ASTM D6671/D6671M-22

Standard Test Method for Mixed Mode I-Mode II Interlaminar Fracture Toughness of Unidirectional Fiber Reinforced Polymer Matrix Composites

SIGNIFICANCE AND USE
5.1 Susceptibility to delamination is one of the major weaknesses of many advanced laminated composite structures. Knowledge of the interlaminar fracture resistance of composites 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 that the composite toughness be measured at various mode mixtures. The toughness contour, in which fracture toughness is plotted as a function of mode mixtures (see Fig. 3), is useful for establishing failure criterion used in damage tolerance analyses of composite structures made from these materials.
FIG. 3 Mixed-Mode Summary Graph  
5.2 This test method can serve the following purposes:  
5.2.1 To establish quantitatively the effects of fiber surface treatment, local variations in fiber volume fraction, and processing and environmental variables on Gc of a particular composite material at various mode mixtures,  
5.2.2 To compare quantitatively the relative values of Gc versus mode mixture for composite materials with different constituents, and  
5.2.3 To develop delamination failure criteria for composite damage tolerance and durability analyses.  
5.3 This method can be used to determine the following delamination toughness values:  
5.3.1 Delamination Initiation—Two values of delamination initiation shall be reported: (1) at the point of deviation from linearity in the load-displacement curve (NL) and (2) at the point at which the compliance has increased by 5 % or the load has reached a maximum value (5%/max) depending on which occurs first along the load deflection curve (see Fig. 4). Each definition of delamination initiation is associated with its own value of Gc and  GII/G calculated from the load at the corresponding critical point. The 5%/Max  Gc value is typically the most reproducible of the three Gc values. The NL value is, however, the more conservative number. When the option of collecting ...
SCOPE
1.1 This test method covers 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 method 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 test 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 Units—The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used independently of the other, and values from the two systems shall not be combined.  
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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.5 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.

General Information

• Status: Published
• Publication Date: 31-Jan-2022
• ICS: 19.060 - Mechanical testing
• ICS: 83.120 - Reinforced plastics
• Technical Committee: D30 - Composite Materials
• Drafting Committee: D30.06 - Interlaminar Properties

Relations

• Refers: ASTM D883-24 - Standard Terminology Relating to Plastics
• Effective Date: 01-Feb-2024
• Refers: ASTM D883-23 - Standard Terminology Relating to Plastics
• Effective Date: 01-Nov-2023
• Refers: ASTM D2734-23 - Standard Test Methods for Void Content of Reinforced Plastics
• Effective Date: 01-Oct-2023
• Refers: ASTM E456-13a(2022)e1 - Standard Terminology Relating to Quality and Statistics
• Effective Date: 01-Apr-2022
• Refers: ASTM D5229/D5229M-20 - Standard Test Method for Moisture Absorption Properties and Equilibrium Conditioning of Polymer Matrix Composite Materials
• Effective Date: 01-Mar-2020
• Refers: ASTM D883-20 - Standard Terminology Relating to Plastics
• Effective Date: 01-Jan-2020
• Refers: ASTM D3878-19a - Standard Terminology for Composite Materials
• Effective Date: 15-Oct-2019
• Refers: ASTM D883-19c - Standard Terminology Relating to Plastics
• Effective Date: 01-Aug-2019
• Refers: ASTM D883-19a - Standard Terminology Relating to Plastics
• Effective Date: 15-Apr-2019
• Refers: ASTM D3878-19 - Standard Terminology for Composite Materials
• Effective Date: 15-Apr-2019
• Refers: ASTM D883-19 - Standard Terminology Relating to Plastics
• Effective Date: 01-Feb-2019
• Refers: ASTM D883-18a - Standard Terminology Relating to Plastics
• Effective Date: 01-Dec-2018
• Refers: ASTM D883-18 - Standard Terminology Relating to Plastics
• Effective Date: 01-Nov-2018
• Refers: ASTM D3878-18 - Standard Terminology for Composite Materials
• Effective Date: 01-Apr-2018
• Refers: ASTM E456-13A(2017)e3 - Standard Terminology Relating to Quality and Statistics
• Effective Date: 01-Oct-2017

Overview

ASTM D6671/D6671M-22 is the standard test method for determining the mixed mode I-mode II interlaminar fracture toughness of unidirectional fiber-reinforced polymer matrix composites. This standard is developed and maintained by ASTM International and provides procedures to evaluate a composite material’s resistance to delamination under varying mode I (opening) and mode II (sliding) loading conditions using the Mixed-Mode Bending (MMB) test.

Delamination resistance is a key property for advanced laminated composite structures, as it influences damage tolerance and durability. By quantifying interlaminar fracture toughness at multiple mode mixtures, this method provides critical data for product development, material selection, and structural safety analysis in engineering applications.

Key Topics

• Purpose and Significance:

  • Addresses the susceptibility of fiber-reinforced polymer composites to delamination, a major reliability concern in structural applications.
  • Enables measurement of delamination fracture toughness (Gc) at various ratios of mode I to mode II loading, representing real-world loading scenarios.
  • Data obtained support the development of damage tolerance and failure criteria, essential for the safe design of composite structures.

• Applicability:

  • Applicable to unidirectional laminates with continuous carbon fiber tape and both brittle and tough single-phase polymer matrices.
  • Also provides useful guidance for testing glass fiber composites and adhesive joints under specified conditions.

• Test Method Overview:

  • Utilizes the Mixed-Mode Bending (MMB) apparatus to load test specimens and record load-displacement curves.
  • Specimens feature a pre-inserted delamination initiator in the midplane to standardize crack initiation.
  • Toughness is determined through key points on the load-displacement curve, such as deviation from linearity and a 5% increase in compliance or maximum load.

• Data Interpretation:

  • Multiple definitions of delamination initiation toughness are reported, offering conservative as well as reproducible values.
  • The test may include optional propagation measurements for further insights, aiding in quality control and failure analysis.

• Best Practices and Limitations:

  • Proper specimen preparation, validation of delamination insert, and adherence to conditioning protocols (such as temperature and humidity) are essential for reliable results.
  • The standard provides procedures for calibration, alignment, and data recording to ensure consistency.
  • Assumes linear elastic behavior; deviations may require further analysis or alternative test setups.

Applications

• Composite Material Selection:
  • Enables manufacturers and engineers to compare the interlaminar fracture toughness of various composite materials, optimizing fiber surface treatments, fiber volume fractions, and processing variables.
• Structural Integrity Analysis:
  • Supplies data for damage tolerance analysis, allowing for accurate modeling of delamination growth and failure modes in aerospace, automotive, marine, and civil engineering structures employing advanced composites.
• Quality Assurance and Research:
  • Assists in characterizing new materials and validating manufacturing processes, contributing to innovations in composite technology.
• Evaluation of Environmental and Processing Effects:
  • Useful for quantifying the effects of environmental exposure, conditioning, and processing techniques on composite performance.

