T. J. Tzong, K. K. Chan, The Boeing Company, Huntington Beach, CA; K. W. Liu, M. Alam, The Boeing Company, Long Beach, CA

Summary:

The aerospace industry continuously investigates and develops efficient designs in order to build cost-effective quality products. Variations in built-up construction of sheet metal parts and lower level assemblies frequently result in fit and form difficulties in major/final assembly of aircraft structures. This multi-part design often prevents the opportunity of applying advanced assembly methods such as Determinant Assembly (DA) that simplifies assemble procedures by eliminating the requirements for complex hard tooling and part locator aides. If part variations and tolerances can be controlled at lower-level assemblies and fabrications, DA on large structures can be achieved. As a result, the manufacturing investments in tooling, processes and floor space can be greatly reduced, and the associated assembly cycle time and costs lowered, which result in significant savings for the final product. For the past few years, Boeing engineers have conducted many laboratory tests and analyses to validate monolithic (unitized) structure designs. To maintain structural integrity, these designs need to satisfy static strength, stiffness, and durability and damage tolerance requirements. The goal of monolithic structure applications is to reduce part count, and lower assembly costs with higher quality products.

Many fuselage frames of large transport aircraft are built-up constructions made of individual parts such as upper and lower caps, vertical stiffeners, shear webs and clips. The assembly of these types of frames requires complicated tooling to position parts and complex assembly methods to drill, clean, seal and fasten the parts. With the current design and manufacturing method, maintaining fit and form between frames and other large structural components is a challenge. In addition, for the optimal operational efficiency, the transport fuselage frame webs are generally designed to allow for elastic buckling in a nominal flight environment. For monolithic structures, potential fatigue damage as a result of repeated buckling, metal grain exposures, and machining radius becomes a concern. It is necessary to have a monolithic design that meets the durability and damage tolerance requirements in a repeated postbuckling environment and satisfies the stiffness and weight constraints. To meet these design challenges, an extensive study of frames was conducted by varying web thickness and sizing and locations of stiffeners. Fatigue allowables were obtained through testing of similar structures with good correlations with analysis results. Nonlinear structural analyses were performed to identify fatigue critical areas and obtain buckling stresses. Taking the weight impact into consideration, an optimal monolithic frame design can be defined. This paper will present engineering and assembly issues associated with the frames, the unitized design study approach, fatigue testing substantiating the design, and applications.