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How to choose non-destructive testing methods for wind turbine blade inspection
Release time：2023-09-26
Wind energy is a green and renewable energy source with good development prospects.
China has enormous potential for developing wind energy and abundant resources, with a total exploitable capacity of approximately 1000-1500 GW. It can be seen that wind power has the potential to become an important component of the future energy structure.
Therefore, the development of wind power generation has also received much attention, and wind turbine blades are an important component of wind turbines, usually made of glass fiber composite materials. Due to the complexity of their manufacturing process, defects are inevitable during the molding process;
In addition, due to the harsh working environment and the complex and variable working conditions, varying degrees of damage can also occur during operation.
Damage and Defect Analysis of Fan Blades
The causes of defects in fan blades are multifaceted, and typical defects such as pores, layering, and inclusions may occur during the production and manufacturing process.
Pore defects are mainly caused by factors such as poor resin and fiber infiltration, incomplete air extrusion, etc; Layered defects are mainly due to insufficient resin dosage, secondary molding, etc; The occurrence of inclusion defects is mainly due to the mixing of foreign objects during the processing.
In addition, during transportation and installation, the blades have a certain degree of elasticity due to their large size and self weight.
Therefore, it is necessary to do a good job in protecting the blades to prevent internal damage.
It is worth noting that the blades of the fan may also experience varying degrees of damage during operation, mainly in the form of cracks, fractures, and matrix aging. External impact is the main cause of cracks, and fractures are usually caused by the accumulation of defect damage. Under normal operating conditions, the blades of the fan will not suddenly fracture, while matrix aging is due to the fan blades working under harsh conditions such as sand, rain, and salt spray corrosion for a long time.
Comparison and Analysis of the Selection of Non destructive Testing Methods
1. X-ray testing technology
For wind turbine blades, researchers such as He Jie have verified through experiments that X-ray technology is a good method for detecting volumetric defects such as pores and inclusions in wind turbine blades. It can detect cracks perpendicular to the surface of the blades, and has certain detection capabilities for resin and fiber aggregation. It can also measure defects such as fiber bending in small thickness wind turbine blade layers, However, it is not sensitive to common delamination defects and cracks parallel to the surface of wind turbine blades. In the literature, defects such as pores and inclusions have been detected, and the existence of defects can be observed from the experimental results, which can meet the requirements of pre factory inspection of blades and enable qualitative analysis.
Researchers at the National Defense Key Laboratory of Electronic Testing at Central North University combine X-ray with modern testing theory. In the digital image processing stage, they use wavelet transform and image decomposition theory to decompose an image into components with different sizes, positions, and directions, changing the size of certain parameters in the wavelet transform domain, and identifying internal defects in X-ray images in real time.
Researchers such as Zhu Province have verified the defect detection situation under different process conditions through experiments, and have shown that process management for radiographic testing is very necessary.
In summary, under experimental conditions, X-ray technology can achieve defect detection of fan blades.
For in-service fan blades, due to the influence of on-site factors and height limitations, it is difficult to achieve on-site inspection using X-ray detection methods. However, it has a certain detection ability for volume defects of fan blades. Due to the limitations of blade size, this method has not yet been widely applied to full size inspection of blades.
2. Ultrasonic testing technology
Ultrasonic testing technology is more suitable for the inspection of fan blades after forming. At this time, the fan blades have not yet been installed, and the purpose of testing is to ensure the factory quality of the fan blades;
The use of ultrasonic testing technology can effectively detect thickness changes and display hidden faults of the product, such as layering, inclusions, pores, lack of adhesive, and weak bonding at the bonding point, thereby greatly reducing the risk of blade failure.
Due to the obvious anisotropy of composite material structures, they can have effects of reflection, scattering, and attenuation, making the propagation of ultrasonic waves in composite multi-layer structures more complex. The main ultrasonic testing methods for fan blade structures include pulse echo method and air coupled ultrasonic guided wave method.
Due to the long detection cycle of this method, different specifications of probes need to be used for different types of defects, and coupling agents need to be used during the detection process, which is also the limitation.
Therefore, for real-time dynamic monitoring, ultrasonic testing technology is difficult to achieve, but it can perform static testing before leaving the factory. Reflection pulses will be formed in the areas where defects exist, so the location of defects can be determined.
Acoustic emission detection technology can dynamically monitor the initiation and propagation of cracks, thereby effectively detecting the overall quality level of fan blade structures, evaluating the actual harm level of defects, and preventing accidents from occurring.
During the detection process, the received signal is spontaneously generated by defects under stress. However, in practical applications, acoustic emission is very sensitive to environmental factors, which can cause interference to the monitoring system and affect the accuracy of detection. Therefore, it is difficult to quantitatively analyze defects. However, it can provide dynamic information of defects under stress, which has certain advantages for life assessment and can be used for safety evaluation of blades.
