Full scale blade testing

From a structural perspective, rotor blades are some of the most critical components of a wind turbine. Fraunhofer IWES is an independent institution which offers structural testing of large-scale rotor blades for both model validation and certification to ensure the blades are reliable. Since 2009, more than 30 blades up to 83 meters in length have been tested.

 

High degree of confidentiality

The testing methods used at Fraunhofer IWES are accredited in accordance with IEC 61400-23. The institute conducts customized and IEC standard blade tests on one of two separate test stands, located in neighboring halls. This ensures that customers have a high level of access to their blades during testing, while guaranteeing a high degree of confidentiality. The facilities of Fraunhofer IWES are located in close proximity to the harbor in Bremerhaven, providing easy access for delivery of larger rotor blades.

 

Static testing

During static testing, loads are applied using hydraulic cylinders and a pulley system. The cylinders are connected to a series of up to eight load frames mounted on the blade. Each load frame is custom designed and built for the specific geometry of the blade being tested. In order to limit forces exerted by the deadweight of the blade, the tests are performed perpendicular to the floor of the test hall. Loads of up to 500 kN can be applied at each load frame. In order to accomodate blades with large deflections under static loading, the larger of the two test blocks can be tilted during static testing. The tests are monitored with the aid of several hundred measuring signals at frequencies of up to 400 Hz, in combination with an optical measurement system that can record three-dimensional deflection of the blade.

 

30 getestete Rotorblätter am Fraunhofer IWES
© Fraunhofer IWES
Anlieferung Rotorblatt am Fraunhofer IWES
© Dieter Hergeth
Test Rotorblatt am Fraunhofer IWES
© Martina Buchholz

Cyclic testing

During uni-axial dynamic fatigue testing, the blade is loaded in the vertical and horizontal direction in turn. A flap-wise (vertical) or lead-lag (horizontal) excitation is applied by means of a servo-hydraulic cylinder coupled to the blade. The cylinder is programmed to excite the blade at its resonant frequency. Consequently, the force on the blade applied by the cylinders is minimized. This method of hydraulic excitation facilitates extremely precise testing.

 

Highly realistic load application

Developments for biaxial blade testing are a further focus of the work. This will lead to more realistic blade loading and also to a reduction in the
test time. The time saving is down to the fact that the two fatigue tests (in flapwise and edgewise direction) can be performed concurrently, not consecutively. The tested part of a blade is expanded, as – in addition to all areas which are loaded in the uni-axial tests – areas away from the main axes
are loaded as well. This results in improved reproduction of the loads and makes the blade test more realistic. Fraunhofer IWES is particularly focusing on the development of procedures which affect the blade´s resonance frequencies for testing. To this end, various procedures are considered in order to apply uni-axial loads.

 

Computational models and virtual testing

One important step here is the numerical design of the planned blade test. All parameters relevant to the test stand and blade are emulated and the fatigue test is simulated in advance. This has already been realized for the uni-axial tests, and the method has been further refined to achieve the target bending moments specified as accurately as possible. This aspect is also set to become more important for biaxial testing in order to make it possible in the first place and effective.

IWES is currently developing methods to include virtual testing as a supplementation to physical testing. Details of the material, manufacture, and design of the component or complete structure will be determined and today´s design models will be supplemented accordingly. The numerical representation of the test specimen and the test setup is validated experimentally, using critical failure modes, in order to evaluate the reliability and validity of the blade model.

At a glance: Innovative testing methods

Ever longer rotor blades demand an excellent understanding of their structure as the reserve load-bearing capacities of the lightweight construction materials need to be utilized particularly efficiently. Fraunhofer IWES is thus constantly working on improving the test methods it employs in order to achieve even more comprehensive results. At its site in Bremerhaven customers benefit from these innovative testing methods. The complementary testing stations allow complete investigation of all aspects of the rotor blade:

Prüfvarianten Rotorblatt am IWES
© Pascal Behning

Equipment

Test Rig I

  • Max. root diameter 4.0 m
  • Max. static root bending moment 50 MNm
  • Max. fatigue root bending moment ±30 MNm
  • Tilt angles 2.5° - 12.5°
  • Max. static tip deflection 17.5 m
  • Max. fatigue tip deflection ±9.5 m
 

Test Rig II

  • Max. root diameter 6.0 m
  • Max. static root bending moment 115 MNm
  • Max. fatigue root bending moment ±30 MNm
  • Tilt angles 0° - 20°
  • Max. static tip deflection 30 m
  • Max. fatigue tip deflection ± 11.0 m
 
 

Animation: Explore the test halls

ISO9001-certified in the areas of “product development up to the prototype stage, technology development and optimization, technology assessments and studies ” as well as “trials in demonstration centers ”.

Accredited due to DIN EN ISO / IEC 17025:2005: mechanical and thermal tests for determining the physical properties of glass fiber and composite materials; mechanical load measurements of wind turbines