Because of their low cost, excellent electrical conductivity, high specific strength (strength/density), and high specific modulus (modulus/density) short metal fiber reinforced composites have enjoyed a widespread use in many critical applications such as automotive industry, aircraft manufacturing, national defense, and space technology. However, little data has been found in the study of short metal fibrous composites. Optimum fiber concentration in a resin matrix and fiber aspect ratio (length-to-diameter ratio) are often not available to a user. Stress concentration at short fiber ends is the other concern when the composite is applied to a load-bearing application. Fracture in such composites where the damage will be initiated or accumulated is usually difficult to be determined. An experimental investigation is therefore carefully designed and undertaken to systematically evaluate the mechanical properties as well as electrical properties. Inconel 601 (nickel based) metal fiber with a diameter of eight microns is used to reinforce commercially available thermoset polyester resin. Mechanical testing such as tensile, impact, and flexure tests along with electrical conductivity measurements is conducted to study the feasibility of using such composites. The advantages and limitations of applying chopped metal fiber reinforced polymeric composites are also discussed

Chung, Wenchiang R.

English

NASA Technical Reports Server

N9()-24357
THE ASSESSMENT OF METAL FIBER REINFORCED POLYMERIC COMPOSITES
Wenchiang R. Chung"
Division of Technology
San Jose State University
San Jose, California
ABSTRACT
Because of their low cost, excellent electrical conductivity,
high specific strength (strength/density), and high specific
modulus (modulus/density) short metal fiber reinforced composites
have enjoyed a widespread use in many critical applications such
as automotive industry, aircraft manufacturing, national defense,
and space technology. However, little data has been found in the
study of short metal fibrous composites. Optimum fiber
concentration in a resin matrix and fiber aspect ratio
(length-to-diameter ratio) are often not available to a user.
Stress concentration at short fiber ends is the other concern when
the composite is applying to a load-bearing application. Fracture
in such composites where the damage will be initiated or
accumulated is usually difficult to be determined. An
experimental investigation is therefore carefully designed and
undertaken to systematically evaluate the mechanical properties as
well as electrical properties. In this study, Inconel 601 (nickel
based) metal fiber with a diameter of eight microns will be used
to reinforce commercially available thermoset polyester resin.
Mechanical testing such as tensile, impact, and flexure tests
along with electrical conductlvity measurements will be conducted
to study the feasibility of using such composites. The advantages
and limitations of applying chopped metal fiber reinforced
polymeric composites will also be discussed.
INTRODUCTION
Over the years, polymers have been well known for their
electrical insulating properties and great strides have been made
in electrical and electronic applications, mainly related to
electrical insulation. Consequently, research has been directed
to improve the dielectric strength of polymers so that they can be
used for better insulators, in the past ten years, with the
advent of electrically conductive polymers, their potential to
perform as active roles in conducting electricity has been
discovered and realized (ref. I) . Recent polymer researches have
revealed that polymers can indeed conduct electricity as well as
metals. Now the electrically conductive polymers can be used as
antistatic coatings, fuel cell catalysts, solar electrical ce_=0
photoelectrodes in a photogalvanic cell, protective coatings on
electrodes in photo_!ectro-chemical cells, and as lightweight,
inexpensive batteries.
* Presentedby Seth P. Bat, cs - San Jc6eSzateUmvemity.
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Add to this developing need for electrophotographic industry, due
to the increasing need of lightweight, low cost, moldable, and
high specific strength for defense and high tech applications, it
is expected that the development of electrically conductive
polymeric materials will grow strongly and significantly. On the
other hand, conductive plastic housings and molded parts can also
be beneficial to the controls of electromagnetic interference
shielding (EMI) and electrostatic charge discharge (ESD).
Advanced research studies have shown that there are three possible
methods to make polymers conductive. The first approach is to
apply a thin conductive coating onto the molded part. This
approach, however, is costly and not efficient because of
involving a two-step operation which increases the difficulties in
obtaining a good adhesion as well as a uniform coating. The
second approach is often held by synthesis or by doping (ref.
