Patent Publication Number: US-10775330-B2

Title: Thermo-chromatic witness features for lightning strike indication in both metallic and composite structures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of and claims the benefit of U.S. patent application Ser. No. 15/143,958, filed on May 2, 2016, now published as U.S. Patent App. Publication No. 2017/0315072, and entitled “Thermo-Chromatic Witness Features for Lightning Strike Indication in Both Metallic and Composite Structures,” the contents of which are hereby incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure is generally related to thermo-chromatic witness features for lightning strike indication in both metallic and composite structures. 
     BACKGROUND 
     With the increasing use of carbon fiber reinforced polymer (CFRP) composite structures in the aerospace and commercial aircraft industry, there is an increasing need for the advancement of efficient non-destructive evaluation (NDE) methods for the detection of composite damage. An especially dangerous form of damage results from lightning strike, which can induce thermal damage that is difficult to detect. 
     Typical inspection methods for thermal damage due to lightning strikes may include using ultrasonic instrumentation to determine whether damage, such as delamination, separation, and/or cracking, has occurred. Another method of inspection includes performing a Fourier transform infrared (FTIR) analysis of a spectral signal taken from the aircraft to correlate a visual inspection to heat damage for specific resin coatings. However, these methods are time consuming and may be inaccurate. 
     Other methods for detecting damage due to lightning strikes may involve determining a position of an initial strike point based on marks, discoloration, or damage caused by the initial lightning strike, and inspecting each component of the aircraft in proximity to the initial strike point. However, many of the components inspected may fall outside of a particular pathway followed by the electrical current from the lightning strike and are therefore unlikely to have been damaged. Because it is unknown which pathway the current may have taken, these components may be needlessly inspected. The inspections may also include time-consuming, expensive, and/or invasive dismantling processes. What is needed is a quick, reliable, and non-invasive method for determining a pathway taken by current from a lightning strike through a structure, and for determining the extent to which components along the pathway may have been damaged. 
     SUMMARY 
     In an embodiment, a system includes a structure and a thermo-chromatic material applied to the structure. The thermo-chromatic material is adapted to change to a color locally in response to localized heating of the structure. The heating may be caused by an electrical current from a lightning strike. By examining the position and extent of the color change, an estimation of damage to the structure due to the lightning strike may be determined. 
     In an embodiment, a method includes changing a material applied to a portion of a structure to a first color in response to heating of the portion of the structure to a first threshold temperature due to an electrical current within the structure. The method further includes detecting a pathway of the electrical current through the structure based on a position of the first color. 
     In some embodiments, the method also includes estimating, based at least on the first color, an extent of damage produced by the electrical current along the detected pathway of the electrical current through the structure. In some embodiments, the method includes changing a material applied to a second portion of the structure to a second color distinct from the first color in response to heating of the second portion of the structure to a second threshold temperature greater than the first threshold temperature. In some embodiments, the method includes successively changing additional materials applied to additional portions of the structure to additional colors distinct from the first color and distinct from each other in response to heating of the additional portions of the structure to additional successively increasing threshold. In some embodiments, the method includes estimating, based at least on the first color and the additional colors, an extent of damage produced by the electrical current along the detected pathway of the electrical current through the structure. 
     In some embodiments, the method includes scanning a surface of the structure with a detector. The method further includes receiving, at the detector, light from the structure. The method also includes locating the position of the first color based on the light from the structure. In some embodiments, the method includes directing a light source at the structure during the scanning. The light source may be an ultraviolet light source and the first color may be substantially invisible under non-ultraviolet light. In some embodiments, the first threshold temperature is between 200° C. and 300° C. 
     In an embodiment, a system includes a structure and a first material applied to a portion of the structure. The material is adapted to change to a first color locally in response to localized heating of the portion of the structure to a first threshold temperature due to an electrical current within the structure. The system further includes a detector configured to receive light from the structure to enable detection of a pathway of the electrical current through the structure based on a position of the first color. 
     In some embodiments, the first color enables estimating an extent of damage produced by the electrical current along the pathway of the electrical current through the structure. In some embodiments, the system further includes a second material applied to a second portion of the structure. The second material is adapted to change to a second color, distinct from the first color, in response to heating of the portion of the structure to a second threshold temperature greater than the first threshold temperature. In some embodiments, the system also includes additional materials applied to additional portions of the structure. The additional materials are adapted to change to additional colors, distinct from the first color and distinct from each other, in response to heating of the additional portions of the structure to additional successively increasing threshold temperatures. In some embodiments, the first color and the additional colors enable estimating an extent of damage produced by the electrical current along the detected pathway of the electrical current through the structure. 
