13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Application of Dang Van criterion to rolling contact fatigue in wind turbine roller bearings Michele Cerullo Department of Mechanical Engineering ,Technical University of Denmark, Lyngby 2100, Denmark mcer@mek.dtu.dk Abstract A 2-D plane strain finite element simulation of rolling contact in wind turbine roller bearings is used to study very high cycle fatigue (VHCF). Focus is on fatigue in the inner ring, where the effect of residual stresses and hardness variation along the depth are accounted for. The purpose here is to ensure that VHCF failure does not initiate. For the purpose the Dang Van multiaxial fatigue criterion is applied, simulating the contact on the bearing raceway by substituting the roller with the Hertzian static pressure distribution. Contact without friction is assumed here and the material used for the simulation is taken to be an AISI 52100 bearing steel. Both an initially stress free bearing and different residual stress distributions are considered. An assumed residual stress distribution, equilibrated by an elastic step calculation, is subsequently subjected to the stresses caused by the contact with the roller. The effect of variable hardness along the depth is also studied, relating its values to the fatigue limit parameters for the material and it is found that its distribution can have a significant influence on the probability of failure for bearings subjected to VHCF loading. Keywords High Cycle Fatigue, Wind turbine, Dang Van 1. Introduction It has been seen [1,2] that one of the important reasons of corrective maintenance for a wind turbine is a failure due to rolling contact fatigue (RCF) in one of the bearings in the gear box [3]. Therefore, the interest on the reliability of gearboxes grew over the last years [4,5]. Though failure rates in electrical systems and other subassemblies in a wind turbine are in fact higher, or at least comparable with faults in the gearbox, recent studies [68] show that the downtime, in terms of hours lost per failure, is much higher for latter ones. This, rather than the failure rate, is therefore one of the main reasons for the industry's focus on these subsystems. In the gearbox, the bearings used are mostly roller bearings, due to the high loads involved. Even if the lubricant is kept clean and the bearing is properly lubricated, roller bearings sometimes experience rolling contact fatigue that appears as a crack starting below the surface of the inner race [9]. Once nucleated, this crack quickly propagates to the surface, resulting in particles of material flaking and leading to the failure of the bearing. Roller bearings for wind turbine applications operate in the fully elastic range and are subjected to a very high number of load cycles, with an expected life of 20 years [10]. However, practical experience show a high life scatter in these machinery elements, with failures that sometimes occur after a few years. The failure of these elements is thought to be due mainly to inhomogeneities and nonmetallic inclusions, that act as sites for crack nucleation under rolling contact fatigue. The cracks usually nucleate around inclusions, where the material experiences high stress concentration and typical butterfly defects are observed. The modelling in the present paper is focused on ensuring that the cyclic stress fields stay within limits so that very high cycle fatigue damage does not initiate. Several multiaxial fatigue models have been developed [1115], and some of them have been applied to RCF problems. The Dang Van criterion [16] and its further modifications has been widely used, over the last decades, in automotive industry [17] and in rolling contact problems as railwails and bearings [18,19]. It seems that the Dang Van criterion is not sufficiently conservative for negative values of the hydrostatic stress, therefore a modified version has been recently proposed [20], predicting a less sensitive behavior with respect to this stress component. This paper also includes a study of the overall effect
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