Related Standards

• ASTM D5528 - Standard Test Method for Mode I Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites.
• ASTM D5229/D5229M - Standard Test Method for Moisture Absorption Properties and Equilibrium Conditioning of Polymer Matrix Composite Materials.
• ASTM D2734 - Test Methods for Void Content of Reinforced Plastics.
• ASTM D3171 - Test Methods for Constituent Content of Composite Materials.
• ASTM D2651 - Guide for Preparation of Metal Surfaces for Adhesive Bonding.

ASTM D6671/D6671M-22 is an essential standard for professionals engaged in composite material testing, structural engineering, and research, enabling robust, reproducible assessment of interlaminar fracture toughness under practical, mixed-mode loading conditions.

Standards Content (Sample)

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: D6671/D6671M − 22
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 D6671/D6671M; 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.
1. Scope 2. Referenced Documents
2.1 ASTM Standards:
1.1 This test method covers the determination of interlami-
D883Terminology Relating to Plastics
nar fracture toughness, G , of continuous fiber-reinforced
c
D2651GuideforPreparationofMetalSurfacesforAdhesive
composite materials at various Mode I to Mode II loading
Bonding
ratios using the Mixed-Mode Bending (MMB) Test.
D2734TestMethodsforVoidContentofReinforcedPlastics
1.2 This test method is limited to use with composites
D3171Test Methods for Constituent Content of Composite
consisting of unidirectional carbon fiber tape laminates with
Materials
brittle and tough single-phase polymer matrices. This test
D3878Terminology for Composite Materials
method is further limited to the determination of fracture
D5229/D5229MTestMethodforMoistureAbsorptionProp-
toughness as it initiates from a delamination insert. This
erties and Equilibrium Conditioning of Polymer Matrix
limited scope reflects the experience gained in round robin
Composite Materials
testing. This test method may prove useful for other types of
D5528TestMethodforModeIInterlaminarFractureTough-
toughness values and for other classes of composite materials;
ness of Unidirectional Fiber-Reinforced Polymer Matrix
however,certaininterferenceshavebeennoted(seeSection6).
Composites
This test method has been successfully used to test the
E4Practices for Force Calibration and Verification of Test-
toughness of both glass fiber composites and adhesive joints.
ing Machines
E6Terminology Relating to Methods of MechanicalTesting
1.3 Units—The values stated in either SI units or inch-
E122PracticeforCalculatingSampleSizetoEstimate,With
pound units are to be regarded separately as standard. The
Specified Precision, the Average for a Characteristic of a
values stated in each system are not necessarily exact equiva-
Lot or Process
lents; therefore, to ensure conformance with the standard, each
E177Practice for Use of the Terms Precision and Bias in
system shall be used independently of the other, and values
ASTM Test Methods
from the two systems shall not be combined.
E456Terminology Relating to Quality and Statistics
1.4 This standard does not purport to address all of the
3. Terminology
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- 3.1 Terminology D3878 defines terms relating to high-
priate safety, health, and environmental practices and deter-
modulus fibers and their composites. Terminology D883 de-
mine the applicability of regulatory limitations prior to use. fines terms relating to plastics. Terminology E6 defines terms
relating to mechanical testing. Terminology E456 and Practice
1.5 This international standard was developed in accor-
E177 define terms relating to statistics. In the event of conflict
dance with internationally recognized principles on standard-
between terms, Terminology D3878 shall have precedence
ization established in the Decision on Principles for the
over the other terminology standards.
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
NOTE 1—If the term represents a physical quantity, its analytical
Barriers to Trade (TBT) Committee.
dimensionsarestatedimmediatelyfollowingtheterm(orlettersymbol)in
fundamental dimension form, using the following ASTM standard sym-
bology for fundamental dimensions, shown within square brackets: [M]
for mass, [L] for length, [T] for time, [u] for thermodynamic temperature,
This test method is under the jurisdiction of ASTM Committee D30 on
Composite Materials and is the direct responsibility of Subcommittee D30.06 on
Interlaminar Properties. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Feb. 1, 2022. Published March 2022. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 2001. Last previous edition approved in 2019 as D6671/D6671M–19. Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/D6671_D6671M-22. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6671/D6671M − 22
and[nd]fornon-dimensionalquantities.Useofthesesymbolsisrestricted
C =calibration specimen compliance, δ/P, mm/N [in./lbf]
cal
to analytical dimensions when used with square brackets, as the symbols
C =system compliance, δ/P, mm/N [in./lbf]
sys
may have other definitions when used without the brackets.
CV=coefficient of variation, %
3.2 Definitions of Terms Specific to This Standard:
E =longitudinalmodulusofelasticitymeasuredintension,
3.2.1 crack opening mode (Mode I), n—fracture mode in
MPa [psi]
which the delamination faces open away from each other and
E =transverse modulus of elasticity, MPa [psi]
no relative crack face sliding occurs.
E =modulus of calibration bar, MPa [psi]
cal
3.2.2 crack sliding mode (Mode II), n—fracture mode in
E =modulus of elasticity in the fiber direction measured in
1f
which the delamination faces slide over each other in the
flexure, MPa [psi]
2 2
direction of delamination growth and no relative crack face
G=total strain energy release rate, kJ/m [in.-lbf/in. ]
opening occurs.
G =shear modulus out of plane, MPa [psi]
G =shear modulus in plane, MPa [psi]
3.2.3 mixed-mode fracture toughness, G [M/T ], n—the
c
critical value of strain energy release rate, G, for delamination G =opening (Mode I) component of strain energy release
I
2 2
rate, kJ/m [in.-lbf⁄in ]
growth in mixed-mode.
G =shear (Mode II) component of strain energy release
II
3.2.4 mixed-mode ratio, G /G [nd], n—the ratio of Mode I
I II
2 2
rate, kJ/m [in.-lbf⁄in ]
strain energy release rate to Mode II strain energy release rate.
G /G=mode mixture
II
3.2.5 mode mixture, G /G [nd], n—fraction of Mode II to
II 2
G =total mixed-mode fracture toughness, kJ/m [in.-lbf/
c
total strain energy release rate.
in ]
est
G = estimated value of total mixed-mode fracture
3.2.5.1 Discussion—The mixed-mode ratio, G/ G,isat
I II c
2 2
times referred to instead of the mode mixture. toughness, kJ/m [in.-lbf⁄in ]
3.2.6 Mode I strain energy release rate, G [M/T ], n—the h=half thickness of test specimen, mm [in.]
I
lossofstrainenergyassociatedwithModeIdeformationinthe L=half-span length of the MMB test apparatus, mm [in.]
test specimen per unit of specimen width for an infinitesimal
m=slope of the load displacement curve, N/mm [lb/in.]
increaseindelaminationlength, da,foradelaminationgrowing
m =slope of the load displacement curve from calibration
cal
under a constant displacement.
test, N/mm [lbf/in.]
n = number of specimens
3.2.7 Mode II strain energy release rate, G [M/T ], n—the
II
P=applied load, N [lbf]
loss of strain energy associated with Mode II deformation in
