Abstract:
The present disclosure describes an apparatus including a composite panel. The apparatus includes a first composite panel including a first optical fiber embedded therein, the first optical fiber being arranged in a pattern, and a first input port connected to a first end of the first optical fiber, the first input port configured to receive an optical signal from an optical time domain reflectometer. The optical time domain reflectometer is configured to send the optical signal through the first input port and measure a strength of a reflected optical signal that is reflected back from the first optical fiber, wherein the strength indicates a measured optical impedance of the first optical fiber. A measured optical impedance that is substantially the same as a baseline optical impedance for the fiber indicates no damage, while a measured optical impedance that differs from the baseline optical impedance by a predetermined threshold indicates damage.

Description:
BACKGROUND 
     Many newer aircraft make extensive use of composite materials to reduce weight. Such composite panels for the skin of aircraft are susceptible to breakage caused by bird strikes, hail, and other debris. In conventional metal panels, such damage is typically readily visible as dents. However, pilots and maintenance crews may not detect the damage of composite panels as readily since composite panels typically spring-back and do not dent. In addition, cracks to composite panels can be difficult to visually detect. This allows the possibility of reduced performance or later catastrophic failure due to an undetected breakage. As such, examining an aircraft made of composite panels for damage is both time consuming and unreliable. 
     Some conventional techniques for detecting damage in composite materials include several examples of using optical fiber embedded in the composite material. Techniques for using the fiber as a sensor include embedding Bragg cells at intervals along the fiber to reflect laser pulses and laying individual fibers in a grid pattern each terminated by a mirrored surface to isolate damage to a particular row and column of fiber. The former has a granularity dependent upon the spacing of the Bragg cells and includes the extra cost of forming them. The latter has a granularity set by the grid density and will only detect situations in which the damage is severe enough to sever the fiber. 
     SUMMARY 
     In view of the foregoing, the present disclosure presents a composite panel embedded with optical fiber and utilizes an optical time domain reflectometer (TDR) to detect any changes in the optical impedance of an optical fiber. If the fiber is embedded close enough to the material surface, this approach is able to not only to detect damage to the composite panel that breaks the fiber but also damage that only pinches, bends, or otherwise disturbs the fiber geometry. One would have the option of switching the TDR among fibers formed into a grid as mention above or to use a single fiber routed to cover the whole surface of a composite component. By knowing the positioning and baseline optical impedance of the fiber in the composite panel, the point of damage can be identified from an optical impedance measurement made by the TDR. Such a system may be self-calibrating by maintaining a baseline against which sudden changes in reflectance can be judged. 
     The present disclosure describes an apparatus including a composite panel. The apparatus includes a first composite panel including a first optical fiber embedded therein, the first optical fiber being arranged in a pattern, and a first input port connected to a first end of the first optical fiber, the first input port configured to receive an optical signal from an optical time domain reflectometer. The optical time domain reflectometer is configured to send the optical signal through the first input port and measure a strength of a reflected optical signal that is reflected back from the first optical fiber, wherein the strength indicates a measured optical impedance of the first optical fiber. A measured optical impedance that is substantially the same as a baseline optical impedance for the fiber indicates no damage, while a measured optical impedance that differs from the baseline optical impedance by a predetermined threshold indicates damage. 
    
    
     
       BRIEF SUMMARY OF THE DRAWINGS 
         FIG. 1  shows an aircraft consisting of a plurality of composite panels. 
         FIG. 2   a  shows a composite panel embedded with optical fiber in a serpentine pattern. 
         FIG. 2   b  shows a composite panel embedded with optical fiber in a circular pattern. 
         FIG. 2   c  shows a composite panel embedded with optical fiber in a grid pattern. 
         FIG. 3   a  shows a composite panel embedded with optical fiber that has been broken. 
         FIG. 3   b  shows a composite panel embedded with optical fiber that has been kinked. 
         FIG. 4  show multiple composite panels embedded with optical fiber connected together in series. 
         FIG. 5  shows a block diagram of a optical time domain reflectometer and a damage detector. 
         FIG. 6A  shows an example of an optical impedance chart of a typical optical fiber with an open end. 
         FIG. 6B  shows an example of a chart of the optical impedance of a fiber that has developed a highly reflective fault. 
         FIG. 6C  shows another example of a chart of measured optical impedance indicating multiple faults. 
