Observations of component and specimen behaviour in fatigue

Trends in structural integrity analysis for design show increased use of e.g. numerical FEM-analysis. FEM analysis produces pointwise strain or stress throughout a component. The distribution through the thickness is nonlinear and the meaning of these numbers compared to traditional stress by formulae call for new, realistic and reliable interpretation. This paper presents a set of observations of component behaviour in fatigue when data is plotted as local strain fluctuation versus cycles to failure. Part of the paper shows the differences in raw data between standard uniaxial test data and component data, when both data are represented by the uniaxial equivalent value for the local maximum peak strain range or amplitude using octahedral shear strain equivalence theory (vMises) for simplicity. The rest of the paper presents rules developed for bringing component data to coincide with standard uniaxial test data - thus providing a way to predict component behaviour based on material uniaxial data - and a new rule to estimate local yielding (plasticity correction) for structural analyses based on elasticity. The development work was done in a European Commission sponsored project "Fatigue Design" (BE 97-4658), which was practically completed in 2002.