ASTM D7779-11(2015)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: D7779 − 11 (Reapproved 2015)
Standard Test Method for
Determination of Fracture Toughness of Graphite at
Ambient Temperature
This standard is issued under the fixed designation D7779; 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.
precrackformetallicmaterials.Thistestmethodforgraphitemakesuseof
1. Scope
a machined notch with sharp cracking at the root of the notch because of
1.1 This test method covers and provides a measure of the
the nature of graphite. Therefore, fracture toughness values determined
resistance of a graphite to crack extension at ambient tempera- with this method may not be interchanged with K as defined in Test
Ic
Method E399.
ture and atmosphere expressed in terms of stress-intensity
factor, K, and strain energy release rate, G.These crack growth
1.4 This test method gives fracture toughness values, K
Ic
resistance properties are determined using beam test specimens
and critical strain energy release rate, G for specific condi-
Ic
with a straight-through sharp machined V-notch.
tions of environment, deformation rate, and temperature. Frac-
ture toughness values for a graphite grade can be functions of
1.2 This test method determines the stress intensity factor,
environment, deformation rate, and temperature.
K, from applied force and gross specimen deflection measured
away from the crack tip. The stress intensity factor calculated
1.5 This test method is divided into two major parts. The
at the maximum applied load is denoted as fracture toughness,
first major part is the main body of the standard, which
K , and is known as the critical stress intensity factor. If the
Ic
provides general information on the test method, the applica-
resolution of the deflection gauge is sensitive to fracture
bility to materials comparison and qualification, and require-
behavior in the test specimen and can provide a measure of the
ments and recommendations for fracture toughness testing.
specimen compliance, strain energy release rate, G, can be
The second major part is composed of annexes, which provide
determined as a function of crack extension.
information related to test apparatus and test specimen geom-
etry.
1.3 This test method is applicable to a variety of grades of
graphite which exhibit different types of resistance to crack
Main Body Section
Scope 1
growth, such as growth at constant stress intensity (strain
Referenced Documents 2
energy release rate), or growth with increasing stress intensity
Terminology 3
(strain energy release rate), or growth with decreasing stress
Summary of Test Method 4
Significance and Use 5
intensity (strain energy release rate). It is generally recognized
Apparatus 6
that because of the inhomogeneous microstructure of graphite,
Test Specimen 7
the general behavior will exhibit a mixture of all three during Procedure 8
Specimen Dryness 9
the test. The crack resistance behavior exhibited in the test is
Calculation of Results 10
usually referred to as an “R-curve.”
Report 11
Precision and Bias 12
NOTE 1—One difference between the procedure in this test method and
Keywords 13
test methods such as Test Method E399, which measure fracture
Annex Annex A1
toughness, K , by one set of specific operational procedures, is that Test
Ic
1.6 The values stated in SI units are to be regarded as
Method E399 focuses on the start of crack extension from a fatigue
standard. No other units of measurement are included in this
standard.
This test method is under the jurisdiction of ASTM Committee D02 on
1.6.1 Measurementunitsexpressedinthesetestmethodsare
Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of
Subcommittee D02.F0 on Manufactured Carbon and Graphite Products. in accordance with IEEE/ASTM SI 10.
CurrenteditionapprovedJune1,2015.PublishedJuly2015.Originallyapproved
1.7 This standard does not purport to address all of the
in 2011. Last previous edition approved in 2011 as D7779 – 11. DOI:10.1520/
safety concerns, if any, associated with its use. It is the
D7779-11R15.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7779 − 11 (Reapproved 2015)
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
D7779 − 11 (2015)
2. Referenced Documents testing. Any contributions from crack branching or other
2 secondary cracking are not included in this measurement.
2.1 ASTM Standards:
3.2.2 crack extension orientation, n—direction of propaga-
C709 Terminology Relating to Manufactured Carbon and
tion in relation to a characteristic direction of the graphite
Graphite (Withdrawn 2017)
specimen. This identification may be designated by a letter or
C1161 Test Method for Flexural Strength of Advanced
letters indicating the plane and direction of crack extension.
