Composite panel comprising a perforated metal foil for lightning strike protection

Abstract

A composite panel comprising a perforated metal foil for lightning strike protection, the composite panel comprising: a plurality of carbon layers; a perforated metal foil comprising a number of apertures and directly fastened to the plurality of carbon layers; and a protective layer made of a resin reinforced with fibers, the protective layer being fastened to the metal foil, wherein the perforated metal foil is embedded into the protective layer through its apertures, a free surface of the protective layer forms a top side of the composite panel, a thickness of the protective layer between the top side of the composite panel and the perforated metal foil is at least 15 micrometers, and a thickness of the perforated metal foil is not more than 30 micrometers, the plurality of apertures defines in total an open area of not more than 40% of the surface area, and a maximum distance between two opposite points on the aperture periphery is equal to or less than 3 mm.

Description

TECHNICAL FIELD

[0001] The present invention relates to a composite panel comprising a perforated metal foil for lightning strike protection of aerospace structures and to a perforated metal foil. BACKGROUND

[0002] For the past 60 years, aluminum has been the primary material used in aircraft and aerospace construction. As interest in constructing more efficient aircraft has grown, manufacturers are designing more components with lightweight composites. In particular, composite panels are currently used to form the skin of the fuselage and wings. However, composites are poor conductors of electrical current. It is known that composites are susceptible to severe damage in the event of a lightning strike without proper protection. To date, aircraft manufacturers have incorporated aluminum or copper mesh foil or interlaced wire mesh into these composite panels to dissipate lightning strike energy and prevent damage to the composite panels.

[0003] In both approaches, mesh metal foil has become the industry standard and is preferred over interlaced wire mesh because mesh metal foil does not unravel or have loose strands that can become problematic during processing into pre-preg materials or dry lay-up as part of the composite manufacturing process. The uniform design of mesh metal foil also ensures that the electrical conductivity is not compromised even when the material is formed into various shapes and contours and it provides a smooth surface on the final product. The mesh metal foil used in this application must be manufactured with strict tolerances to meet specific weight, open area, and electrical conductivity requirements.

[0004] Aircraft manufacturers use design guidelines, such as those specified by the Society of Automotive Engineers International (SAE International) in its Aerospace Recommended Practice (ARP) 5414, which defines lightning strike zones (areas of the aircraft that are more susceptible to lightning strikes, e.g., Zone 1A, 1B). The guidelines also provide the electrical withstand capability required for such struck zones. For example, a material is typically required to have the capability to withstand a Zone 1A strike of 200,000 amperes. For mesh foil, the thinnest material that can meet this standard to date is produced using a 42 micron foil due to the limitations of the meshing process. This material has a weight of 175 grams per square meter, an electrical resistivity of 3.6 milliohms per square meter, and an open area of the foil of 56%. In this application, the method of characterizing the performance of the foil is to evaluate the ratio of the weight to the electrical conductivity (which is the inverse of the electrical resistivity) of the foil and expressed in grams ohms per square meter. For the mesh foil described above, the weight to electrical conductivity ratio is 0.63 grams ohms per square meter.

[0005] Aircraft manufacturers are constantly looking for ways to improve efficiency, reduce costs, improve fuel economy, and reduce CO2 emissions. One clear way to achieve these goals is to reduce the weight of the aircraft. By reducing the weight of the composite panels, the overall weight of the aircraft can be reduced; however, according to SAE ARP514, the required conductivity standards must also be met in the specific impacted areas.

[0006] Therefore, it is desirable to produce lighter composite panels that still meet the required electrical withstand capability.

[0007] We know from patent application US2006 / 051592 an alternative solution that meets the above needs. This solution consists in fixing a patch (i.e. a decal) to a composite panel already manufactured to improve the electrical withstand capability of the composite panel. The patch comprises a metal foil embedded in a layer made of resin and this layer made of resin forms the outer surface of the patch / decal.