     In some embodiments, the system also includes a light source directed toward the structure. The light source may be an ultraviolet light source and the first color may be substantially invisible under non-ultraviolet light. 
     In some embodiments, the structure includes a copper foil applied to a composite aircraft, a conductive mesh of a composite aircraft, a dielectric top of a composite aircraft, a lightning diversion strip, a grounding cable or plate, a portion of a lightning protection layer of an aircraft, or any combination thereof. In some embodiments, the material includes a thermo-chromatic witness material. 
     In an embodiment, a method includes applying a first material to a first portion of a structure. The first material is adapted to change to a first color in response to heating of the first portion of the structure to a first threshold temperature due to an electrical current within the structure. The method further includes applying a second material to a second portion of the structure. The second material is adapted to change to a second color, distinct from the first color, in response to heating of the second portion of the structure to a second threshold temperature. 
     In some embodiments, the first material is incorporated into a first set of applique strips and the second material is incorporated into a second set of applique strips. The first set of applique strips and the second set of applique strips are applied to the structure in a repeating pattern. In some embodiments, the first material is incorporated into a first portion of a coating matrix and the second material is incorporated into a second portion of coating matrix. The coating matrix is applied to the structure. In some embodiments, the first material is incorporated into a first portion of a resin of a lightning protection layer and the second material is incorporated into a second portion of the resin of the lightning protection layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts an embodiment of an aircraft  100  with a thermo-chromatic material. 
         FIG. 2  depicts an embodiment of a system for lightning strike indication. 
         FIG. 2A  depicts a cross-section view of the embodiment of the system for lightning strike indication. 
         FIG. 3  depicts an embodiment of a system for lightning strike indication. 
         FIG. 3A  depicts a cross-section view of the embodiment of the system for lightning strike indication. 
         FIG. 4  depicts an embodiment of a system for lightning strike indication. 
         FIG. 4A  depicts a cross-section view of the embodiment of the system for lightning strike indication. 
         FIG. 5  depicts an embodiment of a system for detecting a position and the extent of potential damage due to lightning strikes. 
         FIG. 6  is a flowchart that depicts an embodiment of a method for lightning strike indication. 
         FIG. 7  is a flowchart that depicts an embodiment of a method for making a lightning strike indication system. 
     
    
    
     While the disclosure is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the disclosure is not intended to be limited to the particular forms disclosed. Rather, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the disclosure as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an embodiment of an aircraft  100  is depicted. The aircraft  100  may include composite materials, metallic materials, other types of building materials, or combinations thereof. A surface  102  of the aircraft  100  may include a thermo-chromatic witness material applied thereto as described herein. At some time during the operations of the aircraft  100 , the surface  102  of the aircraft  100  may get struck by lightning  104 . In response to heat generated within the structure of the aircraft  100 , the thermo-chromatic witness material may change color creating a color changed region  106  of the surface  102 . Because the color changed region  106  is formed due to heat generated by electrical current from the lightning  104 , the color changed region  106  may substantially follow the pathway of the electrical current until the electrical current leaves the surface  102  of the aircraft  100  or dissipates such that it no longer generates sufficient heat to activate the thermo-chromatic witness material. In some embodiments, the thermo-chromatic material is selected such that the color changed region  106  may be visible under ultraviolet light and may be substantially invisible under non-ultraviolet light. 
     An advantage of applying the thermo-chromatic witness material to the surface  102  of the aircraft is that it enables detection of both the strike point of the lightning  104  and the pathway taken by the electrical current through the surface  102  of the aircraft  100 . By locating the position of the color changed region  106 , a more accurate estimation may be made of which components may have been damaged due to the lightning  104 . Thus, time and expense may be saved by inspecting only those components proximate to the color changed region  106 , as those components are more likely to have been overheated or otherwise damages by the electrical current. Other benefits and advantages associated with the embodiment of  FIG. 1  may be apparent to persons of ordinary skill in the relevant art having the benefit of this disclosure. 
     Referring to  FIGS. 2 and 2A , an embodiment of a system  200  for lightning strike indication is depicted.  FIG. 2  depicts a top view of the system  200 .  FIG. 2A  depicts a cross-section view of the system  200  along the cross-section line designated by the symbol A of  FIG. 2 . The system  200  may include a thermo-chromatic witness material  202  applied to a structure  204 . 
