PSI - Issue 31

Vera Friederici et al. / Procedia Structural Integrity 31 (2021) 8–14

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V. Friederici et al. / Structural Integrity Procedia 00 (2019) 000–000

Nomenclature Δ K

delta of stress intensity factor threshold of stress intensity factor maximum of stress intensity factor minimum of stress intensity factor crack driving force parameter number of cycles for crack propagation number of cycles for crack initiation delta of the positive part of the stress intensity factor number of cycles

Δ K th Δ K + K max K min

K*

N

N p N i N t

total number of cycles

R C m α S a

stress ratio

coefficient of Paris equation coefficient of Paris equation

coefficient of crack driving force parameter equation

stress amplitude

stress

σ

1. Introduction Slewing bearings in wind turbines connect the rotor blade to the hub. They are designed to adjust the angle of the blades (pitch) for different wind speeds and thus control the power and loads of the wind turbine. Particularly during oscillating mode in which the bearing is rotated by only a few degrees with each rotation of the hub, the bearing is subjected to high loads at very localized points. Besides failure due to wear and fatigue on the raceways, also severe risk of structural failure (core crushing) is possible: accidents and serious damages have been reported (Caithness Windfarm Information Forum, 2020; Blaue, C., 2018). In the service life calculations of slewing bearings so far only speed, bearing load and temperature have been considered (GMN, 2020). In reality, fatigue failure is much more complex: in addition to the bearing loads also nonmetallic inclusions and the local hardness have a significant influence on the failure behavior (Murakami, Y., 2002). Especially crack initiation time is an underestimated parameter. However, crack initiation is very difficult to measure (Combrade, P., 2019). An attempt to estimate the time needed for crack nucleation is based on the idea of correlating the results of crack propagation measurements and rotating bending tests. At crack surfaces of rotating bending specimen the crack origin and the residual fracture surface is well detectable (Fig. 1a). Connecting these two characteristic sites an approximate crack path (crack length) can be estimated subsequently, see Figure 1a. Assuming a uniformly propagation between initiating site and residual fracture surface on basis of a crack growth rate da / dN vs. Δ K curve, the number of oscillating cycles can be predicted. For first approximation, the fact that the fatigue life of a structural component actually requires a multi-scale approach (Božić, Ž., 2014) is disregarded. In future also fatigue crack propagation limit curves as presented by J. Lukács (2019) could be used as a good statistical tool for structural integrity calculations. The crack initiation time N i (number of oscillating cycles) can be calculated from the difference between the number of oscillating cycles required for crack propagation N p and the number of oscillating cycles until final fracture N t of the rotating bending specimen (Fig. 1b). This will be done for every heat treatment condition by simulation using Abaqus XFEM. The benefits of XFEM simulations are undisputed. A. Solob showed the applicability of XFEM calculation based analyses with regard to stress intensity factor values calculation and for three-dimensional cracks’ paths predictions in complex geometries (2020).