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Enhanced hybrid composite for advanced aerospace applications.

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]
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Author:Tache, Florin; Dobre, Tanase; Chirilus, Alina Alexandra
Publication:Annals of DAAAM & Proceedings
Date:Jan 1, 2008
Words:1273
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