۱٫ Introduction

۲٫ Experimental results

۳٫ Calculated local stresses at the defect tip

۴٫ Multiaxial fatigue criterion

۵٫ Analysis of the proposed multiaxial fatigue criteria

۶٫ Conclusions

References

This study is dedicated to the effect of surface defects on the fatigue behaviour of high strength steels and the evaluation of their sensitivity. For that purpose, defects were introduced by Electro-Discharge Machining (EDM) on the edge of sheet metal test samples. Different defect depths are tested on high strength steel grades. A stress-based multiaxial fatigue life prediction method is then developed to assess the fatigue life. Input data are cyclic stress tensor states at the notch root that are calculated by a FEM analysis using the open-source Salome-meca software. In fact elastic–plastic numerical simulations were performed for each defect geometry to determine stresses distribution around the defect. A new class of multiaxial fatigue criteria extended from classical formulation to new ones with stress gradient terms related to the normal stress component is formulated. The Fogue integral approach based criterion is used since it is proved to give pertinent prevision under rotating principal stress directions multiaxial loading. For any material plane P the local stress gradient in its normal direction is calculated by parabolic interpolation and integrated to the normal stress component within the multiaxial fatigue criterion. For investigated multiaxial criteria, accounting for stress gradient effect is assessed from the experimental campaign results on two very high strength steel grades with occurence of surface defects.

Introduction

Because of economic constraints, there is a strong requirement nowadays for making decrease the material weight of mechanical structures; hence the stress levels induced by the service loading are increasing. The introduction of Very High Strength Steels provides by this way lightweight design and allows cost saving. The VHSS fatigue strength increases with mechanical properties but steel grades becomes also more sensitive to defects. Many industrial components such as chassis parts contain namely defects of different kinds (scratch, sharp notch or sensitivity viewed as defects such as coating thickness, cut edge…). As these surface defects of high strength steels have been shown to be the origin of the failure under cyclic loading, understanding the fatigue process and assessing its effects for engineering components or structures are of great importance for mechanical design in order to guarantee an appropriate in-service durability. Different approaches have been proposed in the literature for materials containing defects. They can be classified into three main groups: empirical approach, approach based on fatigue notch factor and fracture mechanics approach.