Patent Application
20180257790
This invention relates to a metallic foil used for lightning strike protection in composite aerospace structures and more particularly to such a metallic foil which is ultra-thin and light weight and yet has very low resistivity.
Aluminum has been the principal material used in aircraft and aerospace construction for the past 60 years. With the growing interest to construct more efficient aircraft, manufacturers are designing more components out of light-weight composite materials. Current composite structures include the fuselage, wing skins, engine nacelles, control surfaces, wing tips (winglets), and even rotary blades on helicopters and wind turbines. Composites, however, are poor conductors of electrical current. Without proper protection, composite materials are susceptible to severe damage in the event of a lightning strike. To date, aircraft manufacturers have used aluminum or copper expanded foils or woven wire mesh incorporated into the surface of these composite structures to dissipate lightning strike energy and prevent damage to the composite material.
Of the two approaches, expanded metal foils have become the industry standard and are superior to woven wire as they do not unravel or have loose strands that may become problematic during processing into a pre-preg material or when conducting a dry lay-up as part of the composite manufacturing process. The homogenous design of expanded metal foils also ensures uncompromised conductivity even when forming the material into a variety of shapes and contours and it provides a smooth surface on the end product. Expanded metal foils used in this application must be manufactured with tight tolerances to meet a specific weight, open area, and conductivity requirements.
Aircraft manufactures use design guidelines, such as those set forth by SAE International in its Aerospace Recommended Practice (ARP) 5414, which defines lightning strike zones (areas of the aircraft more susceptible to lightning strikes (e.g. Zone 1A, 1B). It also provides required electrical withstand capabilities for such strike zones. For example, often materials are required to have the ability to withstand a Zone 1A strike of 200,000 amps. For expanded foils, due to the limitations of the expansion process, the thinnest material possible to meet this criteria to date has been produced using 42 micron foils. The weight of this material is 175 grams per square meter, the resistivity is 3.6 milliohms per square, and the foil has 56% open area. A way to characterize the performance of foils in this application is to assess the foil's weight to conductivity ratio, with conductivity being the inverse of resistivity and represented in gram-ohms per square. For the above expanded foil, its weight to conductivity ratio is 0.63 gr-ohms.
Aircraft manufacturers are always looking for ways to increase efficiency, reduce costs, improve fuel economy, and reduce the amount of CO2 emissions. One clear way to achieve these objectives is to reduce aircraft weight. By reducing the weight of the composite material, the overall weight of the aircraft may be reduced; however, the conductivity criteria required for specific strike zones per SAE ARP5414 must still be satisfied. Thus, it would be desirable to produce thinner and lighter foils, which still meet the required electrical withstand capabilities. Another way of stating this is that it would be very desirable to minimize the foil weight to conductivity ratio.
The benefits and advantages of the present invention over existing systems will be readily apparent from the Detailed Description to follow. One skilled in the art will appreciate that the present teachings can be practiced with embodiments other than those summarized or disclosed below.
In one aspect the invention features a metallic foil for lightning strike protection in a composite aerospace structure. The metallic foil comprising copper or a copper alloy having a length, a width, a thickness of not more than 30 microns. There are a plurality of pores of a predefined geometric shape extending through the thickness of the metallic foil and being distributed across a surface area defined by the length and the width of the metallic foil. The plurality of pores in the aggregate define an open area of not more than 40% of the surface area. The metallic foil has a weight of not more than 115 g/m2 and a weight to conductivity ratio of not more than 0.40 gram-ohms per square.
In other aspects of the invention one or more of the following features may be included. The predefined geometric shape of the plurality of pores may be non-circular. The predefined geometric shape of the plurality of pores may be one or more of elliptical, diamond, oval, hexagonal, and square. The area of each of the predefined geometric shapes may be between 0.5×10−3 and 5.0×10−3 square inches. A strand width between the predefined geometric shapes may be not greater than 0.050 inches. The metallic foil may have an isotropic resistance. The top surface and the bottom surface may include a coating of an inert material that is resistant to tarnishing and oxidation. The top surface and the bottom surface may include a coating of silane to improve adhesion to other surfaces.
In yet another aspect, the invention features a metallic foil for lightning strike protection in a composite aerospace structure, wherein the metallic foil comprises aluminum or an aluminum alloy having a length, a width, a thickness of not more than 30 microns. There are a plurality of pores of a predefined geometric shape extending through the thickness of the metallic foil and being distributed across a surface area defined by the length and the width of the metallic foil. The plurality of pores in the aggregate define an open area of not more than 40% of the surface area. The metallic foil has a weight of not more than 35 g/m2 and a weight to conductivity ratio of not more than 0.19 gram-ohms per square.
