ASTM D6115-97

Standard Test Method for Mode I Fatigue Delamination Growth Onset of Unidirectional Fiber-Reinforced Polymer Matrix Composites

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 specimen shown in Fig 1. 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.

General Information

• Status: Historical
• Publication Date: 09-Apr-1997
• ICS: 83.120 - Reinforced plastics
• Technical Committee: D30 - Composite Materials
• Drafting Committee: D30.06 - Interlaminar Properties

Relations

• Replaced By: ASTM D6115-97(2004) - Standard Test Method for Mode I Fatigue Delamination Growth Onset of Unidirectional Fiber-Reinforced Polymer Matrix Composites
• Effective Date: 01-Mar-2004
• Referred By: ASTM D4762-18 - Standard Guide for Testing Polymer Matrix Composite Materials
• Effective Date: 10-Apr-1997
• Referred By: ASTM D6856/D6856M-03(2016) - Standard Guide for Testing Fabric-Reinforced “Textile” Composite Materials
• Effective Date: 10-Apr-1997

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: D 6115 – 97
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 D 6115; 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
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 specimen shown in Fig. 1. 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
FIG. 1 DCB Specimen with Piano Hinges
experience gained in round robin testing. This test method may
prove useful for other types and classes of composite materials,
D 3171 Test Method for Fiber Content of Resin-Matrix
however, certain interferences have been noted (see Section 6.5
Composites by Matrix Digestion
of Test Method D 5528).
D 3878 Terminology of High-Modulus Reinforcing Fibers
1.3 The values stated in SI units are to be regarded as
and Their Composites
standard. The values provided in parentheses are for informa-
D 5229/D 5229M Test Method for Moisture Absorption
tion only.
Properties and Equilibrium Conditioning of Polymer Ma-
1.4 This standard does not purport to address all of the
trix Composite Materials
safety concerns, if any, associated with its use. It is the
D 5528 Test Method for Mode I Interlaminar Fracture
responsibility of the user of this standard to establish appro-
Toughness of Unidirectional Fiber-Reinforced Polymer
priate safety and health practices and determine the applica-
Matrix Composites
bility of regulatory limitations prior to use.
E 4 Practices for Force Verification of Testing Machines
2. Referenced Documents
E 6 Terminology Relating to Methods of Mechanical Test-
ing
2.1 ASTM Standards:
E 122 Practice for Choice of Sample Size to Estimate a
D 883 Terminology Relating to Plastics
Measure of Quality for a Lot or Process
D 2584 Test Method for Ignition Loss of Cured Reinforced
E 177 Practice for Use of the Terms Precision and Bias in
Resins
ASTM Test Methods
D 2651 Guide for Preparation of Metal Surfaces for Adhe-
E 456 Terminology Relating to Quality and Statistics
sive Bonding
E 467 Practice for Verification of Constant Amplitude Dy-
D 2734 Test Method for Void Content of Reinforced Plas-
namic Loads on Displacements in an Axial Load Fatigue
tics
Testing System
E 691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
This specification is under the jurisdiction of ASTM Committee D-30 on High
Modulus Fibers and Their Composites and is the direct responsibility of Subcom-
mittee D30.06 on Interlaminar Properties.
Current edition approved April 10, 1997. Published August 1997.
2 5
Annual Book of ASTM Standards, Vol 08.01. Annual Book of ASTM Standards, Vol 15.03.
3 6
Annual Book of ASTM Standards, Vol 08.02. Annual Book of ASTM Standards, Vol 03.01.
4 7
Annual Book of ASTM Standards, Vol 15.06. Annual Book of ASTM Standards, Vol 14.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6115
E 739 Practice for Statistical Analysis of Linear or Linear- 3.3.14 h—thickness of DCB specimen.
ized Stress-Life (S–N) and Strain-Life e-N Fatigue Data 3.3.15 N—number of elapsed fatigue cycles.
E 1049 Practices for Cycle Counting in Fatigue Analysis 3.3.16 N —application dependent value of N at which
a
E 1150 Definitions Relating to Fatigue delamination growth onset will occur.
1 %
3.3.17 N —number of fatigue cycles for the value of P
a max
3. Terminology
at N 5 1 to decrease by 1 %.
ViS
3.3.18 N —number of fatigue cycles at which the onset
3.1 Terminology D 3878 defines terms relating to high- a
of delamination growth is observed.
modulus fibers and their composites. Terminology D 883
5 %
3.3.19 N —number of fatigue cycles for the value of P
defines terms relating to plastics. Terminology E 6 defines a max
at N 5 1 to decrease by 5 %.
terms relating to mechanical testing. Terminology E 456 and
3.3.20 P—applied load.
Practice E 177 define terms relating to statistics. Definition
3.3.21 P —value of load at the onset of delamination
