Abstract:
An apparatus for providing for the ingress/egress of an optical fiber in composite materials wherein flexible tooling protects the ingress and egress point of the optical fiber. The optical fiber is placed into the uncured laminates of the composite material prior to curing such laminates. The ingress/egress point of this fiber is protected from the laminates by enclosing the fiber with a plurality of polyimide and poly(tetrofluoroethylene) tubes. During the curing process, a rubber plate covers the laminates and the fiber optic lead are brought out of the laminates through a plug in the rubber plate and positioned along the rubber plate in grooves to protect the external fiber leads during curing. After curing a strain relief boot is placed over the fiber optic lead where it ingresses/egresses the composite material to relieve the strain generated on the optical fiber lead during operational use.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains generally to composite material and more specifically a technique for the ingress and egress of fiber optic sensor leads from the surface of composite materials. 
     2. Description of the Related Art 
     The purpose of embedding fiber optic sensors in materials is to accurately measure a specific material property at a specific location in the composite specimen. To accomplish this, the fiber optic sensor (FOS) must be placed precisely in the desired location (including depth or layer), and ingress of the optical fiber leads must be accomplished with a minimum of risk to the optical fiber and host composite part. 
     When a fiber optic sensor strand is embedded within a composite part, it is necessary to provide a lead through which the sensors can be interrogated. This lead extends out of the part, and must be of sufficient length and in good condition for cleaving and splicing operations to standard fiber optic connectors. The fiber lead is fragile, being made of glass, with a diameter, generally, of 125 microns (0.005 inches). This lead serves as the only link between the embedded sensors and the readout electronics, if the lead suffers damage then the sensors are lost, as they are irretrievably buried within a high strength composite component. 
     Cure process for many composite materials is an extremely harsh environment for the fiber optic sensor, and the sensor lead. Most high performance composites are consolidated under high pressure and temperature. Pressure is applied often through the use of a vacuum bag, hard tooling with an expanding mandrel, shrink tape, or a combination of the above. Failure of the lead can occur during the application of pressure due to pinching or kinking of the lead between parts of the tooling, due to relative motion of the parts of a given cure fixture. The lead does not have to break to fail; permanent sharp bends (kinks) in the fiber optic lead will render it useless as a waveguide. Likewise, subsequent post cure operations. such as disassembly of the curing fixture, are extremely hard on fiber optic sensor leads due to the tendency of the composite material to bleed or leak resin during the cure process. 
     Another less obvious mode of failure of the lead occurs when an improper tubing schedule is used to protect the leads at the ingress point. Without proper damming, resins will flow up the tubing during the cure cycle through capillary action. If the tubing is sufficiently oversized, air bubbles form within the tubing around the fiber optic cable. Upon later flexing of the lead the fiber optic cable can break at these bubbles, particularly if the tubing is made of such a material as Teflon®. 
     The last major mode of failure is breakage of the lead during handling and machining operations on the composite part. Leads which are not routinely armored and not sufficiently strain relieved will suffer damage under normal handling and machining operations. 
     The most widely used method for ingress and egress of the fiber optic leads are from the edge of a part. this method has been used successfully for test coupons but has limited practical applications because the edge of the coupon cannot be machined without chopping off the fiber optic cable in the process, or leaving an un-machined portion of material around the edge. Also, the fiber optic sensor lead is prone to breakage or severe kinking at the edge of the laminate during vacuum bagging as it is unsupported. Further, if the lead is supported to avoid the previously noted deficiency, then resin from the composite part often flows over the lead, which generally is spooled up at the edge of the laminate. This often causes fiber breakage during de-bagging or mold disassembly; the resin glues the fiber coil to itself and to parts of the mold or vacuum bag assembly. Interlaminar stress concentrations in composite parts becomes extremely large at the edge of a part. Test coupons exhibit this by edge delamination prior to failure. A discontinuity caused by a fiber optic cable at this area makes this condition worse as it acts as a stress riser or defect. 
