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The results show that fibre fractures in the unidirectional (UD) load carrying fibre bundles initiate from off-axis cracks in the thin supporting backing fibre bundles. With an increasing number of fatigue load cycles, the UD fibre fractures progress gradually into the thickness direction of the UD fibre bundles, which eventually results in final fracture of the fibre composite. It is also found that the UD fibre fracture regions generally grow larger and initiate earlier at cross-over regions of the backing fibre bundles than at single backing fibre bundle regions. Furthermore, UD Fibre fractures are only observed to initiate at locations where the backing fibre bundles are ‘in contact’ with a UD fibre bundle. By observing the damage progression in 3D, it is also clear that the UD fibre fractures initiated and progressed as local 3D phenomena rather than being homogeneously distributed within the UD fibre bundles. Hence, the results show the importance of considering the problem in 3D.
The knowledge obtained on the fatigue damage mechanisms during the project can not only be used to improve the materials, but also sets the stage for X-ray CT based modelling. This is a step towards more realistic fatigue life-time modelling of fibre composites used for wind turbine blades, which will make it possible to push the design limits of wind turbine blades and thereby decrease the cost of energy for the wind energy production. In addition, the methods established during the PhD project can be applied to other problems, material systems, and load conditions in the future, which opens up for many new opportunities.
|Publisher||DTU Wind Energy|
|Number of pages||196|
|Publication status||Published - 2017|
|Series||DTU Wind Energy PhD|
Stang, H., Kleis, C., Mikkelsen, L. P., Sørensen, B. F., Toftegaard, H. L., Berggreen, C., Branner, K., Michel, A., Andreassen, M. J., Luczak, M., Chen, X., Bjørnbak-Hansen, J., Legarth, B. N., Waldbjørn, J. P., Bangaru, A. K., Moncy, A. & Quinlan, A.
07/11/2017 → …