Ceramics at Ambient Temperature
The letter or letters represent the direction normal to the crack
C1421 Test Methods for Determination of Fracture Tough-
plane and the direction of crack propagation.
ness of Advanced Ceramics at Ambient Temperature
3.2.2.1 Discussion—The characteristic direction should be
E4 Practices for Force Verification of Testing Machines
associated with the microstructural grain orientation of the test
E177 Practice for Use of the Terms Precision and Bias in
specimen.
ASTM Test Methods
3.2.2.2 Discussion—The crack plane can be defined by
E337 Test Method for Measuring Humidity with a Psy-
letter(s) representing the direction of tensile stress normal to
chrometer (the Measurement of Wet- and Dry-Bulb Tem-
the crack plane. And the direction of crack extension can be
peratures)
defined by letter(s) representing the direction parallel to the
E399 Test Method for Linear-Elastic Plane-Strain Fracture
characteristic grain orientation of the test specimen. As illus-
Toughness of Metallic Materials
trated in Annex A1, the tensile stress direction is notated first,
E561 Test Method forK Curve Determination
R
followed by a hyphen, and then the crack extension direction.
E691 Practice for Conducting an Interlaboratory Study to
The legend given in Test Methods C1421 includes the follow-
Determine the Precision of a Test Method
ing:
E1823 TerminologyRelatingtoFatigueandFractureTesting
E2309 Practices for Verification of Displacement Measuring
Systems and Devices Used in Material Testing Machines
M = molding direction
IEEE/ASTM SI 10 Standard for Use of the International
EX = extrusion direction
System of Units (SI) (The Modern Metric System)
AXL = axial, or longitudinal axis (if M or EX are not
applicable)
3. Terminology
R = radial direction
3.1 Definitions: C = circumferential direction
R/C = mixed radial and circumferential directions
3.1.1 The terms described in Terminology C709 and E1823
are applicable to the test methods prescribed herein.Appropri-
3.2.2.3 Discussion—For a graphite test specimen of rectan-
ate sources for each definition are provided after each defini-
gular cross section, R and C may be replaced by rectilinear
tion in parentheses.
coordinate axes, x and y, corresponding to two adjacent sides
-3/2 -1
3.1.2 crack extension resistance, K [FL ], G [FL ], or
R R
of the test specimen.
-1
J [FL ], n—measure of the resistance of a material to crack
R
3.2.2.4 Discussion—Depending on how test specimens are
extension expressed in terms of the stress-intensity factor, K,
cut from a graphite product, the crack plane may be longitu-
strain energy release rate, G, or values of J derived using the
dinal to the forming direction, or circumferential, or radial, or
J-integral concept. E1823
a mixture of these directions as shown in Annex A1.
3.1.3 R-curve, n—plot of stress intensity or strain energy
3.2.2.5 Discussion—For the test specimen the plane and
releaserateasafunctionofstablecrackextensionandprovides
direction of crack extension with respect to the applied tensile
a measure of crack propagation trend in the material. E561
stress should be recorded. Report the orientation of the
3.1.4 slow crack growth, (SCG), n—sub-critical crack specimen and crack propagation direction with respect to the
grain direction.
growth (extension) which may result from, but is not restricted
to, such mechanisms as environmentally-assisted stress corro- 3.2.2.6 Discussion—Ifthereisnoprimaryproductdirection,
reference axes may be arbitrarily assigned but must be clearly
sion or diffusive crack growth, usually at constant load.
-3/2
identified.
3.1.5 stress-intensity factor, K[FL ], n—magnitude of the
3.2.3 critical crack depth, [L], n—crack depth at which
ideal-crack-tip stress field (stress field singularity) for a par-
catastrophic fracture initiation occurs, corresponding to the
ticular mode in a homogeneous, linear-elastic body. E1823
maximum in the applied load.
3.2 Definitions of Terms Specific to This Standard:
-3/2
3.2.4 fracture toughness, K[FL ], n—property which de-
3.2.1 crack depth, a [L], n—length of the crack in a notched
fines the critical stress intensity factor necessary to initiate a
beam specimen, which includes the machined notched length
crack for subsequent propagation on further loading.
and the crack length which the crack has traveled during
3.2.5 small crack, n—being small when all physical dimen-
sions (in particular, with length and depth of a surface crack)
are small in comparison to a relevant microstructural scale,
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
continuum mechanics scale, or physical size scale.The specific
Standards volume information, refer to the standard’s Document Summary page on
physical dimensions that define “small” vary with the particu-
the ASTM website.
lar material, geometric configuration, and loadings of interest.
