Process for production of holes including microperforations in a composite material, device for its implementation, and soundproofing layer made of the said material

CA 02394046 2002-07-11 1 PROCESS FOR MAKING HOLES ESPECIALLY OF MICRO-PERFORATIONS IN A COMPOSITE MATERIAL, DEVICE FOR ITS IMPLEMENTATION AND ACOUSTIC CUSHIONING LAYER CONSTITUTED OF THE SAID MATERIAL The present invention relates to the manufacture of panels of acoustic attenuation, in particular panels of composite material having a low weight-resistance to stress ratio, intended for example to equip the air inlet or the nacelle of an aircraft engine.

Conventionally, such structures are made of a light metal alloy, one or more sheets of which are perforated in order to obtain acoustic attenuation.

The perforation of metal sheet is carried out by known techniques such as molding, drilling or even punching.

Until now, the use of a composite material, made from fibers impregnated with resin, has not been applicable to the production of such a perforated sheet because the drilling or punching of this type of material leads to breakage. fibers and therefore the loss of the strength qualities of said material.

To make holes in a composite material without breaking the fibers of which it is composed, it is possible to form passages or reserves in the composite material before the latter is impregnated with resin.

The “dry” composite material is placed in a mold, the lower part of which comprises “needles” suitable for slipping between the fibers of the composite material by separating them, which makes it possible not to break them.

An intermediate part CA 02394046 2010-08-06 2 keeps the material in place and the upper part of the mold closes everything.

The resin is then sent under pressure into the mold: it impregnates the fibers, the needles guaranteeing the perforation of the composite material.

This method requires the purchase of a specific mold for composite materials to be provided with through holes.

In addition, the difficulty of demolding is a problem frequently encountered by manufacturers using this method.

From document EP 0314353, a method is known which has been proposed for solving this demolding problem.

However, this method requires specific tools and an implementation preventing the composite material from sticking to the mold, it is therefore complicated and expensive.

Furthermore, the laser is commonly used, including in the field of composite materials, but essentially for making cuts throughout the thickness of a skin made of composite material, for example, due to the property of the laser of concentrating in one. point a very high level of power, which inevitably damages the fibers and causes them to break.

The present invention aims precisely to overcome the various drawbacks of the known techniques of perforation or production of through holes in a sheet material by proposing a method suitable for producing holes and in particular micro-perforations in a composite material in a precise manner, efficient and without damaging the fibers of said composite material.

To this end, the invention as specifically claimed relates to a process for producing holes, in particular micro-perforations, in a composite material, consisting of mineral or organic fibers embedded in a resinous matrix, characterized in that it consists:

CA 02394046 2010-08-06 2a - in a preliminary step, in carrying out tests on a sample of the composite material to be perforated, using a laser beam directed substantially perpendicularly to the surface of said sample, said tests consisting in adjusting the duration of illumination in a determined location of the sample, as well as CA 02394046 2002-07-11 3 the focusing and the power of said laser beam, so as to confine the temperature in the mass of the composite material in a temperature range both above the thermal decomposition temperature of the matrix resin and below the temperature of alteration of the physical properties of the fibers, so as to achieve the '' elimination of the resin without affecting the physical integrity of the fibers, then, - to be reproduced on the material to be treated, in each hole or perforation location to be made, the operating conditions of the laser so as to constitute in said material, at said location, a hole or through passage, following only local burning of the resin of the matrix of the composite material.

Preferably, the step of reproducing on the material to be processed said operating conditions of the laser is carried out automatically by programming, on the one hand, parameters for controlling the operation of the laser and, on the other hand, controlling the displacement of the laser. laser beam opposite the various perforation locations.

The composite material to be treated consists, for example, of a skin formed of a ply or several stacked plies, of a fabric of pre-impregnated fibers.

Such a skin can advantageously constitute the acoustic damping layer of an acoustic damping panel of the sandwich structure type comprising a cellular core flanked, on the side opposite to said damping layer, by a rear reflector.

It is easy to perforate, in accordance with the invention, in such a damping layer, the number of micro-perforations, on which the rate of open area depends, desired for adaptation to the ranges of sound frequencies to be damped, that is ie function of the motor in the case of panels for nacelles.

Such an acoustic damping layer fulfills both a dissipating function through its micro-perforations and a structural function through the fibers which, moreover, are intact and ensure the passage of forces.

