ASTM D6068-96(2002)e1
(Test Method)Standard Test Method for Determining J-R Curves of Plastic Materials
Standard Test Method for Determining J-R Curves of Plastic Materials
SIGNIFICANCE AND USE
A J-R curve produced in accordance with this test method characterizes the crack growth resistances of a wide range of tough polymers and polymer blends (1-5)7 that cannot be obtained in sufficient size and thickness for valid characterization by linear elastic fracture mechanics in Test Methods D 5045.
The J-R curve characterizes, within the limits set forth in this test method, the resistance of a polymeric material to slow stable crack growth after initiation from a preexisting sharp flaw.
A J-R curve can be used as an index of material toughness for blend or alloy design, material selection, materials processing, and quality assurance (6).
The J-R curves from bend specimens represent lower bound estimates of J capacity as a function of crack extension, and have been observed to be conservative relative to those obtained from specimen configurations under tensile loading.
The J-R curves for a given material of constant microstructure tend to exhibit lower slope (flatter) with increasing thickness. Thus, it is recommended that the largest possible specimen with representative microstructure be used.
The J-R curve can be used to assess the stability of cracks in structures in the presence of ductile tearing, with awareness of the differences that may exist between laboratory test and field conditions.
A J-R curve may depend on the orientation and propagation of the crack in relation to the anisotropy of the material which may be induced by specimen fabrication methods.
Because of the possibility of rate dependence of crack growth resistance, J-R curves can be determined at displacement rates other than that specified in this test method (7).
SCOPE
1.1 This test method covers the determination of the J-integral versus crack growth resistance (J-R) curves for polymeric materials.
1.2 This test method is intended to characterize the slow, stable crack growth resistance of bend-type specimens in such a manner that it is geometry insensitive within limits set forth in this test method.
1.3 The recommended specimens are the three-point bend (SE(B)) and pin-loaded compact tension (C(T)) specimens. Both specimens have in-plane dimensions of constant proportionality for all sizes. Specimen configurations other than those recommended in this test method may require different procedures and validity requirements.
1.4 This test method describes a multiple specimen method that requires optical measurement of crack extension from fracture surfaces. It is not recommended for use with materials in which the crack front cannot be distinguished from additional deformation processes in advance of the crack tip.
1.5 The values stated in SI units are to be regarded as the standard.
1.6 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.
Note 1—There is no equivalent ISO standard.
General Information
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
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Designation: D6068 – 96 (Reapproved 2002)
Standard Test Method for
Determining J-R Curves of Plastic Materials
This standard is issued under the fixed designation D6068; 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.
´ NOTE—Editorially corrected figure reference in paragraph 7.3 in November 2002.
1. Scope D5045 Test Methods for Plane-Strain Fracture Toughness
and Strain Energy Release Rate of Plastic Materials
1.1 This test method covers the determination of the
E399 Test Method for Linear-Elastic Plane-Strain Fracture
J-integral versus crack growth resistance (J-R) curves for
Toughness K of Metallic Materials
polymeric materials. Ic
E616 Terminology Relating to Fracture Testing (Discontin-
1.2 This test method is intended to characterize the slow,
ued 1996)
stable crack growth resistance of bend-type specimens in such
E1152 Test Method for Determining-J-R-Curves
a manner that it is geometry insensitive within limits set forth
E1737 Test Method for J-Integral Characterization of Frac-
in this test method.
ture Toughness (Discontinued 1998)
1.3 The recommended specimens are the three-point bend
F1473 Test Method for Notch Tensile Test to Measure the
(SE (B)) and pin-loaded compact tension (C (T)) specimens.
Resistance to Slow Crack Growth of Polyethylene Pipes
Both specimens have in-plane dimensions of constant propor-
and Resins
tionality for all sizes. Specimen configurations other than those
recommended in this test method may require different proce-
3. Terminology
dures and validity requirements.