     Egress from the surface of a part between layers of vacuum bagging materials has caused the following problems. The fiber left a deep imprint on the part surface, and thus created a defect and a possible failure initiation point. Resin flow into the vacuum bag material, i.e., breather (a material that resembles quilt batting) around the ingress-egress point creates a poor housekeeping condition, and the fiber optic cable must be carefully picked out of the resulting resin buildup. The fiber is unprotected from kinking at the egress point during vacuum bagging/mold assembly during this process. Egress points are unprotected after cure, no strain relief devise is used to protect the leads. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of this invention is to provide an apparatus for installing fiber optic leads in a composite material so as to provide a rugged ingress/egress point for the fiber optic lead. 
     Another object of the invention is to provide an apparatus that allows for the installation of fiber optic cables in composite material without causing failure of the lead during the curing process through pinching due to pressure, kinking between parts of the tooling, or due to the relative motion of the parts in the cure fixture, and without causing failure of the fiber optic cables during removal of the finished part from the curing fixture. 
     These and other objectives are accomplished by utilizing flexible tooling and protecting the ingress/egress point of this fiber from the damage by enclosing the fiber with a plurality of polyimide and poly(tetrofluoroethylene) tubes so as to form a protected fiber optic lead. During the curing process, a rubber plate covers the surface of the laminate and the fiber optic leads are brought out of the laminates through a rubber plug in the rubber plate. The leads are then positioned along the rubber plate in grooves to protect the fiber lead during curing. After curing a strain relief boot is bonded over the fiber optic lead where it ingresses/egresses the composite material to protect the leads from damage during normal handling and use. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a longitudinal cross-sectional view of the fiber optic lead ingress/egress device along the axis of the optical fiber. 
     FIG. 2 shows a longitudinal cross-sectional view of a fiber optic cable with layers of protective tubing along the axis of the optical fiber. 
     FIG.  3   a  shows a cross-sectional view of a fiber optic cable with one layer of protective tubing. 
     FIG.  3   b  shows a cross-sectional view of a fiber optic cable with two layers of protective tubing. 
     FIG.  3   c  shows a cross-sectional view of a fiber optic cable with three layers of protective tubing. 
     FIG.  3   d  shows a cross-sectional view of a fiber optic cable with four layers of protective tubing forming a fiber optic sensor lead. 
     FIG.  4   a  shows a template for the construction of a flexible rubber tool. 
     FIG.  4   b  shows a top view of a flexible rubber tool and flexible sheeting. 
     FIG.  4   c  shows a bottom view of a plug cut out from the flexible rubber tool with cavity. 
     FIG.  4   d  shows a metal pattern to make a flexible rubber tool. 
     FIG.  5   a  shows a cross-sectional view of a composite structure with the ingress/egress device and optical fiber lead installed with the flexible rubber tool prepared for compression and curing. 
     FIG.  5   b  shows a groove in the flexible rubber tool into which the optical fiber lead is placed during the pressurization and curing process. 
     FIG.  6   a  shows a top view of a strain relief boot. 
     FIG.  6   b  shows a cross-sectional view of a strain relief boot. 
     FIG.  6   c  shows a three-dimensional view of a strain relief boot. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The device taught by this invention is for the ingress and egress of fiber optic sensor cables from the surface of composite parts while providing protection to the fiber optic cable during part fabrication, autocleave curing processes, and through subsequent handling and machining operations. 
