack branching patterns which, in turn, helps determinetheprimaryfracturesurfacesandcanhelpassessthestressstate if it is not known. Special emphasis should be ondeterminingwhether thefracturepatternindicatesmisalign-ments or breakages at test grips (in tension), at stress concen-trators (neck region in tension), or load application points (inexure and disk tests).NOTE 9For additional information, see X 2.3.5.7.1.7.1Crack patterns can range from very simple to quitecomplex depending upon the specimen or component geometryand the stress states in the body. Multiple fractures are commontohigh-strengthceramicsthat storelargeamountsofelasticenergy during testing. Upon failure, this energy is released andreects fromfree surfaces back through the body of thematerial causing additional fractures. Appendix X6 showsmanypotentialfracturepatternsinsomecommontestspeci-mens. A hierarchy or sequence of crack propagation can assistin backtracking to the primary fracture surfaces. Crack branch-ing can be used to determine the direction of crack propaga-tion. A traveling macrocrack will typically branch into succes-sively more cracks and will rarely rejoin another crack to forma single crack (see Fig. 1). Acrack that intersects another crackat anglescloseto90andstops(doesnot continueintoanadjacent piece) will usually be a secondary crack that can bequickly eliminated since it will not contain the fracture origin.For specimens that do not show macroscopic crack branching,incipient branching in the form of shallow cracks can often befound along the edge of the main crack on the exterior surface.Aswiththemacroscopiccracks, theangleoftheseshallowcracks in relation to the main crack indicate the local directionof crackgrowth. Vicinal illuminationor dyepenetrants, orboth, may be used to make these cracks more easily discern-ible.7.1.7.2Misalignment or deviation from the assumed stressstate can be discerned by fracture surfaces that are at anirregular angle (not 90) to the anticipated maximum principalstress. Branching angles can be helpful in detecting multiaxialstressstates. Frequent breakageat test grips(intension), atstress concentrators (neck region in tension), or load applica-tionpoints(inexureanddisktests)mayindicatemisalign-ment.7.1.7.3Thedetectionofthegeneralregionofthefractureorigin, and the fracture mirror if present, during visual exami-nation depends on the ceramic material being analyzed. Dense,ne-grained, or amorphous ceramics are conducive to fractog-raphyandwill leavedistinct fracturemarkings(hackleandmirror) whichwill aidinlocatingthe origin(see Fig. 2).Hackle lines and ridges on the fracture surface are extremelyhelpful in locating the general vicinity of a fracture origin, evenwhen a fracture mirror is not evident (Fig. 3). They will radiatefrom, and thus point the way back to, the fracture origin. Theyare best highlighted by low incident angle lighting which willcreate useful shadows. Fracture mirrors are telltale features thataretypicallycenteredonthestrength-limitingorigins. Ifthespecimen or component is highly stressed, and the material isne-grained and dense, a distinct fracture mirror will form asshown in Fig. 2. On the other hand, lower energy fractures andthose in coarse-grained or porous ceramics will not leavedistinct fracture markings (Fig. 3). Coarse hackle markings orridges can still be used to determine the vicinity of the fractureorigin, especially with oblique lighting.NOTE 10MTL TR 90-57,8Military Handbook 790, Practice C 1256,and several references in Appendix X2 illustrate and discuss further meansof locating the fracture origin.NOTE 11Coarse-grainedor porous materials mayhave nofracto-graphic markings that permit origin identication, and optical and SEMwill prove useless.7.1.8Optical Microscopy (10 to 2003)Examine bothmatinghalvesoftheprimaryfracturesurface. Thisisoftenperformed in conjunction with the visual inspection. Thepurposeof theoptical examinationistolocatethefractureorigin on the primary fracture surfaces (Table 1, Levels 23)and attempt to characterize the origin. If characterization is notpossible duringthis step, the optical examinationhelps tominimize the time spent during the subsequent SEM examina-tion.TABLE 2 List of Some Commonly Used Storage Media for Fractured Specimens or Components (see Fig. X 4.4)Storage Media Advantages DisadvantagesEnvelopes Convenient for notes, minimal space required, inexpensive Lint contamination, specimen is free to moveGlass vials Very clean, reusable Hard surface could cause secondary fracture, specimen free tomove, expensivePlastic trays Clean, inexpensive, save space Plastic contamination, specimen free to moveTape Inexpensive, mark primary fracture with notes, maintainreconstructed specimenAdhesive contamination, limited shelf lifeFIG. 1 Schematic of Typical Fracture Patterns Showing CrackBranchingC 132257.1.8.1A stereomicroscope is preferred for examining frac-ture surfaces due to its excellent depth of eld. Viewing will bemost effective in the 10 to 2003range since at highermagnications the depth of eld is reduced. A traversing stagecoupled with crosshairs or a graduated reticule in the eyepieceis useful for measuring the size or area, or both, of the mirrorand, if possible, the origin. Illumination should be provided byacommonmicroscopelight sourcewithadjustableintensityand angle of incidence to provide a means of variable lighting.These variations can highlight aspects of the fracture surfacethat may be hidden if one is restricted to a single view.7.1.8.2The specimen should be mounted to viewthefracture andexternal surfaces. Aholder, suchas a simplealligator clip attached to a stand with a exible arm and havingacompliant coatingorsheathcoveringtheteeth, providesasturdy grip (Fig. X4.4) for examination. Viewing both of themating primary fracture surfaces simultaneously can expediteandimprove the qualityof the analysis since what mightappear to be a pore on one half may show an agglomerate onthe other (exure specimens should be mounted tensilesurface-to-tensile surface). Care shall be taken so that extrane-ous damage is not created.NOTE 12DO NOTuseclaysorwaxesformountingbecausethesematerialscancontaminatethefracturesurfaceandareverydifficult toremove. Surface contaminants such as lint and dust can be removed easilywith canned or ltered compressed air.NOTE 13Additional illuminationtechniquesandhelpful proceduresare as listed in X2.1.1.7.1.8.3At the lowest magnication, locate the mirror usingthe hackle on the fracture surface. In high-strength, ne-grained, and dense ceramics the origin will be approximatelycentered in the fracture mirror as shown in Figs. 2b and Figs.2c. Hackle lines and ridges will be very helpful since they willradiate outward fromthe fracture origin and mirror. Asdiscussed in 7.1.7, low energy fractures or fractures in porousor coarse-grained ceramics may not lead to mirror formation,but the same principles of using the hackle lines apply. Twisthacklelinesareespeciallyhelpful andoccur whenacrackNOTE 1(A) A schematic of a aw located at the surface.(B) An optical micrograph of a surface-located aw in a biaxial borosilicate crown glass disc fractured in a biaxial ring-on-ring strength test (s 5 118MPa).(C) Schematic of an origin located in the volume.