۱٫ Introduction

۲٫ Finite element model description

۳٫ Fatigue method

۴٫ Simulation schedule

۵٫ Results

۶٫ Conclusions

Acknowledgements

References

Currently, additive manufacturing with metals is an increasingly popular technique that allows the manufacturing of pieces of difficult shapes, nearly impossible to make with other techniques. Usually, these shapes try to optimize the solid to have the same strength with a lower weight. The fatigue behavior of the material of the components manufactured with this technique is a field in development. On the other hand, fretting fatigue is a common type of fatigue where a “stress concentration” appears due to the contact between two components. There are some procedures used to increase fatigue life in this situation (shot peening, surface knurling, etc.). This paper tries to analyze the possible beneficial effect on fatigue life of introducing voids inside the material in components under fretting, which is feasible now thanks to additive manufacturing. The problem under study is, for now, a 2D simplification where a cylinder is in contact with a half plane and a normal constant load and a variable tangential load are applied. This geometry has been numerically simulated, introducing a circular hole below the surface. The effect of this hole is to make the contact more elastic, which decreases the stresses near the surface. This work analyses and compares the stress and strain fields and Smith-Watson-Topper multiaxial fatigue parameter in the areas sensitives to fretting with respect to a case with homogeneous material (no internal voids). Various configurations changing different parameters like size and position of the hole, friction coefficient and the size of the slip zone have been considered. The problem analyzed in this paper is two-dimensional, therefore there would be no need to use additive manufacturing in a real situation. However, the results obtained in this paper indicate that it could also work in 3D. Actually, it is in a real three-dimensional problem where the additive manufacturing would be necessary for the introduction of voids inside the material to improve fatigue life.

Introduction

Fretting damage is a phenomenon that may occur on surfaces pressed together when subjected to cyclic loads. In addition to the bulk stress and the contact stresses produced by the pressure that keeps the surfaces together, components under the action of fretting are subjected to relative displacements. This combination leads to different types of damage such as wear, oxidation and the nucleation of cracks [1]. Various works shed light on different palliatives in fretting fatigue [2]. The palliatives can be divided between the ones that modify the geometry to in turn change the stress/strain fields, the ones that modify the properties of the material and surfaces (hardness, friction coefficient, etc.) or the ones that introduce residual stresses (shotpeening, etc). This paper focuses strictly on the modification of the geometry of the elements in contact so that it might be possible to mitigate, to the extent possible, the influence of fretting in crack initiation. This idea is linked to progress in additive manufacturing, thanks to which it can be made almost any type of geometry quickly, accurately and with a wide variety of metals, such as steel, aluminium and titanium. However, this is a field that is developing and there are not many accurate data about the behaviour of the material. The fatigue behaviour of the material manufactured in the traditional way differs from the behaviour of the components created by additive manufacturing [3]. This is due to the method of manufacturing layer by layer, causing an apparent anisotropy to the component as well as the inclusion of unwanted defects, porosity and residual stresses. In general, the new material is not completely homogenous and must undergo heat treatments to improve their mechanical properties.