ASTM D6671-01
(Test Method)Standards Test Method for Mixed Mode I-Mode II Interlaminar Fracture Toughness of Unidirectional Fiber Reinforced Polymer Matrix Composites
Standards Test Method for Mixed Mode I-Mode II Interlaminar Fracture Toughness of Unidirectional Fiber Reinforced Polymer Matrix Composites
SCOPE
1.1 This test method describes the determination of interlaminar fracture toughness, Gc, of continuous fiber-reinforced composite materials at various Mode I to Mode II loading ratios using the Mixed-Mode Bending (MMB) Test.
1.2 This test method is limited to use with composites consisting of unidirectional carbon fiber tape laminates with brittle and tough single-phase polymer matrices. This test is further limited to the determination of fracture toughness as it initiates from a delamination insert. This limited scope reflects the experience gained in round robin testing. This method may prove useful for other types of toughness values and for other classes of composite materials; however, certain interferences have been noted (see Section 6). This test method has been successfully used to test the toughness of both glass fiber composites and adhesive joints.
1.3 The values stated in SI units are to be regarded as standard. The values provided in parentheses are for information only.
1.4 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.
General Information
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Standards Content (Sample)
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Designation: D 6671 – 01
Standard Test Method for
Mixed Mode I-Mode II Interlaminar Fracture Toughness of
Unidirectional Fiber Reinforced Polymer Matrix Composites
This standard is issued under the fixed designation D 6671; 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 and their Composites
D 5229 Test Method for Moisture Absorption Properties
1.1 This test method describes the determination of inter-
and Equilibrium Conditioning of Polymer Matrix Com-
laminar fracture toughness, G , of continuous fiber-reinforced
c
posite Materials
composite materials at various Mode I to Mode II loading
D 5528 Test Method for Mode I Interlaminar Fracture
ratios using the Mixed-Mode Bending (MMB) Test.
Toughness of Unidirectional Fiber-Reinforced Polymer
1.2 This test method is limited to use with composites
Matrix Composites
consisting of unidirectional carbon fiber tape laminates with
E 4 Practices for Force Verification of Testing Machines
brittle and tough single-phase polymer matrices. This test is
E 6 Terminology Relating to Methods of Mechanical Test-
further limited to the determination of fracture toughness as it
ing
initiates from a delamination insert. This limited scope reflects
E 122 Practice for Choice of Sample Size to Estimate a
the experience gained in round robin testing. This method may
Measure of Quality for a Lot or Process
prove useful for other types of toughness values and for other
E 177 Practice for Use of the Terms Precision and Bias in
classes of composite materials; however, certain interferences
ASTM Test Methods
have been noted (see Section 6). This test method has been
E 456 Terminology Relating to Quality and Statistics
successfully used to test the toughness of both glass fiber
composites and adhesive joints.
3. Terminology
1.3 The values stated in SI units are to be regarded as
3.1 Terminology D 3878 defines terms relating to high-
standard. The values provided in parentheses are for informa-
modulus fibers and their composites. Terminology D 883
tion only.
defines terms relating to plastics. Terminology E 6 defines
1.4 This standard does not purport to address all of the
terms relating to mechanical testing. Terminology E 456 and
safety concerns, if any, associated with its use. It is the
Practice E 177 define terms relating to statistics. In the event of
responsibility of the user of this standard to establish appro-
conflict between terms, Terminology D 3878 shall have prece-
priate safety and health practices and determine the applica-
dence over the other Terminology standards.
bility of regulatory limitations prior to use.
3.2 Definitions of Terms Specific to This Standard:
2. Referenced Documents 3.2.1 crack opening mode (Mode I)—fracture mode in
which the delamination faces open away from each other and
2.1 ASTM Standards:
2 no relative crack face sliding occurs.
D 883 Terminology Relating to Plastics
3.2.2 crack sliding mode (Mode II)—fracture mode in
D 2651 Guide for Preparation of Metal Surfaces for Adhe-
3 which the delamination faces slide over each other in the
sive Bonding
direction of delamination growth and no relative crack face
D 2734 Test Method for Void Content of Reinforced Plas-
4 opening occurs.
tics
3.2.3 mode mixture, G /G—fraction of Mode II to total
II
D 3171 Test Method for Constituent Content of Composite
strain energy release rate. The mixed-mode ratio, G /G ,isat
I II
Materials
times referred to instead of the mode mixture.