In further aspects of the invention one or more of the following features may be included. The predefined geometric shape of the plurality of pores may be non-circular. The predefined geometric shape of the plurality of pores may be one or more of elliptical, diamond, oval, hexagonal, and square. The area of each of the predefined geometric shapes may be between 0.5×10−3 and 5.0×10−3 square inches. A strand width between the predefined geometric shapes may be not greater than 0.050 inches. The metallic foil may have an isotropic resistance. The top surface and the bottom surface may include a coating of an inert material that is resistant to tarnishing and oxidation. The top surface and the bottom surface may include a coating of silane to improve adhesion to other surfaces.
In an additional aspect, the invention features a method of making a metallic foil for lightning strike protection in a composite aerospace structure. The method includes providing a metallic foil comprising a copper or a copper alloy having a length, a width, and a thickness of not more than 30 microns. The method also includes forming in the metallic foil a plurality of pores of a predefined geometric shape extending through the thickness of the metallic foil and being distributed across a surface area defined by the length and the width of the metallic foil. The plurality of pores in the aggregate define an open area of not more than 40% of the surface area. The metallic foil has a weight of not more than 115 g/m2 and a weight to conductivity ratio of not more than 0.40 gram-ohms per square.
In other aspects of the invention one or more of the following features may be included. The predefined geometric shape of the plurality of pores may be one or more of elliptical, diamond, oval, and square. The area of the predefined geometric shapes may be between 0.5×10−3 and 5.0×10−3 square inches and wherein a strand width between the predefined geometric shapes is not greater than 0.050 inches. The step of forming may include perforating the plurality of pores in the metallic foil. The step of perforating may be accomplished using a perforating die to pierce the metallic foil. The step of perforating may be accomplished using a laser to cut or ablate the metallic foil.
In a further aspect, the invention features method of making a metallic foil for lightning strike protection in a composite aerospace structure, including providing a metallic foil comprising an aluminum or an aluminum alloy having a length, a width, and a thickness of not more than 30 microns. The method includes forming in the metallic foil a plurality of pores of a predefined geometric shape extending through the thickness of the metallic foil and being distributed across a surface area defined by the length and the width of the metallic foil. The plurality of pores in the aggregate define an open area of not more than 30% of the surface area. The metallic foil has a weight of not more than 35 g/m2 and a weight to conductivity ratio of not more than 0.19 gram-ohms per square.
In other aspects of the invention one or more of the following features may be included. The predefined geometric shape of the plurality of pores may be one or more of elliptical, diamond, oval, and square. The area of the predefined geometric shapes may be between 0.5×10−3 and 5.0×10−3 square inches and wherein a strand width between the predefined geometric shapes may be not greater than 0.050 inches. The step of forming includes perforating the plurality of pores in the metallic foil. The step of perforating may be accomplished using a perforating die to pierce the metallic foil. The step of perforating may be accomplished using a laser to cut or ablate the metallic foil.
These and other features of the invention will be apparent from the following detailed description and the accompanying figures.
Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:
The disclosure and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments and examples that are described and/or illustrated in the accompanying drawings and detailed in the following description. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale, and features of one embodiment may be employed with other embodiments as the skilled artisan would recognize, even if not explicitly stated herein. Moreover, all listed values throughout the specification are nominal values and are subject to normal manufacturing tolerances.
Descriptions of well-known components and processing techniques may be omitted so as to not unnecessarily obscure the embodiments of the disclosure. The examples used herein are intended merely to facilitate an understanding of ways in which the disclosure may be practiced and to further enable those of skill in the art to practice the embodiments of the disclosure. Accordingly, the examples and embodiments herein should not be construed as limiting the scope of the disclosure. Moreover, it is noted that like reference numerals represent similar parts throughout the several views of the drawings.
The metallic foil according to an aspect of the invention is designed to be ultra-thin, very light weight, and to have very low resistivity. As a result, very low weight to conductivity ratios may be achieved. With these characteristics the foil may be used to protect aircraft from lightning strikes of a significant magnitude (e.g. Zone 1A strikes of 200 amps or more), while allowing the aircraft manufacturer to produce a lighter weight and more efficient aircraft. The metallic foil is also configured to provide optimal adhesive qualities so that it can be layered into and effectively adhered to composite structures. The typical types of metallic material which may be used are copper, aluminum, and alloys thereof.
For a copper or copper alloy foil herein, the thickness may be not more than 30 microns. The foil may have a plurality of pores which, in the aggregate, define an open area of not more than 40% of the surface area. The weight of the foil may be not more than 115 g/m2. The foil may have a resistance of not more than 3.5 milliohms per square. This yields a weight to conductivity ratio of 0.40 gr-ohms. Compared with the 42 micron thick expanded copper foil described above, which has a 0.63 gr-ohms weight to conductivity ratio, this is over a 35% improvement. With the foils and processing used herein, it is expected that copper or copper alloy foils as thin as approximately 12 microns would be achievable with comparable weight to conductivity ratios.
