Williams, J. H., Jr.

English

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IMPROVEMENT OF COMPOSITE FRACTURE TOUGHNESS
BY FUSIBLE FIBERS AND COATINGS
by
James H. Williams, Jr.
Composite Materials and
Nondestructive Evaluation Laboratory
December 1976
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
^
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(KASA-CE-14S31C) ISPBOVEMJENI OF COMPOSITE
FBACIUfiE TOUGHNESS BY FUSIBLJs FIBiBS AND
COATINGS Final Eeport, Aug. 1975 - Dec.
1S76 (ftassachusetts Inst. of Tech.) 2C p
REPRODUCED BY
NATIONAL TECHNICAL
INFORMATION SERVICE
U. S. DEPARTMENT OF COMMERCE
SPRINGFIELD, VA. 22161
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- 7/^ 76
I M P R O V E M E N T OF C O M P O S I T E F R A C T U R E
T O U G H N E S S BY FUSIBLE FIBERS AND COATINGS
Final Report, August 1975-December 1976
(NASA Grant No. NSG-1217)
Submitted to the
Materials Division of the Langley
Research Center
(Grant Monitor: Dr. Wilbur B. Fichter)
by
James H. Williams, Jr.
Associate "Professor of Mechanical Engineering
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
December 1976
REPRODUCED BY
NATIONAL TECHNICAL
INFORMATION SERVICE
U. S. DEPARTMENT OF COMMERCE I
SPRINGFIELD, VA. 22161
ABSTRACT
The results of a program to improve the fracture toughness of graphite
fiber epoxy composites by thermoplastic fibers and fiber coatings are
summarized. The concept can provide the simultaneous improvement of both
composite strength and toughness, in particular, for laminates containing
fibers coated with polysulfone. Very limited tests indicate that fatigue
behavior may be improved by polysulfone coatings whereas polyvinylalcohol
coatings may degrade the fatigue behavior. The low velocity impact tough-
ness is increased by polyvinyl'alcohol coatings more than the increase
achieved by polysulfone coatings.
The fracture modes of the composites are affected substantially by the
choice of coating and stacking sequence. Thus, in instances where a
particular mode of fracture or crack propagation direction is desired,
the choice of coating, if any is used, is very important.
-1-
ACKNOWLEDGMENTS
This report represents the first general summary of results
relating to a concept which the author originated during 1973 when he
was holder of the Edgerton Young Faculty Professorship. Work on dis-
continuous reinforcing fibers was initiated at the Union Carbide R & D
Center during the summer of 1974 when the author held an NSF Industry
Participation Grant. That work was continued under this Grant (NSG-1217),
resulting in two publications and other preliminary analyses. The work
on the fiber coatings has been totally supported by this Grant which also
has subsidized a theoretical analysis of interlaminar stresses. (Refer
to Cumulated Bibliography of Issued Publications.) The support of the
Materials Division and Dr. W.B. Fichter the grant monitor, of the NASA
Langley Research Center is greatly appreciated.
-2-
TABLE OF CONTENTS
ABSTRACT 1
ACKNOWLEDGMENTS 2
TABLE OF CONTENTS 3
INTRODUCTION 4
MATERIALS •.. ... 5
STATIC STRENGTH AND FRACTURE TOUGHNESS 6
FATIGUE AND IMPACT TESTS 9
Fatigue 9
Impact 10
INTERLAMINAR STRESSES 13
CONCLUSIONS i 14
REFERENCES ... ; 15
CUMULATED BIBLIOGRAPHY OF ISSUED PUBLICATIONS 16
FIGURES 17
-3-
INTRODUCTION
One of the most common structural trade-offs in the utilization
of graphite fiber composites is strength versus toughness. The goal of
this research program was to improve the toughness characteristics of
continuous graphite fiber composites [1]. The attempt to achieve the
toughness improvements was by means of thermoplastic fibers and fiber
coatings. The characteristics of the coatings enable them to (1) control
fiber-matrix debonding; (2) contain individual fiber fractures, thus
decreasing fiber-fiber flaw communication; and (3) control fiber pull-
out. A model of the composite structure is shown in Fig. 1.
The following theoretical and experimental aspects of the proposed
research effort have been undertaken (See the "Cumulated Bibliography of
Issued Publications" section of this report.):
1) theoretical analyses of fiber intersections and the
orientation efficiency factor relating to the thermo-
plastic reinforcing fibers;
2) theoretical and experimental analyses of the static
strength and the fracture toughness of coated graphite
fiber composites;
3) fatigue and impact experiments on coated graphite fiber
composites;
4) a theoretical analysis of the unnotched interlaminar
stresses for arbitrary lamina stacking sequences.
