Fatigue Reliability Analysis of Dynamic Components with Variable Loadings Without Monte Carlo Simulation

US Army helicopters have retirement times assigned for fatigue sensitive components. These retirement times are based on a deterministic approach. They are derived using the Prime Contractor’s safe life methodology which typically includes fatigue testing of production components to determine a working S/N curve, measurements of top of scatter loads from flight load surveys, and construction of a composite usage spectrum from pilot surveys. The philosophy of the safe life approach is to maintain a level of reliability within the design life of a component. However, the level of reliability for each substantiated life has not been defined due to limitations of analytical tool availability and understanding of random variables for each parameter for its reliability evaluation. Monte Carlo Simulation is the most common approach for fatigue reliability analysis. Thompson and Adams [1] have used the simulation approach to demonstrate a six-nines reliability on several UH-60 dynamic components. In contrast to the simulation, this paper introduces an analytical methodology to evaluate the fatigue reliability of a spectrum with variable loading. The analytical approach provides fast execution to obtain a solution in seconds in comparison to long hours of the solution time from a Monte Carlo simulation run when a high degree of the reliability is required. Two assumptions were made in developing this approach. First, a “Constant Damage Rate Tracking” is assumed for each individual load step, and second, the fatigue reliability is based on a weighted average of the damage rates of the spectrum load steps. The analytical approach was validated by comparison to Monte Carlo simulations. This paper presents an accurate and efficient way to evaluate fatigue reliability analysis using the “Constant Damage Rate Tracking” approach. The approach allows a quick determination of a component’s reliability at its retirement time when the fatigue strength, severity of load, load variability, and the usage are characterized. This paper presents the reliability solution of a two step spectrum problem and the AHS Round Robin problem [2] for verification. 1 Presented at the American Helicopter Society 63 Annual Forum, Virginia Beach, Virginia, May 1-3, 2007. Copyright © 2006 by the American Helicopter Society International, Inc. All Rights reserved.