Predicting fatigue lives for bi-modal stress spectral densities

Abstract Fatigue failures of components subject to random stress histories with bi-modal spectral densities are investigated. Such stress spectral densities are often experienced by chassis and suspension components of road vehicles. Some of the methods for predicting fatigue damage using frequency-domain statistics of random stress processes are reviewed. A new ‘bi-modal’ model is devised for calculating the probability density function of stress ranges in the case of a spectral density with two peaks. The method has the attraction of converging to the same results as the well known Rayleigh distribution for narrow band processes, if either of the spectral peaks is small. A component in a heavy road vehicle suspension is used as a case study. Sensitivity analyses are conducted to compare the new method with various other frequency-domain approaches, and with the widely accepted rain-flow method of cycle counting in the time domain.