This article conducts an in-depth failure analysis on the phenomenon of bolt fracture of wind-turbine blades in a certain wind farm. By means of macroscopic inspection, composition analysis, microstructure examination, and mechanical property tests of bolts, the key factors leading to bolt fracture were deeply explored, and corresponding improvement suggestions were put forward. The research results show bolt fracture is mainly closely related to excessive preload and material defects. This article aims to provide useful references for enhancing the safety of wind power equipment.
1 Introduction
Against the backdrop of the increasing global emphasis on renewable energy, especially wind energy, the wind power industry has shown a rapid development trend. However, the reliability of wind-power generation equipment is of vital significance for ensuring the stability of the power supply. In recent years, with the continuous increase in the service life of fans, failure problems of various components of fans have occurred frequently, bringing many challenges to subsequent maintenance work. Especially as the key fasteners of fans, the bolts’ breakage will directly endanger the operational safety of the fans [1-2]. This article conducts an in-depth analysis of the bolt fracture event of a wind-turbine blade in a certain wind farm [3-4], revealing its failure mechanism, with the aim of providing practical and effective suggestions for the maintenance and improvement of similar equipment [5-6].
2 Research Content
The H17 fan is an EN-115/2.2 type fan produced by Envision Energy Co., LTD. Its blade model is TMT-56.5-G170107F, the bolt specification is M36↔518 mm, the performance grade reaches 10.9, and the material selected is 42CrMoA. The nut specification is M36, with a performance grade of 10 and made of No. 45 steel [7-8]. In this incide turbine blades and the pitch bearing, five bolts of Blade B broke, and four were submitted for inspection, including those shown in Figure 1 [9-10].
Macroscopic inspection, elemental composition analysis [11], microstructure analysis, and mechanical property tests were conducted on the broken bolts (eight pieces) and intact bolts (two pieces were randomly inspected) submitted for inspection [12].
2.1 Macroscopic examination
The macroscopic morphology of the fracture surface of the submitted bolts is shown in Figure 2. The opening positions of all the bolts are at the lower part of the picture, and the fracture positions of the bolts on both sides are at the first thread of the nut and the screw at the engagement position.
Typical fatigue fracture characteristics of bolt 2025-JS-007 are visible: fatigue source zone (Zone I), spread zone (visible fatigue arc, Zone II), and instantaneous fracture zone (Zone III). Several stripes (yellow arrows) were observed in the middle of the fracture expansion zone of bolts 2025-JS-006, 2025-JS-010, and 2025-JS-012, indicating, that during the fracture process, the crack underwent multiple expansions and then stopped expanding again. Step morphology appears at the crack source of bolts 2025-JS-006, 2025-JS-008, 2025-JS-010, and 2025-JS-012. Each fracture surface cracks from the lower part of the photo, and the expansion area shows a radial pattern. Moreover, the radial pattern occupies the majority of the overall area of the fracture surface, which conforms to the macroscopic morphological characteristics of rapid fracture.
There was no obvious macroscopic plastic deformation at each fracture. Most of the bolts have rust on the thread between the nut position and the smooth rod, and there are also mechanical scratches at the tip of the thread, as indicated by the white arrow in Figure 2i.
Typical fatigue fracture characteristics of bolt 2025-JS-007 are visible: fatigue source zone (Zone I), spread zone (visible fatigue arc, Zone II), and instantaneous fracture zone (Zone III). Several stripes (yellow arrows) were observed in the middle of the fracture expansion zone of bolts 2025-JS-006, 2025-JS-010, and 2025-JS-012, indicating that, during the fracture process, the crack underwent multiple expansions and then stopped expanding again. Step morphology appears at the crack source of bolts 2025-JS-006, 2025-JS-008, 2025-JS-010, and 2025-JS-012. Each fracture surface cracks from the lower part of the photo, and the expansion area shows a radial pattern. Moreover, the radial pattern occupies the majority of the overall area of the fracture surface, which conforms to the macroscopic morphological characteristics of rapid fracture.
There was no obvious macroscopic plastic deformation at each fracture. Most of the bolts have rust on the thread between the nut position and the smooth rod, and there are also mechanical scratches at the tip of the thread, as indicated by the white arrow in Figure 2i.