[0008] This solution is effective, but the attachment of the patch to the panel requires several steps that must be carried out very carefully and that must be preceded by a surface treatment of the panel. If not carried out with the utmost care, the operator will observe a rapid peeling of the patch. SUMMARY

[0009] The present invention aims to provide a composite panel that is intrinsically lighter than the composite panels of the prior art by its manufacture and that still meets the required electrical withstand capability and that is also resistant to corrosion. To this end, the present invention relates to a composite panel for aerospace structures comprising a plurality of carbon layers, said composite panel further comprising: a perforated metal foil comprising a number of apertures and fastened directly to said plurality of carbon layers; and a protective layer made of resin comprising a fibrous reinforcement, said fibrous reinforcement having a typical unit area weight of 2 g / m2to 20 g / m2, said protective layer being fastened to said metal foil, wherein said perforated metal foil is embedded in said protective layer through its apertures, the free surface of said protective layer forming the top side of said composite panel, the thickness of said protective layer between the top side of said composite panel and said perforated metal foil being at least 15 microns and the thickness of said perforated metal foil not exceeding 30 microns, said apertures having a predetermined geometry, extending through the thickness of said perforated metal foil and being distributed over a surface area defined by the length and width of said perforated metal foil, said plurality of apertures defining in total an open area not exceeding 40% of said surface area and the maximum distance between two opposite points on the perimeter of an aperture being equal to or less than 3 mm. 2 2 BRIEF DESCRIPTION OF DRAWINGS ​​

[0010] Embodiments of the application will now be described, by way of example only, with reference to the accompanying drawings in which:

[0011] • Figure 1 is a cross-sectional view of a composite panel comprising a perforated metal foil according to an aspect of the application;

[0012] • Figure 2 is a perspective view of a perforated metal foil of the composite panel shown in Figure 1

[0013] • Figure 3 is a perspective view of a portion of a mesh foil of the prior art;

[0014] • Figure 4 is a plan view of a portion of the perforated metal foil of Figure 2

[0015] • Figure 5 is a plan view of a portion of the perforated metal foil having diamond shaped apertures;

[0016] • Figure 6 is a plan view of a portion of the perforated metal foil having oval shaped apertures;

[0017] • Figure 7 is a plan view of a portion of the perforated metal foil having circular shaped apertures;

[0018] • Figure 8 is a plan view of a portion of the perforated metal foil having square shaped apertures;

[0019] • Figure 9 is a perspective view of a perforating machine for producing a perforated metal foil.

[0020] • Figure 10 is a view similar to Figure 1 showing a composite panel comprising a perforated metal foil according to another aspect of the application. DETAILED DESCRIPTION

[0021] With regard to Figure 1 A composite panel 10, for example for constructing a skin of a fuselage of an aircraft, comprises a honeycomb core 12 sandwiched between a first plurality of carbon layers 14 and a second plurality of carbon layers 16. Each of the first and second plurality of carbon layers 14, 16 is attached to the honeycomb core 12 using an adhesive 13, for example an epoxy resin. The first plurality of carbon layers 14 is arranged on a back side 11a of the panel 10, which is the side of the composite panel 10 that is cured against a vacuum bag used during manufacturing of the panel 10.

[0022] According to the application, the composite panel 10 comprises:​​

[0023] - a thin perforated metal foil 18 having a number of orifices 20 and directly fastened to a plurality of carbon layers, here the second plurality of carbon layers 16 of the example; Figure 1

[0024] - a protective layer 19 made of resin, for example epoxy resin, fastened to the perforated metal foil 18, in which the perforated metal foil 18 is embedded in the protective layer 19 through its orifices 20. The free surface of the protective layer 19 forms the top side lib of the composite panel 10, i.e. the side that will form the outer face of the skin of the aircraft and thus will be coated with protective and decorative paint when the panel 10 is arranged on the structure of the aircraft.

[0025] In an embodiment that is not part of the invention as claimed, the thin perforated metal foil 18 is indirectly fastened to the plurality of carbon layers, here the second plurality of carbon layers 16, via an optional barrier layer 17 made of electrically insulating material interposed between the perforated metal foil 18 and the plurality of carbon layers.

