ASTM C1678-2007 Standard Practice for Fractographic Analysis of Fracture Mirror Sizes in Ceramics and Glasses.pdf

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1、Designation: C 1678 07Standard Practice forFractographic Analysis of Fracture Mirror Sizes in Ceramicsand Glasses1This standard is issued under the fixed designation C 1678; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year

2、of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice pertains to the analysis and interpretationof fracture mirror sizes in brittle materials. Fracture m

3、irrors(Fig. 1) are telltale fractographic markings that surround afracture origin in brittle materials. The fracture mirror size maybe used with known fracture mirror constants to estimate thestress in a fractured component. Alternatively, the fracturemirror size may be used in conjunction with know

4、n stresses intest specimens to calculate fracture mirror constants. Thepractice is applicable to glasses and polycrystalline ceramiclaboratory test specimens as well as fractured components. Theanalysis and interpretation procedures for glasses and ceramicsare similar, but they are not identical. Di

5、fferent optical micros-copy examination techniques are listed and described, includ-ing observation angles, illumination methods, appropriatemagnification, and measurement protocols. Guidance is givenfor calculating a fracture mirror constant and for interpretingthe fracture mirror size and shape fo

6、r both circular andnoncircular mirrors including stress gradients, geometricaleffects, and/or residual stresses. The practice provides figuresand micrographs illustrating the different types of featurescommonly observed in and measurement techniques used forthe fracture mirrors of glasses and polycr

7、ystalline ceramics.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to

8、use.2. Referenced Documents2.1 ASTM Standards:2C 1145 Terminology of Advanced CeramicsC 1256 Practice for Interpreting Glass Fracture SurfaceFeaturesC 1322 Practice for Fractography and Characterization ofFracture Origins in Advanced Ceramics3. Terminology3.1 Definitions: (See Fig. 1)3.1.1 fracture

9、mirror, nas used in fractography of brittlematerials, a relatively smooth region in the immediate vicinityof and surrounding the fracture origin C 1145, C 13223.1.2 fracture origin, nthe source from which brittlefracture commences. C 1145, C 13223.1.3 hackle, nas used in fractography of brittle mate

10、rials,a line or lines on the crack surface running in the local directionof cracking, separating parallel but noncoplanar portions of thecrack surface. C 1145, C 13223.1.4 mist, nas used in fractography of brittle materials,markings on the surface of an accelerating crack close to itseffective termi

11、nal velocity, observable first as a misty appear-ance and with increasing velocity reveals a fibrous texture,elongated in the direction of crack propagation. C 1145,C 13223.2 Definitions of Terms Specific to This Standard:(See Fig. 1)3.2.1 mirror-mist boundary in glasses, nthe peripherywhere one can

12、 discern the onset of mist around a glass fracturemirror. This boundary corresponds to Ai, the inner mirrorconstant.3.2.2 mist-hackle boundary in glasses, nthe peripherywhere one can discern the onset of systematic hackle around aglass fracture mirror. This boundary corresponds to Ao, theouter mirro

13、r constant.3.2.3 mirror-hackle boundary in polycrystalline ceramics,nthe periphery where one can discern the onset of systematicnew hackle and there is an obvious roughness change relativeto that inside a ceramic fracture mirror region. This boundarycorresponds to Ao, the outer mirror constant. Igno

14、re prematurehackle and/or isolated steps from microstructural irregularitiesin the mirror or irregularities at the origin.1This practice is under the jurisdiction of ASTM Committee C28 on AdvancedCeramics and is the direct responsibility of Subcommittee C28.01 on MechanicalProperties and Performance

15、.Current edition approved Oct. 15, 2007. Published February 2008.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM webs

16、ite.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2.4 fracture mirror constant, n(Fl-3/2) an empirical ma-terial constant that relates the fracture stress to the mirrorradius in glasses and ceramics.4. Summary of Practice4.1 This

17、 practice provides guidance on the measurementand interpretation of fracture mirror sizes in laboratory testspecimens as well as in fractured components. Microscopyexamination techniques are listed. The procedures for glassesand ceramics are similar, but they are not identical. Guidanceis given for

18、interpreting the fracture mirror size and shape.Guidance is given on how to interpret noncircular mirrors dueto stress gradients, geometrical effects, or residual stresses.4.2 The stress at the origin in a component may be estimatedfrom the mirror size.4.3 Fracture mirror constants may be estimated