(D) An optical micrograph of a volume-located origin in a siliconized silicon carbide tension specimen (s5 350 MPa).NOTE 2The mirror can be centered around a portion of the origin and not the entire origin. In ceramic terminology, smooth is a relative term.FIG. 2 Fracture Surfaces of Advanced Ceramics Which Failed in a Brittle MannerC 13226encounters a principal stress eld that is not perpendicular tothe original plane of fracture. Twist hackle commences asnely spaced parallel lines which usually merge in the direc-tion of crack propagation, giving rise to the well known riverpattern as shown in Fig. 4.NOTE 14The merger of twist hackle in the direction of crackpropagationis oppositetothetendencyof macrocracks todivergeasdiscussed in 7.1.7.1. These features are usually well dened in glasses andvery ne grained, fully dense polycrystalline ceramics. Such twist hackleoften occurs on individual grains in coarse-grained polycrystalline ceram-ics. (See X2.1.1 for a discussion and illustration of these features.)7.1.8.4Examinetheexternal surfacesof thespecimenorcomponent if the origin is surface- or edge-located. Aspecimenholder (Fig. X4.4) with a at or vee groove can be used to holdthe entire specimen at a convenient working height to view theexternal surfaces. This examination can be especially helpful ifthe origin is not evident on the fracture surface and handling ormachining damage is suspected. It is also helpful in ascertain-ing if any interaction/reaction has occurred between the mate-rial and the environment.7.1.8.5Characterizethestrength-limitingorigininaccor-dance with7.2. Recordobservations pertainingtofeaturesspecictothelighting, suchascolorandreectivity. Theserecords should include, but not be limited to, notes, sketches,andphotographs. Althoughthis extrastepmayseemtime-consuming, it often leads to greater efficiency in the long run.These records are extremely useful for publication and mini-mizing the search time with the SEM. The latter point can notbe underestimated. Novices often lose much time searching fortheoriginorexaminingthewrongareawiththeSEM. TheSEMimagesarequitedifferent fromoptical images, andareorientation time is sometimes necessary.7.1.8.6Reexamine the specimen fracture surfaces if neces-sary. This will be important if a new material is being examinedor if a particular origin type becomes clear only after some orall of the specimens have been examined.7.1.8.7Photograph the fracture surface, if appropriate (see7.1.10). Photomacrography using a camera with extensiontubes or bellows (Fig. X4.5) is exible in that control of overallresolution and depth of eld is possible and the system is notexpensive. Ontheother hand, theconvenienceof havingacamera mounted directly to the binocular microscope forNOTE 1The coarse hackle lines that emanate from the aw can be used to locate the origin.NOTE 2The coarse hackle lines are obvious (arrows) and clearly indicate the location of the origin (a Knoop indentation-induced pre-crack), eventhough a mirror is NOT readily visible.FIG. 3 (A) Schematic of a Flaw in Which a Mirror Has Not Formed and (B) an Optical Micrograph of a Fracture Surface of a SinteredSilicon Nitride Flexure Specimen (s5 227 MPa)NOTE 1The direction of crack propagation is shown by the arrow.FIG. 4 Schematic of Twist Hackle Lines That Form a RiverPatternC 13227photomicrography (Fig. X4.2) is a great time-saver. Withbuilt-inzoomranges from5to1andbeamsplitters, it ispossibletoframe, focus, andshoot quicklyandefficiently.Modern built-in video cameras with monitors can be coupled tocolor printers which give photograph-size hard copies in lessthanoneminuteandwithouttheneedtodealwithlmandnegatives. These video images, with appropriate software, canalso be stored in a digital format (oppy or laser disk). Suchimages can then be retrieved and displayed on a video monitoror ontheSEMmonitor. This is averyefficient means ofcouplingthe twomethods, andenhancedproductivitywillresult.NOTE 15The Metals Handbook listed in X2.2.3, has some helpful tipson lighting techniques for photomacrography.7.1.8.8For translucent ceramics, it may be useful to illumi-nate the fracture surface from the side with low incident angleillumination. Anopaquecardheldnexttothespecimensidecan block the light entering the specimen bulk. This willminimize light scattering frominside the specimen. Alter-nately, it may be useful to coat the fracture surface withevaporated carbon or sputtered gold-palladium prior to opticalexamination. Thiswill oftenimprovethevisibilityof somecrackpropagationpatterns, eliminatesubsurfacereections,and improve the quality of the photographs taken of thefracture surface.NOTE 16Be careful! Coatings that are too thick can cover or obscuresubmicron pores and subtle features in very high-strength advancedceramics. Intheseinstancesit issuggestedthat theSEMexamination(7.1.9) be carried out on uncoated specimens at a low voltage prior to thiscoating. Also, subtle color or contrast variations will be lost or obscuredif the specimen is coated.7.1.8.9In some applications, replicas of a fracture surfacemay be used advantageously. Although extra preparation stepsareinvolved, celluloseacetate, polyvinyl chloride(PVC), orsilicon elastomer replicas can record important features, bothfor optical and SEMexamination. Advantages include (1)elimination of obscuring subsurface features which may hinderthe optical microscopy of transparent or translucent ceramics;(2) provision of an easily stored record of the fracture surfaceof a critical specimen; (3) greater accessibility of curvedsurfaces to high-magnication optical study; or ( 4) study ofunique specimen geometries. Disadvantages include the risk ofaltering the fracture origin (for example, pull-out of an agglom-erate) and loss of color, contrast, or reectivity discrimination.NOTE 17See X2.1.1 for more details.7.1.9SEMExamination (10 to 20003)Examine bothmating halves of the primary fracture surfaces of some or allspecimens in the SEM. Optical microscopy is not alwaysadequate to characterize fracture origins. This is especially truefor strong materials which have very small mirror regions andsmaller origins. Nevertheless, optical microscopy is an essen-tial adjunct to SEM examination since telltale color, contrast,or reectivity features, as well as subtle features such as mist,may be completely lost in electron-microscope viewing. Onceoptical fractography is complete and the origins are character-izedas well as possible, a subset of specimens shouldbepreparedforSEManalysis. Determinationofthenumberofspecimens which will comprise the subset will depend on theintent of the analysis (see Table 1).7.1.9.1Preparation:7.1.9.2(a) Ifnecessarythespecimensshouldbecut toaconsistent height that allows for ease of installation andmovementintheSEM. Wetcuttingshouldbedonesoastoush away the specimen and cutting wheel debris. They shouldbe cut as at as possible to eliminate problems due to excessivetilt, although a slight tilt backwards can be benecial on exurespecimens(thisallowsfor thesimultaneousviewingof thefracture and tensile surfaces). During the cutting process, everypossiblemeasureshouldbetakentoprevent damagetothefracture and external surfaces.7.1.9.3(b) Cut specimens should be ultrasonically cleanedin water or an alternate uid to remove any cutting solutions orother contaminants. Several cleaning uids are listed in Table3.Specimensshouldthenberinsedinaquicklyevaporatingsolvent to remove any nal residue. Solvents such as acetoneor ethanol are recommended for this step. Once cleaned, eachTABLE 3 Cleaning FluidsAgent AdvantagesADisadvantagesATrichloroethylene Removes oils, adhesives, and grease Toxic, harmful by inhalation, in contact with skin, andif swallowedFast cleaner Causes severe irritationNot ammableXylene and Toluene Removes oils, adhesives, and grease FlammableHarmful by inhalation, in contact with skin, and ifswallowedIrritating to eyes, skin, and respiratory systemReadily absorbed through skinAcetone and Ethanol Inexpensive Does not remove all oils, adhesives, or greasesReadily available Longer cleaning time requiredResidual lm from acetoneFlammableRisk of serious damage to eyesIrritating to respiratory system and skinCleaning powder mixed with distilled water and Inexpensive Potential for soap residue to remain on the specimenheated Readily available Difficulty in removing most oils, adhesives, andgreaseIrritating to eyes, skin, and respiratory systemLonger cleaning time requiredASee Material Safety Data Sheets (MSDS) for further information.C 13228specimen should be properly labeled and placed in a separateglass or plastic container to prevent contamination. All subse-quent handling should only be done with tweezers or lint-freegloves and the specimens should not be brought into contactwith tapes, clays, waxes, or brous materials.7.1.9.4(c) Coatingofaceramiciswidelyusedtoreducechargingofthesurfaceandenhanceresolutionandcontrast.However, some of the newSEMequipment is capable ofoperating at low accelerating voltages which minimizes charg-ing. If suchequipment is available, andtimepermits, it isrecommended that the fracture surfaces rst be viewed withouta coating. The use of low accelerating voltages can provide abetter view of the surface topography. If a coating is needed itshouldbecarefullyapplied. Coatings that aretoothickormultiplecoatingsmayobscurefeaturesandleadtomisinter-pretation of the origins.7.1.9.5(d) Athincoating, typically5nm, of carbonorgold-palladium should be applied onto the specimens using avacuumevaporator or sputter