[0026] One particular purpose of the protective layer 19 is to prevent delamination of the perforated metal foil 18. To this end, the thickness of the protective layer between the top side 11a of the composite panel 10 and the metal foil 10 is at least 15 microns and preferably about 40 microns. In particular, the protective layer 19 contains a fibrous reinforcement to increase the mechanical resistance and thus, even the tendency to delaminate due to corrosion caused by the air flow mixed with dust and water droplets. This reinforcement (woven or non-woven) can be glass or carbon fiber and its typical weight per unit area is between 2 and 20 g / m2.

[0027] During the composite panel 10 manufacturing process, the protective layer 19 pre- impregnates the perforated metal foil 18, or, in one variant, it comes from the resin injected or infused in the composite panel 10 manufacturing process.

[0028] In any case, the weight per unit area of the surface film 19 is between 20 and 200 g / m2.

[0029] The perforated metal foil 18 is designed to have a very low weight to electrical conductivity ratio and, by these characteristics, the perforated metal foil 18 is used to protect the aircraft whose outer skin is formed by the composite panel 10 according to the invention from lightning strikes of considerable amplitude (for example, 200,000 amperes or more of Zone 1A lightning strikes) while allowing the aircraft manufacturer to produce lighter weight and more efficient aircraft.

[0030] Reference is made to Figure 2 ​FIG. 2 shows a perspective view of a portion 30 of a perforated metal foil 18 having circular orifices 20. The portion 30 of the perforated metal foil 18 is shown having a length L and a width W, which define a surface area A as L * W. A plurality of orifices 20 are distributed over the surface area A, which extend through the thickness T of the perforated metal foil 18.

[0031] The thickness T is no more than 30 microns, in order to limit the weight of the composite panel 10. By way of comparison, and with respect to Figure 3 The thinnest thickness TE achievable for a portion 40 of currently manufactured mesh material is approximately 40 microns. The mesh material is produced by simultaneously tearing and stretching the material. After such a process, the material will exhibit a plurality of holes / openings 42, which are Figure 3 diamond shaped in the present example. Due to the limitations of the meshing process, this is the thinnest material achievable, while still meeting the low resistivity / high conductivity and strength requirements.

[0032] Thinner metal perforated foils 18 are achievable; however, the size and spacing of the orifices, as well as the amount of open area relative to the total foil area must be optimized to achieve the desired weight, strength, adhesion capability, and conductivity. In this example, the portion 30 of the perforated metal foil 18 has uniformly spaced oval orifices, which can be formed in the foil by mechanically stamping the material or using a laser to cut or ablate the material to form the holes. From Figure 2 and Figure 3 it is evident that the portion 40 of the mesh metal material has a larger amount of open area (defined by the total area of the holes 42) relative to the total area AE (LE * WE), while the portion 30 of the perforated metal foil has a smaller amount of open area (defined by the total area of the holes 32) relative to the total area A. Limiting the open area allows the perforated metal foil 18 to be manufactured thinner, while still achieving the required strength and resistivity characteristics.

[0033] The open area of the perforated metal foil 18 typically needs to be equal to or lower than 40% of the total surface area of the perforated metal foil 18, in order to obtain a good compromise between weight and electrical performance.

[0034] The perforated metal foil 18 is also configured to provide the best adhesion quality, so that it can be laminated and effectively embedded in the resin layer (such as the protective layer 19). For the purposes of our description, good adhesion of the perforated metal foil 18 to the composite panel 10 after the curing cycle is the adhesion that prevents the perforated metal foil 18 from peeling off significantly (e.g., less than 25%) when a high water jet having the following parameters is applied: pressure, duration, distance, cleaning angle, cleaning frequency, water temperature, maximum peeled surface. The values of these parameters can be defined based on the requirements of the specific application.

