ASTM C1256-93(2019)

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: C1256 − 93 (Reapproved 2019)
Standard Practice for
Interpreting Glass Fracture Surface Features
This standard is issued under the fixed designation C1256; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3.1.3 forking angle—the angle subtended by two immedi-
ately adjacent fractures which have just branched or forked.
1.1 Fracture features on the surface of a crack reflect the
3.1.4 fracture mirror constant—a constant, characteristic of
nature and course of the fracture event associated with the
a given glass composition, which, when divided by the square
breakage of a glass object. This practice is a guide to the
root of the fracture mirror radius, will yield the fracture stress.
identification and interpretation of these fracture surface fea-
tures.
3.1.5 fracture mirror radius—a dimension of the fracture
mirror as measured along the original specimen surface. It is
1.2 The practice describes the various fracture surface
defined as the distance from the origin to the first detectable
features as to their appearance, the process of formation and
mist.
their significance.
3.1.6 fracture surface markings—features of the fracture
1.3 The practice does not provide the procedural informa-
surface produced during the fracture event which are useful in
tion necessary for a complete fractographic analysis. Such
determining the origin and the nature of the local stresses that
information is available in the general literature. (See Glossary
produced the fracture.
for suggested literature).
3.1.7 fracture system—the fracture surfaces that have a
1.4 This international standard was developed in accor-
common cause or origin.
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the 3.1.8 terminal velocity—the uppermost limiting velocity at
Development of International Standards, Guides and Recom- which a crack can propagate in a material, the approach to
mendations issued by the World Trade Organization Technical which is marked on the fracture generated surface by the
Barriers to Trade (TBT) Committee. presence of mist. The terminal velocity is approximately one
half the velocity of sound in the material.
2. Referenced Documents
3.1.9 uniform stress—a state of stress that does not change
2.1 ASTM Standards: within the region of concern.
C162Terminology of Glass and Glass Products
4. Summary
3. Terminology
4.1 This practice is intended to aid in the identification of
fracturesurfacemarkingsaswellastoassistintheunderstand-
3.1 Definitions:
ing of their formation and significance.
3.1.1 bending stress—a continuously and linearly changing
stress across the thickness of a glass body, varying from
5. Significance and Use
compression on one surface to tension on the opposite surface.
5.1 Fractography is often used to help identify the events
3.1.2 forking—amechanismwherebyapropagatingfracture
thathaveresultedinthefractureofaglassobject.Thispractice
branches into two fractures, separated from each other by an
defines the appearance of various fracture surface features, as
acute angle.
well as their method of formation. Thus, there can be a
common understanding of their relationship to the fracture
1 process as well as a common terminology.
ThispracticeisunderthejurisdictionofASTMCommitteeC14GlassandGlass
Products and is the direct responsibility of Subcommittee C14.04 on Physical and
Mechanical Properties
6. Fracture Surface Markings
Current edition approved Aug. 1, 2019. Published August 2019. Originally
6.1 Origin:
approved in 1993. Last previous edition approved in 2013 as C1256–93 (2013)
DOI: 10.1520/C1256-93R19.
6.1.1 Identification—The origin is almost always found at
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
the junction where the fracture-generated surface meets a free
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
surface or a dissimilar material. Commonly, the origin is
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. symmetrically located near the apex of the mirror and it is
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1256 − 93 (2019)
usually small compared to the mirror. Fig. 1 shows typical
origins and mirrors bounded by mist.
6.1.2 Formation—The origin represents the single, unique
location at which every fracture system begins to form.
6.1.3 Significance—The origin defines the location where
the fracture began. It may contain the stress concentrator or it
may be the stress concentrator.
6.2 Mist Region:
6.2.1 Identification—Under low power (5 − 50 × )
magnification, it has a misty appearance. Proceeding away
from the origin, it becomes more fibrous in appearance and
elongated in the direction of crack spread. (See Fig. 2.)
6.2.2 Formation—It is produced as the crack front breaks
into numerous segments, which then round into one another.
Theirpropagationabortsasthecrackfrontapproachesterminal
velocity.
