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AIAA

Computer Aided Design Report
Vol. 11, No. 2, February 1991

SKIN AND RIB STRUCTURES

Thin shell structures with integral ribs or stiffeners carry loads more efficiently than most other types. For this reason, such structures have been used frequently in aircraft engines, rockets, and similar high-performance vehicles. However, because of their efficient load-carrying properties, integrally stiffened structures may find future application in automobiles, ships, and other vehicles where excess weight penalizes performance and operating cost.

One reason reinforced shell structures have not caught on is that they're not easy to design. Over the years, designers of reinforced plate-and-shell structures, such as ships and storage tanks, have developed various cookbook rules for the placement of welded stiffeners. Such rules work well enough when stiffeners are laid out in a rectangular grids and large factors of safety are employed. However, in aircraft structures, where factors of safety are smaller and where integral stiffeners may be laid out in triangular patterns, more thorough analysis is needed.

Performing detailed stress analysis of integrally reinforced shells with conventional finite element modelers is a tough job. Simply creating the geometry for these complex structures is very time consuming using interactive methods. In like fashion it takes much time to draw these structures using two-D or three-D CAD tools. Making changes, such as adding a hole for a pipe or duct, takes hours (if not days) of work with conventional CAD.

PS Associates (PS.A), a consulting engineering firm, has developed a highly integrated collection of programs for designing integrally plate and shell structures. The system, called "Isogrid-SST Services," starts with the external loads on the structure, The geometric constraints, and other design criteria, such as whether the structure can yield under the given loading conditions or what stresses are allowed by high-cycle fatigue criteria. Then, with some guidance from the engineer, Isogrid-SST Services automatically lays out the reinforcing grid of the structure and optimizes it for stiffener size and spacing. Once the grid is defined, the system automatically designs the reinforcing needed around the penetrations and provides a detailed stress report.

The wire-frame geometry produced by Isogrid-SST Services can be rendered in Hewlett-Packard graphics language (HPGL) for plotting or for use in technical reports. (Most word processors and technical publishing systems accept HPGL.) Isogrid-SST Services geometry also can be translated to Computervision, Unigraphics, and AutoCAD. An interface has been developed for the Intelligent Modeler solid model system (a program not yet marketed commercially) and other solid interfaces are possible.

Isogrid-SST Services also carries design data through to manufacturing activities. In aerospace structures, integrally reinforced shells are typically made by milling or chemically etching pockets from a solid sheet of material, leaving the reinforced structure in place. Isogrid-SST Services can select appropriate stiffener cross sections to suit the planned manufacturing method.

With current manufacturing practices, the reinforcing grids for cylindrical structures are generally milled from flat stock. Then the stock is bent, welded, and annealed to relieve residual stresses. The Isogrid-SST software generates flat patterns for the structure automatically. The software also automates NC tool path generation when used in conjunction with some leading CAD/CAM systems. PS.A has written GRIP programs which feed Unigraphics the appropriate mix of geometric information and NC programming commands to automatically cut all the pockets of a structure. The company developed a similar capability for Computervision's CADDS system using "Execute" files.

The payoff from this integrated set of applications is very large. Using Isogrid-SST Services an engineer can design a reinforced shell in a few days. With conventional CAD/CAM tools, such a project may require more than twenty times the labor and schedule time. Isogrid-SST works so efficiently because it solves a limited set of problems in a highly automated fashion. However, the scope of the problem is limited too. For example, Isogrid-SST can currently handle flat-sided, conic and cylindrical structures. The company is currently improving the software to handle any ruled surface bounded by bi-cubic curves, such as elliptically shaped ducts.

PS.A prefers to use the software to perform design services rather than to license it for use by others. Paul Slysh, the company's president, says fees for service range from "a few thousand to a few hundred thousand dollars" depending on the scope of the work required. The higher range applies to projects for which new software must be developed. If clients are willing to work with basic geometry which the program can already handle, fees will be at the lower end of the range. PS.A limits licensing of its software to customers who have paid for consulting services and want to use the software to maintain or modify existing structures. If your firm is currently struggling with reinforced shell structures, or planning to use them in future products, Isogrid-SST Services may be a big help.