P =critical load at 5%/max point of loading curve, N
the test specimen per unit of specimen width for an infinitesi-
5%/max
mal increase in delamination length, da, for a delamination [lbf]
P =estimated value of critical load, N [lbf]
growing under a constant displacement.
est
2 P =weight of lever and attached apparatus, N [lbf]
g
3.2.8 strain energy release rate, G [M/T ], n—the loss of
P =criticalloadatnonlinearpointofloadingcurve,N[lbf]
strain energy, dU, in the test specimen per unit of specimen nl
P =expected load on the loading tab, N [lbf]
width for an infinitesimal increase in delamination length, da, tab
P =criticalloadwhendelaminationisobservedtogrow,N
for a delamination growing under a constant displacement; in
vis
[lbf]
mathematical form,
S =standard deviation
n-1
1 dU
G52 (1) t=thickness of calibration bar, mm [in.]
b da
U=strain energy, N-mm [in.-lbf]
where:
V=fiber volume fraction, %
a = delamination length, mm [in.],
xi = measured or derived property for an individual speci-
b = width of specimen, mm [in.],
men from the sample population
2 2
G = total strain energy release rate, kJ/m [in.-lbf/in. ], and
x¯ = sample mean (average)
U = total elastic strain energy in the test specimen, N-mm
α=mode mixture transformation parameter for setting lever
[in.-lbf].
length
β=non-dimensional crack length correction for mode mix-
ture
χ=crack length correction parameter,
3.3 Symbols:
a=delamination length, mm [in.]
E Γ
a =initial delamination length, mm [in.]
o χ 5Œ 3 22
H S D J
11G 11Γ
a =propagation delamination lengths, mm [in.]
1-25
b=width of specimen, mm [in.] δ=load point deflection, mm [in.]
b =width of calibration specimen, mm [in.] δ =estimated load point deflection, mm [in.]
cal
est
c=lever length of the MMB test apparatus, mm [in.] δ =maximumallowableloadpointofdeflection,mm[in.]
max
c =lever length to center of gravity, mm [in.]
Γ=transverse modulus correction parameter,
g
C=compliance, δ/P, mm/N [in./lbf]
D6671/D6671M − 22
=E E
11 22
Γ 51.18
G
4. Summary of Test Method
4.1 TheMixed-ModeBending(MMB)testapparatusshown
in Fig. 1 is used to load split laminate specimens to determine
the delamination fracture toughness at various ratios of Mode
I to Mode II loading. The composite test specimen, shown in
FIG. 2 MMB Test Variables
Fig. 2, consists of a rectangular, uniform thickness, unidirec-
tional laminated composite specimen, containing a nonadhe-
Since delaminations can be subjected to and extended by
sive insert at the midplane which serves as a delamination
loadings with a wide range of mode mixtures, it is important
initiator. Loading forces are applied to the MMB specimen via
that the composite toughness be measured at various mode
tabsthatareappliedneartheendsofthedelaminatedsectionof
mixtures. The toughness contour, in which fracture toughness
thespecimenandthroughrollersthatbearagainstthespecimen
is plotted as a function of mode mixtures (see Fig. 3), is useful
in the nondelaminated region.The base of the MMB apparatus
for establishing failure criterion used in damage tolerance
holds the specimen stationary while the MMB lever loads the
analyses of composite structures made from these materials.
specimen. The base attaches to the bottom specimen tab and
also bears on the specimen near the far end with a roller. The
5.2 This test method can serve the following purposes:
lever attaches to the top tab and bears down on the specimen
5.2.1 To establish quantitatively the effects of fiber surface
halfway between the base roller and the tabs. The lever roller
treatment, local variations in fiber volume fraction, and pro-
actsasafulcrumsobypushingdownontheleverarmopposite
cessing and environmental variables on G of a particular
c
the tab, the tab is pulled up.The length of the lever arm, c, can
composite material at various mode mixtures,
bechangedtovarytheratiooftheloadpullingonthetabtothe
5.2.2 To compare quantitatively the relative values of G
c
load bearing through the roller, thus changing the mode
versus mode mixture for composite materials with different
mixture of the test. The load shall be applied to the lever such
constituents, and
that the load remains vertical during the loading process. To
5.2.3 To develop delamination failure criteria for composite
reduce geometric nonlinear effects as a result of lever rotation,
damage tolerance and durability analyses.
the lever shall be loaded such that the height of loading is
5.3 This method can be used to determine the following
slightly above the pivot point where the lever attaches to the
delamination toughness values:
test specimen (1, 2).
5.3.1 Delamination Initiation—Two values of delamination
4.2 A record of the applied load versus opening displace-
initiation shall be reported: (1) at the point of deviation from
ment is recorded on an x-y recorder, or equivalent real-time
linearity in the load-displacement curve (NL) and (2)atthe
plotting device or stored digitally and post-processed. The
pointatwhichthecompliancehasincreasedby5%ortheload
interlaminar fracture toughness, G , and mode mixture, G /G,
c II has reached a maximum value (5%/max) depending on which
are calculated from critical loads read from the load displace-
occurs first along the load deflection curve (see Fig. 4). Each
ment curve.
definition of delamination initiation is associated with its own
value of G and G /G calculated from the load at the
c II
5. Significance and Use
correspondingcriticalpoint.The5%/Max G valueistypically
c
5.1 Susceptibility to delamination is one of the major
the most reproducible of the three G values. The NLvalue is,
c
weaknessesofmanyadvancedlaminatedcompositestructures.
however, the more conservative number. When the option of
Knowledge of the interlaminar fracture resistance of compos-
collecting propagation values is taken (see 5.3.2), a third
ites is useful for product development and material selection.
initiation value may be reported at the point at which the
delamination is first visually observed to grow on the edge of
thespecimen.TheVISpointoftenfallsbetweentheNLandthe
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
5%/Max points.
this standard.
FIG. 1 MMB Apparatus FIG. 3 Mixed-Mode Summary Graph
D6671/D6671M − 22
toughness values could result in nonconservative growth pre-
dictions in these select materials. The use of longer initial
delaminations increases the tendency for stable delamination
growth.
7. Apparatus
7.1 The mixed-mode bending fixture, as seen in Fig. 5, uses
a lever to load the MMB specimen. Using one applied load at
the end of the lever, a downward load is applied to the
specimen center creating Mode II, while an upward force is
FIG. 4 Load-Displacement Curves
applied to the split end of the laminate creating Mode I.
Machine drawings for an example of MMB apparatus may be
5.3.2 Propagation Option—In the MMB test, the delamina- found in Appendix X2, but other designs that perform the
tion will grow from the insert in either a stable or an unstable
necessary functions are acceptable.The half-span length of the
manner depending on the mode mixture being tested. As an MMBApparatusL(seeFig.2)shallbe50mm[2in.].Tokeep
option, propagation toughness values may be collected when
geometric nonlinear effects small, the loading height (the
delaminations grow in a stable manner. Propagation toughness height of the loading point above the hinge point attaching the
values are not attainable when the delamination grows in an
lever to the test specimen, as shown in Fig. 1) shall be 0.3 L,
