Structural Integrity of Flange Connection in Onshore Wind Turbine Towers: Investigating Premature Bolt Failures

Abstract

Premature bolt failure on onshore wind turbine towers is a major issue and hamper field operation safety. As such, this article examines the structural integrity of 32CrB4 steel M56 high strength grade 10.9 flange connection bolts as applied on wind turbine towers. By comprehensively analyzing characteristics like mechanical properties, chemistry and microstructure, this study hopes to point out what is behind the failure of these bolts. A range of testing was utilized to determine the strength and ductility of these bolts. Analysis of the chemical composition conducted with scanning electron microscopy and energy dispersive spectroscopy--found elemental distributions that could affect material performance. Tests that revealed defects or inconsistencies were performed using optical microscopy, scanning electron microscopy, and electron backscatter detection. This showed that while the bolts met their mechanical requirements, there were aspects which might indicate hydrogen embrittlement. These include intergranular cracking near the edges of the fracture surface on all of the bolts. In addition, the variations in size and distribution of gaps (especially one test piece) indicated weakness. In spite of these findings, it has not yet been possible to pin down the cause or causes of bolt failures conclusively due to limitations inherent in the data as well as the complex interplay between different mechanisms leading to failure. This study underscores the urgent need to execute further research in this area to understand all factors affecting the fracture in theese bolts. By providing interesting parties in the wind turbine industry insights that they can use to develop more efficient strategies for maintenance and ultimately make turbine maintenance safer and more stable.

Premature bolt failure on onshore wind turbine towers is a major issue and hamper field operation safety. As such, this article examines the structural integrity of 32CrB4 steel M56 high strength grade 10.9 flange connection bolts as applied on wind turbine towers. By comprehensively analyzing characteristics like mechanical properties, chemistry and microstructure, this study hopes to point out what is behind the failure of these bolts. A range of testing was utilized to determine the strength and ductility of these bolts. Analysis of the chemical composition conducted with scanning electron microscopy and energy dispersive spectroscopy--found elemental distributions that could affect material performance. Tests that revealed defects or inconsistencies were performed using optical microscopy, scanning electron microscopy, and electron backscatter detection. This showed that while the bolts met their mechanical requirements, there were aspects which might indicate hydrogen embrittlement. These include intergranular cracking near the edges of the fracture surface on all of the bolts. In addition, the variations in size and distribution of gaps (especially one test piece) indicated weakness. In spite of these findings, it has not yet been possible to pin down the cause or causes of bolt failures conclusively due to limitations inherent in the data as well as the complex interplay between different mechanisms leading to failure. This study underscores the urgent need to execute further research in this area to understand all factors affecting the fracture in theese bolts. By providing interesting parties in the wind turbine industry insights that they can use to develop more efficient strategies for maintenance and ultimately make turbine maintenance safer and more stable.