Bladena tests blade with combined loading which confirms that D-Stiffener™ avoids cracks on the trailing edge panels

As the awareness about blade failures caused by combined loads increases, the need for changing blade design standards and certification has become a very discussed topic. As part of the EUDP RATZ project, Bladena did a full-scale test with combined loading at BLAEST on a 34m blade. Also, the structural impact of Bladena’s D-Stiffener™ was tested and the expected increase in buckling capacity of the trailing edge area was demonstrated.

New test design with combined loading

​​Traditionally, blades are tested under pure edge and flapwise loads in test centers. The intention of testing in these directions is to validate the design assumptions and to pass the required certification process. It is well known and proved that the simplified flap and edgewise tests does not represent the loads that the blade experiences during operation. Previous studies funded by EUDP e.g. the Full-scale Test SSP 34m blade at Risø or Experimental Blade Research Phase II at Risø, have shown a critical angle between the pure edge and pure flap area on blades, where the trailing edge buckles at rather low loads.

Traditional blade test

​​Full-scale test with combined loading
The test was loaded with a combined edge and flapwise loading. The load was selected with the angle that gives higher loads. The load was applied at four radial positions of the blade. 
The classical way of applying loads in a full-scale blade is by load clamps. Using clamps will restrict the blade to deform and thus prevent it to get the failure modes to evaluate if they occur during the test. ​

​The impact of D-Stiffeners™ 
15 D-Stiffeners™ were installed in the trailing edge region mid span of the blade. To measure the effect of D-Stiffeners™ 12 strain gauges were mounted back-to-back in the trailing edge. 

Two full-scale tests were performed. The one without D-Stiffeners™ showed a non-linear strain behavior at 48% loads. A non-linear strain behavior is well-known in the literature as a clear sign that the structures have buckled and  the risk for failure is likely if additional loads are applied. This was the case at Risø DTU where a similar blade was tested using the same test setup including load clamps. In the test with D-stiffeners™ installed there was no non-linear strain behavior, hence there was no buckling. ​

​Owners requirements to existing Blade design and certification standards
Owner requirements are a set of additional requirements to the existing certification requirements. They concern mainly wind turbine blade testing in order to minimize the issues appearing in the field after the certification process. Owners requirements is basically a set of calculations/measurements/upgradings of testing methods which, if performed, will address some of the failure modes not captured by today’s standards.
​Birgit Junker, blade specialist at E.ON, recognises the need for add-on criteria to the exiting certification requirements e.g. the full-scale test with combined loads: “The importance of changing the requirements for full-scale testing, so that they include combined loading, is essential to reduce the risk of blade damages in the field. As the size of the blades increases we come across larger blade damages if this is not addressed in the future, she says.”

Conclusion
A full-scale test with combined static loads was performed at the certified blade test center BLAEST as part of the EUDP project called RATZ. Measurements in the trailing edge of a 34 meter blade were taken to capture bending/buckling of the trailing edge panels. The load applied was a combination of flap and edgewise loading, which is known to be a more critical load than the traditional pure flap and edge. 
The implementation of the tests was proven to not be very demanding making it possible to implement in a short period of time.

Strain measurements showed a clear effect of the D-Stiffeners™. Based on these measurements it was concluded that the actual blade, which showed buckling at a load level at 48%, had no buckling with the D-Stiffeners™ installed when the test was stopped at approximate 60% loading.