Failure Analysis of a MW-Class Wind Turbine Blade Root Bolt
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TECHNICAL ARTICLE—PEER-REVIEWED
Failure Analysis of a MW-Class Wind Turbine Blade Root Bolt Zhunbei Zheng . Xiaochen Wang . Wenbin Guo . Zhanjun Yang . Xingxin Sun . Xiuguang Li
Submitted: 9 April 2018 / in revised form: 19 May 2019 Ó ASM International 2019
Abstract Failure analysis of a fractured high strength bolt used in a MW Class wind turbine blade connection system was conducted to clarify the failure cause and to determine the prevention method. Applying detailed mechanical characteristics and metallurgical investigations, the bolts failure cause can be attributed to bending fatigue. Evidenced by the nut polished markings, insufficient tightening was considered as the important reason for the bolt failure. The thread microstructure indicated that the fractured bolt thread was rolled rather than machined. However, rolling induced cracks locating on the thread crest and non-uniformly distributed decarburized layer on the bolt thread root existed. Thus, it can be concluded that the bolt failure was caused by the insufficient tightening and improper manufacturing procedure. Keywords Wind turbine blade High strength bolt Insufficient tightening Decarburized layer Cyclic bending fatigue
Introduction As an electric generator, wind turbine has been rapidly developed recently. Although such type electric generator has many advantages, such as less environmental pollution, fast installation and low maintenance expense, its main Z. Zheng X. Wang (&) W. Guo Z. Yang X. Sun X. Li China Datang Northwest Electric Power Test and Research Institute, Xi’an, China e-mail: [email protected] X. Wang School of Automation, Xi’an University of Post and Telecommunication, Xi’an, China
disadvantage, temporary variation nature of wind flow, can cause many unpredictable accidents affecting the equipment efficient utilizing [1]. As one of the most important components of a wind turbine, each blade with specific airfoil configuration can absorb energy from flow field to the turbine rotor section. Thus, the rotor-blade connection system may experience high-level alternating/fluctuating stress, and cause mechanical fatigue failure problems. Liu has conducted detailed stress analysis of a fractured wind turbine blade root bolt, and the main fracture reason was contributed to the structure’s low-temperature fracture toughness [2]. Applying detailed fractographic study and metallurgical analysis, Zhang et al. [3] investigated some fractured bolts assembled on the fan used in the internal combustion engine. The results indicated that appearance of the microcracks in the fractured bolts thread tip makes the stress at the thread regions intensively increase. Thus, the fatigue cracks can be initiated from the thread regions, which should be responsible for the formation of the cracks. Zhang et al. [4] also conducted detailed metallurgical analysis and mechanical testing to analyze a fractured bolt, and the fracture reason was attributed to the improper heat treatment, which caused the decarburized layer beneath the entirety of
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