The resistive layer of the panel thus formed has a truly optimal low weight-resistance to stress ratio, much lower than that CA 02394046 2010-08-06 4 of the known resistive layers with equivalent dissipating property consisting of a metallic fabric, of a perforated metal sheet, or two layers, one structural and the other porous and dissipative, superimposed.

The invention as described also relates to a device for implementing the above method and more particularly for implementing the industrial phase, that is to say the reproduction on the composite material to be treated, at desired locations, laser operating conditions determined in the prior testing phase.

Such a device uses a laser of suitable characteristics and is characterized in that it comprises means for controlling the power and focusing of the laser, means for controlling the relative displacement between the head of the laser and the material to be treated, consisting of at least one woven web, in at least two orthogonal directions parallel to the plane of said web and computer means for controlling said piloting means according to a pre-established program.

Other characteristics and advantages of the invention will emerge from the following description of an embodiment of the method defined above, description given by way of example only and with reference to the appended drawing in which:

- Figure 1 is a schematic sectional view of a conventional non-machined composite material;

FIG. 2 represents the material of FIG. 1 after machining according to the method of the invention, and FIG. 3 is a diagram showing a device suitable for carrying out the machining illustrated by FIG. 2.

FIG. 1 represents a block of composite material formed from a stack of fabrics made up of fibers pre-impregnated with a suitable resin.

By fibers is meant a set, of square, round or rectangular section, of threads, strips of threads, wicks, braids or strands of threads, of different nature, for example carbon, glass, nylon or of CA 02394046 2002-07-11 "Kevlar", provided that the fibers calcine at a temperature above the melting temperature of the resin.

The fiber impregnation resin is preferably a thermoplastic resin, but the process of the invention can optionally be applied to thermosetting resin composite materials.

The composite material of FIG. 1 is shown in section after polymerization or consolidation of the impregnation resin and comprises a stack of fabrics formed of warp fibers 1 and weft fibers 2, embedded in a resinous matrix 3.

In Figure 2, there is shown respectively at 4, 5, 6 and 7 through holes or perforations through the entire thickness of the block of composite material, perpendicular to the plane of the stacked fabrics (1,2).

In accordance with the invention, these through passages are produced using a laser, the thermal action of which is controlled so as to occasionally modify the adhesion characteristics of the resin 3, in other words by locally bringing the temperature of the material. composite above the decomposition or pyrolysis temperature of the resin, which is of the order of 350 to 450 C in an oxidizing atmosphere, depending on the types of resin, while remaining notably below the temperature liable to alter the physical properties of the fibers, in particular the calcination temperature.

This fiber calcination temperature is markedly higher than that of pyrolysis of conventional resins and is for example of the order of 1500 ° C. for carbon fibers.

Regulation of the laser power so that at all points of the mass of the composite material, where the effects of the laser are manifested, the maximum local temperature reached is confined to a temperature range significantly higher than the pyrolysis temperature of the resin inter-fibers while remaining well below the temperatures for reaching the integrity of the fibers, is therefore possible.

Thus, by applying a laser beam substantially perpendicular to one of the faces of the composite material, by correctly focusing said beam, said well-shaped passages (4 to 7) are produced in the mass of the composite material (4 to 7) CA 02394046 2002- 07-11 6 approximately cylindrical, formed by removing the resin (3), only the fibers (1,2), intact, remaining inside and / or on the sides of said passages.

Due to the fact that the fibers (1,2) are not strictly aligned and rectilinear, that the weaving is not absolutely regular and that its characteristics may vary from one composite material to another, that the diameter of the perforations to be made may vary and the position of the axis of the laser beam is random with respect to the meshes of the tissues, the geometry of the perforations (4 to 7) may vary greatly from one perforation to another, a perforation may be, as illustrated in Figure 2, more encumbered than another by bare fibers, not degraded, the main thing being that the continuity of the passage is ensured from one face to the other of the composite material and that the overall rate of open surface of the composite material thus perforated corresponds to the desired acoustic characteristics, this being easily measured using an air flow resistance bench.

The perforations (4 to 7) have a section of the order of a few square millimeters and are for example approximately cylindrical and with a diameter between 2 and 3 mm, or else of square or rectangular section (3mm x 2mm for example).

FIG. 3 schematically represents a device suitable for implementing the method of the invention on a composite material 8 constituted by a consolidated flat skin formed of one or more folds each formed of a fabric of fibers impregnated with a resin. thermoplastic.

The skin 8 is placed on a machining table 9 of a numerically controlled machine for example.

