Optimized CFRAL composite for advanced aerospace applications.

Tache, F. ; Dobre, T. ; Tache, A.A. 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 way parts are designed, mainly due to their heterogeneity and anisotropy (Zgura & Moga, 1999).

The material being analyzed in this paper, namely CFRAl (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 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. are 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 could be bulletproof plates and any kind of containers that would successfully endure the explosion of a bomb inside them. The material has been under development for the past few years, showing promising results from its very beginnings and present paper focuses on describing the latest improvements regarding CFRAl's properties.

The study and selection of materials that form a composite material is of paramount importance. A careful mixture of matrix and reinforcing elements, according to the destination of the composite to be produced, can lead to a material which has its constituents in complete accordance with each other and thus displaying very good properties.

Another important aspect in the development of a composite material is the technique and methodology employed when producing it. It is well known that, in general, the assembly of several elements has properties better than the sum of its constituents' properties. A good and simple example is that in the case of a fiber reinforced composite subjected to tensile--compressive loads, the matrix basically carries the compressive forces, while fibers have an excellent response to the tensile loads. Taken separately. The matrix (namely epoxy resin) would not withstand tension and the fibers would easily be torn apart by compression.

Developing a composite material includes, besides theoretical estimates regarding its composition and properties, experimental tryouts. Consequently, this paper also presents several tests conducted on different composite samples, ranging from standard tensile tests to the material's behaviour when a speeding bullet tries to penetrate a CFRAl plate.

Composites are considered to be heterogeneous and anisotropic materials obtained by the macroscopic combination of two or more phases that have a separation interface or interphase and which sinergetically cooperate, leading to the final properties of the whole composite material (Zgura & Moga, 1999).

2. CFRAl Early Development

The material being investigated involves reinforcing an aluminum matrix with carbon fibers. In order to make the two elements work, the electro-chemical incompatibility between carbon and aluminum had to be overcome. From the different solutions investigated (Tache et al., 2006), it has been decided to use aluminum in the form of powder and an epoxy resin based matrix that completely envelops the fine aluminum particles and, at the same time, effectively adheres to the carbon fibers. Therefore, CFRAl is a hybrid composite, being made of three different constituent elements.

Even from the earliest stage of development, the final destination of a composite has to be considered. Details such as what kind of loads will act on the structure made of CFRAl, their order of magnitude, direction etc. have to be analyzed. This particular material is intended to have a good behaviour in case of impact with small dimension high velocity objects and even resist to the destructive force of an explosion. Simple examples to its application in practice would be bulletproof plates and containers that can endure the effects of an explosion inside them.

The first CFRAl samples produced in the UPB laboratory were made of 5 layers of [+ or -]45[degrees] carbon fibers fabrics joined together by the epoxy resin covering the aluminum powder. The composite was produced using the hand lay-up method, which presents the advantage of allowing one to modify the continuous medium percentage within a single composite section and also to change it from one layer of the composite to another. The method is used to obtain parts having, for instance, soft core and tough outer skin, that would withstand corrosion and other hostile external actions (Tache et al., 2006).

Several samples of the material were prepared for testing in various conditions, thus undergoing not only standard tensile tests, but also high speed bullet impact tests. The tensile tests revealed an ultimate tensile strength of around 125 MPa, while standard 7.65 and 9 mm calibre bullets penetrated the 5-layer material when shot from a 10-metre distance.

[FIGURE 1 OMITTED]

The value for the tensile strength mentioned above is not high at all, but the main advantage of this new material is its specific tensile strength, namely the ratio between ultimate tensile strength and density, which is around 65.8 x [10.sup.3] N * m/kg, a good starting point in the material development, showing that it has potential.

3. Enhanced CFRAl Development

Analyzing the first samples proved beneficial as lessons were learned regarding the strong and weak points of this new material. Solutions are now being developed to enhance its manufacturing technology and properties.

A comprehensive theoretical approach is presented, as well as some new samples of CFRAl and the first experimental results regarding standard tensile, shock, thermal and ballistic tests performed on these specimens.

3.1 Theoretical Aspects

Having in mind the first tryouts regarding the production and testing of CFRAl, solutions have been sought out to enhance the material's properties. All aspects have been carefully analyzed, from selecting the materials to conducting the tests on the samples.

