EP3921144A1

EP3921144A1 - Composite material for lightening various structures

Info

Publication number: EP3921144A1
Application number: EP20705125.1A
Authority: EP
Inventors: Roberto Papetti; Vittorio SISTO; Francesco DAVID; Vincenzo Tagliaferri
Original assignee: Frattelli Mazzocchia SpA
Current assignee: Frattelli Mazzocchia SpA
Priority date: 2019-02-06
Filing date: 2020-02-06
Publication date: 2021-12-15
Also published as: WO2020161220A1; IT201900001681A1

Abstract

A method for preparing a composite material comprising a felt made of fibers is here described. Such composite material is aimed at manufacturing light, strong and low cost structures, to be used in the automobile, rail and naval transport and in general for all those industrial applications where light and very resistant structures are required.

Description

COMPOSITE MATERIAL FOR LIGHTENING VARIOUS STRUCTURES

FIELD OF THE INVENTION

The present invention relates to the production of composites for the structural use characterized by the utilization of fibers (optionally recycled) with three-dimensional geometry. The invention relates to new materials and new processes aimed at producing such materials useful for light, strong and low cost structures, to be used in the automobile, rail and ship transport and in general for all those industrial applications where light and very resistant structures are required.

BACKGROUND OF THE INVENTION

In materials science, a composite material is a heterogeneous material, that consists of two or more phases with different physical properties, wherein said properties are better than those of the phases that constitute it. Usually, the different steps in the composite are constituted of different materials, such as in the case of carbon fiber and epoxy resin composites. There are, however, exceptions wherein the different phases are made of the same material, such as SiC/SiC and self-reinforced polypropylene (SRPP).

The single materials forming the composites are called constituents, and according to their function, they are called matrix and reinforcement (or charge).

The matrix is the polymeric part (for example the resin), while the reinforcement are the fibers.

The combination of these two parts is a product able to guarantee high mechanical properties and lower density that is the reason why the composites are largely used in applications where lightness and strength are crucial, such as aeronautics.

The matrix consists of a continuous homogeneous phase, which has the task of:

enclosing the reinforcement, ensuring the cohesion of the composite material (and of any layers of which it is composed, in the case of laminated composite);

ensuring that the reinforcing particles or fibers have the correct dispersion inside the composite and that there is no segregation.

In most cases, the matrixes are polymeric, because they guarantee low density (and therefore lightness of the final material): however, they have the inconvenience to drastically reduce the performance when the temperature rises.

In polymer matrix composite materials, epoxy resins can be used (as a matrix) (the same used in some adhesives and polyesters) or the phenolic resins, optionally added with other polymers (such as PVB), which contribute to improve the mechanical characteristics (such as flexibility) of the composite material while maintaining adhesion to the reinforcement.

The reinforcement is represented by a dispersed phase, which is in fact dispersed in various ways within the matrix and has the task of ensuring rigidity and mechanical strength, taking on itself most of the external load. According to the type of reinforcement, the composite materials are divided in:

particle composites;

fiber-reinforced composites;

structured composites (for example sandwich panels, laminated composite materials and aluminum composite panels).

In the particle-composites the reinforcement consists of "particles", which (unlike the fibers) can be considered to be equiaxial. The chemical-physical properties of the particle composite materials depend on the geometry of the particle system, that is: size and shape of the particles;

concentration, distribution and orientation of the particles within the matrix phase.

Fiber-reinforced composites can in turn be classified in:

continuous (or long) fiber composites;

discontinuous (or short) fiber composites aligned with each other;

discontinuous (or short) fiber composites arranged randomly.

Composite materials with a fibrous dispersed step have a marked anisotropy. Said anisotropy does not occur (or at least very negligibly) in particle composites, to the extent that said particles are equiaxial. The anisotropy, if under control may constitute an advantage: the material is reinforced in those directions where it is known that it will be loaded and therefore the performances are optimized (as in the case of continuous fiber composites). If, however, it is due to phenomena more difficult to control (for example plastic flow of the material in a mold, as in the case of short fiber composites) it becomes problematic, because the orientation of the directions of maximum reinforcement hardly coincides with the desired one.

