ANALYSIS OF FRACTURE DAMAGE IN HIGH-STRENGTH BOLTS USED IN WIND TURBINES

Abstract

This study presents a multi-faceted experimental damage analysis based on field data in high-strength bolts used in wind turbines. The relationship between production parameters and field performance, which is missing in the literature, has been demonstrated for the first time through comprehensive experimental investigations on M56x370-10.9 DAST21 bolts. Chemical composition, microstructure analysis, microhardness measurements, stereo microscope, SEM and EDS methods were applied on six broken bolts. The findings revealed that the cracks mostly started from the bottom of the teeth and the progression behavior showed brittle-ductile fracture characteristics. Thread base microhardness values were measured in the range of 403–460 HV0.3, exceeding the maximum value of 390 HV0.3 specified in the EN ISO 898-1 standard. In addition, the Si equivalent (Si_eq) between 0.215–0.234% has led to intense cracks in the galvanized coating and progressive damage to the steel. The unique contribution of the study is that multifaceted analyses supported by field data show that the interaction of bottom of the tooth hardness-galvanizing layer defects-hydrogen embrittlement plays a critical role in the fracture mechanism. In practical terms, these results; It reveals that heat treatment control and galvanizing process optimization for manufacturers and torquing procedures for users are critical for safety. Thus, the study makes important contributions to the literature by explaining the relationship between production parameters and service performance with quantitative data.

Keywords

• Broken Bolt
• Damage Analysis
• Fracture Mechanism
• Microstructure Investigation
• Renewable Energy
• Wind turbine.

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