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Engineers conducting NDT tests at a wind turbine power station. Image Credits: William Perugini/shutterstock.com
Whether it is to reduce the dependence on fossil fuels, cut carbon emissions or simply seize on a growing industry, consumers, businesses, communities and investors have all been embracing wind power as a legitimate next-generation source of electricity.
According to the American Wind Energy Association (AWEA), during 2012 enough wind turbines were built to generate over 13,000 megawatts, representing a 28 percent increase in annual growth. That same year, wind turbines were the primary source of new US energy and comprised 3.5 percent of the total American energy mix.
Making Turbine Blades
There were at least 550 wind-related manufacturing facilities in 2012, according to the AWEA, and many of these facilities construct wind turbine blades, the most visible and important part of a wind turbine.
Improvements have led to the construction of longer and longer blades made with stronger and lighter materials. The longer and more rigid a blade is, the more electricity it can harvest from the wind. Blades are also designed to be as light and resistant to damage as possible, as strong winds and lightning strikes can quickly compromise a blade’s structural integrity.
Over the years, the blade-making process has also been refined through a series of new methods and technological advances. These refinements were designed not only to reduce costs, but also to eliminate any defects that may negatively affect the mechanical properties of the blades before they are actually put in service.
During both the manufacturing process and routine maintenance, checking for defects and faults is crucial to extending the life of a turbine blade and engineers have needed to develop systems to inspect blades for even the smallest of problems.
Renewitt: An EU Effort to Non-Invasively Test Turbine Blades
In recent years, engineers have developed methods capable of testing blades not only during the manufacturing process, but also in the field. These methods must use non-destructive testing (NDT) techniques, as a service technician will not be able to open a blade during operation to check for structural damage.
Around a decade ago, the European Union launched the ‘Development of new and novel automated inspection technology for glass reinforced plastic wind turbine blades’ (Renewitt) project, an endeavor designed to study solutions for inspecting wind turbine blades. The EU said the project was specifically developed to maximize the efficiency of European blade manufacturers and increase their competitiveness.
The project was successfully able to investigate several different methods for in-service blade inspection. One such inspection method is digital radiography. Digital radiography was designed to replace film-based X-ray technology and it produces fast computer images that can be sent, modified or stored in a similar way to any another type of digital media.
The EU project also investigated a technique called laser shearography, which provides information about internal defects in non-homogenous materials like turbine blades. In the shearography procedure, a strain is placed on the test surface and if underlying problems are present, the surface will deform at these locations. These deformations can then be detected using laser light.
Another method investigated by the EU team was pulsed thermography. This technique requires the application of an energy source on a surface and subsequently measuring the resulting thermal contrast between background material and the area of interest. The heating pulse is typically very quick, from milliseconds to a few seconds, and only increases the temperature of the material a few degrees, making it highly desirable as a non-destructive technique.
The Renewitt team also investigated two different kinds of ultrasound technology. The EU announced the Renewitt researchers were successfully able to combine all of the methods into a system that could be used in both manufacturing and maintenance processes. The composite process is software controlled and incorporated into a robotic scanner. The scanner can reach all areas of the blade, producing a comprehensive picture of its integrity.
Focusing on Ultrasound
With Renewitt being a European endeavor, some manufacturers, particularly non-European, don’t rely on its composite system. Perhaps influenced by its use in the aviation industry, some in-service inspections of turbine blades utilize ultrasound to locate defects.
This type of ultrasonic technology is founded on the basic concept of wave physics. When a transducer generates a high frequency sound wave into a fiberglass or composite material, the wave will move in a line directly perpendicular to the surface until it runs into a boundary, such as a far wall or a different material. When this occurs, the sound wave will bounce back in a noticeable way. This is detected by a receiver and allows an ultrasound device to evaluate the round-trip time of the pulse.
Since sound waves will reflect from voids or cracks in a material, shifts in the ultrasound echo pattern reveal changes in the internal structure of a material being tested. In evaluating the fiberglass and composite materials of wind turbine blades, the device normally looks for the existence of echoes within certain parameters to signify the defective interior of the test piece. While the non-homogeneous nature of a blade often scatter noise reflections, cracks around the diameter of the sound beam normally return strong and specific local indications that can be identified by a trained operator.
Inspectors typically choose the frequency of the sound wave being used and the probe type based on the material being tested. To search for smaller internal defects, an inspector might use a smaller probe and a higher frequency. If an inspector is looking for a larger defect, he or she might use a larger probe and lower sonic frequency. Incidentally, lower frequency waves can penetrate deeper into a blade.
A Range of NDT Solutions are Available for In-Service Inspections
From ultrasound to digital radiography, the wind turbine industry has a range of options available for those looking to inspect turbine blades after they are installed. These methodologies are constantly evolving and the goal is to try and keep pace with the evolution of materials used to make these blades.
Sources and Further Reading
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