ASTM C1678-21

Standard Practice for Fractographic Analysis of Fracture Mirror Sizes in Ceramics and Glasses

SIGNIFICANCE AND USE
5.1 Fracture mirror size analysis is a powerful tool for analyzing glass and ceramic fractures. Fracture mirrors are tell-tale fractographic markings in brittle materials that surround a fracture origin as discussed in Practices C1256 and C1322. Fig. 1 shows a schematic with key features identified. Fig. 2 shows an example in glass. The fracture mirror region is very smooth and highly reflective in glasses, hence the name “fracture mirror.” In fact, high magnification microscopy reveals that, even within the mirror region in glasses, there are very fine features and escalating roughness as the crack advances away from the origin. These are submicrometer in size and hence are not discernable with an optical microscope. Early investigators interpreted fracture mirrors as having discrete boundaries including a “mirror-mist” boundary and also a “mist-hackle” boundary in glasses. These were also termed “inner mirror” or “outer mirror” boundaries, respectively. It is now known that there are no discrete boundaries corresponding to specific changes in the fractographic features. Surface roughness increases gradually from well within the fracture mirror to beyond the apparent boundaries. The boundaries were a matter of interpretation, the resolving power of the microscope, and the mode of viewing. In very weak specimens, the mirror may be larger than the specimen or component and the boundaries will not be present. Eq 1 is hereafter referred to as the “empirical stress – fracture mirror size relationship,” or “stress-mirror size relationship” for short. A review of the history of Eq 1, and fracture mirror analysis in general, may be found in Refs (1)3 and (2).  
5.5 A, the “fracture mirror constant” (sometimes also known as the “mirror constant”) has units of stress intensity (MPa√m or ksi√in.) and is considered by many to be a material property. As shown in Figs. 1 and 2, it is possible to discern separate mist and hackle regions and the apparent boundaries between them i...
SCOPE
1.1 This practice pertains to the analysis and interpretation of fracture mirror sizes in brittle materials. Fracture mirrors (Fig. 1) are telltale fractographic markings that surround a fracture origin in brittle materials. The fracture mirror size may be used with known fracture mirror constants to estimate the stress in a fractured component. Alternatively, the fracture mirror size may be used in conjunction with known stresses in test specimens to calculate fracture mirror constants. The practice is applicable to glasses and polycrystalline ceramic laboratory test specimens as well as fractured components. The analysis and interpretation procedures for glasses and ceramics are similar, but they are not identical. Different optical microscopy examination techniques are listed and described, including observation angles, illumination methods, appropriate magnification, and measurement protocols. Guidance is given for calculating a fracture mirror constant and for interpreting the fracture mirror size and shape for both circular and noncircular mirrors including stress gradients, geometrical effects, residual stresses, or combinations thereof. The practice provides figures and micrographs illustrating the different types of features commonly observed in and measurement techniques used for the fracture mirrors of glasses and polycrystalline ceramics.
FIG. 1 Schematic of a Fracture Mirror Centered on a Surface Flaw of Initial Size (a)
Note 1: The initial flaw may grow stably to size ac prior to unstable fracture when the stress intensity reaches KIc. The mirror-mist radius is Ri, the mist-hackle radius is Ro, and the branching distance is Rb. These transitions correspond to the mirror constants, Ai, Ao, and Ab, respectively.  
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.3 This standard does not purport to address all of the safet...

General Information

• Status: Published
• Publication Date: 30-Jun-2021
• ICS: 81.060.20 - Ceramic products
• Technical Committee: C28 - Advanced Ceramics
• Drafting Committee: C28.03 - Physical Properties and Non-Destructive Evaluation

Relations

• Refers: ASTM C1256-93(2019) - Standard Practice for Interpreting Glass Fracture Surface Features
• Effective Date: 01-Aug-2019
• Refers: ASTM C1322-15(2019) - Standard Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
• Effective Date: 01-Jul-2019
• Refers: ASTM C1145-19 - Standard Terminology of Advanced Ceramics
• Effective Date: 01-Jul-2019
• Refers: ASTM C1322-15 - Standard Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
• Effective Date: 01-Jul-2015
• Refers: ASTM C1256-93(2013) - Standard Practice for Interpreting Glass Fracture Surface Features
• Effective Date: 01-Oct-2013
• Refers: ASTM C1145-06(2013)e1 - Standard Terminology of Advanced Ceramics
• Effective Date: 01-Feb-2013
• Refers: ASTM C1145-06(2013) - Standard Terminology of Advanced Ceramics
• Effective Date: 01-Feb-2013
• Refers: ASTM C1322-05b(2010) - Standard Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
• Effective Date: 15-Jul-2010
• Refers: ASTM C1256-93(2008) - Standard Practice for Interpreting Glass Fracture Surface Features
• Effective Date: 01-Apr-2008
• Refers: ASTM C1145-06 - Standard Terminology of Advanced Ceramics
• Effective Date: 01-Jan-2006
• Refers: ASTM C1322-05be1 - Standard Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
• Effective Date: 01-Jul-2005
• Refers: ASTM C1322-05b - Standard Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
• Effective Date: 01-Jul-2005
• Refers: ASTM C1322-05a - Standard Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
• Effective Date: 01-Feb-2005
• Refers: ASTM C1145-05 - Standard Terminology of Advanced Ceramics
• Effective Date: 01-Jan-2005
• Refers: ASTM C1322-05 - Standard Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
• Effective Date: 01-Jan-2005

Overview

ASTM C1678-21 provides a standard practice for the fractographic analysis of fracture mirror sizes in ceramics and glasses. This industry-recognized standard is essential for professionals working in the field of material science, failure analysis, and quality control of brittle materials such as glass and ceramics. Fracture mirrors are distinctive markings formed around the fracture origins and serve as key indicators for understanding the stress conditions leading to material failure.

By analyzing the size and shape of fracture mirrors, users can estimate stress at the origin point of a fracture or calculate material-specific fracture mirror constants. These techniques, supported by optical microscopy and systematic measurement protocols, enhance both research and practical evaluation of brittle material performance.