The "ingredients" has to be in close relation to the composite's purpose, in this case to have a good behaviour in the event of an explosion inside a container made of CFRAl. An explosion is a gathering of several harsh effects that are not easy to investigate altogether, but can be well analyzed separately. There are three main destructive factors that the material should account for in the event of an explosion:

* High thermal gradient--a sudden rise in temperature right after the moment of the explosion.

* High pressure gradient--considering a closed container, the pressure inside it increases to high values in a very short period of time.

* Impact with short fragments--small dimension and irregular form objects hit the material while travelling at very high speeds.

Each of these aspects is thoroughly analyzed and presented in the following pages.

[FIGURE 2 OMITTED]

3.1.1 High Thermal Gradient

Carbon fibers are known to have a very good endurance to high temperatures, being able to withstand up to 2000 K or even more. So the thermal concern with CFRAl relates to its matrix, namely the aluminum powder enveloped in epoxy resin. The resin is actually quite bad when it comes to thermal resistance, having a melting point of around 500 K, while the aluminum powder can go up to 930 K, depending on its purity. Overall, the thermal endurance of the composite is a combination of the three ingredients' endurance s, with the continuous medium having the role of only keeping stuck together the carbon fibers layers which ultimately help maintain the structural integrity of the material within acceptable parameters, so the overall structure that is made of CFRAl manages to keep it's shape intact.

even if the inner layers who are most affected by the temperature rise caused by the explosion would lose their integrity while the thermal wave penetrates the material, its intensity diminishes as the energy is consumed by the previously affected layers. So, in general, one can assume that the first 2/3 layers of carbon fibers fabrics are completely destroyed by the thermal effect of the explosion, while the other ones remain intact and contain the explosion within the composite tank.

3.1.2 High Pressure Gradient

the effect of the rising pressure on the material in case of an explosion is of paramount importance and therefore is thoroughly investigated. A spherical container made of CFRAl is considered, like the one shown in Figure 2, with an explosion taking place in its center. Assuming a uniform distribution of the pressure on the wall, the sphere tends to increase its volume, so the composite layers that make up the container's wall are being subjected to tension loads. The detailed explanation of this analogy is presented below.

[FIGURE 3 OMITTED]

By imagining a sectioning plane through the sphere center (Figure 3 a) and isolating half of the sphere and of the explosion (pressurized gas) as being a free body (Figure 3 b), the loads acting on this body are the tension loads within the wall and the pressure caused by the explosion (Gere, 2002).

Pressure p acts perpendicular to the planar circular area of gas in the hemisphere and, by considering a uniform distribution of p, the resulting pressure force is

P = p * ([pi][r.sup.2]), (1)

where r is the internal radius of the sphere.

Due to the symmetry of the container and load distribution, the tension load a is uniformly distributed onto the entire circumference. Further, the wall being a thin one (r/1 > 10), one can consider with good accuracy that the effort is uniformly distributed over the whole wall thickness t. The thinner the wall, the better this approximation is. The resultant of the tension loads within the wall is a horizontal force numerically equal to the stress [sigma] multiplied by the area of the surface on which it acts, according to the formula

[SIGMA] = [sigma](2[pi][r.sub.m]t), (2)

in which the mean radius [r.sub.m] is r +1/2.

From the equilibrium equation of forces on the horizontal direction

[sigma] * (2[pi][r.sub.m]t) - p * ([pi] [r.sup.2] = 0, (3)

the tension load within the container wall results to be as follows:

[sigma] = [pr.sup.2]/[2r.sub.m]t. (4)

Because a thin wall has been considered, one can neglect the small difference between the two radii, by replacing the mean radius with the internal radius or vice versa. Although any alternative is satisfactory for the approximate analysis, the stress values are closer to the exact ones if the internal radius is used instead of the mean radius. Consequently, the formula for calculating the tension loads within the spherical container wall becomes

[sigma] = pr/2t. (5)

Due to the symmetry of the spherical container, relation (5) is the same independent of the direction of the sectioning plane, as long as it passes through the sphere center. The concluding remark is therefore that the wall of a spherical pressurized tank is subjected to tension loads a uniform in all directions (Figure 3 c). These efforts, tangent to the curved surface of the sphere, are called membranar loads, as they are the only true loads acting on real membranes, such as soap bubbles or thin rubber layers.