[0035] The open area is maximized to allow the perforated metal foil 18 to be stretched and thus applied to a surface having a double curvature, such as can be the composite panel 10, to form an aircraft skin. The size and shape of the apertures 20 can vary, as will be explained below. However, when the perforated metal foil 18 of the composite panel 10 is subjected to a peel force, the shearing of the surface film thickness occurs at the top of the edge of the aperture 20. Therefore, to reduce the shearing constraint encountered by the protective layer 19 and thus increase the resistance to peeling, it is advantageous to maximize the sum of the perimeters of all the apertures 20 of each surface cell. For a given open area, this is obtained by reducing both the aperture size and the pitch. Therefore, the aperture size of the apertures 20 of the perforated metal foil 18 is defined so that the maximum distance between two opposite points on the perimeter of the aperture 20 is between 0.25 mm and 3 mm.

[0036] The limiting factor as to how small the apertures 20 can be made belongs to the perforation process and its associated speed.

[0037] In the case of perforation by mechanical punching / perforation, such as die-based perforation, the ability to manufacture small punches and the ability of the puncher will limit the minimum size of the apertures 20. It has been found that apertures 20 of 1 mm in size are close to the realistic limit.

[0038] In the case of perforation by laser etching, the apertures 20 can be made much smaller, i.e. between 0.25 mm and 1 mm.

[0039] Taking into account the ability of the perforated metal foil 18 to be stretched, the resistance to peeling, and the constraints from the perforation process, it has been found that a pattern realized by apertures 20 of approximately 1 mm in size and approximately 30% open area is convenient.

[0040] Typical metal materials that can be used for the perforated foil are copper, aluminum, and alloys thereof.

[0041] For a copper or copper alloy perforated foil 18, the plurality of apertures 20 collectively defines an open area of no more than 40% of the surface area. The perforated copper foil 18 has a weight of no more than 300 g / m 2 , preferably 115 g / m 2 . The resistance of such 115 g / m 2 foil 18 is no more than 3.5 milliohms per square meter. This results in a weight to conductivity ratio of 0.40 gram ohms per square meter. This is an improvement of more than 35% over the 40 micron thick meshed copper foil described above, which has a weight to conductivity ratio of 0.63 gram ohms per square meter. Using the perforated metal foil 18 and the processes used herein, it is contemplated that copper or copper alloy foils 18 as thin as approximately 12 microns can be realized at equivalent weight to conductivity ratios.

[0042] For the aluminum or aluminum alloy perforated foil 18 herein, the plurality of apertures 20 collectively define an open area of no more than 40% of the surface area. The perforated aluminum foil 18 can have a weight of no more than 250 g / m 2 . The perforated foil 18 can have a resistance of no more than 5.5 milliohms per square meter. This results in a weight-to-conductivity ratio of 0.19 grams ohm per square meter. This is an increase of over 70% compared to the meshed copper foil of 0.63 grams ohm per square meter. Using the perforated metal foil 18 and processing used herein, it is expected that aluminum or aluminum alloy foils as thin as approximately 12 microns can be realized with equivalent weight-to-conductivity ratios.

[0043] In the past, lightning strike applications have primarily utilized interwoven materials (resulting in a square pattern) or meshed materials (resulting in a diamond or hexagonal shape). By using perforated apertures, and with respect to Figures 4 to 8 , the open area in the perforated metal foil 18 can be formed by apertures of different shapes, sizes, spacing, and patterns.

[0044] In terms of electric field lines, the most efficient shape is circular, as the sharp corners that are necessarily prevalent in interwoven or meshed materials create higher flux densities in the area of the sharp corners. These areas of higher flux naturally increase the measured resistivity of the material.

[0045] Referring to Figure 4 , the portion 60a of the perforated metal foil 18 is shown to include oval-shaped apertures 20 of the same size. The size of the apertures 20 can be varied as needed for a particular application, so long as the maximum distance (length A) between two opposing points on the perimeter of the aperture 20 is between 0.25 mm and 3 mm, as this is needed to increase the peel tolerance.

