| This dissertation investigates the feasibility and potential of life extension control logic for reducing fatigue within aerospace vehicle structural components. A key underpinning of this control logic is to exploit nonintuitive, optimal loading conditions which minimize nonlinear crack growth behavior, as predicted by analytical fatigue models with experimentally validated behavior. A major simplification in the development of life extension control logic is the observation and justification that optimal stress loading conditions, as described by overload magnitude ratio and application interval, are primarily independent of crack length and therefore, component age. This weak relationship between optimal stress loading and structural age implies the life extension control logic does not require tight integration with real-time health monitoring systems performing crack state estimation from measurement and model simulation. At a fundamental level, the life extension control logic conducts load alleviation and/or amplification tailoring of external and internal excitations to optimally exploit nonlinear crack retardation phenomenon. The life extension control logic is designed to be a simple, practical modification applied to an existing flight control system. A nonlinear autopilot for the nonlinear F-16 dynamics, coupled with a separate flexible F-16 wing model and a state space crack growth model, are used to demonstrate the life extension control concept. Results indicate that significant structural life savings is obtained by integrating life extending control logic dedicated for critical structural components to the existing flight control system. On the other hand, some components under life extending control showed minor reductions of structural life, particularly when the components are located in a low stress region where fatigue damage is of lower concern. Further, to achieve enhanced long-term structural integrity with life extending control, tradeoffs with flight system stability and performance may be required. Careful consideration is thus necessary when applying life extending logic to the aircraft flight control system. Although life extending control appears feasible with significant potential, full implementation of the concept requires further study. |