unstablemanner.Propagationtoughnessvaluesmaybeheavily
and the applied load shall remain vertical as the apparatus is
influenced by fiber bridging which is an artifact of the
loaded. The load application to the lever and to the test
zero-degree-type test specimen (3-5). Since they are often
specimen should allow sliding with minimal friction. In the
believed to be artificial, propagation values must be clearly
pictured apparatus, this is accomplished with roller bearings,
markedassuchwhentheyarereported.Oneuseofpropagation
but equivalent means are acceptable.
values is to check for problems with the delamination insert.
7.2 Testing Machine—A properly calibrated test machine
Normally, delamination toughness values rise from the initia-
shall be used which can be operated in a displacement control
tion values as the delamination propagates and fiber bridging
mode with a constant displacement rate in the range of 0.5 to
develops.Whentoughnessvaluesdecreaseasthedelamination
5.0 mm/min [0.02 to 0.20 in./min]. The testing machine shall
grows, a poor delamination insert is often the cause. The
conform to the requirements of Practices E4. The testing
delamination may be too thick or deformed in such a way that
a resin pocket forms at the end of the insert. For accurate
initiation values, a properly implanted and inspected delami-
nation insert is critical (see 8.2).
5.3.3 Precracked Toughness—Under rare circumstances,
toughness may decrease from the initiation values as the
delamination propagates (see 5.3.2). If this occurs, the delami-
nation should be checked to ensure that it complies with the
insert recommendations found in 8.2. Only after verifying that
the decreasing toughness was not due to a poor insert, should
precracking be considered as an option. With precracking, a
delamination is first extended from the insert in Mode I, Mode
II,ormixedmode.Thespecimenisthenreloadedatthedesired
mode mixture to obtain a toughness value.
6. Interferences
6.1 Linear elastic behavior is assumed in the calculation of
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 D5528.
6.3 The nonlinear (NL) initiation value of toughness is
normallythemoreconservativevalue,butafewmaterialshave
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
toughness values from being determined. The use of initiation FIG. 5 Mixed-Mode Bending Fixture
D6671/D6671M − 22
machine shall be equipped with a clevis which can be attached 8. Sampling and Test Specimens
to the loading yoke of the MMB apparatus and an anvil on
8.1 Test laminates must contain an even number of plies,
which the base of the MMB apparatus can be placed.
and shall be unidirectional, with delamination growth occur-
ring in the 0° direction.
7.3 Load Indicator—The testing machine load-sensing de-
vice shall be capable of indicating the total load carried by the
8.2 Anonadhesiveinsertshallbeinsertedatthemidplaneof
testspecimen.Thisdeviceshallbeessentiallyfreefrominertia
the laminate during layup to form an initiation site for the
lag at the specified rate of testing and shall indicate the load
delamination(seeFig.6andFig.7).Thefilmthicknessshallbe
with an accuracy over the load range(s) of interest of within
no greater than 13µm [0.0005in.]. Specimens should not be
61% of the indicated value.
precracked.Bynotprecracking,aninitiationvaluefreeoffiber
bridging may be obtained (see 5.3.2). A polymer film is
7.4 Load Point Displacement Indicator—The load point
recommended for the insert to avoid problems with folding or
displacement may be taken from the crosshead separation of
crimping at the cut end of the insert as was observed for
theloadframeorfromanexternalgaugeattachedtotheMMB
aluminum foil inserts during round robin testing of DCB
apparatus. If the crosshead separation is used as the measure-
specimen, Test Method D5528 (6). For epoxy matrix compos-
ment of load point displacement, correction must be made for
ites cured at relatively low temperatures, 177°C [350°F] or
the compliance of the loading system, C which includes the
sys
less, a thin film made of polytetrafluoroethylene (PTFE) is
compliance of the load frame and the MMB apparatus. The
recommended. For composites with polyimide, bismaleimide,
compliance of the loading system must be measured at each
or thermoplastic matrices that are manufactured at relatively
leverlength ctobeusedduringtesting(see11.5).The C will
sys
high temperatures, greater than 177°C [350°F], a thin poly-
beusedintheequationforspecimenmodulustocorrectforthe
imide film is recommended. For materials outside the scope of
load system compliance.
this standard, different film materials may be required. If a
7.4.1 The load point displacement may be obtained from a
polyimidefilmisused,thefilmshallbepaintedorsprayedwith
properlycalibratedexternalgaugeortransducerattachedtothe
amoldreleaseagentbeforeitisinsertedinthelaminate.(Mold
MMB apparatus such as the linearly variable displacement
release agents containing silicone may contaminate the lami-
transducer (LVDT) shown in Fig. 1. The displacement indica-
nate by migration through the individual layers. It is often
tor shall indicate the load point displacement with an accuracy
helpful to coat the film at least once and then bake the film
of within 61% of the indicated value once the delamination
before placing the film on the composite. This will help to
occurs.Iftheloadpointdisplacementistakenfromanexternal
prevent silicone migration within the composite.)
gauge or transducer, the C value should be set to zero in the
sys
8.3 Specimen Dimensions:
specimen modulus equation (Eq 10).
8.3.1 As indicated in Fig. 6 and Fig. 7, the overall length of
7.5 Load Versus Load Point Displacement Record—An x-y
the specimen is not critical but will normally be around
plotter, or similar device, shall be used to make a permanent
137mm [5.5in.]. The width of the specimen shall be between
record during the test of load versus opening displacement at
20 to 25mm [0.8 to 1.0in.], inclusive.
the point of load application. Alternatively, the data may be
NOTE2—RoundrobintestingonnarrowandwideDCBspecimens,Test
stored digitally and postprocessed.
Method D5528, yielded similar results. Since the MMB specimen is
similar, the width of the MMB specimen is not considered a critical
7.6 Optical Microscope (Only for Propagation Option)—A
parameter.
traveling optical microscope with a magnification no greater
than 70×, or an equivalent magnifying device, shall be posi-
tionedononesideofthespecimentoobservethedelamination
front as it extends along one edge during the test visually.This
device shall be capable of pinpointing the delamination front
with an accuracy of at least 60.5 mm [60.02 in.]. A mirror
may be used to determine any discrepancy visually in delami-
nation onset from one side of the specimen to the other. Other
methods, such as crack length gauges bonded to a specimen
edge, may be used to monitor delamination length provided
their accuracy is as good as the optical microscope so that
delaminationlengthmaybemeasuredtotheaccuracyspecified
above.
7.7 The micrometer(s) shall use a suitable size diameter
ball-interface on irregular surfaces such as the bag side of a
laminate and a flat anvil interface on machined edges or very
smooth tooled surfaces. The accuracy of the instruments shall
be suitable for reading to within 1% of the sample width and
thickness.Fortypicalspecimengeometries,aninstrumentwith
an accuracy of 60.025 mm [0.001 in.] is desirable for