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 CFRAl's endurance to the pressure generated by an explosion is its tensile strength, much easier to evaluate and quantify by conducting standard tensile tests.

[FIGURE 4 OMITTED]

The composite being made by overlapping several lamina groups, it can be considered, regarding the tensile strength, as a straight rectangular beam with nonhomogeneous section and made of several elements with different properties, these elements being the lamina groups. considering the classical theory of a beam under tension loads, one can say that the tensile strength of the laminate is the combination of the tensile strengths of the lamina groups that make up the composite.

Considering a simultaneous cure of the resin for all the layers, forming spatial bonds, it is certain that the connections between layers are chemical in nature. Therefore, the lamina groups are perfectly bonded and cannot slide relative to one another while exterior loads are acting on them. In other words, there is no delamination. So the strain is continuous when passing from one layer to the next, and the composite strain sc is equal to the strain of the lamina groups [[epsilon].sub.l]:

[[epsilon].sub.c] = [[epsilon].sub.l]. (6)

If F is the tension force acting on the composite, then the axial forces within the lamina groups that make up the composite are evenly distributed:

F = [F.sub.i] + [F.sub.2] + ... + [F.sub.n] = [n.summation over (l=1)] [F.sub.l]. (7)

Because the tension only varies from one layer to the next, being constant within each separate layer, the forces acting on the lamina groups can be replaced with the product between the tension induced inside the group and its cross-section area:

F = [[sigma].sub.1][A.sub.1] + [[sigma].sub.2][A.sub.2] + ... + [[sigma].sub.n] [A.sub.n] = [n.summation over (l=1)] [[sigma].sub.l][A.sub.l]. (8)

All lamina groups have the same width w, equal to the width of the composite. only the thickness can differ from one group to another, though for the specimens made so far, the same thickness has been used for all layers. So relation (8) becomes

F/w = [n.summation over (l=1)] [[sigma].sub.l][t.sub.l], (9)

where n is the total number of laminas in one cfrAl composite plate.

The force divided by the width unit can be written for the ultimate tensile strength of the composite as follows (Zgura & Moga, 1999):

[[bar.F].sub.t] = [n.summation over (l=1)] [[sigma].sub.tl][zt.sub.l]. (10)

3.1.3 Impact with Short Fragments

The effects of the short fragments resulted from an explosion on a cfrAl plate are analyzed by making an analogy with something far easier to quantify, test, measure etc., namely the resistance of this composite material when being impacted by speeding bullets.

Standard ballistic tests involve shooting a bullet from a 10-metre distance into a CFRAl plate fixed with a special substance, called ballistic paste, on a wooden surface. The paste keeps the sample stuck to the wooden surface where it is mounted even after the impact with the bullet.

Two widely-used bullet calibres have been used for these ballistic tests: 7.65 mm and 9 mm. A few parameters for these two bullets are listed in the following table.

For each of the considered bullets, the energy to be dissipated at the moment of impact with the CFRAl plate is given by the formula

[E.sub.bullet] = 1/2 * [m.sub.bullet] * [V.sup.2.sub.bullet], (11)

where:

[E.sub.bullet]--kinetic energy of speeding bullet, in Joules; [m.sub.bullet]--mass of bullet, in kg; [V.sub.bullet]--velocity of bullet, in m/s.

With the values in Table 1, the energy that needs to be dissipated by the composite plate is calculated to be around 220.9 J for a 7.65 mm calibre bullet and 302.6 J for a 9 mm calibre bullet.

The first CFRAl samples produced, made of 5 layers of carbon fibers fabrics, proved to be insufficient for stopping both types of bullets, as it can be seen in the right side image of Figure 1 .

For making new and improved CFRAl samples, two different types of carbon fibers fabrics have been used. 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, with carbon fibers actually covering four different orientation directions.

The endurance of the composite material to bullet impact is mainly given by the carbon fibers layers. Therefore, the following paragraphs describe in detail some of the aspects related to this matter, with the resistance of the epoxy-aluminum matrix being minimized. It is not entirely neglected, as the continuous medium has the role of transmitting the loads from one composite layer to the next one and, at the same time, maintaining the general shape of the whole material by keeping the carbon fibers layers stuck to one another (Tache et al., 2008).

