Advanced Analysis Methods and Nondestructive Inspection Technology Under Development in the NASA Airframe Structural Integrity Program

An advanced analytical methodology has been developed for predicting the residual strength of stiffened thin-sheet riveted shell structures such as those used for the fuselage of a commercial transport aircraft. The crack-tip opening angle elastic-plastic fracture criterion has been coupled to a geometric and material nonlinear finite element shell code for analyzing complex structural behavior. An automated adaptive mesh refinement capability together with global-local analysis methods have been developed to predict the behavior of fuselage structure with long cracks. This methodology is currently being experimentally verified.

Advanced nondestructive inspection technology has been developed that will provide airline operators with the capability to conduct reliable and economical broad-area inspections of aircraft structures. Standard methods based on ultrasonics, thermal diffusivity, radiography, coherent optics, and electromagnetics have been modified for airframe geometries and aircraft inspection environments to detect disbonds at tear straps and lap splices, fatigue cracks at rivets, and corrosion throughout the airframe structure. Advanced signal processing methods have been combined with computational models to provide a quantitative engineering record of the damage in the structure. These methods are currently being demonstrated in the field. Laboratory methods are also being developed to characterize defects in materials and strain and displacement fields in structures.