Micromechanical study of the effect of inclusions on fatigue failure in a roller bearing

Abstract

Purpose – The purpose of this paper is to carry out a set of micromechanical analyses to study the effect of small inclusions on fatigue life of wind turbine bearings. Design/methodology/approach – The local stress concentrations around an inclusion are determined from a characteristic unit cell model containing a single inclusion, using the approximation of a 2D plane strain numerical analysis. The Dang Van multiaxial fatigue criterion is used for the local stresses in the matrix material, to ensure that the stresses remain within the fatigue limit. The matrix material is taken to be one of the most commonly used bearing steels, AISI 52100, and two different types of inclusions are considered. The macroscopic stress histories applied correspond to either a Hertzian or an elastohydrodynamic (EHL) contact pressure distribution under the rollers. Findings – The paper shows that sub-surface fatigue failure due to rolling contact is more likely to develop close to the inclusion-matrix interface, at particular angles that depend on the material and on the inclusion orientation. Originality/value – Inclusions represent an important issue in the design of wind turbine bearings, that are supposed to work in the very high cycle regime (N>109 cycles). This paper develops a micromechanical study that provides a deeper understanding on effect of inclusions on the fatigue life, according to one of the most used multiaxial fatigue criteria.

Influence of hard and soft inclusions inside a ferritic matrix

“…2a). It can be concluded that the Al2O3 inclusions are in general more harmful since they trigger the generation of plastic deformation at lower stress regime, confirming the conclusion reported in [2,3]. However, depending on the Euler angles set, the differences in nominal stress between the Al2O3 and the MnS inclusions seem to become smaller when the BCC cubic structure is rotated to give an alignment between cubic diagonal and the pulling direction.…”

“…The ratio between the sphere radius and the cylinder radius is 0.20, giving a volume fraction of 0.005. According to Cerullo and Tvergaard [2], increasing sizes of the volume fraction of the inclusion reduced the damage factor estimated with the modified Dang Van criterion, probably due to the fact that a bigger inclusion reduces the stress peak for the matrix in the reference cell. Future works will investigate this aspect.…”

“…), which often represent the locus for the micro-crack formation. Several studies [1][2][3] investigated the role of the material defects in the crack formation, focusing the attention on the individuation of the main factors responsible for the limitation of the fatigue life. An exhaustive overview by Murakami [4] individuated the defect characteristics that influence the most the fatigue life of metallic components in: type of defect (inclusion, pore, shrinkage, etc.…”

“…Two loading conditions were analysed: uniaxial tension loading and torsional loading, with the defect perfectly centred inside the specimen. Cerullo and Tvergaard [2] analysed the effect of small inclusions on the fatigue life of wind turbine bearings by means of micromechanical analyses. A modified version Dang Van fatigue criterion [8,9] was adopted to individuate the load level below which the material failure is avoided.…”

Roles of microstructure, inclusion, and surface roughness on rolling contact fatigue of a wind turbine gear

“…The ratio of the fatigue limits ( τ −1 / σ −1 ) is usually regarded as an empirical value for fatigue evaluation, lying in the range 0.57 to 0.8 for steels. And the fatigue performances of wind turbine gears and roller bearings have been successfully evaluated with setting the ratio to 0.577. The ratio in the Dang Van criteria is also set as 0.577 in this study.…”