     PREPARATION OF A FIBER OPTIC STRAND TO FORM A FIBER OPTIC LEAD 
     In the preferred embodiment of the device  10 , the first step is the preparation of a fiber optic sensor lead  13  by placing four layers of tubing  14 - 22  over the fiber optic sensor strand  12 , as shown in FIG.  1 . For a standard telecommunications grade optical fiber 0.005 inches in diameter, the preferable tubing layers  12 - 22  are (1) a 0.012 to 0.007 inch interior dimension (ID) polyimide tubing having a 0.001 to 0.0017 inch wall thickness  14 , (2) a 0.014 to 0.009 inch ID polyimide tubing having a 0.001 to 0.0017 inch wall thickness  16 , (3) a 39 Gage poly(tetrofluoroethylene) (Teflon®) heat shrink tubing, or 30 Gage dual heat shrink tubing  18 , and ( 4 ) a 28-30 Gage thin wall poly(tetrofluoroethylene) (Teflon®) tubing  22 . The tubing  14 - 22  is applied in a step fashion, as shown in FIG.  2  and FIGS.  3   a  through  3   d , with the innermost polyimide tubing  14  extending for approximately 0.75 inches within the laminates  24   a  through  24   g  of composite material  24 . The second layer of polyimide tubing  16  extending approximately 0.5 inches within the laminates  24   a-g , and the Teflon® tubing  18  and  22  flush or approximately 0.1 inches within the laminates  24   a-g . A damming material  26 , such as General Electric RTV 30, RTV 61, or strain gage cement made by Micromeasurements Group, Inc., is applied between the layers of tubing  16  and  18 , and layers  18  and  22  to prevent capillary resin flow up the tubing surrounding the fiber optic cable  12  and layered tubing  14 - 22  forming the fiber optic lead  13 . Damming normally is not performed between the fiber optic cable  12  and polyimide tubing  14  and between the polyimide tubing  14  and  16  because resin  34  flow during the composite material  24  curing process up these tight fitting tubes  14 - 16  is beneficial to the strength of the fiber optic lead  13  after the composite material  24  cure. However, some low viscosity resin systems may require damming of the polyimide due to the high flow characteristics. The polyimide tubing  14  and  16  must be tacked  36  to the fiber optic  13  at the far end  28  away from the egress point  32  to prevent slippage using strain gage cement or five minute epoxy by Duro Corp. 
     The innermost layers of polyimide tubing  14  and  16  prevent the fiber lead  13  from kinking as it traverses the ply  24   a - 24   g  in the composite material  24  and protects it from breakage during the threading operation. Epoxy and cyanate ester resins, such as 954-2A by Hexcell Corp., bond well to polyimide tubing, and thus anchors it within the laminates  24   a-g  after cure. The first layer of Teflon® tubing  18  provides the lead  13  with structural strength, protection from damage at the surface of the part, and prevents resins from adhering the lead  13  to the molds (not shown) or other materials. The purpose of the final layer  22  is to provide protection from inadvertent resin flow from the composite laminates  24   a-   24   g  and other materials. The final thin wall of outermost Teflon® tubing  22  provides extra protection, and is expendable. The last layer  22  provides protection from inadvertent resin flow from the composite material  24 ; after cure it is readily stripped off the lead  26  along with any debris that happens to adhere to it. 
     PREPARATION OF A TOOL FOR EMBEDDING A FIBER OPTIC CABLE IN A FIBER REINFORCED COMPOSITE PART 
     The second step in constructing the preferred embodiment of the device  10  is to prepare a flexible tool  51 , a top view of which is shown in FIG.  4   b , for illustrative purposes the flexible tool  51  may described as for a flat plate. The techniques described here may be applied to complex parts and can be modified for hard tooling. A Mylar ® template  78 , as shown in FIG.  4   a , which delineates the path of the fiber optic sensor strand  12  through the laminate  24 , is plotted full scale. Ingress and egress points  47  are clearly marked, as are the sensor  49  locations and areas to be avoided because of future machining operations. The first step is to construct a pattern  79  for the fiber lead  13 , and for the strain relief at the ingress point  47 . These patterns are bonded to a flat metal tool  48 , see FIG.  4   d , which is marked according to the template  46  part. Next, flexible sheeting for a flexible tool  51  (reinforced room temperature vulcanizing material (RTV) sheeting is adequate to accomplish this requirement) is cut out to the size of the flexible rubber tool  48  to be fabricated. The sheeting for the flexible tool  48  should be approximately 0.250 inch thick and capable of withstanding elevated temperatures (above 350° F.), and may be made of any pourable mold material which is temperature resistant, such as RTV 60 made by General Electric Corp., or any pourable flexible material which has high temperature resistance and is compatible with the composite material  24  resin system. Square holes  52  are then cut in the flexible tool  48  around the ingress and egress points  47  of the leads  13  as indicated on the template  46 . The flexible tool  48  sheeting is then placed over the top of the metal plate  78 , as shown in FIG.  4   b . Next, channels or grooves  54  are cut in the top surface of the flexible tool  48  material to a depth which will allow the tubing patterns  79 , as shown in FIG.  4   d , to lie flush with the surface of the tool material  48  (nominally 0.063 inches deep). Special attention is to be given to the prevention of sharp beds or kinks in the pattern. A compatible two part liquid RTV, such as RTV 60, is then mixed and poured into the square holes  52  and over the fiber lead patterns  79  taped into the cut groves  54 . Excess is squeegeed away, the tool  48  is vacuum bagged  66  with a caul plate  62 , as shown in FIG.  5   a , and the two part RTV is allowed to cure. Once cured, the caul plate  62  is removed, and the square plugs  56  at the ingress points  52  are cut out with a scalpel, slit where the fiber lead  13  passes through the plug  56 , and identified as to which part of the flexible tool  51  it is associated with. The fiber patterns are released from their filled grooves  54 , leaving a tubular cavity of the exact diameter of the fiber sensor lead  13 , as shown in FIG  5   b . The rubber plugs  56  now contain the cavity for the strain relief  58 , and a hollow tube which leads from the strain relief to the matching cavity on the surface tool  48 . A similar process may be used to fabricate tooling utilizing flexible mold materials for fiber optic egress from a more complex part. Existing hard tools for close molded parts may be modified by adding a soft plug with a strain relief at the egress site. 
