Fatigue Reliability Assessment of Gas Equipment under Randomly Fluctuating Stress Amplitude in Small Range

A new method is proposed to assess the fatigue reliability of gas equipment subjected to a load with a randomly fluctuating stress amplitude in a relatively small range. The Langevin equation for fatigue damage evolution was derived on the basis of the Palmgren-Miner's rule and the S-N diagram under constant amplitude loading. A stochastic partial differential equation, called the Fokker-Planck equation, for the evolution of the probability density function of cummulated fatigue damage was obtained from the Langevin equation. The probability density function of cummulated dàmage was expressed in a Gaussian distribution function. The mean of the remaining fatigue life can be evaluated on the basis of the concept of first passage time. The new method was applied to the fatigue data of an aluminum alloy subjected to a random loading. The scatter of the fatigue life can be obtained by introducing a magnification factor of the intensity of actual randomly fluctuating stress. The magnification factor corresponds to the degree of the contributions by factors other than the fluctuating load.