Guided wave-based approach for health monitoring of composite structures; Application to wind turbine blades

Doctoral thesis, 2018

The use of composite materials has increased in manufacturing of large structures. These structures can be subjected to different types of defects, for which detection has always been a challenge. Wind turbine blades are a perfect example of these large structures that can be exposed to different defects such as delamination, debonding or ice accumulation during the turbine operation. Tools based on guided waves can be used for health monitoring of such structures. The special characteristics of guided wave-based tools such as mobility, rapidity and cost efficiency make them interesting to be used for non-destructive testing and structural health monitoring. Besides these general characteristics, what makes the guided waves a good choice for health monitoring of large structures is that they have the ability to propagate long distances. The size of these structures and their high damping require the use of low frequency guided waves, which are in focus of the present project. The ultimate goal of this work is to investigate the application of guided waves for health monitoring of large composite structures with focus on wind turbine blades. The investigation has been carried out using calculations of dispersion relations and numerical simulations of guided wave propagation in composite materials. In order to make the numerical model computationally efficient, the possibility of using material homogenization methods has been investigated for guided waves propagating in the low range of frequencies. The homogenization methods are aimed at being used for both composite laminates and delamination regions. A similar study is performed on sandwich structures since they are used as parts of large structures. In order to overcome the effects of high attenuation properties of the structures a design optimization method is developed and proposed to create special arrays of transducers for generation of guided waves. The main aim of the designed arrays is to evenly distribute the wave energy in the area of interest in a domain. The developed methods and conclusions have been used to study the propagation of guided waves in wind turbine blades with focus on detection of defects and accumulated ice. The research has been supported by experimental studies for different part of the project. Results show that the developed numerical methods can be used to simulate guided wave propagation in composite laminates. Moreover, the specifically designed arrays of transducers are able to evenly distribute the wave energy in large structures. Finally, it is observed that guided waves can be used to develop multifunctional smart systems for defect and ice detection on wind turbine blades.