Verifiable quick patch repair for composite structures

Abstract

A method for patching a hole in a composite parent structure. The method comprises: inserting an insert in the hole; placing a composite patch having a multiplicity of curved flexible members on one side of the composite parent structure in a position where a central portion of the composite patch overlies the insert and the flexible members confront opposing portions of the composite parent structure providing adhesive between the composite patch, the insert, and the composite parent structure; pressing the composite patch against the composite parent structure with sufficient pressure to force the flexible members to conform to the shape of the composite parent structure; and while the flexible members are in a stressed state, curing the adhesive in a manner that causes the flexible members to bond to the parent structure.

Claims

1. A method for patching a hole in a composite parent structure, said method comprising: inserting an insert in the hole; while the insert is in the hole, placing a composite patch having a multiplicity of curved flexible members on one side of the composite parent structure in a position where a central portion of the composite patch overlies the insert and the flexible members contact opposing portions of the composite parent structure disposed around the insert; providing adhesive between the composite patch and the insert, and between the composite patch and the composite parent structure; and pressing the composite patch against the composite parent structure with sufficient pressure to force the flexible members to conform to the shape of the surface of the opposing portions of the composite parent structure while the adhesive therebetween is curing.

2. The method as recited in claim 1, further comprising supporting the insert on one side while the composite patch is pressing on another side of the insert.

3. The method as recited in claim 1, wherein the flexible members of the composite patch become less curved or straight during said pressing step.

4. The method as recited in claim 1, further comprising non-destructive evaluation of the integrity of bond lines between the flexible members of the composite patch and the opposing portions of the composite parent structure.

5. The method as recited in claim 1, wherein said pressing step comprises applying pressure using mechanical force.

6. The method as recited in claim 1, wherein said pressing step comprises applying pressure using magnetic force.

7. The method as recited in claim 1, wherein said pressing step comprises applying vacuum pressure.

8. The method as recited in claim 1, further comprising placing a bladder over said composite patch, wherein said pressing step comprises applying pressure to said bladder.

9. The method as recited in claim 1, wherein the composite parent structure and the composite patch are made of fiber-reinforced composite material.

10. A method for patching a hole in a composite parent structure, said method comprising: inserting an insert in the hole; while the insert is in the hole, placing a composite patch having a multiplicity of curved flexible members on one side of the composite parent structure in a position where a central portion of the composite patch overlies the insert and the flexible members contact opposing portions of the composite parent structure disposed around the insert; providing adhesive between the composite patch and the insert, and between the composite patch and the composite parent structure; pressing the composite patch against the composite parent structure with sufficient pressure to force the flexible members to conform to the shape of the surface of the opposing portions of the composite parent structure so that spaces between the flexible members and the composite parent structure are filled with adhesive; and curing the adhesive while the flexible members are conformed to the shape of the surface of the opposing portions of the composite parent structure in a manner that causes the flexible members to bond to the parent structure.

11. The method as recited in claim 10, wherein after the flexible members have been bonded to the parent structure by said curing, each flexible member is in a respective pre-stressed flexed state with a potential to deform toward an unflexed state in the event that a strength of the bond between that flexible member and an opposing portion of said parent structure becomes zero.

12. The method as recited in claim 10, further comprising supporting the insert on one side while the composite patch is being pressed against another side of the insert.

13. The method as recited in claim 10, wherein the flexible members of the composite patch become less curved or straight during said pressing step.

14. The method as recited in claim 10, wherein said pressing step comprises applying pressure using mechanical force.

15. The method as recited in claim 10, wherein said pressing step comprises applying pressure using magnetic force.

16. The method as recited in claim 10, wherein said pressing step comprises applying vacuum pressure.

17. The method as recited in claim 10, further comprising placing a bladder over said composite patch, wherein said pressing step comprises applying pressure to said bladder.

18. The method as recited in claim 10, further comprising placing randomized fibers within the adhesive before the adhesive is cured.

19. The method as recited in claim 10, wherein the composite parent structure and the composite patch are made of fiber-reinforced composite material.

