Cumulative-Damage Reliability for Random-Independent (Normal- Or Weibull-Distributed) Fatigue Stress, Random-Fixed Strength, and Deterministic Usage

To determine helicopter component retirement intervals, it is common for fatigue engineers to utilize Miner's rule for cumulative damage. Time consuming Monte Carlo simulations are considered the state of the art method for determining reliability in the presence of load variations. However, a new set of methods employed on a recent AHS fatigue reliability round robin problem provide an indication that near term advances are on the horizon. This paper presents the development of a set of analytical formulas for the calculation of cumulative damage per Miner's rule for the case with randomly varying oscillatory fatigue loads applied to a given component selected randomly from a population of various components. Although further development is required to cover all material characterizations, the method shows great promise in providing efficient fatigue reliability calculations during iterations of the design process. This paper provides a detailed derivation of the method, useful charts and tables, and guidance regarding implementation of the formula in various computer applications.