The last approved version of this historical standard is referenced on
www.astm.org. E1823
D7779 − 11 (2015)
3.2.6 stable crack extension, n—crack propagation which 5.2 The fracture toughness value provides information on
provides measurable data of the dependence of stress intensity the initiation of fracture in graphite containing a straight-
factor on crack extension and which occurs over some mea- through notch; the information on stress intensity factor
surable time duration. beyond fracture toughness as a function of crack extension
provides information on the crack propagation resistance once
3.2.7 three-point flexure, n—flexure configuration where a
a fracture crack has been initiated to propagate through the test
beam test specimen is loaded at a location midway between
specimen.
two support bearings. C1161
3.2.8 unstable crack extension—uncontrollablecrackpropa-
6. Apparatus
gation which yields no measurable data of the dependence of
6.1 Testing—Test the specimens in a testing machine that
stress intensity factor on crack extension.
has provisions for autographic recording of force applied to the
3.3 Symbols:
test specimen versus time and actuator displacement or deflec-
3.3.1 a—crack depth, including the machined notch (see
tion of the specimen, or both, in the notch plane. The testing
Fig. 1).
machine shall conform to the requirements of Practice E4.
3.3.2 a/W—normalized notch depth.
6.2 Deflection Measurement—The deflection gauge should
3.3.3 B—the specimen width (see Fig. 1).
be capable of resolving 0.001 mm. Practices E2309 cover
3.3.4 g(a/W)—geometric function of the ratio a/W. procedures and requirements for the calibration and verifica-
tion of displacement measuring systems.
3.3.5 L—test specimen length (see Fig. 1).
6.3 Recording Equipment—Provide digital data acquisition
3.3.6 P—force.
for automatically recording the applied force versus displace-
3.3.7 P —maximum force.
max
ment.
3.3.8 S—support span (see Fig. A1.2).
6.4 Fixtures—Use a three-point test fixture constructed with
3.3.9 W—the specimen depth (see Fig. 1).
high stiffness materials (see Fig. A1.2). Choose the outer
support span, S, such that 5≤ (S/W)≤ 10.The outer two rollers
4. Summary of Test Method
shall be free to roll outwards from support locations to
4.1 This test method involves an application of force to a
minimize friction effects. The middle flexure roller shall be
beam test specimen in three-point flexure. The test specimen
fixed. The specimen should overhang each of the outer rollers
contains a straight-through notch in the center. The equations
by a minimum distance equal to the specimen dimension, W.
for calculating the fracture toughness have been established on
6.5 Dimension-Measuring Devices—Measure and report all
the basis of linear-elastic stress analyses.
applicable specimen dimensions to an accuracy of 0.013 mm.
4.2 Notched Beam Method—A straight-through notch is
Flat, anvil-type micrometers shall be used for measuring test
machined in a beam test specimen. The applied force on the
specimen dimensions. Ball-tipped or sharp-anvil micrometers
notched test specimen as a function of time and actuator
are not recommended as they may damage the test specimen
displacement or specimen deflection in three-point flexure, or a
surface by inducing localized cracking. Non-contacting (for
combination thereof, are recorded for analysis. The fracture
example, optical comparator, light microscopy, etc.) measure-
toughness, K , is calculated from the maximum (fracture)
Ic ments are recommended for notch depth measurements. Mea-
force, the test specimen dimensions, the measured notch depth,
sure and report the notch depth to an accuracy of 0.0025 mm.
and the support span of the test fixture. Calculation of strain
energy release rate, G, requires a determination of specimen
7. Test Specimen
compliance, and crack length at each load point of the load
7.1 Test Specimen Configuration—The specimen shall have
versus displacement curve. The maximum G derived from the
a straight-through machined V-notch with a maximum notch
strain energy release rate versus crack growth curve is re-
root radius of 0.10 mm. The notch may be sharpened by
corded.
drawing an industrial razor blade or similar device across the
notch tip to encourage stable crack extension from the as-
5. Significance and Use
machined notch tip.
5.1 This test method may be used for guidance for material
7.1.1 The included angle of the razor blade edge should be
development to improve toughness, material comparison, qual-
less than the specimen notch angle. I
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