[FIGURE 5 OMITTED]

Figure 5a shows a plain weave carbon fibers fabric with the fibers being orientated in perpendicular directions to one another. The fibers in one direction are woven to those in the other direction. The representation in the image is intentionally exaggerated with respect to the thickness of the lamina for visualization purposes.

When hitting this fabric, the bullet tries to penetrate it by several means. One is the shearing force that appears in the exact section where the contact takes place between the edge of the bullet and the carbon fibers. It is well known that carbon fibers are very strong when subjected to tension, but very poor when it comes to shearing forces.

Another damaging factor is represented by the shock waves that are formed in the moment of impact and propagate along the first carbon fibers hit by the bullet. These waves arrive at the first weave point and, due to the bending radius of the fibers, they break the fibers in that location.

Last but not least, the bullet penetrates the fibers by simply displacing them from the point of impact around its diameter, making enough room in the fabric just to get through and arrive at the next layer, where the penetrating process starts again. But this time with less energy remained to be dissipated, as part of it was already consumed on penetrating the previous layer.

A similar process takes place in the case of a satin weave (Figure 5b). Nevertheless, there are some important differences. The fibers are not woven to one another, but are kept together by being woven with a very thin glass fiber string. This string goes from a layer of carbon fibers orientated in one direction, practically a unidirectional carbon fibers layer, to an adjacent layer of carbon fibers orientated perpendicular to the direction of the other layer. There is no true carbon fibers fabric in this case, but a fabric of two layers of uni-directional fibers.

In this case, the shearing force is the same, as the bullet hits directly some of the fibers. But there is also the distinction that it first shears some fibers on the layer closest to the impact point side of one lamina, while the other layer is still intact and, by the time the bullet gets to it, some of its energy had already been spent cutting the first layer. Then the process repeats itself for the next lamina, practically shearing separately each uni-directional carbon fibers layer that makes the satin weave.

A big advantage of this kind of weave is the fact that the shock waves produced at the moment of impact can ride smoothly on each carbon fiber until they reach the edge of the material. There are no more bending radii to concentrate the efforts.

Penetration by just displacing the fibers around the bullet as it passes from one layer to the other is similar to the case of a plain weave, with the observation that the satin weave used in making CFRAl is much more dense than the plain weave.

3.2 Experimental Results

Having in mind the theoretical aspects already presented, new CFRAl samples have been produced in the UPB laboratory. In brief, they are presented on the following pages.

3.2.1 Making Enhanced CFRAl

As mentioned before, the first CFRAl samples were made by using the hand lay-up method. This meant that the carbon fibers fabrics were laid on a flat and smooth surface and the resin was simply spread onto one fabric, then onto the next layer and so on. After all desired layers were arranged together, the material was just pressed and allowed to cure at room temperature.

This method is very easy to use and quite handy, but it has some major disadvantages, such as void spaces within the composite, due to the fact that each layer is laid after the previous one and there is no method to get rid of all air inherently caught inside. Another drawback is the curing at room temperature, which takes a long time and does not ensure a complete and correct reaction. This can lead to imperfect bonds between matrix and reinforcement and eventually to delamination.

The new CFRAl samples benefit not only from a more thorough theoretical approach, but also from the new manufacturing technique, that now guarantees a complete reaction and a smaller number of voids inside the composite mass.

The conclusions drawn from the results on the first samples helped understand the material's behaviour to short fragments impact and, together with the tensile tests, represent a baseline for improving CFRAl regarding selection of constituents, lay-up, heating and pressing processes, all in close relation to the properties required for this material.

3.2.1.1 Reinforcement Agent

A composite material needs to be designed and produced having in mind its final destination, namely the functional role it will have in a certain structure. The intention for CFRAl is to make it as resistant as possible to the effects of an explosion, which demand good tensile strength and resistance to short fragments impact and to a high thermal gradient. A straightforward result from these requirements is that the carbon fibers have to cover as many directions as possible, as the tension loads within the material would be acting in all directions tangent to the spherical container's surface.