Only items (2), (3) and (4) will be reviewed briefly here as item (1) is
already a part of the open technical literature.
-4-
MATERIALS
The specimens for all the tests were cut from nine (9) graphite
fiber epoxy panels (23cm x 23cm x 0.24cm) which were fabricated by
Fiber Materials Incorporated. The fibers were- Celanese GY-70. In
some of the panels, the fibers were coated with either polyyinylalcohol
(PVOH) or polysulfone (PPS). In all tests the specimens were grouped
according to the lamination construction (A: [0°, + 45°] ; .
s
B: [0°, ± 45°, 0°] ; C: Unidirectional) and further classified by
S
the fiber coatings (I: PVOH ; II: PPS ; III: No Coating). Extensive
details concerning the constituents, panels and fabrication procedures
are given in [ 2 ],
.-5-:
STATIC STRENGTH AND FRACTURE TOUGHNESS
The static strength and notched fracture toughness characteristics
are detailed in [2], These results are summarized in Table I below
where for each material the maximum value of the tensile strength (af )
for three unnotched specimens and the maximum value of the fracture
toughness for two notched specimens are reported. Note that the tensile
strength is normalized with respect to the fiber volume fraction. It can
be seen that the fiber coatings provide significant increases in both
°fr 3nd V
In Table II the typical fracture characteristics of the various
types of composites are summarized. Although, these descriptions are more
prominent in the notched specimens, they are typical for unnotched specimens
as well. Photographs of fractured specimens and additional discussions are
given in [2].
-6-
TABLE I
Maximum Values of Unnotched Static Strength
and Notched Fracture Toughness
La
m
in
at
e
i
[0°, + 45°]
s
[0°, ± 45°, 0°]s
[0 ]
PVOH
328
17.0
0.60:0.11
624
21.2
0.58:0.11
418
21.3
0.65:0.12
Fiber Coating
PPS
452
13.9
0.64:0.14
540
17.2
0.70:0.13
587
27.1
0.67:0.14
No Coating
375
8.5
0.52:0
396
8.7
0.46:0
664
6.8
0.42:0
•
°fr/Vf
KQ
VVc
KQ
°fr/V£
KQ
Vf:Vc
Unnotched Fracture Stress:
Approximate Fiber Volume Fraction:
a (MN/m )
V.
TIC
Approximate Coating Volume Fraction: V
Mode I Fracture Toughness: K (MN/m3-/2) 5
Q c
a is the notched fracture stress referred to the unnotched cross-
sectional area of the specimen.
c is the notch length having 0° flank angle.
All notched specimens except the PPS[0°] had a 3.68 cm width and
a 0.92 cm notch length; the corresponding dimensions for the
PPS[0°] notched specimens were 2.54 cm and 0.64 cm, respectively.
Matrix Elastic Modulus: 3930 MN/m2; Matrix Fracture Stress: 75 MN/m2.
Sll specimens had a total thickness of 0.24 cm.
—7—
TABLE II
Typical Fracture Characteristics of Composite Specimens
0)
4->
a
-c
•H
e
cs
i-q
[0°, + 45°]
~ s
[0°, + 45°, 0°]
s
[0°]
Fiber Coating
PVOH
Large amounts of
delamination. 0°
laminae shatter
extensively.
Major cracks prop-
agate along
interfaces* in the
+ 45° and - 45°
directions of the
respective laminae.
Major cracks, prop-
agate along inter-
faces of the + 45°
and - 45° laminae,
respectively,
accompanied by
moderate delam-
ination. 0° laminae
fracture same as
adjacent + 45° or
- 45° laminae.
Later delamination
ends, single saw-
tooth major crack
propagates hori-
zontally.
A single crack
propagates verti-
cally (parallel
to load axis)
along interfaces
all the way to the
specimen tabs,
resulting in a
complete separa-
tion of the speci-
men and tabs.
PPS
A single crack
propagates along
either a + 45° or
- 45° interface
with small amounts
of delamination.
When delamination
occurs,, major
cracks propagate
along interfaces
in both + 45° and
- 45° laminae
simultaneously.
Moderate delamina-
tion accompanied
by simultaneous
major cracks along
interfaces of
respective + 45°
and - 45° laminae.
0° laminae fracture
along + 45° or
- 45° direction in
accordance with
adjacent +45°
or - 45° laminae.