E 1150 defines terms relating to fatigue. In the event of conflict
cr
growth from the insert in the quasi-static tests.
between terms, Terminology D 3878 shall have precedence
3.3.22 P —maximum cyclic load.
over the other terminology standards.
max
3.3.23 R—ratio of minimum and peak loads P /P .
3.2 Definitions of Terms Specific to This Standard: min max
3.3.24 SD—standard deviation.
3.2.1 crack opening mode (Mode I)—fracture mode in
3.3.25 U—strain energy.
which the delamination faces open away from each other and
3.3.26 V —fiber volume fraction, %.
in which these faces do not undergo any relative sliding.
f
3.3.27 d—load point deflection.
3.2.2 cycles to onset of delamination growth, N —the num-
a
3.3.28 d —value of displacement at the onset of delamina-
ber of fatigue cycles elapsed until the onset of delamination
cr
tion growth from the insert in a quasi-static test.
growth from an implanted thin insert.
3.3.29 d —maximum value of cyclic displacement.
3.2.3 fatigue delamination growth onset relationship,
max
3.3.30 d —mean value of cyclic displacement.
G–N—the relationship between the peak cyclic value of strain mean
3.3.31 d —minimum value of cyclic displacement.
energy release rate to the number of fatigue cycles until the
mm
3.3.32 D—effective delamination extension to correct for
onset of delamination growth, N .
a
rotation of DCB arms at delamination front.
3.2.4 mode I interlaminar fracture toughness, G —the
Ic
3.3.33 [D] —average value of D from the quasi-static tests.
critical value of G for delamination growth because of an av
opening load or displacement.
4. Summary of Test Method
3.2.5 strain energy release rate, G—the loss of strain
4.1 The Double Cantilever Beam (DCB) shown in Fig. 2 is
energy, dU, in the test specimen per unit of specimen width for
described in Test Method D 5528.
an infinitesimal increase in delamination length, da, for a
delamination growing under a constant displacement. In math-
ematical form:
1 dU
G52 (1)
b da
where:
U 5 total elastic strain energy in the test specimen,
b 5 specimen width, and
a 5 delamination length.
3.3 Symbols:
3.3.1 a—delamination length.
3.3.2 a —initial delamination length.
3.3.3 b—width of DCB specimen.
3.3.4 C—compliance, d/P, of DCB specimen.
3.3.5 CV—coefficient of variation, %.
FIG. 2 G–N Curve
3.3.6 da—infinitesimal increase in delamination length.
3.3.7 dU—infinitesimal increase in strain energy.
3.3.8 E —modulus of elasticity in the fiber direction. 4.2 The DCB specimen is cycled between a minimum and
II
3.3.9 G—strain energy release rate. maximum displacement, d , and d , at a specified fre-
min max
3.3.10 G —opening mode I interlaminar fracture tough- quency. For linear elasticity and small deflections (d/a < 0.4)
Ic
ness. the displacement ratio, d / d , is identical to the R-ratio.
min max
3.3.11 [G ] —average values of G from the quasi-static The number of displacement cycles at which the onset of
Ic av Ic
tests. delamination growth occurs, N , is recorded. The mode I cyclic
a
3.3.12 G —maximum or peak cyclic mode I strain en- strain energy release rate, for example the maximum value,
Imax
ergy release rate. G is calculated using a modified beam theory or other
Imax
3.3.13 G–N—relationship between the cyclic strain energy methods described in Test Method D 5528. By testing several
release rate and the number of cycles to onset of delamination specimens a relationship is developed between G and N
Imax a
growth. for the chosen frequency.
D6115
5. Significance and Use a 1 % decrease in the maximum cyclic load; and (3) the
number of cycles until the compliance has increased by 5 %,
5.1 Susceptibility to delamination is one of the major
N5% (this is approximately equivalent to a 5 % decrease in the
weaknesses of many advanced laminated composite structures. a
maximum cyclic load). The three techniques gave different
Knowledge of a laminated composite material’s resistance to
1%
results but the N value is typically the lowest of the three
a
interlaminar fracture under fatigue loads is useful for product
values and is recommended for generating a conservative
development and material selection. Furthermore, a measure-
criterion for avoiding onset of fatigue delamination growth in
ment of the relationship of the mode I cyclic strain energy
durability and damage tolerance analyses of laminated com-
release rate and the number of cycles to delamination growth
posite structures. Because of the difficulties in visually moni-
onset, G–N, that is independent of specimen geometry or
toring the end of a delamination during a fatigue test, the visual
method of load introduction, is useful for establishing design
method is not included in this test method.
allowables used in damage tolerance analyses of composite
6.4 The test frequency may affect results. If the test fre-
structures made from these materials.