In the case of fiber-reinforced composites, the reinforcement can for example consist of:

glass fiber;

carbon fiber (consisting of graphitic carbon and amorphous carbon);

ceramic fibers (for example silicon carbide or alumina);

aramid fiber (like Kevlar);

basalt fiber.

To date the polymeric composite materials for structural uses consist of two phases: a thermosetting or thermoplastic polymer matrix and fibers of different nature with high tensile strength and low density. The matrix has the function of impregnating and holding the fibers together, allowing the shaping of the material, in relation to the component to be made, and of transferring to the fibers the stresses applied to the component during use. In order for the fibers to contribute to the structural strength of the composite, the adhesion between the fibers and the matrix must be very high. Fiber-reinforced polymeric composites are a class of materials widely used in the aerospace sector, as they are characterized by lightness and high specific structural strength (ratio between strength and density), which allows to reduce energy consumption and/or to increase the useful load of the aircraft. At present, in this context, composites with a thermosetting matrix reinforced with carbon fibers are particularly studied for the high performance they offer. The diffusion on other means of transport on land and sea is currently limited by the high cost of carbon fibers and the production technologies of composite products, which are generally not very flexible and in any case can be automated only for some product geometries.

DESCRIPTION OF THE INVENTION

The present invention relates to polymeric composites for structural uses comprising inorganic or synthetic fibers, preferably from waste recycling, having three-dimensional geometry. Furthermore, the present invention relates to a method for the preparation of said composite materials.

The composite materials according to the present invention are useful in the automotive, aeronautical, railway, naval fields and, in general, for all those industrial uses where light and resistant structures are required.

The composite material object of the present invention is prepared using a process called infusion.

Infusion is a technique for creating composites, which involves laying the reinforcement fibers on the dry mold, i.e. without resin, and subsequently adding resin to the mold by depression. In other words, an area with a lower pressure than the surrounding atmospheric one is created inside the mold, capable of attracting the resin, for which flow channels have been set up, going from the resin storage tanks to the mold. This depression, commonly called "vacuum", is obtained by covering the mold with a plastic film, commonly called "vacuum bag", effectively connected to the mold to avoid air infiltration. The air is sucked from the area between the vacuum bag and the mold by means of an electromechanical pump and suction ducts. In this way the vacuum bag will adhere strongly to the mold by squeezing the fibers and creating the depression necessary to make the resin flow. Once the resin begins to flow into the mold, it must be able to travel the entire surface of the mold, that is, impregnating all the fibers, in less time than the curing time.

In fact, the resin must remain in the liquid state throughout the infusion process to allow complete impregnation of all areas; only at this point the catalysing process of the resin can begin which then passes to the solid state with an exothermic reaction. In general, the resin passes from the storage tanks to the mold through suitably sized and positioned tubes. Once inside the mold, however, the resin must be able to flow easily and therefore resin diffusion methods must be provided which vary according to the type of infusion adopted.

It is therefore an object of the present invention a composite material comprising a felt made of fibers, wherein said felt is obtained as reported below:

STEP 1 - Working the fibers for obtaining the felt

_• Step 1A - CUTTING THE FIBERS

The fibers are cut through a cutting unit (rotary cutter), in order to obtain fibers of a size between 1-600 mm; preferably 15-300 mm; most preferably 30-60 mm in length.

Said fibers are selected from the group comprising: carbon, glass, ceramic, aramid, basalt fibers; carbon fibers are preferred; carbon fiber sheets from aerospace industry waste are highly preferred.

_• Step IB - OPENING THE BUNDLE OF FIBERS

Once cut, the fibers are collected in a carding machine that allows the opening the bundle of fibers and untangle them.