D 3878 Terminology for High-Modulus Reinforcing Fibers
3.2.4 Mode I strain energy release rate, G —the loss of
I
strain energy associated with Mode I deformation in the test
specimen per unit of specimen width for an infinitesimal
This test method is under the jurisdiction of ASTM Committee D30 on
Composite Materials and is the direct responsibility of Subcommittee D30.06 on
increase in delamination length, da, for a delamination growing
Interlaminar Properties.
under a constant displacement.
Current edition approved May 10, 2001. Published August 2001.
Annual Book of ASTM Standards, Vol 08.01.
Annual Book of ASTM Standards, Vol 15.06.
4 6
Annual Book of ASTM Standards, Vol 08.02. Annual Book of ASTM Standards, Vol 03.01.
5 7
Annual Book of ASTM Standards, Vol 15.03. Annual Book of ASTM Standards, Vol 14.02.
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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.
D6671–01
3.2.5 Mode II strain energy release rate, G —the loss of SD = standard deviation
II
strain energy associated with Mode II deformation in the test t = thickness of calibration bar, mm
specimen per unit of specimen width for an infinitesimal U = strain energy, N-mm
increase in delamination length, da, for a delamination growing V = fiber volume fraction, %
under a constant displacement. x = crack length correction for crack tip rotation
3.2.6 mixed-mode fracture toughness, G —the critical value d = load point deflection, mm
c
of strain energy release rate, G, for delamination growth in G = transverse modulus correction parameter
mixed-mode.
4. Summary of Test Method
3.2.7 mixed-mode ratio, G /G —the ratio of Mode I strain
I II
4.1 The Mixed-Mode Bending (MMB) test apparatus shown
energy release rate to Mode II strain energy release rate.
in Fig. 1 is used to load split laminate specimens to determine
3.2.8 strain energy release rate, G—the loss of strain
the delamination fracture toughness at various ratios of Mode
energy, dU, in the test specimen per unit of specimen width for
I to Mode II loading. The composite test specimen, shown in
an infinitesimal increase in delamination length, da, for a
Fig. 2, consists of a rectangular, uniform thickness, unidirec-
delamination growing under a constant displacement. In math-
tional laminated composite specimen, containing a nonadhe-
ematical form,
sive insert at the midplane which serves as a delamination
1 dU
G 5 (1) initiator. Loading forces are applied to the MMB specimen via
bda
tabs that are applied near the ends of the delaminated section of
the specimen and through rollers that bear against the specimen
where:
a = delamination length, in the nondelaminated region. The base of the MMB apparatus
b = width of specimen, and holds the specimen stationary while the MMB lever loads the
U = total elastic strain energy in the test specimen.
specimen. The base attaches to the bottom specimen tab and
3.3 Symbols:
also bears on the specimen near the far end with a roller. The
a = delamination length, mm
lever attaches to the top tab and bears down on the specimen
a = initial delamination length, mm
halfway between the base roller and the tabs. The lever roller
o
a = propagation delamination lengths, mm
acts as a fulcrum so by pushing down on the lever arm opposite
1-25
ã = nondimensional delamination length value, a/hx
the tab, the tab is pulled up. The length of the lever arm, c, can
b = width of specimen, mm
be changed to vary the ratio of the load pulling on the tab to the
b = width of calibration specimen, mm
load bearing through the roller thus changing the mode mixture
cal
c = lever length of the MMB test apparatus, mm
of the test. The load shall be applied to the lever such that the
c = lever length to center of gravity, mm
load remains vertical during the loading process. To reduce
g
C = compliance, d/P, mm/N
geometric nonlinear effects as a result of lever rotation, the
C = calibration specimen compliance, d/P, mm/N
lever shall be loaded such that the height of loading is slightly
cal
C = system compliance, d/P, mm/N
above the midplane of the test specimen (~15 mm) (1).
sys
CV = coefficient of variation, %
4.2 A record of the applied load versus opening displace-
E = longitudinal modulus of elasticity measured in tension,
11 ment is recorded on an x-y recorder, or equivalent real-time
MPa
plotting device or stored digitally and postprocessed. The
E = transverse modulus of elasticity, MPa
interlaminar fracture toughness, G , and mode mixture, G /G,
c II
E = modulus of calibration bar, MPa
are calculated from critical loads read from the load displace-
cal
E = modulus of elasticity in the fiber direction measured in
ment curve.
1f
flexure, MPa
2 5. Significance and Use
G = total strain energy release rate, kJ/m
5.1 Susceptibility to delamination is one of the major
G = shear modulus out of plane, MPa
weaknesses of many advanced laminated composite structures.
G = shear modulus in plane, MPa
G = opening (Mode I) component of strain energy release
I
2 8
The boldface numbers in parentheses refer to a list of references at the end of
rate, kJ/m
this standard.