         FIG. 7  shows multiple composite panels embedded with optical fiber connected together in parallel. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an aircraft  100  that has an outer skin consisting of a plurality of composite panels  105   a - h . For simplification, only a small section of composite panels that may exist on aircraft  100  are shown. While  FIG. 1  is directed at an aircraft implementation, it should be noted that the composite panels and techniques discussed herein may be applicable for any use of composite panels, vehicular or otherwise. As mentioned above, many newer aircraft make extensive use of composite materials to reduce weight. In general, a composite material is any material that is made from two or more materials with significantly different physical or chemical properties. As one example, modern aircraft use a composite material made from carbon fiber embedded in resin. Other examples of composite materials include, but are not limited to, fiberglass or Kevlar embedded in resin, fiber-reinforced polymers (where the fiber may include wood), carbon-fiber reinforced plastic, and glass-reinforced plastic. 
       FIG. 2   a  shows a composite panel embedded with an optical fiber according to one embodiment of the disclosure. Composite panel  200  includes an optical fiber  201  that is embedded in the composite panel and arranged in a specific pattern. As shown in  FIG. 2   a , optical fiber  201  is arranged in a serpentine pattern. Optical fiber  201  may be any gauge of optical fiber that is able to be embedded in the composite panel and that is capable of carrying an optical signal to the end of the optical fiber given the pattern chosen. Preferably, optical fiber  201  is embedded into the composite panel at a depth where damage to the composite panel is reflected to the optical fiber as a break, pinch, bends, kink or other disturbance to the optical fiber geometry. The sensitivity to damage is increased if the depth of the optical fiber is closer to the surface of the panel that may suffer damage. This depth may be anywhere to a few millimeters to the entire thickness of the panel. 
     One end of the optical fiber may be left as an open end  210 . The other end of the optical fiber is connected to an input port that, is configured to receive optical signal  221  from optical time domain reflectometer  220 . Optical time domain reflectometer is configured to send an optical signal  221  through optical fiber  201  and measure the strength of any reflected signals coming from the fiber. As the optical fiber shown in  FIG. 2   a  is substantially free of disturbances to its geometry and is left with an open end  210 , the majority of any reflected signal  222  would come off normal discontinuities and minor defects experienced in the manufacture of the optical fiber. 
     The strength of reflected signals received back by optical time domain reflectometer  220  is integrated over a period of time. This integrated strength can then be used to give an indication of the optical impedance of the optical fiber over the length of its distance.  FIG. 6A  shows an example of an optical impedance chart of a typical optical fiber with an open end as shown in  FIG. 2A . As shown in this chart, an input reflection is typically seen toward the beginning length of the fiber due to the connection of the optical time domain reflectometer transmitter. The strength of the reflected signal gradually decreases over the length of the fiber and then dips further at the end of the fiber as the fiber has an open end in this example. An open end would result in fewer signal reflections. If the optical fiber ended in a connection (such as to another optical fiber) or ended with a reflector, the optical impedance chart would be expected to show a stronger reflected signal at the end of the fiber. The chart of optical impedance shown in  FIG. 6A  may be thought of as the baseline impedance of the optical fiber  201  in  FIG. 2A . This baseline optical impedance may be measured against future measurements of optical impedance to determine if there is damage to the fiber, and thus by inference, damage to the composite panel. 
       FIG. 6B  shows an example of a chart of the optical impedance of a fiber that has developed a highly reflective fault, such as a kink. An example, of a fiber with a kink is shown in  FIG. 3B . As seen in the chart, there is a spike  610  in reflected signal strength between the beginning and end of the fiber. This spike suggests a highly reflective fault such as a kink, bend, or other discontinuity in the fiber that causes signal to be reflected back to the optical time domain reflectometer. The optical impedance chart of  FIG. 6B  could then be compared to the baseline optical impedance chart in  FIG. 6A  to show that the impedance of the optical fiber has changed. This optical impedance change would also show the distance from the beginning of the fiber where the fault is located. As such, by knowing the length and pattern of the fiber, the point on the fiber at which the optical impedance has changed would also indicate where on the composite panel damage is believed to exist. 
     It would be preferable to set a threshold for the amount of change necessary to signify that the change in impedance measured is actually due to damage to the panel. It is foreseeable that changes in optical impedance of the fiber may be due to typical environmental conditions and do not actually indicate any damage. The threshold chosen to actually signify damage may be chosen such that a desired level of sensitivity is achieved. The lower the threshold of optical impedance change, the higher the sensitivity. 
       FIG. 6C  shows another example of a chart of measured optical impedance with multiple faults, including highly reflective faults (e.g., bends, kinks, discontinuities) and minimally reflective faults (e.g., a break in the fiber). As can be seen in the  FIG. 6C , multiple highly reflective and minimally reflective faults may be discovered by an optical time domain reflectometer in a single measurement. As such, a single measurement of optical impedance may be used to locate multiple damage locations on a composite panel. 