_• Step 1C - ADDING AN EMULSIFYING PRODUCT

The untangled fibers are subjected to a step called "enzymic step" in which, thanks to a pneumatic conveying system, the fibers are transported in a hopper in which, with the help of specific nozzles, an emulsifying product is nebulized with the dual objective to favour the sliding of the fibers and to reduce the electrostatic charge. By emulsifying product it is meant a substance capable of stabilizing an emulsion, acting as a surfactant or stabilizer.

_• Step ID - INCREASING THE ADHESIVENESS OF THE FIBERS

Optionally, in this step a resinoid substance/primer is applied, useful for promoting the adhesion of the fibers to the matrix. This step is particularly important, because it allows to safeguard the mechanical properties of the fibers and makes them more wettable.

_• Step IE - ADDING AUXILIARY SUPPORT FIBERS

Subsequently, the fiber mass of the previous steps, is mixed by means of a mixing unit with auxiliary support fibers selected from the group comprising: natural fibers, such as cotton or flax; organic polyester fibers (PES); polyestereketone fibers (PEEK); inorganic glass, metal and/or aramid fibers; wherein said auxiliary support fibers are present in a dose of 0.5-50%, preferably 1-10%; most preferably 5% of the total composition. The auxiliary support fibers allow homogeneous reinforcement and guarantee specific functional properties. Step IF - CARDING

The fibers thus obtained/treated of the previous steps feed a carding machine, preferably aerodynamic carding, in which the fibers are untangled, air-formed and distributed according to a partially parallel orientation. The advantage of aerodynamic carding, in addition to being cheaper, is that it preserves the length of the fibers, since contact with the mechanical parts is minimized. From the aerodynamic carding step, a veil of fibers oriented according to a three-dimensional and one-way architecture is obtained.

• Step 1G - STRA TIFYING VEILS OF FIBERS

The veil of fibers is subsequently layered and joined together with other veils to create a fabric with a flat textile structure having the desired thickness and weight.

• Step 1H - WELDING THE VEILS

To promote cohesion, the different layers of veils are heated until the fibers are thermally welded to each other due to the effect of heating, thus obtaining a non-woven fabric.

• Step 1L - COMPRESSING THE WELDED VEILS

To increase the compactness of the structure, the non-woven fabric of the previous step is passed through rollers, obtaining a non-woven felt (three-dimensional mat) whose physical and mechanical properties vary according to the application needs in terms of weight, thickness, grammage, percentage of fibers, auxiliary additives and degree of fiber orientation.

It is further object of the present invention a process for preparing a composite material comprising the following steps:

STEP 1 - Working the fibers for obtaining the felt

This STEP is the one described above.

STEP 2 - Making a mold

The molds can be made of wood, epoxy boards, metal or composite material. Generally, metal or composite material molds are used to produce numerous components, while wooden molds are used for the production of prototypes.

STEP 3 - Assembling the mold

The mold is assembled, if composed of several parts, and covered with a release product to facilitate the detachment of the finished product when the lamination is finished.

STEP 4 - Cutting the felt

The felt (i.e. the three-dimensional mat obtained at the end of STEP 1), as reinforcement material, is cut to obtain reinforcement shapes that adapt to the mold made and prepared during STEPS 2 and 3.

STEP 5 - Lying the felt on the mold (lamination)

The application of such reinforcement material (the felt) on the mold is carried out dry, facilitating its positioning on said mold and allowing the use of high grammage reinforcement material. Said positioning provides for the placing of several layers of reinforcing material allowing to make a laminate.

STEP 6 - optional - Covering the mold If the positioning of the reinforcement materials of step 5 continues over time, it is advisable to cover the mold with plastic sheeting between one working session and the next, to avoid that residues of fibers or dust settle on the reinforcement material.

STEP 7 - optional - Keeping the felt in position on the mold

It must be ensured that the reinforcing materials positioned on the mold during step 5 do not move during the subsequent steps. Inert spray adhesives that do not interfere with the resin are used to hold the fabrics in place. Alternatively, specially designed pins, particularly long and thin, can be used to hold the fabrics together.

