Heat blanket buffer assembly

An apparatus and method is disclosed using a blanket (50) which is an assembly of at least one graphite fiber element of high thermal conductivity and at least one metal sheet element such as a copper sheet element to repair an aircraft structure (10) made of composite fiber materials. The blanket (50) is placed in thermal contact with a silicone rubber heat blanket (22), having an internal heat source, and the aircraft structure, such as composite laminates (12, 14). Use of the blanket (50) increases the rate at which the temperature of the laminates can be elevated to the proper range for repair and transfers heat from hot spots to cold spots to ensure a uniform temperature throughout the laminates.

TECHNICAL FIELD OF THE INVENTION 
This invention relates to the repair of aircraft structures. 
BACKGROUND OF THE INVENTION 
Aircraft structure formed of carbon fiber composite laminates are becoming 
more common, particularly in high performance military aircraft. These 
materials replace the traditional materials, such as aluminum, from which 
aircraft structures have traditionally been made. 
One of the advantages of the use of carbon fiber materials is the ease of 
repair should the aircraft structure become damaged, as by an accident 
such as a tool or the like damaging the surface or even combat related 
damage. The repair can be performed by providing sufficient bonding resins 
and carbon fiber material to the place of repair and bonding the carbon 
fibers with heat to cure the bonding materials. 
In performing such repairs, it is desirable to maintain the temperature of 
the aircraft structure being repaired at a precise, and uniform 
temperature consistent with the curing requirements of the bonding 
material. In the past, it has been common to use a silicone rubber heat 
blanket with heater wires distributed therethrough to heat the aircraft 
structure to a desired temperature. However, because of integral heat 
sinks and nonuniform structure in the aircraft assemblies, it has been 
difficult to achieve a uniform temperature distribution. A need exists for 
an apparatus and method to provide a more uniform heat distribution to 
provide a uniform temperature for the repair. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, an apparatus is 
provided for repairing an aircraft structure which is repairable with the 
use of heat. The apparatus includes a heat source and a blanket of at 
least one graphite fiber element and at least one thin flexible metal 
sheet element, such as a copper sheet element, with the blanket in thermal 
contact with the heat source and the aircraft structure. The blanket of 
graphite fiber and copper sheet elements delivers heat to the aircraft 
structure and ensures a uniform temperature at the aircraft structure for 
repair. In accordance with another aspect of the present invention, a 
silicone rubber heat blanket is provided in thermal contact with the 
blanket of graphite fiber and copper sheet elements. 
In accordance with another aspect of the present invention, a method is 
provided for repairing an aircraft structure which is repairable with the 
use of heat, including the step of placing a blanket of at least one 
graphite fiber element and at least one metal sheet element such as a 
copper sheet element in thermal contact with the aircraft structure to be 
repaired and placing a heat source in thermal contact with the blanket of 
the graphite fiber and copper sheet elements. The method further includes 
a step of heating the aircraft structure with heat from the heat source 
distributed uniformly by the blanket of graphite fiber and copper sheet 
elements to provide a uniform temperature in the aircraft structure for 
repair. A further aspect of the method is the placement of a silicone 
rubber heat blanket in thermal contact with the blanket of the graphite 
fiber and copper sheet elements. The copper sheet element provides a 
thermal mass that, in conjunction with the high conductivity graphite 
fiber, provides uniform temperature in the aircraft structure for repair.