coater. The gold-palladiumcoating is recommended for imaging purposes since it providesbetter conductivity. Carbon coatings deposited by evaporationarepreferredfor X-rayemissionanalysisbecausecarbonisnearly transparent to X rays. Athermal evaporation method formetal coatings can be used with a specimen tilted relative to themetal source, creating an oblique deposition. This can be usedtocreateshadows that highlight verynemarkings onthespecimen.NOTE 18See X2.2.3 for additional information.7.1.9.6(e) Specimens may be mounted for examinationeither singly or multiply on stubs using conductive paints. Bothmating halves of the primary fracture surface of each specimenshall bemounted. Specimensshall bemountedwiththecutsurface down and care shall be taken to avoid getting conduc-tive paint on the fracture surface or upper portion of theexternal surfaces. The specimens shall be mounted in asystematic fashion to permit rapid orientation by the observer.For example, exure bars should be aligned with their tensilesurfaces the same way. If a pencil is used to mark the specimenorientation or the approximate location of the origin, exercisecarethat notracesofthepencil material get onornearthefracturesurface. Oncemounted, specimensmaybesprayedwithcompressedair toremoveanylint or lightlyclingingdebris.7.1.9.7ExaminationBegintheexaminationbyorientingthe specimen in the monitor while viewing the specimen at thelowest magnication. Locate the fracture mirror at the lowestmagnication. It is often useful to use an optical photograph asaguidewhentryingtolocatethefracturemirror. Adjustthecontrast andbrightnesstoprovidethemaximumamount ofinformation. Theentiresurfaceshouldbephotographedat alowmagnicationtoprovideaframeof referencefor laterwork. Conventional practice is to orient the specimen image ina consistent manner, that is, place the tensile surface of aexure specimen at the bottom of the photograph.7.1.9.8(a) The SEM may be used either in the secondaryelectron or backscattered electron modes. The former gives afully illuminated image of the surface topography with betterspatial resolution while the latter provides greater heightcontrast due to its sensitivity to the detector orientation.Features not in direct line with the detector are darker or evenin shadow. Backscattered electrons carry both topographic andcompositionaldata. Thisisvaluablefordetectinginhomoge-neities and inclusions. The topographic and compositionalsignals can be separated for further analytical exibility. If theanalystisunsuccessfulincharacterizingtheoriginusingthesecondary electron mode, then the backscattered electron modeshould be tried, or vice versa.NOTE 19See X2.2.2.7.1.9.9(b) Locate, characterize, andphotographthefrac-tureorigin. It shouldbeapproximatelyinthemiddleofthefracture mirror if a mirror exists. Hackle lines which typicallyradiatefromthefractureorigincanalsobeusedtondtheorigin.7.1.9.10(c) Characterize the origin in accordance with 7.2.It maybe necessarytoacquire anenergy- or wavelength-dispersive X-ray analysis of both the origin and the backgroundto determine whether there are any chemical differences.Conventional energy-dispersiveX-rayanalyzersareusedtoobtainanX-rayspectrumfor sodium( z511) andhigheratomic number elements. The spatial resolution is of the orderof1mwithapenetrationof1to2mbelowthesurface.Wavelength-dispersive X-ray analyzers are available whichcandetect elementsdowntoboron(z55). Thesearelesscommonly used since they require extremely at and smoothsurfacesandcrystal spectrometersthat aretunedtospecicwavelengths (elements). Direct correlations between structureand composition can be made by directing X-ray returns ontotheSEMmonitortherebycreatinganX-raydot mapoftheelements present.7.1.9.11(d) Examine the external surfaces of the specimenor component if the origin is surface located. In some cases,such as when handling or machining damage are suspected, itmay be necessary to tilt the specimen slightly in order to viewa portion of the external surfaces. Sometimes a 180 rotationcan help discern subsurface machining-related cracks.7.1.9.12(e) Photograph the fracture origin. This will typi-callybeinthe200to10003range. Useamagnicationinwhich the origin accounts for approximately one third of theframe area. Aphotographshowingthe fracture mirror andsome hackle is also very helpful for later reassessment of anorigin. In many cases, photographs at varying magnicationsare necessary to furnish all the required information regardingthe failure of the specimen. It is recommended that, wheneverpossible, a consistent set of magnications and orientations beusedtopermit comparativeassessmentsbetweenspecimens.Stereo photographic pairs sometimes can reveal topographicaldetails that are important to origin characterization.7.1.9.13(f) Maintain notes and records of the fractographicndings. These may include sketches of the fracture surface,notesontheorigintypeandappearance, locationofphoto-graphs taken, magnication and reference numbers of photo-graphs, whetherornotX-rayspectrawereacquired, andthelocation used to acquire the spectra. When maintaining notes ofacquiredX-rayspectra,alwaysincludetheacceleratingvolt-age, probe current, magnication, dead time, counts and scanC 13229time, working distance, and whether the spectra was taken inscan or spot mode.7.1.9.14(g) Repeat the steps in the SEMexamination(7.1.9.7) for the mating half of the primary fracture surface.7.1.9.15(h) Examine the region in the vicinity of thefracture origin to detect any evidence of stable crack extensionor slow crack growth (SCG). If an origin is surface located, itmay be susceptible to environmentally assisted SCG. If frac-ture is at elevated temperatures, SCG can occur from surface-or volume-located origins. Intergranular crack features near theorigin surrounded by transgranular or mixed transgranular plusintergranular fracture often are suggestive of SCG. However,intergranular markings maybedifficult todistinguishfrommicroporosity in some materials.7.1.9.16(i) OptionalIn polycrystalline ceramics, ob-serve and record the mode of crack propagation (transgranularorintergranular)inthevicinityoftheoriginandalsointheregion outside the mirror.7.1.9.17(j) OptionalIt is highly recommended that esti-matesof thefracturemirror size(mist-hackleboundary) bemade for some or all of the specimens in the sample set or inthecomponents. Uniformguidelinesfor suchmeasurementscurrentlydonot exist, andthefractographer shouldclearlystate inthe report what criteria were usedandillustrativepictures or sketches shall be prepared.7.1.10RecordingFractographic ObservationsIt is rec-ommended that, whenever possible, three photographs be takenof each fracture surface (one set per pair of fracture halves isadequate). As seen in the schematic Fig. 5, these shouldinclude, but not be limited to:(1) Aphotograph(optical or SEM) of theentirefracturesurface;(2) A photograph of the fracture mirror and some surround-ing detail; and(3) A photograph of the origin.NOTE 20This idealized procedure of three photographs per fracturesurfaceisthemost comprehensiverecordkeepingpractice. It maybeimpractical or too time-consuming to perform this on every specimen ina sample set. At a minimum, it should be done for several representativespecimens. In many instances, a reexamination or reappraisal of an originis needed, anda single closeupphotographof anapparent originisinadequatesincethephotographmaybeincompleteor of thewrongfeature. In such instances, photographs of the whole fracture surface andmirror region are invaluable.7.1.11It is highly recommended that a representative pol-ished section be made and photographed to reveal the normalmicrostructure of the ceramic andallowanassessment ofwhether theoriginsareabnormal or normal microstructuralfeatures. The polished section should be thermally or chemi-cally etched if necessary.7.2Origin Characterization:7.2.1GeneralThe fracture origin in each specimen/component shall be characterized by the following threeattributes: identity, location, and size, as summarized in Table4. See Fig. 6 and Fig. 7. For example, pore, volume-distributed;near surface; 30m. Origins areeither inherentlyvolume-distributedthroughoutthebulkofthematerial(forexample,agglomerates, largegrains, or pores) or inherentlysurface-distributed on the material (for example, handling damage, pitsfrom oxidation, or corrosion). An inherently volume-distributedoriginina ceramic material can, inanysinglespecimenorcomponent, bevolume-located, surface-located,nearsurface-located, oredge-located, asseeninFig. 