Blades of offshore turbines already exceed 60 meters, and the trend goes even further. New and bigger blades put forward new demands in several areas. To meet these demands, Risø DTU has established an experimental research facility for full scale testing of blades.
Test of structural strength
One of the main purposes of the test facility is to examine the structural strength of wind turbine blades in different load combinations. The combination of flap wise, edgewise and tensional load gives a more realistic picture of the loads a blade will experience during its lifetime. Also the way of applying and controlling the loads is different from existing test techniques.
Towards new structural design and better certification methods
When we combine advanced measurements with reliable numerical simulations (FEM), we are able to identify the weak spot of the blade and suggest new structural designs that take care of this particular weakness.
During the last 3-5 years, seven reinforcements of blades (all patented) have been developed at Risø DTU. Some of the ideas still need more experimental validation.
Also the new test facility will enable us to evaluate and improve certification test methods.
The building can hold a blade at the size of around 30-40 meters. The blade shown in the picture is donated for research purposes by SSP Technology A/S. It is made of glass fiber reinforced epoxy. The original length is 34 meters with the tip cut off at 25 meters. The weight is 4.5 ton and the diameter at the root is 1.8 meter. The blade is mounted at the rig for edgewise test (The blade is pulled towards the floor).
The test facility makes it possible to repeat test under various condition validating different solutions conducted to confirm hypothesis as well as testing the influence of different ways to apply load.
The figure above presents different ways to apply load to the blade structure. Fig. a: Loading clamp used at Blaest test centre. Fig. b: The anchor plates allow transverse shear distortion of the profile, which is expected to be a realistic failure mechanism. Fig. c: A close-up of an anchor plate glued to the blade at 19m.
The measuring equipment is essential for the research on structural behaviour of wind turbine blades at Risø DTU. The Wind Energy Division benefits from the large number of equipment available both from other divisions at Risø DTU and also from other departments at DTU especially DTU Mek and DTU Byg. Through this collaboration is has been possible to acquire state of the art equipment for a system capable of handle a very large number of deformation and force sensors. To be able to use so many sensors efficiently and without mistakes an integrated system with dedicated software is used. The system is used to handle the setting on the amplifiers for each sensor and also collect the data from the experiments. It also displays the measurement results and performs post processing of the data real time during the test.
Mechanical displacement sensors are used for measurement of local and global deformations.
More than 350 strain gauges and around 40 displacements transducers are mounted at the blade.
Left: the measurement equipment mounted inside the blade. Right: Set of measuring sensors applied for an key cross-section of the examined blade.
DIC - Digital Image Correlation system
The Aramis system is an optical measurement system able to measure 3D displacement and surface strain of an object by image processing. It uses two CCD (Charge Couple Device) cameras to detect the difference in the measured pattern.
This picture shows the results of the optical DIC (Digital Image Correlation) measurements. A spectral pattern is painted on the blade so the system can measure the out of plane deformation.
Acoustic Emission is used as another Non Destructive Test method. The main purpose of this system is to detect when cracks start to progress, and especially to give an early warning when the cracks start to grow in an unstable manner. . In the AE method, the damage in the blade structure is recorded with microphones. When this is the case, it is time to stop the test and verify the local damage progression.
The Acoustic Emission measurements were performed in cooperation with the Division of Materials Research at Risø DTU.Read article (in Danish) at the newspaper Ingeniøren's homepage