Abstract:
A method for repairing a composite material having an internal void includes identifying the location of the internal void and forming a single channel extending from a surface of the composite material to the internal void. The method further includes pulling a vacuum in the internal void and inserting a repair material through the channel into the internal void.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates generally to the field of composite materials and more particularly to a method for composite material repair. 
     BACKGROUND OF THE INVENTION 
     Many manufacturing processes today call for the fabrication of components from “composite” materials, also known as fiber-reinforced polymers. Fiber-reinforced polymers are comprised of reinforcing fibers that are positioned in a polymer matrix. Commonly, the reinforcing fibers are fiberglass, although high strength fibers such as aramid and carbon are used in advanced applications, such as aerospace applications. The polymer matrix is typically a thermoset resin, such as polyester, vinyl ester, or epoxy. Specialized resins, such as, phenolic, polyurethane and silicone are used for certain applications. 
     Composite materials may be formed using numerous types of fabrication process. One such process is a wet lay-up process. In a wet lay-up process, layers of dry reinforcing fiber are laid on a mold by hand or by a placement machine. Liquid resin is then poured on the fiber materials such that the resin fills the spaces between the fibers. The materials may then be cured at room temperature or in an autoclave and the liquid resin turns into a solid thermoset. The fibers are thus embedded in the solid thermoset resin and reinforce the resin. Alternatively, layers of fibers can be pre-impregnated with resin and then partially cured to form layers of “prepreg” material. After this partial curing, the resin has not completely set and the prepreg layers are flexible and can be shaped in or around a mold or forming tool. Once the prepreg layers are so shaped, the prepreg is then completely cured in an autoclave to form a fiber-reinforced laminate. 
     A common defect associated with composite structures is air inclusions or voids located inside the composite material. Such voids weaken the composite material and sometimes must be repaired. Another scenario requiring repair is when the composite material is impacted during service, resulting in delamination or delaminations between layers of the material. Such damage is typically referred to as interlaminar defects or interlaminar damage. 
     One type of repair for voids and interlaminar defects is resin injection. During one type of resin injection repair, two holes are drilled through the composite material to the void or delamination inside the composite material. The two holes are typically drilled at opposite ends of the defect. Resin is then either driven into one hole using pressure until it exits the second hole, or resin is drawn into one hole by applying a vacuum to the second hole. When using such a two-hole process, air entrapment in the void is common and therefore the resin does not completely fill the void. In addition, since two holes must be drilled in the structure, the already weak structure is further weakened. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method for composite material repair is provided that substantially eliminates or reduces disadvantages or problems associated with previously developed methods. In particular, the present invention contemplates a method of repairing a void in a composite material using a repair material injected into the void through a single channel. 
     In one embodiment of the present invention, a method for repairing a composite material having an internal void includes identifying the location of the internal void and forming a single channel extending from a surface of the composite material to the internal void. The method further includes pulling a vacuum in the internal void and inserting a repair material through the channel into the internal void. 
     Technical advantages of the present invention include a method for composite material repair that provides an improved method of repairing voids in composite materials over the prior art. Unlike the prior art, the present invention only requires the formation of one hole or channel in the composite material, and thus minimal additional damage is caused by the repair process. Furthermore, the method of the present invention allows the creation of a complete or almost complete vacuum in the void, causing most of the air molecules to be removed from the void before the repair material is injected. Therefore, the amount of air remaining in the void after the repair material has been injected is minimized and the void is entirely or almost entirely filled with the repair material. 
     Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and for further features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a cross-section of a composite material repair set-up according to teachings of the present invention; 
     FIG. 2 is a plan view of portions of the composite material repair set-up of FIG. 1; and 
     FIG. 3 is a flowchart illustrating a method of composite material repair according to teachings of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention and its advantages are best understood by referring now in more detail to FIGS. 1 through 3 of the drawings. 
     FIGS. 1 and 2 illustrate cross-sectional and plan views, respectively, of a composite material repair set-up  10  according to one embodiment of the present invention. FIG. 3 is a flowchart illustrating a method of composite material repair according to one embodiment of the present invention. FIGS. 1-3 will be described together. Set-up  10  includes a composite material  12  having an internal void  14 . Void  14  may have been formed during the formation of composite material  12 . For example, void  14  may have been formed due to the failure of two layers of composite material to bond together in the area at which void  14  is located. Alternatively, void  14  may have been formed due to delamination between two layers of composite material  12 . Such delamination may be caused by impact to composite material  12 . The term “void” indicates any type of defect forming a cavity in composite material  12 , whether it be an air inclusion formed during the fabrication of material  12 , a interlaminar defect formed due to impact or other stress, or any other cause of a cavity within material  12 . In addition, the present invention may be used in conjunction with composite materials formed using any appropriate fabrication process. 
     The presence of void  14  in composite material  12  causes weakness in material  12  and may lead to a failure of material  12 . Therefore, it is preferable that void  14  be filled to strengthen material  12 . Before void  14  is repaired by filling, the location thereof is identified at step  100  of the method illustrated by the flowchart of FIG. 3 using ultrasonic equipment or any other appropriate method. After the location of void  14  is identified, a channel  16  is formed at step  102  that extends from a surface  18  of material  12  to void  14 . Channel  16  is typically formed by drilling through material  12  or by any other appropriate method. Channel  16  intersects void  14  at any point; however, it is preferable that channel  16  intersects void  14  near the center thereof. 
     When appropriate, channel  16  is cleaned to remove any loose material. The location of channel  16  is then identified by drawing lines  19  (illustrated in FIG. 2) on surface  18  of composite material  12  that intersect with the channel  16 . Lines  19  are needed to identify the location of channel  16  when the channel cannot be seen through the bleeder material  22  as described below. The channel locations lines  19  should be long enough to extend past the area of surface  18  that will be covered by bleeder material  22 . Alternatively, any other appropriate method of marking or otherwise determining the location of channel  16  are also included within the scope of the present invention. 