     The thermo-chromatic witness material  202  may include any substance that changes color due to a change in temperature. For lightning detection applications, the color change may be sufficiently permanent to remain changed until inspection of the system  200  may take place. In some embodiments, the thermo-chromatic witness material  202  may change color or fluoresce at a threshold temperature between 200° C. and 300° C. 
     Different types of thermo-chromatic witness materials may be associated with different time intervals during which heat is applied in order to activate. In some embodiments, in order to detect lightning strikes, the thermo-chromatic witness material  202  is activated during the short time interval associated with a lightning strike. For example, the thermo-chromatic witness material  202  may activate within a time period between 0.1 second and 1 second. Further, the thermo-chromatic witness material  202  may retain its changed color for a sufficient time interval to enable inspection of the system  200 . For example the thermo-chromatic witness material may retain its changed color for up to several days or longer. 
     The structure  204  may include any structure for which lightning strike protection may be desirable. In some embodiments, the structure  204  may be a portion of an aircraft. For example, the structure  204  may include a copper foil applied to a composite aircraft, a conductive mesh of a composite aircraft, a dielectric top of a composite aircraft, a lightning diversion strip, a grounding cable or plate, a portion of a lightning protection layer of an aircraft, another portion of an aircraft, or combinations thereof. 
     At some time during operation, the system  200  may be struck by lightning. The lightning may generate an electrical current that enters the structure  204  at an entrance point  210 . The current may travel through the structure  204  along a pathway  208  before exiting through an exit point  212 . In some embodiments, the electrical current may dissipate, either partially or entirely, along the pathway  208 . For example, the structure  204  may be designed to redirect and/or spread the electrical current over a large area resulting in a reduction of the electrical current. In some instances, the electrical current may be completely dissipated before reaching the exit point  212 . 
     As the electrical current moves through the structure  204  along the pathway  208 , a portion  206  of the structure  204  in proximity to the pathway may become heated. When the portion  206  reaches a threshold temperature, the thermo-chromatic witness material  202  applied at the portion  206  may change to another color, forming a color changed region  214 . The hatching shown in  FIGS. 2 and 2A  indicate that a color of the color changed region  214  differs from the rest of the thermo-chromatic witness material  202 . 
     An advantage of the system  200  is that by forming a color change region  214  in the thermo-chromatic witness material  202 , the system  200  provides a quick, reliable, and non-invasive method for determining a pathway taken by current from the lightning strike through the structure  204 , and for determining the extent to which components along the pathway  208  may have been damaged. Other benefits and advantages of the system  200  may be apparent to persons of ordinary skill in the relevant art having the benefit of this disclosure. 
     Referring to  FIGS. 3 and 3A , an embodiment of a system  300  for lightning strike indication is depicted. The system  300  may include a first set of applique strips  302 - 307  and a second set of applique strips  312 - 317 . The first set of applique strips  302 - 307  may include a first thermo-chromatic witness material that changes color at a first threshold temperature. The second set of applique strips  312 - 317  may include a second thermo-chromatic witness material that changes color at a second threshold temperature that is greater than the first threshold temperature. 
     The electrical current passing through the structure  204  may be the most intense near the entrance point  210 . As it travels along the pathway  208 , it may lose power as it heats the structure  204  and/or dissipates through a large conductive area. As such, the structure  204  may be heated to higher temperatures along the pathway  208  near the entrance point  210  as compared to near the exit point  212 . 
     As depicted in  FIGS. 3 and 3A , the electrical current may heat portions of the structure  204  in contact with some of the applique strips (e.g., the applique strips  302 - 304 ) to a temperature beyond the first threshold temperature. In response to the heat, the first thermo-chromatic witness material included in the applique strips  302 - 304  may change color to form color changed regions  322 - 324  over the heated portions of the structure  204 . Likewise, the current may heat other portions of the structure  204  in contact with other applique strips (e.g., the applique strips  313 - 315 ). Near the entrance point  210 , the heat may exceed the second threshold temperature causing the second thermo-chromatic witness material included in the applique strip  313  to change color to form another color change region  326 . As the pathway  208  moves away from the entrance point  210 , the heat generated by the electrical current may decrease. As such, further from the entrance point  210 , the heat may not exceed the second threshold temperature, resulting in the formation of no color change regions within some of the applique strips (e.g., the applique strips  314 ,  315 ) despite being in contact with heated portions of the structure  204 . 