6.2.3 Significance—It defines the limit of the mirror region
and indicates that the crack has nearly reached terminal
velocity, or both.
FIG. 2 An Origin Area, with Mirror and Mist
6.3 Mirror:
6.3.1 Identification—The mirror is a smooth portion of the
mirror was formed. If the mirror is symmetrical, then use the
fracture surface surrounding the origin (see Fig. 2). It is
radius to the mist boundary.To calculate the stress at breakage
commonly bounded by mist, but mist may not form when the
when the mirror is not symmetrical, the mirror radius is best
local stress at the fracture front diminishes as the crack
determined by dividing the mirror diameter by two. A more
extends.
detailed description of the relationship between the mirror and
6.3.2 Formation—It represents the initial portion of the
the breaking strength for various glasses is found on p. 364 of
propagating crack where the velocity is accelerating from the
(1) and in (2) and (3). Further discussion on quantitative
origin to a value sufficient to induce turbulence at the crack
fracture analysis techniques is well summarized in (4).
front, that is, approaching terminal velocity, where mist and
6.4 Wallner Lines:
forking may appear.
6.4.1 Identification—Wallnerlines,alsocalledripplemarks,
6.3.3 Significance—It is often helpful in locating the origin.
are rib-shaped marks, frequently appearing as a series of
The shape defined by the mist boundary is indicative of the
curved lines resembling ripples created when an object is
uniformityofthestressfieldatthetimeoffailure,forexample;
dropped into still water. (See Figs. 3-8.)
anopenmirror,definedbymistonlyalongtheoriginalsurface,
6.4.2 Formation—They are produced when the plane of the
implies bending; a semicircular mirror implies uniform ten-
propagating crack front is temporarily altered by an elastic
sion: (See Fig. 1) The mirror dimensions may be used to
pulse.
calculate the stress at breakage, because the mirror radius is
6.4.3 Significance—The direction of local propagation is
inverselyproportionaltothesquareofthestressatthetimethe
perpendicular to the Wallner lines; it proceeds from the
concave to the convex side of the line. The shape of the line
indicatesthedirectionofstressesatvariouspointsonthecrack
FIG. 1 Origin Areas Produced Under Various Stress Functions FIG. 3 Primary Wallner Lines Generated From a Surface Noncon-
and Their Typical Fracture Features formity and an Inclusion
C1256 − 93 (2019)
FIG. 4 Primary Wallner Lines Generated; (a) From Surface
Scratches, (b) A Bubble Generating Gull Wings
FIG. 5 Secondary Wallner Lines Generated From Mist Formation
front. The more advanced portions of the line generally 6.5.1 Identification—PrimaryWallnerlinesareusuallyquite
correspond to regions of higher tension. distinct and always have their source associated with some
discontinuity which was present before fracture. Examples
6.5 Wallner Lines, Primary:
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FIG. 6 Secondary Wallner Lines Generated From Mist Formation
FIG. 8 Tertiary Wallner Lines
profile at the crack front. In instances where the mist hackle
band is quite narrow, they verify its presence.
6.7 Wallner Lines, Tertiary:
6.7.1 Identification—These are a complex set of lines,
exhibiting a periodicity and an intensity which may diminish
within the pattern.They are neither hook-shaped nor trace to a
discontinuity as the source of an elastic pulse. (See Fig. 7 and
Fig. 8.)
6.7.2 Formation—They result from an interaction at the
crack front with sonic waves from an external shock or from
stress release at the onset of cracking.
6.7.3 Significance—They indicate that the failure resulted
fromamechanicalshock,whereanelasticpulsewasgenerated
outside the plane of crack propagation.
FIG. 7 Tertiary Wallner Lines Created by Sonic Pulses Produced
from Mechanical Shock Which Broke the Material 6.8 Dwell Mark:
6.8.1 Identification—Dwell marks, also called arrest lines,
have a similar rib-shaped contour to that of Wallner lines but
would include bubbles or other inclusions, surface damage or are distinctly sharper, often exhibiting a noticeable change in
an abrupt change in surface contour. (See Fig. 3 and Fig. 4.) fracture plane after the mark
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