For an aluminum or aluminum alloy foil herein, the thickness may be not more than 30 microns. The foil may have a plurality of pores which, in the aggregate, define an open area of not more than 40% of the surface area. The weight of the foil may be not more than 35 g/m2. The foil may have a resistance of not more than 5.5 milliohms per square. This yields a weight to conductivity ratio of 0.19 gr-ohms. Compared with the 0.63 gr-ohms of the expanded copper foil, this is over a 70% improvement in the weight to conductivity ratio. With the foils and processing used herein, it is expected that aluminum or aluminum alloy foils as thin as approximately 12 microns would also be achievable with comparable weight to conductivity ratios.
Foil characteristics such as thickness, weight, and resistivity are readily quantifiable; however, adhesion is not. For purposes of our description, good adhesion of the foil to a composite panel after a curing cycle is one that prevents substantial peeling (e.g. less than 25%) of the foil upon application of a high water jet device following, for example, Volvo STD1049.5134, with the following parameters: pressure, duration, distance, sweeping angle, sweeping frequency, water temperature, maximum peeled surface. The values for these parameters may be defined based of the requirements of the particular application.
To provide context, the metallic foil according to an aspect of the invention is shown embedded in a cross-section of a composite panel 10,
Referring to
Thinner foils are achievable according to this invention; however, the pore size and spacing, as well as the amount of open area relative to total foil area must be optimized to achieve desired weight, strength, adhesion capability, and conductivity. Portion 30 of foil 18 in this example has uniformly spaced elliptical-shaped pores, which may be formed in the foil by mechanically punching the material or using a laser to cut or ablate the material to form the pores. It is apparent from
Before describing in more detail below how the open area in metallic foil 18 may be formed with pores of different shapes, sizes, spacing and patterns the manufacturing process for such metallic foils is described.
A method of manufacturing of the ultra-thin metallic foil according to an aspect of this invention may include an in-line process of perforating the metallic foil with specifically sized and spaced holes of various geometric shapes to achieve desired performance characteristics. This may be accomplished using a perforating machine, such as machine 50 depicted in
A key element of the die-based perforation process is the use of a backing material, such as 40 lb Kraft paper, to “carry” the ultra-thin foils through the perforating process. The use of the carrier essentially adds thickness to the material being perforated, which prevents tearing and also helps to provide a precise, sharp cut with minimal burring. Referring again to
An alternate approach to using a mechanical die-based perforating machine, such as machine 50, would be to use a reel-to-reel galvanometric laser to accomplish the precision cutting/ablating to manufacture the metallic coils according to an aspect of this invention. Although more costly, laser technologies have advanced to the point where this process is an effective alternative to die-based perforating, especially since the material is thin and pore sizes are relatively small.
The metallic foil 52 may be pretreated to improve its durability and adhesion characteristics. In addition, the metallic foil may be passivated, which creates an inert surface that is resistant to tarnishing and oxidation. Also, the metallic foil may be coated with silane, which helps to provide the material with improved adhesion characteristics.
By perforating the thin metallic foil 52, a plurality of openings of a predefined geometric shape extending through the thickness of the metallic foil and being distributed across a surface area may be formed. The aggregate amount of open area formed by the openings is configured to provide maximum conductivity while maintaining as light a weight as possible, which may be as light as 115 grams per square meter or less. Sizing of the openings/perforations is very important in order to maintain desired strength, conductivity, and effective adhesion properties and the following sections describe exemplary methods for designing the open area for metallic foils according to this invention.
The open area of the metallic foil according to this invention generally needs to be below 40% of the overall surface area of the metallic foil in order to maintain sufficient strength and conductivity. Without sufficient strength, the foil will tear or break when incorporating it into into a pre-preg or surfacing film and/or during manufacturing the composite laminate structure. Without enough foil material, the required amount of conductivity needed to absorb the significant electrical current generated during a lightning strike will not be achieved. At the same time, it is desired to maximize open area to minimize the weight and allow the laminate adhesives to effectively seep through the perforations and create “wet-out” uniformly to the outer surface of the structure, releasing any air bubbles and creating a smooth, homogeneous strong bond between the layers of the composite. In comparison, prior to the current metallic foils, the lowest amount of open area in expanded foils is approximately 55% open area, but thicker and heavier material must be used to provide the required strength and conductivity.