     PREPARATION OF THE LAMINATES FOR CURING AND IMPLANTATION OF A FIBER OPTIC LEAD 
     Referring again to FIG. 1, assume that the fiber optic sensor strand  12  is to be embedded at the mid-point of the composite material  24  laminates  24   a - 24   g , with an equal number of composite plys on top and bottom. The top and bottom halves of the laminates  24   a - 24   c  and  24   d - 24   g , respectfully, are laid up according to the overall ply schedule, and then the template  46  which delineates the path of the fiber optic sensor strand  12  through the laminate  24   a - 24   c  is placed over the “top” half of the laminate  24   c , facing up. The laminates  24   a - 24   c  are then placed on top of the soft rubber tool  48  such that the square holes line up with the egress points  47 , and against ply  24   a . The laminates  24   a - 24   c  are taped to the tool  48  to prevent sliding. The laminates  24   a - 24   c  are then pierced at the egress points  47  with an awl of diameter 0.06 inches, and a small piece of Teflon® tubing  68  immediately inserted within the hole. (Laminates tend to “heal” due to the tackiness of the uncured resin.) The template  46  is then carefully cut along the intended fiber path so as to provide a profile of the sensor layout. The template  46  is then laid onto the composite material  24 , and the sensor strands  12  are fed through the ingress points  47  and down through the square holes  52  in the tool  48 . The polyimide tubing, which may be obtained through the Cole Parmer Corp., at the ingress points  47  are tacked into position by a small patch of film adhesive which is compatible with the composite resin system to ensure proper depth penetration within the laminate  24  of the polyimide coated section of the lead  13 . The fiber optic sensors (not shown) are then positioned as indicated by the template  46 . Complex sensor patterns may be require inking the path on the composite material  24  and may also require the use of a compatible film adhesive (unsupported) to tack the fiber optic sensor (not shown) into place. Once the sensors (not shown) have been placed and secured, the template  46  is removed, and the bottom half of the laminate  24   d - 24   g  is then placed over the sensors (not shown) and rolled or ironed into place. The finished laminates  24   a - 24   g  are then taped around the edges to the rubber tool  48  to prevent sliding, and the entire assembly is carefully turned over onto the cure plate  64 . 
     The rubber plugs  56  are then installed at the egress points  47 ,  52 , and the fiber optic cable leads  13  are gently pushed into the grooves  54  in the surface of the tool  48  Release film and then the caul plate  62  is placed on top of the tool  48 , and the art to be bagged  66  for the autoclave process. 
     The tool  48  provides a strain relief cavity at the surface of the composite material  24 . This is needed to strengthen the part in this area to prevent kinking of the fiber optic sensor lead  13  in this critical zone where it leaves the surface of the composite material  24 . The strain relief area also relaxes the accuracy required for the location of the fiber lead  13  as it leaves the composite material  24 ; the fiber lead  13  may exit the composite material  24  surface anywhere within the strain relief area  58 . The strain relief area  58  fills with resin that flows during the curing process. 