STEP 8 - Laying infusion aids on the mold

Once the lamination step of the reinforcement material (the felt) has been completed, the mold must be covered with the infusion auxiliaries, i.e. tools that allow the flow of the resin inside the laminate and therefore the execution of the infusion process.

STEP 9 - Placing an infusion network on the mold

The infusion network consists of pipes, fittings, valves, couplings and traps that allow vacuum suction and resin injection into the vacuum bag. The resin infusion channels generally consist of tubes that start from the storage tanks and arrive at the article.

After spreading the infusion network, it is necessary to prepare the suction channels that are necessary for the creation of the depression or "vacuum".

STEP 10 - Laying the vacuum bag

The next step consists in laying out the vacuum bag, which is placed above all the layers previously described. The vacuum bag consists of an air-impermeable plastic film, which is spread over the mold and fixed along its entire perimeter to prevent the entry of air.

The vacuum bag must then be shaped, placed on top of the mold and cut along the perimeter. Finally, the perimeter of the vacuum bag must be fixed to the flange of the mold outside the suction lines.

STEP 11 - Producing the vacuum

When the previous steps have been completed correctly, creation of the vacuum can be carried out, using a pumping station consisting of one or more pumps to allow the air present between the vacuum bag and the mold to flow.

STEP 12 - Infusing a resin

The resin must first be poured into the containers and then added to the catalyst and any additives, such as accelerators or retarders. The temperature range within which the infusion can be carried out ranges from 16° C to 32° C.

As soon as the catalyst has been added, the stop taps of the infusion lines must be opened to allow the resin to begin to flow to the mold.

The infusion can be said to be completed when all the areas have been reached by the resin; at this point it is noted that the resin begins to enter the suction lines and at the same time the pressure value undergoes a sudden drop.

Said preparation process is characterized in that: - the auxiliary support fibers used during STEP 1 are selected from the group comprising: natural fibers, such as cotton or flax; organic polyester fibers (PES); polyestereketone fibers (PEEK); inorganic glass, metal and/or aramid fibers;

- at the end of STEP 1, the fibers have a filiform structure having a completely random positioning;

- in STEP 10 the perimeter flange of the mold has a size of at least 150mm.

- in STEP 11 the value of depression for the realization of the vacuum varies from 0.4 bar to 0.65 bar, a very preferable value is 0.55 bar;

- in STEP 12 the temperature range within which the infusion can be carried out ranges from 16 °C to 32 °C;

- the hardening period of the resin used in STEP 12 vary from 6h to 24h;

- in STEP 12 the infusion period depends on the propagation speed of the resin, and wherein said propagation speed is affected by:

room temperature;

viscosity of the resin;

the reinforcement materials used;

the arrangement of the infusion channels;

the extent of the depression and

the complexity of the surface.

It is a further object of the present invention a process for preparing a composite material wherein, the resin used in STEP 12 is selected from the group comprising: epoxy resin and/or epoxy-vinylester resin.

It is a further object of the present invention a process for preparing a composite material wherein, before starting the infusion of STEP 12, amino catalysts are added to the resin.

It is a further object of the present invention a process for preparing a composite material wherein, before starting the infusion of STEP 12, additives selected from the group comprising: accelerating and retarding additives are added to the resin.

It is a further object of the present invention a process for preparing a composite material wherein the molds are made of wood, epoxy boards, metal, composite material and/or their alloys.

It is a further object of the present invention a process for preparing a composite material wherein the infusion method is selected from the group comprising: Resin Transfer Molding, Vacuum Bag Infusion, Resin Film Infusion, Resin Liquid Infusion, Resin Injection between Double Flexible Tooling, Seeman's Composite Resin Infusion Molding Process and High Pressure Resin Transfer Molding.

It is a further object of the present invention the use of the composite material according to the invention in the field of: automotive, railway, naval, air transport and in industrial applications where light and resistant structures are required.