20. A method for patching a hole in a composite parent structure, said method comprising: inserting an insert in the hole; while the insert is in the hole, placing a first composite patch having a multiplicity of curved flexible members on one side of the composite parent structure in a position where a central portion of the first composite patch overlies the insert and the flexible members contact opposing portions of the composite parent structure disposed around the insert; while the insert is in the hole, placing a second composite patch having a multiplicity of curved flexible members on another side of the composite parent structure in a position where a central portion of the second composite patch underlies the insert and the flexible members of the second patch contact opposing portions of the composite parent structure disposed around the insert; providing adhesive between the first composite patch and the insert, and between the first composite patch and the composite parent structure, between the second composite patch and the insert, and between the second composite patch and the composite parent structure; and pressing the first and second composite patches toward each other with sufficient pressure to force the flexible members of both patches to conform to the shapes of confronting surfaces of the composite parent structure while the adhesive therebetween is curing.

21. The method as recited in claim 20, further comprising curing the adhesive while the flexible members of the first and second composite patches are conformed to the shapes of the surfaces of the opposing portions of the composite parent structure in a manner that causes the flexible members of the first and second composite patches to bond to the composite parent structure.

22. The method as recited in claim 21, wherein after the flexible members of the first and second composite patches have been bonded to the parent structure by said curing, each flexible member is in a respective pre-stressed flexed state with a potential to deform toward an unflexed state in the event that a strength of the bond between that flexible member and an opposing portion of said parent structure becomes zero.

23. The method as recited in claim 20, wherein the composite parent structure and the first and second composite patches are made of fiber-reinforced composite material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A and 1B are diagrams respectively showing outer and inner exploded views of unassembled components of a verifiable composite patch repair of composite parent structure in accordance with one embodiment. (Adhesive is not shown in FIGS. 1A and 1B.)

(2) FIGS. 2A and 2B are diagrams respectively showing outer and inner views of the composite patch repair components shown in FIGS. 1A and 1B after the patches and parent structure have been bonded together.

(3) FIG. 3 is a diagram showing a scenario in which disbonds present in a composite patch repair of the type shown in FIG. 2A are detectable due to the pre-stressed fingers of the composite repair patch.

(4) FIG. 4 is a diagram showing an exploded sectional view of an arrangement in which bladders, pressure plates and a center rod are used to apply uniform pressure during bonding of pre-stressed composite patches to opposing sides of a composite parent structure. (Adhesive is not shown in FIG. 4.)

(5) FIG. 5 is a diagram showing an exploded sectional view of an arrangement in which bladders, pressure plates and magnets are used to apply uniform pressure during bonding of pre-stressed composite patches to opposing sides of a composite parent structure. (Adhesive is not shown in FIG. 5.)

(6) FIG. 6 is a diagram showing an exploded sectional view of an arrangement in which a bladder, two pressure plates and two magnets are used to apply uniform pressure during bonding of a single pre-stressed composite patch to one side of a composite parent structure. (Adhesive is not shown in FIG. 6.)

(7) FIG. 7 is a diagram showing an exploded sectional view of an arrangement in which a vacuum bag, a bladder, and two pressure plates are used to apply uniform pressure during bonding of a single pre-stressed composite patch to one side of a composite parent structure. (Adhesive is not shown in FIG. 7.)

(8) Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals.

DETAILED DESCRIPTION

(9) Pre-stressed composite repair patches, repaired composite structures, and methods for repairing composite structures using pre-stressed composite patches will now be described with reference to specific embodiments. These embodiments illustrate some but not all means and methods for reducing to practice the concepts disclosed herein. The patches, patched structures and patching methods disclosed herein can be applied to any composite structure which has been damaged, and is particularly suitable for aircraft structures made of composite material.

(10) In the implementations disclosed in detail below, a composite parent structure having a hole, an insert in the hole and a composite patch are bonded together using adhesive. The composite patch includes features that facilitate improving bond line control between the components and/or enable potential disbond locations to be easily detected along a joint created between the components. In accordance with the specific embodiments disclosed hereinafter, the patch comprises a central portion and a multiplicity of flexible members that extend outwardly from the outermost portion of the central portion. These flexible members are stressed and then bonded to the parent structure while in a stressed state. The flexible members are stressed by pressing the patch against the parent structure, thereby causing the flexible members to flex from an unstressed state to a pre-stressed state. The flexible members are bonded to the parent structure while in the pre-stressed state. In the bonded state, the pre-stress in the flexible members exerts a force tending to restore the flexed members to their unstressed (i.e., unflexed) state were the bond strength to decrease to zero.

