Fracture failure analysis of high strength bolts on foundation flange of large wind turbine

Bolt fastener is commonly used to fasten and connect two parts with through holes, which is indispensable mechanical parts. It is widely used in assembly of many machinery industries such as automobile, internal combustion engine, aviation and wind turbine. Wind turbine is a large equipment, and high-strength bolt is an important part of it, which directly affects the bearing capacity, service life and safety performance of wind turbine. Because of the random change of wind speed and strength, flange bolts of wind turbine need to bear the alternating load during the work process. It is a weak link in the wind turbine system, so the failure analysis and prevention of the connecting bolts is very important.

The whole machine collapse accident occurred in 2MW wind turbine of a wind farm. According to the analysis of the collapse accident, the wind turbine is dumped from the connection between the foundation and the tower. There are many phenomena of fracture, bending, nut falling off and foundation flange turning out of the high strength connecting bolts between the foundation and the tower. Therefore, it is of great significance to study the failure mechanism of foundation flange bolts to prevent the wind turbine from causing major operation accidents due to bolt fracture. In this paper, the strength of the foundation flange bolt is calculated and analyzed under two working characteristics, and the corresponding relationship between the external bending moment and the bolt stress is obtained. The results are verified by the numerical calculation results, and the results are consistent with each other. Then, the fracture morphology of bolt is analyzed theoretically, and the fracture of the same material sample is compared and analyzed by static load tensile test, and the fracture properties of the bolt on the accident site are determined. On this basis, the foundation flange is analyzed and numerical calculation is used to reproduce the phenomenon of foundation flange turning. The calculation results show that the yield deformation occurs at the neck of the foundation collar, which leads to the foundation flange turning out and the bolt fracture. Finally, it is determined whether the foundation ring can be recycled or not, and the technical performance indexes of the basic ring are tested and evaluated.

Strength calculation of flange bolt

Working characteristics

According to GL wind turbine certification specification. 51 it can be seen that under the external load Z in the two-axis direction, the working model of the foundation flange and bolt connection of the wind turbine is shown in Figure L.
20210509144607 47673 - Fracture failure analysis of high strength bolts on foundation flange of large wind turbine
When the external load increases, the flange joint surface is separated from each other, as shown in Figure 1, the lever effect is formed with the inside of flange as the fulcrum, the bolt as the spring and the tower wall as the external load application point. In summary, the calculated external bending moment of flange and bolt under different working conditions is shown in Table 1 without considering the additional bending moment, external torque and shear force. Based on the fitting principle of cubic polynomial, the curve of external bending moment value MXY and bolt stress is drawn, as shown in Figure 2.
20210509145115 38355 - Fracture failure analysis of high strength bolts on foundation flange of large wind turbine
From the comparison of the bolt subjected to the general ultimate load (bending moment, shear force and torque) and its magnitude, it can be seen that the bending moment at the bottom of the tower is much greater than the shear and torque at the place, that is, the bending moment is the dominant factor. When the shear force and torque acting on the bolt increase 10 times, the influence of shear force and torque on the stress distribution of bolts should be considered.
In summary, when the external load Z value is too large or the pre tightening force F fails (no matter how to fail), the working state of the bolt, flange and flange joint surface will be changed significantly, and the bolt group will fail. Meanwhile, from the general limit load, bending moment is the main reason for the failure and failure of the bolt.

Analysis of fracture morphology of bolts

In the structure of wind turbine, there are many kinds of bolt connection. The basic types of bolt failure are overload fracture failure, fatigue failure, stress corrosion fracture failure, etc. [7,8]. The flange joint bolts in wind turbine are mainly fatigue fracture and overload fracture. The fracture of the coupling bolt under alternating load is fatigue fracture.

Analysis of foundation flange

Stress analysis of flange

As mentioned above, when the flange joint surface is in the state of mutual stripping, the bolt, flange and tower wall form a similar “leverage effect”. As shown in Figure 3, the tower flange is under tension F1, the supporting force F2 provided by the inner side of the foundation flange, the tension F3 for the bolt to resist the bending deformation of the flange (bending moment caused by wind load acting on the tower wall), and the resultant force F4 for the bolt to resist the extrusion force (caused by flange overturning) and shear force (caused by wind load acting on the tower wall). It can be seen from figure 8 that the overturning of foundation flange is mainly caused by the reaction of F2 and F3 and the combined action of F4. When the angle theta of the joint surface between the foundation flange and the tower flange is small, the corresponding flange overturning phenomenon is not obvious. At the same time, if the value of theta is small, the bending moment can be ignored, so it can be considered that F1 + F2 is about equal to F3. In the stress state of the above flange and bolt, F2 and F3 play a role in restraining flange eversion. The above stress analysis shows that during the collapse of the wind turbine, due to the action of F4, the bolts are bent and deformed, resulting in different degrees of bending of the bolts at the accident site.
20210509145812 11417 - Fracture failure analysis of high strength bolts on foundation flange of large wind turbine