The composite material obtained at the end of the procedure described above was subjected to physico- mechanical tests, such as tensile tests with strain gauge, bending, Flat-top cylinder Indenter for Mechanical Characterization (FIMEC) and modal analysis, to evaluate its mechanical strength.

The present invention will now be described, for illustrative but not limitative purposes, with particular reference to the Figures here attached and described below. DESCRIPTION OF THE FIGURES

Figure 1 shows the felt (three-dimensional mat) obtained through the working of carbon fiber, in which the fibers are oriented according to a three-dimensional and random architecture.

Figure 2 shows a functional scheme of the infusion system comprising:

a mold (1);

a flange (2) of the mold;

a vacuum bag (3);

for vacuum bag sealer (4);

a suction line (5);

an infusion line (6);

a suction station (7);

resin (8).

Figure 3 shows the composite material obtained at the end of the process of the present invention.

Figure 4 shows in a graph the result of the tensile tests in the same infusion direction as the resin on 4-layer panels.

Figure 5 shows in a graph the result of the tensile tests in the 45° direction with respect to the infusion direction of the resin on 4-layer panels.

Figure 6 shows in a graph the result of the tensile tests in the 90° direction with respect to the infusion direction of the resin on 4-layer panels

Figure 7 shows in a graph the result of the bending tests in the same infusion direction as the resin on 4- layer panels.

Figure 8 shows in a graph the result of the bending tests in the 45° direction with respect to the infusion direction of the resin on 4-layer panels.

Figure 9 shows in a graph the result of the bending tests in the 90° direction with respect to the infusion direction of the resin on 4-layer panels.

EXAMPLES

EXAMPLE 1 - Process for the preparation of a carbon fiber felt

• CUTTING THE CARBON FIBERS

The recycled carbon fibers were collected to be cut through a cutting unit (rotary cutter) so as to obtain a homogeneous size of the fibers, having a length of 45 mm.

• OPENING THE BUNDLE OF THE CARBON FIBERS

Once the carbon fibers were cut, they were collected in a carding machine that allowed to open and untangle the bundle of fibers to face the next stages.

• ADDING AN EMULSIFYING PRODUCT

Subsequently, using a pneumatic conveying system, the carbon fibers were transported in a hopper in which, with the help of specific nozzles, an emulsifying product was sprayed. INCREASING THE ADHESIVENESS OF THE FIBERS

A resinoid substance/primer useful to promote the adhesiveness of the fibers to the matrix was applied, to protect the mechanical properties of the carbon fibers and to make them more wettable.

• ADDING AUXILIARY SUPPORT FIBERS

Subsequently, the mass of carbon fibers was mixed, through a mixing unit, for 5% of the total composition, with flax fibers.

• CARDING

The fibers thus obtained were processed with an aerodynamic carding machine, in which the fibers were untangled, air-formed and distributed according to a partially parallel orientation. Following aerodynamic carding, a veil of fibers oriented according to a three-dimensional and one-way architecture was obtained (see Figure 1).

• STRA TIFYING VEILS OF FIBERS

The veil of fibers was subsequently stratified and joined together with three other veils of fibers to create a non-woven fabric with a flat textile structure.

• WELDING THE VEILS

To promote cohesion, the four fiber veils were heated by thermal means. Due to the heating effect, the fibers that formed the veils were welded to each other, resulting in a non-woven fabric.

• COMPRESSING THE WELDED VEILS

To increase the compactness of the structure, the non-woven fabric was passed through rollers, obtaining a non-woven felt (three-dimensional mat) having a 1 mm thickness.

At the end of STEP 1, the carbon fibers of the starting material were thus transformed into a non-woven felt (three-dimensional mat).

EXAMPLE 2 - Process for preparing a panel made of composite material

A panel made of composite material was made, using 4 200 g layers of carbon fibers felt worked as described in Example 1. The sizes of each single layer of felt were as follows: 300 mm x 185 mm and 1 mm thickness.

Said layers were oriented at a 0°/90°/0°/90° to obtain an almost isotropic laminate.