DETAILED DESCRIPTION OF THE INVENTION 
With reference now to the accompanying figures and the following detailed 
description, an apparatus and method for uniformly heating an aircraft 
structure 10 to provide a uniform temperature in this structure for 
repairing the structure is disclosed. 
In FIGS. 1 and 2, the aircraft structure 10 includes a left hand composite 
laminate 12 and a right hand composite laminate 14 separated by an 
aluminum heat sink 16. The laminates 12 and 14 and heat sink 16 rest on a 
composite base panel 18. However, it will be understood that the present 
invention need not use a heat sink 16 and the laminate or laminates being 
repaired can be of any configuration. 
Lying atop the laminates 12 and 14 and heat sink 16 is a graphite fiber 
blanket 20 having highly conductive pitch graphite fibers. Lying atop the 
blanket 20 is a silicone rubber heat blanket 22. Blanket 22 is of the type 
commonly known in the industry that contains heater wires therein through 
which current can pass to provide heat to perform a repair on the 
laminates 12 and 14. 
In the absence of the graphite fiber blanket 20, as the laminates 12 and 14 
are heated by the heat blanket 22, the temperature distribution in the 
laminates and aluminum heat sink will vary considerably with time, as seen 
in FIG. 3. However, by use of the graphite fiber blanket 20, the 
temperature distribution between the aluminum bar heat sink 16 and the 
laminates 12 and 14 and within the laminates is much more uniform, as seen 
in FIG. 4, providing a much more uniform temperature throughout the 
laminates. In addition, the temperature rise of the entire aircraft 
structure is much quicker, providing for a faster and more uniform bonding 
of the repair material to the aircraft structure. The graphite fiber 
blanket transfers heat from hot spots to cold spots and assists in 
maintaining uniform temperature with resulting improved repair patch 
integrity. Of course, after the repair is completed, the blanket 20 and 
silicone rubber heat blanket 22 are removed. 
The graphite fiber blanket 20 utilized is formed of the type of material 
discussed in U.S. Pat. No. 5,316,080, which patent is incorporated herein 
by reference in its entirety, which has a very high thermal conductivity, 
exceeding even that of copper. The thermal conductivity of the fibers can 
be, for example, about three times that of copper. 
One graphite fiber blanket 20 found usable in the process of the present 
invention is manufactured by AMOCO Performance Products, Inc. as 
Thermalgraph.TM. fabric under the trademark Thornel.RTM.. Both the 
Thermalgraph.TM. fabric EWC-300X and the Thermalgraph.TM. fabric EWC-500X 
have been found suitable. The test illustrated in FIG. 4 was with EWC-300X 
fabric. These materials are pitch fiber based high thermal conductivity 
woven fabrics. Due to the orthotopic nature of the weave and the high 
longitudinal thermal conductivity of the fibers, biaxial thermal 
conductivity is achieved. The EWC-300X is a plain weave fabric constructed 
from four-thousand filament continuous pitch tows. EWC-500X is available 
as an eight harness satin weave fabric constructed from two-thousand 
filament continuous pitch tows. The EWC-300X material has a count (warp 
and fill) of 11.times.10 tows per inch and a weight of 599 g per square 
meter. The fabric electrical resistivity (warp and fill) is 0.05 
.OMEGA./sq. The density is 2.1 g per cubic centimeter and the yam 
electrical resistivity is 4.0-5.0 micro ohm meters. The estimated thermal 
conductivity is 200-300 W/m.degree. K. The EWC-500X material has a count 
(warp and fill) of 20.times.20 tows per inch and a weight of 485 g per 
square meter. The thickness is 0.61 mm and the resistivity is 0.03 
.OMEGA./sq. The density is 2.15 g per cubic centimeter and the yarn 
electrical resistivity is 2.3 to 2.8 micro ohm meters. The estimated 
thermal conductivity is 400-500 W/m.degree. K. 
In the preferred embodiment, a three-ply graphite fiber blanket 20 is 
formed composed of three plies of the EWC-300X material stitched together. 
In the assembly illustrated in FIGS. 1 and 2, the silicone rubber heat 
blanket 22 is a rectangle of 12 inches by 18 inches. The aluminum heat 
sink is a bar 5/16 inch thick by 21/2 inches wide by 18 inches long. 
With reference to FIGS. 1-5, a modified blanket 50 will be described which 
incorporates graphite fiber such as used in blanket 20 and metal sheet. 
The blanket 50 is used in the same manner as blanket 20, as illustrated in 
FIGS. 1 and 2 and is a substitute for blanket 20. The blanket 50 can be 
used in the repair of aircraft structures of composites or metals. The 
metal sheet can be copper, silver, platinum or other high thermal density 
materials. 
With reference to FIG. 5, one blanket 50 constructed in accordance with the 
teachings of the present invention is shown with the blanket 50 
constructed with two sheets 52 and 54 of copper 0.016 inches thick (16 
mils), a single-ply sheet 56 of graphite fiber of thickness about 0.010 
inches (10 mils), two sheets 58 and 60 of copper 0.016 inches thick, a 
three-ply sheet 62 of graphite fiber of thickness about 0.030 inches (30 
mils), two sheets 64 and 66 of copper sheet 0.016 inches thick, a 
three-ply sheet 68 of graphite fiber of thickness about 0.030 inches (30 
mils), a single sheet 70 of copper sheet 0.016 inches thick, a three-ply 
sheet 72 of graphite fiber of thickness about 0.030 inches (30 mils), and 
two sheets 74 and 76 of copper sheet 0.016 inches thick. The blanket is 
encased within a Teflon (polytetrafluoroethylene) cover 78 having a 
thickness of 5 mils (0.005 inches). 
The use of a metal in sheet form, such as copper, silver, platinum or other 
high thermal density material, provides the advantage of combining a 
relatively high thermal mass that, in conjunction with the high 
conductivity graphite fiber, provides uniform temperature in the aircraft 
structure for repair. The metal, such as copper, can have various forms. 
For example, it can be sheet (generally considered to be of thickness 
0.010 inches or more), foil (generally considered to be of thickness less 
than 0.010 inches), braid, or mesh. For purposes of the application, 
including the claims, sheet will refer collectively to sheet, foil, braid 
or mesh. The metal, such as copper, not only acts to distribute thermal 
energy, but also as a thermal storage medium as a reservoir of thermal 
energy. 
The metal sheet will preferably have sufficient thickness to add to the 
performance of the blanket 50, but will not be so thick as to be too heavy 
or inflexible as many of the structures to be repaired have a curvature 
and it is desirable for the blanket 50 to conform to the surface of the 
structure being repaired. For copper, it is expected that a range of 
thickness between about 0.008 inches (8 mils) and about 0.020 inches (20 
mils) would be satisfactory. Of course, if the surface of the structure to 
be repaired is flat, flexibility of blanket 50 is not critical and the 
thickness of the metal sheets can be any thickness desired. 
It is preferable to encase the modified blanket 50 within a Teflon 
(polytetrafluoroethylene) cover 78. The Teflon (polytetrafluoroethylene) 
does not bond to the resins used in the repair and is capable of 
withstanding the temperature of repair, typically 350 to 500.degree. F. If 
a Kapton (polyimide resin in the form of a film) cover is used in 
substitution for Teflon (polytetrafluoroethylene) cover 78, repair 
temperatures up to about 700.degree. can be employed. The three-ply 
graphite fiber material is preferably stitched together in the Z 
(thickness) direction. 
Any number of layers of graphite fiber and metal sheet can be used. The 
sheets of the blanket 50, such as sheets 52-78, can be secured together at 
a single center tie point for ease of handling but are preferably not 
otherwise secured together. 
Although a single embodiment of the invention has been illustrated and 
described with numerous specific details in the forgoing description and 
accompanying drawings, it will be understood that the invention is not 
limited to the embodiment disclosed, but is capable of numerous 
rearrangements, modifications and substitutions of parts and elements 
without department from the spirit and scope of the invention.