Prediction of fatigue life based on level crossings and load history

Abstract

Models commonly used in fatigue life prediction are based on cycles counted in different ways. The most used method is based on Rain Flow counting which takes care of the stress history in a very specific way. This method has three main drawbacks. It is an ad-hoc way to produce cycles from a continuously varying stress curve. It introduces a memory in the cycle counting in a very rigid way and the algorithm is quite complicated. A model based on level crossings is on the other hand easy to apply but the level crossing spectrum does not contain enough information about the stress history. Here a model is proposed where the damage accumulation depends on the actual level crossing and the stress history condensed in a state variable, as well. The proposed model has the following properties. Failure occurs when the total damage exceeds a given value. Every stress change causes a non-negative damage which depends only on the actual stress, its change and the stress state variable. In a specific application the state variable can be interpreted as the opening stress of a crack. The model is time invariant in the sense that the damage does not change if the time scale is transformed. Hence the life is determined by the sequence of maxima and minima of the stress. In general the dynamics of the state variable must be time invariant and stable in the sense that a stationary and ergodic random stress function shall generate a stationary and ergodic state variable. In this case it is possible to predict fatigue life in terms of a damage intensity, which is the expected damage per time unit. Transactions on Engineering Sciences vol 6, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533 114 Localized Damage INTRODUCTION Fatigue life prediction under variable amplitude is usually based on the material properties, the stress history and some assumptions regarding the damaging process. The material properties are usually obtained from constant amplitude tests and the stress history is condensed in a suitable way by some counting technique. The damaging process is modelled by the Palmgren-Miner rule in connection with some additional assumptions. The commonly used model for fatigue life prediction at variable amplitude is the cycle counting approach. Here some cycle counting technique is defined and the stress history is condensed into a number of cycles at M levels. The total damage is then calculated according to the Palmgren-Miner rule,