Two different types of carbon fibers have been used, a 0/90[degrees] plain weave (Figure 6a) and a [+ or -]45[degrees] satin weave (Figure 6b). In the 3D computer representation,

[FIGURE 6 OMITTED]

3.2.1.2 Continuous Medium

The CFRAl matrix has to endure high temperatures and keep the carbon fibers layers together, maintaining the general shape of the structure, until the energy of the explosion is dissipated.

Therefore, a special system has been used as continuous medium, consisting of epoxy resin that envelops aluminum powders. Aluminum has a melting point of 660[degrees]C and a density of 2700 kg/[m.sup.3], very low relative to other metals. The main characteristics of the matrix are:

--possibility of curing at room temperature;

--can be taken out of the mould at room temperature;

--very good thermal properties after a post-curing heat treatment;

--total absence of aromatic compounds.

3.2.1.3 Constructing the Samples

The easiest shape to be produced in the laboratory is a rectangular contour plate, so the carbon fibers fabrics were cut into rectangular shapes. For pressing the material, several moulds have been used, made of 4 mm thick steel plates, fastened together with M6 screws and nuts. Composite sticking onto the moulds was avoided by using a de-bagging paste. M10 nuts were placed on each fastener, between the sheet metal plates, to ensure constant thickness of the composite material. Furthermore, in order to have smooth surfaces of CFRAl, the composite was covered with thin aluminum sheets and each inner side of the moulds was covered with Fibrefrax FT ceramic fibers paper 3 mm thick.

Several samples have been produced, with 5, 10 and 14 layers, respectively, using both types of weaves or a single one, in order to show up the differences between their properties. In any case, the samples were pressed between the steel moulds and put in an oven for curing. A post-curing heat treatment has been applied to all samples, to ensure a complete reaction.

After curing, the raw CFRAl samples were cut into finite rectangular plates, getting rid of irregular edges. Standard specimens have been prepared for tensile tests and wider plates for conducting ballistic tests, thermal measurements and standard impact tests on a special laboratory machine.

[FIGURE 7 OMITTED]

3.2.2 CFRAl Parameters

Some of the most important parameters of this composite material are its density and the fractions of the constituents within one composite, with respect to the entire mass and volume of the composite, respectively (mass fraction of matrix and fibers, volume fraction of matrix and fibers). They are all listed in Table 2.

CFRAl density has been improved especially in the last specimens produced, namely the 14-layer samples. It is important to notice the value of around 1.65 g/[cm.sup.3], very close to the density of a classical carbon-epoxy composite material. Even though CFRAl has aluminum in its composition, the metal's density being very low with respect to other metals (2.7 g/[cm.sup.3]) keeps the overall density of the hybrid composite close to the density of a composite with no metal inside.

An important aspect is also the fact that even if the density has been reduced from the first produced samples to the last ones, the resin completely envelops the carbon fibers, as it can be seen in Figure 8, which presents a cross-section view of a CFRAl specimen (left image), magnified 200 times using a BX 51 Olympus microscope (right image).

[FIGURE 8 OMITTED]

3.2.3 Tensile Strength Assessment

The tensile strength of the first 5-layer samples was presented in the previous chapter as being around 125 MPa. These samples also proved to be ineffective when impacted with a 7.65 and a 9x18 mm calibre bullet.

The improved samples, with 10 and 14 carbon fibers layers and a new curing cycle to obtain a complete reaction have also been tested and the results differ very much from the first ones. Best tensile strength results were recorded for the 10-layer specimens and are detailed below, while the 14-layer samples performed very good during ballistic tests (presented in the next chapter).

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

The ultimate tensile strength of CFRAl is around 319 MPa for specimens # 3 & 4 (10 layers of 0/90[degrees] plain weave) and 276 MPa for the other specimens (made of 5 layers of 0/90[degrees]plain weave and 5 layers of [+ or -]45[degrees] satin weave). The Young's modulus is around 36 GPa for specimens # 3 and 4 and 27 GPa for the others.

Regarding the specific tensile strength of the new 10-layer samples, namely the ratio between their ultimate tensile strength and their density, the values are now 190.603 x [10.sup.3] N*,m/kg for samples # 3 & 4, and 158.621 x [10.sup.3] N*m/kg in the case of specimens # 1, 2, 5 & 6. It is important to note the enormous progress from the first samples, the specific tensile strength now being three times higher than that of the preliminary samples. These values were a little lower in the case of the 14-layer specimens (best tensile strength of 208 MPa and specific tensile strength of 125.226 x [10.sup.3] N * m / kg), but further tests are planned.