8. Thevariety of locations for a volume-distributed origin is aconsequenceof therandomsamplingprocedureincurredinpreparing specimens or components (for example, machining).NOTE 21AppendixX2listsseveralexcellentreferencesconcerningaws in ceramics, their formation, and their characterization.7.2.2Origin CharacterizationIdentity:7.2.2.1Characterize the origin by a phenomenological ap-proachwhichidenties what the originis andnot howitFIG. 5 Schematic of the Three Photographs Suggested forRecording Fractographic ObservationsC 132210appearsunderaparticularmodeofviewing. Descriptionsofthe mode of viewing may be used as qualiers, for example,pores that appear white when viewed optically, but use of onlythe appearance, white spots, should be avoided. (This approachischosensinceoriginsappeardrasticallydifferentinopticalversus electron microscopy.)7.2.2.2UsethenomenclaturesystemofSection3ifpos-sible. The nomenclature is designed to identify the origin byname (for example, pore, inclusion) and is classied based onthe inherent spatial distribution as discussed in 5.9 and 7.2.1. Itshould be recognized that not all origins can be so character-ized and that some origins may be specic to a material and itsprocess history (see 3.11).7.2.2.3There may be multiple origin types coincident at afractureorigin. Whensuchmixedattributecasesarise,somejudgment is required as to which origin is primary or intrinsic.The fractographer shall determine which origin type is primaryand use an ampersand (&) between the primary and secondaryorigincodesfor reportingandgraphical representationpur-poses. (For example, PV&LGVdenotes the origin is primarilya volume-distributed pore but with some associated largegrains.)NOTE 22Originscansometimesbedifficult tocharacterizeif theyhavemixedattributes. For example, porous regions oftenhaveporesassociated with them. If there is any doubt about the origin characteriza-tion, a more complete description of the origin type should be containedin the report.7.2.2.4Insomemixedattributecases it is impossibletodetermine which origin type is primary. The fractographer shallthenuseabackslash(/) betweentheidentitycodesinthereport and graphical representation, (agglomerate or pore,AV/PV) to indicate the identity of the origin could be one or theother.7.2.2.5Some high strength ceramics (s $ 1000 MPa) mayfractureduetothecombinedeffectsofmultipleorigintypeswhichare centrallylocatedinthe fracture mirror. Fromafracture mechanics analysis neither origin type is large enoughto initiate fracture, but together they are large enough to causefracture. A plus sign (+) shall be used in the report and graphrepresentation to indicate that these origin types linked togetherto limit the strength of the ceramic. (For example, PV+ MDSindicatesvolume-distributedporecombinedwithmachiningdamage to become the fracture origin.)7.2.2.6Insomeceramicmaterialstheremaybemultipleorigin populations within the same origin type, (large aluminagrains or largezirconiagrains inazirconia-toughenedalu-mina), which limit the strength of the material. In suchinstances a subscript shall be used to differentiate each popu-lation (LGVa indicates large alumina grains and LGVa indicateslarge zirconia grains).7.2.2.7In instances where the specimen is examined but theorigin identity cannot be determined, the origin shall bedesignated as an unidentiable origin, as listed in 3.10.1 and aquestion mark (?) will be used in the report or graphicalrepresentation as shown in Fig. 9.7.2.2.8Incases where the identityof the origincanbeestimated, but is not certain, a question mark may be appendedto the identity code, for example, Pore(?) or PV?.7.2.2.9When a specimen has not been examined, it shall berecorded as not examined and a hyphen (-) will be used in thereport and graphical representation to denote this.7.2.3Origin CharacterizationLocation:7.2.3.1Characterize the location of a specic origin quali-tativelyinagivenspecimen/component. Theoriginshall becharacterized as being volume-located (bulk-located), surface-located, near surface-located, or edge-located (if an edgeexists), for example, pore (volume-distributed), surface-located.NOTE 23The origin location, which species only the location of thestrength-limiting aw in a given specimen, shall not be used to statisti-cally differentiate origin populations.7.2.3.2Origins shall be considered surface-located in aspecimen or component if the origin is in direct contact with anexternal surface. If therearetwoor moretypesof externalsurfaces (a rectangular exure specimenthat has side andtensile surfaces), these surfaces shall be differentiates. Originswhich are located at the juncture of two external surfaces (thechamferorcornerofaexureortensilespecimen)shall beconsidered edge-located.7.2.3.3In some instances, it is useful to specify the originlocation if it is near, but not in direct contact with the externaltensilesurface. Thislocationcategoryshall betermed, nearsurface (NS)-located. This additional specication of locationis important for fracture mechanics evaluation of origins andservice-performanceissues.Forexample,somenearsurface-locatedorigins maybemoresusceptibletotime-dependentcrackgrowththanequivalent volume-locatedorigins. Nearsurface-locatedorigins mayalsobe likelytolinkupwithsurface machining or impact damage or to extend subcriticallytothesurfaceprior tocatastrophicfracture. Inorder tobeconsiderednear surface-locatedrather thanvolume-located,the origin shall be no more than one times the size of the origindiameter or major axis below the tensile surface. The proximityto the tensile surface shall be noted by estimating the perpen-diculardistancefromthissurfacetotheclosest point oftheorigin, see Fig. 6. If the results of the fractographic analysis aretobeusedfor designpurposes(Table1, Level 3) thenthefractographer maywishtoconsult further withthe designengineer regarding the near-surface classication. Alternativecriteria for the NS classication may apply in some instances.Thiscriteria, withsupportingreasoning, shallbeincludedinthe report section.7.2.4Origin CharacterizationSize:7.2.4.1Characterizetheoriginsize. Thesizeneednot bemeasured precisely as this characterization is intended toTABLE 4 Origin Characterization SchemeIdentity Location SizeNomenclature andinherent spatialdistribution:Spatial location of anindividual origin in aspecic specimen:Estimate of thediameter for equiaxedorigins, orVolume-distributed, orsurface-distributedVolume-located, orsurface-located, ornear surface-located,or edge-locatedMinor and major axesof volume-distributedorigins, or depth andwidth of surface-distributed originsSee Fig. 6 and Fig. 7C 132211describethegeneral natureof theorigins (the20-mporeversus the 1-m porosity). A fully quantitative size character-ization is permitted (but not required) by this practice.NOTE 24Preciseoriginmeasurementsareusuallynot helpful sincethe origins true size may not be revealed on the fracture surface, and exactfracture mechanics analyses of most origins are not possible due to theircomplexshape. Animportant exceptiontothis is machiningdamagewherein the origin size measurement may be very useful for the estimationof fracture toughness.7.2.4.2Measureandrecordtheorigindepth( a) and, ifpossible, the width (2c) in cases when the origins are inherentlysurface-distributed, such as machining damage or pits. See Fig.7. Use the depth (a) in Eq. 1 and Eq. 2.NOTE 25Full characterization to determine the appropriate shapefactor (Y) for KIccalculations requires the width of the origin (2c) to bemeasured in addition to the crack depth (a). See Fig. 7 and the paper byRajuandNewmanlistedinX2.8.3for semicircular or semiellipticalsurface-crack stress intensity factors.7.2.4.3Measure and record the origin diameter (2a) if theoriginisinherentlyvolume-distributedandisapproximatelyequiaxed, as illustrated in Fig. 6 and Fig. 7. However, use theorigin radius in Eq. 1 and Eq. 2. If a volume-distributed originis oblong or asymmetrical, report the approximate minor andmajor axis lengths (2a and 2 c) (for example, a 25 by 60-mpore), seeFig. 6andFig. 