Fatigue damage and life under random loading.
Abstract:ESDU 06009 is the first in a series of Items devoted to fatigue damage under random loading. Introductory concepts associated with acoustic fatigue and with stationary, Gaussian random processes are defined in this Item. The main aim, however, is to provide a method for the estimation of the fatigue life of structural components subjected to acoustic fatigue.
Two approaches to the estimation of the mean fatigue life are considered. The first approach refers to structures in which the responses have a predominant bending mode and the second takes into account the probabilistic nature of the random response of the structure. In the case of a stiffened panel, which can have one predominant bending mode, the r.m.s. stress depends upon the level of acoustic excitation, the damping of the panel and the frequency of the lowest bending mode, as described in ESDU 72005. When the r.m.s. stress in a stiffened panel is known, together with the S-N curves for acoustic fatigue and the frequency of the lowest bending mode, the fatigue life of the panel is determined.
For the structures with multi-modal response to random loading, the Palmgren-Miner fatigue damage rule is generalised to take into account the stochastic character of the structural response. Consequently, the probability density function (PDF) for stress ranges of rainflow cycles is introduced. The rainflow counting technique is analysed more thoroughly in ESDU 06010. Examples of two-dimensional (joint) PDFs for stress ranges of rainflow cycles and the mean stress values of the cycles are shown for different processes. However, fatigue life is estimated using the one-dimensional PDF for stress ranges of damage equivalent rainflow cycles, thus enabling the influence of the mean stress of the rainflow cycles to be taken into account.
Other Items are concerned with cycle counting, spectral and crack propagation methods for the estimation of fatigue life. The four parts of the series are:
|Data Item ESDU 06009