Reduction of Stress Concentration Factor (SCF) on the Bolted Joint Connection for a Large Wind Turbine Rotor Blade through Various Design Modifications

Kwangtae Ha

doi:10.3390/app10186588

Abstract

The importance of a reliable blade root connection has grown due to the higher-gravity-induced edgewise loads on the blade root that resulted from the recent increased size and weight of a wind turbine rotor blade. To avoid the loosening of a bolt joint connection or even consecutive blade failures, the stress concentration factor (SCF) at the bolt thread root that is sensitive to fatigue should be understood comprehensively. In this work, two-dimensional and three-dimensional finite element (FE) analysis methods were used to determine the SCF at the bolt threads both between an insert and a M42 bolt used for a large offshore blade, and between a M42 bolt and a nut. The effect of various geometric parameters on the SCF were also investigated, which included shank diameter, nut height, nut type, and relief cone. Results showed that the decreased diameter of a M42 bolt shank diameter was the dominant design driver in reducing the stress concentration factor by 40%, from 3.94 to 2.32. The round nut type was also a recommended factor to be implemented to connect bolts and inner pitch bearing with an additional 10% SCF reduction. The relief cones applied to bolt threads and insert threads also contributed to the reduction of SCF to 2.01, a 49% reduction in total. This work not only provides guidelines by which to choose the proper geometry of the bolt and nut for a large blade, but also could be beneficial in designing bolted joint connections of segment or modular blades.

Highlights

IntroductionWind turbine generators (WTGs) have gained much attention because they utilize one of the cleanest and most environmentally friendly energy resources as an alternative to fossil fuels [1]
Wind turbine generators (WTGs) have gained much attention because they utilize one of the cleanest and most environmentally friendly energy resources as an alternative to fossil fuels [1].A modern wind turbine typically consists of blades, a nacelle including a generator and gearbox, and a substructure such as a tower
Before closing the investigation on stress concentration factors, the convergence of 3D finite element analysis depending on the mesh size was checked

Summary

Introduction

Wind turbine generators (WTGs) have gained much attention because they utilize one of the cleanest and most environmentally friendly energy resources as an alternative to fossil fuels [1]. Two types of root joint connections are widely used in commercial wind turbine blades: steel insert type and T-bolt ( referred to as “IKEA” connection) type, as shown in Figure 1 [3,4]. There is, little work on the stress concentration factor of a bolt–nut joint and a bolt–insert joint connection that depends on the varying geometric parameters of bolts, nut, and insert. The stress concentration factors on bolted connections will be fully investigated blades, which could reduce the BCD and weight as anjoint alternative to T-bolted connections [9]. Thatblades is, as the effect of the higher edgewise fatigue loading on the blade root region becomes critical, the stress concentration factor (SCF) at the

Numerical

Blade Root Connection Model

Finite Element Model

Numerical Analysis Results

Nut Type Effect on SCF

Discussion