10 Predicting Full-Field Strain on a Wind Turbine for Arbitrary Excitation. . . 109 Fig. 10.8 A photo of the test set-up showing arbitrary impacts made on the three blades using three impact hammers and the associated displacement vectors of optical targets due to the impacts at one instant in time method generated strain with an accuracy up to 5 microstrain even for the large field of view. Furthermore, the technique does not need the distributed pattern surface preparation that is required for DIC measurement. This paper has demonstrated the strong potential of the proposed technique for full-field strain extraction by using displacements of optical targets. 10.7 Conclusions In this paper, an algorithm to monitor the dynamic response of operating wind turbines is proposed and experimentally verified. This algorithm combines a non-contacting measurement approach and an expansion technique that can be used to extract full-field dynamic strain on rotating structures. It was shown that the proposed approach could predict dynamic strain with accuracy higher that digital image correlation. Furthermore, the proposed approach is not limited to the line of sight of the cameras and can predict strain on locations where no other sensor can be mounted. The results of the paper indicate the strong potential of the approach to be used to measure the dynamics of rotating structure and assess their structural health during operation. The predicted full-field response can also be used to promote the understanding of the loading conditions that rotating structures experience and improve the performance of these structures. Acknowledgements This research presented in this paper is partly supported by the National Science Foundation under Grant Number 1230884 (Achieving a Sustainable Energy Pathway for Wind Turbine Blade Manufacturing). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
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