Carbon fibers were obtained from several manufacturers. Surface treatments were performed on these fibers by anodization. The surfaces of these fibers were analyzed by X-ray photoelectron spectroscopy and wetting force measurement. The breaking strength of these fibers was measured at 2.5 cm length. It was seen that the surface treatments reduces the strength of the fibers. It was also seen that the Hercules fibers had a higher breaking strength than the Union Carbide fibers. Fiber critical length measurements showed no difference in critical lengths between AS-4 and AU-4 fibers embedded in polysulfone. However, the fiber lengths were much shorter for the surface treated fibers. This effect could be related to increased adhesion between fiber and matrix, or it could be due to the lower breaking strength of the surface treated fiber

Devilbiss, T. A.

Wightman, J. P.

English

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VIRGINIA TECH
CENTER FOR ADHESION SCIENCE
[NASA-CR-177271) FRACTURE SURFACE ANALYSIS N86-27427
IN COMPOSITE AND TITANIUM BONDING Annual
Report (Virginia Polytechnic lest. ,and State
Oniv.) 14 p HC A02/MF &01 CSC1 11D Unclas
G3/24 43133
ANNUAL REPORT
FRACTURE SURFACE ANALYSIS IN
COMPOSITE AND TITANIUM BONDING
BY
T. A. DeVILBISS AND J. P. WIGHTMAN
VIRGINIA POLYTECHNIC INSTITUTE
AND STATE UNIVERSITY
216 NORRIS HALL
BLACKSBURG, VIRGINIA 24061
Telephone: (703) 961-6824
Telex: 7108753699
https://ntrs.nasa.gov/search.jsp?R=19860017955 2020-03-20T14:10:44+00:00Z
ANNUAL REPORT
FRACTURE SURFACE ANALYSIS IN COMPOSITE
AND TITANIUM BONDING
BY
T. A. DeVILBISS AND J. P. WIGHTMAN
v
 PREPARED FOR
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
NASA - LANGLEY RESEARCH CENTER
MATERIALS DIVISION
HAMPTON, VA 23665
D. J. PROGAR
GRANT #NAG-1-343
from
Chemistry Department
Virginia Polytechnic Institute and State University
Blacksburg, VA 24061
April, 1986
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INTRODUCTION
>k \i-pry
Thermoplastic matrices used for composites do not undergo polymerization
reaction after being incorporated into the composite as thermosetting resins
do. This fact leads to several consequences. The first is that
thermoplastics will be less likely to form chemical bonds with the fiber.
Another is that these thermoplastics will have a much higher viscosity when
melted to apply them to the fiber than the thermoset monomers (oligomers).
Thus, it will be much more difficult to obtain contact between fiber and
matrix.
The interphase between fiber and matrix can have a large influence on the
mechanical properties of composites. The properties of this interphase can be
controlled by surface treating the fibers and by sizing the fiber. Surface
treatment can change the functionality of the fiber surface as well as its
morphology. Sizing typically consists either of a reactive low molecular
weight compound, or else a dilute solution of a thermoplastic resin. The
reactive groups on the low molecular weight compound are chosen so that they
can react with both the fiber surface and the matrix. The work reported here
is aimed at studying the effect of the properties of the carbon fiber surface
on the fiber/matrix interaction.
EXPERIMENTAL
Carbon Fibers
The fibers used in this study included Hercules AS-4 and AU-4, two types
of Union Carbide T-300 one untreated and one surface treated but unsized.
Surface Treatment
Surface treatments were performed on Hercules AU-4 carbon fibers. The
treatments included anodization in 0.5M sodium hydroxide (0.5M NaOH) and 0.5M
sulfuric acid (0.5M H2$04) at 6 volts for 2 min. The current densities were
approximately 3.3 and 4.8 amperes per square meter, respectively.
X-ray Photoelectron Spectroscopy
The surfaces of the fibers were analysed by X-ray photoelectron
spectroscopy (XPS) in a Perkin-Elmer Phi 5300 electron spectrometer. The
fibers were mounted on the instrument probe with silver paint.
Wetting Force Measurement
The wetting force of carbon fibers were measured in five liquids of
varying polar and dispersive components using a Perkin-Elmer TGA-2
microbalance. The fibers were glued to a wire hook with a cyanoacrylate