thickness and width measurements. FIG. 6 Specimen—MMB Test (SI Units)
D6671/D6671M − 22
L = half-span length of the MMB test apparatus, mm
[in.],
P = estimated value of critical load, N [lbf],
est
χ = crack length correction parameter,
δ = estimated load point of deflection, mm [in.], and
est
Γ = transverse modulus correction parameter.
8.5 Itisrecommendedthatvoidcontentandfibervolumebe
reported. Void content may be determined using the equations
of Test Methods D2734. The fiber volume fraction may be
determinedusingadigestionprocessperTestMethodsD3171.
8.6 Sampling—Test at least five specimens per test condi-
tionunlessvalidresultscanbegainedthroughtheuseoffewer
specimens, such as in the case of a designed experiment. For
statisticallysignificantdata,theproceduresoutlinedinPractice
E122 should be consulted. The method of sampling shall be
reported.
FIG. 7 Specimen—MMB Test (Inch-Pound Units)
8.7 Load Introduction—Load shall be introduced through
applied tabs. The tabs may be made from piano hinges as
shown in Fig. 6 and Fig. 7, or end blocks. The tabs shall be
applied such that the initial delamination length, measured
8.3.2 Panelsshallbemanufactured,andspecimenscutfrom
fromtheloadlinetotheendoftheinsert,is0.45L< a< L-3 h.
the panels as shown in Fig. 6 and Fig. 7. The insert length is
Thetabsshallbeatleastaswideasthespecimen(20to25mm
approximately 50mm [2in.] which corresponds to an initial
[0.8 to 1.0in.]). The tabs shall be made of a metal with
delamination length of approximately 25 mm [1in.] plus the
modulus greater than 60 000MPa, and shall be capable of
extra length required to apply the tabs. The end of the insert
sustaining the applied load without incurring damage across
should be accurately located and marked on the panel before
thewidth.Thetabsmaybeadhesivelybondedormechanically
cutting specimens.
applied. The load transfer region should not extend more than
8.4 Thelaminatethicknessshallnormallybebetween3and
3mm [0.1 in.] past the center of the loading axis toward the
5mm [0.12 and 0.2in.]. The variation in thickness for any
delamination tip to reduce specimen stiffening effects. To
given specimen shall not exceed 0.1mm [0.004in.]. The
reduce geometric nonlinearity, the center of the loading axis
thickness of the specimen may need to be increased to avoid
shall also be within 4mm [0.15in.] of the midplane of the
large applied displacements and therefore geometric nonlinear
specimen leg.An estimate of the load to be carried by the tab
errors as described in 13.2. Eq 2 and 3 can be used to select a
in the MMB test can be calculated from estimated values of
specimenthicknesstoachieveapermissibleamountofapplied
modulus, E and toughness, G , using the following equation:
11 c
displacement.
2 3 est
4c b h E G
2 3
11 c
4 3c 2 L a1hχ
~ ! ~ !
P 5 (4)
Œ
2 2
tab
a 117c 254cL121L
P
est
δ 5 (2)
3 2
est
8bE h L
3 4
2 3 3 where:
1~c1L! ~2L 13~a10.42hχ! !
P = expected load on the loading tab, N [lbf].
tab
8.7.1 Bonded Tabs—The bonding surfaces of the tabs and
the specimen shall be properly cleaned before bonding to
est 2 3 2 ensure load transfer without debonding of the tabs from the
G b E h L
C 11
specimen during the test. If debonding occurs, the specimen
P 5 (3)
est
should not be reused if there is physical evidence that a
2 2 2 2
!
~3c 2 L! ~a1hχ! 1 ~c1L! ~a10.42hχ!
delamination initiated when the bond failed, or if an increased
compliance is observed upon reloading.
where:
8.7.1.1 Surface Preparations of the Specimen—Thebonding
a = delamination length, mm [in.],
surfaceofthespecimenmaybelightlygritblastedorscrubbed
b = width of specimen, mm [in.],
c = lever length of the MMB test apparatus, mm [in.], with sandpaper, then wiped clean with a volatile solvent, such
E = longitudinal modulus of elasticity measured in as acetone or methylethylketone (MEK), to remove any con-
tension, MPa [psi], tamination.
E = transverse modulus of elasticity, MPa [psi],
22 8.7.1.2 Surface Preparation of the Loading Tabs—Theload-
G = shear modulus out of plane, MPa [psi],
ing tabs may be cleaned as in 8.7.1.1. If this procedure results
est
G = estimated value of total mixed-mode fracture
c
in a bond failure between the specimen and the tabs, it may be
2 2
toughness, kJ/m [in.-lbf⁄in. ],
necessary to apply a more sophisticated cleaning procedure
h = half thickness of test specimen, mm [in.],
based on degreasing and chemical etching. Consult Guide
D6671/D6671M − 22
D2651 for the surface preparation procedure that is most visual detect
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D6671/D6671M − 19 D6671/D6671M − 22
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 D6671/D6671M; 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.
1. Scope
1.1 This test method covers the determination of interlaminar fracture toughness, G , of continuous fiber-reinforced composite
c
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 method 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 test 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 Units—The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in
each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used
independently of the other, and values from the two systems shall not be combined.
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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.5 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.
2. Referenced Documents
2.1 ASTM Standards:
D883 Terminology Relating to Plastics
D2651 Guide for Preparation of Metal Surfaces for Adhesive Bonding
D2734 Test Methods for Void Content of Reinforced Plastics
D3171 Test Methods for Constituent Content of Composite Materials
D3878 Terminology for Composite Materials
This test method is under the jurisdiction of ASTM Committee D30 on Composite Materials and is the direct responsibility of Subcommittee D30.06 on Interlaminar
Properties.
Current edition approved March 15, 2019Feb. 1, 2022. Published April 2019March 2022. Originally approved in 2001. Last previous edition approved in 20132019 as
ε1
D6671/D6671M – 13D6671/D6671M – 19. . DOI: 10.1520/D6671_D6671M-19.10.1520/D6671_D6671M-22.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6671/D6671M − 22
D5229/D5229M Test Method for Moisture Absorption Properties and Equilibrium Conditioning of Polymer Matrix Composite
Materials
D5528 Test Method for Mode I Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites
E4 Practices for Force Calibration and Verification of Testing Machines
E6 Terminology Relating to Methods of Mechanical Testing
E122 Practice for Calculating Sample Size to Estimate, With Specified Precision, the Average for a Characteristic of a Lot or
Process
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E456 Terminology Relating to Quality and Statistics
3. Terminology
3.1 Terminology D3878 defines terms relating to high-modulus fibers and their composites. Terminology D883 defines terms
relating to plastics. Terminology E6 defines terms relating to mechanical testing. Terminology E456 and Practice E177 define terms
relating to statistics. In the event of conflict between terms, Terminology D3878 shall have precedence over the other terminology
standards.
NOTE 1—If the term represents a physical quantity, its analytical dimensions are stated immediately following the term (or letter symbol) in fundamental
dimension form, using the following ASTM standard symbology for fundamental dimensions, shown within square brackets: [M] for mass, [L] for length,