Compared with ultrasonic method, this method has no advantage in detecting the quality of static blades; However, due to its low requirement for the proximity of the inspected parts, this technology is more suitable for real-time monitoring of in-service fan blades. By adopting a multi sensor long-distance arrangement, the acoustic emission signals generated by the blades during operation can be received. Through post-processing, dynamic information of the damaged parts can be obtained.
The main reason for using this method to monitor blades is that they will be subjected to external forces during operation, resulting in stress concentration. Defects under external forces will spontaneously generate signals, which can determine the location of defects.
4. Fiber optic sensor technology
Embedding a fiber optic sensor array at the critical position of the fan blade to detect the changes in internal stress and strain during the dynamic process of processing, forming, and service. Real time monitoring of deformation and cracks caused by external forces, fatigue, etc. can be achieved to monitor the condition and evaluate the damage of the fan blade.
Fiber optic has the characteristics of small size, light weight, high sensitivity, and resistance to electromagnetic interference. It is not only a sensor, but also capable of transmitting optical signals, making it easy to bury in components without affecting the overall strength of the components. Moreover, fiber optic can continuously and real-time detect changes in internal structural parameters, and can detect internal damage to materials and structures caused by various reasons.
Therefore, this method has good development prospects, but due to the performance stability and price issues of fiber optic sensors, its application is greatly limited.
5. Infrared non-destructive testing technology
Many domestic studies and literature research have shown that infrared thermal imaging detection technology can detect several typical defects in glass fiber blades.
Moreover, the larger the defect size and shallower the depth, the greater the maximum surface temperature difference formed during the cooling process, making it easier to detect using an infrared thermal imager. For glass fiber reinforced composite materials used in manufacturing wind turbine blades, thermal imaging technology is a relatively applicable non-destructive testing method, especially suitable for common delamination and adhesive type defects.
Compared with other detection methods, this method has the characteristics of non-contact, large area remote detection, simple operation, and easy real-time observation, making it more suitable for on-site detection of fan blades;
However, due to the height limitation of the tower, there are certain limitations in on-site inspection. Considering factors such as light exposure and small temperature difference on the blade surface, this can have adverse effects on the inspection results, making it difficult to detect and qualitatively analyze defects.
Therefore, further development and research are needed in the application of this method, which has significant research significance.
China has enormous potential for developing wind energy and abundant resources, with a total exploitable capacity of approximately 1000-1500 GW. It can be seen that wind power has the potential to become an important component of the future energy structure.
Therefore, the development of wind power generation has also received much attention, and wind turbine blades are an important component of wind turbines, usually made of glass fiber composite materials. Due to the complexity of their manufacturing process, defects are inevitable during the molding process;
In addition, due to the harsh working environment and the complex and variable working conditions, varying degrees of damage can also occur during operation.
Damage and Defect Analysis of Fan Blades
The causes of defects in fan blades are multifaceted, and typical defects such as pores, layering, and inclusions may occur during the production and manufacturing process.
Pore defects are mainly caused by factors such as poor resin and fiber infiltration, incomplete air extrusion, etc; Layered defects are mainly due to insufficient resin dosage, secondary molding, etc; The occurrence of inclusion defects is mainly due to the mixing of foreign objects during the processing.
In addition, during transportation and installation, the blades have a certain degree of elasticity due to their large size and self weight.
Therefore, it is necessary to do a good job in protecting the blades to prevent internal damage.
It is worth noting that the blades of the fan may also experience varying degrees of damage during operation, mainly in the form of cracks, fractures, and matrix aging. External impact is the main cause of cracks, and fractures are usually caused by the accumulation of defect damage. Under normal operating conditions, the blades of the fan will not suddenly fracture, while matrix aging is due to the fan blades working under harsh conditions such as sand, rain, and salt spray corrosion for a long time.
Comparison and Analysis of the Selection of Non destructive Testing Methods
1. X-ray testing technology
For wind turbine blades, researchers such as He Jie have verified through experiments that X-ray technology is a good method for detecting volumetric defects such as pores and inclusions in wind turbine blades. It can detect cracks perpendicular to the surface of the blades, and has certain detection capabilities for resin and fiber aggregation. It can also measure defects such as fiber bending in small thickness wind turbine blade layers, However, it is not sensitive to common delamination defects and cracks parallel to the surface of wind turbine blades. In the literature, defects such as pores and inclusions have been detected, and the existence of defects can be observed from the experimental results, which can meet the requirements of pre factory inspection of blades and enable qualitative analysis.
