Design Optimization of Airframe Structure using a Material Modeling Approach

The aerospace industry is increasingly using composites for structural applications. The 787 aircraft, for example, makes extensive utilization of composite materials for the primary structure. This ever-increasing consumption of composite materials in aerospace has caused a corresponding increase in physical testing to ensure parts have sufficient strength. However, systematic physical testing can be very expensive and contribute significantly to the overall cost of a new airplane program.

The ability to predict strength of composite parts and structures provides a significant competitive advantage. Not only can expensive physical tests be minimized, but the vast design space of potential composite configurations can be explored far beyond what is capable from physical testing alone.

To enable this predictive capability, simulation methods have been developed at Boeing that employ a material modeling approach. In this approach, material properties that can be obtained from simple coupon tests are generalized for 3-D application using the principles of continuum mechanics. Algorithms for failure initiation and propagation are encapsulated into ‘material subroutines’ that can be interfaced with commercial finite element analysis software such as ABAQUS. Stress analysts can then utilize the modeling capabilities of ABAQUS, together with customized material subroutines, to predict the strength of any type of airframe structure in a highly generalized manner.

The presentation will include details of these methods as well as a brief overview of past successful applications to composite coupon strength prediction. Building up on this capability, these methods will be illustrated at the assembly level for a single-shear joint. Design variations will demonstrate the transfer of failure from composite to metal as the design parameters are varied. Additionally, a skin-stringer assembly from a commercial airplane will be modeled and the predicted strength will be compared with that from physical testing, further demonstrating realistic airframe structural applications of this material modeling approach.