3.2.4 Ballistic Tests Results

Several CFRAl specimens have been tested in order to evaluate their response in case of a bullet impact. Results on the first samples were shown in Figure 1 of chapter 2. Both bullets penetrated the composite plates.

The new and improved samples proved much better regarding bullet impact resistance. The 10-layer samples managed to successfully stop a 7.65 mm calibre bullet, while 14-layer CFRAl stopped a 9x18 mm calibre bullet, both shot from a standard 10-metre distance.

[FIGURE 11 OMITTED]

Figure 11 shows on the left side (10-layer specimen) that the bullet only affects the first layers it encounters in its path and, as it penetrates them, it eventually breaks apart inside CFRAl, leaving the last layers on the other side of the plate intact, no delamination being detected. For the 9 mm bullet impact test a CFRAl plates made of 14 carbon fibers layers were used, as the 10-layer setup proved insufficient.

The 9x18 mm (Makar[degrees]v) bullet produced more damage than the smaller bullet, but it was stopped by two different 14-layer CFRAl samples, separately. One of the samples was made of only one type of carbon fibers fabrics, namely satin weave, where fibers in one direction do not bend over those in the other direction, ensuring a better dissipation of the impact energy, which goes straight towards the edges of the material, having no turns in its pathway.

The smaller sample on the right side of Figure 11, made of two different types of carbon fibers fabrics (plain weave and satin weave), also proved successful in stopping the 9x18 mm bullet, but in this case the performance is credited to the high volume fraction of the fibers that compensated for the woven fibers bended over one another. In this sample, the volume fraction of the fibers in the composite is 53.18%, higher than that in the 14-layer satin weave sample of only 45.76%.

Consequently, with both types of samples successfully withstanding the impact of a bullet, four different orientation directions are accounted for in the case of a structure made of the two different types of carbon fibers fabrics, thus proving that CFRAl has a good potential to endure the effects of an explosion in the vicinity or inside a container made of this hybrid composite material (Tache et al., 2008).

3.2.5 Laboratory Impact Tests

One of the requirements for CFRAl is to withstand impacts. Besides ballistic tests presented above, standard laboratory experiments have been carried out on a 14layer CFRAl plate ([+ or -]45[degrees] satin weave). Plate thickness was 11 mm and length and width 120x100 mm in order to fit properly on the testing machine.

[FIGURE 12 OMITTED]

Though it has been impacted seven times, the composite plate has a good endurance to mechanical shock, represented by a load of roughly 8.5 KN to which corresponds an energy of 5.14 kg m (graph in Figure 12, test # 7--strongest hit) and a print of the hitting object on the composite plate with a diameter of 10.5 mm and a depth of 1.7 mm.

The strain produced by each impact is very small and localized in impact areas. Seen in a side view, the plate shows no deformation on the face opposite to the impact area. The relatively long horizontal segment (about 3 miliseconds) present during each test can be explained by the fact that the material responds to the shock by distributing the impact energy in all its lamina layers. A major contribution is that of aluminum powders, which help in reducing vibrations right after impact.

3.2.6 Thermal Characteristics

Temperature values were recorded during the heating and cooling of a 14-layer CFRAl plate. Several experimental rigs were used, with and without a heatsink, with two values for the electrical current powering a resistor acting as heater and placed at the end of an aluminum bar of square cross-section. This bar is at the other end in contact with the CFRAl plate, thus ensuring a uniform heat distribution in the contact area. Based on these measurements, a numerical code is being developed in an attempt to establish the thermal conductivity of the hybrid composite through the dynamic method. Though results are raw for now, some details are presented below.

[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]

4. Financial Aspects

In the developing stage of a composite material, at least some aspects regarding its cost have to be considered, estimated and ways to diminish the costs for the material's production and maintenance sought out.

It is a long way from the raw constituents (resin and fibers) to finite test samples, employing resources which are not always easily assessed. Some of these resources are: materials for the moulds, screws and nuts for fastening the moulds, ceramic papers to ensure a smooth surface of the produced plates, tools for the lay-up process and preparation for curing in the oven, extra resin and fibers from the gross plates irregular margins, energy consumed by the oven for curing and after-curing heat treatment, cutting gross samples into standard test specimens, time etc.