Researchers at the National Defense Key Laboratory of Electronic Testing at Central North University combine X-ray with modern testing theory. In the digital image processing stage, they use wavelet transform and image decomposition theory to decompose an image into components with different sizes, positions, and directions, changing the size of certain parameters in the wavelet transform domain, and identifying internal defects in X-ray images in real time.
Researchers such as Zhu Province have verified the defect detection situation under different process conditions through experiments, and have shown that process management for radiographic testing is very necessary.
In summary, under experimental conditions, X-ray technology can achieve defect detection of fan blades.
For in-service fan blades, due to the influence of on-site factors and height limitations, it is difficult to achieve on-site inspection using X-ray detection methods. However, it has a certain detection ability for volume defects of fan blades. Due to the limitations of blade size, this method has not yet been widely applied to full size inspection of blades.
2. Ultrasonic testing technology
Ultrasonic testing technology is more suitable for the inspection of fan blades after forming. At this time, the fan blades have not yet been installed, and the purpose of testing is to ensure the factory quality of the fan blades;
The use of ultrasonic testing technology can effectively detect thickness changes and display hidden faults of the product, such as layering, inclusions, pores, lack of adhesive, and weak bonding at the bonding point, thereby greatly reducing the risk of blade failure.
Due to the obvious anisotropy of composite material structures, they can have effects of reflection, scattering, and attenuation, making the propagation of ultrasonic waves in composite multi-layer structures more complex. The main ultrasonic testing methods for fan blade structures include pulse echo method and air coupled ultrasonic guided wave method.
Due to the long detection cycle of this method, different specifications of probes need to be used for different types of defects, and coupling agents need to be used during the detection process, which is also the limitation.
Therefore, for real-time dynamic monitoring, ultrasonic testing technology is difficult to achieve, but it can perform static testing before leaving the factory. Reflection pulses will be formed in the areas where defects exist, so the location of defects can be determined.
3. Acoustic emission detection technology
Acoustic emission detection technology can dynamically monitor the initiation and propagation of cracks, thereby effectively detecting the overall quality level of fan blade structures, evaluating the actual harm level of defects, and preventing accidents from occurring.
During the detection process, the received signal is spontaneously generated by defects under stress. However, in practical applications, acoustic emission is very sensitive to environmental factors, which can cause interference to the monitoring system and affect the accuracy of detection. Therefore, it is difficult to quantitatively analyze defects. However, it can provide dynamic information of defects under stress, which has certain advantages for life assessment and can be used for safety evaluation of blades.
Compared with ultrasonic method, this method has no advantage in detecting the quality of static blades; However, due to its low requirement for the proximity of the inspected parts, this technology is more suitable for real-time monitoring of in-service fan blades. By adopting a multi sensor long-distance arrangement, the acoustic emission signals generated by the blades during operation can be received. Through post-processing, dynamic information of the damaged parts can be obtained.
The main reason for using this method to monitor blades is that they will be subjected to external forces during operation, resulting in stress concentration. Defects under external forces will spontaneously generate signals, which can determine the location of defects.
4. Fiber optic sensor technology
Embedding a fiber optic sensor array at the critical position of the fan blade to detect the changes in internal stress and strain during the dynamic process of processing, forming, and service. Real time monitoring of deformation and cracks caused by external forces, fatigue, etc. can be achieved to monitor the condition and evaluate the damage of the fan blade.
Fiber optic has the characteristics of small size, light weight, high sensitivity, and resistance to electromagnetic interference. It is not only a sensor, but also capable of transmitting optical signals, making it easy to bury in components without affecting the overall strength of the components. Moreover, fiber optic can continuously and real-time detect changes in internal structural parameters, and can detect internal damage to materials and structures caused by various reasons.
Therefore, this method has good development prospects, but due to the performance stability and price issues of fiber optic sensors, its application is greatly limited.
5. Infrared non-destructive testing technology
Many domestic studies and literature research have shown that infrared thermal imaging detection technology can detect several typical defects in glass fiber blades.
Moreover, the larger the defect size and shallower the depth, the greater the maximum surface temperature difference formed during the cooling process, making it easier to detect using an infrared thermal imager. For glass fiber reinforced composite materials used in manufacturing wind turbine blades, thermal imaging technology is a relatively applicable non-destructive testing method, especially suitable for common delamination and adhesive type defects.
Compared with other detection methods, this method has the characteristics of non-contact, large area remote detection, simple operation, and easy real-time observation, making it more suitable for on-site detection of fan blades;
However, due to the height limitation of the tower, there are certain limitations in on-site inspection. Considering factors such as light exposure and small temperature difference on the blade surface, this can have adverse effects on the inspection results, making it difficult to detect and qualitatively analyze defects.
Therefore, further development and research are needed in the application of this method, which has significant research significance.
