Abstract
For the service life estimation of metallic components under cyclic loading according to strain-based approaches, a simulation of the elastic-plastic stress–strain path at the point of interest is necessary. An efficient method for determining this stress–strain path is the use of the load–notch-strain curve, as this is also implemented within the FKM guideline nonlinear. The load–notch-strain curve describes the relationship between the load on the component and the local elastic-plastic strain. On the one hand, this can be estimated from loads or theoretical elastic stresses by using notch root approximations. On the other hand, this can be determined in a finite element analysis based on the elastic-plastic material behaviour. This contribution describes how this latter option is carried out in general and how it can be optimised in such a way that the FEA requires significantly less calculation time. To show the benefit of this optimisation, a comparative calculation on an exemplary geometry is carried out.
Highlights
The assessment of fatigue strength is one of the most important aspects in the design of safety-related components
It is only necessary to carry out the finite element analyses (FEA) with the double stress–strain curve according to Equation (6) for the hysteresis branch; the load–notch-strain curve for the initial load can be calculated from this
The reference load–notch-strain curve is regarded as the “true” load–notch-strain curve and is determined by FEA with a very fine mesh
Summary
The assessment of fatigue strength is one of the most important aspects in the design of safety-related components. The variant using PRAJ has the advantage that load sequence effects can be taken into account, which promises a better accuracy, especially for load sequences with changing statistic characteristics Both described calculation procedures within the FKM guideline nonlinear have in common the fact that notch root approximations are used as standard approaches for determining the local stresses and strains. A disadvantage of this is the significantly higher calculation effort required in terms of computational resources and calculation time compared to FEA with linear-elastic material behaviour and a subsequent notch root approximation, especially when finite element models with many elements are considered—e.g., for complex geometries This is the reason why the standard use of FEA with elastic-plastic material behaviour to determine local stresses and strains has not yet found its way into regulations for the assessment of fatigue strength.
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