For the infusion process, an epoxy-vinylester resin was used.

At the end of the process, the panel was found to have a mass of 283 g.

The geometric and physical characteristics of the resulting panel were the following:

Size of the panel: 300 mm x 185 mm

Thickness: 4.5 mm

Weight: 283 g

Density: 1,133 kg/mc

% by weight of reinforcement: Pf = 15%

% by volume of reinforcement: Vf = 9.5%

Number of layers of 200gr/mq: 4

Sheets orientation: 0°/90°/0°/90° EXAMPLE 3 - Characterization of the composite material

Using the process of Example 2, ten panels of the four-layer composite material according to the present invention were prepared.

These panels were characterized by carrying out the following mechanical tests:

• tensile tests in the same direction as the resin infusion flow: indicated with "T 0°" (see Figure 4);

• tensile tests in the 45° angular direction with respect to the resin infusion flow: indicated with "T

45°" (see Figure 5);

• tensile tests in the direction perpendicular to the resin infusion flow: indicated with "T 90°" (see Figure 6);

• bending tests in the same direction as the resin infusion flow: indicated with "F 0°" (see Figure 7);

• bending tests in the 45° angular direction with respect to the resin infusion flow: indicated with "F

45°" (see Figure 8); and

• bending tests in the direction perpendicular to the resin infusion flow: indicated with "F 90°" (see Figure 9).

At the end of the tests, the following mechanical characteristics reported in Table 1 and in Table 2 were found. Tables 1 and 2 report the mean values obtained by characterizing the 10 panels of composite material obtained with the process of the present invention.

Table 1 - Tensile tests in the 0 °, 45 ° and 90 ° directions of the composite material object of the present invention.

Table 2 - Bending tests in the 0°, 45° and 90° directions of the composite material object of the present invention.

Tables 3 and 4 report the mean values obtained by characterizing 10 panels of composite materials made of the classic carbon fiber sheets. Table 3 - Tensile tests in the 0°, 45° and 90° directions of a composite material made with classic carbon fiber sheets.

Table 4 - Bending tests in the 0°, 45° and 90° directions of a composite material made with classic carbon fiber sheets.

By comparing Table 1 with Table 3 and Table 2 with Table 4 it was noted that, with the same carbon concentration, the composite material object of the present invention had 20% better mechanical characteristics in bending, compared to a composite material made using classic carbon fiber sheets.

EXAMPLE 4 - Experimental modal analysis for the characterization of a panel made with 4 layers of carbon fiber felt

A panel of composite material, made with 4 200 g layers of carbon fiber felt and with epoxy-vinylester infusion was characterized, through experimental modal analysis.

Said layers were oriented at 0°/90°/0°/90° to obtain an almost isotropic laminate. Said panel had a mass of 283 g.

The geometric and physical characteristics of the analysed panel were the following:

• Size of the panel: 300 mm x 185 mm

• Thickness: 4.5 mm

• Weight: 283 g

· Density: 1,133 kg/mc

• % by weight of reinforcement (estimated): Pf = 15%

• % by volume of reinforcement (estimated): Vf = 9.5%

• Number of layers of 200 gr/mq: 4

• Sheets orientation: 0°/90°/0°/90°

The results were aimed at verifying the operating elastic constants through correlation with the corresponding modal analysis performed numerically, on the Finite Element model (FE) of the same panel. By means of the consolidated method of correlation between the experimental data of the modal response of the panel, and those obtained numerically, through the accurate tuning of the relative FE model, it was possible to verify values of the real elastic constants of the material examined, by completing the picture with the damping characteristics associated with the frequencies of the proper modes considered in the characterization, in the frequency range between 0 and 1000 Hz.

The use of a rectangular panel, with the dimensions of 300 xl85 mm, with an average thickness of 4.5 mm, allowed to simplify the experimental analysis, presenting a series of typical and repetitive modal forms. The acquisition step was facilitated by the two-dimensional shape of the panel, which allowed the use of a single accelerometer in the normal direction of the plane, on 15 acquisition points equally spaced in the grid, and an excitation point with force detection via load cell (impulsive type, with instrumented hammer with load cell).

