Patent Publication Number: US-2010108342-A1

Title: Layered structure with outer lightning protection surface

Description:
BACKGROUND 
     The present application relates generally to the protection of structures that may be subjected to lightning. In particular, the application subject matter reduces the occurrence of micro-cracking in such structures when a sacrificial metal layer outer surface is used for lightning protection. 
     Many structures may need protection from lightning. For example, lightning protection is a requirement on many Fiber Reinforced Plastic (FRP) aerospace structures and other composite parts that may be subjected to lightning. While the FRP matrix may be conductive, the FRP structure may not disperse the highly concentrated energy from a lightning strike quickly enough to prevent delamination and embrittlement of the structure. A lightning strike on an unprotected FRP structure may thus result in complete failure, leaving a hole in the FRP structure. 
     Historically, one engineering approach to protecting FRP structures from lightning has been to include a thin layer of metal foil or screen in the outer layer of the composite. When struck by lightning, the metal layer is vaporized into a plasma ball which disburses the energy, thereby sacrificially protecting the FRP matrix underneath from severe damage. The metal outer surface layer may be solid foil, expanded foil, woven wire screen, wire interwoven into the FRP matrix, or have some other configuration. 
     The metals used in the sacrificial metal outer surface layer are selected for their ability to absorb energy, electrical conductivity, and chemical inertness relative to the graphite fibers or other components in the FRP. However, these metals may not have the same coefficient of expansion (COE) as the rest of the FRP structure. As the protected FRP structure undergoes changes in temperature during its lifetime, the COE difference between layers may result in independent movement between the sacrificial metal outer surface layer and the FRP matrix, inducing stresses that can lead to micro-cracking within the FRP structure. These micro-cracks can lead to discoloration and corrosion and may reduce the strength of the FRP structure, or lead to delamination and complete failure. 
     The COE difference between the sacrificial metal outer surface layer and the rest of the FRP structure is believed to be a driving force behind micro-cracking. However, the inventors herein believe (without being bound by that theory) that the existence of sharp edges or points on the metal foil may serve as stress concentration sites, which may initiate micro-cracking. A reduction in the number or severity of these stress concentration sites could greatly reduce or eliminate the degree of micro-cracking. 
     SUMMARY 
     According to one aspect of the present invention, a layered structure is provided including an underlying structure to be protected from lightning and an electrically conductive outer surface layer attached to the underlying structure. The electrically conductive outer surface layer is subjected to a stress concentration site reducing operation. 
     According to a further aspect of the present invention, a method of making a layered structure is provided. An electrically conductive outer surface layer is formed, stress concentration sites on the electrically conductive outer surface layer are reduced, and the electrically conductive outer surface layer is combined with an underlying structure to be protected from lightning. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross section drawing of layers of an exemplary Fiber Reinforced Plastic (FRP) composite structure with an electrically conductive outer surface layer; 
         FIG. 2  is a drawing of an exemplary expanded metal foil, illustrating exemplary stress concentration sites; 
         FIG. 3  is a cross section drawing of a metal strand of an exemplary expanded metal foil, taken along line  3 - 3  in  FIG. 2 ; 
         FIG. 4  is a cross section drawing of the metal strand of  FIG. 3  interfaced with an FRP composite structure; 
         FIG. 5  is a cross section drawing of the metal strand of  FIG. 3  immersed into an FRP composite structure; 
         FIG. 6  is a cross section drawing of a metal strand of an exemplary expanded metal foil in which the number or severity of stress concentration sites has been reduced; and 
         FIG. 7  is a cross section drawing of the metal strand of the exemplary expanded metal foil of  FIG. 6  interfaced with an FRP composite structure. 
     
    
    
     DESCRIPTION 
     An exemplary Fiber Reinforced Plastic (FRP) composite structure  100  is shown in  FIG. 1 . The composite structure  100  is shown with an FRP matrix layer  102  and an electrically conductive outer surface layer  104 . The FRP matrix layer  102  is attached to the electrically conductive outer surface layer  104  at interface  106 . The electrically conductive outer surface layer  104  protects the FRP matrix layer  102  from damage when the composite structure  100  is subjected to lightning  108 . 