(11) FIGS. 1A and 1B are diagrams respectively showing outer and inner exploded views of unassembled components of a verifiable patch repair of a parent structure 10 made of composite material in accordance with one embodiment. The parent structure 10 depicted in FIG. 1A has a hole 11, which may have been formed in the parent structure 10 by machining to remove remnants of a site of damage. An insert 12 made of composite material will be inserted into hole 11 during the repair process. Then a pair of patches 2 and 4, also made of composite material, are placed on opposite sides of the parent structure 10 in respective positions above and below the insert 12. The parent structure 10 may be a skin of an aircraft structure, such as a skin of a fuselage. The composite material may comprise fiber-reinforced plastic.

(12) As best seen in FIG. 1A, patch 2 comprises a central portion have a center opening 18 and a perimeter portion having a multiplicity of stress-relief apertures 14 and a multiplicity of slits 8. The slits 8 extend from respective apertures 14 to a peripheral edge of the patch. In an initial state prior to installation as part of a repair, patch 2 has an overall shape which is not planar but, as described in more detail below, patch 2 may comprise a planar portion surrounded by a non-planar portion, which non-planar portion is capable of being flexed under pressure so that it adopts a planar configuration.

(13) In accordance with one embodiment, the central portion of patch 2 comprises an innermost portion 5 and an outermost portion disposed between the innermost portion 5 and a hypothetical circle (not shown) which can be drawn tangent to apertures 14 to represent the outermost portion of the central portion. The inner surface of the innermost portion 5 is preferably planar in an unstressed state if the opposing surface of insert 12 is planar. In this case, the innermost portion 5 can be flat. In the alternative, if the opposing surface of insert 12 has a curved contour, then the inner surface of the innermost portion 5 (in an unstressed state) may be provided with a matching contour.

(14) The perimeter portion of patch 2 comprises a multiplicity of curved flexible members 6 (i.e., fingers) arranged in side-by-side relationship. Adjacent flexible members 6 extend outward from the outermost portion of the central portion. A distal portion of each flexible member 6 may have a thickness which decreases gradually with increasing distance (i.e., tapers) from the center to the periphery of the patch 2. Preferably the flexible members 6 are integrally formed with the central portion of patch 2. The flexible members are separated by slits 8 and apertures 14 therebetween.

(15) Each flexible member 6 is curved and resists flexure in an elevational direction of decreasing curvature and twisting when in the unflexed state depicted in FIG. 1A, but each flexible member 6 is sufficiently flexible that it can twist, bend and otherwise conform to the shape of an opposing surface when pressed against that surface with sufficient force. More specifically, the flexible members 6 are capable of changing shape from curved to flat or less curved when pressed against a surface. In particular, the flexible members 6 have the property that, when the flexible members 6 are placed in contact with a planar surface, the flexible members will change shape from curved to flat if the composite patch is pressed against the planar surface with sufficient force. For the purpose of illustration, it will be assumed that the surfaces of parent structure 10 are planar, in which case the innermost portion 5 of patch 2 will be flat when patch 2 is in an unflexed state. In contrast, the perimeter portion of patch 2 is curved when the composite patch is in an unflexed state and will become flat when pressed against a planar surface of the parent structure 10.

(16) As best seen in FIG. 1B, patch 4 may have a construction similar, if not nearly identical, to patch 2. Accordingly, the foregoing description of patch 2 is equally applicable to patch 4. Although the patches 2 and 4 may be circular in shape, circularity is not required and in some applications non-circular shapes (e.g., elliptical) may be appropriate.