In the acquisition step, the responses and excitations were directly acquired, in the form of transfer functions in the frequency domain, acceleration/force, through the multichannel analyzer "GenRad 2515 C.A.T. System" with appropriate windows on the impact signal of the instrumented hammer and the accelerometer response, performing an average of 5 useful hits for each point of acquisition. The "coherence" function, controlled on all acquisition points in the selected acquisition field, between 0 and 1,000 Hz, always had values equal to, or close to, "1", which indicated an excellent correlation in the transfer between the excitation and response energy, with good linearity and, in general, absence of effects unrelated to the desired excitation in the transfer functions.

The modal analysis, with the acquired functions, were performed using the computer integrated in the G.R. 2515 system, using the analysis software "Modal Plus" (SDRC).

A total of 10 ways of vibrating the panel were extracted, with their own frequencies, the damping and deformations that, through the animation of the modal model, allowed its complete identification and sequence, providing the data for the precise tuning of the corresponding FE model, with determination of the elastic constants of the composite material in question.

The panel was placed horizontally on a layer of polyurethane foam shaped in cusps. This solution was sufficient to approximate a free-free mass condition, in which the rigid modes of the same on the support occurred at a much lower frequency than that of the first elastic mode of the structure, ensuring non interference.

The panel was ideally divided into 15 points of acquisition of the response to excitation, equally spaced in 2 directions, X (long side) and Y (short side).

The impact technique was used as excitation, using a PCB instrumented hammer, with nominal 100 N/Volt load cell, while the 15 point response was acquired with an ICP 303A3 piezoelectric accelerometer, of PCB, fixed, from time to time, by means of a "Petro Wax" adhesive from PCB ("beeswax" synthetic). For each point an average on 5 hits considered valid was performed.

The response accelerometer was previously calibrated against a reference accelerometer PCB 302A07 (100 mV/G), via schaker B-K 4809, driven by a B-K 2706 amplifier with sinusoidal input at 160 Hz (1,000 rad/s) by Pintek 3200 generator, at an acceleration of 1 Grms, equal to 1.41 peak-peak.

Calibration, acquisitions and subsequent modal analysis processing, were performed using a 8 channels Computer Aided System "C.A.T GenRad 2515", with a software RTA 3.0, and for signal analysis and data acquisition, a "Modal Plus" program (SDRC).

An acquisition session was performed on 15 points, using the excitation point "1Z-". The transfer functions were stored for subsequent modal analysis. The coherence function was always evaluated and displayed, always close to the value 1 at the peaks, thus indicating the correct transfer of energy between excitation and response. The functions were displayed in Bode graphs, complete with the progress of the output/input step. The frequency range selected was 0÷l,000Hz, in which 8 modes of vibrating were present, a number more than sufficient to identify the modal response of the panel, to be used as the basis of "tuning" of the corresponding FE model, with the determination of the sought elastic constants.

The transfer functions obtained for the 15 response points on the panel were transferred within the "Modal Plus" program for the actual modal analysis called the panel.

Given the good general quality of the transfer functions, obtained in this case, it was possible to proceed with the extraction of 10 modes, very well defined and classifiable, using the "Search Peak" technique (SP - which operates in the frequency domain) in the range frequency analysed from 0 to 1,000Hz (see Table 5).

