Static structures (e.g. – buildings) – Composite prefabricated panel including adjunctive means – Sandwich or hollow with sheet-like facing members
Reexamination Certificate
1999-12-08
2001-09-11
Stephan, Beth A. (Department: 3635)
Static structures (e.g., buildings)
Composite prefabricated panel including adjunctive means
Sandwich or hollow with sheet-like facing members
C052S793110, C052S799110, C052S799140, C052S799130, C052S799120
Reexamination Certificate
active
06286289
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
BACKGROUND
Field of the Invention
This invention relates to fabricated isogrid structures, specifically to an improved design and method of manufacturing isogrid structures.
BACKGROUND
Description of the Prior Art
The concept of an isogrid structure originated from work sponsored by the U.S. military in the early 1970's. The focus of the work was to develop light weight structures with isotropic mechanical properties for aerospace and military hardware applications. This work resulted in the current concept for design and manufacture of isogrid structures. The current concept used in design and manufacture of isogrid structures is as follows.
An isogrid panel is a plate called a face sheet with a triangular array of integral stiffening ribs called stringers. The stringers have a stiffener called a flange on the side opposite the face sheet. In cross section the stringer has the appearance of an I Beam. The stringer forms the I beam web. The flanges are formed by the face sheet and the stiffener flange. The face sheet is continuous between all the stringers.
For a an isogrid structure to have isotropic mechanical properties the stringers must be arranged in equilateral triangles. However, other stringer structures are often used such as squares and rectangles. These other structures can perform quite well when design requirements do not need true isotropic mechanical properties.
Isogrid structures have been previously manufactured by two methods. The first method is by machining the structure from a thick plate of material. For aerospace applications this is done by taking a large plate of material and milling the isogrid panel from the plate using high speed milling machines that have been developed specifically for this purpose. The quality of product is excellent using this method but it has two major disadvantages. First, the material yield is very low. For expensive materials this greatly adds to the cost of the part. Second, milling isogrid panels is expensive due to the cost of custom built milling machines and the long machine time required to remove the bulk of material from the plate. Both of these disadvantages result in expensive parts fabricated by this method. This has limited the use of this method of manufacturing.
The second method of manufacturing isogrid structures is by casting. If the flange on the stringer is removed and draft angle is added to the sides of the stringer to allow mold extraction, then an isogrid structure can be cast using a permanent mold casting method. This design for an isogrid reduces the stiffness and the isotropic mechanical properties. However, for some applications this is acceptable. The use of casting to produce isogrid structures can result in a low cost mass produced product. It does have disadvantages. The first is that product design changes require the manufacture of a new mold. This is an expensive and lengthy process. It reduces the ability for this manufacturing process to be used for one of a kind and small quantity product. A second disadvantage of the casting process for manufacture of isogrid structures is the loss of mechanical properties due to the removal of the stinger flange.
The use of isogrid structures manufactured by the above processes are found in patent records. These are U.S. Pat. No. 4,040,333 ISOGRID SHELL GUN MOUNT filed Oct. 18, 1976; U.S. Pat. No. 5,485,723 VARIABLE THICKNESS ISOGRID CASE filed Apr. 29, 1994: and U.S. Pat. No. 5,787,654 ISOGRID TILE filed Sep. 21, 1995. These patents show the usefulness of light weight high strength isogrid structures. The disadvantages of the current design and fabrication methods has limited their application. These disadvantages are itemized below.
(a) The material yield for machined isogrid structures is low. For expensive materials this greatly increases the cost of the finished product.
(b) Machining of isogrid structures is an inherently expensive process.
(c) Cast isogrid structures do not attain isotropic mechanical properties.
(d) A cast isogrid structure requires a new mold each time the product design is changed. Mold making is an expensive process. This limits the use of this method to medium and high production quantities.
(e) Both methods of manufacture cannot achieve very thin sections in the face sheet, the ribs, and the flanges. This puts a lower limit on the weight of the structure that can be designed using these methods.
SUMMARY
In accordance with the present invention an isogrid structure is comprised of multiple face sheets with multiple stringers between the face sheets. Tabs on the edges of the stringers fit through mating slots in the face sheets. The tabs on the stringers are joined to the face sheets to form a ridged isogrid structure.
OBJECT AND ADVANTAGES
Accordingly, several objects and advantages of the present invention are:
a) to provide an isogrid structure that is very light weight and strong with true isotropic mechanical properties;
b) to provide an isogrid structure with reduced weight by using material cut outs in the face sheets and stringers;
c) to provide an isogrid structure that is manufactured from components that are cut out of flat sheet greatly reducing material costs and waste;
d) to provide an isogrid structure that can be inexpensive and quickly manufactured in quantities as low as one to mass production quantities;
e) to provide an isogrid structure with face sheets and stringers that can have a thinner wall thickness than can be achieved with existing methods of manufacture;
f) to provide an isogrid structure design that lends itself to design and fabrication using computer aided design (CAD) and computer aided manufacture (CAM):
g) to provide an isogrid structure design that can use high speed inexpensive cutting methods such as computer controlled water jet or plasma arc cutting instead of high speed milling; and
h) to provide an isogrid structure design that can use computer controlled joining processes such as computer controlled welding or gluing to attach the stringers to the face sheets.
Further objects and advantages will become apparent from a consideration of the ensuing description and drawings.
REFERENCES:
patent: 1576559 (1926-03-01), Swift
patent: 2193994 (1940-03-01), Weaver
patent: 3940891 (1976-03-01), Slysh
patent: 4040333 (1977-08-01), Slysh
patent: 4055924 (1977-11-01), Beaver, Jr.
patent: 4173103 (1979-11-01), Sargis
patent: 5471367 (1995-11-01), Krumweide et al.
patent: 5485723 (1996-01-01), McCoy et al.
patent: 5487930 (1996-01-01), Lockshaw et al.
patent: 5787654 (1998-08-01), Drost
Dvornak Matthew J
Hisel Stanley D
Powell John W.
Dorsey Dennis L.
Stephan Beth A.
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