adhesive. The hook was then placed on the microbalance and the liquid raised
up to the wetting liquid. Six fibers were measured for each surface treatment
and liquid. The liquids used were: water, ethylene glycol, formamide,
methylene iodide, and bromonaphthalene. Polar and dispersive components of
the fiber surface energy were determined using the technique described in the
previous report(l).
Breaking Strength
The breaking strength of the fibers were measured at 2.54 cm length. The
fibers were glued across a 2.5 cm. hole in a paper tab. A cyanoacrylate
adhesive was used as the adhesive. The paper tab was then mounted in a table
model Instron with a 20 gm capacity load cell using alligator clips. The
paper was burned away with a nichrome wire attached to a wall outlet rheostat
set at about 10 volts. The fiber was then pulled in tension with a crosshead
speed of 5 mm per minute. 20-30 fibers were broken for each fiber of
interest. Breaking strength was also measured as a function of length. The
fibers were mounted in paper tabs similar to the ones described above except
that they had either 12.5 mm or 6 "mm holes in them.
Fiber Critical Length
To measure the interaction of the matrix with the fiber, a fiber critical
length test was performed as described by Wadsworth and Spilling (2). In this
test, an annealed (500°F, 30 min-oven cool) coupon (15 cm X 15 cm) of A1100
aluminum was coated with a 5 gm/100 ml solution of UDEL F1700 polysulfone in
methylene chloride. Fibers were then individually placed on the coated
aluminum spaced about 5 mm apart. The fibers were then coated with the 5
gm/100 ml solution. After the solvent was allowed to evaporate, the coupons,
fiber, polymer system was annealed at 250°C in a vacuum oven for 12 hours.
The coupons were cooled in the oven. After cooling, the coupons were cut into
2.5 cm X 15 cm strips and pulled in tension to 30% strain in an Instron
testing machine. The broken fiber lengths were measured on a microscope with
a micrometer stage.
RESULTS AND DISCUSSION
X-Ray Photoelectron Spectroscopy
The XPS results are shown in Figure 1. This figure shows the carbon Is
photopeaks for each fiber. Also listed next to the photopeaks are the atomic
percentages of elements detected. It can be seen that AS-4 has a higher
nitrogen content than AU-4. It was shown in the previous report (1) that the
AS-4 surface was rich in amine groups. This is most likely a result of the
proprietary surface treatment. The surface treatment on the T-300 fiber
increased the oxygen content. The functionality is thus due to oxygen
containing compounds. The sulfuric acid anodization increased the oxygen
content of the AU-4 fiber. It can also be seen from the shape of the carbon
peak that there seems to be more carbon in the higher oxidation states than
the other treatments.
Wetting Force Measurements
The polar (y ) and dispersive (Yd) components of the surface energy of
the carbon fibers are listed in Table I. There is a lot of scatter in this
data and it is doubted if any of the differences are statistically
significant. The only conclusion that may be drawn is that the T-300 fibers
appear to have a lower polar contribution to their surface energy.
Breaking Strength
The breaking strengths of several of the carbon fibers (length: 2.54 cm)
listed in Table II. It can be seen from this that the surface treatments used
are reducing the strength of the fiber slightly. The breaking strengths as a
function of length are plotted in Figure 2. The means and standard deviations
of each plotted point are tabulated in Table III. It appears that the AS-4
fiber increases in strength sharply at shorter lengths. The AU-4 increases
less sharply. It should be noted that the AS-4 and AU-4 were obtained from
different batches. The differences observed may result from this fact rather
than from the effect of the surface treatment. The breaking strength of the
Union Carbide fibers was independent of length. These effects are due to the
density and distribution of flaws on the surface of and within the fiber.
Care should be used in analysing these data since the standard deviations
listed in Table III are rather large. In fact from these, no real significant
