Reliability-based design optimization of airframe components

Abstract

A stochastic design optimization (SDO) methodology has been developed to design components of an airframe structure. The design is obtained as a function of the risk or reliability. Uncertainties in load, strength, and material properties are treated as distribution functions. Design constraints and optimum weight become functions of reliability. Weight versus reliability traces out an inverted S-shaped graph. The centre of the graph corresponds to 50 per cent probability of success. A heavy design with weight approaching infinity could be produced for a near-zero rate of failure. Weight can be reduced to a small value for the most failure-prone design. Reliability can be changed for different components of an airframe structure. The SDO capability is obtained by combining three codes. MSC/Nastran is the deterministic analysis tool, the fast probability integration of the NESSUS (numerical evaluation of stochastic structures under stress) software is the probabilistic calculator, and NASA (National Aeronautics and Space Administration) Glenn Research Center's testbed CometBoards is the optimizer. The SDO capability requires a finite-element model, a material model, a load model, and a design model. The stochastic optimization concept is illustrated considering an academic example and a real-life raked wingtip structure of the Boeing 767—400 extended range airliner made of metallic and composite materials.