2-5). Synthesis is done by side reactions. One of the major side
reactions involves the benzene ring. Other reactions lead to
branched and cross-linked polymers. Doping involves oxidation and
reduction reactions. This method, usually produces polymeric
compounds such as polyacetylene and polyphenylene, althoughitlhasbeen[
proven:.to be effective and has been widely used, problems rise
from conductive polymers themselves such as their processability,
stability, mechanical and physical properties, etc. The last
approach, proposed in this study, is to incorporate electrically
conductive fillers in the polymeric resin matrix. Many conductive
materials such as carbon, metals, metal-coated fillers in the form
of particles, particulates, and fibers can be randomly dispersed
into a resin matrix and form a so-called "conductive composite."
This approach so far appears to be a viable solution to the
development of conductive polymers. Due to the lack of systematic
research study in this area, limited data can be found to help
research scientists, engineers in industries in the application of
these materials. More importantly, much research is urgently
needed to fully understand the interrelationships among structure,
property and processing prior to their commercial utilizations
(ref. 1-6).
The conductive polymeric composite was first presented in
1966 by Garland (ref. 7). He used silver particles, approximately
50 to 200 microns in diameter, to reinforce a thermoset
phenol-formaldehyde (Bakelite) resin matrix. His experimental
data indicated that metal-filled polymers undergo a sharp
transition from an insulator to a conductor at a critical volume
concentration of metal fillers. In his study the electrical
resistivity remained almost constant until the silver volume
concentration of 38% is reached - then it droped drastically and
the whole composite became an electrical conductor. Since
Gurland's work, many other researchers have reported different
sharp transition from insulators to conductors at different volume
concentrations (ref. 8-15) .
46
Among their studies, Dearaujo and his co-investigator (ref. I0)
had found that normally at least 40% volume fraction of metal
fillers was needed in order to make a composite conductive. In
the curing process of a resin matrix, they also suggested that a
slow curing rate at lower temperatures would greatly enhance the
conductivity of the composite.
Recently, because short fiber reinforced composites can
offer design flexibility, weight reduction, energy savings and
high-volume production for structural applications, they are
widely used in automotive, recreation, business machinery,
electrical appliance7 and military applications. Metal fiber
reinforced composites become highly desirable to meet the
aforementioned requirements not only for load-bearing capability
but electrical conductivity as well, which normally metal particle
reinforcement cannot achieve. However, not much work has been
done in this area. Experimental data were found only limited to
individual cases. Davenport (ref. 16) mentioned in his study that
the metal fiber length (L) to diame-ter (D) ratio (known as aspect
ratio) in a composite must have i00 or more in order to induce
electrical conductivity. He demonstrated the electrical
conductivity should be a function of L/D. In addition, the fiber
packing density is a significant factor which is closely
associated with the ratio of L/D (ref. 17). Bigg and Stutz
investigated a stainless steel fiber (8 micrcns in diameter,
aspect ratio: 750) reinforced ABS system, and found that the
composite had an electrical resistivity of 0.70 ohm-cm at the
fiber volume concentration of 1% (ref. 18). They also claimed in
their research that a highly conductive composite can be achieved
with a low concentration of metal fibers by simply using high
aspect ratio fibers. Their work seems very promising,
yet needs to be proved. Most of the metal fiDer reinforced
composites emphasized the electrical properties rather
than the mechanical properties.
Nickel has long been considered as a preferred metal because of
its low electrical resistivity. In this study, Inconel 601 nickel
based fiber with a dimeter of 8 microns and an aspect ratio of 125
was heavily used to reinforce a commercially available thermoset
polyester resin. Composite samples were made in coupon shapes
depending cn the test requirements. Both mechanical and
electrical measurements were further conducted to help understand
the micromechanical behavicr as well as electrical conductivity.
47
SPECIMEN PREPARATION AND TESTING
Chopped Inconel 601 metal fibers were donated by Bekaert
Fiber Technologies. To prevent the sizing effect from the
interfacial bonding between fiber and resinmatrix, a thin
water-soluble PVA (polyvinyl alcohol) coating originally attached
to fibers was removed from Inconel fibers prior to the process.
Fiber volume concentration, varied from 0% to 50%, was carefully
controlled asa material parameter to conduct this study. Metal