7, andusehalf oftheminoraxislength in Eq. 1 and Eq. 2.7.2.4.4If fracture mechanics data are available for theparticular material, the size of the fracture origin may beestimated using at least one of the following fracture mechan-ics techniques.NOTE 26The fracture mechanics calculation is used here as a meanstoverifythat thecorrect feature(s)havebeenidentiedasthefractureorigin. A detailed analysis and discussion of complications in comparingcalculated and measured origin sizes are inAppendix 2of ARL-TR-656,6VAMAS Report No. 19,7and the paper by Quinn and Swab cited inX2.7.7.7.2.4.5(a) Origin Size Estimated from Fracture Toughnessor Fracture EnergyFracture toughness (KIC) can be used toestimate the size of the fracture origin from Eq. 1:a 5 @KIC/~sY!#2(1)where:a 5measure of the origin size (that is, depth for asurface crack, or radius or half minor-axis length foravolume-distributedorigin, seeFig. 6andFig. 7(m),KIC5fracture toughness, MPa* =m,s 5fracture stress at the origin location, MPa, andY 5stress intensity shape factor for the origin,dimensionless.NOTE 27In Eq. 1, the factor Y incorporates all stress state, specimen,and crack geometric factors. In some references in the literature, Y is usedsomewhat differently. The fracture mechanics literature should beconsultedtondvaluesofYforspecicstressdistributions,specimen,and crack geometries. Fig. 7 illustrates several crack geometries and theassociatedYfactors. TheYfactorsmayvaryaroundtheperipheryofacrack front. In each instance, the maximum Y should be used. AppendixX2containsseveralreferenceswhichlistseveralcompilationsofstressintensity factors.NOTE 28The stress intensityfactors inFig. 7are for specimensloaded in direct tension. They may be used for origins in exurally loadedspecimens, providedthattheoriginsaresmallrelativetothespecimencross-section size. For exurally loaded specimens, the stress at the originlocationshouldbeusedinEq. 1. If theoriginislargerelativetothespecimen cross-section size, consult the references in the FractureMechanics section of Appendix X2 for appropriate stress intensity factors.NOTE 29Eq.1canbeusedtoestimatethefractureoriginsize,butcomplications often hamper exact calculations. Most origins are tooirregular to permit accurate shape factor (Y) determination. Fig. 7 showssome simple crack shapes which can be used for guidance, but these are2-dimensional cracks which may not adequately match real 3-dimensionalorigins.Fracturetoughnessisrelatedtofractureenergyforcracksloadedinplane-strain conditions by Eq. 2:KIC 5 @~2Egf !/~1 2 n2!#1/2(2)NOTE 1Origins can be characterized as near-surface (NS) depending upon whether they are within the distances illustrated. The origin size is thediameter for equiaxed origins, and is the length of the minor and major axes of an elongated origin. All measurements dimensions are approximate only.FIG. 6 Schematic Showing Origins and Their Dimensions Relative to the Specimen SurfaceC 132212where:E 5elastic modulus, MPa,gf5fracture energy, MN/m or MJ/m2, andn 5Poissons ratio, dimensionless.and thus:a 5 @~2Egf!/Ys ~1 2 n2!#2(3)7.2.4.6(b) Origin Size Estimated from the Fracture MirrorSizeIf a fracture mirror is evident, it can be used to estimatean origin size. The ratio of the outer mirror (mist-hackleboundary) tooriginradiusistypically13to1(for glasses,single crystals, andpolycrystalline ceramics) andthe innermirror (mirror-mist boundary) ratio is between 10 to 1 (glasses)and 6 to 1 (polycrystalline ceramics).7.2.4.7(c) Compare the measured origin size to the valueobtainedfromEq. 1or Eq. 2. If thesevaluesdonot agreewithinafactorof2or3, itishighlyrecommendedthatthefracture origin be reexamined to verify that the correctfeature(s) have been identied as the origin. If thereexamination shows that the origin has been correctlyidentied and measured, the variation in these sizes should benoted in the report and explanations given to account for thediscrepancy.NOTE 30Stablecrackextensionpriortofracture,link-upofanear-surfaceawwithasurface, risingR-curvecrackextensionresistance,residual stresses, or local variations in fracture toughness, can complicatethe comparison of the measured fracture origin size and that estimated byNOTE 1Ymax is shown for each example. The Y at the other points of the crack periphery is shown (in parentheses) for comparison in a few examples.FIG. 7 Stress Intensity Factors (Y) for Penny-Shaped (Circular) and Elliptical Cracks or Semicircular and Semielliptical Surface Cracksin Tension Stress FieldsC 132213Eq. 1 and Eq. 2. Details on these and other complications can be found inAppendix 2 of ARL-TR-6566or VAMAS Report No. 19.7NOTE 31There is often good correlation of the fractographicmeasurement andfracture mechanics size estimates for semiellipticalsurface cracks from machining damage.7.2.4.8(d) Component AnalysisThefailurestressinacomponent may not be known, making it difficult to estimatethe origin size using Eq. 1 or Eq. 2. However, an estimate of thefailure stress can be made from the mirror radius according toEq. 4:s 5 @A/=r# (4)where:r 5mirror radius, m, andA 5mirror constant (mirror-mist or mist-hackle), MPa*=m.Alist of mirror constantsfor someglassesandadvancedceramics is given in Appendix X7.8. Report8.1GeneralA samplereportingformatisshowninFig.10. The report shall contain the following information:8.1.1Fractographers identity;8.1.2Equipment used;8.1.3Overall origin types identied;8.1.4The inspection criteria in accordance with Table 1;8.1.5Theoriginidentity, location, size, andthemodeofviewing (optical or SEM, or both) for each specimen;8.1.6Estimatedoriginsizes fromfracturemechanics foreach specimen (include the technique used to make suchestimates) and a comparison of these estimates to the measuredfracture origin sizes;8.1.7Ageneral statement shall be made regarding theapproximate condence levels for the identity classication ofeach origin type, or if necessary, each individual origin. (Thepores were quite distinct and all classications are reasonablycertain unless appended by the8? symbol); and8.1.8Supplemental observations suchas transgranular orintergranular fracture (or the approximate ratio of each) in thevicinity of the origin (inside the mirror) and outside the fracturemirror, fracture mirror measurements, and the criteria used tomeasure them, if such information is available.8.2 To the extent possible, couple the fractographicobservations directly to process history and resultantmicrostructure. Representative micrographs of polishedsections of the microstructure showing porosity and grain sizedistribution are highly recommended.8.3Couplethefractographicobservations directlytothemechanical test results. FractographicmontagesandlabeledWeibullorotherstrengthgraphs(Fig. 9andFig. 11)areanexceptionally versatile means of accomplishing this. Montagespresent the fractographic results in a comprehensive manner.9. Keywords9.1 advanced ceramics; aws; fractography; fracturemechanics; fracture mirrors; fracture origins; microscopyNOTE 1A) volume-located;B) edge-located;C) surface-located; andD) near surface-located.FIG. 8 Schematic Which Shows the Four Possible Locations of aVolume-Distributed Fracture OriginC 132214NOTE 1Origin identity and location keys are added for ease in interpretation. The majority of the origins identied in this example arevolume-distributed, although as the location column shows some of the individual origins were located at the specimen surface. The fractographic analysiscriterion was Level 2 (Materials Development), and thus the location and size were not determined for every specimen. The superscript V stands forinherently volume-distributed origins and the superscript S for inherently surface-distributed origins. In contrast, the V, S, and E designations in thelocation column refer to the location of the strength-limiting origin in a specic specimen.FIG. 9 A Labeled Weibull Graph Including a Listing of Strength Values, Identied Origin Types, and Their Associated Locations andSizesC 132215NOTE 1This report is complimentary to mechanical property test result reports such as used in Test Method C 1211.FIG. 10 A Sample Reporting FormatC 132216APPENDIXES(Nonmandatory Information)X1. EXAMPLES OF FRACTURE ORIGINS IN ADVANCED CERAMICSX1.1 See Figs. X1.1-X1.15.NOTE 1Calculations of mirror and origin sizes, fracture mechanics estimates, and other information can be made in the sides and margins of thisworksheet. A photograph of microstructure including porosity and grain size should also be included on the montage as illustrated on the lower right.FIG. 11 A Schematic of a Working Fractographic Montage