[0046] The open area of the material can be determined by the following equation:

[0047] (1) Open Area = 1 - Desired Weight I (Thickness x Density)

[0048] As an example, take a desired weight of 100 grams per square meter (gsm) and use 17 micron thick copper as the desired material, which has a density of 8.89 x 106 g / m 3 , then the open area is determined to be:

[0049] (2) Open Area = 1 - 100 I (17 x 10 - 6 x 8.89 x 106) = 33.8%

[0050] A 17 micron thick copper material with a weight of 100 g / m 2 would be suitable for producing the perforated metal foil 18 according to the present application.

[0051] Once the target open area, size, and shape of the aperture are selected, the spacing of the apertures can be determined to achieve the final configuration. Continuing with the example above, for an elliptical aperture, the calculation is as follows:

[0052] (3) Elliptical area: pi (pi) * Aperture Length (A) / 2 * Aperture Height (B) / 2

[0053] (4) Open Area = Elliptical Area * 2 / (X) * (Y)

[0054] Referring to Figure 5 , portion 60b of perforated metal foil 18 is shown to include equi-sized diamond-shaped apertures 20, which can be used to form a 17-micron thick copper foil suitable for lightning strike applications. Referring to Figure 6 , portion 60c of perforated metal foil 18 is shown to include equi-sized oval-shaped apertures 20, which can also be used to form a 17-micron thick copper foil suitable for lightning strike applications.

[0055] As shown in Figures 4 to 6 , the use of elliptical, oval, or diamond-shaped apertures can be advantageous to obtain a material with a fine-tuned anisotropy in electrical resistance. The resistivity along the "X" direction will differ from the resistivity along the "Y" direction depending on the spacing and ratio of length to height of the apertures 20. At times it is desirable to use a perforated metal foil 18 with electrical conductivity properties that are approximately equal in all directions (i.e., "isotropic"). To achieve the isotropic feature of electrical conductivity, a symmetric pattern of circular or square form can be used.

[0056] Referring to Figure 7 , portion 70a of perforated metal foil 18 is shown to include equi-sized and symmetrically spaced circular apertures 20. Referring to Figure 8 , portion 70b of perforated metal foil 18 is shown to include equi-sized and symmetrically spaced square apertures 20.

[0057] While not shown, other shapes of apertures 20 (e.g., polygonal) are also possible in order to meet the needs of a particular electrical resistance of the perforated metal foil 18.

[0058] The method of manufacture of perforated metal foil 18 includes an in-line process of perforating a metal foil with apertures of specific sizes and spacing of various geometrical shapes to achieve the desired performance characteristics. This can be done using a perforating machine, such as the machine 50 depicted in Figure 9 .

[0059] According to this manufacturing method, a wide web of thin metal foil 52, less than 30 microns in thickness (with backing material (not visible)), is shown being fed under rollers 54 and into a cutting die 56. The backing material also helps to control the tension of the metal foil 52 as it is passed laterally through the cutting die 56. Although not shown in this view, the web of solid foil 52 can be processed in a reel to reel operation. In other words, the solid metal foil 52 with backing material can be fed into the machine 50 via an open reel, and the web of perforated metal foil can be received from the machine and collected on a take-up reel.

[0060] The metal foil 52 can be pre-treated to improve its durability and adhesion properties. In addition, the metal foil 52 can be passivated, which creates an inert surface that is resistant to rust and oxidation. Furthermore, the metal foil 52 can be coated with silane, which helps to provide the material with improved adhesion properties.

[0061] By perforating the thin metal foil 52, a perforated metal foil 18 is obtained having a plurality of pre-determined geometrically shaped apertures 20 extending through the thickness of the metal foil 52 and distributed over the entire surface area. The total amount of open area created by the apertures 20 is configured to provide maximum electrical conductivity while maintaining as light a weight as possible. The size of the apertures 20 is very important in order to maintain the desired strength, electrical conductivity, and effective adhesion properties, and the following sections describe exemplary methods of designing the open area of the perforated metal foil 18 according to the present application.

[0062] According to an aspect of the present application, an alternative method (not shown) to using a mechanical die based perforator, such as the machine 50, is to use a roll-to-roll galvanometer laser to accomplish precision cutting / ablation to manufacture the metal foil.