     Returning to  FIG. 2   a , optical time domain reflectometer  220  in this example is affixed to composite panel  200 .  FIG. 2   b , shows an alternative embodiment where optical time domain reflectometer  270  is configured to be detachably affixble to composite panel  250 . That is, rather than designing each panel with its own optical time domain reflectometer, a single reflectometer may be used to discretely check each panel manually.  FIG. 2   b  also shows an alternative circular pattern for optical fiber  251 . Again, optical time domain reflectometer  270  sends optical signal  261  through input  271  and receives back reflected signal  262 , which may be substantially reflected off of reflector  260 .  FIG. 2   c  shows another alternative pattern for arranging the optical fibers. In this case, composite panel  280  has multiple optical fibers arranged in a grid pattern, each with their own input ports and reflectors. Such a pattern may provide better precision and localization of damage detection at the expense of increased complexity. 
       FIG. 3   a  shows a composite panel  300  embedded with optical fiber  310  that has been broken. As shown in this example, the optical signal  311  produced by optical time domain reflectometer  320  is not substantially reflected at the break. Since the strength of reflected signal  312  would be lower than that of a baseline optical impedance reading for the same fiber, a comparison of the baseline optical impedance and the measured optical impedance would indicate a minimally reflective fault at that location of the fiber. This location of the minimally reflective fault would give an indication of the location of damage to the composite panel. 
     Similarly,  FIG. 3   b  shows a composite panel  350  embedded with optical fiber  370  that has been kinked. As shown in this example, the optical signal  371  produced by optical time domain reflectometer  360  is substantially reflected at the kink. Again, since the strength of reflected signal  312  at that location of the fiber would be substantially higher than the baseline impedance of the same fiber at that location, a comparison of the baseline optical impedance and the measured optical impedance would indicate a highly reflective fault at that location of the fiber. This location of the highly reflective fault would give an indication of the location of damage to the composite panel. 
       FIG. 4  shows an example where the optical fibers of two or more panels are connected together. In order to conserve the number of optical time domain reflectometers used and the amount of measurements taken, panels  401 ,  402 , and  403  may connected together in such a way that their embedded optical fibers also connect together with optical continuity. As shown in  FIG. 4 , composite panel  401  includes an optical fiber that has a reflector  440  at one end and connects to the optical fiber of composite panel  402  at input port  420 . Similarly, the optical fiber of composite panel  402  connects to the optical fiber of composite panel  403  at input  420 . Optical time domain reflectometer may than be attached to the opposite end of the optical fiber in panel  403 . This arrangement has the advantage of using only one reflectometer for three panels. 
       FIG. 7  shows another example of a configuration where the number of optical time domain reflectometers may be conserved. In this example, composite panels  701 - 703  are not affixed to their own optical time domain reflectometer. Instead, the panels are affixed with optical transmitters and receivers  710   a - c . As an example, the optical transmitter may be an LED or a laser, however any type of optical transmitter suitable for transmitting a signal along an optical fiber may be used. The optical receiver may be any type of circuit suitable for detecting a reflected signal from an optical fiber, such as a photodiode. Optical time domain reflectometer  730  may be configured to sequentially control the transmission and reception of optical transmitter and receiver  710   a - c  through a switch  720 . In this scenario, the transmission and reception of the optical signal is distributed through each of a group of panels, while the measurement of the reflected signal strength and calculation of the optical impedance of the fiber over distance is handled by a single optical time domain reflectometer for that group of panels. Essentially,  FIG. 7  shows composite panels whose optical fibers are connected to an optical time domain reflectometer in parallel. 
       FIG. 5  shows a block diagram of an optical time domain reflectometer and a damage detector according to one embodiment of the disclosure. Optical time domain reflectometer  500  receives reflected signal  540  back from the optical fiber and integrates the strength of that signal over time. This integrated strength is then converted to a measured optical impedance  502  of the optical fiber over its length the measured optical impedance is stored in memory  501 . At some time after that calculation, whether it be immediately, periodically, or manually, measured optical impedance  502  and some form of panel number  504  which identifies the panel from which optical impedance was measured is communicated over communication link  550  to damage detector  510 . Communication link  550  may be any type of communication link, including wired and wireless links. In addition, damage detector  510  may be a discrete component from optical time domain reflectometer  500 , or may be contained within the same housing. 
     Damage detector  510  than uses the panel number  504  to look up a baseline optical impedance  503  of the optical fiber in that panel from a panel database  520 . The baseline optical impedance is than compared to the measured optical impedance to determine if that panel has suffered any damage and where that damage of located on the panel based on the length and pattern of the optical fiber. The operation of damage detector  510  may be carried out by an ASIC, FPGA, dedicated hardware, firmware, or software running on a microprocessor.