FATIGUE LIFE RELIABILITY BASED ON MEASURED USAGE, FLIGHT LOADS AND FATIGUE STRENGTH VARIATIONS

The Navy has developed a structural usage monitoring system based on the regime recognition concept for fatigue tracking of individual dynamic components with the objectives of maximizing the safety, reliability, and readiness of the fleet in an environment of limited defense resources. The first Structural Data Recording Set (SDRS) was installed in the AH-1W fleet in February 1993. Fifty aircraft have been equipped with SDRS thus far, and a total of 3,400 flight hours of usage have been acquired. The usage received from individual aircraft shows significant variations, with damaging regime usage being lower than the non-damaging. The measured usage from individual aircraft is presently combined with severe component damage rates to compute fatigue damage accrued by each component in the fleet. Because of the inherent conservatism in component fatigue life due to the severe usage spectrum, it is being replaced by variable measured usage, and its impact on dynamic component reliability needs to be assessed. To achieve these objectives, the variations in usage of the individual regimes (such as level flight, hover, turns, gunnery turns, pullup, climb, dive, etc.) that constitute the spectrum were studied. The usage variations and the variability in alternating component loads associated with these regimes were modeled using a Weibull distribution. The fatigue test data of over 264 aluminum components from U.S. Navy helicopters were utilized to represent the variations in component fatigue strength. The usage, flight loads, and fatigue strength variations were combined according to Palmgren-Miner's cumulative damage theory to compute component fatigue life. The fatigue life computations were performed with a computer program developed for this purpose, which randomly combined the variations in usage, loads, and fatigue strength in increments. The computed component fatigue lives were ordered with their associated joint probability of failure, which was utilized to arrive at reliabilities for the component lives. The present modeling for each basic regime reveals that the flight load variations are related to the regimes, which are in turn related to the helicopter usage variation in the fleet. The component lives versus reliability curve was established which will help the Navy to plan component retirements with estimated risks.