19、frommatched sets of fracture stresses and mirror sizes.5. Significance and Use5.1 Fracture mirror size analysis is a powerful tool foranalyzing glass and ceramic fractures. Fracture mirrors aretelltale fractographic markings in brittle materials that surrounda fracture origin as discussed in Practic

20、es C 1256 and C 1322.Fig. 1 shows a schematic with key features identified. Fig. 2shows an example in glass. The fracture mirror region is verysmooth and highly reflective in glasses, hence the name“fracture mirror.” In fact, high magnification microscopyreveals that, even within the mirror region i

21、n glasses, there arevery fine features and escalating roughness as the crackadvances away from the origin. These are submicrometer insize and hence are not discernable with an optical microscope.Early investigators interpreted fracture mirrors as havingdiscrete boundaries including a “mirror-mist” b

22、oundary andalso a “mist-hackle” boundary in glasses. These were alsotermed “inner mirror” or “outer mirror” boundaries, respec-tively. It is now known that there are no discrete boundariescorresponding to specific changes in the fractographic features.Surface roughness increases gradually from well

23、within thefracture mirror to beyond the apparent boundaries. The bound-aries were a matter of interpretation, the resolving power of themicroscope, and the mode of viewing. In very weak specimens,the mirror may be larger than the specimen or component andthe boundaries will not be present.5.2 Figs.

24、3-5 show examples in ceramics. In polycrystallineceramics, the qualifier “relatively” as in “relatively smooth”must be used, since there is an inherent roughness from themicrostructure even in the area immediately surrounding theorigin. In coarse-grained or porous ceramics, it may beimpossible to id

25、entify a mirror boundary. In polycrystallineceramics, it is highly unlikely that a mirror-mist boundary canbe detected due to the inherent roughness created by thecrack-microstructure interactions, even within the mirror. Theword “systematic” in the definition for “mirror-hackle bound-ary in polycry

26、stalline ceramics” requires some elaboration.Mirror boundary hackle lines are velocity hackle lines createdafter the radiating crack reaches terminal velocity. However,premature, isolated hackle can in some instances be generatedwell within a ceramic fracture mirror. It should be disregardedwhen jud

27、ging the mirror boundary. Wake hackle from anisolated obstacle inside the mirror (such as a large grain oragglomerate) can trigger early “premature” hackle lines. Stepsin scratches or grinding flaws can trigger hackle lines thatNOTEThe initial flaw may grow stably to size acprior to unstable fractur

28、e when the stress intensity reaches KIc. The mirror-mist radius is Ri, themist-hackle radius is Ro, and the branching distance is Rb. These transitions correspond to the mirror constants, Ai,Ao, and Ab, respectively.FIG. 1 Schematic of a Fracture Mirror Centered on a Surface Flaw of Initial Size (a)

29、.C1678072emanate from the origin itself. Sometimes the microstructureof polycrystalline ceramics creates severe judgment problemsin ceramic matrix composites (particulate, whisker, or platelet)or self-reinforced ceramics whereby elongated and interlockinggrains impart greater fracture resistance. Mi

30、rrors may beplainly evident at low magnifications, but accurate assessmentof their size can be difficult. The mirror region itself may besomewhat bumpy; therefore, some judgment as to what is amirror boundary is necessary.5.3 Fracture mirrors are circular in some loading conditionssuch as tension sp

31、ecimens with internal origins, or they arenearly semicircular for surface origins in tensile specimens, orif the mirrors are small in bend specimens. Their shapes canvary and be elongated or even incomplete in some directions ifthe fracture mirrors are in stress gradients. Fracture mirrorsmay be qua

32、rter circles if they form from corner origins in aspecimen or component. Fracture mirrors only form in mod-erate to high local stress conditions. Weak specimens may notexhibit full or even partial mirror boundaries, since the crackmay not achieve sufficient velocity within the confines of thespecime

33、n.5.4 Fracture mirrors not only bring ones attention to anorigin, but also give information about the magnitude of thestress at the origin that caused fracture and their distribution.The fracture mirror size and the stress at fracture are empiri-cally correlated by Eq 1:s=R 5 A (1)where:s = stress a

34、t the origin (MPa or ksi),R = fracture mirror radius (m or in),A = fracture mirror constant (MPa=morksi=in).Equation 1 is hereafter referred to as the “empirical stress fracture mirror size relationship,” or “stress-mirror size rela-tionship” for short.Areview of the history of Eq 1, and fracturemir