2.2 Composition analysis
The composition analysis of the base material of the submitted bolts was conducted, and the results showed the chemical composition of the base material of the submitted bolts all met the standard requirements.
2.3 Microstructure analysis
The typical microstructure of the fracture surface of a bolt is shown in Figure 3. Step morphology can be seen in the crack source areas of multiple bolts, such as bolt 2025-JS-006 (arrow in Figure 3a), and mechanical damage traces can be seen in the fatigue source area of bolt 2025-JS-007 (arrow in Figure 3b).
Combined with the macroscopic observation results, the bolt cracks originate from the defects at the bottom of the thread (such as damage notches, etc.). In the radial area of the fracture surface, secondary cracks can be observed (arrows in Figures 3c, 3d, and 3f), as well as the morphology of nucleation at inclusions (Figure 3g). At the same time, there are a large number of tear ridges, a small number of dimms (bands), and small facets. These features conform to the typical morphology of quasi-cleavage fracture surfaces. The typical microstructure of the fracture section of a bolt is shown in Figure 4. The microstructure near the bottom of the thread near the bolt fracture surface is refined, and the grain deformation streamline is consistent with the thread shape, and no obvious decarburization or carburization layer is observed (Figure 4b). The bottom of the thread near the bolt fracture surface is uneven at several places, with defects such as protrusion, missing blocks, and folding (Figure 4c, Figures 4e-4h). These defects are prone to becoming the source of crack initiation. There are a few inclusions in the core of the bolt, and the core structure is tempered sorbite. No obvious abnormalities are found in the metallographic structure (Figures 4i and 4j).
2.4. Mechanical property test
2.4.1 Brinell hardness test
The Brinell hardness test was conducted on the cross-section of the threaded area of the broken bolt, and the measurement results are shown in Table 1. The test results show the hardness of the inspected bolts all meets the hardness requirements (316-375HBW) for bolts with performance grade 10.9 as stipulated in GB/T 3098.1-2010.
2.4.2 Tension test at room temperature
Room temperature tensile tests were conducted on the randomly inspected bolts, and the test results are shown in Table 2, except that the tensile strength of two tensile specimens of bolt 2025-JS-003 (with intact shape) and one tensile specimen of bolt 2025-JS-009 (broken) does not conform to the provisions of GB/T 3098.1-2010 for performance bolts of grade 10.9, the tensile strength, specified plastic elongation strength, and elongation after fracture of other specimens all meet the requirements of relevant standards.
2.4.3 Low temperature impact test
The low-temperature (minus-20°C) Charpy pendulum impact test was conducted on a random inspection of the submitted bolts. The test results are shown in Table 3. The test results show the randomly inspected bolts all meet the low-temperature impact performance requirements (KV2³27 J) for bolts with performance grade 10.9 as stipulated in GB/T 3098.1-2010.
3 Conclusions
1: The results of the component analysis show the base material composition of the submitted bolts all comply with the standard requirements.
2: The results of mechanical property tests show: 1) The hardness, impact toughness, specified plastic extension strength, elongation after fracture, and low-temperature impact performance of the inspected bolts all meet the relevant standard requirements. 2) For one broken bolt (2025-JS-009, tensile strength 1030 MPa) and one unbroken bolt (2025-JS-003), the tensile strengths of the two tensile specimens (with tensile strengths of 1,038 MPa and 1,039 MPa) were slightly lower than the provisions of GB/T 3098.1-2010 for grade 10.9 performance bolts (Rm³1040 MPa), while the tensile strengths of the other specimens all met the requirements of relevant standards. Considering the design safety factor, the tensile strength of the three samples not meeting the standard is basically unrelated to the bolt fracture this time.
3: The metallographic examination results show the microstructure near the bottom of the adjacent thread on the longitudinal section of the bolt is refined, the grain deformation streamline is consistent with the thread shape, and no obvious decarburization or carburization layer is observed. The bottom of several parts of the bolt near the thread is uneven, with multiple defects such as protrusion, missing blocks, and folding, which can easily become the crack source for crack initiation. There are a few inclusions in the core of the bolt. The core structure is tempered sorbite, and no abnormalities are found in the metallographic structure.