     The electrically conductive outer surface layer  104  may be produced from a metal. For example, the electrically conductive outer surface layer  104  may be in the form of a solid foil, an expanded foil, a woven wire screen, or a wire interwoven into the FRP matrix layer  102 . Exemplary metals that may be used for the electrically conductive outer surface layer  104  include aluminum (and its alloys), copper (and its alloys such as brass and bronze), nickel (and its alloys such as monel), tantalum, stainless steel, niobium, and titanium. These and any other metals that may be used for the electrically conductive outer surface layer  104  may be passivated through processes such as anodization or other oxidization methods. Ideally, the outer surface layer  104  is made from a material or combination of materials that is chemically inert with respect to the underlying structure  102 , has a relatively large heat of fusion and heat of vaporization, and a relatively low electrical resistance. 
     The electrically conductive outer surface layer  104  may alternatively be made from a non-metal, yet electrically conductive, material or combination of materials. For example, the electrically conductive outer surface layer  104  may be an expanded polymer plastic that has been coated with an external metal layer, for example, via plating or vapor deposition. 
       FIG. 2  is a drawing of an exemplary expanded metal foil  200  that can be used as the electrically conductive outer surface layer  104 . The expanded metal foil  200  may have a lattice-like grid of metal strands  202  separated by openings  204 . For example, as depicted in  FIG. 2 , the metal strands  202  may be configured with diamond-shaped openings  204 . However, the expanded metal foil  200  may be configured with metal strands  202  and openings  204  of any shape and size suitable for a particular application. 
     The expanded metal foil  200  may be produced, for example, by a method of slit and stretch. In this manner, a precision die can slit and stretch the metal material in as little as one operation. The metal material can then be directed through a set of rollers to adjust the metal material to a final thickness for the expanded metal foil  200 . The shape, form, and number of openings are dictated by the particular tool used and may be modified or changed to suit a particular application. 
     As with many metal forming operations, the resultant expanded metal foil  200  may have various burrs, chads, or sharp edges along the metal strands  202 . In general, such stress concentration sites may be expected and accepted by the user of the finished part. These characteristics can vary in many ways, such as shape, size, number, location, and severity.  FIG. 3  is a cross section drawing of a metal strand  202  in an exemplary expanded metal foil  200 , taken along line  3 - 3  in  FIG. 2 , showing such stress concentration sites. For example, a burr  204  is shown extending from the surface of the metal strand  202 ; a chad  206  is shown hanging from the edge of the metal strand  202 ; and sharp edges  208  are shown on the edges of the metal strand  202 . For illustration purposes, the burr  204  and chad  206  are drawn relatively large, but they may also be very small, and in many cases imperceptible without magnification. Although the exemplary stress concentration sites shown in  FIG. 3  are along the sides of the metal strands  202 , the burrs  204 , chads  206 , or sharp edges  208  may occur anywhere throughout the expanded metal foil  200 , including on the relatively flat sides of the strands  202  and in the areas where two or more metal strands  202  intersect, forming corners in the grid. 
     As can be appreciated by referring to the cross section drawing of a composite structure  400  in  FIG. 4 , an electrically conductive outer surface layer  404  may be the expanded metal foil  200  with metal strands  202 . It should be evident that the burrs  204 , chads  206 , sharp edges  208  and similar stress concentration sites along the metal strands  202  located at interface  406  will be in contact with an FRP matrix layer  402 .  FIG. 4  shows the composite structure  400  with the electrically conductive outer surface layer  404  and the FRP matrix layer  402  interfacing along a line at the interface  406 . 
     Alternatively, as shown for example in  FIG. 5 , a composite structure  500  may include an electrically conductive outer surface layer  504 , such as the expanded metal foil  200 , interwoven or immersed into the surface of an FRP matrix layer  502 . In this way, a portion or all of the expanded metal foil  200  may be disposed within the FRP matrix layer  502 . This results in an interface  506  that at least partially surrounds the immersed metal strand  202  surfaces of the expanded metal foil  200 . 