Linking Fractographs and Strength PlotC 132217FIG. X1.1 Examples of PoresC 132218FIG. X1.2 Examples of Porous SeamsC 132219FIG. X1.3 Examples of Porous RegionsC 132220FIG. X1.4 Examples of AgglomeratesC 132221FIG. X1.5 Examples of InclusionsC 132222FIG. X1.6 Examples of Compositional InhomogeneitiesC 132223FIG. X1.7 Examples of Large GrainsC 132224FIG. X1.8 Examples of CracksC 132225FIG. X1.9 Examples of Machining DamageC 132226FIG. X1.10 Examples of Machining Damage (See Fig. X1.9.)C 132227FIG. X1.11 Examples of Handling DamageC 132228FIG. X1.12 Examples of PitsC 132229NOTE 1Courtesy of A. Pasto, GTE Laboratory, now with Oak Ridge National Laboratory.FIG. X1.13 Examples of Surface VoidsC 132230FIG. X1.14 Examples of Less Common Other FlawsC 132231FIG. X1.15 Examples of Flaws with Mixed AttributesC 132232X2. A SELECT BIBLIOGRAPHY ON FRACTOGRAPHY AND ORIGINS IN CERAMICSINTRODUCTIONThe references listed as follows are included for the benet of users who wish to inquire furtherabout fractographyof ceramicsingeneral, microscopictechniques, fractureoriginsandawsinadvanced ceramics, fracture mirrors, and fracture mechanics and its application to advanced ceramics.X2.1Books on Advanced Ceramics FractographyX2.1.1 Frechette, V. D., Failure Analysis of BrittleMaterials, Advances in Ceramics, Vol 28, American CeramicSociety, Westerville, OH, 1990.X2.1.1.1Amust for theseriousfractographer. Thisbookcovers all aspects of the fractographyof glasses includingfundamental markings on crack surfaces (Wallner lines, hackle,and so forth), crack forking, failure origins, estimates of stressat fractureandfractographictechniques. Superblyillustratedwith a number of service failures and case histories presented.X2.1.2Fractography of Glasses and Ceramics, Advances inCeramics, Vol 22, Varner, J., and Frechette, V., eds., AmericanCeramic Society, Westerville, OH, 1988.X2.1.2.1Eight papersonceramicsfromasymposiuminPhiladelphia in April 1982. Includes the comprehensive reviewpaper by Rice, and papers by Pantano and Kelso, and Healyand Mecholsky (cited below).X2.1.3Fractography of Ceramic and Metal Failures,Mecholsky, Jr., J., andPowell, Jr., S., eds., ASTMSTP827,ASTM, Philadelphia, PA, 1984.X2.1.3.1Proceedings of aconferenceof thesamenameheldat AlfredUniversityin1988. Sectionson: fundamentalphenomena, high-temperature fracture, fractography andfracture mechanics, fractography in materials development andtesting, and component failures.X2.1.4 Concepts, Flaws and Fractography, FractureMechanics of Ceramics, Vol 1, Bradt, R., Hasselman, D., andLange, F., eds., Plenum Press, NY, 1974.X2.1.4.1Proceedings of a conference at Pennsylvania StateUniversity in 1973 with 23 papers on fracture mechanicsapplied to origin detection and fractography in ceramics. Thelater volumesof thisseriesalsohaverelevant fractographypapers.X2.2Microscopic TechniquesX2.2.1Pantano, C. G., and Kelso, J. F., Chemical Analysisof FractureSurfaces,Fractographyof CeramicandMetalFailures, ASTM STP 827, ASTM, 1984, pp. 139156.X2.2.1.1 The applicability of various instrumentaltechniques for chemical analysis of fracture surfaces isreviewed. The relative merits and spatial and depth resolutionsof Auger microscopyandenergyor wavelengthdispersiveelectron microscopy are given.X2.2.2Healy, J. T., andMecholsky, Jr., J. J., ScanningElectron Microscopy Techniques and Their Application toFailure Analysis of Brittle Materials, Fractography ofCeramicandMetal Failures, ASTMSTP827, ASTM, 1984,pp. 157181.X2.2.2.1Discusses cleaning, coating, and other proceduresfor SEM specimens. The merits and differential emphases ofsecondary and backscattered electron imaging are presented.X2.2.3Fractography, Metals Handbook, 9th ed., Vol 12,ASM, Metals Park, OH, 1987.X2.2.3.1An excellent handbook on fractography of metals.Some generic sections including photographic, opticalinspection, andelectronmicroscopytechniques aredirectlyapplicable to ceramic fractography. Light, secondary electron,and backscattered electron photos of identical locations inmetal specimens are compared. Caution: Some cleaning andpreparationtechniques suchas surfacecoatings, replicatingtapes, replicating tape stripping, and aggressive detergentcleaning which are prescribed for metals are not recommendedfor ceramic fracture surfaces.X2.3Fractography of CeramicsOverview PapersX2.3.1 Mecholsky, Jr., J. J., and Freiman, S. W.,Determinationof Fracture Mechanics Parameters ThroughFractographic AnalysisofCeramics,FractureMechanicsofCeramics Applied to Brittle Materials, Freiman, S., ed., ASTMSTP 678, ASTM, 1979, pp. 136150.X2.3.1.1Ashort but useful overviewof the utility offractography as a quantitative tool to determine strength-limiting origins, the stress at failure, and critical fracturetoughness.X2.3.2 Rice, R. W., Fractographic Identication ofStrength Controlling Flaws and Microstructure, FractureMechanics of Ceramics, Vol 1, Bradt, R., Hasselman, D., andLange, F., eds., Plenum Press, NY, 1974, pp. 323345.X2.3.2.1Ashort but valuable discussionof several keyorigins (pores, pore groups, and large grains) and theirrelationship to fracture energy. The fracture energy can eitherbe a single-crystal or polycrystalline value depending upon therelative sizes of origin and microstructure.X2.3.3Quinn, G. D., Swab, J. J., andSlavin, M. J.,AProposed Standard Practice for Fractographic Analysis ofMonolithic Advanced Ceramics, MTL TR 90-57, November1990, NTIS Access No. ADA-231989.X2.3.3.1The basis for this practice. Discusses essentialbackgroundinformationandtherationaleforconsistencyincharacterization. From this information a standardnomenclature and origin characterization scheme are created.Alsoincludes adetailedbibliographyandexamples of thevarious types of origins.X2.3.4Rice, R. W., Topography of Ceramics, in Surfacesand Interfaces of Glass and Ceramics, Frechette, V., LaCourse,W., and Burdick, V., eds., PlenumPress, NY, 1974, pp.439472.X2.3.4.1A veryhelpfulintroductiondescribestheroleofunaidedeye, handlens, optical, scanning, andtransmissionC 132233electron microscopy. Fig. 1 shows optical and SEM photos ofthe same origin. Fracture surface features such as transgranularandintergranular fracture, crackmicrostructureinteractions,crackbranching, mirrors, andsinglecrystalfractographyarediscussed.X2.3.5 Rice, R. W., Ceramic Fracture Features,Observations, Mechanism and Uses, Fractography ofCeramicandMetal Failures, ASTMSTP827, ASTM, 1984,pp. 5103.X2.3.5.1A lengthyreviewpaperwithadetailedtechnicaldiscussion of fracture mirrors and related features (mist,hackle, and branching) in glasses, polycrystals, and singlecrystals. The bluntness of origins (round pores versus sharpmachining cracks) will alter the mirror-to-origin radius ratio. Auseful table of branch angle as a function of mode of loading(exure, tensile, biaxial, thermal) for several materials is given.X2.3.6Richerson, D. W., Failure Analysis, ModernCeramic Engineering, Marcel Dekker, Inc., NY, 1982, pp.325375.X2.3.6.1An outstanding presentation of both intrinsic andextrinsicawsinsiliconnitrideandsiliconcarbide. Richlyillustrated, this chapter carefully relates aws to processing andservice conditions.X2.3.7 Failure Analysis, Engineering MaterialsHandbook, Vol 4, CeramicsandGlasses, Schneider, S., ed.,ASM, Metals Park, OH, 1991, pp. 629673.X2.3.7.1Chapter 9 includes excellent articles on descriptivefractography by J. Varner, quantitative analysis by T.Michalske, optical ber analysis by J. Mecholsky, glassceramic failure analysis by B. Adams and S. DeMartino, andthe application of fracture mechanics by I. Bar-on.X2.4Origins in Advanced CeramicsX2.4.1 Kirchner, H., Gruver, R., and Sotter, W.,Characteristics of Flaws at Fracture Origins and FractureStressFlawSizeRelationsinVariousCeramics,MaterialScience and Engineering, Vol 22, 1976, pp. 147156.X2.4.1.1Aconcisebut useful report onstrength-limitingorigins in alumina, silicon nitride, and silicon carbide with adetailed tabulation of different types of origins. Emphasis is onporosity, largegrains, andmachiningorigins. Animportantobservation (Fig. 1 b) is that origins in the center of fracturemirrorsmayintersectthefracturesurfaceatanangle, andatrue view of the origin may not be seen.X2.4.2Baumgartner, H., and Richerson, D., InclusionEffects on the Strength of Hot Pressed Si3N4, FractureMechanics of Ceramics, Vol 1, 1974, pp. 367386.X2.4.2.1Goodcharacterizationofmachiningdamageandinclusions in silicon