Key Topics

Fracture Mirror Analysis:

• Fracture mirrors are smooth, reflective zones surrounding the origin of cracks in brittle materials.
• The analysis focuses on measuring these regions to gather insights about stress distribution and fracture mechanics.
• Fracture mirrors help identify the magnitude of stresses that caused failure.

Mirror Boundaries and Features:

• Boundaries such as mirror-mist and mist-hackle are traditionally observed in glasses; only mist-hackle is discernible in ceramics.
• Surface roughness generally increases gradually from the mirror region outward, rather than displaying sharply defined boundaries.

Microscopy Examination Techniques:

• Optical microscopy is the preferred method for characterizing fracture mirrors, with specific guidance provided for both glasses and ceramics.
• Techniques include adjusting observation angles, illumination methods, and magnification to reveal relevant features.
• Stereo microscopes, compound microscopes, and specialized lighting (e.g., grazing, vicinal) are employed depending on the material type.

Measurement Protocols:

• Measurements are ideally made using calibrated reticules or digital image analysis under carefully controlled microscopy conditions.
• Radii are generally measured along directions of constant stress, and shapes (circular, elliptical, or semicircular) are noted.
• The standard outlines procedures for both direct and indirect measurement, emphasizing accuracy, consistency, and thorough documentation.

Applications

Quality Control in Manufacturing:

• Evaluate and assure the mechanical reliability of ceramics and glasses used in structural and optical applications.
• Used by manufacturers for post-fracture analysis of test specimens and in-service components.

Failure Analysis:

• Aid forensic investigations by determining fracture origins and estimating fracture stresses using the empirical stress–mirror size relationship.
• Supports identifying material defects, production issues, or service-related stress factors.

Research and Development:

• Support the development of stronger, more reliable glass and ceramic materials by systematically cataloging material-related fracture constants.
• Enables benchmarking of new materials and processing methods.

Materials Testing Laboratories:

• Standardizes the interpretation of fracture surfaces in tensile, flexural, and bending tests, ensuring reliable comparison across different studies and industries.

Related Standards

• ASTM C1145: Terminology of Advanced Ceramics - Provides definitions and descriptive terms relevant to ceramics.
• ASTM C1256: Practice for Interpreting Glass Fracture Surface Features - Offers detailed insights for analyzing glass fracture markings.
• ASTM C1322: Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics - Focuses on advanced ceramics and their fracture origins.