Table 5 - Values of modal parameters extracted with the "Search Peak" technique, in the frequency domain, with reference to the "llz +" function

The calibrated elastic constants of the 4-layer panel, which most reduced the deviations on the calculated natural frequencies compared to the experimental ones, were the following:

elastic module in X direction: Ex = 9,400MPa

elastic module in Y direction: Ey = 9,800MPa

intralaminar cutting module XY: Gxy = 3,000MPa

Poisson ratio: nuxy= 0.3

% by volume of reinforcement: Vf = 9.7%

% by weight of reinforcement: Pf = 15%

Damping ratio: 0.7%

The average deviation on the calculated natural frequencies compared to the experimental ones (Table 6), for the first 10 ways of vibrating, was not greater than 1.4%, therefore, with excellent reliability of the evaluation of the elastic characteristics. Table 6 - Comparison between the results of the experimental modal analysis and the results of the FE modal analysis with calibrated values of the elastic characteristics of the materials

Claims

1. A method for preparing a composite material comprising a felt made of fibers, comprising the following steps:

STEP 1- Working the fibers for obtaining the felt;

STEP 2 - Making a mold;

STEP 3 - Assembling the mold;

STEP 4 - Cutting the felt;

STEP 5 - Laying the felt on the mold;

STEP 6 - optional - Covering the mold;

STEP 7 - optional - Keeping the felt in position on the mold;

STEP 8 - Laying infusion aids on the mold;

STEP 9 - Placing a network of infusion on the mold;

STEP 10 - Laying the vacuum bag;

STEP 11 - Producing the vacuum;

STEP 12 - Infusing a resin;

in which said method for preparation is characterized by the fact that:

- said STEP 1 comprises the following processing steps:

Step 1A - Cutting the fibers;

Step IB - Opening the bundles of fibers;

Step 1C - Adding an emulsifying product;

Step ID - Increasing fibers adhesiveness;

Step IE - Adding auxiliary support fibers;

Step IF - Carding;

Step 1G - Stratifying veils of fibers;

Step 1H - Welding the veils;

Step 1L - Compressing the welded veils;

- the fibers of Step 1A are selected from the group comprising: carbon fiber, fiberglass, ceramic fiber, aramidic fiber, basalt fiber;

- the auxiliary support fibers of the SteplE:

- are selected from the group comprising: natural fibers, organic fibers of polyester (PES), poliesterechetone fibers (PEEK); inorganic fibers of glass, metals and/or aramid fibers; and

- are present in a dose of 0.5-50%, preferably 1-10%, most preferably 5% of total composition;

- the fibers, at the end of step 1, have a filiform structure having a random placement;

- in STEP 10 the perimeter flange of the mold, has a size of at least 150 mm.

- in STEP 11 the value of depression for the realization of vacuum, varies from 0,4 bar to 0,65 bar, most preferable is a value of 0,55 bar;

- in STEP 12, the temperature range within which can be performed the infusion varies from 16°C to 32°C;

- the hardening time of the resin used in STEP 12, varies from 6h to 24h;

- in STEP 12, the infusion time depends on the speed of propagation of the resin, wherein the velocity of propagation is influenced:

by the room temperature;

by the viscosity of the resin; by the fibers used;

by the laying of the infusion network;

by the entity of the depression and

by the complexity of the surface.

2. The method of claim 1, in which the resin used in the step 12 is selected from the group comprising: epoxy resin and epoxy-vinylester resin.

3. The method of claim 1, in which before starting the infusion of the step 12, amine catalysts are added to the resin.

4. The method of claim 1, in which before starting the infusion of stage 12, are added to the resin, additives selected in the group comprising: accelerators additives or retarders additives.

5. The method of claim 1, in which the molds are made of wood, epoxy boards, metal, in composite material and/or their alloys.

6. The method of claim 1, in which the carbon fiber of step 1, may be replaced by a material selected from the group comprising: glass fiber, ceramic fiber, aramidic fiber, basalt fiber.

7. The method of claim 1, in which the infusion method is selected from the group comprising: Resin

Transfer Moulding, Vacuum Bag Infusion, Resin Film Infusion, Resin Liquid Infusion, Resin Injection between Double Flexible Tooling, Seeman's Composite Resin Infusion Moulding Process and/or High Pressure Resin Transfer Moulding.

8. A composite material obtained by the method according to any of the preceding claims.

9. Use of the composite material of claim 8 for: automotive transport, rail transport, marine transport, aircraft transport and in industrial applications in which are required lightweight and resistant structure.