ASTM D6068-96

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: D 6068 – 96
Standard Test Method for
Determining J-R Curves of Plastic Materials
This standard is issued under the fixed designation D 6068; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope E 399 Test Method for Plane-Strain Fracture Toughness of
Metallic Materials
1.1 This test method covers the determination of the
E 616 Terminology Relating to Fracture Testing
J-integral versus crack growth resistance (J-R) curves for
E 1152 Test Method for Determining J-R Curves
polymeric materials.
E 1737 Test Method for J-Integral Characterization of Frac-
1.2 This test method is intended to characterize the slow,
ture Toughness
stable crack growth resistance of bend-type specimens in such
F 1473 Test Method for Notch Tensile Test to Measure the
a manner that it is geometry insensitive within limits set forth
Resistance to Slow Crack Growth of Polyethylene Pipes
in this test method.
and Resins
1.3 The recommended specimens are the three-point bend
(SE (B)) and pin-loaded compact tension (C (T)) specimens.
3. Terminology
Both specimens have in-plane dimensions of constant propor-
3.1 Definitions—Terminology related to fracture testing
tionality for all sizes. Specimen configurations other than those
contained in Terminology E 616 is applicable to this test
recommended in this test method may require different proce-
method.
dures and validity requirements.
3.2 Definitions of Terms Specific to This Standard:
1.4 This test method describes a multiple specimen method
−1
3.2.1 J-integral, J (FL )—a mathematical expression, a
that requires optical measurement of crack extension from
line or surface integral over a path that encloses the crack front
fracture surfaces. It is not recommended for use with materials
from one crack surface to the other, used to characterize the
in which the crack front cannot be distinguished from addi-
local stress-strain field around the crack front. See Terminology
tional deformation processes in advance of the crack tip.
E 616 for additional discussion.
1.5 The values stated in SI units are to be regarded as the
3.2.2 J-R curve—a plot of resistance to stable physical crack
standard.
extension, Da .
p
1.6 This standard does not purport to address all of the
3.2.3 net thickness, B (L)—the distance between the roots
N
safety concerns, if any, associated with its use. It is the
of the side grooves in side grooved specimens.
responsibility of the user of this standard to establish appro-
3.2.4 original crack size, a (L)—the physical crack size at
priate safety and health practices and determine the applica-
the start of testing.
bility of regulatory limitations prior to use.
3.2.5 original uncracked ligament, b (L)—the distance
NOTE 1—There is no equivalent ISO standard.
from the original crack front to the back edge of the specimen
(b =W−a8 ).
0 0
2. Referenced Documents
3.2.6 physical crack extension, Da (L)—an increase in
p
2.1 ASTM Standards:
physical crack size (Da =a −a ).
p p 0
D 618 Practice for Conditioning Plastics and Electrical
3.2.7 physical crack size, a (L)—the distance from a
p
Insulating Materials for Testing
reference line to the observed crack front. The distance may be
D 4066 Specification for Nylon Injection and Extrusion
a calculated average of several measurements along the crack
Materials
front. The reference line depends on the specimen geometry
D 5045 Test Methods for Plane-Strain Fracture Toughness
and is normally defined as in 3.2.10. The reference line is
and Strain Energy Release Rate of Plastic Materials
defined prior to specimen deformation.
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
This test method is under the jurisdiction of ASTM Committee D-20 on Plastics
of the test specimen (shown in Fig. 2).
and is the direct responsibility of Subcommittee D20.10 on Mechanical Properties.
Current edition approved Dec. 10, 1996. Published January 1997.
Annual Book of ASTM Standards, Vol 08.01.
Annual Book of ASTM Standards, Vol 08.02.
Annual Book of ASTM Standards, Vol 03.01.
Annual Book of ASTM Standards, Vol 08.03.
Annual Book of ASTM Standards, Vol 08.04.
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NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D 6068
length and crack extension on the fracture surfaces after each
test.
5. Significance and Use
5.1 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) that cannot
be obtained in sufficient size and thickness for valid character-
ization by linear elastic fracture mechanics in Test Methods
D 5045.
5.2 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.
5.3 A J-R curve can be used as an index of material
toughness for blend or alloy design, material selection, mate-
rials processing, and quality assurance (6).
5.4 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.
5.5 The J-R curves for a given material of constant micro-
structure tend to exhibit lower slope (flatter) with increasing
thickness. Thus, it is recommended that the largest possible
FIG. 1 Specimen Configurations
specimen with representative microstructure be used.
5.6 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.
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. Load versus load-line displacement may be recorded
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.
FIG. 2 Bending Rig 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).
3.2.10 specimen width, W(L)—a physical dimension on a
6.3 Bend Test Fixture—A suggested fixture for SE (B)
test specimen measured from the rear surface of the specimen
specimens is shown in Fig. 2. The fixture may have either
to a reference line (for example, the front edge of a bend
stationary or moving rollers of sufficiently large diameter to
specimen or the load line of a compact specimen).
minimize excessive plastic indentation.
6.4 Grips for C (T) Specimens—A suggested clevis and pin
4. Summary of Test Method
arrangement for gripping compact specimens is given in Test
4.1 This test method describes a multiple specimen tech-
Method E 399. This arrangement accommodates specimens
nique for determining the J-R curve for polymeric materials.
with B = 0.5 W.