By adhering to ASTM C1678-21, professionals can achieve a consistent, scientifically robust approach to the fractographic analysis of ceramic and glass components, leading to improved material performance and failure prevention strategies. This standard is vital for advancing material science, enhancing product safety, and supporting quality assurance in the production and application of brittle materials.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: C1678 − 21
Standard Practice for
Fractographic Analysis of Fracture Mirror Sizes in Ceramics
and Glasses
This standard is issued under the fixed designation C1678; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
1.1 This practice pertains to the analysis and interpretation
Barriers to Trade (TBT) Committee.
of fracture mirror sizes in brittle materials. Fracture mirrors
(Fig. 1) are telltale fractographic markings that surround a
2. Referenced Documents
fractureorigininbrittlematerials.Thefracturemirrorsizemay
2.1 ASTM Standards:
be used with known fracture mirror constants to estimate the
C1145 Terminology of Advanced Ceramics
stress in a fractured component. Alternatively, the fracture
C1256 Practice for Interpreting Glass Fracture Surface Fea-
mirror size may be used in conjunction with known stresses in
tures
test specimens to calculate fracture mirror constants. The
C1322 Practice for Fractography and Characterization of
practice is applicable to glasses and polycrystalline ceramic
Fracture Origins in Advanced Ceramics
laboratorytestspecimensaswellasfracturedcomponents.The
analysis and interpretation procedures for glasses and ceramics
3. Terminology
are similar, but they are not identical. Different optical micros-
copy examination techniques are listed and described, includ-
3.1 Definitions: (See Fig. 1)
ing observation angles, illumination methods, appropriate 3.1.1 fracture mirror, n—as used in fractography of brittle
magnification, and measurement protocols. Guidance is given
materials, a relatively smooth region in the immediate vicinity
for calculating a fracture mirror constant and for interpreting of and surrounding the fracture origin. C1145, C1322
the fracture mirror size and shape for both circular and
3.1.2 fracture origin, n—the source from which brittle
noncircular mirrors including stress gradients, geometrical
fracture commences. C1145, C1322
effects, residual stresses, or combinations thereof. The practice
3.1.3 hackle, n—as used in fractography of brittle materials,
providesfiguresandmicrographsillustratingthedifferenttypes
alineorlinesonthecracksurfacerunninginthelocaldirection
of features commonly observed in and measurement tech-
of cracking, separating parallel but noncoplanar portions of the
niques used for the fracture mirrors of glasses and polycrys-
crack surface. C1145, C1322
talline ceramics.
3.1.4 mist, n—as used in fractography of brittle materials,
1.2 The values stated in SI units are to be regarded as
markings on the surface of an accelerating crack close to its
standard. No other units of measurement are included in this
effective terminal velocity, observable first as a misty appear-
standard.
ance and with increasing velocity reveals a fibrous texture,
1.3 This standard does not purport to address all of the
elongated in the direction of crack propagation. C1145, C1322
safety concerns, if any, associated with its use. It is the
3.2 Definitions of Terms Specific to This Standard:
responsibility of the user of this standard to establish appro-
(See Fig. 1)
priate safety, health, and environmental practices and deter-
3.2.1 mirror-mist boundary in glasses, n—the periphery
mine the applicability of regulatory limitations prior to use.
where one can discern the onset of mist around a glass fracture
1.4 This international standard was developed in accor-
mirror. This boundary corresponds to A, the inner mirror
i
dance with internationally recognized principles on standard-
constant.
ization established in the Decision on Principles for the
3.2.2 mist-hackle boundary in glasses, n—the periphery
where one can discern the onset of systematic hackle around a
This practice is under the jurisdiction of ASTM Committee C28 on Advanced
Ceramics and is the direct responsibility of Subcommittee C28.03 on Physical
Properties and Non-Destructive Evaluation. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved July 1, 2021. Published July 2021. Originally approved contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
in 2007. Last previous edition approved in 2015 as C1678 – 10 (2015). DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/C1678-21. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1678 − 21
NOTE 1—The initial flaw may grow stably to size a prior to unstable fracture when the stress intensity reaches K . The mirror-mist radius is R, the
c Ic i
mist-hackle radius is R , and the branching distance is R . These transitions correspond to the mirror constants, A,A , and A , respectively.
o b i o b
FIG. 1 Schematic of a Fracture Mirror Centered on a Surface Flaw of Initial Size (a)
glass fracture mirror. This boundary corresponds to A , the tell-tale fractographic markings in brittle materials that sur-
o
outer mirror constant. round a fracture origin as discussed in Practices C1256 and
C1322. Fig. 1 shows a schematic with key features identified.
3.2.3 mirror-hackle boundary in polycrystalline ceramics,
Fig. 2 shows an example in glass.The fracture mirror region is
n—the periphery where one can discern the onset of systematic
very smooth and highly reflective in glasses, hence the name
new hackle and there is an obvious roughness change relative
“fracture mirror.” In fact, high magnification microscopy
to that inside a ceramic fracture mirror region. This boundary
reveals that, even within the mirror region in glasses, there are
corresponds toA , the outer mirror constant. Ignore premature
o
very fine features and escalating roughness as the crack
hackle and/or isolated steps from microstructural irregularities
advances away from the origin. These are submicrometer in
in the mirror or irregularities at the origin.
size and hence are not discernable with an optical microscope.
–3/2
3.2.4 fracture mirror constant, (Fl ), n—an empirical
Early investigators interpreted fracture mirrors as having
material constant that relates the fracture stress to the mirror
discrete boundaries including a “mirror-mist” boundary and
radius in glasses and ceramics.
also a “mist-hackle” boundary in glasses. These were also
termed “inner mirror” or “outer mirror” boundaries, respec-
4. Summary of Practice
tively. It is now known that there are no discrete boundaries
4.1 This practice provides guidance on the measurement
correspondingtospecificchangesinthefractographicfeatures.
and interpretation of fracture mirror sizes in laboratory test
Surface roughness increases gradually from well within the
specimens as well as in fractured components. Microscopy
fracture mirror to beyond the apparent boundaries. The bound-
examination techniques are listed. The procedures for glasses
arieswereamatterofinterpretation,theresolvingpowerofthe
and ceramics are similar, but they are not identical. Guidance
microscope,andthemodeofviewing.Inveryweakspecimens,
is given for interpreting the fracture mirror size and shape.
the mirror may be larger than the specimen or component and
Guidance is given on how to interpret noncircular mirrors due
the boundaries will not be present.
to stress gradients, geometrical effects, or residual stresses.
5.2 Figs. 3-5 show examples in ceramics. In polycrystalline
4.2 Thestressattheorigininacomponentmaybeestimated