(17) During the repair process, a first layer of adhesive is provided between patch 2 and parent structure 10 and between patch 2 and insert 12; and a second layer of adhesive is provided between patch 4 and parent structure 10 and between patch 4 and insert 12. This adhesive is not shown in FIGS. 1A and 1B. The adhesive is preferably applied on the inner surface of patches 2 and 4, so that when the flexible members 6 are pressed against the confronting surfaces of the parent structure 10, some of the adhesive on the inner surfaces of the flexible members 6 will be squeezed into the slits 8, providing a visual indication of adequate adhesive coverage. Preferably the adhesive has a scrim (e.g., comprising random fibers) so that all of the resin cannot be squeezed out. This features enables control of the minimum bond line thickness to conform to bond line thickness specifications.

(18) In accordance with the embodiment depicted in FIGS. 1A and 1B, the hole 11 in the parent structure 10 is formed by machining a local area of the parent structure which has suffered damage, such as cracking or delaminations. The damaged area is removed and the resulting hole is shaped to receive an insert. In the example depicted in FIGS. 1A and 1B, hole 11 and insert 12 are both circular. After the preparation of hole 11 by machining, a repair process comprising the following steps may be performed.

(19) First, a center opening (not shown in FIG. 1A) is formed the insert 12 and then the insert 12 is inserted in the hole 11. Optionally, adhesive is placed on either the peripheral surface of hole 11 or on the peripheral surface of the insert 12 or both before the insert 12 is inserted in hole 11. Next adhesive is applied on the inner surfaces of patches 2 and 4. Then the patches 2 and 4 are placed over and under the insert in positions such that the center opening 18 and 20 of the patches are aligned with the center opening in the insert 12. More specifically, patch 2 is placed on one side of the parent structure 10 in a position where the flat innermost portion 5 of patch 2 overlies the insert 12 and the flexible members 6 of patch 2 confront opposing portions of parent structure 10 disposed around the insert 12; and patch 4 is placed on the other side of the parent structure 10 in a position where the flat innermost portion 5 of patch 4 underlies the insert 12 and the flexible members 6 of patch 4 confront opposing portions of parent structure 10 disposed around the insert 12. A threaded rod 16 is then passed through the aligned center openings of the insert and patches. The threaded rod 16 has a length such that a portion of the threaded shaft protrudes beyond the innermost portion 5 of patch 4 when patches 2 and 4 are in unflexed states in contact with the opposing surfaces of the parent structure 10. A nut (not shown in FIG. 1A) in then screwed onto the protruding end of the shaft of threaded rod 16. The nut is then tightened until the flat innermost portions 5 of patches 2 and 4 are in contact with the insert 12. Tightening of the nut produces a mechanical force which presses the patches 2 and 4 against the parent structure 10 and causes the patches to flatten. More specifically, the head of the threaded rod 16 exerts a contact force on the portion of patch 2 that surrounds center opening 18 while the turning nut exerts an equal and opposite contact force on the portion of patch 4 that surrounds center opening 20. As part of this flattening process, the flexible members 6, which were initially curved, become flat. When the patches 2 and 4 are completely flattened, the slits 8 form gaps between the flexible members 6.

(20) As previously mentioned, for the purpose of illustration it has been assumed that the parent structure has planar surfaces, in which case the flexible members 6 become flat. Alternatively, if a surface of the parent structure 10 is not planar, then any flexible member 6 in contact with such non-planar surface will conform to that surface. For example, the curved flexible members 6 may become less curved, but not flat.

(21) Returning to the repair process, the patches 2 and 4 are pressed together for a duration of time sufficient to allow the adhesive to cure. When the adhesive has fully cured, the patches 2 and 4 will be bonded to the parent structure 10 in a flattened state. FIGS. 2A and 2B show outer and inner views of the composite patch repair components shown in FIGS. 1A and 1B after the patches 2 and 4, insert 12 and parent structure 10 have been bonded together. The threaded rod 16 can now be removed and the bore formed by the center openings of the patch/insert/patch sandwich can be plugged in a manner that leaves no protrusion on any surface of the parent structure 10 which will be exposed to airflow. In some cases, the head of the threaded rod 16 can be cut off.

(22) Still referring to FIGS. 2A and 2B, slits 8 and apertures 14 provide disbond isolation in the bond lines between the flexible members 6 and confronting portions of the parent structure 10 by restricting any disbond from spreading beyond each flexible member 6. Isolating the flexible members 6 from each other restricts disbond growth by preventing any disbond from propagating in a direction transverse to a flexible member 6.

