1、Designation: C1678 10Standard Practice forFractographic Analysis of Fracture Mirror Sizes in Ceramicsand Glasses1This standard is issued under the fixed designation C1678; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of
2、 last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice pertains to the analysis and interpretationof fracture mirror sizes in brittle materials. Fracture mirr
3、ors(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 known s
4、tresses 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. Diffe
5、rent 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 for b
6、oth 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 polycryst
7、alline ceramics.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.3 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 t
8、o establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2C1145 Terminology of Advanced CeramicsC1256 Practice for Interpreting Glass Fracture SurfaceFeaturesC1322 Practice for Fractography
9、and Characterization ofFracture Origins in Advanced Ceramics3. Terminology3.1 Definitions: (See Fig. 1)3.1.1 fracture mirror, nas used in fractography of brittlematerials, a relatively smooth region in the immediate vicinityof and surrounding the fracture origin C1145, C13223.1.2 fracture origin, nt
10、he source from which brittlefracture commences. C1145, C13223.1.3 hackle, nas used in fractography of brittle materials,a line or lines on the crack surface running in the local directionof cracking, separating parallel but noncoplanar portions of thecrack surface. C1145, C13223.1.4 mist, nas used i
11、n fractography of brittle materials,markings on the surface of an accelerating crack close to itseffective terminal velocity, observable first as a misty appear-ance and with increasing velocity reveals a fibrous texture,elongated in the direction of crack propagation. C1145,C13223.2 Definitions of
12、Terms Specific to This Standard:(See Fig. 1)3.2.1 mirror-mist boundary in glasses, nthe peripherywhere one can 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
13、 the onset of systematic hackle around aglass fracture mirror. This boundary corresponds to Ao, theouter mirror constant.3.2.3 mirror-hackle boundary in polycrystalline ceramics,nthe periphery where one can discern the onset of systematic1This practice is under the jurisdiction of ASTM Committee C28
14、 on AdvancedCeramics and is the direct responsibility of Subcommittee C28.03 on PhysicalProperties and Non-Destructive Evaluation.Current edition approved Jan. 1, 2010. Published March 2010. Originallyapproved in 2007. Last previous edition approved in 2009 as C1678 09. DOI:10.1520/C1678-10.2For ref
15、erenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, We
16、st Conshohocken, PA 19428-2959, United States.new hackle and there is an obvious roughness change relativeto that inside a ceramic fracture mirror region. This boundarycorresponds to Ao, the outer mirror constant. Ignore prematurehackle and/or isolated steps from microstructural irregularitiesin the
17、 mirror or irregularities at the origin.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 practice provides guidance on the measurementand interpretation of fracture mi
18、rror 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 interpreting the fracture mirror size and shape.Guidance is given on how to int
19、erpret 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 frommatched sets of fracture stresses and mirror sizes.5. Significance and Use5
20、.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 Practices C1256 and C1322.Fig. 1 shows a schematic with key features identified. Fig.
21、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 in glasses, there arevery fine features and escalating roughness as the crackadvan
22、ces 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” boundary andalso a “mist-hackle” boundary in glasses. These were alsotermed “inner
23、 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 within thefracture mirror to beyond the apparent boundaries. The bound-aries were
24、 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. 3-5 show examples in ceramics. In polycrystallineceramics, the qualifier “relativ
25、ely” 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 identify a mirror boundary. In polycrystallineceramics, it is highly unlikely that
26、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 polycrystalline ceramics” requires some elaboration.Mirror boundary hackle lines are vel
27、ocity hackle lines createdafter the radiating crack reaches terminal velocity. However,premature, isolated hackle can in some instances be generatedNOTEThe initial flaw may grow stably to size acprior to unstable fracture when the stress intensity reaches KIc. The mirror-mist radius is Ri, themist-h
28、ackle 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).C1678 102well within a ceramic fracture mirror. It should be disregardedwhen ju
29、dging 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 thatemanate from the origin itself. Sometimes the microstructureof polycrys
30、talline ceramics creates severe judgment problemsin ceramic matrix composites (particulate, whisker, or platelet)or self-reinforced ceramics whereby elongated and interlockinggrains impart greater fracture resistance. Mirrors may beplainly evident at low magnifications, but accurate assessmentof the
31、ir 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 specimens with internal origins, or they arenearly semicircular for surface origin
32、s 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 quarter circles if they form from corner origins in aspecimen or component. Fractur
33、e 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 thespecimen.NOTE(a) shows the whole fracture surface and the fracture mirror (arrow) which
34、 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 Micrographs of a Fracture Mirror in a Fused Silica Glass Rod Broken in Flexure at 122 MPa Maximum Stress on theBottom.C1678 1035.4
35、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:NOTENotice how clear the mirror is
36、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 difference compared to the roughness inside the mirror region.FIG. 3 Silicon Carbide Tension Strength Specimen (371 MPa) with a Mirror Centered on a Composi
37、tional Inhomogeneity Flaw.C1678 104s=R 5 A (1)where:s = stress at 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-tio
38、nship” for short.Areview of the history of Eq 1, and fracturemirror analysis in general, may be found in Refs 1 and 2.5.5 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 property.
39、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 constant is
40、designated Ai. The mist-hackle boundary isreferred to as the outer mirror boundary, and its mirror constantis designated Ao. The mirror-mist boundary is usually notperceivable in polycrystalline ceramics. Usually, only themirror-hackle boundary is measured and only an Aofor themirror-hackle boundary
41、 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 based on Eq 1 and the A values, oralternatively stress intensity factors, matc
42、h 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 should be directly related to a more fundamen-tal parameter based on stress i
43、ntensity 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 sizes and known stresses fromlaboratory test specimens may also be used to co
44、mpute 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. PracticeNOTE The mirror boundary is difficult to delineate in this material. (a) shows the uncoate
45、d fracture surface of a 2.8 mm thick flexural strengthspecimen 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
46、elongated somewhat into the depth due to the stressgradient. 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).C1678 105C1322 has a comprehensive
47、list of fracture mirror constants fora 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 optical microsco
48、py is notfeasible. A differential interference contrast optical microscopeis optional.NOTEThe mirror is incomplete into the bend stress gradient, but the mirror sides can be used to construct boundary arcs in (c) (b) and (c) are close-upsof (a). Radii are measured in the direction of constant stress
49、 along the bottom.FIG. 5 Silicon Nitride Bend Bar with a Knoop Surface Crack in a Silicon Nitride (449 MPa).C1678 1066.1.2 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 a research compound micro-scope may be used for glasses. It should not be used forceramics since it is not suitable for rough ceramic fracturesurfaces.6.1.3.1 Interference contras
