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Significance
Composite wind turbine blades undergo various structural failures which affect their performance. In particular, trailing edge failures have been widely reported in rotor blades. As such, several experimental and numerical based studies have been conducted to explore the mechanisms and causes of the training edge failures.
In a recently published literature, buckling effects due to the ultimate static loading has been identified as the main cause of the failures. Additionally, subcomponent testing has also been carried out to examine the structural behaviors of composite blades. In addition to the full-scale blade tests, this is also an important element in the certification of wind turbine blades.
Unlike the full-scale tests, subcomponents can be used to effectively evaluate the structural integrity of different trailing edges sections thus providing more understanding of the failure mechanisms. However, despite the available knowledge on the structural response of the trailing edge sections in the subcomponent tests, the entire failure sequence in the trailing edge section has not been fully explored. This should further take into account the buckling response in both the pre-peak and post-peak regimes. Considering the significance of the post-peak regime especially in determining the failure modes and characteristics, investigation of the post-peak response of the trailing edge sections will enable understanding of the failure sequences and the underlying mechanisms which facilitate damage-tolerant blade designs.
On the other hand, predicting the failure modes and sequences have remained a challenging task due to the presence of multiple structural nonlinearities induced by the changes in the boundary and load conditions. Even though finite element models have been used to predict the buckling response and stress-strain distribution, it is not suitable for predicting highly nonlinear structural behaviors associated with failures of different materials.
Recently, scientists at Technical University of Denmark: Dr. Xiao Chen, Mr. Peter Berring, Mr. Steen Hjelm, Dr. Kim Branner and Mr. Sergei Semenov explored the structural failure of a trailing edge section to understand the failure sequence and the buckling modes taking into account the multiple structural nonlinearities. Specifically, an advanced finite element model was developed to investigate the entire failure sequence of a blade subcomponent specimen. Furthermore, Digital Image Correlation (DIC) was used to measure strain distributions and buckling deformation during the loading history.
The developed finite element modeling technique exhibited good capability for predicting strain distributions, buckling deformation and post-peak responses that constitute the key influencers of the failure modes and characteristics in trailing edge sections. Additionally, both the buckling-driven failure phenomenon and the surface contact of sandwich panels were found to significantly contribute to the failure process of the trailing edge sections. For instance, composite and adhesive materials failed in the post-peak regime while foam materials failed in the pre-peak regime.
In summary, Technical University of Denmark researchers presented a detailed study on the failure of trailing edge sections by evaluating the critical loading conditions. In general, the authors pointed out that failure behavior in different blades will vary depending on the material properties, geometric profiles, and loading conditions. Altogether, the presented numerical method will pave the way for advancing structural analysis and evaluation of composite rotor blades. “By combining advanced subcomponent testing and high-fidelity numerical modeling, we can reshape the conventional testing pyramid of composite rotor blades.” Dr. Xiao Chen said in a statement to Advances in Engineering, “Eventually, this allows us to develop more reliable and cost-effective composite structures in a highly efficient way despite the ever-increasing sizes of rotor blades.” The work is currently published in the journal, Composite Structures.
Reference
Chen, X., Berring, P., Madsen, S., Branner, K., & Semenov, S. (2019). Understanding progressive failure mechanisms of a wind turbine blade trailing edge section through subcomponent tests and nonlinear FE analysis. Composite Structures, 214, 422-438.
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