Researchers at the Technical University in Denmark (DTU) are currently involved in testing wind turbine blades at a test hall on Risø Campus to determine why a blade buckles under the constant pressure from the wind and gravity, or why it collapses under the extreme load applied by sudden and powerful gusts of wind.
The researchers use heavy equipment to pull on a blade until it breaks with a loud crash, DTU says. In the course of the past four years, three blades have been subjected to this harsh treatment, which has provided essential knowledge for blade manufacturers and can lead to better, cheaper and safer design, DTU says.
A blade is composed of several thousand parts glued together, with the glue joints often being the weak points of the blade. However, there may also be other reasons for blades buckling or breaking off.
When running a destruction test, the turbine blade is strapped onto a test bench, where it can be twisted and turned at various angles to simulate the power of the wind combined with gravity. A static load exerts downward pressure at four selected points along the blade, distorting the blade several metres from its standard position until it finally succumbs to the pressure and fragments. The entire process is documented with both video cameras and a variety of specially developed measuring instruments.
Before the huge traction machines are attached to the blades, researchers perform a series of calculations intended to identify the weakest spot of the blade in question.
“Wind turbine blades work just like the wings of an aircraft in that they are subjected to suction on the upper side and pressure on the underside when the wind strikes the blade. And what causes an aircraft to take off, causes the rotor on a wind turbine to turn. This load is called the wind load. The second load to which the blade is subjected is gravity. If the load exceeds the blade strength, it breaks—and it is precisely the combination of the two loads on which we have made our calculations,” said Senior Researcher Kim Branner, DTU Wind Energy, the man behind the four-year research project ‘Experimental Blade Research.’
Images: DTU