     As the composite structure  400  or  500  undergoes changes in temperature during its lifetime, any coefficient of expansion (COE) difference between the electrically conductive outer surface layer  404 ,  504  and the FRP matrix layer  402 ,  502  could result in independent movement and stress at the interface  406 ,  506 , potentially resulting in micro-cracking within the composite structure  400 ,  500 , and in particular, the FRP matrix layer  402 ,  502 . Burrs  204 , chads  206 , sharp edges  208  and similar stress concentration sites on the metal strands  202  of the expanded metal foil  200  serve as stress concentration sites for micro-cracks  410 ,  510 . For illustration purposes, the micro-cracks  410 ,  510  are drawn relatively large, but they may be very small, and in many cases may be imperceptible without magnification. The micro-cracks  410 ,  510  are also drawn originating from the tips of the burr  204 , chad  206 , or sharp edge  208 . However, the micro-cracks  410 ,  510  can originate from any area where burrs  204 , chads  206 , sharp edges  208  or similar stress concentration sites make contact with the FRP matrix layer  402 ,  502 . In some cases, a micro-crack  410 ,  510  can start in the area around the burr  204 , chad  206 , or sharp edge  208 , but not originate directly from the surface of the burr  204 , chad  206 , or sharp edge  208 . Consequently, any burrs  204 , chads  206 , sharp edges  208  or the like occurring within the interface  406 ,  506  area are potential initiation sites for micro-cracking  410 ,  510 . 
     A reduction in the number or severity of these stress concentration sites may greatly reduce the degree and severity of micro-cracking  410 ,  510 . Once initiated, micro-cracks  410 ,  510  typically propagate further when exposed to additional expansion and contraction stress, vibration, other materials, or other stresses. 
     Burrs  204 , chads  206 , sharp edges  208  and similar stress concentration sites on the expanded metal foil  200  may be reduced and their effects neutralized by post expansion processing. Such processing may reduce either the number, or severity, or both, of the stress concentration sites. Chemical and electrochemical etching are exemplary methods of post expansion processing of the expanded metal foil  200 . Other potential post expansion processing methods include mechanical micro-deburring.  FIG. 6  shows a metal strand  602  of a processed expanded metal foil  600 . For example, the processed metal strand  602  in  FIG. 6 , when compared to the unprocessed metal strand  202  of  FIG. 3 , shows the effects of post expansion processing on the expanded metal foil  600 . In particular, the burr  204  is processed into a rounded hump  612 , the chad  206  is removed and replaced with a dull edge  614 , and the sharp edges  208  have also been processed into dull edges  614 . 
       FIG. 7  shows a composite structure  700  with an electrically conductive outer surface layer  704  and an FRP matrix layer  702  interfacing along a line at interface  706 . The composite structure  700  is shown with the metal strand  602  of the processed expanded metal foil  600  from  FIG. 6  as the electrically conductive outer surface layer  704 . For example, the processed metal strand  602  in  FIG. 7 , when compared to the unprocessed metal strand  202  of  FIG. 4 , shows the effects of post expansion processing on the composite structure  700 : a reduction in the number and severity of the stress concentration sites, thus reducing or eliminating micro-cracks  710 . 
     Chemical etching may be done using a variety of chemical or electrochemical processes that preferentially free or remove material from burrs  204 , free chads  206 , and remove material from sharp edges  208  of the expanded metal foil  200 . Mechanical micro-deburring may remove burrs  204  and chads  206  and dull the sharp edges  208  of the expanded metal foil  200  without changing the general shape of the expanded metal foil  200 . These processes result in a reduction in the number and severity of stress concentration sites on the expanded metal foil  200  and, consequently, reduce the likelihood of micro-crack initiation sites forming within the composite structure  100 ,  400 ,  500 ,  700 . 
     Although embodiments of the invention have been shown and described, it is understood that equivalents and modifications will occur to others in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications.