Monitoring Strain Concentration at Fixtures of Wind Turbine Blades Using Fibre Bragg Grating Sensors

Fibre Bragg grating (FBG) sensing is proposed to measure the strain induced at fixture points during static load edgewise tests of wind turbine blades. An FBG sensor is placed on the carbon fibre composite upper spar cap surface covered by a fixture device, where traditional sensors (e.g. strain gauges) cannot be usually positioned due to the presence of metal parts in the fixtures. The strain response of a full scale 56.8 m-long blade is studied by comparing the measured FBG strain signal (under the fixture) with the strain measured by two adjacent strain gauges when applying different loads, ranging from 20% up to 100% of the maximum blade design load. Experimental results demonstrate that FBG strain values do not fall on the straight line defined by the two measured gauge strains when the applied load increases from 40% up to 100% in edgewise tests. It is found out that the deviation of the measured FBG strain with respect to the expected value increases with the applied load indicating the concentration of bending additional strain at the fixture point. This phenomenon may result in damages of the wind blade under test. It is then expected that the FBG monitoring approach here proposed could be of great help in the design of more suitable fixtures and in the optimisation of a loading strategy for static load tests of big wind blades.