3.1 Definitions—Terminology related to fracture testing
1.4 This test method describes a multiple specimen method
contained in Terminology E616 is applicable to this test
that requires optical measurement of crack extension from
method.
fracture surfaces. It is not recommended for use with materials
3.2 Definitions of Terms Specific to This Standard:
in which the crack front cannot be distinguished from addi-
−1
3.2.1 J-integral, J (FL )—a mathematical expression, a
tional deformation processes in advance of the crack tip.
line or surface integral over a path that encloses the crack front
1.5 The values stated in SI units are to be regarded as the
from one crack surface to the other, used to characterize the
standard.
localstress-strainfieldaroundthecrackfront.SeeTerminology
1.6 This standard does not purport to address all of the
E616 for additional discussion.
safety concerns, if any, associated with its use. It is the
3.2.2 J-Rcurve—aplotofresistancetostablephysicalcrack
responsibility of the user of this standard to establish appro-
extension, Da .
p
priate safety and health practices and determine the applica-
3.2.3 net thickness, B (L)—the distance between the roots
N
bility of regulatory limitations prior to use.
of the side grooves in side grooved specimens.
NOTE 1—There is no equivalent ISO standard.
3.2.4 original crack size, a (L)—the physical crack size at
the start of testing.
2. Referenced Documents
3.2.5 original uncracked ligament, b (L)—the distance
2.1 ASTM Standards:
from the original crack front to the back edge of the specimen
D618 Practice for Conditioning Plastics for Testing
(b =W−a8 ).
0 0
D4066 Classification System for Nylon Injection and Ex-
3.2.6 physical crack extension, Da (L)—an increase in
p
trusion Materials (PA)
physical crack size (Da =a −a ).
p p 0
3.2.7 physical crack size, a (L)—the distance from a
p
reference line to the observed crack front.The distance may be
This test method is under the jurisdiction ofASTM Committee D20 on Plastics
a calculated average of several measurements along the crack
and is the direct responsibility of Subcommittee D20.10 on Mechanical Properties.
front. The reference line depends on the specimen geometry
Current edition approved November 10, 2002. Published January 2003. Origi-
and is normally defined as in 3.2.10. The reference line is
nallyapprovedin1996.Lastpreviouseditionapprovedin1996asD6068 - 96.DOI:
10.1520/D6068-96R02E01. defined prior to specimen deformation.
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
Standards volume information, refer to the standard’s Document Summary page on Withdrawn. The last approved version of this historical standard is referenced
the ASTM website. on www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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D6068 – 96 (2002)
3.2.8 specimen span, S(L)—the distance between specimen
supports for the SE (B) specimen.
3.2.9 specimen thickness, B(L)—the side-to-side dimension
of the test specimen (shown in Fig. 2).
3.2.10 specimen width, W(L)—a physical dimension on a
test specimen measured from the rear surface of the specimen
to a reference line (for example, the front edge of a bend
specimen or the load line of a compact specimen).
4. Summary of Test Method
4.1 This test method describes a multiple specimen tech-
nique for determining the J-R curve for polymeric materials.
The J-R curve consists of a plot of J versus crack extension in
the region of J-controlled growth as determined by the data
qualification requirement of 9.2.
4.2 This test method uses optical measurements of crack
length and crack extension on the fracture surfaces after each
test.
5. Significance and Use
FIG. 2 Bending Rig
5.1 A J-R curve produced in accordance with this test
method characterizes the crack growth resistances of a wide
5.3 A J-R curve can be used as an index of material
range of tough polymers and polymer blends (1-5) that cannot
toughness for blend or alloy design, material selection, mate-
be obtained in sufficient size and thickness for valid character-
rials processing, and quality assurance (6).
ization by linear elastic fracture mechanics in Test Methods
5.4 The J-R curves from bend specimens represent lower
D5045.
bound estimates of J capacity as a function of crack extension,
5.2 TheJ-Rcurvecharacterizes,withinthelimitssetforthin
and have been observed to be conservative relative to those
this test method, the resistance of a polymeric material to slow
obtained from specimen configurations under tensile loading.
stable crack growth after initiation from a preexisting sharp
5.5 The J-R curves for a given material of constant micro-
flaw.
structure tend to exhibit lower slope (flatter) with increasing
thickness. Thus, it is recommended that the largest possible
4 specimen with representative microstructure be used.
