Enhanced hybrid composite for advanced aerospace applications.
Tache, Florin ; Dobre, Tanase ; Chirilus, Alina Alexandra 等
1. INTRODUCTION
In the last few decades, composite materials have been used more
and more often wherever lightweight, yet strong structures are needed
(Beukers & van Hinte, 2005). There are numerous types of composite
materials, differing by type itself, production method, uses etc.
(Beukers, 2005). Regardless of type, composites have induced a major
change in the classical way of parts design, mainly due to their
heterogeneity and anisotropy (Zgura & Moga, 1999).
The material being analysed in this paper, namely CFRA1 (Carbon
Fibers Reinforced Aluminum), involves reinforcing an aluminum matrix
with carbon fibers. In order to make the two constituents work properly
with one another, a fundamental issue must be surpassed, which is the
electro-chemical incompatibility between aluminum and carbon. A handy
solution has been employed with respect to this matter. Even in the
early stages of a composite material development, its final destination
must be considered. Aspects like what kind of loads will act on the
composite structure, their order of magnitude etc. should be taken into
account and thoroughly examined. The material considered herein is
supposed to withstand the impact of small-dimension high-velocity
objects and to have a good behaviour in the event of an explosion in its
vicinity. Practical examples regarding its application would be
bulletproof plates and any kind of containers that would successfully
endure the explosion of a bomb inside them.
2. MAKING CFRA1
For making the material, two types of carbon fibers fabrics have
been chosen. They differentiate from one another by having different
fiber orientations, 0/90[degrees] and [+ or -]45[degrees], and by
fabrics type, plain weave and satin weave (Wikipedia, 2008). By
combining the two, a better resistance to an explosion is conferred to
the composite material, the carbon fibers actually covering four
different orientation directions. The matrix involved herein consists of
an epoxy resin having aluminum powder inside it. The metal particles are
completely enveloped by the resin, thus avoiding a direct contact
between carbon and aluminum, and leading to a delamination-free hybrid
composite material that benefits from all of its constituents
properties. And by a carefully studied manufacturing technology, CFRA1
develops properties well above the sum of its constituents properties.
It takes tenacity from aluminum, strength from carbon fibers and ease of
making from the epoxy resin.
[FIGURE 1 OMITTED]
The images in figure 1 show the two different types of carbon
fibers fabrics used in making CFRA1 (real material and symbolically
depicted) and different test specimens for conducting standard tensile
and bullet impact tests.
3. MECHANICAL PROPERTIES ASSESSMENT
Having in mind CFRA1's aforementioned purpose, a proper way to
evaluate its behaviour in the event of an explosion is to consider a
spherical container made of CFRA1, with a high pressure gradient in its
center. In this model, the thermal and short fragments impact effects
are neglected and will be dealt with later. Hence, figure 2 shows a
symbolic representation of the forces being developed and the tensions
induced by these forces within the container wall. Considering a thin
wall with respect to the sphere radius, the only true loads acting on it
are tension loads uniform in all directions, called membrane tension
loads (Gere, 2002).
[FIGURE 2 OMITTED]
The tension load within the sphere wall, acting in all directions,
is:
[sigma] = pr/2t, (1)
where p is the uniform pressure generated by the explosion, r is
the sphere radius and t the wall thickness.
Extrapolating and generalizing the model, by considering a sphere
with a very long radius, the spherical shape can be approximated with a
planar surface. Consequently, a good indication of CFRA1 resistance to
an explosion is its tensile strength, much easier to be evaluated by
conducting standard tensile tests. Several tensile test specimens have
been tested and the results will be thoroughly analysed. Preliminary
data reveal tensile strength of a 10-layer CFRA1 plate above 300 MPa.
With an excellent behaviour of carbon fibers at high temperatures,
only one aspect remains to be assessed, namely the resistance to short
fragments impact during the explosion. For this, bullet impact tests
have been and are to be conducted on different CFRA1 plates.
Ballistic tests have revealed an outstanding CFRA1 resistance
against a 7.65 mm calibre bullet shot from a standard 10-metre distance
(figure 3) and room for improvement when it comes to a 9 mm calibre
bullet. For the latter one, new samples have been prepared, being made
up of 14 carbon fibers layers and arranged in such a manner that the
shock waves produced by the impact with the speeding bullet are better
dissipated in the entire composite mass. Later tests with these new
improved samples have also proven excellent resistance to a 9 mm calibre
bullet. As it can be seen from the picture above, the bullet only
affects the first layers it encounters in its path and, as it penetrates
them, it eventually breaks apart inside CFRA1, leaving the last layers
intact. The picture on the right shows the last 4 layers with no
delamination, similar behaviour being observed in the 9 mm test as well,
but with a CFRA1 plate made of 14 carbon fibers layers.
While the thickness of the material has grown, its density has been
actually reduced, now being roughly 1650 kg/[m.sup.3], by adopting a
higher volume fraction of the fibers (above 50%) inside the composite
material. Nevertheless, a cross-section view of a CFRA1 specimen,
magnified 200 times using a BX 51 Olympus microscope (figure 4), shows
the matrix completely enveloping the carbon fibers.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
It is a long way from the raw materials (resin and fibers) to the
10-layer test specimen shown in figure 5, employing resources which are
not always easy to quantify. Some of these are: materials for the
moulds, screws and nuts, tools for the lay-up process and preparation
for curing in the oven, cutting gross samples into standard test
specimens, time etc.
Considering that at least some of the abovementioned resources
costs would be reduced in an eventual CFRA1 mass production, the price
of this material is given below, for different lay-ups, in Euros per
mass unit and per area unit.
[FIGURE 5 OMITTED]
5. CONCLUSIONS
Though it is a daring challenge, Carbon Fibers Reinforced Aluminum
is a new type of composite material being developed by the Chemical
Engineering Department in cooperation with the Faculty of Aerospace
Engineering at University 'POLITEHNICA' of Bucharest, Romania.
The research so far shows promising results, CFRA1 having foreseeable
excellent behaviour in applications requiring a reliable, yet
lightweight, reinforcement material that can also withstand powerful
thermal shocks. Current study represents a continuation of the work
started within the last two years at UPB and is an integral part of the
first author's PhD study.
6. REFERENCES
Beukers, A. & van Hinte, Ed. (2005). Flying Lightness, 010
Publishers, ISBN 90-6450-538-1, Rotterdam
Beukers, A. (2005). Engineering with Composites, Lecture Notes,
Delft University of Technology, Faculty of Aerospace Engineering
Gere, J. M. (2002). Mechanics of Materials, 5th SI Edition, Nelson
Thornes Ltd., ISBN 0-7487-6675-8, Cheltenham, UK,
Zgura Gh., Moga V. (1999). Bazele proiectarii materialelor
compozite (Basics of Composite Materials Design), Editura Bren, ISBN
973-9493-01-7, Bucuresti.
*** http://en.wikipedia.org/wiki/Plain_weave, Plain
weave--Wikipedia, the free encyclopedia, accessed 2008-06-03
Table 1. CFRA1 price for different lay-ups
CFRA1 type Price Price
[Euro/kg] [Euro/
[m.sub.2]]
CFRA1 5 x [+ or -] 45[degrees] 35.98 331.84
CFRA1 5 x 0/90[degrees] 30.85 289.83
CFRA1 10 x [+ or -] 45[degrees] 52.22 536.84
CFRA1 10 x 0/90[degrees] 43.32 452.80
CFRA1 5 x [+ or -] 45[degrees] 36.09 494.82
+ 5 x 0/90[degrees]