ASTM D6115-97(2011)
(Test Method)Standard Test Method for Mode I Fatigue Delamination Growth Onset of Unidirectional Fiber-Reinforced Polymer Matrix Composites
Standard Test Method for Mode I Fatigue Delamination Growth Onset of Unidirectional Fiber-Reinforced Polymer Matrix Composites
SIGNIFICANCE AND USE
Susceptibility to delamination is one of the major weaknesses of many advanced laminated composite structures. Knowledge of a laminated composite material’resistance to interlaminar fracture under fatigue loads is useful for product development and material selection. Furthermore, a measurement of the relationship of the mode I cyclic strain energy release rate and the number of cycles to delamination growth onset, G–N, that is independent of specimen geometry or method of load introduction, is useful for establishing design allowables used in damage tolerance analyses of composite structures made from these materials.
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–N of a particular composite material.
5.2.2 To compare quantitatively the relative values of G–N for composite materials with different constituents.
5.2.3 To develop criteria for avoiding the onset of delamination growth under fatigue loading for composite damage tolerance and durability analyses.
SCOPE
1.1 This test method determines the number of cycles (N) for the onset of delamination growth based on the opening mode I cyclic strain energy release rate (G), using the Double Cantilever Beam (DCB) specimen shown in . This test method applies to constant amplitude, tension-tension fatigue loading of continuous fiber-reinforced composite materials. When this test method is applied to multiple specimens at various G-levels, the results may be shown as a G-N curve, as illustrated in Fig 2.
1.2 This test method is limited to use with composites consisting of unidirectional carbon fiber tape laminates with single-phase polymer matrices. This limited scope reflects the experience gained in round robin testing. This test method may prove useful for other types and classes of composite materials, however, certain interferences have been noted (see Section 6.5 of Test Method D 5528).
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.
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Standards Content (Sample)
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Designation: D6115 − 97 (Reapproved 2011)
Standard Test Method for
Mode I Fatigue Delamination Growth Onset of Unidirectional
Fiber-Reinforced Polymer Matrix Composites
This standard is issued under the fixed designation D6115; 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 D2584 Test Method for Ignition Loss of Cured Reinforced
Resins
1.1 This test method determines the number of cycles (N)
D2651 GuideforPreparationofMetalSurfacesforAdhesive
for the onset of delamination growth based on the opening
Bonding
mode I cyclic strain energy release rate (G), using the Double
D2734 TestMethodsforVoidContentofReinforcedPlastics
Cantilever Beam (DCB) specimen shown in Fig. 1. This test
D3171 Test Methods for Constituent Content of Composite
method applies to constant amplitude, tension-tension fatigue
Materials
loading of continuous fiber-reinforced composite materials.
D3878 Terminology for Composite Materials
When this test method is applied to multiple specimens at
D5229/D5229M TestMethodforMoistureAbsorptionProp-
various G-levels, the results may be shown as a G–N curve, as
erties and Equilibrium Conditioning of Polymer Matrix
illustrated in Fig. 2.
Composite Materials
1.2 This test method is limited to use with composites
D5528 TestMethodforModeIInterlaminarFractureTough-
consisting of unidirectional carbon fiber tape laminates with
ness of Unidirectional Fiber-Reinforced Polymer Matrix
single-phase polymer matrices. This limited scope reflects the
Composites
experience gained in round robin testing.This test method may
E4 Practices for Force Verification of Testing Machines
proveusefulforothertypesandclassesofcompositematerials,
E6 Terminology Relating to Methods of Mechanical Testing
however,certaininterferenceshavebeennoted(seeSection6.5
E122 Practice for Calculating Sample Size to Estimate,With
of Test Method D5528).
Specified Precision, the Average for a Characteristic of a
1.3 The values stated in SI units are to be regarded as Lot or Process
E177 Practice for Use of the Terms Precision and Bias in
standard. No other units of measurement are included in this
standard. ASTM Test Methods
1.3.1 Exception—Thevaluesprovidedinparenthesesarefor E456 Terminology Relating to Quality and Statistics
E467 Practice for Verification of Constant Amplitude Dy-
information only.
namic Forces in an Axial Fatigue Testing System
1.4 This standard does not purport to address all of the
E691 Practice for Conducting an Interlaboratory Study to
safety concerns, if any, associated with its use. It is the
Determine the Precision of a Test Method
responsibility of the user of this standard to establish appro-
E739 PracticeforStatisticalAnalysisofLinearorLinearized
priate safety and health practices and determine the applica-
Stress-Life (S-N) and Strain-Life (ε-N) Fatigue Data
bility of regulatory limitations prior to use.
E1049 Practices for Cycle Counting in Fatigue Analysis
E1150 Definitions of Terms Relating to Fatigue (Withdrawn
2. Referenced Documents
1996)
2.1 ASTM Standards:
D883 Terminology Relating to Plastics
3. Terminology
3.1 Terminology D3878 defines terms relating to high-
modulus fibers and their composites. Terminology D883 de-
This specification is under the jurisdiction of ASTM Committee D30 on
fines terms relating to plastics. Terminology E6 defines terms
Composite Materials and is the direct responsibility of Subcommittee D30.06 on
relating to mechanical testing. Terminology E456 and Practice
Interlaminar Properties.