ceramics, the qualifier “relatively” as in “relatively smooth”
from the mirror size.
must be used, since there is an inherent roughness from the
4.3 Fracture mirror constants may be estimated from
microstructure even in the area immediately surrounding the
matched sets of fracture stresses and mirror sizes.
origin. In coarse-grained or porous ceramics, it may be
impossible to identify a mirror boundary. In polycrystalline
5. Significance and Use
ceramics, it is highly unlikely that a mirror-mist boundary can
5.1 Fracture mirror size analysis is a powerful tool for be detected due to the inherent roughness created by the
analyzing glass and ceramic fractures. Fracture mirrors are crack-microstructure interactions, even within the mirror. The
C1678 − 21
NOTE1—(a)showsthewholefracturesurfaceandthefracturemirror(arrow)whichiscenteredonasurfaceflaw;(b)isaclose-upofthefracturemirror
which 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 the
Bottom
word “systematic” in the definition for “mirror-hackle bound- somewhat bumpy; therefore, some judgment as to what is a
ary in polycrystalline ceramics” requires some elaboration. mirror boundary is necessary.
Mirror boundary hackle lines are velocity hackle lines created
5.3 Fracture mirrors are circular in some loading conditions
after the radiating crack reaches terminal velocity. However,
such as tension specimens with internal origins, or they are
premature, isolated hackle can in some instances be generated
nearly semicircular for surface origins in tensile specimens, or
well within a ceramic fracture mirror. It should be disregarded
if the mirrors are small in bend specimens. Their shapes can
when judging the mirror boundary. Wake hackle from an
vary and be elongated or even incomplete in some directions if
isolated obstacle inside the mirror (such as a large grain or
the fracture mirrors are in stress gradients. Fracture mirrors
agglomerate) can trigger early “premature” hackle lines. Steps
may be quarter circles if they form from corner origins in a
in scratches or grinding flaws can trigger hackle lines that
specimen or component. Fracture mirrors only form in mod-
emanate from the origin itself. Sometimes the microstructure
erate to high local stress conditions. Weak specimens may not
of polycrystalline ceramics creates severe judgment problems
exhibit full or even partial mirror boundaries, since the crack
in ceramic matrix composites (particulate, whisker, or platelet)
may not achieve sufficient velocity within the confines of the
orself-reinforcedceramicswherebyelongatedandinterlocking
specimen.
grains impart greater fracture resistance. Mirrors may be
plainly evident at low magnifications, but accurate assessment 5.4 Fracture mirrors not only bring one’s attention to an
of their size can be difficult. The mirror region itself may be origin, but also give information about the magnitude of the
C1678 − 21
NOTE 1—Notice 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
forms and 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 Inhomogeneity Flaw
C1678 − 21
NOTE 1—The mirror boundary is difficult to delineate in this material. (a) shows the uncoated fracture surface of a 2.8 mm thick flexural strength
specimen that fractured at 486 MPa. Vicinal illumination brings out the markings. (b) shows a mirror-hackle boundary where systematic new hackle is
detected (small white arrows) as compared to the roughness inside the mirror. The marked circle is elongated somewhat into the depth due to the stress
gradient. 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)
stress at the origin that caused fracture and their distribution. separate mist and hackle regions and the apparent boundaries
The fracture mirror size and the stress at fracture are empiri- between them in glasses. Each has a corresponding mirror
cally correlated by Eq 1:
constant, A. The most common notation is to refer to the
mirror-mist boundary as the inner mirror boundary, and its
σ=R 5 A (1)
mirror constant is designated A. The mist-hackle boundary is
i
where:
referredtoastheoutermirrorboundary,anditsmirrorconstant
σ = stress at the origin (MPa or ksi), is designated A . The mirror-mist boundary is usually not
o
R = fracture mirror radius (m or in.), and perceivable in polycrystalline ceramics. Usually, only the
A = fracture mirror constant (MPa√morksi√in.).
mirror-hackle boundary is measured and only an A for the
o
mirror-hackle boundary is calculated. A more fundamental
Eq 1 is hereafter referred to as the “empirical stress –
relationship than Eq 1 may be based on the stress intensity
fracture mirror size relationship,” or “stress-mirror size rela-
tionship”forshort.AreviewofthehistoryofEq1,andfracture factors (K ) at the mirror-mist or mist-hackle boundaries, but
I
mirror analysis in general, may be found in Refs (1) and (2). Eq 1 is more practical and simpler to use.
5.5 A, the “fracture mirror constant” (sometimes also
5.6 The size predictions based on Eq 1 and theAvalues, or
known as the “mirror constant”) has units of stress intensity
alternatively stress intensity factors, match very closely for the
(MPa√morksi√in.)andisconsideredbymanytobeamaterial
limiting cases of small mirrors in tension specimens. This is
property. As shown in Figs. 1 and 2, it is possible to discern
also true for small semicircular mirrors centered on surface
flaws in strong flexure specimens. So, at least for some special
3 mirror cases,Ashould be directly related to a more fundamen-
The boldface numbers in parentheses refer to a list of references at the end of
this standard. tal parameter based on stress intensity factors.
C1678 − 21
NOTE 1—The 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-ups of (a)). Radii are measured in the direction of constant stress along the bottom.
FIG. 5 Silicon Nitride Bend Bar with a Knoop Surface Crack in a Silicon Nitride (449 MPa)
5.7 The size of the fracture mirrors in laboratory test 6. Procedure
specimen fractures may be used in conjunction with known
6.1 Use an optical microscope whenever possible.
fracture mirror constants to verify the stress at fracture was as
6.1.1 For glasses, use a compound optical microscope in
expected. The fracture mirror sizes and known stresses from
bright field mode with reflected light illumination. A scanning
laboratorytestspecimensmayalsobeusedtocomputefracture
electron microscope may be used if optical microscopy is not
mirror constants, A.
feasible.Adifferential interference contrast optical microscope
5.8 The size of the fracture mirrors in components may be is optional.
used in conjunction with known fracture mirror constants to 6.1.2 Forceramics,useastereoopticalmicroscopewithlow
estimate the stress in the component at the origin. Practice angle illumination. Low angle illumination is occasionally
C1322hasacomprehensivelistoffracturemirrorconstantsfor called “grazing illumination.” A scanning electron microscope
a variety of ceramics and glasses. may be used if optical microscopy is not feasible.
C1678 − 21
6.1.3 Differential interference contrast (DIC, also known as options are available to accentuate subtle surface roughness
Nomarski) mode viewing with a research compound micro- and topography features.
scope may be used for glasses. It should not be used for 6.3.2 For ceramics, accentuate the hackle lines. Ceramics
ceramics since it is not suitable for rough ceramic fracture should not be uniformly and directly illuminated such as by a
surfaces. ring light, since the light will reduce contrast especially in
6.1.3.1 Interference contrast mode of viewing can discern translucent or transparent materials. Ceramics shall be illumi-
