Fully-Stressed Design of Airframe Redundant Structures.

摘要

This paper presents a description of two automated, fully stressed design procedures for airframe redundant structures along with wing box beam examples. One of the methods is a variation of the other; both are based upon the displacement method and are essentially digital computer adaptations of a traditional optimization technique. Using the simpler, 'average stress' method, one analyzes a structure that has been idealized as a system of discrete elements and then resizes the members of the structure based upon the average stresses occurring in each member, together with some failure criteria. It has been found that in applying this method to aircraft wings, the final structure resulting from many iterations sometimes exhibits discontinuities in material distributions that are sufficiently severe to be unacceptable to stress analysts for use in design. This effect is evident in the accompanying examples. Even in such cases, however, useful results for rough work (e.g., preliminary design) may be obtained by cutting off the iteration procedure after approximately three cycles. The alternative approach is based upon "nodal stresses" in the manner of the force method rather than average element stresses, and it converges to reasonably smooth material distributions. It has the disadvantage, however, of requiring additional programming for its implementation. Consequently, thus far it has been applied only to wing-type structures. Both procedures have been in use recently at Grumman on various preliminary designs.