nitride. The inclusions are much smallerthan expected (based on a penny-shaped crack model),evidently the result of a locally degraded fracture toughness.X2.4.3Gee, M. G., andMorrell, R., FractureMechanicsand Microstructures, Fracture Mechanics of Ceramics, Vol 8,Bradt, R., Evans, A., Hasselman, D., and Lange, F., eds.,Plenum Press, NY, 1986, pp. 122.X2.4.3.1Principally a discussion of the application offracture mechanics theories to strength. Microstructuralinuences will signicantly complicate this and may limitutilitytoqualitative issues. The nature of strength-limitingorigins and their severity is discussed. In some instances, sharpcracks will not form until the stress is applied.X2.4.4Evans, A. G., Structural Reliability, A ProcessingDependent Phenomenon,Journal of theAmericanCeramicSociety, Vol 65, No. 3, 1982, pp. 127139.X2.4.4.1Emphasis on the micromechanics of fracture withagooddiscussionof theeffect of thermal andmechanicalproperty mismatches between a origin and the matrix. Only afew photos, but includes some excellent schematics. Includes awell-known graph of stress versus origin size for silicon nitrideshowing the relative severity of different origins (WC, Fe, Si,C inclusions, porosity and machining damage).X2.4.5 Rice, R. W., Failure Initiation in Ceramics:Challenges of NDE and Processing, and CeramicDevelopments, Sorrell, C., and Ben-Nissan, B., eds.,Materials Science Forum, Vol 3436, Trans. Tech. Publ. Ltd.Switzerland, 1988, pp. 10571064.X2.4.5.1 Acomprehensive, well-illustrated review offailure-initiating origins. Nearly an encyclopedia of aws.Origins include: agglomerates, pores, large grains, inclusions,machining damage, handling damage, thermocouple beads,ball mills, dandruff, insects, feces, inadequate mixing ofconstituents, and so forth.X2.4.6Rice, R. W., Processing Induced Sources ofMechanical FailureinCeramics,Processingof CrystallineCeramics, Palmour, H., Davis, R., and Hare, T., eds., PlenumPress, NY, 1978, pp. 303319.X2.4.6.1A short, well-illustrated review of origins. A goodstarting point.X2.4.7Rice, R. W., Mecholsky, Jr., J. J., and Becher, P. F.,The Effect of GrindingDirectiononFlawCharacter andStrength of Single Crystal and Polycrystalline Ceramics,Journal of Material Science, Vol 16, 1981, pp. 853862.X2.4.7.1Machining damage in a variety of ceramics iswell-illustrated by nine gures.X2.4.8Mecholsky, Jr., J. J., Freiman, S. W., and Rice, R. W.,Effects of Grinding on Flaw Geometry and Fracture of Glass,Journalofthe AmericanCeramicSociety, Vol60,Nos.34,1977, pp. 114117.X2.4.8.1Twoprimarysets of cracks result fromsurfacegrinding. Theseareschematicallyshownandcomplementedby SEM photos and related to fracture mechanics parameters.X2.4.9 Rice, R. W., Pores as Fracture Origins inCeramics,Journal of Material Science, Vol 19, 1984, pp.895914.X2.4.9.1A well-illustratedexaminationofporesinglassyand polycrystalline materials. Pores tend to be sharper in thelatter than in the former.X2.4.10Munz, D., Rosenfelder, O., Goebells, K., andReiter, H., Assessment of Flaws in Ceramic Materials on theBasis of Non-Destructive Evaluation, Fracture Mechanics ofCeramics, Vol 1, Bradt, R., Hasselman, D., and Lange, F., eds.,Plenum Press, NY, 1986, pp. 265283.X2.4.10.1Sixdifferent awtypes were characterizedinreaction bonded and sintered silicon nitrides. Some aws werearticiallycreatedtosupport afracturemechanicsanalysis.Pores had a different effect upon strength than inclusions.C 132234X2.5Fracture MirrorsX2.5.1Mecholsky, Jr., J. J., Freiman, S. W., and Rice, R. W.,Fracture Surface Analysis of Ceramics, Journal of MaterialScience, Vol 11, 1976, pp. 13101319.X2.5.1.1Adetailed correlation of origin size, fracturemirror sizes and characterization, and fracture mechanicsparameters for single and polycrystalline ceramics. A table ofmirror constantsisgivenfor arangeof ceramics, andit isdemonstrated that the outer mirror (hackle) to origin size ratiois about 13 to 1. The inner mirror (mist) ratio is 610 to 1.X2.5.2Mecholsky, Jr., J. J., Rice, R. W., and Freiman, S. W.,Prediction of Fracture Energy and Flaw Sizes in Glasses fromMeasurements of Mirror Size, Journal of the AmericanCeramic Society, Vol 57, No. 10, 1974, pp. 440443.X2.5.2.1Detailsof fracturemirror featuresarediscussedandrelatedtofracturemechanicsparametersfor glasses. Atable of mirror constants for glasses is included. Anow famousschematicrenditionofafracturemirrorshowingtheorigin,mist, and hackle is presented.X2.5.3Kirchner, H. P., Gruver, R. M., andSotter, W. A.,Fracture StressMirror Size Relations for PolycrystallineCeramics, Philosophical Magazine, Vol 33, No. 5, 1976, pp.775780.X2.5.3.1Many mirror constants for a range of ceramics.X2.5.4Kirchner, H. P., andConway, Jr., J. C., FractureMechanics of Crack Branching in Ceramics, Fractography ofGlass and Ceramics, Advances in Ceramics, Vol 22, AmericanCeramic Society, Westerville, OH, 1988, pp. 187213.X2.5.4.1Analysisshowsthat fracturemirror featuresarecontrolled by stress intensity.X2.5.5 Mecholsky, Jr., J. J., and Freiman, S. W.,Determinationof Fracture Mechanics Parameters ThroughFractographic Analysis of Ceramics, Fracture MechanicsApplied to Brittle Materials, ASTM STP 678, Freiman, S., ed.,ASTM, 1979, pp. 136150.X2.5.5.1A short discussion of fracture mirrors and mirrorconstants with a comparative table of mirror constants.Comments on useful techniques to measure mirror parameters.X2.6Fracture MechanicsEstimates of Flaw SizeX2.6.1Richerson, D. W., ModernCeramic Engineering,Marcel Dekker Inc., NY, 1982.X2.6.1.1Chapter 3 is a good primer on strength and fracturetoughness measurements and their applicability tofractographic analysis. Several numerical examples are givenfor estimatingthestrengthof aspecimenonthebasisof afracturemechanicscalculationusingthemeasuredawsize.Richerson uses fracture energies, which are readily related tofracture toughness by equation 3.15 in the book or Eq. 2 in thispractice.X2.6.2 Baumgartner, H. K., and Richerson, D. W.,InclusionEffects onthe Strengthof Hot PressedSi3N4,Fracture Mechanics of Ceramics, Vol 1, Bradt, R., Hasselman,D., and Lange, F., eds., Plenum Press, NY, 1974, pp. 367386.X2.6.2.1Appliesfracturemechanicstooneclassofawswith several numerical examples. The strength-limitinginclusions were smaller than expected from fracturemechanics, suggesting that the fracture toughness was alteredin the vicinity of the inclusions.X2.6.3Mecholsky, Jr., J. J., Freiman, S. W., and Rice, R. W.,Fracture Surface Analysis of Ceramics, Journal of MaterialScience, Vol 11, 1976, pp. 13101319.X2.6.3.1 Compares measured aw sizes to fracturemechanics estimates for a range of ceramics and glasses.X2.6.4Kirchner, H. P., Gruver, R. M., andSotter, W. A.,Characteristics of Flaws at Fracture Origins and FractureStress-FlawSize Relations inVarious Ceramics, MaterialScience and Engineering, Vol 22, 1976, pp. 147156.X2.6.4.1Measuredawsizes werecomparedtofracturemechanics estimates for several different types of aws inalumina, silicon nitride, and silicon carbide.X2.6.5 Evans, A. G., and Tappin, G., Effects ofMicrostructureontheStress toPropagateInherent Flaws,Proceedings of BritishCeramic Society, Vol 20, 1972, pp.275297.X2.6.5.1Discusses aws in alumina ceramics and comparesthe stress needed to cause fracture to fracture mechanicsestimates. Microstructural factors such as aw linking prior tocatastrophic fracture are discussed.X2.6.6Munz, D., Rosenfelder, O., Goebells, K., and Reiter,H., Assessment of Flaws in Ceramic Materials on the Basis ofNon-Destructive Evaluation, Fracture Mechanics ofCeramics, Vol 1, Bradt, R., Hasselman, D., and Lange, F., eds.,Plenum Press, NY, 1986, pp. 265283.X2.6.6.1Asuperb, comprehensive fracture mechanicsanalysis of sixdifferent awtypes intwosiliconnitrides.Fractographic size measurements agreed with fracturemechanics estimates for some aw types, but not others. Over100 specimens. Discusses the different crack models which canbe used to simulate real aws as well as the shortcomings ofsuchmodels. Includes Raju-Newmanandelliptical internalaw stress intensity factor solutions.X2.6.7G. D. QuinnandJ. J. Swab, FractographyandEstimates of Fracture Origin Size from Fracture Mechanics,Ceram. Eng. and Sci. Proc., Vol 17, 3, pp. 5158, 1996.X2.6.7.1Fracturemechanics shouldbeusedroutinelyinfractographicanalysestoverifythat thecorrect featurehasbeen identied as the fracture origin. In many instances,however, the calculated