very subtle mist features in glasses, but the threshold of mist nated with low angle illumination. Thin gold or carbon
detectability is highly dependent upon how the polarizing coatings may be applied to translucent or transparent ceramics
elementsarepositioned.Therefore,usethepolarizingelements as needed.
in a grey mode (non-color, remove the lambda plate for color
6.4 Use an appropriate magnification.
control)andslowlyincreaselightintensity,butnotethathigher
6.4.1 For glasses, use a magnification such that the fracture
light intensity can hide details. Rotate the analyzer until one
mirror area occupies about 75 % to 90 % of the width of the
can determine repeatably consistent boundary conditions, for
field of view. Fracture mirrors are reasonably easy to see in
example, mist and hackle. Typically, more details will be
glasses, and magnifications should be used such that the
evident, but when properly used, DIC viewing can produce
fracture mirrors nearly fill the field of view.
consistentmirrorradiimeasurements.Notethattheseradiimay
6.4.2 For ceramics, use a magnification such that the frac-
be smaller than those obtained with conventional viewing
ture mirror area occupies about 33 % to 67 % of the width of
modes. Thus, mirror-mist fracture mirror constants may be
the field of view. Mirror interpretation is more problematic
slightly smaller than those obtained with bright field illumina-
with polycrystalline ceramics. Even though a mirror may be
tion. Therefore, it shall be stated in the report if interference
obvious at low or moderate magnification, at high magnifica-
contrast techniques were used.
tion it may be impossible to judge a boundary. It is more
6.1.4 Dark field illumination may be used for glasses, but
practical to view the mirror region and the natural microstruc-
some resolution may be lost with glasses and radii may be
turalroughnessthereinrelativetothehackleoutsidethemirror.
slightly larger as a result. Dark field is very effective with
“Stepping back” and using the 33 % to 67 % rule should help
highly reflective mirror surfaces of ceramic single crystals.
an observer better detect the topography differences. Supple-
6.1.5 Scanning electron microscope images of mirrors are
mentallower-magnificationimagesmayaidinterpretation.The
not recommended for glasses, since the mirror-mist boundary
magnification of the supplemental images should differ from
is usually indiscernible. SEM images often appear flat and do
that of the main measurement image by no more than a factor
not have adequate contrast to see the fine mist detail at the
of five, otherwise it is difficult to correlate features between the
ordinary magnifications used to frame the whole mirror. SEM
images.
images may be used with very small mirrors that would be
6.5 Measure the mirror size while viewing the fracture
difficult to see with optical microscopy, for example, high-
surface with an optical microscope whenever possible.
strength optical fibers. Scanning electron microscope images
6.5.1 For both glasses and ceramics, use either calibrated
may be used for ceramics if necessary, but contrast and
reticules in the eyepieces or traversing stages with micrometer-
shadowing should be enhanced.
positioning heads. Alternatively, measurements may be made
6.1.6 It is recommended that the report state the inspection
ondigitalimagesonahigh-resolutioncomputermonitor,while
method/instrument used.
the fracture surface can be simultaneously viewed through the
6.2 The fracture surface should be approximately perpen-
microscope eyepieces in order to aid judgment.
dicular to the microscope optical path or camera.
NOTE 1—Mirror size measurements made on computer monitor screens
6.2.1 This requirement poses a small problem if the mirrors
are subject to inaccuracies, because they are two-dimensional renditions
in ceramics are examined with a stereo binocular microscope.
of a three-dimensional fracture surface. Nevertheless, high-resolution
This microscope has two different tilted optical paths. If
cameras and monitors are beginning to match the capabilities and
viewing with both eyes in a stereo microscope, the specimen
accuracy of an observer peering through the optical microscope.
should be flat and facing directly upwards. The observer’s
6.5.2 Measurements from photos or digitally recorded im-
brain will interpret the image as though the observer is facing
ages may be used as a last resort if the steps in 6.5.1 cannot be
it directly. Alternatively, if a camera is mounted on one light
followed. This may be necessary for very small specimens or
path of the stereo microscope, and it is used to capture or
very strong specimens with tiny mirrors where a scanning
display the mirror, then the specimen should be tilted so that
electron microscope must be used to photograph the mirror.
the camera axis is normal to the fracture surface. For example,
Measurements from other devices may be used provided that
slightly tilt the specimen to the right if the camera is attached
the criterion used for identifying the mirror boundary is
to the right optical path.
carefully documented. Complementary high and low magnifi-
6.3 Optimize the illumination to accentuate topographical cation images may be used to help aid in interpretation. Mirror
detail.
size measurements from photographs are usually less accurate
6.3.1 For glasses, accentuate the mist and hackle features. or precise. They frequently overestimate mirror sizes unless
Glasses may either be illuminated from directly down onto a conditions are carefully optimized to accentuate contrast and
fracture surface or by a low grazing angle. Low angle illumi- topographic detail. Two-dimensional photographic renditions
nationislessconvenientwithcompoundlightmicroscopes,but of a three-dimensional fracture surface usually lose much of
the observer should experiment with whatever illumination the topographic detail discernable by the eye with a compound
C1678 − 21
optical or stereo microscope. Video cameras shall not be used as the fiber diameter . The mirror radius should simply be
to capture mirror images, since they lack adequate resolution. measured from the origin to the mirror-mist or mist-hackle
6.5.3 In ceramics, the fracture mirror regions may have an boundary on the opposite side of the fiber, R , as shown in Fig.
d
intrinsic roughness due to the microstructure. The mirror 6.
boundary is judged to be the point where systematic radiating
6.6.3 Mirror shapes are commonly affected by stress gradi-
new hackle commences and there is an obvious roughness
ents in a plate or a beam. Mirror radii are elongated in the
change relative to the inside-mirror region. The new hackle
direction of decreasing stress. In such cases, measure the
that generates the mirror boundary is formed by the radiating
mirror radius along the tensile surface where the stress is
crack running at or near terminal velocity. Ignore premature
constant. Do not measure the mirror radii into the stress
isolated hackle that may be generated well within a mirror.
gradient. See Annex A1 for more information on how to
Wakehacklefromanobstacleinsidethemirror(suchasalarge
interpret elongated mirrors and mirrors in stress gradients.
grain or agglomerate) can trigger early premature hackle lines.
6.6.4 Fracture mirrors in glasses that are centered around a
Steps in scratches or grinding flaws can trigger premature
surface-located origin may have a slight inward pinch towards
hackle that emanate from the origin itself.
the origin or a “cusp” due to free surface effects. Fig. 2 and
several figures in AnnexA2 illustrate such cusps. Truncate the
6.6 Measure radii in directions of approximately constant
