Volume 5, Issue 4 (April 2008)
The Influence of Residual Stress on the Design of Aircraft Primary Structure
Aggressive performance and weight objectives are driving aircraft manufacturers toward the use of advanced materials and structural concepts that may have inherent, process induced residual stresses in localized, but critical areas. Certification of these structures will require that the influence of these residual stresses be properly accounted for during design. One example of this circumstance is the unitization of lugs and fittings with primary spars and bulkheads. This is being done in order to reduce part count, which, in turn, reduces the necessity for large numbers of fasteners and the associated hole preparation/mating requirements. Such unitization can be achieved through the use of large forgings, which experience has shown may have significant residual stresses in localized areas, even after final machining. For man-rated flight vehicles, primary structural elements are typically designed based on damage tolerance concepts. This requires that fatigue crack growth analysis and testing be used for certification of the structure. Thus, for advanced design concepts based on unitized structure, the influence of residual stress on fatigue crack growth must be addressed. A substantial body of work has been developed over the past three decades by numerous researchers in the field of fracture mechanics with regard to residual stress. In what has become the standard approach to the problem, the residual stress field is used to estimate a residual stress intensity factor (SIF) using weight function or Green’s function techniques. The residual SIF is superimposed with the applied SIF due to service loading and the total is then used in an otherwise unmodified, LEFM-based fatigue crack growth analysis. In this paper, we describe current research directed toward the formal inclusion of residual stress effects in the design of aircraft primary structure. This effort has three focus areas. The first is the extraction of confounding residual stress effects during the characterization of the fundamental fatigue crack growth rate behavior of a critical aluminum alloy. The second is the quantification, both by analysis and experiment, of the location, spatial magnitude, and stress magnitude of the residual stress fields in a candidate forged/machined part. The third is the development of improved fatigue crack growth analysis methods that selectively account for the presence of residual stresses. Each of the three focus areas provides a critical ingredient to a proposed design analysis method in which components are analyzed using intrinsic (residual stress free) material data, with residual stresses then explicitly introduced only in those areas where they are known to exist. The discussion includes the results of a trade study on a wing spar showing potential optimization, both in terms of weight savings in over-designed areas, and service life/damage tolerance enhancement in under-designed areas.