K. Chan, The Boeing Company, Huntington Beach, CA; J. Harter, USAF AFRL, Wright-Patterson AFB, OH

Summary:

The use of integral structures on aircraft has grown rapidly in recent years. This is due, in part, to manufacturing technology advancements such as high-speed machining, chem-milling, friction stir welding, superplastic forming, and large castings. The benefits of integral structures include reducing manufacturing cycle time and costs, saving component fabrication and assembly costs by 50%, offering new design flexibility and weight saving opportunities, improving ergonomics and reducing worker’s injuries, increasing Determinant/Precision Assembly (DA) opportunities, improving fits between parts, simplifying assembly processes and reducing reworks, and enhancing reverse engineering processes for part repairs and replacements.

In order to realize the benefits of integral structures, several technical challenges need to be met which include: (1) developing design concepts that meet airworthiness requirements, provide safety, and save weight, (2) producing structures with long periods of crack free life, slow crack growth rate with large size damage containment for inspectability and repairability, and (3) validating design tools and analysis methods to efficiently qualify integral structure designs.

New methods of determining and demonstrating damage tolerance, residual strength and inspection intervals based on integral structure designs and characteristics are therefore needed. The External K-Solver Project, sponsored under the USAF AFRL/VA, has developed a method to analyze crack growth in parametrically defined integral structure. Using the USAF computer code AFGROW as the life analysis platform, the External K-solver includes StressCheck (ESRD, Inc.) finite-element modeling capability, and Microsoft Component Object Model (COM) technology. COM technology allows AFGROW and StressCheck to communicate in real-time so that finite-element meshes can be built ‘on the fly’, based on key features such as stiffeners and crack fronts. Auto-meshing capabilities, especially when combined with p-version finite elements, can greatly reduce engineering labor costs.

The External K-Solver allows users to obtain accurate stress intensity solutions as cracks propagate through complex structures. The benefits of the program to the analyst community include improved life assessments and an ability to evolve design and service life requirements for new or existing integral structure designs.