stress whenever possible. A mirror diameter may be measured cusps when interpreting the arc of the fracture mirror bound-
aries as discussed in Annex A2.
and halved to estimate the radius if the origin site is indistinct
or complex.
6.6.5 Residual stresses may alter fracture mirror s
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: C1678 − 10 (Reapproved 2015) C1678 − 21
Standard Practice for
Fractographic Analysis of Fracture Mirror Sizes in Ceramics
and Glasses
This standard is issued under the fixed designation C1678; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This practice pertains to the analysis and interpretation of fracture mirror sizes in brittle materials. Fracture mirrors (Fig. 1)
are telltale fractographic markings that surround a fracture origin in brittle materials. The fracture mirror size may be used with
known fracture mirror constants to estimate the stress in a fractured component. Alternatively, the fracture mirror size may be used
in conjunction with known stresses in test specimens to calculate fracture mirror constants. The practice is applicable to glasses
and polycrystalline ceramic laboratory test specimens as well as fractured components. The analysis and interpretation procedures
for glasses and ceramics are similar, but they are not identical. Different optical microscopy examination techniques are listed and
described, including observation angles, illumination methods, appropriate magnification, and measurement protocols. Guidance
is given for calculating a fracture mirror constant and for interpreting the fracture mirror size and shape for both circular and
noncircular mirrors including stress gradients, geometrical effects, and/or residual stresses. residual stresses, or combinations
thereof. The practice provides figures and micrographs illustrating the different types of features commonly observed in and
measurement techniques used for the fracture mirrors of glasses and polycrystalline ceramics.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of
the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
C1145 Terminology of Advanced Ceramics
C1256 Practice for Interpreting Glass Fracture Surface Features
This practice is under the jurisdiction of ASTM Committee C28 on Advanced Ceramics and is the direct responsibility of Subcommittee C28.03 on Physical Properties
and Non-Destructive Evaluation.
Current edition approved July 1, 2015July 1, 2021. Published September 2015July 2021. Originally approved in 2007. Last previous edition approved in 20102015 as
C1678 – 10.C1678 – 10 (2015). DOI: 10.1520/C1678-10R15.10.1520/C1678-21.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1678 − 21
NOTE 1—The initial flaw may grow stably to size a prior to unstable fracture when the stress intensity reaches K . The mirror-mist radius is R , the
c Ic i
mist-hackle radius is R , and the branching distance is R . These transitions correspond to the mirror constants, A , A , and A , respectively.
o b i o b
FIG. 1 Schematic of a Fracture Mirror Centered on a Surface Flaw of Initial Size (a)
C1322 Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
3. Terminology
3.1 Definitions: (See Fig. 1)
3.1.1 fracture mirror, n—as used in fractography of brittle materials, a relatively smooth region in the immediate vicinity of and
surrounding the fracture originorigin. C1145, C1322
3.1.2 fracture origin, n—the source from which brittle fracture commences. C1145, C1322
3.1.3 hackle, n—as used in fractography of brittle materials, a line or lines on the crack surface running in the local direction of
cracking, separating parallel but noncoplanar portions of the crack surface. C1145, C1322
3.1.4 mist, n—as used in fractography of brittle materials, markings on the surface of an accelerating crack close to its effective
terminal velocity, observable first as a misty appearance and with increasing velocity reveals a fibrous texture, elongated in the
direction of crack propagation. C1145, C1322
3.2 Definitions of Terms Specific to This Standard:
(See Fig. 1)
3.2.1 mirror-mist boundary in glasses, n—the periphery where one can discern the onset of mist around a glass fracture mirror.
This boundary corresponds to A , the inner mirror constant.
i
3.2.2 mist-hackle boundary in glasses, n—the periphery where one can discern the onset of systematic hackle around a glass
fracture mirror. This boundary corresponds to A , the outer mirror constant.
o
3.2.3 mirror-hackle boundary in polycrystalline ceramics,,ceramics, n—the periphery where one can discern the onset of
systematic new hackle and there is an obvious roughness change relative to that inside a ceramic fracture mirror region. This
boundary corresponds to A , the outer mirror constant. Ignore premature hackle and/or isolated steps from microstructural
o
irregularities in the mirror or irregularities at the origin.
C1678 − 21
–3/2 -3/2
3.2.4 fracture mirror constant, (Fl ), n—(Fl ) an empirical material constant that relates the fracture stress to the mirror radius
in glasses and ceramics.
4. Summary of Practice
4.1 This practice provides guidance on the measurement and interpretation of fracture mirror sizes in laboratory test specimens
as well as in fractured components. Microscopy examination techniques are listed. The procedures for glasses and ceramics are
similar, but they are not identical. Guidance is given for interpreting the fracture mirror size and shape. Guidance is given on how
to interpret noncircular mirrors due to stress gradients, geometrical effects, or residual stresses.
4.2 The stress at the origin in a component may be estimated from the mirror size.
4.3 Fracture mirror constants may be estimated from matched sets of fracture stresses and mirror sizes.
5. Significance and Use
5.1 Fracture mirror size analysis is a powerful tool for analyzing glass and ceramic fractures. Fracture mirrors are telltaletell-tale
fractographic markings in brittle materials that surround a fracture origin as discussed in Practices C1256 and C1322. Fig. 1 shows
a schematic with key features identified. Fig. 2 shows an example in glass. The fracture mirror region is very smooth and highly
reflective in glasses, hence the name “fracture mirror.” In fact, high magnification microscopy reveals that, even within the mirror
region in glasses, there are very fine features and escalating roughness as the crack advances away from the origin. These are
submicrometer in size and hence are not discernable with an optical microscope. Early investigators interpreted fracture mirrors
as having discrete boundaries including a “mirror-mist” boundary and also a “mist-hackle” boundary in glasses. These were also
termed “inner mirror” or “outer mirror” boundaries, respectively. It is now known that there are no discrete boundaries
corresponding to specific changes in the fractographic features. Surface roughness increases gradually from well within the fracture
mirror to beyond the apparent boundaries. The boundaries were a matter of interpretation, the resolving power of the microscope,
and the mode of viewing. In very weak specimens, the mirror may be larger than the specimen or component and the boundaries
will not be present.
5.2 Figs. 3-5 show examples in ceramics. In polycrystalline ceramics, the qualifier “relatively” as in “relatively smooth” must be
used, since there is an inherent roughness from the microstructure even in the area immediately surrounding the origin. In
coarse-grained or porous ceramics, it may be impossible to identify a mirror boundary. In polycrystalline ceramics, it is highly
unlikely that a mirror-mist boundary can be detected due to the inherent roughness created by the crack-microstructure interactions,