For proper adhesion, the pore area is optimal when it is between 0.5×10−3 and 5.0×10−3 square inches. Historically, lightning strike applications have primarily utilized either a woven material, which yields a square pattern, or expanded material, which yields a diamond or hexagonal shapes. By using perforated pores, different shapes may be used, including ovals, circles, and elliptical shapes. The most efficient shapes, in terms of electric field lines, are rounded, as sharp corners that are necessarily prevalent in woven or expanded materials create higher flux density in the areas of the sharp corners. These higher flux areas naturally increase the measured resistivity of the material.
Referring to
The open area of material may be determined by the following equation:
Open Area=1−Desired Weight/(Thickness×Density) (1)
As an example, taking a desired weight of 100 grams per square meter (gsm) and using 17 micron thick copper as the desired material, which has a density of 8.89×106 g/m3, the open area is determined to be:
Open Area=1−100/(17×10−6×8.89×106)=33.8% (2)
As noted above, open area of less than 40% of the overall surface area of the metallic foil is required in order to maintain sufficient strength and conductivity. Therefore, 17 micron thick copper material having a weight of 100 gsm will be suitable to produce a metallic foil according to the invention.
Once the targeted open area, pore size and shape are selected, the spacing of the pores can be determined to achieve the resulting configuration. Carrying on with the example above, for an elliptical shape pore, calculations are as follows:
Area of ellipse: pi*pore length(A)/2*pore height(B)/2 (3)
Open area=ellipse area*2/(X)*(Y) (4)
With spacing between pores being equal in both X and Y directions:
X=SQRT((3.1416*A*B/2)/(open area)) (5)
Using as an example, an elliptical pore size of 0.060×090 inches, with a desired open area of 33.8%:
X=SQRT(3.1416×0.06 in×0.09 in/2/0.32)=0.158 in (6)
With a pore spacing of 0.158 inches, this configuration measures a strand width (“S”) of approximately 0.049 inches or 31% of X/Y. Since 0.050 inches is the largest targeted strand width, S, as noted previously, this pore size would approximately be the maximum size recommended for this material for use in a lightning strike application.
Similar configurations can be implemented with the above pore dimensions using different shapes, including diamonds and ovals. Referring to
Using ovals, ellipses, or diamond shape pores, as shown in
Referring to
The following is a table of exemplary configurations using 18 micron copper foil, including differing geometric shapes and sizes that have been determined to optimally meet adhesion, thickness, weight, and conductivity requirements according to this invention.
In Table 1 are included the specifications of the pores contained in various 18 micron foils made of copper, including perforation shape, size (dimensions and area), and spacing. Also included are the amount of open area in each foil design, weight, maximum resistance, and resistance characteristics, i.e. isotropic or anisotropic. With the anisotropic foils the resistance ratio across the width of the foil relative to the resistance along the length of the foil differs (e.g. 1.3:1), while the isotropic foils have a ratio of 1:1.
The following is a table of exemplary configurations using 25 micron aluminum foil, including differing geometric shapes and sizes that have been determined to optimally meet adhesion, thickness, weight, and conductivity requirements according to this invention.
While we have described several specific examples of metallic foils, these are not intended to limit the invention and, using the design criteria provided herein, many different configurations of foils may be produced for various applications. Accordingly, many foil designs falling within the bounds of the invention may be produced with very beneficial attributes; namely, foils that are ultra-thin (i.e. 30 microns or less), very light weight (i.e. 115 grams/m2 or less), and very low weight to conductivity ratios (i.e. 0.40 gram-ohms). Notwithstanding broad range of foil designs possible, the following general parameters for an optimized design are provided: opening size area of 0.5×10−3 to 5.0×10−3 square inches, open area between 25% and 35%, and spacing maintained between 0.100 and 0.300 inches. Foils with these criteria allow for very good adhesion and conductivity in both directions while also providing a very light weight foil.
It should be noted that some possible foil designs may have higher resistivity values, such as the 25 micron aluminum films in Table 2. These foils have resistivity values from 4.5-6.3 milliohms per square, which exceeds the maximum value of 3.5 milliohms per square for zone 1A lightning strike protection. However, these foils still have very low weight to conductivity ratios and are very good candidates for less intense lightning strike protection, e.g. Zone 2 lightning strike protection.
In comparison to the prior art expanded foil, the weight to conductivity ratio is significantly improved, with the current foil while still maintaining good adhesion properties. This is due to the reduced foil thickness and specific pore geometry and spacing. For the 18 micron copper foil samples from Table 1, improvements in weight to conductivity ratios of greater than 50-65% relative to the 42 micron expanded foil described above may be obtained. And, even greater improvements can be obtained with the aluminum foil. For the 25 micron aluminum foil samples from Table 2, improvements in weight to conductivity ratios of greater than 75-80+% relative to the 42 micron expanded foil may be obtained
While the foregoing description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiments and examples herein. The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto. The invention is therefore not limited by the above described embodiments and examples.