     After channel  16  has been formed through composite material  12 , preparations are made to create a vacuum in void  14 . In the illustrated embodiment, a vacuum bag setup is used to create a vacuum in void  14 . In this setup, bleeder material  22  is placed over channel  16  on surface  18  at step  104 . Bleeder material  22  is a thick, felt-like cloth that absorbs excess repair material  32 , described below. Bleeder material  22  also functions as a breather, providing a continuous air path for creating the vacuum in void  14 . Overlaying the bleeder material  22  is a vacuum hag  20 . Bag sealant tape  24  is placed around bleeder material  22  at step  106 . Sealant tape  24  is a putty-like material that is used to create a seal between vacuum bag  20  and surface  18 . 
     Vacuum bag  20 , which is typically a thick plastic material, is placed over bleeder material  22  at step  108  and is pressed in contact with sealant tape  24  at step  110 . By pressing vacuum bag  20  against sealant tape  24 , a seal is formed between bag  20  and surface  18 . Alternatively, sealant tape  24  is attached to vacuum bag  20 , and then the bag and sealant tape  24  are pressed against surface  18 . Although the present invention encompasses using any thickness of material for the vacuum bag, the vacuum bag  20  preferably has a thickness of at least 0.5 millimeters. 
     As illustrated in FIGS. 1 and 2, a vacuum hose  26  is inserted through vacuum bag  20 . Vacuum hose  26  is inserted through vacuum bag  20  by means of a vacuum port or any other appropriate method. A vacuum pump  27  (illustrated in FIG. 2) is attached to the vacuum hose  26  to draw or pull a vacuum through bleeder material  22  in void  14  and channel  16  at step  112 . 
     To locate channel  16  (either before or after forming the vacuum) a straight edge is aligned with lines  19  such that the parts of the lines obscured by the bleeder material  22  can be re-drawn on top of the vacuum bag. The point at which these re-drawn lines intersect should indicate the location of channel  16 , which is not visible through the vacuum bag and the bleeder material. Again, any other appropriate method of locating channel  16  is encompassed by the present invention. 
     After locating channel  16 , a syringe  28  is used to inject a repair material  32 , such as a resin or any other appropriate material, through channel  16  into void  14  at step  114 . A needle  30  attached to the syringe is inserted into the channel  16  to inject the repair material. The needle  30  is long enough to pierce bag  20  and bleeder material  22  and enter channel  16 , and has an inside diameter sufficient to allow the flow of repair material  32  into the void  14 . In one embodiment of the invention, a needle having an inside diameter of 1.0 millimeters is used, however the invention is not limited to any one needle diameter. 
     Before inserting needle  30  through vacuum bag  20  and bleeder material  22 , a small amount of repair material  32  may be ejected through the needle to ensure that no air bubbles are contained in the repair material inside the syringe  28 . In addition, it is preferable that a small amount of repair material  32  be ejected on top of vacuum bag  20  at the point at which needle  30  is to be inserted through the vacuum bag. This forms a small bead  34  of repair material on the surface of vacuum bag  20  thereby providing a seal to prevent air leakage around the needle  30  when inserted through bag  20 . 
     To fill void  14 , needle  30  is inserted through bead  34 , vacuum bag  30  and bleeder material  22  into channel  16 . As a result of the previously pulled vacuum in void  14  and channel  16 , repair material  32  is drawn into void  14  from syringe  28 . When the repair material begins to fill channel  16  after filling void  14 , vacuum pump  27  may be shut off at step  116 , allowing the vacuum head to release gradually. Slight pressure is applied to a plunger  36  of syringe  28  until repair material  32  overflows into the bleeder material  22 . Pressure is applied to overcome the friction of plunger  36  in syringe  28  and to cause repair material  32  to flow into void  14  and channel  16 . When the vacuum head is released, atmospheric pressure drives the resin into any remaining unfilled regions of void  14 . If a sufficient vacuum has been pulled in void  14  and channel  16 , these areas will be completely filled with repair material  32 . 
     When the vacuum head has been released, the injection process is complete. Needle  30  is removed from channel  16 , and vacuum bag  20 , bleeder material  22  and tape  24  are removed from surface  18  at step  118 . If appropriate, the repair material  32  that fills void  14  and channel  16  may be hardened by curing. Furthermore, any excess repair material  32  coming out of channel  16  onto surface  18 , may be sanded or otherwise removed from surface  18  at step  120 . 
     The method of the present invention results in a very high percentage of void  14  being filled with repair material  32 . It has been found through experimentation that there is an approximately one-to-one correlation between the percentage of air evacuated from void  14  and the percentage of the void volume that is filled with repair material when using the present invention. Therefore, if a sufficient vacuum is drawn in void  14  and channel  16 , these areas will be almost completely filled with repair material  32 . For example, a typical two-stage vacuum pump will evacuate more than 99.9% of the air inside of void  14  and channel  16 . In this example, since 99.9% of the air is evacuated from void  14  and channel  16 , approximately 99.9% of void  14  and channel  16  will be filled with repair material after the process is complete. Since virtually all of the air molecules are removed from void  14  and channel  16  before repair material injection begins, there is virtually no air entrapment in void  14  and channel  16 . This ensures that composite material  12  will regain the greatest possible amount of strength after the repair process is complete. In addition, since the process described above requires only one channel  16 , the loss of material strength caused by the repair process is minimized. 
     Although the present invention has been described with selected embodiments, a myriad of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present invention encompass such changes, variations, alterations, transformations, and modifications as fall within the spirit and scope of the appended claims.