INTEGRITY OF AIRCRAFT STRUCTURAL ELEMENTS WITH MULTISITE FATIGUE DAMAGE

Integrity evaluation of aging structures is extremely important to ens ure economic and safe operation of the flight vehicle. A two step anal ytical approach has been developed to estimate the residual strength o f pressurized fuselage stiffened shell panels with multi-bay fatigue c racking. A Global Finite Element Analysis is first carried out to obta in the load flow pattern through the damaged panel. This is followed b y the Schwartz-Neumann Alternating Method for local analysis to obtain crack tip stresses and the relevant crack tip parameters that govern the onset of fracture. Static residual strength is evaluated using fra cture mechanics based, as well as net section yield based, criteria. T he presence of holes, with or without Multi-Site Damage (MSD), ahead o f a dominant crack is found to significantly degrade the capacity of t he fuselage structure to sustain static internal pressure. An Elastic- Plastic Alternating Method has been developed and applied, to evaluate the residual strength of flat panels with multiple cracks. The comput ational methodologies presented herein are marked improvements to the present state-of-the-art, and are extremely efficient, both from engin eering man-power, as well as computational costs, points of view.