35、ror analysis in general, may be found in Refs 1 and 2.NOTE(a) shows the whole fracture surface and the fracture mirror (arrow) which is centered on a surface flaw. (b) is a close-up of the fracture mirrorwhich is elongated slightly into the interior due to the flexural stress gradient.FIG. 2 Optical

36、 Micrographs of a Fracture Mirror in a Fused Silica Glass Rod Broken in Flexure at 122 MPa Maximum Stress on theBottom.C16780735.5 A, the “fracture mirror constant” (sometimes alsoknown as the “mirror constant”) has units of stress intensity(MPa=morksi=in) and is considered by many to be amaterial p

37、roperty. As shown in Figs. 1 and 2, it is possible todiscern separate mist and hackle regions and the apparentboundaries between them in glasses. Each has a correspondingmirror constant,A. The most common notation is to refer to themirror-mist boundary as the inner mirror boundary, and itsmirror con

38、stant is designated Ai. The mist-hackle boundary isreferred to as the outer mirror boundary, and its mirror constantNOTENotice how clear the mirror is in the low power images in (a) and (b). The mirror boundary (arrows in c) is where systematic new hackle formsand there is an obvious roughness diffe

39、rence compared to the roughness inside the mirror region.FIG. 3 Silicon Carbide Tension Strength Specimen (371 MPa) with a Mirror Centered on a Compositional Inhomogeneity Flaw.C1678074is designated Ao. The mirror-mist boundary is usually notperceivable in polycrystalline ceramics. Usually, only the

40、mirror-hackle boundary is measured and only an Aofor themirror-hackle boundary is calculated. A more fundamentalrelationship than Eq 1 may be based on the stress intensityfactors (KI) at the mirror-mist or mist-hackle boundaries, butEq 1 is more practical and simpler to use.5.6 The size predictions

41、based on Eq 1 and the A values, oralternatively stress intensity factors, match very closely for thelimiting cases of small mirrors in tension specimens. This isalso true for small semicircular mirrors centered on surfaceflaws in strong flexure specimens. So, at least for some specialmirror cases, A

42、 should be directly related to a more fundamen-tal parameter based on stress intensity factors.5.7 The size of the fracture mirrors in laboratory testspecimen fractures may be used in conjunction with knownfracture mirror constants to verify the stress at fracture was asexpected. The fracture mirror

43、 sizes and known stresses fromlaboratory test specimens may also be used to compute fracturemirror constants, A.5.8 The size of the fracture mirrors in components may beused in conjunction with known fracture mirror constants toestimate the stress in the component at the origin. PracticeC 1322 has a

44、 comprehensive list of fracture mirror constantsfor a variety of ceramics and glasses.6. Procedure6.1 Use an optical microscope whenever possible.6.1.1 For glasses, use a compound optical microscope inbright field mode with reflected light illumination. A scanningelectron microscope may be used if o

45、ptical microscopy is notfeasible.6.1.2 For ceramics, use a stereo optical microscope with lowangle grazing (vicinal) illumination. A scanning electron mi-croscope may be used if optical microscopy is not feasible6.1.3 Differential interference contrast (DIC, also known asNomarski) mode viewing with

46、a research compound micro-scope is not recommended for either glasses or ceramics. It isnot suitable for rough ceramic fracture surfaces. It also createscomplications with glass fracture surfaces. There is no questionthat DIC mode viewing can discern very subtle mist features inglasses, but the thre

47、shold of mist detectability is highlydependent upon how the polarizing sliders are positioned.Hence, DIC measured radii are quite variable. DIC measuredNOTE The mirror boundary is difficult to delineate in this material. (a) shows the uncoated fracture surface of a 2.8 mm thick flexural strengthspec

48、imen that fractured at 486 MPa. Vicinal illumination brings out the markings. (b) shows a mirror-hackle boundary where systematic new hackle isdetected (small white arrows) as compared to the roughness inside the mirror. The marked circle is elongated somewhat into the depth due to the stressgradien

49、t. The radius in the direction along the bottom surface (a region of constant stress) was 345 mm.FIG. 4 A Fracture Mirror in a Fine-Grained 3 Mol % Yttria-Stabilized Tetragonal Zirconia Polycrystal (3Y-TZP).C1678075radii can be substantially smaller than those obtained withconventional viewing modes. It also must be borne in mind thatnot all users have access to interference contrast microscopes.6.1.4 Dark-field illumination may be used for glasses, butsome resolution may be lost with glasses and radii may beslightly larger as a result. Dark field is very effective wit