[T] for time, [u] for thermodynamic temperature, and [nd] for non-dimensional quantities. Use of these symbols is restricted to analytical dimensions when
used with square brackets, as the symbols may have other definitions when used without the brackets.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 crack opening mode (Mode I)—I), n—fracture mode in which the delamination faces open away from each other and no
relative crack face sliding occurs.
3.2.2 crack sliding mode (Mode II)—II), n—fracture mode in which the delamination faces slide over each other in the direction
of delamination growth and no relative crack face opening occurs.
3.2.3 mixed-mode fracture toughness, G [M/T ]—], n—the critical value of strain energy release rate, G, for delamination growth
c
in mixed-mode.
3.2.4 mixed-mode ratio, G /G [nd]—[nd], n—the ratio of Mode I strain energy release rate to Mode II strain energy release rate.
I II
3.2.5 mode mixture, G /G [nd]—[nd], n—fraction of Mode II to total strain energy release rate.
II
3.2.5.1 Discussion—
The mixed-mode ratio, G / G , is at times referred to instead of the mode mixture.
I II
3.2.6 Mode I strain energy release rate, G [M/T ]—], n—the loss of strain energy associated with Mode I deformation in the test
I
specimen per unit of specimen width for an infinitesimal increase in delamination length, da, for a delamination growing under
a constant displacement.
3.2.7 Mode II strain energy release rate, G [M/T ]—], n—the loss of strain energy associated with Mode II deformation in the
II
test specimen per unit of specimen width for an infinitesimal increase in delamination length, da, for a delamination growing under
a constant displacement.
3.2.8 strain energy release rate, G [M/T ]—], n—the loss of strain energy, dU, in the test specimen per unit of specimen width
for an infinitesimal increase in delamination length, da, for a delamination growing under a constant displacement; in mathematical
form,
1 dU
G 52 (1)
b da
where:
a = delamination length, mm [in.],
b = width of specimen, mm [in.],
D6671/D6671M − 22
2 2
G = total strain energy release rate, kJ/m [in.-lbf/in. ], and
U = total elastic strain energy in the test specimen, N-mm [in.-lbf].
3.3 Symbols:
a = delamination length, mm [in.]
a = initial delamination length, mm [in.]
o
a = propagation delamination lengths, mm [in.]
1-25
b = width of specimen, mm [in.]
b = width of calibration specimen, mm [in.]
cal
c = lever length of the MMB test apparatus, mm [in.]
c = lever length to center of gravity, mm [in.]
g
C = compliance, δ/P, mm/N [in./lbf]
C = calibration specimen compliance, δ/P, mm/N [in./lbf]
cal
C = system compliance, δ/P, mm/N [in./lbf]
sys
CV = coefficient of variation, %
E = longitudinal modulus of elasticity measured in tension, MPa [psi]
E = transverse modulus of elasticity, MPa [psi]
E = modulus of calibration bar, MPa [psi]
cal
E = modulus of elasticity in the fiber direction measured in flexure, MPa [psi]
1f
2 2
G = total strain energy release rate, kJ/m [in.-lbf/in. ]
G = shear modulus out of plane, MPa [psi]
G = shear modulus in plane, MPa [psi]
2 2
G = opening (Mode I) component of strain energy release rate, kJ/m [in.-lbf ⁄in ]
I
2 2
G = shear (Mode II) component of strain energy release rate, kJ/m [in.-lbf ⁄in ]
II
G /G = mode mixture
II
2 2
G = total mixed-mode fracture toughness, kJ/m [in.-lbf/in ]
c
est 2 2
G = estimated value of total mixed-mode fracture toughness, kJ/m [in.-lbf ⁄in ]
c
h = half thickness of test specimen, mm [in.]
L = half-span length of the MMB test apparatus, mm [in.]
m = slope of the load displacement curve, N/mm [lb/in.]
m = slope of the load displacement curve from calibration test, N/mm [lbf/in.]
cal
n = number of specimens
P = applied load, N [lbf]
P = critical load at 5%/max point of loading curve, N [lbf]
5%/max
P = estimated value of critical load, N [lbf]
est
P = weight of lever and attached apparatus, N [lbf]
g
P = critical load at nonlinear point of loading curve, N [lbf]
nl
P = expected load on the loading tab, N [lbf]
tab
P = critical load when delamination is observed to grow, N [lbf]
vis
S = standard deviation
n-1
t = thickness of calibration bar, mm [in.]
U = strain energy, N-mm [in.-lbf]
V = fiber volume fraction, %
xi = measured or derived property for an individual specimen from the sample population
x¯ = sample mean (average)
α = mode mixture transformation parameter for setting lever length
β = non-dimensional crack length correction for mode mixture
χ = crack length correction parameter,
E Γ
χ5Œ 32 2
H S DJ
11G 11Γ
δ = load point deflection, mm [in.]
δ = estimated load point deflection, mm [in.]
est
δ = maximum allowable load point of deflection, mm [in.]
max
D6671/D6671M − 22
Γ = transverse modulus correction parameter,
=E E
11 22
Γ5 1.18
G
4. Summary of Test Method
4.1 The Mixed-Mode Bending (MMB) test apparatus shown in Fig. 1 is used to load split laminate specimens to determine the
delamination fracture toughness at various ratios of Mode I to Mode II loading. The composite test specimen, shown in Fig. 2,
consists of a rectangular, uniform thickness, unidirectional laminated composite specimen, containing a nonadhesive insert at the
midplane which serves as a delamination initiator. Loading forces are applied to the MMB specimen via tabs that are applied near
the ends of the delaminated section of the specimen and through rollers that bear against the specimen in the nondelaminated
region. The base of the MMB apparatus holds the specimen stationary while the MMB lever loads the specimen. The base attaches
to the bottom specimen tab and also bears on the specimen near the far end with a roller. The lever attaches to the top tab and bears
down on the specimen halfway between the base roller and the tabs. The lever roller acts as a fulcrum so by pushing down on the
lever arm opposite the tab, the tab is pulled up. The length of the lever arm, c, can be changed to vary the ratio of the load pulling
on the tab to the load bearing through the roller, thus changing the mode mixture of the test. The load shall be applied to the lever
such that the load remains vertical during the loading process. To reduce geometric nonlinear effects as a result of lever rotation,
the lever shall be loaded such that the height of loading is slightly above the pivot point where the lever attaches to the test
specimen (1, 2).
4.2 A record of the applied load versus opening displacement is recorded on an x-y recorder, or equivalent real-time plotting device
or stored digitally and post-processed. The interlaminar fracture toughness, G , and mode mixture, G /G, are calculated from
c II
critical loads read from the load displacement curve.
5. Significance and Use
5.1 Susceptibility to delamination is one of the major weaknesses of many advanced laminated composite structures. Knowledge
of the interlaminar fracture resistance of composites 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 that the composite toughness
be measured at various mode mixtures. The toughness contour, in which fracture toughness is plotted as a function of mode
mixtures (see Fig. 3), is useful for establishing failure criterion used in damage tolerance analyses of composite structures made
from these materials.
5.2 This test method can serve the following purposes:
5.2.1 To establish quantitatively the effects of fiber surface treatment, local variations in fiber volume fraction, and processing and
environmental variables on G of a particular composite material at various mode mixtures,
c
