Multi-life-stage monitoring system based on fibre bragg grating sensors for more reliable wind turbine rotor blades: Experimental and numerical analysis of deformation and failure in composite materials

One of today’s greatest global challenges is the need for clean, reliable, and renewable sources of energy, and wind energy has a key role in this process. However, in order to compete with other less ”green” sources of energy the cost to produce wind made electricity needs to be reduced. One way to achieve this is by improving the reliability of wind turbine components and optimising operation and maintenance strategies.

This PhD project is part of the European research project MareWint, where the main objective is to develop an innovative approach for coupled multi physics cosimulation, testing, design and optimisation of offshore wind turbines. The MareWint main scientific objective is to optimise the design of offshore wind turbines, maximise reliability, and minimise maintenance costs.

Integrated within the innovative rotor blades work-package, this PhD project is focused on damage analysis and structural health monitoring of wind turbine blades. The work presented sets the required framework to develop a monitoring system based on fibre Bragg gratings (FBG), which can be applied to the different life stages of a wind turbine blade.

In this concept, the different measured physical parameters are used to improve the design process, and the implemented sensor are used to control the manufacturing and operation stage of a wind turbine rotor blade. The FBG sensors measurement principle is analysed from a multi-life-stage (design, material testing, manufacturing, and operation) perspective, and supported/validated by numerical models, software tools, signal post-processing, and experimental validation.

The damage in the wind turbine rotor blade is analysed from a material perspective (fibre reinforced polymers) and used as a design property, meaning that damage is accepted in an operational wind turbine as long as it is monitored. Thus, a novel crack/damage detection method using FBG sensors is presented, and software/tools are developed for signal simulation and post-processing.

The first part of the thesis is an introduction to the multi-life-stage monitoring system based on FBG sensors and the damage tolerant design of fibre reinforced materials, where the different theory and numerical models used are presented. The second part of the thesis is a compilation of scientific journal papers, in which the use of FBG sensors to monitor the different life-stages of the wind turbine rotor blade is described in more detail.

In Paper P1, a methodology for reliable design and maintenance of wind turbine rotor blades based on a damage tolerance and structural health monitoring approach is presented. Paper P2 presents a novel method to obtain independent strain and temperature measurements using embedded FBG sensors in polymeric tensile tests.

In paper P3, a novel method for assessing crack growth in fibre reinforced polymer or structural adhesive bonded structures by combining conventional measured parameters with parameters associated with measurement errors is presented. Paper P4 presents a FBG signal post-processing tool. In paper P5, a software development tool to simulate the FBG signal from a finite element method model is described.

Paper P6 fits within the manufacturing stage, describing a residual strain measurement solution based on FBG sensors. In paper P7, the fracture process zone length in double cantilever beam specimens is analysed analytically and numerically.