The combination of loads, says NaREC, is a more realistic simulation of the loadings a blade experiences when mounted on a wind turbine than fatigue testing of one direction only. The existing widely used method for blade testing is to run the blade through one million cycles of bending along one axis, then rotate it by 90 degrees and move it one million times in the perpendicular axis. Although this discovers most problems with blades, it is a time consuming process and does not absolutely correspond to the conditions experienced by a wind turbine in real life situations. As a result it can lead to blades being over designed, adding unnecessary extra cost.
"Resonance fatigue testing has the benefit of reduced energy requirements to perform the test -- and by combining the flap and edge testing, the time to perform fatigue testing can be reduced by up to 40%," says NaREC. The company's resonant fatigue system is hydraulically powered and uses moving masses mounted on the blade with saddles. The position on the blade and the size of the masses are tuned to provide the test required. The test is monitored by a combination of a real-time vision tracking system that uses a high definition video camera to monitor blade deflections, and strain sensors mounted on the blade.
NaREC's belief is that its testing method, compared with more conventional approaches, will provide greater confidence to blade manufacturers and wind farm operators that their blades can withstand the rigours of at least 20 years of operation. NaREC, a not for profit company, was formed in 2002 and is one of five "centres of excellence" set up by the regional development agency for North East England, One Northeast. The agency has also granted £5 million to US wind turbine maker Clipper for development of a 7.5 MW turbine at Blyth for the offshore market (Windpower Monthly, October 2007).