M. A. James, R. Schultz, R. Bucci, M. Heinimann, M. Kulak, Alcoa, Inc., Alcoa Center, PA
Virtually all future air-vehicles have challenging design goals to reduce weight and cost of structure. Both unitized structure and thick shaped parts offers opportunity for simplified assemblies and reduce parts count, which in turn leads to further cost-saving potential via buy-to-fly cost reductions that derive from advanced joining methods and innovative use of material form. However, design for lower weight generally translates to higher operating stresses, and for unitized structure as well as for thick and shaped parts this means damage tolerance becomes a key design driver. This presentation describes results from several programs investigating the importance of residual stress in unitized structure and thick shaped parts. In one example we investigate integral-stiffened wing and fuselage cover applications for large transport-type aircraft. Several of the concept variants employed integral stiffened extrusions that were friction stir weld joined to form an ultra wide advanced concept panel. These latter tests were particularly enlightening in two important regards: 1) the tests demonstrated advance concept weight saving potential in excess of 20% over current state-of-the-art structure sized by damage tolerance, and 2) the proper accounting of residual stress effects in both test interpretation and predictive modeling is essential to understanding and capturing full benefit of the advanced design approaches. In another example we describe the modeled and measured residual stress state of large die forgings. We show that Alcoa’s new signature cold work forging process significantly reduces residual stress.