Current edition approved Aug. 1, 2011. Published December 2011. Originally E177 define terms relating to statistics. Definitions E1150
approved in 1997. Last previous edition approved in 2004 as D6115 – 97 (2004).
defines terms relating to fatigue. In the event of conflict
DOI: 10.1520/D6115-97R11.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The last approved version of this historical standard is referenced on
the ASTM website. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6115 − 97 (2011)
3.3.10 G —opening mode I interlaminar fracture tough-
Ic
ness.
3.3.11 [G ] —average values of G from the quasi-static
Ic av Ic
tests.
3.3.12 G —maximum or peak cyclic mode I strain en-
Imax
ergy release rate.
3.3.13 G–N—relationship between the cyclic strain energy
release rate and the number of cycles to onset of delamination
growth.
3.3.14 h—thickness of DCB specimen.
3.3.15 N—number of elapsed fatigue cycles.
FIG. 1 DCB Specimen with Piano Hinges
3.3.16 N —application dependent value of N at which
a
delamination growth onset will occur.
1 %
3.3.17 N —number of fatigue cycles for the value of P
a max
at N = 1 to decrease by 1 %.
between terms, Terminology D3878 shall have precedence
ViS
3.3.18 N —numberoffatiguecyclesatwhichtheonsetof
a
over the other terminology standards.
delamination growth is observed.
3.2 Definitions of Terms Specific to This Standard:
5 %
3.3.19 N —number of fatigue cycles for the value of P
a max
3.2.1 crack opening mode (Mode I)—fracture mode in
at N = 1 to decrease by 5 %.
which the delamination faces open away from each other and
in which these faces do not undergo any relative sliding.
3.3.20 P—applied load.
3.2.2 cycles to onset of delamination growth, N —the num-
3.3.21 P —value of load at the onset of delamination
a
cr
ber of fatigue cycles elapsed until the onset of delamination
growth from the insert in the quasi-static tests.
growth from an implanted thin insert.
3.3.22 P —maximum cyclic load.
max
3.2.3 fatigue delamination growth onset relationship,
3.3.23 R—ratio of minimum and peak loads P /P .
min max
G–N—the relationship between the peak cyclic value of strain
energy release rate to the number of fatigue cycles until the 3.3.24 SD—standard deviation.
onset of delamination growth, N .
a
3.3.25 U—strain energy.
3.2.4 mode I interlaminar fracture toughness, G —thecriti-
Ic
3.3.26 V —fiber volume fraction, %.
f
cal value of G for delamination growth because of an opening
3.3.27 δ—load point deflection.
load or displacement.
3.3.28 δ —value of displacement at the onset of delamina-
cr
3.2.5 strain energy release rate, G—the loss of strain
tion growth from the insert in a quasi-static test.
energy, dU, in the test specimen per unit of specimen width for
3.3.29 δ —maximum value of cyclic displacement.
an infinitesimal increase in delamination length, da, for a
max
delamination growing under a constant displacement. In math- 3.3.30 δ —mean value of cyclic displacement.
mean
ematical form:
3.3.31 δ —minimum value of cyclic displacement.
mm
3.3.32 ∆—effective delamination extension to correct for
1 dU
G52 (1)
rotation of DCB arms at delamination front.
b da
3.3.33 [∆] —average value of∆ from the quasi-static tests.
av
where:
U = total elastic strain energy in the test specimen,
4. Summary of Test Method
b = specimen width, and
a = delamination length. 4.1 The Double Cantilever Beam (DCB) shown in Fig. 2 is
described in Test Method D5528.
3.3 Symbols:
3.3.1 a—delamination length.
4.2 The DCB specimen is cycled between a minimum and
maximum displacement, δ , and δ , at a specified fre-
3.3.2 a —initial delamination length. min max
quency. For linear elasticity and small deflections (δ/a < 0.4)
3.3.3 b—width of DCB specimen.
the displacement ratio, δ / δ , is identical to the R-ratio.
min max
3.3.4 C—compliance, δ/P, of DCB specimen.
The number of displacement cycles at which the onset of
3.3.5 CV—coefficient of variation, %.
delaminationgrowthoccurs, N ,isrecorded.ThemodeIcyclic
a
strain energy release rate, for example the maximum value,
3.3.6 da—infinitesimal increase in delamination length.
G is calculated using a modified beam theory or other
Imax
3.3.7 dU—infinitesimal increase in strain energy.
methods described in Test Method D5528. By testing several
3.3.8 E —modulus of elasticity in the fiber direction.
II
specimens a relationship is developed between G and N
Imax a
3.3.9 G—strain energy release rate. for the chosen frequency.
D6115 − 97 (2011)
versus the peak cyclic strain energy release rate for the DCB is
very high. Therefore, small variations in the peak cyclic strain
energy release rate will result in large changes in the delami-
nationgrowthrate.Forthesetworeasons,thistestmethoddoes
not monitor the fatigue delamination growth rate. Instead, this
test method monitors the number of cycles until the onset of
delamination growth from the end of a thin insert. A value of
G may be defined such that delamination growth will not occur
until N cycles have elapsed, where N is defined by the
a a
application, Fig. 1.
