Current structural design and analytical procedures used by the aeronautics industry are largely semiempirical, even though significant improvements have occurred in structural analysis methodology over the last two decades. Preliminary design efforts require estimates using relatively quick, easy to use, and insightful tools to allow sensitivity studies among different structural design options. Final design efforts require tools that provide a precise and accurate assessment of the structural design.

Finite element analysis methods are routinely used for predicting the stress, strain, and displacement fields in complex structural geometries. Superior graphical interfaces have significantly improved pre-and post-processing of data files. Automated mesh generation, mesh refinement, and automated adaptive remeshing have resulted in major improvements in the efficiencies of model development and analysis and in the accuracies of the numerical solutions. Post-processing algorithms and graphical interfaces have significantly enhanced the ability of the analyst to interpret the results of the stress analysis. Along with these improved analytical and software tools, advances in the available computing capabilities have been rapid. In spite of these advances, the reliable prediction of structural failure modes, ultimate strength, residual strength, and fatigue life has remained elusive to the structural engineer. Standard practice still relies heavily on extensive testing at the subelement, element, subcomponent, component, and full-scale levels. Design details are frequently optimized through test programs. Scale-up effects are handled through a building-block approach that relies on testing to verify the anticipated structural performance at each scale level. Full-scale static and fatigue tests are conducted to identify "hot spots," to verify adequate structural integrity for design limit and ultimate loads, and to verify durability and damage tolerance requirements. Hence the designation "semiempirical" for current practice, which is necessarily expensive and time-consuming.

In spite of over 60 years of experience by researchers and industry in designing metallic structure and 25 years of experience designing composite structure, the lack of rigorous analytical methods to predict residual strength and fatigue life can inhibit the cost-effective introduction of advanced materials with superior specific strength and stiffness relative to current conventional materials. Continuing efforts are needed to develop—and insert into standard engineering practice— advanced mechanics-based analytical prediction methodologies that would allow innovative designs to be evaluated and optimized at acceptable cost. Such methods would also provide the means to assess the effects of service history on the durability and damage tolerance performance of the structure. This chapter examines current issues and assess the impact of advanced methods on the introduction of new materials and the design of more-structurally efficient and cost-effective primary structures on next-generation aircraft.

Metallic materials tend to fail due to the formation and growth of a dominant microcrack that eventually reaches a critical length and then more rapidly propagates to failure. However, the recently recognized phenomenon of multiple-site damage has been shown to be a critical issue in aging of commercial transport aircraft. While fracture mechanics is now a mature part of standard practice in engineering, rigorous prediction methodology only exists for brittle materials that exhibit limited plasticity. The fatigue crack growth behavior and fracture processes exhibited by ductile materials are reasonably well understood. However, the development of rigorous elastic-plastic analytical methods has been hampered by the complicated three-dimensional effects present in most structures. In thin-sheet planar structural components, local constraint effects on the development of plastically deformed material frequently result in highly inaccurate solutions when obtained from two-dimensional plane-stress or plane-strain assumptions. Three-dimensional analyses of cracks in geometries such as lugs and fittings may be inaccurate due to uncertainties in modeling the crack-front singularity.

Three-dimensional models of the geometry and crack configurations are computationally intensive and have been impractical for the practicing engineer. Therefore, improved

Front Matter (R1-R14)

Executive Summary (1-4)

1 Requirements and Drivers (5-10)

2 Structural Concepts (11-25)

3 Metallic Materials and Processes (26-35)

4 Polymetric Composite Materials and Processes (36-44)

5 Environmentally Compliant Materials and Processes (45-48)

6 Methodologies for Assessment of Structural Performance (49-56)

7 Aircraft Maintenance and Repair (57-66)

8 Nondestructive Evaluation (67-70)

9 Committee Findings (71-76)

References (77-82)

Appendix: Biographical Sketches of Committee Members (83-84)