1、Designation: C 1678 09Standard 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 () 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 mi
3、rrors(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
4、 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. Dif
5、ferent 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
6、 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 polycry
7、stalline 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
8、 to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2C 1145 Terminology of Advanced CeramicsC 1256 Practice for Interpreting Glass Fracture SurfaceFeaturesC 1322 Practice for Fractogr
9、aphy 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 C 1145, C 13223.1.2 fracture ori
10、gin, nthe source from which brittlefracture commences. C 1145, C 13223.1.3 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. C 1145, C 13223.1.4 mist,
11、 nas used in 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. C 1145,C 13223.2 De
12、finitions of 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 on
13、e can discern the onset of systematic hackle around a1This practice is under the jurisdiction of ASTM Committee C28 on AdvancedCeramics and is the direct responsibility of Subcommittee C28.03 on PhysicalProperties and Non-Destructive Evaluation.Current edition approved May 1, 2009. Published June 20
14、09. Originallyapproved in 2007. Last previous edition approved in 2007 as C 1678 07.2For 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 p
15、age onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.glass fracture mirror. This boundary corresponds to Ao, theouter mirror constant.3.2.3 mirror-hackle boundary in polycrystalline ceramics,nthe periphery where one
16、 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. Ignore prematurehackle and/or isolated steps from microstructural irregularitiesin the mirror or
17、 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 mirror sizes
18、 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 interpret non
19、circular 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 Use5.1 Fractur
20、e 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 C 1256 and C 1322.Fig. 1 shows a schematic with key features identified. Fig. 2shows a
21、n 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 crackadvances away
22、 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 mirror”
23、 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 a matte
24、r 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 “relatively” as
25、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 a mirror
26、-mist boundary canbe detected due to the inherent roughness created by thecrack-microstructure interactions, even within the mirror. TheNOTEThe initial flaw may grow stably to size acprior to unstable fracture when the stress intensity reaches KIc. The mirror-mist radius is Ri, themist-hackle radius
27、 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).C1678092word “systematic” in the definition for “mirror-hackle bound-ary in polycrystalline
28、 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 judging the
29、 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 polycrystalline c
30、eramics 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 their size c
31、an 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 origins in tens
32、ile 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. Fracture mirrors
33、 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 is cente
34、red 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.C16780935.4 Fracture m
35、irrors 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 in the low
36、 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 Compositional Inh
37、omogeneity Flaw.C1678094s=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-tionship” for
38、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. As shown in
39、 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 designated
40、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 is calcula
41、ted. 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, match very clos
42、ely 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 intensity fa
43、ctors.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 compute fract
44、uremirror 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 uncoated fracture
45、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 elongated s
46、omewhat 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).C1678095C 1322 has a comprehensive list of fra
47、cture 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 optical microscopy is notfe
48、asible. 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 along the
49、bottom.FIG. 5 Silicon Nitride Bend Bar with a Knoop Surface Crack in a Silicon Nitride (449 MPa).C16780966.1.2 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 contrast mode of viewing
