Joints and connections – Interfitted members – Peripheral enlargement – depression – or slot on one member is...
Reexamination Certificate
1998-09-22
2001-08-21
Kim, Harry C. (Department: 3629)
Joints and connections
Interfitted members
Peripheral enlargement, depression, or slot on one member is...
C403S267000, C244S119000, C244S131000
Reexamination Certificate
active
06276866
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to joint structures for composite materials, and, more particularly, to a joint that is resistant to shear and peel failures and whose strength does not deteriorate over time.
In the manufacture of structures made of composite materials, large parts may often be fabricated as integral subassemblies, avoiding the need for as many mechanical joints as are usually found in metallic structures. However, it is still necessary to join the subassemblies to each other with mechanical joints. Experience has shown that adhesively bonded mechanical joints are usually preferred to joints using fasteners, because the adhesive spreads the structural loadings over a relatively large area to reduce stress concentrations in the joint. However, adhesives have the disadvantage that they may degrade by embrittlement or other mechanisms over a period of time, raising questions about the long-term integrity of the joint. Also, a bonded joint, if failed at the edge, will peel, which is a progressive failure across the joint, during subsequent repeated application of a load that is significantly lower than the normal failure load.
In one type of joint that may be discussed as exemplary, a base leg butts against and is joined to a cross piece, defining a T-shaped joint. This type of joint has many applications, such as, for example, the joining of a pressure bulkhead to an annular skin shell structure. In one application, an aircraft built of composite materials has a fuselage in the form of a thin annular shell or skin. Within the fuselage, an internal bulkhead, also made of composite materials, separates the pressurized passenger compartment from the unpressurized environment, from a fuel tank, or from some other compartment. The pressure bulkhead must be able to sustain, with a safety factor, a pressure differential of as much as about 10 pounds per square inch.
The bulkhead is joined to the annular shell structure around the periphery of the bulkhead, so as to provide both structural support and a pressure seal. This joint is in the form of a T-joint, when viewed in circumferential section.
Because the pressurization load is critical to safely (joint failure is catastrophic) and because this load is continuous for most of the duration of any flight, this joint is extremely important. It is not generally acceptable to use a bonded-only thin joint for the bulkhead/fuselage joint. Instead, the bonded joint is typically made oversize or backed up with additional structure. In one approach, for example, a backup using a large number of fasteners is employed, adding substantial weight to the aircraft.
In another approach, the bulkhead is joined to the inside wall of the annular shell using a flange on the circumference of the bulkhead or a series of tapes. In the flange technique, the outer circumference of the bulkhead is terminated in a flange whose outer surface is adhesively joined to the inner surface of the annular shell. The flange is made sufficiently wide so that the available adhesive bonding area will carry the loads of the bulkhead. The flange approach is operable, but it adds excessive weight to the structure and also is subject to deterioration if the strength of the adhesive degrades over time.
In the tape approach, tapes are formed of a number of lapped plies of composite material. The tapes are corner structural elements at the interface of the bulkhead and the skin, either bonded or laid up in place. Each tape is attached on one side to one face of the bulkhead, bent in the middle section of the tape, and attached on the other side to the inside of the annular shell. The attachment of the tape to the bulkhead and the tape to the inside of the annular shell is performed with adhesives. Tapes are placed on each side of the bulkhead. The joint is sealed with extra adhesive material, if necessary. When the bulkhead is loaded in service, it deforms in bending responsive to the pressure differential. The bending tends to elongate the tapes on the higher-pressure side of the bulkhead, and places the adhesive attachments of the tapes into shear loading. The predominant failure mode is an adhesive peel failure at the tape attachments. Because the aircraft must be built for extended-term service and because the shear properties of the adhesive material may deteriorate over time, the conventional practice is to overbuild the joint with extra tapes and extra adhesive attachment area.
While these approaches to the joint structure are operable, they cannot be certified for manned pressurized aircraft, because there remains the concern that some combination of circumstances may lead to unexpectedly accelerated deterioration of the adhesive and failure of the joint between the bulkhead and the annular skin shell. There is therefore a need for an improved joint design for composite materials to join a bulkhead to an annular shell. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a joint structure useful in joining two pieces of composite material together. Very little weight is added to the structure by the presence of the joint. An adhesive may be, and preferably is, used at the interface, but the adhesive is present primarily as a sealant. The loads at the joint are carried largely by mechanical interlocking, so that little if any load is borne by the adhesive. Consequently, deterioration of the strength of the adhesive over time is of much less concern than in conventional joints. The joint structure is useful in joining bulkheads to annular shells, as in aircraft and in tankage, as well as other applications.
In accordance with the invention, a structure comprises an exterior piece having an exterior piece inner surface comprising an exterior piece inner surface first ridge, an exterior piece inner surface second ridge, and an exterior piece inner surface valley between the exterior piece inner surface first ridge and the exterior piece inner surface second ridge. The exterior piece further includes an exterior piece outer surface oppositely disposed from the exterior piece inner surface. The structure also comprises an interior piece having an interior piece outer surface in facing contact to the exterior piece inner surface. The interior piece outer surface comprises an interior piece outer surface first valley, an interior piece outer surface second valley, and an interior piece outer surface ridge between the interior piece outer surface first valley and the interior piece outer surface second valley. The interior piece outer surface first valley is in engaging contact with the exterior piece inner surface first ridge, the interior piece outer surface second valley is in engaging contact with the exterior piece inner surface second ridge, and the interior piece outer surface ridge is in engaging contact with the exterior piece inner surface valley. A first tensioned element contacts the exterior piece outer surface at a location oppositely disposed from the exterior piece inner surface first ridge.
This approach may be applied in joining a bulkhead to an annular shell structure. Such a structure comprises a bulkhead having an outwardly facing outer periphery and an outwardly facing bulkhead ridge on the outer periphery of the bulkhead. There is an annular shell having a shell inwardly facing surface and a shell outwardly facing surface, a first inwardly facing shell ridge formed into the annular shell and extending circumferentially around the annular shell, and a second inwardly facing shell ridge formed into the annular shell and extending circumferentially around the annular shell. The second inwardly facing shell ridge is spaced apart from the first inwardly facing shell ridge to define a shell valley therebetween. The shell valley receives the bulkhead ridge therein. A first tensioned band contacts the shell outwardly facing surface and extends circumferentially around the shell outwardly facing surface at a location overlying the first inwardly faci
Garmong Gregory
Kim Harry C.
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