The boldface numbers given in parentheses refer to a list of references at the
5.6 The J-R curve can be used to assess the stability of
end of the text.
cracks in structures in the presence of ductile tearing, with
awareness of the differences that may exist between laboratory
test and field conditions.
5.7 A J-R curve may depend on the orientation and propa-
gation of the crack in relation to the anisotropy of the material
which may be induced by specimen fabrication methods.
5.8 Because of the possibility of rate dependence of crack
growth resistance, J-R curves can be determined at displace-
ment rates other than that specified in this test method (7).
6. Apparatus
6.1 Measurements of applied load and load-line displace-
ment are needed to obtain the total energy absorbed by the
specimen.Loadversusload-linedisplacementmayberecorded
digitally or autographically.
6.2 Testing Machine—The J-integral tests are to be con-
ducted under displacement control to maximize the attainable
amount of stable crack extension in the test specimens.
NOTE 2—The extent to which the crack grows in a stable manner is
dependent on the machine stiffness (8) and the mode of control of loading
(9).
6.3 Bend Test Fixture—A suggested fixture for SE (B)
specimens is shown in Fig. 2. The fixture may have either
stationary or moving rollers of sufficiently large diameter to
FIG. 1 Specimen Configurations minimize excessive plastic indentation.
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D6068 – 96 (2002)
6.4 Gripsfor C(T)Specimens—Asuggestedclevisandpin
arrangement for gripping compact specimens is given in Test
Method E399. This arrangement accommodates specimens
with B = 0.5 W.
6.5 Displacement Measurement:
6.5.1 Load-line displacement measurements are needed to
calculate J from the area under the load-displacement record.
6.5.2 The remote displacement measurement can be per-
formed using the stroke or position transducer on the testing
machine. Data obtained in this manner must be corrected for
extraneous displacements (such as indentation effects, pin
penetration, or machine compliance) by conducting a separate
indentation measurement described in 8.7.
6.5.3 Adirect displacement measurement can be performed
using a separate displacement transducer. This arrangement is
showninFig.2forSE(B)specimens.ForC(T)specimens,the
displacement gage should be placed in the notch on the load
line.
7. Specimen Configuration, Size, and Preparation
7.1 Specimen Size:
7.1.1 The initial selection of specimen size and dimensions
can only be based on J results estimated from previous
experience. Generally, the largest available specimens are
recommended for testing in order to obtain a larger portion of
the J-R curve and to obtain the most conservative estimate of
crack growth resistance.
7.1.2 Any thickness may be used with the understanding
FIG. 3 Arrangements for Finding Indentation Displacement
that the J-R curve will be limited by the maximum crack
extension considerations of 9.2 and that the J-R curve is only
appropriate for the thickness that is being evaluated.
where conditioning is required. In cases of disagreement, the
7.2 Specimen Configurations:
tolerances shall be 61°C and 62 % relative humidity.
7.2.1 The recommended SE (B) and C (T) specimens are
7.4.2 Note that for some hygroscopic materials, such as
similar to the configurations in Test Methods D5045 and are
nylons, the material specifications (for example, Specification
shown in Fig. 1. The specimens can be modified to permit
D4066) call for testing “dry as molded specimens.” Such
load-line displacement measurement. Suggested modifications
requirements take precedence over the above routine precon-
are given in Test Method E1152.
ditioning to 50 % relative humidity and require sealing the
7.2.2 All in-plane dimensions are proportional to the speci-
specimens in water vapor-impermeable containers as soon as
men width, W. The thickness is nominally B = 0.5 W.
molded and not removing them until ready for testing.