(23) In addition, the adhesive squeezed out at the peripheral edges of the patches and in the gaps between the flexible members help ensure bond line coverage across the patches.

(24) Moreover, because the previously curved flexible members 6 are now bonded in a flattened state to the parent structure 10, the flexible members are pre-stressed at the bond line and will tend to return to a curved state if the bond strength becomes zero (i.e., if the flexible member or a portion thereof releases from the underlying parent structure). If a sufficiently large disbond occurs between a flexible member 6 and the confronting portion of the parent structure 10, then at least a portion of that flexible member will move as it seeks to return to its unflexed state. In many cases, the resulting change in shape of the flexible member in the area of the disbond will be visible, providing a bond failure indication to an inspector. Such movement and deformation can prevent the formation of a kissing bond, which can be missed by current NDE methods. Instead a disbond is formed which can be readily detected using NDE methods due to the absence of contact (i.e., due to the presence of a gap at the interface) between the released portion of the flexible member and the parent structure at the location of the disbond. FIG. 3 shows a scenario in which disbonds 22 and 24 between patch 2 and parent structure 10 are visible and detectable by NDE methods.

(25) FIG. 4 is an exploded sectional view of unassembled components of a verifiable patch repair arrangement in accordance with an alternative embodiment. Again the object being repaired is a composite parent structure 10 having a hole 11 in which a composite insert 12 will be inserted. Again, a pair of composite patches 2 and 4 will be placed on opposite sides of the parent structure 10 in respective positions above and below the insert 12. The patches 2 and 4 may have the same structure as the patches previously described and shown in FIG. 1A. Again the adhesive which will bond the patches 2 and 4 to the insert 12 and to the parent structure 10 is not shown. The parent structure 10 may be a skin of an aircraft structure, such as a skin of a fuselage. The composite material may comprise fiber-reinforced plastic.

(26) In accordance with an alternative embodiment shown in FIG. 4, means are provided for applying a uniform pressure on the flexible patches. The means for applying uniform pressure comprise a pair of bladders 30 and 32, which will be placed in contact with patches 2 and 4 respectively, and a pair of pressure plates 26 and 28, which will be placed in contact with bladders 30 and 32 respectively. Each of pressure plates 26 and 28, bladders 30 and 32, patches 2 and 4, and insert 12 has a center opening through a threaded rod 16 is passed. Again the nut which couples to the shaft of the threaded rod 16 is not shown, but the mechanism for generating a mechanical force to press the patches 2 and 4 against the parent structure 10 is analogous to what has been previously described with respect to the embodiment shown in FIG. 1A. Tightening of the nut produces a mechanical force which presses the pressure plates 26 and 28 against bladders 30 and 32, which in turn press against the patches 2 and 4 respectively, causing the patches 2 and 4 to contact and conform to the surfaces of the parent structure 10. More specifically, the head of the threaded rod 16 exerts a contact force on the portion of pressure plate 26 that surrounds its center opening 18 while the turning nut exerts an equal and opposite contact force on the portion of pressure plate 28 that surrounds its center opening. As the pressure plates 26 and 28 exert pressure on the bladders 30 and 32, the bladders 30 and 32 distribute those pressures uniformly over the entire surface areas of the patches 2 and 4. The patches 2 and 4 are pressed together for a duration of time sufficient to allow the adhesive to cure. Then the repair set-up can be dismantled by unscrewing the nut from the threaded rod 16, removing the threaded rod 16, and then removing the bladders and pressure plates. The bore formed by the center openings of the patch/insert/patch sandwich can be plugged as previously described.