G = shear (Mode II) component of strain energy release
II
rate, kJ/m
G = total mixed-mode fracture toughness, kJ/m
c
h = half thickness of test specimen, mm
L = half-span length of the MMB test apparatus, mm
m = slope of the load displacement curve, N/mm
P = applied load, N
P = critical load at 5 %/max point of loading curve, N
5 %/max
P = expected value of critical load, N
exp
P = weight of lever and attach apparatus, N
g
P = critical load at nonlinear point of loading curve, N
nl
P = expected load on the loading tab, N
tab
P = critical load when delamination is observed to grow, N FIG. 1 MMB Apparatus
vis
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.
D6671–01
FIG. 2 MMB Test Variables
FIG. 4 Load-Displacement Curves
Knowledge of the interlaminar fracture resistance of compos-
ites is useful for product development and material selection.
Since delaminations can be subjected to and extended by
loadings with a wide range of mode mixtures, it is important 5.3.2 Propagation Option—In the MMB test, the delamina-
that the composite toughness be measured at various mode
tion will grow from the insert in either a stable or an unstable
mixtures. The toughness contour, in which fracture toughness manner depending on the mode mixture being tested. As an
is plotted as a function of mode mixtures (see Fig. 3), is useful
option, propagation toughness values may be collected when
for establishing failure criterion used in damage tolerance delaminations grow in a stable manner. Propagation toughness
analyses of composite structures made from these materials.
values are not attainable when the delamination grows in an
5.2 This test method can serve the following purposes: unstable manner. Propagation toughness values may be heavily
5.2.1 To establish quantitatively the effects of fiber surface
influenced by fiber bridging which is an artifact of the
treatment, local variations in fiber volume fraction, and pro- zero-degree-type test specimen (2-4). Since they are often
cessing and environmental variables on G of a particular
believed to be artificial, propagation values must be clearly
c
composite material at various mode mixtures, marked as such when they are reported. One use of propagation
5.2.2 To compare quantitatively the relative values of G values is to check for problems with the delamination insert.
c
versus mode mixture for composite materials with different Normally, delamination toughness values rise from the initia-
constituents, and tion values as the delamination propagates. When toughness
5.2.3 To develop delamination failure criteria for composite values decrease as the delamination grows, a poor delamina-
damage tolerance and durability analyses. tion insert is often the cause. The delamination may be too
5.3 This method can be used to determine the following
thick or deformed in such a way that a resin pocket forms at the
delamination toughness values: end of the insert. For accurate initiation values, a properly
5.3.1 Delamination Initiation—Two values of delamination
implanted and inspected delamination insert is critical (see
initiation shall be reported: (1) at the point of deviation from 8.2).
linearity in the load-displacement curve (NL) and (2)atthe
5.3.3 Precracked Toughness—Under rare circumstances,
point at which the compliance has increased by 5 % or the load toughness may decrease from the initiation values as the
has reached a maximum value (5 %/max) depending on which
delamination propagates (see 5.3.2). If this occurs, the delami-
occurs first along the load deflection curve (see Fig. 4). Each nation should be checked to insure that it complies with the
definition of delamination initiation is associated with its own
insert recommendations found in 8.2. Only after verifying that
value of G and G /G calculated from the load at the the decreasing toughness was not due to a poor insert, should
c II
corresponding critical point. The 5 %/Max G value is typically
precracking be considered as an option. With precracking, a
c
the most reproducible of the three G values. The NL value is, delamination is first extended from the insert in Mode I, Mode
c
however, the more conservative number. When the option of
II, or mixed mode. The specimen is then reloaded at the desired
collecting propagation values is taken (see 5.3.2), a third mode mixture to obtain a toughness value.
initiation value may be reported at the point at which the
delamination is first visually observed to grow on the edge of 6. Interferences
the specimen. The VIS point often falls between the NL and the
6.1 Linear elastic behavior is assumed in the calculation of
5 %/Max points.
G used in this test method. This assumption is valid when the
c
zone of damage or nonlinear deformation at the delamination
front, or both, is small relative to the smallest specimen
dimension, which is typically the specimen thickness for the
MMB test.
6.2 The application to other materials, layups, and architec-
tures is the same as described in Test Method D 5528.
6.3 The nonlinear (NL) initiation value of toughness is
normally the more conservative value, but a few materials have
exhibited lower propagation toughness values, particularly in
the high Mode II regime. In the high Mode II regime, the
delamination growth is often unstable, precluding propagation
FIG. 3 Mixed-Mode Summary Graph toughness values from being determined. The use of initiation
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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