7.2.3 The original crack length, a , shall be greater than 0.5
7.5 Notching:
W, but less than 0.65 W.
7.5.1 The objective of the notching procedure is to obtain
7.2.4 The span, S, to width, W, ratio in SE (B) specimens
the sharpest possible crack with minimal damage to the
shall be 4.
material in advance of the crack tip.
7.2.5 Side Grooves—Specimens may need side grooves to
7.5.2 Machine a pre-notch into the specimen to a depth of
promote straighter crack fronts during testing. The side
0.5 W using either a saw or a single-point flycutter.
groovesshouldbeequalindepthandhaveanincludedangleof
7.5.3 Create a natural crack by inserting a razor blade into
45 6 5° with a root radius of 0.25 m 6 0.05 mm. The total
the pre-notch and tapping it into the specimen and forcing the
thickness reduction may not exceed 0.20 B. Side grooves must
crack to grow in advance of the razor blade tip.
be used when the crack front requirements of 9.2.3 cannot be
7.5.4 If a natural crack cannot be successfully generated by
met with plane sided specimens.
tappingtherazorblade,slideafresh,unusedrazorbladeacross
7.2.6 Alternative specimens may have 2# W/B# 4.
the root of the machined pre-notch.
7.3 Indentation Correction Specimens—Separately pre-
pared unnotched test specimens are used for indentation 7.5.5 Thelengthoftherazorcrackshallnotbelessthan5 %
of the total original crack length, a .
displacementandenergycorrections.Thespecimensareshown
in Fig. 3. 7.5.6 Alternative Notching Techniques:
7.4 Conditioning: 7.5.6.1 Fatigue pre-notching is permissible. Suggested
7.4.1 Condition the test specimens at 23 6 2°C and 50 6 notching conditions are given in Test Method E1152. Because
5 % relative humidity for not less than 40 h prior to test in of the possibility of hysteretic heating leading to subsequent
accordance with Procedure A of Practice D618, for those tests damage, frequencies less than 4 Hz are recommended.
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D6068 – 96 (2002)
7.5.6.2 Pressing a fresh razor blade into the notch is also /4, B /2, and 3B /4) from a side of the specimen along the
N N
permissible provided that damage to the material is minimized. original crack front on the fracture surface (Fig. 4).
SuggestednotchingconditionsandequipmentaregiveninTest 8.6.2 Alongthefrontoftheregionofstablecrackextension,
Method F1473. measure the crack length at five equally spaced points centered
about the specimen centerline and extending to 0.005W from
8. Procedure the surfaces of plane sided specimens or from the roots of the
side grooves in grooved specimens (Fig. 5). Calculate the
8.1 TestingProcedure—Theobjectiveofthisprocedureisto
average physical crack size, a , as follows: average the two
p
develop a J-R curve consisting of J-integral values at spaced
near-surface measurements, combine the result with the re-
crack extensions, Da , as described in 9.3.2. In the multi-
p
maining three measurements, and determine the average of
specimen method, each test specimen is to develop a single
these four values.
point on the J-R curve. A series of specimens are loaded to
8.6.3 Calculate the average physical crack extension, Da
p
different displacements using crosshead or displacement con-
(=a −a ).
p 0
trol. The resulting crack fronts are marked (as described in
8.7 Indentation Correction:
Appendix X1) and the crack extensions are measured from the
8.7.1 Use an unnotched calibration specimen (shown in Fig.
fracture surface. An independent indentation measurement is
3) that is of the same geometry as the individual specimens
also conducted to correct for non-fracture related energy
used in the J tests.
dissipation. The J value is then calculated from the indentation
8.7.2 For the unnotched bend specimens, the fixtures shall
correctedenergyforfracture.Eachspecimenhasthusprovided
be positioned to minimize the amount of specimen bending by
a set of J, Da values to describe the J-R curve.
p
either positioning the bottom supports of the ben
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