5.2.2 To compare quantitatively the relative values of G versus mode mixture for composite materials with different constituents,
c
and
FIG. 1 MMB Apparatus
The boldface numbers in parentheses refer to athe list of references at the end of this standard.
D6671/D6671M − 22
FIG. 2 MMB Test Variables
FIG. 3 Mixed-Mode Summary Graph
5.2.3 To develop delamination failure criteria for composite damage tolerance and durability analyses.
5.3 This method can be used to determine the following delamination toughness values:
5.3.1 Delamination Initiation—Two values of delamination initiation shall be reported: (1) at the point of deviation from linearity
in the load-displacement curve (NL) and (2) at the point at which the compliance has increased by 5 % or the load has reached
a maximum value (5%/max) depending on which occurs first along the load deflection curve (see Fig. 4). Each definition of
delamination initiation is associated with its own value of G and G /G calculated from the load at the corresponding critical point.
c II
The 5%/Max G value is typically the most reproducible of the three G values. The NL value is, however, the more conservative
c c
number. When the option of collecting propagation values is taken (see 5.3.2), a third initiation value may be reported at the point
at which the delamination is first visually observed to grow on the edge of the specimen. The VIS point often falls between the
NL and the 5%/Max points.
5.3.2 Propagation Option—In the MMB test, the delamination will grow from the insert in either a stable or an unstable manner
depending on the mode mixture being tested. As an option, propagation toughness values may be collected when delaminations
grow in a stable manner. Propagation toughness values are not attainable when the delamination grows in an unstable manner.
Propagation toughness values may be heavily influenced by fiber bridging which is an artifact of the zero-degree-type test
specimen (3-5). Since they are often believed to be artificial, propagation values must be clearly marked as such when they are
reported. One use of propagation values is to check for problems with the delamination insert. Normally, delamination toughness
values rise from the initiation values as the delamination propagates and fiber bridging develops. When toughness values decrease
as the delamination grows, a poor delamination insert is often the cause. The delamination may be too thick or deformed in such
a way that a resin pocket forms at the end of the insert. For accurate initiation values, a properly implanted and inspected
delamination insert is critical (see 8.2).
FIG. 4 Load-Displacement Curves
D6671/D6671M − 22
5.3.3 Precracked Toughness—Under rare circumstances, toughness may decrease from the initiation values as the delamination
propagates (see 5.3.2). If this occurs, the delamination should be checked to ensure that it complies with the insert
recommendations found in 8.2. Only after verifying that the decreasing toughness was not due to a poor insert, should precracking
be considered as an option. With precracking, a delamination is first extended from the insert in Mode I, Mode II, or mixed mode.
The specimen is then reloaded at the desired mode mixture to obtain a toughness value.
6. Interferences
6.1 Linear elastic behavior is assumed in the calculation of G used in this test method. This assumption is valid when the zone
c
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 architectures is the same as described in Test Method D5528.
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 toughness values from being determined. The use of initiation toughness values could
result in nonconservative growth predictions in these select materials. The use of longer initial delaminations increases the
tendency for stable delamination growth.
7. Apparatus
7.1 The mixed-mode bending fixture, as seen in Fig. 5, uses a lever to load the MMB specimen. Using one applied load at the
end of the lever, a downward load is applied to the specimen center creating Mode II, while an upward force is applied to the split
end of the laminate creating Mode I. Machine drawings for an example of MMB apparatus may be found in Appendix X2, but
other designs that perform the necessary functions are acceptable. The half-span length of the MMB Apparatus L (see Fig. 2) shall
be 50 mm [2 in.]. To keep geometric nonlinear effects small, the loading height (the height of the loading point above the hinge
FIG. 5 Mixed-Mode Bending Fixture
D6671/D6671M − 22
point attaching the lever to the test specimen, as shown in Fig. 1) shall be 0.3 L, and the applied load shall remain vertical as the
apparatus is loaded. The load application to the lever and to the test specimen should allow sliding with minimal friction. In the
pictured apparatus, this is accomplished with roller bearings, but equivalent means are acceptable.
7.2 Testing Machine—A properly calibrated test machine shall be used which can be operated in a displacement control mode with
a constant displacement rate in the range of 0.5 to 5.0 mm/min [0.02 to 0.20 in./min]. The testing machine shall conform to the
requirements of Practices E4. The testing machine shall be equipped with a clevis which can be attached to the loading yoke of
the MMB apparatus and an anvil on which the base of the MMB apparatus can be placed.
7.3 Load Indicator—The testing machine load-sensing device shall be capable of indicating the total load carried by the test
specimen. This device shall be essentially free from inertia lag at the specified rate of testing and shall indicate the load with an
accuracy over the load range(s) of interest of within 61 % of the indicated value.
7.4 Load Point Displacement Indicator—The load point displacement may be taken from the crosshead separation of the load
frame or from an external gauge attached to the MMB apparatus. If the crosshead separation is used as the measurement of load
point displacement, correction must be made for the compliance of the loading system, C which includes the compliance of the
sys
load frame and the MMB apparatus. The compliance of the loading system must be measured at each lever length c to be used
during testing (see 11.5). The C will be used in the equation for specimen modulus to correct for the load system compliance.
sys
7.4.1 The load point displacement may be obtained from a properly calibrated external gauge or transducer attached to the MMB
apparatus such as the linearly variable displacement transducer (LVDT) shown in Fig. 1. The displacement indicator shall indicate
the load point displacement with an accuracy of within 61 % of the indicated value once the delamination occurs. If the load point
displacement is taken from an external gauge or transducer, the C value should be set to zero in the specimen modulus equation
sys
(Eq 10).
7.5 Load Versus Load Point Displacement Record—An x-y plotter, or similar device, shall be used to make a permanent record
during the test of load versus opening displacement at the point of load application. Alternatively, the data may be stored digitally
and postprocessed.
7.6 Optical Microscope (Only for Propagation Option)—A traveling optical microscope with a magnification no greater than 70×,
or an equivalent magnifying device, shall be positioned on one side of the specimen to observe the delamination front as it extends
along one edge during the test visually. This device shall be capable of pinpointing the delamination front with an accuracy of at