Above the table 9 is mounted a laser head 10 with a vertical axis connected by an optical fiber 11 to a laser source 12, for example of the Yag type.

The laser head 10 is movable horizontally in X under the action of means symbolized at 13, while the table 9 is movable horizontally in Y, that is to say perpendicular to the plane of FIG. 3, under the action of means symbolized in 14.

CA 02394046 2002-07-11 7 The displacement means 13 and 14 are commanded, monitored and synchronized using computer program-controlled means 15, also connected, on the one hand, to means 16 for controlling the power of the laser 12 and for focusing the beam of the head 10 and, on the other hand, to means symbolized at 17 for controlling the scanning of the laser beam.

Advantageously, below the table 9, which is a metal grid for example, is arranged a system 18 for sucking the gases and pyrolysis residues generated by the burning of the resin of the composite material.

The method of the invention comprises a preliminary phase of determining the operating parameters of machining, namely the instantaneous power of the laser, the focusing of the beam, possibly the scanning of the beam when the diameter of the beam is less than the size of the perforation. at. achieve and the duration of illumination of the composite material by the laser beam.

To this end, instead of the skin 8, a sample of the skin will be placed on the table 9 and a series of tests will be carried out until perforations are obtained which meet the conditions, namely complete elimination. locally of the resin without affecting the physical integrity of the fibers present.

The useful parameters being recorded for each test, those corresponding to the conclusive tests will be taken into account and these parameters will be programmed in the computer system 15, as well as the movements of the table 9 and of the head 10 to achieve the necessary number of perforations. in the skin 8, a function of the ranges of sound frequencies to be damped.

The skin 8 to be machined is then placed on the table 9 and the machine will automatically make said perforations.

Preferably, the laser used will be a pulsed mode laser, more practical to drive than a continuous laser.

The duration of illumination of each location of the skin 8 is of the order of a few seconds and it is possible without problem to perforate skin with a thickness which may reach a few millimeters.

CA 02394046 2002-07-11 8 Being a thermoplastic resin composite material, it is easy to make the necessary perforations on a flat skin 8, then to assemble this skin with the other constituents, honeycomb core and rear reflector , in an appropriate mold under the usual conditions for polymerization of binding resins, the melting point of which, of the order of 180 C, is lower than the melting temperature, of the order of 280 C, usual thermoplastic resins.

The perforations made beforehand in the acoustic attenuation layer, constituted by said perforated skin, will not be blocked by the resin of the skin during the production of the panel.

It is thus possible to make the perforations in the skin 8 only at the time of manufacture of the panel, the acoustic adaptation of the skin thus being able to be adjusted to the expected characteristics of the panel, depending on the location where it will be mounted and of its environment, which gives a lot of flexibility to the process.

For example, for air inlets of the same dimensions intended for engines with different characteristics, it is possible to produce panels with acoustic attenuation layers of identical structure and simply differing in the number, or even the morphology and the distribution, of the perforations.

The skin 8 machined in accordance with the invention can be used alone or in combination in any other acoustic attenuation structure of the sandwich or multilayer type.

The method of the invention can also be implemented on composite materials with thermosetting resin, although the operation is a little more delicate because the perforations can only be carried out on a skin or layer in shape, once carried out the assembly of the various components of the acoustic panel.

Said perforations must in fact be made after the last autoclave heating operation of the assembly, otherwise the perforations would risk being blocked during passage in the autoclave.

This therefore requires special tools for controlling the laser on a shaped part.

CA 02394046 2002-07-11 9 The method of the invention makes it possible to produce acoustic attenuation panels with a single resistive layer which does not require the addition of a structural layer since said single resistive layer simultaneously plays the role of acoustic dissipation and that of structural reinforcement, with a layer thickness that can be reduced to a minimum, that is to say genuinely ensuring an extremely low weight-resistance ratio.

A resistive layer according to the invention is finally, in addition to the weight, more advantageous than a resistive layer consisting of a simple fabric or metal foil on other points.

The layer according to the invention only of composite material makes it possible to have good homogeneity of a panel, the other constituents of which are made of identical or compatible materials, since the bonding or assembly of these various constituents will be of good quality.

As the panel is made with identical or compatible materials, the problems of corrosion, galvanic couples and differential thermal expansion are avoided, so there is no risk of breakage or kinks.

Finally, the repair of such a panel is easier than the repair of a panel using a metallic fabric, in fact, a patch of composite material is easier to stick on a composite material to be repaired than a patch of metallic fabric. on a metallic fabric to be repaired.