(27) In accordance with a further alternative embodiment shown in FIG. 5, coupled magnets 34 and 36 can be used to exert magnetic forces on the pressure plates 26 and 28 instead of using a threaded rod and nut to produce a mechanical force. The magnets 34 and 36 may be high-strength permanent magnets (for example, rare-earth magnets such as those made of neodymium, iron and boron), electro-magnets, or electro-permanent magnets. In this implementation, the pressure plates 26 and 28, patches 2 and 4, and insert 12 do not require center openings, thereby eliminating the need to plug a bore formed by center openings of the patch/insert/patch sandwich. If the parent structure 10 is relatively thin, two magnets may not be needed; instead the opposing pressure plate 28 can be made of a paramagnetic material, such as 400 series stainless steel. Again the pressures exerted by the pressure plates 26 and 28 (due to the attractive forces produced by the magnets) are uniformly distributed over the surface areas of patches 2 and 4 by the bladders 30 and 32 during flattening of the patches 2 and 4 and curing of the adhesive (not shown).

(28) The embodiments shown in FIGS. 3-5 each involve two repair patches 2 and 4 bonded to opposite sides of a parent structure 10. In many cases, however, a single patch 2 (see FIG. 6) placed on one side of the parent structure 10 will suffice. The embodiment depicted in FIG. 6 comprises a pair of pressure plates 26 and 28 disposed on opposing sides of the parent structure, a single bladder 30 disposed between the single patch 2 and the pressure plate 26, and a pair of magnets 34 and 36 coupled across the repair to exert magnetic forces that will move pressure plate 26 toward the parent structure 10 to flatten patch 2 and clamps pressure plate 28 against the lower surface of parent structure 10. The pressure exerted by pressure plate 26 is uniformly distributed over the surface area of patch 2 by bladder 30. In this embodiment, the pressure plate 28 is in contact with and bears against the lower surface of the parent structure 10. The adhesive between patch 2 and insert 12 and between patch 2 and parent structure 10 is not shown in FIG. 6.

(29) In an alternative embodiment not shown, a single repair patch can be applied using a threaded rod and nut arrangement to press pressure plate 26 toward the parent structure 10 to flatten the patch 2.

(30) In accordance with another alternative embodiment, a single patch can be applied using vacuum bag methods, as shown in FIG. 7. The set-up depicted in FIG. 7 comprises the following components on one side of the parent structure: a single patch 2, an upper pressure plate 26, a bladder 30 disposed between patch 2 and pressure plate 26, and a vacuum bag 38 having a perimeter which will be hermetically sealed to the upper surface of the parent structure 10 by means of a seal 40. On the other side of the parent structure 10, the set-up includes a pressure plate 28 having a perimeter which will be hermetically sealed to the lower surface of the parent structure 10 by means of a seal 42. When the pressure plate 28 and the vacuum bag 38 are sealed to the parent structure 10, they form a vacuum chamber. This vacuum chamber can be coupled to a vacuum source (not shown) by means of a vacuum probe (not shown) that passes through an opening in the vacuum bag 38 and is connected to the vacuum source by a hose (not shown). Evacuation of the vacuum chamber causes the vacuum bag 38 to pull the pressure plate 26 toward the parent structure 10, thereby producing a pressure that is uniformly distributed over the surface area of patch 2 by the bladder 30. Evacuation of the vacuum chamber also clamps pressure plate 28 against the lower surface of parent structure 10. The adhesive between patch 2 and insert 12 and between patch 2 and parent structure 10 is not shown in FIG. 7.

(31) The center rod, magnet, and vacuum bag methods disclosed above are all innovative ways to simplify the repair process, thereby making it more robust and reducing re-repairs.

(32) A single composite repair patch of the type disclosed above could also be used in cases where the site of damage is a depression, not a through-hole. In such cases, an insert would be provided that fits in the depression. In the case where the pressure applicator comprises a threaded rod and a nut, the insert and the patch would both have a center opening for passage of the threaded rod. Alternatively, if the pressure applicator comprises magnets, such center openings would not be needed.

(33) While apparatus and methods for patching a hole in composite structure have been described with reference to various embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the teachings herein. In addition, many modifications may be made to adapt the concepts and reductions to practice disclosed herein to a particular situation. Accordingly, it is intended that the subject matter covered by the claims not be limited to the disclosed embodiments.

(34) The method claims set forth hereinafter should not be construed to require that the steps recited therein be performed in alphabetical order (any alphabetical ordering in the claims is used solely for the purpose of referencing previously recited steps) or in the order in which they are recited. Nor should they be construed to exclude any portions of two or more steps being performed concurrently or alternatingly.