differences can be observed. It is felt however that a distribution analysis
of the breaking strengths will show that there are some differences.
Fiber Critical Lengths
Carbon fibers normally break at around 5% strain. It would be expected
that stretching to 30% strain would result in the fiber breaking once and then
no further breaks observed since the stress cannot be transferred through a
broken fiber. However, the matrix can transfer stress to the fiber.
Measurement of the lengths of the broken fibers is thus a measure of the
ability of the matrix to transfer stress to the fiber. One problem that
arises in the analysis is that the strength of the fiber will also influence
the length at which the fibers break. Since the fibers are breaking at around
0.25-0.5 mm, it would be necessary to know the breaking strength of the fibers
at this short length.
The fiber critical lengths are tabulated in Table IV. There is
apparently no difference between the AS-4 and AU-4. The surface treated
fibers had a shorter fiber length indicating that there was stronger
interfacial adhesion to these fibers. However, the strengths of these fibers
have not been taken into consideration. Therefore, it cannot be concluded at
this time that the surface treatments used did in fact promote adhesion to the
fiber.
SUMMARY
Carbon fibers were obtained from several manufacturers. Surface
treatments were performed on these fibers by anodization. The surfaces of
these fibers were analysed by XPS and wetting force measurement. The breaking
strength of these fibers was measured at 2.5 cm length. It was seen that the
surface treatments reduces the strength of the fibers. It was also seen that
the Hercules fibers had a higher breaking strength than the Union Carbide
fibers. Fiber critical length measurements showed no difference in
critical lengths between AS-4 and AU-4 fibers embedded in polysulfone.
However, the fiber lengths were much shorter for the surface treated fibers.
This effect could be related to increased adhesion between fiber and matrix,
or it could be due to the lower breaking strength of the surface treated
fiber.
REFERENCES
1) DeVilbiss, T. A. and Wightman, J. P., "Fracture Surface Analysis in
Composite and Titanium Bonding", Grant #NAG-l-343, September (1985).
2) Wadsworth, N. J. and Spilling, I., "Load Transfer from Broken
Fibers in Composite Materials", Brit. J. Appld. Phys. (J. Phys. D)
ser. 2, 1, 1049 (1968).
TABLE I
SURFACE ENERGY ANALYSIS
Fiber Tp 'd
Cel ion 6000
AS-4
AU-4
NaOH Anodized
HpSO* Anodized
HN03 Boi led
T-300 No Shear
T-300 Shear
18.1
22.0
18.2
21.0
22.4
18.1
9.5
11.1
29.6
29.4
28.1
28.8
33.7
29.6
32.9
35.8
TABLE II
BREAKING STRENGTH OF CARBON FIBERS
Fiber
Celion 6000
AS- 4
AU-4
NaOH Anodized
H2S04
T-300 No Shear
T-300 Shear
S(GPa)
2.48
2.87
2.71
2.21
2.12
2.20
2.24
0
0.51
0.87
0.79
0.78
0.95
0.39
0.38
2.54 cm length
TABLE III
MEAN BREAKING STRENGTH (X) AND STANDARD DEVIATION (a) OF
CARBON FIBERS AT VARIOUS LENGTHS
2.54 cm
Fiber
AS-4
AU-4
T-300
T-300
X
14
13
11
shear 11
.7
.9
.3
.5
0
4.
4.
2.
1.
5
1
0
9
X
16.
16.
10.
11.
1.27 c
a
7
1
8
4
3.
3.
2.
2.
1
1
7
9
X
20
17
13
12
0.63
.4
.1
.6
.8
cm
0
3.2
3.3
2.4
3.0
TABLE IV
FIBER CRITICAL LENGTHS OF FIBERS CAST IN 5% UDEL P1700
Fiber Fiber Length (mm) a
AS-4 0.45 0.12
AU-4 0.42 0.11
H2S04 Anodized 0.27 ' 0.07
NaOH Anodized 0.34 0.08
Hercules AU-4
C-79.0 %
0- U.2 %
N- 1.8%
Si- 5.0 %
•300 295 290 285
Binding Energy (evl
230
Hercules AS-4
C-80.0%
o- u.s%
N- 5.5%
300 295 290 28S
Binding Energy 1«V|
280
Union Carbide T-300
C - 9 5 . 8 %
o- 2.0%
N- 3.1 %
300 295 290 235
Binding Energy (eVI
230
Union Carbide T-300 s
C - 7 8 . 3 %
0- 19.5%
N- 2.2%
300 29S 290 285
Binding Energy ( e V t
230
(0.5M)
C- 68 .4%
0- 28.9%
N - 1.5%
S- 0 .0%
Si- 1.2%
295 290 235
Binding Energy leVl
230
C - 7 5.6 %
0 - 17.0%
N - 0.9%
Na- 6.
290
Figure 1. XPS C Is photopeak of commercially available and anodized carbon
fibers.
OCD
—i
(D
C
Z. CH
ato
a
3
c/i
O - A S - 4
A -AU-4
D -T-300
o -T -3005
0.25 0.5
L ena f h ( inches )
1.0
Figure 2. Breaking strength versus length of carbon fibers.


Fracture surface analysis in composite and titanium bonding

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