A single crack
propagates verti-
cally (parallel
to load axis)
toward tabs along
interfaces, then
horizontally, then
vertically in the
opposite direction
of initial motion.
(Occasionally ,
specimen behaves
same as" [0°] - No
Coating specimen.)
No Coating
A single crack
propagates
horizontally
.(orthogonal to
load axis) accom-
panied by small
amounts of delami-
nation in vicinity
of crack path.
A single crack
propagates hori-
zontally (ortho-
gonal to load
axis) accompanied
by small amounts
of delamination in
vicinity of crack
path.
A single crack
propagates horizon-
tally (orthogonal
to load axis) .
Smooth cleavage-
type surfaces are
created.
* Interfaces refer to the intralaminar. fiber-matrix, fiber-coating and
coating-matrix surfaces along the fiber direction.
-r-
FATIGUE AND IMPACT TESTS
Due to materials limitations, all the composites could not be
impact tested and only very abbreviated fatigue tests were conducted.
-These are summarized below.
Fatigue;
All specimens were sinusoidally fatigue loaded at 30 Hz in
a cantilevered bending mode with zero mean stress. Emphasis was
placed on testing two types of specimens, namely Bill ([0°, + 45°, 0°]
and no coating) and CI (Unidirectional and PVOH coating). In both cases
the 0° laminae were parallel to the longitudinal axis of the specimen.
Sufficient data to establish S-N curves were not generated in either
case as only ten (10) specimens of each material were available.
The Bill material was tested without, fracture to 5 x 10 cycles
when the oscillating stress in the extreme fibers was 0.75 of the
uniaxial static fracture stress (0" ). At a fatigue stress of 0.80 0 ,
the specimen fractured at 1.7 x 10 cycles, and at a fatigue stress of
4
0.90 a , the specimen fractured at about 2 x 10 cycles. In both the
fractured and unfractured specimens, no delamination was observed.
Fatigue data for the CI material was 1.2 x 10 cycles at 0.80 of
without rupture; 7.4 x 10 cycles at 0.88 a, without rupture; but almost
immediate rupture when the fatigue stress exceeded 0.90 a . Despite the
fact that the specimens did not rupture in a number of the tests,
significant fiber/coating/matrix debonding occurred. Also, in the
ruptured specimens, fiber pullout was evident.
-9-
Finally, in addition to the above tests, two (2) C II specimens
(unidirectional and PPS coating) were fatigued to 3 x 10 cycles at 0 9 a
fr
without rupture and with no visible debonding damage.
Impact
A pendulum impact tester located at the U.S. Army Materials &
Mechanics Research Center (Watertown, Mass.) was used. The rated capac-
ity of the tester was 22.4 J with an impact head weight of. 3.6 kg. The
impact velocity was 3.4 m/sec. The specimen geometry and loading are
shown in Fig. 2.
Because the impact tester was not in a state"of calibration, the
data obtained must be interpreted officially in a qualitative sense*.
Despite this, significant. trends, are indicated by the data obtained.
The average normalized^impact energies for a minimum of three of each
type of specimen are given in Table III.
Unnotched specimens were tested to determine the notch sensitiv-
ity. The results indicate that .the unnotched specimens had normalized
impact energies that were approximately 60% to 70% higher than those of
their notched counterparts.
This statement is made in accordance with an agreement dated
May 24, 1976 and signed by the author.
t The impact energies are normalized with respect to the net cross-
sectional area at the notch. These values have not been normalized
with respect to the fiber volume fractions.
-10-
TABLE III
Approximate Notched Impact Energies (kJ/m )
'
L
am
in
at
e
[0°, + 45°]
s
[0°, ± 45°, 0°]
S
[0°]
Fiber Coating
PVOH
No Data
49 kJ/m2
57 kJ/m2
PPS
34 kJ/m2
21 kJ/m2
30 kJ/m2
No Coating
6 kJ/m2
5 kJ/m2
No Data
-11-
The impact fracture behavior of the various specimens is quite
consistent with the descriptions in Table II. PVOH coating resulted
in significant intra-ply shattering and delamination; PPS coating
produced no intra-ply shattering but some delamination along the
fracture path; and the uncoated fiber composites displayed a single
cleavage-type fracture plane with no visible damage away from the
fracture surface.
