Issue 33

J. Toribio et alii, Frattura ed Integrità Strutturale, 33 (2015) 434-443; DOI: 10.3221/IGF-ESIS.33.48 434 Focussed on multiaxial fatigue Role of multiaxial stress state in the hydrogen-assisted rolling-contact fatigue in bearings for wind turbines J. Toribio, M. Lorenzo, D. Vergara Fracture of Materials and Structural Integrity Research Group, University of Salamanca, Spain toribio@usal.es, mlorenzo@usal.es , dvergara@usal.es A BSTRACT . Offshore wind turbines often involve important engineering challenges such as the improvement of hydrogen embrittlement resistance of the turbine bearings. These elements frequently suffer the so-called phenomenon of hydrogen-assisted rolling-contact fatigue (HA-RCF) as a consequence of the synergic action of the surrounding harsh environment (the lubricant) supplying hydrogen to the material and the cyclic multiaxial stress state caused by in-service mechanical loading. Thus the complex phenomenon could be classified as hydrogen-assisted rolling-contact multiaxial fatigue (HA-RC-MF). This paper analyses, from the mechanical and the chemical points of view, the so-called ball-on-rod test, widely used to evaluate the hydrogen embrittlement susceptibility of turbine bearings. Both the stress-strain states and the steady-state hydrogen concentration distribution are studied, so that a better elucidation can be obtained of the potential fracture places where the hydrogen could be more harmful and, consequently, where the turbine bearings could fail during their life in service. K EYWORDS . Hydrogen-assisted rolling-contact multiaxial fatigue; Wind turbines; bearings; Numerical analysis. I NTRODUCTION ffshore wind turbines often involve important engineering challenges [1], one of the most important being the improvement of hydrogen embrittlement resistance of the turbine bearings, a key issue in the evaluation of the structural integrity of such components. These elements are prone to suffer the so-called phenomenon of hydrogen-assisted rolling-contact fatigue (HA-RCF) [2, 3] as a consequence of the synergic action of the surrounding harsh environment (the lubricant) supplying hydrogen to the material and the cyclic multiaxial stress state caused by in- service mechanical loading [2, 3]. Thus the complex phenomenon of progressive damage could be classified as hydrogen- assisted rolling-contact multiaxial fatigue (HA-RC-MF). Three important aspects linked with bearing failures are being extensively researched: (i) rolling contact fatigue (RCF) [4- 7], (ii) influence of carbide particles on fatigue life [8,9], and (iii) local microplastic strain accumulation via ratcheting [10- 12]. To achieve a better assessment of the structural integrity of such components, the analysis of hydrogen accumulation (revealing the prospective damage places) arises as a key issue. In previous studies [2, 3], the widely used RC-MF ball-on-rod test [12-15] was simulated by the finite element method (FEM) in order to obtain the stress-strain state inside the bearings during life in-service. From these states, the hydrogen distribution corresponding to the steady-state in the radial direction of the bearing was obtained. This paper goes further in the study developed in previous research [2, 3] including the analysis of the hydrogen distributions in hoop and axial directions, in order to obtain the potential fracture places where the hydrogen embrittlement phenomenon initiates. O