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It is widely recognized that the accuracy of notch fatigue calculations can be improved significantly when those calculations are based on the elastic‐plastic response strain at the notch root, as opposed to the remotely applied loads or stresses. Two of the most widely used approximations for this response are Neuber's rule and Glinka's equivalent strain energy density method. In the present work, a survey of some of the many published evaluations of these methods was first conducted, and then, additional detailed comparisons with elastic‐plastic finite element analyses for a series of semicircular and V‐shaped notch configurations were performed. Based on the observed limitations of both the Neuber and Glinka approaches, and with the guidance of the elastic‐plastic finite element results, a new (and more robust) approach for the estimation of notch response strains is proposed. This approach calls for the definition of a generalized notch response curve (GNRC), which is dependent on both the material stress–strain curve and the notch geometry. Once defined, the GNRC allows the determination of the response strain for any applied stress.

The fully effective utilization of large aluminum forgings in aerospace structures has been hampered in the past by inadequate understanding of, and sometimes inaccurate representation of, bulk residual stresses and their impact on both design mechanical properties and structural performance. In recent years, significant advances in both computational and experimental methods have led to vastly improved characterization of residual stresses. As a result, new design approaches which require the extraction of residual stress effects from material property data and the formal inclusion of residual stresses in the design analysis, have been enabled. In particular, the impact of residual stresses on durability and damage tolerance can now be assessed, and more importantly, accounted for at the beginning of the design cycle.

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.

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