[0063] The present application has been described above when the composite panel 10 comprises a honeycomb core 12. In another variant of the present application, and with reference to Figure 10 , the composite panel 110 is monolithic and comprises a plurality of carbon layers 116 arranged on the back face 111a of the panel 110.

[0064] The thin perforated metal foil 118, which features exactly the same as the perforated metal foil 18 described in connection with Figures 1 to 8 the features of the perforated metal foil 18 described in connection with

[0065] Finally, a protective layer 119 made of resin (for example, epoxy-based resin) is fastened to the perforated metal foil 118, in which the perforated metal foil 118 is embedded in the protective layer 119 by passing through its orifices 120. The free surface of the protective layer 119 forms the top side 111b of the composite panel 110 (i.e., the side that, when the panel 110 is arranged on the structure of the aircraft, will form the outer face of the skin of the aircraft and will therefore be coated with protective and decorative paint).

[0066] The purpose of the protective layer 119 is to prevent the delamination of the perforated metal foil 118. To this end, the thickness of the protective layer between the top side 111a of the composite panel 110 and the metal foil 10 is at least 15 microns and preferably about 40 microns. In particular, the protective layer 119 can contain a fibrous reinforcement to increase the mechanical resistance and therefore even reduce the tendency to delaminate due to corrosion caused by the air flow mixed with dust and water droplets. This reinforcement (woven or non-woven) can be glass or carbon fiber and its typical weight per unit area is between 2 and 20 g / m2.

[0067] During the manufacture of the composite panel 110, the protective layer 119 (as a resin film) can be applied separately from the perforated metal foil 118, or it can also pre-impregnate the perforated metal foil 118, or it can also come from the resin injected or poured during the manufacture of the composite panel 110.

Claims

1. A composite panel (10, 110) for aerospace structures, the composite panel comprising a plurality of carbon layers (16, 116), characterized in that, The composite panel includes: a perforated metal foil (18, 118) having a plurality of apertures (20, 120) and directly fastened to the plurality of carbon layers (16, 116); and a protective layer (19, 119) made of a resin containing fiber reinforcement, the fiber reinforcement typically having a basis weight of 2 g / m². 2 Up to 20g / m 2 The protective layer (19, 119) is fastened to the metal foil (18, 118), wherein the perforated metal foil (18, 118) is embedded in the protective layer (19, 119) through its openings (20, 120), the free surface of the protective layer forms the top side (11b, 11b) of the composite panel (10, 110), and the thickness of the protective layer (19, 119) between the top side (11b, 11b) of the composite panel (10, 110) and the perforated metal foil (18, 118) is at least [missing information]. The thickness of the perforated metal foil (18, 118) is no more than 30 micrometers, the orifices have a predetermined geometry, extend through the thickness (T) of the perforated metal foil (18, 118), and are distributed on a surface area defined by the length (L) and width (W) of the perforated metal foil (18, 118), the plurality of orifices (20, 120) collectively define an opening area of ​​no more than 40% of the surface area, and the maximum distance between two opposite points on the periphery of the orifice (20) is equal to or less than 3 mm.

2. The composite panel (10) according to claim 1, characterized in that, The perforated metal foil (18) is made of copper or a copper alloy and weighs no more than 300 g / m. 2 The weight-to-conductivity ratio does not exceed 0.40 g ohms per square meter.

3. The composite panel (10) according to claim 1, characterized in that, The perforated metal foil (18) is made of aluminum or aluminum alloy and weighs no more than 250 g / m. 2 The weight-to-conductivity ratio does not exceed 0.19 g ohms per square meter.

4. The composite panel (10) according to any one of claims 1 to 3, wherein, The predetermined geometry of the plurality of orifices (20) is circular.

5. The composite panel (10) according to any one of claims 1 to 3, wherein, The predetermined geometry of the plurality of orifices (20) is non-circular.

6. The composite panel (10) according to claim 5, wherein, The predetermined geometry of the plurality of orifices (20) is one of ellipse, rhombus, oval and square.

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