aw size is different fromtheempirically measured aw size. This paper reviews the factorsthat may cause the discrepancies.X2.7Fracture MechanicsStress Intensity FactorsX2.7.1Stress IntensityFactors Handbook, Vols1and2,Murakami, Y., ed., Pergamon Press, NY, 1986.X2.7.1.1A collection of stress intensity factors for variouscrack congurations under different loading conditions.X2.7.2Rooke, D. P., and Cartwright, D. J., Compendium ofStress Intensity Factors, Her Majestys Stationary Office,London, 1976.X2.7.2.1A collection of stress intensity factors for variouscrack congurations under different loading conditions.X2.7.3 Newman, Jr., J. C., and Raju, I. S., AnExperimental Stress-Intensity Factor Equation for the SurfaceCrack, Engineering Fracture Mechanics, Vol 15 [12], 1981,pp. 185192.X2.7.3.1Presents anequationfor the calculationof theC 132235shape factor (Y) for origins which are essentially semicircularor semielliptical and located at the surface. The Y is determinedwhere the origin meets the surface and at the deepest point ofthe origin. The highest value is then used in fracture mechanicscalculation.X2.7.4Tada, H., Paris, P. C., and Irwin, G. R., The StressAnalysis of Cracks Handbook, Del Research Corp., St. Louis,MO, 1973.X2.7.5 Bar-on, I., Applied Fracture Mechanics,Engineered Materials Handbook, Vol 4, Ceramics andGlasses, Schneider, S., ed., ASM, Metals Park, OH, 1991, pp.645651.X2.7.5.1Agood primer on the applications of fracturemechanics analysis toidealizedcrackcongurations. Stressintensity shape factors are given for through slits, surfacecracks, and pores with rim cracks.X3. SYNOPSIS OF ARL-TR-656X3.1 This practice was derived fromMILHDBK-790(Fractography and Characterization of Fracture Origins inAdvanced Ceramics) which was prepared by G. D. Quinn, J. J.Swab, and M. J. Slavin. A round-robin exercise sponsored bytheVersaillesProject onAdvancedMaterialsandStandards(VAMAS) was conductedtodetermine the applicabilityofMilitary Handbook 790 and to attempt to clarify anyambiguoussectionsorissues. Thenalreportofthisround-robin is ARL-TR-656, Fractography of Advanced StructuralCeramics: Results fromthe VAMAS Fractography RoundRobin Exercise, which was also published as VersaillesProject on Advanced Materials and Standards (VAMAS)Report No. 19, in February 1995. These reports are on le atASTM Headquarters as research reports for this practice. Thenal report, which is well-illustrated, outlines the round-robinand the responses of the participants in considerable detail. Inmanyinstances, aninterpretationconcensuswasreached, inothers it was not.X3.1.1Theexercisewasdividedintothreetopics. TopicNo. 1 addressed the detection and interpretation of machiningdamageonphotographsof ceramicspecimens. TopicNo. 2dealt withthefractographicanalysis of ceramicspecimens.TopicNo. 3, whichwas optional, askedtheparticipants toperform fractography on a ceramic material of their choice.X3.1.2The results from Topic No. 1 showed that there areproblemsindetectingandinterpretingmachiningdamageinadvancedceramics. Theseproblemsoftenstemmedfromtheparticipants inexperience in howmachining damage canmanifest itself in various ceramic materials. Topic No. 2indicated that the guidelines and characterization schemeoutlined in the handbook were adequate to completelycharacterize fracture origins in ceramics, but some renementsarenecessary. Therewas agoodtoexcellent consensus inorigincharacterizationinmanycases. Theinstances whereconcurrence was not forthcoming helped the organizers makeimprovements tothe procedures of the handbookandalsohighlighted the key steps that should be emphasized.X3.1.3The general conclusions were as follows:X3.1.3.1The guidelines and characterization scheme inMIL HDBK-790 were adequate for the completecharacterization of fracture origins in advanced ceramics, butsome renements were needed for each attribute in the scheme.X3.1.3.2Fracture mechanics should be used more often toassist in the characterization of fracture origins.X3.1.3.3 Fractographers should use all availableinformation about the material and its history during thecharacterization.X3.1.3.4Characterization of origins from photographs or asingle specimen can be misleading.X3.1.3.5Fractographers should examine both mating halvesof the primary fracture surface.X3.1.3.6 Fractographers should examine the externalsurfaces of the specimen or component, especially if the originis surface-located.X3.1.3.7Fractographers should have adequate time toconduct the analysis to ensure a correct appraisal.X3.1.3.8There is a paucity of mirror constants for modernadvanced ceramic materials.X4. FRACTOGRAPHIC EQUIPMENTX4.1 See Figs. X4.1-X4.5C 132236FIG. X4.1 Binocular Stereomicroscope with DirectionallyAdjustable Fiber-Optical Light Source and Variable MagnicationBetween 5 and 803.NOTE 1This type of system is excellent for instructional purposes.FIG. X4.2 Dual Station, Binocular Stereomicroscope with Two Directionally Adjustable Light Sources, Video Camera, Monitor, andInstant Photographic CapabilityC 132237FIG. X4.3 Scanning Electron Microscope with Energy DispersiveSpectroscopic Capabilities, Low-Energy Operation, andMagnication Between 20 and 20 0003NOTE 1(A) Hand-held and tabletop magnifying glass; (B) Variable-angle grips with compliant surface; (C) Fixtures to support specimens to viewmachinedsurfaces;(D)Compressedair;(E) Tweezersforspecimenmanipulation;(F)Plasticstoragetrays;(G)Glassvialsforstorageoffracturedspecimens prior to SEM analysis.FIG. X4.4 Peripheral Equipment to Assist in Fractography and Storage of Fractured Specimens and ComponentsC 132238X5. COMMON CONTAMINANTS ON CERAMIC FRACTURE SURFACESX5.1 See Figs. X5.1-X5.5.FIG. X4.5 Macrophotographic Camera Stand for InstantPhotographsNOTE 1These typically appear as globules, but since pencil graphiteusually has a clay binder, it must be treated with caution.FIG. X5.1 Contamination from Particles of Graphite from aCommon Leaded PencilNOTE 1Masking tape is sometimes used to hold pieces of a fracturedspecimentogether, but shouldbeavoidedonthefractureandtensilesurfaces. The smear blends into the fracture surface and is partiallytransparent to X rays as shown. An energy dispersive analysis identiedthe smear as having potassium, chlorine, and sulfur. Trichloroethylene isan effective solvent to remove the resin.FIG. X5.2 Contamination from a Smear of Masking Tape Resin(White Arrows) Near a ChamferC 132239NOTE 1These are easy to blow off or eliminate by a sonic bath.FIG. X5.3 Contamination from Particles of Paper Lint (BlackArrows) from a Common Manila Specimen EnvelopeNOTE 1What might be the most pernicious contaminant in thefractographic laboratory: mounting clay. The white arrows in (a) show aregion where clay was dabbed on with tweezers. The clay appears to be agenuineinclusionthatblendsdirectlyintotheunderlyingceramic.Itisextremely difficult to remove once it gets onto the specimen and it looksquite appropriate on the fracture surface. It should not be used. (b) is aclose-up of the region of the small arrow from (a). An energy-dispersiveanalysis revealed silicon, aluminum, and titanium. The Si isindistinguishable from the silicon nitride specimen.FIG. X5.4 Contamination from Mounting ClayC 132240X6. TYPICAL FRACTURE PATTERNS IN CERAMIC TEST SPECIMENSX6.1 See Fig. X6.1 and Fig. X6.2FIG. X5.5 Contamination from Human Skin (Courtesy of A. Pasto,GTE Laboratory, now with Oak Ridge National Laboratory)C 132241FIG. X6.1 Typical Fracture and Crack Patterns of Flexure SpecimensC 132242X7. MIRROR CONSTANTS FOR SOME GLASSES AND ADVANCED CERAMICSX7.1 Table X7.1 lists some published fracture mirrorconstantsforarangeofglassesandadvancedceramics. ThemirrorconstantlistedisAotheoutermirrorboundary(mist/hackle). At the present time there are no consistent guidelinesor procedures and techniques for determining fracture mirrorconstants. Different specimen geometries, test techniques(exure, tension), and procedures were used for the followingceramics. In each instance, the original reference is cited.X7.1.1Themirrorconstantshavethesamedimensionsasfracturetoughness: MPa =m. Thenumerical valueof themirrorconstantisalwayshigherthanthefracturetoughness.For glasses and polycrystalline ceramics, the outer mirrorboundary (mist/hackle) constant is typically 3 times larger, butcanrangefrom2times to5times larger thanthefracturetoughness. Inner mirror boundary (mirror/mist) constants are 2times to