High-strength bolts break in every wind farm to case wind turbine and tower failure. As the main connecting parts of wind power generation equipment, they are the guarantee for the safety of wind power generation equipment. Frequent fractures of high-strength bolts, especially blade bolts, are causing seriously trouble to wind farm investors and OEMs.
One of AGICO’s major products is high-strength bolts, which are designed for wind power applications. These include wind blade bolts, wind tower bolts, and anchor cage foundation bolts. As critical components in wind turbines and anchor cage foundations, their main functions are connections between blades, towers, hubs, and nacelles. So the integrity of wind turbine bolts directly affects the structural stability and operational safety.
In many wind tower projects, fractures in turbine bolts have become a persistent issue. Such fractures might have many casues, for example complex loads, harsh environmental conditions, and latent defects. AGICO will investigate the root causes of these failures through real-world case studies and propose actionable solutions.
Multifactorial Mechanisms Behind Bolt Fractures
High Strength Bolt Manufacturing Flaws: Hidden Weaknesses
Inherent material defects such as porosity or inclusions disrupt the homogeneity of the metal matrix, creating localized stress concentrations. For instance, a fractured 35CrMo steel bolt from a turbine tower foundation exhibited surface carbon content (0.28%) below the GB/T 3077-2015 standard, compromising its fatigue resistance. Additionally, machining imperfections—like tool marks at thread roots—act as initiation sites for microcracks. Studies indicate stress concentration factors of 3–5 at these points, accelerating fatigue failure.
Environmental Degradation Factors to Wind Turbine Bolt
- Corrosion: Wind turbine bolt exposure to marine or humid environments induces pitting corrosion, which can reducing effective cross-sectional areas and lead to crack formation. Turbine bolts in salt-laden atmospheres such as marine environments may experience 30–50% shorter fatigue lives.
- Low-Temperature Embrittlement: Subzero temperatures (e.g., below -20°C) diminish turbine bolt toughness, increasing susceptibility to brittle fracture under impact loads.
Decarburization: A Stealthy Threat
Surface decarburization is more likely to form a ferrite layer on the bolt with a thermal expansion coefficient mismatched to the pearlite core, generating localized stresses. In one fraction case, a wind tower bolt’s decarburization depth (0.1 mm) exceeded the 0.05 mm limit, enabling microcrack propagation under cyclic loading.
Installation and Operational Errors
Improper preload is a common problem when the wind turbine bolts are installed with the wrong method. Over-tightening (e.g., >20% above design specifications) induces plastic deformation at thread roots, while under-tightening causes joint slippage and shear stresses. According to the record, installation errors reportedly contribute to 15% of bolt failures in wind tower projects.
Case Study: Fractured Wind Tower Foundation Bolt
A failed 35CrMo hex bolt from a turbine tower base was revealed after the investigation, and the conclusion came out to be: combined material defects, poor machining, and environmental corrosion led to low-cycle fatigue failure. Here is the analyze details:
- Chemical Deviation: Surface carbon content (0.28%) fell below the required 0.32%, reducing the foundation bolt strength.
- Metallurgical Defects: Thread decarburization depth reached 0.1 mm, with networked ferrite structures.
Fractography: Scanning electron microscopy (SEM) identified fatigue striations originating from the intersection of decarburized zones and machining flaws.
How to Prevention Bolt Failure with Advanced Detection
After the analysis of high strength bolt fraction failure in wind farm projects, AGICO engineers has summed up 3 effective methods to avoid bolt failure for wind towers, here are the details:
Full-cycle quality control
- Material optimization: Use vacuum degassing smelting technology to control inclusion levels (such as Class B ≤ 1.5);
- Process improvement: The rolling thread process is introduced to replace cutting processing to make the surface roughness Ra≤0.8μm and reduce stress concentration;
- Surface protection: Apply Dacromet coating or hot-infiltration zinc process, and the salt spray resistance test time is increased to more than 1,000 hours.
Smart Installation and Monitoring
A hydraulic torque wrench is used in conjunction with an ultrasonic axial force measuring instrument to control the preload force error within ±5%. Introducing IoT technology to monitor the loosening status of bolts in real time.
Multimodal Inspection Protocols in Bolt Quality
Use more comprehensive test methods to test the bolt products used in wind towers and turbines, choose bolt with higher class if the current one is not the best choice, here are the parameter details:
- Mechanical Testing: Ensure tensile strength ≥1,040 MPa and elongation ≥12%.
- Chemical Analysis: Use optical emission spectrometry for precise elemental quantification.
- Microstructural Evaluation: Employ metallography (500× magnification) for decarburization assessment and SEM for fractography.
- Hydrogen Embrittlement Mitigation: Apply slow strain rate testing (SSRT) to evaluate hydrogen-induced cracking risks.