     By including a first thermo-chromatic witness material in some of the applique strips and including another thermo-chromatic witness material in other applique strips, a determination may be made of the extent of heating within the structure  204  at multiple portions of the structure  204  along the pathway  208 . For example, a determination may be made that the portions of the structure  204  that are adjacent to the applique strips  302 ,  303 , and  313  may have been subjected to more heating than the portions of the structure  204  adjacent to the applique strips  304 ,  314 ,  315  because the temperatures near the applique strip  313  exceeded the second threshold temperature while the temperatures near the applique strips  314 ,  315  did not exceed the second threshold temperature. Thus, components within the structure  204  near the applique strips  302 ,  303 ,  313  may have been subjected to more heat and consequently may be more likely to have been damaged by the lightning strike. Colors of the first thermo-chromatic witness material and the second thermo-chromatic witness material may be distinct from each other after their respective threshold temperatures are reached in order to enable a determination of temperature ranged reached at each portion of the structure  204 . 
     As shown in  FIG. 3 , the applique strips  302 - 307  that include the first thermo-chromatic witness material and the applique strips  312 - 317  that include the second thermo-chromatic witness material may be alternately applied to produce an even heat sampling for both temperature thresholds across the structure  204 . In some embodiments, more than two types of thermo-chromatic witness materials may be used. For example, additional applique strips including additional thermo-chromatic witness materials may be applied to additional portions of the structure  204 . The additional thermo-chromatic witness materials may be successively adapted to change to additional colors in response to heating of the additional portions of the structure  204  to additional successively increasing threshold temperatures. The additional applique strips may be sequentially laid along the structure  204  to produce an even heat sampling for each temperature threshold. Further, each of the thermo-chromatic witness materials may change to colors distinct from each other. As such, a range of temperatures experienced by the structure  204  may be mapped. 
     An advantage associated with alternating between applique strips that include a first thermo-chromatic witness material and applique strips that include a second thermo-chromatic witness material is that the system  300  may provide a quick, reliable, and non-invasive method for determining the pathway  208  taken by current from a lightning strike through the structure  204 , and for determining the extent to which components along the pathway  208  may have been damaged. Further, because the system  300  incorporates applique strips, it may be easily constructed. Other benefits and advantages of the system  300  may be apparent to persons of ordinary skill in the relevant art having the benefit of this disclosure. 
     Referring to  FIGS. 4 and 4A , an embodiment of a system  400  for lightning strike indication is depicted. The system  400  may include a matrix material  402  applied to the structure  204  with multiple thermo-chromatic witness particles  404  incorporated therein. 
     In some embodiments, the matrix material  402  may include a resin or another type of coating applied to the structure  204 . The matrix material  402  may be applied to the structure  204  by spraying, brushing, another type of application process, or combinations thereof. The thermo-chromatic witness particles  404  may be incorporated into the matrix material  402  before application of the matrix material  402  to the structure  204 . Alternatively, in some embodiments, the thermo-chromatic witness particles  404  may be incorporated into the matrix material  402  after the application thereof. 
     Some of the thermo-chromatic witness particles  404  may include a first thermo-chromatic witness material and some of the thermo-chromatic witness particles  404  may include a second thermo-chromatic witness material. The first thermo-chromatic witness material may change to a first color when heated to a first threshold temperature and the second thermo-chromatic witness material may change to a second color, distinct from the first color, when heated to a second threshold temperature, different than the first threshold temperature. For example, upon heating due to the current along the pathway  208 , some of the particles  404  may change to a first color forming color changed regions (e.g., the color changed regions  412 ,  414 ,  417 ) of the particles. Other color changed regions (e.g., the color changed regions  413 ,  415 ) may correspond to a second color distinct from the first. 
     In some embodiments, as depicted in  FIGS. 4 and 4A , the particles  404  are mixed into the matrix material  402  haphazardly. Alternatively, in some embodiments, the first set of particles  412 ,  414 ,  416 ,  417  are incorporated into a first portion of the matrix material  402  and the second set of particles  413 ,  415  are incorporated into a second portion of the matrix material  402 , with the first and second portions initially separated. Each portion of the matrix material  402  may then be applied to the structure  204  in a pattern. Although not depicted in  FIG. 4 , in some embodiments, some of the particles  404  include additional thermo-chromatic witness materials that change to additional colors, distinct from the first color and second color and distinct from each other, in response to heating of the additional portions of the structure  204  to additional successively increasing threshold. 
     As electrical current passes from the entrance point  210  to the exit point  212  along the pathway  208 , it may heat the structure  204 . If portions of the structure  204  are heated beyond the first threshold temperature, then the thermo-chromatic witness particles  404  may change color based on a temperature range associated with a respective thermo-chromatic witness particles material. The change in color may enable a determination of the position of potential damage to the structure  204  and/or the extent of damage. Other benefits and advantages of the system  400  may be apparent to persons of ordinary skill in the relevant art having the benefit of this disclosure. 