     The rubber tool  48  fully protects the fiber optic cable lead  13  during the cure process from pinching, kinking and breakage due to its soft nature. The tool  48  also provides an optimal straight protected trench for the entire length of the fiber optic lead  13 . The rubber tool  48  fits snugly around the lead  13 , thus protecting the lead  13  from excessive resin flow. Very little resin can become entrapped between the tool  48  and the fiber optic lead  13 . What little that does is easily wiped off, or comes off with the sacrificial layer of Teflon® tubing. 
     The tool  48  protects the surface of the composite material  24  from lead “print through”, a depressing of the outer layer of laminate  24   a  of the composite material  24  due to the presence of the lead  13 , during cure. It presents both the composite material  24  surface and caul plate  62  with a uniformly flat surface. 
     Most fiber optic sensor leads  13  are broken during de-bagging operations/mold disassembly. This soft rubber tool  48  entirely envelops the lead  13 , and the first item removed during the breakout are the rubber plugs  56  around the strain relief  58  (primary area of failure). There is little chance of damaging the lead  13  during disassembly due to the soft nature of the tool  48  and the lack of resin stuck to the leads  13 . Furthermore, most epoxy resins will not bond to RTV, particularly when it has been treated with a release agent. 
     Most fiber optic sensor leads  13  are broken during de-bagging operations/mold disassembly. This soft rubber tool  48  entirely envelops the lead  13 , and the first item removed during the breakout are the rubber plugs  56  around the strain relief  58  (primary area of failure). There is little chance of damaging the lead  13  during disassembly due to the soft nature of the tool  48  and the lack of resin stuck to the leads. Furthermore, most epoxy resins will not bond to RTV, particularly when it has been treated with a release agent. 
     STRAIN RELIEF BOOT 
     The final step in the installation of the fiber optic leads  13  is the fabrication of a strain relief boot  42  to protect the lead egress point, and to armor the fiber optic lead  13 . The boot  42 , as shown in FIGS.  6   a - 6   c , is cast with a two part aerospace epoxy, such as EA 9394 made by Dexter Hysol. The dimensions are roughly 1.0 inches by 0.5 inches in width, other applications could be ½ to ¼ this size. The strain relief boot  42  is cast in any material needed for the specific application in molds similar to those described herein. The boot  42  has an internal cavity  43  which is sized to fit over the egress strain relief nib  38  left on the surface of the tool  48  described above. 
     The tip of the boot  42  is designed to capture a cone shaped RTV plug (not shown), which is installed as a strain relief between the hard boot  42  and the fiber optic lead  13 , if needed. This surface boot  42  device is used with the standard fiber optic armor, or any other protective tubing scheme as required. There should be at this stage a minimum of three layers of tubing on the fiber optic cable  12 , two layers of polyimide, and on top of these a layer of Teflon®. Depending upon the final application, two or three more layers of heat shrink tubing my be added, or the boot  42  can be bonded over the existing tubing using a material such as TM BOND 2151 made by Dexter Lysol. More layers of tubing equals more protection from handling damage. When the lead  13  has been prepared with armor, the lead  13  is threaded through the boot  42 , and the boot  42  is then slid into place and potted with an aerospace epoxy, such as TMBOND 2151 or EAQ309.3 made by Dexter Lysol, so that the boot  42  is bonded well to the surface of the composite material  24  and to the strain relief nub  58  created by the soft tool  48 . This completes the embedding process. 
     Fiber optic sensors are in general very expensive, as are advance composite components. This surface ingress-egress embedding technique has a high survival rate for fragile optic sensors and leads. The surface mounted boot  42  and the molds to create it protect the egress  47  point of the fiber optic sensor lead  13 . The boot  42  makes the egress point  47  rugged, i.e., able to withstand normal handling without damage. This surface boot  42  device may also be used with standard fiber optic armor, or any other protective tubing scheme as required. 
     The flexible tooling is reusable and several parts may be run off of the same tool. The tooling provides a strain relief cavity at the surface of the part. This is required to strengthen the part in this area, and to prevent kinking of the fiber optic sensor in this critical zone where it leaves the surface of the composite. The strain relief area also relaxes the accuracy required for the location of fiber leads as they leave the laminate, the fiber may exit the composite surface anywhere within the strain relief area. This strain relief area fills with resin that flows during the curing process.