6.3 Three definitions to determine the number of cycles
until the onset of delamination growth were used during an
investigative round robin. These include: (1) the number of
cycles until the delamination was visually observed to grow at
FIG. 2 G–N Curve
ViS
the edge, N ;(2) the number of cycles until the compliance
a
1%
hadincreasedby1 %, N (thisisapproximatelyequivalentto
a
5. Significance and Use
a 1 % decrease in the maximum cyclic load; and (3) the
5.1 Susceptibility to delamination is one of the major number of cycles until the compliance has increased by 5 %,
weaknesses of many advanced laminated composite structures. 5%
N (this is approximately equivalent to a 5 % decrease in the
a
Knowledge of a laminated composite material’s resistance to maximum cyclic load). The three techniques gave different
1%
interlaminar fracture under fatigue loads is useful for product
results but the N value is typically the lowest of the three
a
development and material selection. Furthermore, a measure-
values and is recommended for generating a conservative
ment of the relationship of the mode I cyclic strain energy
criterion for avoiding onset of fatigue delamination growth in
release rate and the number of cycles to delamination growth
durability and damage tolerance analyses of laminated com-
onset, G–N, that is independent of specimen geometry or
posite structures. Because of the difficulties in visually moni-
method of load introduction, is useful for establishing design
toringtheendofadelaminationduringafatiguetest,thevisual
allowables used in damage tolerance analyses of composite
method is not included in this test method.
structures made from these materials.
6.4 The test frequency may affect results. If the test fre-
5.2 This test method can serve the following purposes:
quency is high, heating effects may occur in the composite. To
5.2.1 To establish quantitatively the effects of fiber surface
avoid these effects, frequency should be chosen to be between
treatment, local variations in fiber volume fraction, and pro-
1and10cyclespersecond(Hz)andshouldbechosensuchthat
cessing and environmental variables on G–N of a particular
there is no temperature change of the specimen. Other test
composite material.
frequencies may be used if they are more appropriate for the
5.2.2 To compare quantitatively the relative values of G–N
application. The test frequency shall be reported.
for composite materials with different constituents.
6.5 The displacement ratio, δ / δ , may have a large
min max
5.2.3 To develop criteria for avoiding the onset of delami-
effect on the results. Because the DCB specimen cannot be
nation growth under fatigue loading for composite damage
tested in compression the displacement ratio must remain
tolerance and durability analyses.
withinthefollowingrange:0≤δ /δ <1.Thedisplacement
min max
ratio shall be reported. Large deflections may be considered by
6. Interferences
using the corrections given in the Annex of Test Method
6.1 Linear elastic behavior is assumed in the calculation of
D5528.
G used in this test method. This assumption is valid when the
6.6 The application to other materials, lay-ups and architec-
zone of damage or non-linear deformation at the delamination
tures is described in Test Method D5528.
front, or both, is small relative to the smallest specimen
dimension, which is typically the specimen thickness for the
7. Apparatus
DCB test.
7.1 Testing Machine—A properly calibrated test machine
6.2 As the delamination grows under fatigue, fiber bridging
shall be used that can be operated in a displacement control
observed in quasi-static testing (see Test Method D5528) may
mode. The testing machine shall conform to the requirements
also occur. Fiber bridging inhibits the fatigue delamination
of Practices E4 and E467. The testing machine shall be
growth resulting in slower growth rates than if there was no
equipped with grips to hold the loading hinges, or pins to hold
bridging.Thisresultsinartificiallyhighthresholdvalueswhere
4 the loading blocks, that are bonded to the specimen.
the delamination ceases to grow or grows very slowly. In
7.2 Load Indicator—The testing machine load sensing de-
addition, the rate of change of the delamination growth rate
vice shall be capable of indicating the total load carried by the
test specimen. This device shall be essentially free from
Martin, R. H. and Murri, G. B., “Characterization of Mode I and Mode II
Delamination Growth and Thresholds in AS4/PEEK Composites,” Composite
Materials: Testing and Design (9th Volume), ASTM STP 1059, S. P. Garbo, Ed.,
1990, pp. 251 –270. Preliminary data from D30.06 round robin.
D6115 − 97 (2011)
inertia-lag at the specified rate of testing and shall indicate the 9. Calibration
load with an accuracy over the load range(s) of interest of
9.1 The accuracy of all measuring equipment shall have
within 61 % of the indicated value. The peak cyclic load shall
certified calibrations that are current at the time of use of the
not be less than 10 % of the full scale of the load cell. Section
equipment.
8.2detailshowtoestimatetheexpectedpeakcyclicload.Ifthe
current load cell capacity of the test stand is too large, a low
10. Conditioning
load capacity load cell may be placed in series.
10.1 Standard Conditioning Procedure—Condition in ac-
7.3 Opening Displacement Indicator—The opening dis-
cordance with Procedure C of Test Method D5229/D5229M
placement may be estimated as the crosshead separation or
unless a different environment is specified as part of the
actuator displacement provided the deformation of the testing
experiment. Store and test specimens
...
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