     Referring to  FIG. 5 , an embodiment of a system  500  for detecting a position and the extent of potential damage due to lightning strikes is depicted. The system  500  may include a controller  502 , a light source  506 , and a detector  508 . 
     The controller  502  may perform control functions as described herein. In some embodiments, the controller is implemented using a processor and memory. For example, the processor may include a central processing unit (CPU), a graphical processing unit (GPU), a peripheral interface controller (PIC), another type of processing unit, or combinations thereof. In some embodiments, the controller is implemented using programmable or fixed circuit logic such as a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), an integrated circuit (IC) device, another type of circuit logic, or combinations thereof. 
     The controller  502  may include a position determination module  504  that receives input from the detector  508 . Based on the input, the position determination module  504  may determine a position of the color changed region  214 . The controller  502  may further include a damage assessment module  505  that determines the extent of potential damage to the structure  204  at the position. In some embodiments, the controller may generate a visual depiction that maps the position and an estimate of the extent of potential damage to a three-dimensional model of the structure  204 . 
     During operation, the light source  506  may be directed at the structure  204 . In some embodiments, the light source  506  may include an ultraviolet light lamp. The light source  506  may emit light  510  (e.g. ultraviolet light) which may cause the thermo-chromatic witness material  202  to fluoresce. Reflected and/or additional light  511  showing the fluoresced thermo-chromatic witness material may be received at the detector  508 . The detector may send image data based on the light  511  to the position determination module  504 . 
     In order to determine position information, the controller  502  may control the detector  508  to scan a surface of the thermo-chromatic witness material  202 . For example, the detector  508  may scan in the direction indicated by the arrow  512 . Scanning may include rotational movements, translational movements, other types of movements, or combinations thereof. In order to effectuate the scanning, the controller  502  may include gears, motors, or other mechanical devices (not shown) to apply movement to the detector  508 . 
     Based on the light from the structure  204 , the controller  502  may locate the position of the color changed region  214 . Further, the controller  502  may analyze the colors present at the color changed region  214  to determine the extent of heating of the structure  204  throughout each position of the structure  204  and thereby generate an estimate of damage that may have occurred at one or more components of the structure  204 . 
     Referring to  FIG. 6  an embodiment of a method  600  for lightning strike indication is depicted. The method  600  may include changing a material applied to a portion of a structure to a color in response to heating of the portion of the structure to a first threshold temperature due to an electrical current within the structure, at  602 . For example, the thermo-chromatic witness material  202  may change to the color represented by the hatching of  FIG. 2  at the color changed region  214  in response to heating of the portion  206  of the structure  204  proximate to the electrical current path  208 . 
     The method  600  may further include scanning a surface of the structure with a detector, at  604 . For example, the surface of the structure  204  may be scanned by the detector  508 . 
     The method  600  may also include receiving, at the detector, light from the structure, at  606 . For example, the light  511  may be received at the detector  508 . 
     The method  600  may include locating the position of the color based on the light from the structure receiving, at the detector, light from the structure, at  608 . For example, the position of the color changed region  214  may be determined at the position determination module  504 . 
     The method  600  may further include detecting a pathway of the electrical current through the structure based on a position of the first color, at  610 . For example, the pathway  208  may be detected based on the pathway of the color changed region  214 . 
     The method  600  may also include estimating, based at least on the color, an extent of damage produced by the electrical current along the detected pathway of the electrical current through the structure, at  612 . For example, the damage assessment module  505  may determine an extent of damage along the color changed region  214  based on the color of the color changed region  214 . 
     Referring to  FIG. 7  an embodiment of a method  700  for making a lightning strike indication system is depicted. The method  700  may include applying a first material to a first portion of a structure, at  702 . The first material may adapted to change to a first color in response to heating of the first portion of the structure to a first threshold temperature due to an electrical current within the structure. For example, the applique strips  302 - 307  may be applied to the structure  204 . As another example, the chromatic witness particles  412 ,  414 ,  416 ,  417  may be applied to the structure  204 . 
     The method  700  may further include applying a second material to a second portion of the structure, at  704 . The second material may be adapted to change to a second color, distinct from the first color, in response to heating of the second portion of the structure to a second threshold temperature. For example, the applique strips  312 - 217  may be applied to the structure  204 . As another example, the chromatic witness particles  413 ,  415  may be applied to the structure  204 . 
     Although various embodiments have been shown and described, the present disclosure is not so limited and will be understood to include all such modifications and variations are would be apparent to one skilled in the art.