1. Field of the Invention
This invention generally relates to the field of materials construction and, more specifically, to an apparatus and method for constructing a corrugated composite stiffener.
2. Description of the Prior Art
Composite structures are desirable in many industries for many applications. For example, aircraft, space, and land/sea vehicles employ a variety of curved and multiple-contoured surface structures in their fabrication. Composite materials are commonly used for these structures because, among other desirable attributes, composite materials have high strength-to-weight ratios. Even so, composite structures formed from composite materials still need to be stiffened in some instances. Therefore, manufacturers of composite structures are continually searching for better and more economical ways of stiffening composite structures.
There are various systems and methods of stiffening composite structures. For example, in an aircraft context, hat and blade stiffeners are sometimes utilized. A blade stiffener is a thin element, often T-shaped in cross-section, that is affixed to a structure the upper portion of the T-shape, the leg of the T-shape forming the blade and extending outward away from the structure, the stiffener extending along the length of the structure. The blade increases the moment of inertia of the entire cross-section to increase stiffness. Likewise, a hat stiffener has a three-dimensional cross-section that is typically symmetrical. The hat cross-section usually has a center portion from which two legs depend and terminate in feet. The hat stiffener may be attached to the structure at either the feet or the center portion. A problem with hat and blade stiffeners is that they can have special peel problems at their ends. Furthermore, when utilizing hat and blade stiffeners for strength in perpendicular directions, it becomes very expensive from a manufacturing standpoint because hat and blade stiffeners do not lend themselves well to criss-cross patterns.
Another common stiffening method is the use of honeycomb structures. However, honeycomb structures are usually manufactured as a sandwich structure, which means that they can trap moisture within them, leading to degradation, and are limited in depth because of volume and weight considerations. In addition, manufacturing honeycomb structures can be very expensive.
An additional method of stiffening composite structures is the use of corrugated stiffeners bonded to the structures. However, prior methods of constructing corrugated composite stiffeners are limited to hand layups, which are time consuming and produce stiffeners of uneven quality. Corrugated stiffeners of other materials, e.g., paper, may be created by feeding the material between toothed rollers, the teeth engaging and pressing the desired shape into the material. This method is not useful for resilient materials, such as prepreg composites.
An apparatus and method for fabricating corrugated composite stiffeners provides a base tool having grooves formed in an outer surface and a roller having a rotating shaft and a removable, lobed, elastomeric member wrapped around the shaft. The lobes of the elastomeric member form a radial array around the shaft and match the contours of the grooves in the base tool. A section of prepreg fabric is placed between the roller and the base tool, and the roller is rotated and moved along the tool to sequentially engage the lobes with the grooves. The lobes press the fabric into the grooves with the fabric conforming to the contours of the grooves. Several layers of fabric are applied, then the elastomeric member is unwrapped from the shaft and placed on the fabric on the base tool, the lobes locating in the grooves. The base tool, fabric, and elastomeric member are enclosed in a vacuum bag and heated to cure the fabric.
The novel features believed to be characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings.
As shown in
Referring to
Roller 27 comprises rotating shaft 39 and a surrounding elastomeric member 41. Shaft 39 has a circular cross-section and rotates on its central axis, member 41 rotating with shaft 39. Member 41 has four lobes 43, labeled “A” through “D,” extending from its outer surface, though member 41 may have more or less lobes 43 to preferably match the number of grooves 33 in tool 25. Lobes 43 are shown as having a continuous, sine-wave profile, but lobes 43 may have other shapes to conform to grooves 33 of tool 25. Member 41 is formed as a generally-planar strip and is wrapped around shaft 39, the ends of member 41 aligning and being releasably secured at joint 45. Member 41 may be removed from shaft 39 by releasing the ends at joint 45. When removed and laid flat, as shown in
The central axis of roller 27 translates along path 47, which has a portion inclining downward toward the first groove 33 and a second portion that is straight and parallel with the tops of grooves 33. As shown in
The steps for forming stiffener 15 are shown in
In
Roller 27 is shown in
To begin the second layer of tape 34, roller 27 is moved back to the beginning of path 47. A second layer of tape 34 is dispensed from roll 35 and retained within clamp 37. Roller 27 is moved toward tool 25 until lobe D engages the first groove 33, though roller 27 can be rotated such that any of lobes A, B, C, D engage the first groove 33. Roller 27 translates along path 47 and rotates for lobes D, A, B, and C to engage the first through the fourth grooves, respectively, conforming tape 34 to pattern 29 as described above for the first layer of tape 34. As many layers as necessary can be laid on tool 25 to produce a desired thickness of stiffener 15.
Once the desired number of layers of tape 34 are applied, stiffener 15 is compacted and cured.
The final step for forming stiffener 15 is to enclose tool 25 and member 41 in a vacuum bag 53 for compacting stiffener 15 during curing. The air is removed from bag 53, and the outside air pressure forces member 41 toward tool 25, compacting the layers of stiffener 15. Preferably, the assembly is then placed in an autoclave to provide heat and additional pressure during curing of the resin in stiffener 15. After curing, bag 53 and member 41 are removed, and stiffener 15 is lifted from tool 25. Stiffener 15 is then trimmed to fit the skin that requires stiffening and bonded to the skin, as shown in
The advantages to using the present invention are many. The tools and semi-automated method for producing corrugated stiffeners will reduce the time needed to produce these stiffeners and improve the quality control during manufacture. The elastomeric member may be used in debulking steps between applications of layers, as well as in a final compaction and curing step. The tools are adaptable to a range of patterns of grooves and lobes and can be scaled in size to meet application requirements.
While the invention has been shown in only some of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit thereof. For example, the base tool may have a curvature in its upper surface for producing curved stiffeners. Also, rather than releasing the elastomeric member from the shaft after it has pressed the layers into the grooves, a separate elastomeric strip could be laid flat across the grooves to apply pressure.
Number | Date | Country | |
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Parent | 09998890 | Oct 2001 | US |
Child | 10925454 | Aug 2004 | US |