fibers were completely mixed with an appropriate amount of polyester
resin and MEKP (Methyl Ehtyl Ketone Peroxide) curing agent in a
chemical beaker based on a predetermined volume ratio. The mixure
was then poured into an aluminum mold for cure. Traditional
compression molding practice was employed in the curing process;
pressure was around 17 psi (1.17 x l0 s Pa) and temperature was
set at 356°F (180°C). Specimen dimensions were carefully
prepared according to ASTM standard test methods; they were 6 x
3/4 x 1/8 in. (152.4 x 19.05 x 3.18 mm) size for tensile test, 5 x
1/2 x 1/4 in. (127 x 12.7 x 6.35 mm) size for Izod impact test,
and 4 x I/Z x 1/2 in. (101.6 x 3.18 x 12.7 mm) for flexure bars.
The tensile and flexure tests were performed in a screw-driven
computer-asisted Satec testing machine. A testing speed of 0.i
in./min. (2.54 mm/min.) was used for tensile and flexure tests.
The ASTM method D257 was also followed to measure the volume
resistance of each sample. The test data were collected and
discussed in the following sections.
RESULTS AND DISCUSSION
Tensile test data, as shown in Table l, have demonstrated
that fiber concentration can indeed increase the tensile strength
of the composite. Young's modulus is also improved as well. It
is interesting to note that the small fiber concentration at the
ratio lower than i0 volume percent will not contribute to the
increase of entire tensile strength. According to the study,
fiber concentration at 25 % has the maximum UTS. It is found that
fiber orientation along the pulling direction will have
significant effect to tensile properties. Since the specimens are
prepared through a casting process, the fiber orientation in all
directions is assumed the equal. Impact test data (in Table 2)
reveal that impact strength increases with the addition of metal
fibers. However, there is a limitation set at 35%. Low fiber
concentration impairs the impact strength of the composite.
Optical microscopy indicates that because of the existing of metal
fibers, small air bubbles _r_ attached to fiber ends, which is
believed to be responsible f:r the degraded impact strength.
Thr_e point 'z.ex_ure) test data show tha_ fiber fillers can
improve the f!exural strength and tangen_ modulus of _he composite,
_s shown in Table 3. _ is a .... noticed that metal fibers can
dissipate scme energy i:: a cr_ck propagation.
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In other words, with the addition of metal fibers the crack
pattern of a given composite shifted from a pure tension failure
mode toward a more shear failure modu, which increases the
flextural properties. Fiber pull-outs and fiber breakage are some
evidence. In the electrical measurements, a critical fiber
concentration is recorded. Electrical resistivity of 1.0 ohm-cm
is measured at the fiber volume ratio of 45%, which is unexpectedly
high. Figure 1 shows fiber concentration below 30%; the electrical
resistance remains almost constant, that is the composite is still an
electrical insulator. In this study, metal fiber reinforced
composites did undergo a sharp transition which is in concert with
Garland's work (ref. 7).
CONCLUSION
Because of excellent electrical conductors, metal fibers are
suitable additives for inducing electrical conductivity in
traditionally known insulators, polymer materials. Inconel metal
fibers, although proved to be effective reinforcing elements, are
considerably more dense than expected. It is found, during this
study, that the explanation of mechanical properties is often
difFicult_ to make because it involves many unseen factors
such as stress concentration, orientati0h effect, viscoelastic
behavior etc. While significant progress has been made, much work
still needs to be .done. A systematic approach including
experimental and theoretical techniques should be developed to
help understand the micromechanisms and to clarify the
interrelationships among structure, property and processing.
Several factors such as fiber concentration, fiber aspect ratio,
compatibility between fiber and marix can then be studied under
this guidance. The aforementioned suggestions, if applicable, may
lead to a ccmp!ete data bank setup which may eventually benefit
all the designers, engineers, and scientists who are using
conductive composites in their work.
ACKNOWLEDGMENTS
The author gratefully acknowledges the financial support
provided by the School of Applied Arts and Sciences at San Jose
State University, and Inconel 601 steel fibers supplied by Mr.
Steven J. Kidd of Bekaert Fiber Technologies, Marietta, Georgia.
The author is indebted to Mr. Jeff Worneck and Mr. Bart Weinsink
of San Jose State University for their work on specimen
preparation. Informative discussions regarding this work with Mr.
Steven I. Kidd are also gratefu!!7 appreciated.
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The assessment of metal fiber reinforced polymeric composites

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