With Carbon Fibers Reinforced Aluminum being still under development and the produced samples being actually prototypes, only raw materials costs are taken into account for now, when calculating the composite material's price. The costs of all other equipment used to fabricate CFRAl are neglected, considering that at least some of the aforementioned resources costs would be reduced in an eventual CFRAl large scale production.

The price of this material, is given in Table 3, for different lay-ups, in Euros per mass unit and per area unit, as CFRAl is now in the prototype phase. Even in this early stage of development, the prices are comparable to those of already produced bullet-proof vests and other similar ballistic protection materials. And they will indubitably go down in case of a large scale industrialized production of this material in the following years, considering also the fact that the price of currently produced composite materials worldwide has dropped significantly over the last decades.

5. Conclusions

Composite materials are heterogeneous and anisotropic materials made of two or more components that remain macroscopically separated and distinct, but which at the same time form a single material. Carbon fibers composites are often used in various applications, as carbon fibers possess properties such as good strength, corrosion resistance and ability to withstand high temperatures. The range of their applications varies from industrial uses in domains such as aerospace and chemistry to sports articles and other common goods.

The advantages of composite materials in general and those of CFRAl in particular over classical materials, such as metals, are:

--high specific strength and high specific stiffness;

--low weight;

--slow crack growth cauzed by material fatigue;

--possibility to orientate fibers in most loaded directions;

--good corrosion resistance;

--ability to produce complex shapes, thus reducing the number of parts;

--excellent impact behaviour;

--possibility to use the material at high temperatures;

--low thermal expansion coefficient;

--good electrical and thermal conductivity. The weak points of CFRAl are:

--cost generally higher than that of metals;

--necessity to use some specific methods for production, testing and repairing;

--relatively low toughness;

--low electrical and magnetic protection for electrical systems;

--creep rupture.

Carbon Fibers Reinforced Aluminum is a composite material that began to be developed in the Mass Transfer Laboratory of the Faculty of Applied Chemistry and Materials Science in cooperation with the Faculty of Aerospace Engineering from University "POLITEHNICA" of Bucharest, Romania, at the end of 2005, research being financed by the Romanian Space Agency (Tache et al., 2006). The material is a hybrid composite, containing carbon fibers, epoxy resin and aluminum powder which, since the beginning of its development, has shown promising properties, with foreseeable excellent results in applications requiring a reliable, yet light, reinforcement material that can also withstand powerful thermal shocks. CFRAl development continued in the following years, supported by CNCSIS-TD CH450710 and CH450807 research grant for PhD students and is an integral part of the first author's PhD study (Tache et al., 2008).

Results obtained thus far and the material itself will continue to be analyzed in the future and the concluding remarks will help further improve CFRAl regarding its constituent elements and manufacturing technology. Additionally, ways to reduce costs will be sought out and solutions will be implemented in an eventual large scale production, with CFRAl having the potential of being used in domains varying from common goods and sports articles to bullet-proof vests, bomb-proof containers and space vehicles' outer hulls.

DOI: 10.2507/daaam.scibook.2009.53

6. References

Beukers, A. & van Hinte, Ed. (2005). Flying Lightness, 010 Publishers, ISBN 906450-538-1, 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

Tache, F., Stanciu, V., Chiciudean, T. G., Toma, A. C., Stoica, A. & Dobre, T. (2006). Statistical Risks in High Performance Nano-Composites Technology for Space Structures (IAC-06-C2.8.04) presented in October 2006 at the 57th International Astronautical Congress in Valencia, Spain and published in the official IAC 2006 DVD proceedings

Tache, F., Dobre, T. & Chirilus, A. A. (2008). Optimization and Modeling of CFRAl Properties and Manufacturing Technology (IAC-08-C2.I.5), presented in October 2008 at the 59th International Astronautical Congress in Glasgow, Scotland and published in the official IAC 2008 DVD proceedings

Zgura Gh. & Moga V. (1999). Basics of Composite Materials Design (in Romanian), Editura Bren, ISBN 973-9493-01-7, Bucuresti

*** http://en.wikipedia.org/wiki/Plain_weave, Plain weave--Wikipedia, the free encyclopedia (2008). Accessed on 2008-08-26

