Abstract:
A method of restoring a section of a composite includes the steps of removing an undesirable section of a perforated composite and depositing a repair material in the location of the removed, undesirable section. The repair material is then cured. Before completing the curing step, one or more opening are formed through the repair material to produce one or more perforations through the repair material upon completing the curing step.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to composite materials and, more particularly, to repairing composite sandwich structures. 
     Composite structures are commonly known and used in, for example, gas turbine engines for aerodynamic fairings. One type of composite is a sandwich structure having a face skin and a back skin that are bonded to a honeycomb core. The face skin and back skin may be laminated structures with a resin matrix, such as epoxy, with one or more plies of reinforcement fibers. 
     Holes through the face skin are typically mechanically machined or laser drilled or formed in a process known as pin forming. In the pin forming method, a series of pins are pushed through resin impregnated woven fiber plies and left in place during curing. The pins are then removed after curing, leaving a hole through the outer skin. Skins made with the pin forming method are generally structurally superior because the reinforcement fibers remain intact and continuous, whereas the mechanical machining process and laser drilling severs the fibers, thereby diminishing the reinforcing properties of the fibers. 
     In some instances, it is desirable to repair a section of the face skin that has become damaged from an impact, elevated temperatures, abrasion, erosion, or other phenomenon rather than replace the entire face skin. A typical repair of a pin-formed face skin includes removing the section of the perforated face skin and repairing it with a solid laminate patch. Sometimes the patch is left solid, which leads to loss in acoustic absorption. Other times, new perforations are mechanically machined into the patch. Disadvantageously, the patch is made thicker to make up for the loss in strength caused by the mechanical machining. A thicker patch can also cause a loss in acoustic absorption by increasing the thickness to diameter ratio for the skin. Thus, there is a need for a repair method that allows replacement of a damaged section of the perforated skin while maintaining structural, acoustic, and dimensional properties. This invention addresses those needs while avoiding the shortcomings and drawbacks of the prior art. 
     SUMMARY OF THE INVENTION 
     An example method of restoring a section of a composite includes steps of removing an undesirable section of a perforated skin, and depositing repair a material in the location of the removed undesirable section. The repair material is then cured. Before completing the curing process, one or more opening are formed through the repair material to produce one or more perforations through the repair material. 
     An example perforated composite component includes a honeycomb, a perforated skin bonded to the honeycomb, and a repair section attached to the perforated skin. The repair section includes one or more perforations and one or more continuous reinforcement fibers that have been displaced to accommodate the perforation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows. 
         FIG. 1  illustrates an example perforated composite having an undesirable section. 
         FIG. 2  illustrates an example method for repairing the perforated composite shown in  FIG. 1 . 
         FIG. 3  illustrates a perspective view of the perforated composite after a removal step of the repair process. 
         FIG. 4  illustrates a cross-sectional view of the perforated composite after the removal step of the repair process. 
         FIG. 5  illustrates an example of the perforated composite during the repair process. 
         FIG. 6  illustrates an example of displaced continuous fibers of a repair material during the repair process. 
         FIG. 7  illustrates the perforated composite after the repair process. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates selected portions of an example perforated composite  20 , such as an acoustic composite used in a fan-bypass area or nacelle of a gas turbine engine for noise attenuation. In this example, the perforated composite sandwich structure  20  includes a perforated skin  22  and a solid back skin  24  that are bonded to a machined face of a honeycomb core  26 . In the disclosed example, each of the perforated skin  22  and back skin  24  include composite layers  28 , such as a composite made of a resin matrix with continuous woven fiber reinforcement. The perforated skin  22  includes perforations  30  that extend through the thickness of the perforated skin  22  from an airflow side, A, to the honeycomb core  26 . The perforations  30  of the perforated composite sandwich structure  20  in conjunction with cell chambers  31  of the honeycomb core  26  operate in a known manner to attenuate noise. 
     In the disclosed example, the perforated composite sandwich structure  20  includes an undesirable section  40  that has, for example, become damaged from an impact, elevated temperatures, abrasion, erosion, or other phenomenon. As will be described below, the undesirable section  40  is removed and replaced with a repair material  51  ( FIG. 5 ) such that the perforated composite sandwich structure  20  maintains its structural, acoustic, and dimensional properties. In the disclosed example, the repair material  51  is made of a similar or identical composite layer as the perforated skin  22  (i.e., having a resin matrix with continuous woven fiber reinforcement). 
     Referring to the example shown in  FIG. 2 , the perforated composite sandwich structure  20  is repaired in several steps. The undesirable section  40  is removed from the perforated composite  20  at step  42 . The undesirable section  40  is replaced with a repair section  51  at step  44 , and perforations are formed through the central area of the repair material  51  at step  46 . At step  48 , the repair material  51  is cured to form a repaired perforated composite  20  that has similar structural, acoustic, and dimensional properties as the original perforated composite  20  (e.g., before the repair and damage to the undesirable section  40 ). 
       FIGS. 3 and 4  show a perspective view and a cross-sectional view, respectively, of the perforated composite  20  after the removal step  42 . The undesirable section  40  can be removed in a variety of different ways. In one example, the undesirable section  40  is mechanically removed using a grinding, sanding, or other known mechanical process. In this example, the undesirable section  40  is removed such that the remaining perforated skin  22  has a tapered surface  50  that gradually slopes from the outer surface of the perforated skin  22  toward the honeycomb core  26 . 