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. 7
The boldface numbers given in parentheses refer to a list of references at the
4.2 This test method uses optical measurements of crack end of the text.
NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D 6068
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 A direct displacement measurement can be performed
using a separate displacement transducer. This arrangement is
shown in Fig. 2 for SE (B) specimens. For C (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
FIG. 3 Measurement of Initial Crack Length (a )
o
recommended for testing in order to obtain a larger portion of
the J-R curve and to obtain the most conservative estimate of
D 4066) call for testing “dry as molded specimens.” Such
crack growth resistance.
requirements take precedence over the above routine precon-
7.1.2 Any thickness may be used with the understanding
ditioning to 50 % relative humidity and require sealing the
that the J-R curve will be limited by the maximum crack
specimens in water vapor-impermeable containers as soon as
extension considerations of 9.2 and that the J-R curve is only
molded and not removing them until ready for testing.
appropriate for the thickness that is being evaluated.
7.5 Notching:
7.2 Specimen Configurations:
7.5.1 The objective of the notching procedure is to obtain
7.2.1 The recommended SE (B) and C (T) specimens are
the sharpest possible crack with minimal damage to the
similar to the configurations in Test Methods D 5045 and are
material in advance of the crack tip.
shown in Fig. 1. The specimens can be modified to permit
7.5.2 Machine a pre-notch into the specimen to a depth of
load-line displacement measurement. Suggested modifications
0.5 W using either a saw or a single-point flycutter.
are given in Test Method E 1152.
7.5.3 Create a natural crack by inserting a razor blade into
7.2.2 All in-plane dimensions are proportional to the speci-
the pre-notch and tapping it into the specimen and forcing the
men width, W. The thickness is nominally B = 0.5 W.
crack to grow in advance of the razor blade tip.
7.2.3 The original crack length, a , shall be greater than 0.5
7.5.4 If a natural crack cannot be successfully generated by
W, but less than 0.65 W.
tapping the razor blade, slide a fresh, unused razor blade across
7.2.4 The span, S, to width, W, ratio in SE (B) specimens
the root of the machined pre-notch.
shall be 4.
7.5.5 The length of the razor crack shall not be less than 5 %
7.2.5 Side Grooves—Specimens may need side grooves to
of the total original crack length, a .
promote straighter crack fronts during testing. The side
7.5.6 Alternative Notching Techniques:
grooves should be equal in depth and have an included angle of
7.5.6.1 Fatigue pre-notching is permissible. Suggested
45 6 5° with a root radius of 0.25 m 6 0.05 mm. The total
notching conditions are given in Test Method E 1152. Because
thickness reduction may not exceed 0.20 B. Side grooves must
of the possibility of hysteretic heating leading to subsequent
be used when the crack front requirements of 9.2.3 cannot be
damage, frequencies less than 4 Hz are recommended.
met with plane sided specimens.
7.5.6.2 Pressing a fresh razor blade into the notch is also
7.2.6 Alternative specimens may have 2 # W/B # 4.
permissible provided that damage to the material is minimized.
7.3 Indentation Correction Specimens—Separately pre-
Suggested notching conditions and equipment are given in Test
pared unnotched test specimens are used for indentation
Method F 1473.
displacement and energy corrections. The specimens are shown
in Fig. 3.
8. Procedure
7.4 Conditioning:
7.4.1 Condition the test specimens at 23 6 2°C and 50 6 8.1 Testing Procedure—The objective of this procedure is to
5 % relative humidity for not less than 40 h prior to test in develop a J-R curve consisting of J-integral values at spaced
accordance with Procedure A of Practice D 618, for those tests crack extensions, Da , as described in 9.3.2. In the multi-
p
where conditioning is required. In cases of disagreement, the specimen method, each test specimen is to develop a single
tolerances shall be 61°C and 62 % relative humidity. point on the J-R curve. A series of specimens are loaded to
7.4.2 Note that for some hygroscopic materials, such as different displacements using crosshead or displacement con-
nylons, the material specifications (for example, Specification trol. The resulting crack fronts are marked (as described in
NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D 6068
Appendix X1) and the crack extensions are measured from the
fracture surface. An independent indentation measurement is
also conducted to correct for non-fracture related energy
dissipation. The J value is then calculated from the indentation
corrected energy for fracture. Each specimen has thus provided
a set of J, Da values to describe the J-R curve.
p
8.2 Measure specimen dimensions B, B , and W to the
N
nearest 0.050 mm or 0.5 % accuracy, whichever is larger.
8.3 Because of the viscoelastic nature of polymers, the J-R
curve may be dependent on test temperature and displacement
rate. Therefore, record these conditions with the results.
8.3.1 Provided that stable, well-defined crack growth can be
achieved, any test temperature may be used.
8.3.2 Similarly, any test speed that leads to stable, well-
defined crack growth may be used. However, test speeds that
lead to loading times (time to maximum load) that are less than
1 ms are not recommended for this procedure due to dynamic
effects on the loading signal that can lead to erroneous results.
8.3.3 For general characterization, the suggested test condi-
FIG. 4 Measurement of Crac
...