A wind turbine is essentially held together with bolts, the reliability of a joint possibly being compromised by bolt relaxation, vibration, fatigue, and corrosion. Because of the nature of wind turbines, they must be designed to tolerate variable and extreme vibration levels. Two studs, located on the blade to hub interface, have failed. The initial investigation involved verifying that the material is within the design specification. Having confirmed this, a root cause investigation was launched.
A wind turbine uses moving air to turn blades, which spin a shaft that is connected to a generator to make electricity. There are basically two types of wind turbine design; horizontal and vertical axis. The most common type is the horizontal axis turbine. The name comes from the fact that the shaft of the turbine is horizontal to the ground. All the components (blades, shaft and generator) are at the top of the tower, and normally face into the wind. Inside the nacelle (the head) is a wind vane (to determine wind direction), anemometer (to determine wind speed) and controller. When the wind changes direction a yaw motor turns the nacelle so that the blades are facing into the wind. In case of extreme winds, the turbine has a braking system that can slow the shaft speed.
There are many different types of electrical and optical sensors used in wind turbines. In general, they detect, monitor, and communicate information about parameters. These parameters include changes in the distance between two components, levels of vibration that, if excessive, can cause major damage and changes in temperature, pressure, and mechanical stresses.
The engineers at One Eighty received three studs – the two failed studs as well as an additional un-used stud. A visual inspection and calibrated measurements to confirm stud dimensions.
- Mechanical testing in the form of impact, hardness and tensile testing. Optical Emission Spectrography to determine chemical composition Optical Light Microscopy to investigate the microstructure
- Stereo Microscopy for viewing and characterising fracture face defects
- Comparative analysis of stud dimensions between failed and unused stud.
- Comparative analysis of mechanical properties between failed studs and unused stud as well as the standard specification.
- Comparative analysis of chemical properties between failed studs and unused stud as well as the standard specification.
- Comparative analysis of the microstructure between failed studs and unused stud as well as the standard specification
- Characterising fracture face defects and morphology on both failed studs (macroscopic level using Stereo Microscope).
The material is within specification, with no material degradation visible. A summary of the results follows:
- Chemical results show that the material is within specification for both failed studs as well as the unused stud.
- Hardness, tensile and impact values on all studs indicate that the material conforms to the mechanical specification.
- The microstructure consists of tempered martensite, which is normal for this application.
Ratchet marks, beach marks and striations are visible on both samples and indicate fatigue. No evidence of fast brittle or yield in ductility was observed. It is therefore concluded that the failure mechanism was due to fatigue.
It is furthermore noted that both samples show low nominal stress characteristics, with medium to high stress concentration. One sample failed because of rotational bending, while the other sample failed because of unidirectional bending or repeated tension-compression cycles.
The nature of wind turbine operation is such that very little manual control occurs. Safety systems like sensors and overspeed protection devices limit the turbine to predefined operating parameters. Great pains are taken during the initial set-up phase to design a wind turbine with adequate protection and sensor systems, specific to the location. Operational and control deviations cannot be ruled out, due to the variability of the environment.
The fatigue failure therefore could only have been initiated by a system failure or assembly failure that enabled a cyclic load greater than the fatigue endurance limit on the bolts.
Stud tensioning is a complex science that requires an experienced operator with a keen understanding of the process. It is recommended that repeated spot checks occur during installation to verify that best practices are being followed.
A re-evaluation of the design and operating conditions should be undertaken if repeated failures continue to occur.