4: Macroscopic inspection revealed the eight bolts submitted for inspection were distributed on the side of the pitch bearing and the blade side. The fracture occurred at the first thread of the engagement position between the nut and the screw, which was the location with the greatest stress on the bolts. No obvious macroscopic plastic deformation, such as necking, was observed at each fracture surface. Most of the bolts have rust on the thread between the nut position and the smooth rod, and there are mechanical scratches at the tip of the thread.
5: The fracture analysis results show: 1) Macroscopically, the bolt 2025-JS-007 exhibits typical macroscopic fatigue fracture characteristics. Several stripes were observed in the middle of the fracture expansion zone of bolts 2025-JS-006, 2025-JS-010, and 2025-JS-012, indicating the crack expanded several times and then stopped expanding again during the fracture process. The step morphology at the crack source of bolts 2025-JS-006, 2025-JS-008, 2025-JS-010, and 2025-JS-012 can be seen. The expansion zones of each fracture surface present radial patterns, which conform to the macroscopic morphological characteristics of rapid fracture. The rapidly expanding area of the radial pattern occupies the main part of the overall area of the fracture surface, indicating the bolt is subjected to a relatively large stress, that is, the preload force on the bolt is too large. 2) Microscopically, mechanical damage traces can be seen in the crack source area of bolt 2025-JS-007. Combined with macroscopic observation results, the bolt cracks initiate from the defects at the bottom of the thread (such as wear notches, etc.), the morphology of secondary cracks and nucleation at inclusions can be observed in the radial area of the fracture surface. A large number of tear ridges, a small number of dimples (bands), and small facets coexist simultaneously, which conforms to the typical quasi-cleavage fracture surface characteristic morphology.
According to the tensile test results, it can be known the plasticity of the bolts is good, but there is no obvious plastic deformation at the fracture surface macroscopically, and the radial area of most bolts occupies most of the fracture surface area. Microscopically, it shows a quasi-cleavage fracture morphology, indicating the fracture force of most bolts is relatively large.
Based on the above analysis, the main reasons for the bolt fracture are excessive preload on the bolt and the existence of original defects such as protrusion, missing pieces, and folding at the bottom of the thread. At the position of the first thread where the bolt and nut are engaged, the bolt bears the maximum tensile stress. Under the action of preload and operational fatigue load, cracks initiate at the defects caused by thread processing at the bottom of the teeth, etc. Some bolts experience fatigue under the alternating stress caused by the rotation of the blades, leading to crack propagation. Some bolts, due to the changes in force caused by blade movement (such as retracting the blade, etc.), sometimes pause or resume crack propagation. When the effective cross-sectional area of a fatigued bolt is insufficient as the fatigue crack expands, it will break. When the force on other bolts undergoes sudden changes (for example, another bolt loosens or breaks), it will break rapidly when the force exceeds the material strength.
4 Suggestions
1. Strictly control the preload of the bolts: During the installation process, it is essential to use torque tools that meet the standards to ensure the applied preload is strictly controlled within the design range, so as to avoid the bolts being subjected to excessive stress due to excessive preload.
2. Material procurement and inspection: During the acceptance process of new materials upon their entry into the warehouse, the comprehensive performance of tensile strength and toughness should be strictly controlled. Especially for the bolt materials used in the working environment where fans are prone to fatigue, it is necessary to ensure they have sufficient strength and toughness to resist the influence of fatigue loads.
3. Conduct two inspections of the torque and appearance of the pitch bolts of the wind turbine each year: For wind turbines with broken blade bolts, the frequency of torque inspection is increased to check whether the bolts are loose or damaged. We regularly carry out ultrasonic testing of bolts in accordance with the requirements of the regulations to detect bolts that have developed cracks but have not yet broken. Especially when there are significant changes in working conditions, the wind force is about to reach the design value, or the vibration signal is abnormal, bolt inspection should be carried out. If the bolts of the fan crack frequently, it is recommended to install a real-time monitoring device for bolt load to diagnose the working conditions that cause sudden changes in bolt load.
*This article (https://iopscience.iop.org/article/10.1088/1742-6596/3159/1/012054) is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). The article has been edited to conform to the style of Wind Systems magazine.
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