quency is high, heating effects may occur in the composite. To
5.2 This test method can serve the following purposes:
avoid these effects, frequency should be chosen to be between
5.2.1 To establish quantitatively the effects of fiber surface
1 and 10 cycles per second (Hz) and should be chosen such that
treatment, local variations in fiber volume fraction, and pro-
there is no temperature change of the specimen. Other test
cessing and environmental variables on G–N of a particular
frequencies may be used if they are more appropriate for the
composite material.
application. The test frequency shall be reported.
5.2.2 To compare quantitatively the relative values of G–N
6.5 The displacement ratio,d / d , may have a large
min max
for composite materials with different constituents.
effect on the results. Because the DCB specimen cannot be
5.2.3 To develop criteria for avoiding the onset of delami-
tested in compression the displacement ratio must remain
nation growth under fatigue loading for composite damage
within the following range: 0 # d /d < 1. The displace-
tolerance and durability analyses. min max
ment ratio shall be reported. Large deflections may be consid-
ered by using the corrections given in the Annex of Test
6. Interferences
Method D 5528.
6.1 Linear elastic behavior is assumed in the calculation of
6.6 The application to other materials, lay-ups and architec-
G used in this test method. This assumption is valid when the
tures is described in Test Method D 5528.
zone of damage or non-linear deformation at the delamination
front, or both, is small relative to the smallest specimen
7. Apparatus
dimension, which is typically the specimen thickness for the
7.1 Testing Machine—A properly calibrated test machine
DCB test.
shall be used that can be operated in a displacement control
6.2 As the delamination grows under fatigue, fiber bridging
mode. The testing machine shall conform to the requirements
observed in quasi-static testing (see Test Method D 5528) may
of Practices E 4 and E 467. The testing machine shall be
also occur. Fiber bridging inhibits the fatigue delamination
equipped with grips to hold the loading hinges, or pins to hold
growth resulting in slower growth rates than if there was no
the loading blocks, that are bonded to the specimen.
bridging. This results in artificially high threshold values where
8 7.2 Load Indicator—The testing machine load sensing de-
the delamination ceases to grow or grows very slowly. In
vice shall be capable of indicating the total load carried by the
addition, the rate of change of the delamination growth rate
test specimen. This device shall be essentially free from
versus the peak cyclic strain energy release rate for the DCB is
inertia-lag at the specified rate of testing and shall indicate the
very high. Therefore, small variations in the peak cyclic strain
load with an accuracy over the load range(s) of interest of
energy release rate will result in large changes in the delami-
within 61 % of the indicated value. The peak cyclic load shall
nation growth rate. For these two reasons, this test method does
not be less than 10 % of the full scale of the load cell. Section
not monitor the fatigue delamination growth rate. Instead, this
8.2 details how to estimate the expected peak cyclic load. If the
test method monitors the number of cycles until the onset of
current load cell capacity of the test stand is too large, a low
delamination growth from the end of a thin insert. A value of
load capacity load cell may be placed in series.
G may be defined such that delamination growth will not occur
7.3 Opening Displacement Indicator—The opening dis-
until N cycles have elapsed, where N is defined by the
a a
placement may be estimated as the crosshead separation or
application, Fig. 1.
actuator displacement provided the deformation of the testing
6.3 Three definitions to determine the number of cycles
machine, with the specimen grips attached, is less than 2 % of
until the onset of delamination growth were used during an
the maximum cyclic opening displacement of the test speci-
investigative round robin. These include: (1) the number of
men. If not, then the opening displacement shall be obtained
cycles until the delamination was visually observed to grow at
ViS
from a properly calibrated external gage or transducer attached
the edge, N ; (2) the number of cycles until the compliance
a
1% to the specimen. The displacement indicator shall indicate the
had increased by 1 %, N (this is approximately equivalent to
a
crack opening displacement with an accuracy of within 61%
of the indicated value once the delamination occurs.
7.4 Micrometers—As described in Test Method D 5528.
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.
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