Requirements for stress gradient‐based fatigue assessment of notched structures according to theory of critical distance

Moritz Braun,Sören Ehlers,Aleksandar‐Saša Milaković,André Mischa Müller,Wolfgang Fricke

doi:10.1111/ffe.13232

Moritz Braun, Sören Ehlers + Show 3 more

Open Access

https://doi.org/10.1111/ffe.13232

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Abstract

AbstractNotches, local stress raisers within structural components, are one of the most important locations for fatigue crack initiation. It is well known that fatigue is governed by the effective stresses in the vicinity of notches. Within this study, differences in prediction accuracy between different types of theory of critical distance methods, that is, point and line methods, are systematically investigated in conjunction with a sensitivity study regarding mesh refinement, assumed strength hypothesis and material behaviour. For this purpose, a finite element analysis parameter study on notched structures is performed and recommendations for the application of stress gradient methods are presented. Difference in effective stress of up to 30%, and hence a significant difference in fatigue life (e.g., 185% for a slope of S‐N curve of k = 4), is found for typical notch shapes, for example, in welded joints.

Highlights

Notches, a form of geometric discontinuity within structural components, are one of the most significant locations for fatigue crack initiation due to local stress and strain concentrations
This study aims to contribute to the applicability of theory of critical distances (TCD) for fatigue assessment by analysing the effect of notch geometry on fatigue strength, using stress gradient methods
The requirements for fatigue assessment by means of stress gradient methods according to the TCD were investigated with regard to mesh requirements for numerical simulation and differences in prediction accuracy between the two most commonly applied versions of the TCD—point method (PM) and line method (LM)

Summary

Introduction

A form of geometric discontinuity within structural components, are one of the most significant locations for fatigue crack initiation due to local stress and strain concentrations. As notches lead to high local stresses and a multiaxial stress/strain state, they represent an area which is of great interest for fatigue limit state design of structures; this is especially true because their fatigue life is significantly reduced compared with unnotched structures. In fatigue assessment of notched structures, it is well known that the fatigue strength is not governed by the linear elastic maximum stress at the notch tip, but rather by the stress field surrounding the notch.[1] The main reasons are the high stress field gradients at sharp notches, the local plastic zone and initial damage in the form of microscopic cracks.[2] assessment based on the elastic peak stresses fails to predict size effects.[3]

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