least 60.5 mm [60.02 in.]. A mirror may be used to determine any discrepancy visually in delamination onset from one side of
the specimen to the other. Other methods, such as crack length gauges bonded to a specimen edge, may be used to monitor
delamination length provided their accuracy is as good as the optical microscope so that delamination length may be measured to
the accuracy specified above.
7.7 The micrometer(s) shall use a suitable size diameter ball-interface on irregular surfaces such as the bag side of a laminate and
a flat anvil interface on machined edges or very smooth tooled surfaces. The accuracy of the instruments shall be suitable for
reading to within 1 % of the sample width and thickness. For typical specimen geometries, an instrument with an accuracy of
60.025 mm [0.001 in.] is desirable for thickness and width measurements.
8. Sampling and Test Specimens
8.1 Test laminates must contain an even number of plies, and shall be unidirectional, with delamination growth occurring in the
0° direction.
8.2 A nonadhesive insert shall be inserted at the midplane of the laminate during layup to form an initiation site for the
delamination (see Fig. 6 and Fig. 7). The film thickness shall be no greater than 13 μm [0.0005 in.]. Specimens should not be
precracked. By not precracking, an initiation value free of fiber bridging may be obtained (see 5.3.2). A polymer film is
recommended for the insert to avoid problems with folding or crimping at the cut end of the insert as was observed for aluminum
foil inserts during round robin testing of DCB specimen, Test Method D5528 (6). For epoxy matrix composites cured at relatively
low temperatures, 177 °C (350 °F)[350 °F] or less, a thin film made of polytetrafluoroethylene (PTFE) is recommended. For
composites with polyimide, bismaleimide, or thermoplastic matrices that are manufactured at relatively high temperatures, greater
than 177 °C (350 °F),[350 °F], a thin polyimide film is recommended. For materials outside the scope of this standard, different
film materials may be required. If a polyimide film is used, the film shall be painted or sprayed with a mold release agent before
D6671/D6671M − 22
FIG. 6 Specimen—MMB Test (SI Units)
FIG. 7 Specimen—MMB Test (Inch-Pound Units)
it is inserted in the laminate. (Mold release agents containing silicone may contaminate the laminate by migration through the
individual layers. It is often helpful to coat the film at least once and then bake the film before placing the film on the composite.
This will help to prevent silicone migration within the composite.)
8.3 Specimen Dimensions:
8.3.1 As indicated in Fig. 6 and Fig. 7, the overall length of the specimen is not critical but will normally be around 137 mm
[5.5 in.]. The width of the specimen shall be between 20 to 25 mm [0.8 to 1.0 in.], inclusive.
NOTE 2—Round robin testing on narrow and wide DCB specimens, Test Method D5528, yielded similar results. Since the MMB specimen is similar, the
width of the MMB specimen is not considered a critical parameter.
8.3.2 Panels shall be manufactured, and specimens cut from the panels as shown in Fig. 6 and Fig. 7. The insert length is
approximately 50 mm [2 in.] which corresponds to an initial delamination length of approximately 25 mm [1 in.] plus the extra
length required to apply the tabs. The end of the insert should be accurately located and marked on the panel before cutting
specimens.
8.4 The laminate thickness shall normally be between 3 and 5 mm [0.12 and 0.2 in.]. The variation in thickness for any given
specimen shall not exceed 0.1 mm [0.004 in.]. The thickness of the specimen may need to be increased to avoid large applied
D6671/D6671M − 22
displacements and therefore geometric nonlinear errors as described in 13.2. Eq 2 and 3 can be used to select a specimen thickness
to achieve a permissible amount of applied displacement.
2 3
4~3c 2 L! ~a1hχ!
P
est
δ 5 (2)
est 3 2
8bE h L
11 3 4
2 3 3
1 c1L 2L 13 a10.42hχ
~ ! ~ ~ ! !
est 2 3 2
G b E h L
C 11
P 5 (3)
est
2 2 2 2
! 3c 2 L a1hχ 1 c1L a10.42hχ
~ ! ~ ! ~ ! ~ !
where:
a = delamination length, mm [in.],
b = width of specimen, mm [in.],
c = lever length of the MMB test apparatus, mm [in.],
E = longitudinal modulus of elasticity measured in tension, MPa [psi],
E = transverse modulus of elasticity, MPa [psi],
G = shear modulus out of plane, MPa [psi],
est 2 2
G = estimated value of total mixed-mode fracture toughness, kJ/m [in.-lbf ⁄in. ],
c
h = half thickness of test specimen, mm [in.],
L = half-span length of the MMB test apparatus, mm [in.],
P = estimated value of critical load, N [lbf],
est
χ = crack length correction parameter,
δ = estimated load point of deflection, mm [in.], and
est
Γ = transverse modulus correction parameter.
8.5 It is recommended that void content and fiber volume be reported. Void content may be determined using the equations of Test
Methods D2734. The fiber volume fraction may be determined using a digestion process per Test Methods D3171.
8.6 Sampling—Test at least five specimens per test condition unless valid results can be gained through the use of fewer specimens,
such as in the case of a designed experiment. For statistically significant data, the procedures outlined in Practice E122 should be
consulted. The method of sampling shall be reported.
8.7 Load Introduction—Load shall be introduced through applied tabs. The tabs may be made from piano hinges as shown in Fig.
6 and Fig. 7, or end blocks. The tabs shall be applied such that the initial delamination length, measured from the load line to the
end of the insert, is 0.45L < a < L - 3 h. The tabs shall be at least as wide as the specimen (20 to 25 mm [0.8 to 1.0 in.]). The tabs
shall be made of a metal with modulus greater than 60 000 MPa, and shall be capable of sustaining the applied load without
incurring damage across the width. The tabs may be adhesively bonded or mechanically applied. The load transfer region should
not extend more than 3 mm [0.1 in.] past the center of the loading axis toward the delamination tip to reduce specimen stiffening
effects. To reduce geometric nonlinearity, the center of the loading axis shall also be within 4 mm [0.15 in.] of the midplane of the
specimen leg. An estimate of the load to be carried by the tab in the MMB test can be calculated from estimated values of modulus,
E and toughness, G , using the following equation:
11 c
2 3 est
4c b h E G
11 c
P 5 (4)
Œ
tab 2 2
a 117c 2 54cL121L
where:
P = expected load on the loading tab, N [lbf].
tab
8.7.1 Bonded Tabs—The bonding surfaces of the tabs and the specimen shall be properly cleaned before bonding to ensure load
transfer without debonding of the tabs from the specimen during the test. If debonding occurs, the specimen should not be reused
if there is physical evidence that a delamination initiated when the bond failed, or if an increased compliance is observed upon
reloading.
D6671/D6671M − 22
8.7.1.1 Surface Preparations of the Specimen—The bonding surface of the specimen may be lightly grit blasted or scrubbed with
sandpaper, then wiped clean with a volatile solvent, such as acetone or methylethylketone (MEK), to remove any contamination.
8.7.1.2 Surface Preparation of the Loading Tabs—The loading tabs may be cleaned as in 8.7.1.1. If this procedure results in a bond
failure between the specimen and the tabs, it may be necessary to apply a more sophisticated cleaning p
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