System level reliability analyses and predictions in a varying stress environment

Abstract

New flexible methods have been developed to predict reliability and estimate failure time distribution parameters for equipment and systems that are to be exposed to more stressful and diverse usage conditions in the future. Decades ago, the design reference mission for steam catapults and arresting gear on aircraft carriers was quite simple. The design engineers of these systems had a good understanding of the loads and added factors of safety to be cautious. Move ahead to the present, the mixture of aircraft has changed drastically, sortie rate has increased and so has the kinetic energy imparted to these critical systems. This has led to a need to develop a more generalized and flexible reliability predictive tool. This tool can be described as a stress-sensitive Weibull distribution. The entire process is outlined for this innovative technique. It includes the options for several methods of analysis. The base model is a Weibull distribution based solely on failure data without modifications. The first method is also a Weibull distribution, but the Weibull scale parameter is modified by a stress ratio, using end speed and the aircraft weight. The second method uses mean and standard deviation of end speed and aircraft weight to modify the Weibull scale parameter. These scale parameter modifiers are calculated based on an assumed general log-linear model and maximum likelihood estimation tools. The third method decouples any correlation that may exist between aircraft weight and end speed by binning aircraft launches into groups and calculating their proportion of the total. Once evaluated, these methods are able to extrapolate future failures at different levels of stress all across these critical systems.