     As shown in  FIG. 5 , first pins  52   a  are then inserted through the remaining portions of the perforations  30  in the tapered surface  50 . The repair material  51  is placed into the location of the removed undesirable section  40 . The tapered surface  50  at least partially supports the repair material  51  and provides overlap between the composite layers  28  of the remaining perforated skin  22  and layers  28 ′ of the repair material  51 . The overlap ultimately attaches the repair material  40  to the perforated composite sandwich structure  20 . The repair material  51  is worked over the pins  52   a  and seated over the honeycomb  26 . A template  54  is then placed over the repair material  51  using the first pins  52   a  to locate the template. In this example, the template  54  includes a pattern of openings  55  that corresponds to the pattern of the perforations  30  through the perforated skin  22  (e.g., as seen in  FIG. 3 ). The template  54  is used to determine locations to insert second pins  52   b  where the perforated skin  22  has been entirely removed and no perforations  30  remain (i.e. at the center of the repair material  51 ). Thus, the first pins  52   a  extend through some of the openings  55  of the template  54  while leaving other openings  55  empty. The second pins  52   b  are then inserted through the empty openings of the template  54  and through the repair material  51 . The template  54  thereby provides the benefit of locating the second pins  52   b  within the perforation  30  pattern over an area of the perforated skin  22  that has been entirely removed. 
       FIG. 6  shows fibers  56  of the repair material  51 . In this example, the pin  52  (i.e., pin  52   a  or  52   b ) displaces the fibers  56  within the repair material  51  as the pin  52  is inserted through a repair material  51 . In one example, a tapered end  57  of the pin  52  ( FIG. 5 ) wedges through the repair material and moves the fibers  56  aside such that the fibers  56  curve around the pin  52 , while remaining intact and continuous. This provides a benefit of maintaining the structural integrity and reinforcing properties of the fibers  56 . 
     Referring again to  FIG. 5 , a cushion  58  is then placed over the repair section  51  such that the ends of the pins  52   a  and  52   b  extend into the cushion  58 . In the illustrated example, the cushion  58  is made of an elastic material, such as rubber, to absorb the ends  57  of the pins  52   a  and  52   b , to protect the pins  52   a  and  52   b , and to prevent the pins  52   a  and  52   b  from contacting other articles. Optionally, a breather ply  59  may be added between the cushion  58  and the repair material  51  to provide a means to apply vacuum and to provide a mechanism to remove excess resin during compaction to the repair material  51 . 
     In the illustrated example, the area of the repair material  51  is sealed within a vacuum bag  60  in a known manner. The vacuum bag  60  is evacuated to a desired level, and the resulting vacuum within the vacuum bag  60  produces a net atmospheric pressure on the cushion  58  that compresses the repair material  51 . This squeezes some resin from the repair material  51  and consolidates it. At least the area of the repair material  51  is then heated to cure the repair material  51 . Alternatively, the repair material  51  may utilize a resin that does not require heating. The repair material  51  may be any of a variety of different types of material. In one example, the repair material  51  is a composite of an uncured or partially cured resin matrix with continuous woven fiber reinforcement (i.e., a prepreg). In another example, the repair material  51  includes a dry ply or plies of continuous woven fibers and a liquid resin that is cured after placing the dry ply or plies onto the pins  52   a  and  52   b.    
     After the curing step  48 , the vacuum bag  60 , cushion  58 , and template  54  are removed from the perforated composite  20 . The pins  52   a  and  52   b  are also removed, leaving behind perforations  30 ′ that extend through the repair material  51  and through a portion of the original perforated skin  22 . Thus, the pins  52   a  and  52   b  function as a type of place holder during the curing of the repair material  51  to form the perforations  30 ′. 
     Alternatively, instead of using the vacuum bag  60 , the perforated composite  20  is mechanically pressed or clamped to exert the pressure on the cushion  58  during curing of the repair material  51 , or no pressure is used to consolidate the repair material  51 . Optionally, the pins  52   a  and  52   b  have a taper from one end of the pins  52   a  and  52   b  to the other end to facilitate removal of the pins  52   a  and  52   b  from the perforated composite  20  after curing. In one example, the taper is about  30 . The pins  52   a  and  52   b  can also be lubricated to further facilitate removal from the repair material  51 . 
     In another example, an adhesive  62  ( FIG. 5 ) is used in addition to the repair material  51 . In this example, the adhesive  62  is placed between the remaining perforated skin  22 , such as on the tapered surface  50 , and the repair material  51 . The adhesive  62  promotes bonding between the remaining perforated skin  22  and the repair material  51 . In one example, the adhesive  62  is a polymeric material that is compatible with the base resin materials used for the repair material  51  and the perforated skin  22 . A compatible adhesive  62  may be any material that provides a desired degree of bonding between the repair material  51  and the remaining perforated skin  22 . 
       FIG. 7  illustrates the perforated composite  20  after the repair process. In this example, the repair material  51  has a thickness dimension D 1  that is approximately equal to a thickness dimension D 2  of the original perforated skin  22 . Furthermore, since the repair material  51  was formed without cutting the fibers  56  (i.e., the fibers  56  were displaced by pins  52 ), the repair material  51  has structural properties similar to the original perforated skin  22 . Additionally, since the perforations  30 ′ of the repair material  51  are matched with the pattern of the preexisting perforations  30  of the perforated skin  22 , the acoustics of the perforated composite  20  after the repair are similar to the original perforated composite  20 . Thus, the disclosed examples provide a method of repairing the perforated composite  20  such that the structural, acoustic, and dimensional properties are maintained. 
     Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.