-12-
INTERLAMINAR STRESSES
A theoretical formulation of the interlaminar stresses in a
fiber composite laminate having an arbitrary stacking sequence and
subjected to uniform axial extension has been developed [3]. Assuming
a set of three-dimensional displacement functionals, a variational
principle was used to derive the governing equations which were solved
in closed-form. A set of examples remains to be computed.
-13-
CONCLUSIONS
The results of a program to improve the fracture toughness of
graphite fiber epoxy composites by thermoplastic fibers and fiber
coatings have been reviewed. The effects of polyvinylalcohol and
polysulfone fiber coatings were investigated. Polyvinylalcohol coatings
provide the greater impact toughness improvement although it appears
that it degrades the fatigue strength. Polysulfone coatings enhance
strength, notched fracture toughness and low-velocity impact toughness.
Although the effect of polysulfone coatings on fatigue behavior is not
clear, it appears to be favorable.
The various aspects of the program which have been considered
are represented by the cumulated bibliography. Further development
of the concept appears to be attractive although its effectiveness
must be compared with those of competing concepts.
-14-
REFERENCES
1. Williams, J.H., Jr., "Improvement of Composite Fracture
Toughness by Fusible Fibers and Coatings", Proposal submit-
ted to the Materials Division of the NASA Langley Research
Center, March 1975.
2. Kousiounelos, P.N., "Fracture of Graphite Fiber Composites",
S.B. and S.M. Thesis, Massachusetts Institute of Technology,
May 1976.
3. Bin Ahmad, Z. and Williams, J.H., Jr., "Theoretical Formula-
tion of Interlaminar Stresses in Arbitrarily Laminated
Composites", Composite Materials and Nondestructive Evaluation
Laboratory, Department of Mechanical Engineering, Massachusetts
Institute of Technology, December 1976.
-15-
..CUMULATED BIBLIOGRAPHY OF ISSUED PUBLICATIONS
The documents listed below were issued during the conduct of the
research program and were supported either wholly or partially by funds
from the subject grant.
1. Williams, J.H., Jr., Lee, S.S., and Wasserman, C.S., "Fibre
Intersections in a Planar Randomly Oriented Fibre Composite",
(To appear in Fibre Science and Technology).
2. Williams,' J.H. Jr. and Bosy, B.J., "Fibre Orientation
Efficiency Factor for Planar Randomly Oriented Discontinuous
Fibre Composites", (To appear in Fibre Science and Technology).
3. Kousiounelos, P.N., "Fracture of Graphite Fiber Composites",
S.B. and S.M. Thesis, Department of Mechanical Engineering,
Massachusetts Institute of Technology, May 1976.
4. Williams, J.H., Jr. and Kousiounelos, P.N., "Thermoplastic
Fiber Coatings Enhance Composite Strength and Toughness",
(Submitted for publication).
5. Kousiounelos, P.N. and Williams, J.H., Jr., "Heterogeneous
Anisotropic Model for Notched Fiber Composites", (Submitted
for publication).
»
6. Bin Ahmad, Z. and Williams, J.H., Jr., "Theoretical Formula-
tion of Interlaminar Stresses in Arbitrarily Laminated
Composites", Composite Materials and Nondestructive Evaluation
Laboratory, Department of Mechanical Engineering, Massachusetts
Institute of Technology, December 1976.
-16-
Continuou's
Graphite Fibers
X
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Reinforcing
Thermoplastic -
Fibers
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4 ;yI* ''*7 '-'
*.• ;'
X
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•«.
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^
X.
--*•
s^
f* .:
- •'
1
*
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\
vj
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'•«=
\/
J
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\ i
i -
I \k. <
T- S
, ;
f
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I ^
ni ts
MMpi'
p:
[Jj
i( i-'/ -/
If-!
'Hi
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^^
—
—
>^
fi
Pt
,;,
i
\ -
I
H
1i" -
~\ • 'i
I ;
-" ";
i, ^
(^
" Thermoplastic
Rber Coatings
,^
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\
^ - " -
Thermoset Matrix
Fig. 1 Model of unidirectional graphite fiber composite showing thermoplastic
fiber coatings and thermoplastic reinforcing fibers.
-17-
Support
E
0
10
'
t
—
S '
Support/
ftx
t0.2cm
t 1 cm
Orientation of
0° Laminae
/•
Direction and Location of
< n i i -, Imnnft J-nrf^Oi
«
• ^
f"^ Specimen Thickness
^ 0.24 cm
Fig. 2 Sketch of impact specimen showing geometry, orientation, loading
and support.
-18-


Improvement of composite fracture toughness by fusible fibers and coatings

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