This Publication has to be referred as: Tache, F[lorin]; Dobre, T[anase] & Tache, A[lina] A[lexandra] (2009). Optimized CFRAl Composite for Advanced Aerospace Applications, Chapter 53 in DAAAM International Scientific Book 2009, pp. 531548, B. Katalinic (Ed.), Published by DAAAM International, ISBN 978-3-90150969-8, ISSN 1726-9687, Vienna, Austria

Authors' data: MSc. Dipl.-Ing. PhD student Tache, F[lorin|; Univ. Prof. Dipl.-Ing. Dr. Sc. Dobre, T[anase]; MSc. Dipl.-Ing. Tache, A[lina] Alexandra], University ,,POLITEHNICA" of Bucharest, Faculty of Applied Chemistry and Materials Science, Mass Transfer Laboratory, 1-7 Gh. Polizu str., sector 1, 011061, Bucharest, Romania; flotasoft@yahoo.com, tdobre@mt.pub.ro, ada_chirilus@yahoo.com

Tab. 1. Test bullet parameters

Parameter

            Speed   Mass   Maximum     Effective
Bullet      [m/s]   [g]    range [m]   range [m]

7.65        305     4.75   1500        50
9x18        315     6.10   1500        50

Tab. 2. CFRA1 parameters

Parameter

                       Density         [m.sub.m]   [m.sub.f]
Composite              [kg/[m.sup.3]   [%]         [%]

CFRA1  5 x             1839.02         75.51       24.49
[+ or -] 45[degrees]

CFRA1  5 x             1739.17         75.97       24.03
[+ or -] 45[degrees]

CFRA1  5 x             1658.28         61.88       38.12
[+ or -] 45[degrees]
+ 5 x 0/90[degrees]

CFRA1  5 x             1654.82         63.50       36.50
[+ or -] 45[degrees]
+ 5 x 0/90[degrees]

CFRA1 10 x             1673.63         57.64       42.36
0/90[degrees]

CFRA1  5 x             1751.53         73.27       26.73
[+ or -] 45[degrees]
+ 5 x 0/90[degrees]

CFRA1  5 x             1732.92         72.70       27.30
[+ or -] 45[degrees]
+ 5 x 0/90[degrees]

CFRA1 14 x             1661.00         50.62       49.38
[+ or -] 45[degrees]

CFRA1  7 x             1633.01         44.44       55.56
[+ or -] 45[degrees]
+ 7 x 0/90[degrees]

Parameter

                       [V.sub.m]   [V.sub.f]
Composite              [%]         [%]

CFRA1  5 x             74.98       25.02
[+ or -] 45[degrees]

CFRA1  5 x             76.78       23.22
[+ or -] 45[degrees]

CFRA1  5 x             64.88       35.12
[+ or -] 45[degrees]
+ 5 x 0/90[degrees]

CFRA1  5 x             66.45       33.55
[+ or -] 45[degrees]
+ 5 x 0/90[degrees]

CFRA1 10 x             60.62       39.38
0/90[degrees]

CFRA1  5 x             73.99       26.01
[+ or -] 45[degrees]
+ 5 x 0/90[degrees]

CFRA1  5 x             73.72       26.28
[+ or -] 45[degrees]
+ 5 x 0/90[degrees]

CFRA1 14 x             54.24       45.76
[+ or -] 45[degrees]

CFRA1  7 x             46.82       53.18
[+ or -] 45[degrees]
+ 7 x 0/90[degrees]

Tab. 3. CFRA1 price for different lay-ups

CFRA1 type             Euro/kg   Euro/[m.sup.2]

CFRA1  5 x             30.85     289.83
[+ or -] 45[degrees]

CFRA1  5 x             52.22     536.84
[+ or -] 45[degrees]
+ 5 x 0/90[degrees]

CFRA1 10 x             43.32     452.80
0/90[degrees]

CFRA1  5 x             36.09     494.82
[+ or -] 45[degrees]
+ 5 x 0/90[degrees]

CFRA1 14 x             58.35     722.22
[+ or -] 45[degrees]

CFRA1  7 x             57.90     616.06
[+ or -] 45[degrees]
+ 7 x 0/90[degrees]