even within the mirror. The word “systematic” in the definition for “mirror-hackle boundary in polycrystalline ceramics” requires
some elaboration. Mirror boundary hackle lines are velocity hackle lines created after the radiating crack reaches terminal velocity.
However, premature, isolated hackle can in some instances be generated well within a ceramic fracture mirror. It should be
disregarded when judging the mirror boundary. Wake hackle from an isolated obstacle inside the mirror (such as a large grain or
agglomerate) can trigger early “premature” hackle lines. Steps in scratches or grinding flaws can trigger hackle lines that emanate
from the origin itself. Sometimes the microstructure of polycrystalline ceramics creates severe judgment problems in ceramic
matrix composites (particulate, whisker, or platelet) or self-reinforced ceramics whereby elongated and interlocking grains impart
greater fracture resistance. Mirrors may be plainly evident at low magnifications, but accurate assessment of their size can be
difficult. The mirror region itself may be somewhat bumpy; therefore, some judgment as to what is a mirror boundary is necessary.
5.3 Fracture mirrors are circular in some loading conditions such as tension specimens with internal origins, or they are nearly
semicircular for surface origins in tensile specimens, or if the mirrors are small in bend specimens. Their shapes can vary and be
elongated or even incomplete in some directions if the fracture mirrors are in stress gradients. Fracture mirrors may be quarter
circles if they form from corner origins in a specimen or component. Fracture mirrors only form in moderate to high local stress
conditions. Weak specimens may not exhibit full or even partial mirror boundaries, since the crack may not achieve sufficient
velocity within the confines of the specimen.
C1678 − 21
NOTE 1—(a) shows the whole fracture surface and the fracture mirror (arrow) which is centered on a surface flaw.flaw; (b) is a close-up of the fracture
mirror which 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 the
Bottom
5.4 Fracture mirrors not only bring one’s attention to an origin, but also give information about the magnitude of the stress at the
origin that caused fracture and their distribution. The fracture mirror size and the stress at fracture are empirically correlated by
Eq 1:
=
σ R 5 A (1)
where:
σ = stress at the origin (MPa or ksi),
R = fracture mirror radius (m or in),
R = fracture mirror radius (m or in.), and
A = fracture mirror constant (MPa√m or ksi√in).
A = fracture mirror constant (MPa√m or ksi√in.).
C1678 − 21
NOTE 1—Notice 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
forms and 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 Inhomogeneity Flaw
C1678 − 21
NOTE 1—The mirror boundary is difficult to delineate in this material. (a) shows the uncoated fracture surface of a 2.8 mm thick flexural strength
specimen that fractured at 486 MPa. Vicinal illumination brings out the markings. (b) shows a mirror-hackle boundary where systematic new hackle is
detected (small white arrows) as compared to the roughness inside the mirror. The marked circle is elongated somewhat into the depth due to the stress
gradient. 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)
Eq 1 is hereafter referred to as the “empirical stress – fracture mirror size relationship,” or “stress-mirror size relationship” for
short. A review of the history of Eq 1, and fracture mirror analysis in general, may be found in Refs (1) and (2).
5.5 A, the “fracture mirror constant” (sometimes also known as the “mirror constant”) has units of stress intensity (MPa√m or
ksi√in)ksi√in.) and is considered by many to be a material property. As shown in Figs. 1 and 2, it is possible to discern separate
mist and hackle regions and the apparent boundaries between them in glasses. Each has a corresponding mirror constant, A. The
most common notation is to refer to the mirror-mist boundary as the inner mirror boundary, and its mirror constant is designated
A . The mist-hackle boundary is referred to as the outer mirror boundary, and its mirror constant is designated A . The mirror-mist
i o
boundary is usually not perceivable in polycrystalline ceramics. Usually, only the mirror-hackle boundary is measured and only
an A for the mirror-hackle boundary is calculated. A more fundamental relationship than Eq 1 may be based on the stress intensity
o
factors (K ) at the mirror-mist or mist-hackle boundaries, but Eq 1 is more practical and simpler to use.
I
5.6 The size predictions based on Eq 1 and the A values, or alternatively stress intensity factors, match very closely for the limiting
cases of small mirrors in tension specimens. This is also true for small semicircular mirrors centered on surface flaws in strong
flexure specimens. So, at least for some special mirror cases, A should be directly related to a more fundamental parameter based
on stress intensity factors.
The boldface numbers in parentheses refer to a list of references at the end of this standard.
C1678 − 21
NOTE 1—The mirror is incomplete into the bend stress gradient, but the mirror sides can be used to construct boundary arcs in (c) [(b)((b) and (c) are
close-ups of (a)].(a)). Radii are measured in the direction of constant stress along the bottom.
FIG. 5 Silicon Nitride Bend Bar with a Knoop Surface Crack in a Silicon Nitride (449 MPa)
5.7 The size of the fracture mirrors in laboratory test specimen fractures may be used in conjunction with known fracture mirror
constants to verify the stress at fracture was as expected. The fracture mirror sizes and known stresses from laboratory test
specimens may also be used to compute fracture mirror constants, A.
5.8 The size of the fracture mirrors in components may be used in conjunction with known fracture mirror constants to estimate
the stress in the component at the origin. Practice C1322 has a comprehensive list of fracture mirror constants for a variety of
ceramics and glasses.
C1678 − 21
6. Procedure
6.1 Use an optical microscope whenever possible.
6.1.1 For glasses, use a compound optical microscope in bright field mode with reflected light illumination. A scanning electron
microscope may be used if optical microscopy is not feasible. A differential interference contrast optical microscope is optional.
6.1.2 For ceramics, use a stereo optical microscope with low angle grazing (vicinal) illumination. illumination. Low angle
illumination is occasionally called “grazing illumination.” A scanning electron microscope may be used if optical microscopy is
not feasible.
6.1.3 Differential interference contrast (DIC, also known as Nomarski) mode viewing with a research compound microscope may
be used for glasses. It should not be used for ceramics since it is not suitable for rough ceramic fracture surfaces.
6.1.3.1 Interference contrast mode of viewing can discern very subtle mist features in glasses, but the threshold of mist
detectability is highly dependent upon how the polarizing elements are positioned. Therefore, use the polarizing elements in a grey
mode (non-color, remove the lambda plate for color control) and slowly increase light intensity, but note that higher light intensity
can hide details. Rotate the analyzer until one can determine repeatably consistent boundary conditions, e.g., for example, mist and
hackle. Typically, more details will be evident, but when properly used, DIC viewing can produce consistent mirror radii
measurements. Note that these radii may be smaller than those obtained with conventional viewing modes. Thus, mirror-mist