Calculation and analysis of flange value

The material of base ring and base flange is Q345D, and its chemical composition is (mass fraction): 0.15% C, 1.5% Mn, 0.5% Si, 0.05% P and 0.03% s. At room temperature, the tensile strength (sigma B) is 680.94 MPa, the yield strength (sigma S) is 345 MPa, and the elastic modulus (E) is 181 GPa. Establish a three-dimensional numerical calculation model of the foundation ring and flange, as shown in Figure 4. Take a hole on the flange surface as the center, take 450 kn as the first group of data, and then reduce the load value by 0.9 times in turn. Load the foundation ring with 16 groups of symmetrical loads. Through continuously adjusting the load value, the eversion result of the foundation flange can be reproduced. The eversion deformation result of the foundation flange is calculated, as shown in Figure 9, The maximum deformation occurred at the inner side of the foundation flange, which is 3.613mm (the displacement is from the inside to the outside), resulting in the overturning of the foundation flange.
20210509145904 56790 - Fracture failure analysis of high strength bolts on foundation flange of large wind turbine
At the same time, von Mises equivalent stress on the foundation flange is calculated (as shown in Figure 10). The maximum stress (sigma max) on the neck of the foundation ring is 604 MPa, which has exceeded its yield limit (sigma S = 345 MPa), but has not yet reached the tensile limit of the material (sigma B = 680.94 MPa). It shows that the yield deformation of the foundation ring has occurred, but there is no fracture failure, resulting in the mutual peeling of the flange joint surface, and the stress on the bolt increases significantly, Therefore, flange overturning deformation and bolt fracture failure are caused by the foundation
It was caused by obvious yield deformation at the neck. At the same time, it can be seen from Figure 5 that the strength and stiffness of the foundation flange are much greater than the corresponding performance of the foundation ring steel plate.
20210509150017 75372 - Fracture failure analysis of high strength bolts on foundation flange of large wind turbine

Quality inspection of foundation ring

Due to the large tensile stress on the foundation ring after the collapse of the wind turbine on site, in order to judge whether the foundation ring can be recycled, it is necessary to further test the flatness (including inclination), Ovality and foundation levelness of the foundation ring, and conduct ultrasonic testing (UT) and magnetic particle testing (MT) on the welds to judge whether they meet the technical performance requirements. Through the ultrasonic flaw detection of the foundation girth weld, no over standard defects are found, and the results meet the requirements of JB / t4730.3-2005 standard; Through magnetic particle flaw detection for the 100 mm width area of the foundation girth weld, no surface cracks and over standard defects were found, and the results met the requirements of I’4730.4-2005 standard of JB factory.
The flatness of the flange on the base ring is measured by laser leveling instrument, and the deviation is 4.17mm, which is 2mm beyond the technical requirements; The maximum eversion is 3.88mm. The maximum deviation of levelness measurement is 2.5mm, which also exceeds 2mm allowed by technical requirements; The diameter deviation of the pitch circle of the bolt hole measured is very small, which meets the requirements of the drawing, but a few bolt holes are extruded and deformed, and they are all at or near the toppling position of the tower.
To sum up, through the detection of the above items, the ovality of the foundation ring and the ut / mt detection of the flange girth weld meet the requirements of the technical specifications. The test results of flatness, bolt hole diameter and foundation levelness do not meet the technical performance requirements and need to be further repaired and reused.

Conclusion

  • 1) The strength calculation of flange bolts shows that when the preload f fails or the external load Z is overloaded, the flange joint surfaces will peel off from each other, and the load on the bolts will increase significantly. When the load exceeds the tensile strength of the bolt material, the fracture failure will occur, leading to the collapse of the wind turbine. The fracture property of the foundation flange bolt is overload fracture. At the same time, in the bending moment, shear force and torque load, the bending moment is the dominant factor leading to bolt fracture failure.
  • 2) From the observation of bolt fracture morphology, the fracture morphology is consistent with that of overload fracture. The static tensile test not only further verified the results, but also measured the yield strength and tensile strength of the bolt material, which showed that the bolt met the installation requirements of strength grade. The falling off phenomenon of some nuts on site is mainly due to the too small preload or failure during assembly, which leads to the bolt bearing too much load. When the load is greater than the tensile strength of the bolt, the bolt breaks. Therefore, too small preload is one of the reasons for bolt fracture.
  • 3) Through the numerical calculation of the stress state of the flange, the eversion phenomenon of the foundation flange is reproduced. The calculation shows that the stress on the neck of the foundation ring exceeds the yield limit, which indicates that the yield deformation of the foundation ring has occurred, which directly leads to the eversion phenomenon of the foundation flange. At the same time, the joint surface of the flange peels off each other, and the stress on the bolt increases significantly.
  • 4) The flatness (including inclination), ovality, foundation levelness and weld ut / mt of the foundation ring at the accident site were tested. The results show that the ovality of the base ring and the girth weld of the flange meet the requirements of the technical specifications. The test results of flatness, bolt hole diameter and foundation levelness do not meet the technical requirements and need to be further repaired.

Authors: Zhou Zhou, Yang Licheng, Liang Yong, Zeng Yi

Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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