Probabilistic reliability modeling of fatigue on the H-46 tie bar

Abstract

The H-46 helicopter has experienced early in-service failures in its tie bar. The tie bar is a multi-component system that is a critical part of the linkage, which attaches the rotor blade to the rotating hub of the helicopter. This research developed methodology to predict the life of the tie bar under nominal operational flight loads. A probability model is indispensable because a revised design has yet to accumulate field data, and laboratory testing can never be sufficiently extensive for non-parametric reliability prediction. An algorithm was developed for three and four component systems that will generate the probability of system failure based on the probability of failure in its components. Finite element analysis was conducted on the tie bar to determine stress on each component for all possible damage configurations of the tie bar. A given set of flight loads was resolved into boundary conditions for the stress analysis. A methodology was developed to determine the probability of failure of each component using an idealized load history, based on the expected stress- life (S-N) relation of the component at the stress levels experienced by the component. The result is a prediction method that can fortify laboratory results to predict the probability of failure of a system given the system load history. This model will be verified using the early in-service failure statistics of the current design and can be used to assess revised designs. The model will provide a prediction of the failure distributions, (the bell-shaped distribution) as a fabrication of flight hours, or one, two, three, and four elements of failures within the leaves of the tie bar. (MM)