Strain measurements and damage detection by fibre optics sensors in large structures

By means of a novel and robust automated technique based on strain field pattern recognition, a defect detectability study was performed for two 13.5-metre-long wind turbine blade prototypes under different load scenarios. Fibre optic sensors (FOS) were embedded into the blades (fully made of composite materials] during the manufacturing process, which consisted in an experimental RTM light procedure. Two different FOS technologies were used: fibre Bragg gratings (FBGs) and distributed sensing by means of an optical backscatter reflectometer (OBR). Some challenges associated with the integration of the sensor within the structure had to be faced in order to guarantee the appropriate operation of the sensors network during the service life of the blades (in this case, during the experiments). Several static tests were conducted, including a test campaign in a wind turbine test laboratory with known artificial damages induced into the structure, and the sensitivity of the technique was evaluated. The results showed that each damage could be detected using both sensing techniques.