fracture mirror constants may be slightly smaller than those obtained with bright field illumination. Therefore, it shall be stated
in the report if interference contrast techniques were used.
6.1.4 Dark-field Dark field illumination may be used for glasses, but some resolution may be lost with glasses and radii may be
slightly larger as a result. Dark field is very effective with highly-reflective highly reflective mirror surfaces of ceramic single
crystals.
6.1.5 Scanning electron microscope images of mirrors are not recommended for glasses, since the mirror-mist boundary is usually
indiscernible. SEM images often appear flat and do not have adequate contrast to see the fine mist detail at the ordinary
magnifications used to frame the whole mirror. SEM images may be used with very small mirrors that would be difficult to see
with optical microscopy, e.g., for example, high-strength optical fibers. Scanning electron microscope images may be used for
ceramics if necessary, but contrast and shadowing should be enhanced.
6.1.6 It is recommended that the report state the inspection method/instrument used.
6.2 The fracture surface should be approximately perpendicular to the microscope optical path or camera.
6.2.1 This requirement poses a small problem if the mirrors in ceramics are examined with a stereo binocular microscope. This
microscope has two different tilted optical paths. If viewing with both eyes in a stereo microscope, the specimen should be flat and
facing directly upwards. The observer’s brain will interpret the image as though the observer is facing it directly. Alternatively, if
a camera is mounted on one light path of the stereo microscope, and it is used to capture or display the mirror, then the specimen
should be tilted so that the camera axis is normal to the fracture surface. For example, slightly tilt the specimen to the right if the
camera is attached to the right optical path.
6.3 Optimize the illumination to accentuate topographical detail.
6.3.1 For glasses, accentuate the mist and hackle features. Glasses may either be illuminated from directly down onto a fracture
surface or by grazing angle, vicinal illumination. Vicinal a low grazing angle. Low angle illumination is less convenient with
compound light microscopes, but the observer should experiment with whatever illumination options are available to accentuate
subtle surface roughness and topography features.
6.3.2 For ceramics, accentuate the hackle lines. Ceramics should not be uniformly and directly illuminated such as by a ring light,
since the light will reduce contrast especially in translucent or transparent materials. Ceramics shall be illuminated with grazing
angle, vicinal low angle illumination. Thin gold or carbon coatings may be applied to translucent or transparent ceramics as needed.
6.4 Use an appropriate magnification.
C1678 − 21
6.4.1 For glasses, use a magnification such that the fracture mirror area occupies about 75 % to 90 % of the width of the field of
view. Fracture mirrors are reasonably easy to see in glasses, and magnifications should be used such that the fracture mirrors nearly
fill the field of view.
6.4.2 For ceramics, use a magnification such that the fracture mirror area occupies about 33 % to 67 % of the width of the field
of view. Mirror interpretation is more problematic with polycrystalline ceramics. Even though a mirror may be obvious at low or
moderate magnification, at high magnification it may be impossible to judge a boundary. It is more practical to view the mirror
region and the natural microstructural roughness therein relative to the hackle outside the mirror. “Stepping back” and using the
33 % to 67 % rule should help an observer better detect the topography differences. Supplemental lower-magnification images may
aid interpretation. The magnification of the supplemental images should differ from that of the main measurement image by no
more than a factor of five, otherwise it is difficult to correlate features between the images.
6.5 Measure the mirror size while viewing the fracture surface with an optical microscope whenever possible.
6.5.1 For both glasses and ceramics, use either calibrated reticules in the eyepieces or traversing stages with micrometer-
positioning heads. Alternatively, measurements may be made on digital images on a high-resolution computer monitor, while the
fracture surface can be simultaneously viewed through the microscope eyepieces in order to aid judgment.
NOTE 1—Mirror size measurements made on computer monitor screens are subject to inaccuracies, because they are two-dimensional renditions of a
three-dimensional fracture surface. Nevertheless, high-resolution cameras and monitors are beginning to match the capabilities and accuracy of an
observer peering through the optical microscope.
6.5.2 Measurements from photos or digitally recorded images may be used as a last resort if the steps in 6.5.1 cannot be followed.
This may be necessary for very small specimens or very strong specimens with tiny mirrors where a scanning electron microscope
must be used to photograph the mirror. Measurements from other devices may be used provided that the criterion used for
identifying the mirror boundary is carefully documented. Complementary high and low magnification images may be used to help
aid in interpretation. Mirror size measurements from photographs are usually less accurate or precise. They frequently overestimate
mirror sizes unless conditions are carefully optimized to accentuate contrast and topographic detail. Two-dimensional photographic
renditions of a three-dimensional fracture surface usually lose much of the topographic detail discernable by the eye with a
compound optical or stereo microscope. Video cameras shall not be used to capture mirror images, since they lack adequate
resolution.
6.5.3 In ceramics, the fracture mirror regions may have an intrinsic roughness due to the microstructure. The mirror boundary is
judged to be the point where systematic radiating new hackle commences and there is an obvious roughness change relative to the
inside-mirror region. The new hackle that generates the mirror boundary is formed by the radiating crack running at or near
terminal velocity. Ignore premature isolated hackle that may be generated well within a mirror. Wake hackle from an obstacle
inside the mirror (such as a large grain or agglomerate) can trigger early premature hackle lines. Steps in scratches or grinding flaws
can trigger premature hackle that emanate from the origin itself.
6.6 Measure radii in directions of approximately constant stress whenever possible. A mirror diameter may be measured and
halved to estimate the radius if the origin site is indistinct or complex.
6.6.1 Measurements should be taken from the center of the mirror region, but some judgment may be necessary. A common
procedure is to make a judgment whether a mirror is indeed approximately semicircular or circular. If it is, then multiple radii
measurements may be made in different directions and averaged to obtain the mirror size estimate. The center of the mirror may
not necessarily be the center of the flaw at the origin. Careful inspection of tiny localized fracture surface markings (Wallner lines
and micro hackle lines) may reveal that fracture started at one spot on a flaw periphery. For example, fracture from grinding or
impact surface cracks in glass often starts from the deepest point of the flaw and not at the specimen outer surface. Fig. 2 shows
an example in glass. Large pores often trigger unstable fracture from one side. If an exact mirror center cannot be determined,
measure a mirror diameter and halve the measurement. This is commonly do
...