Fundamentals for remote structural health monitoring of wind turbine blades - a preproject. Annex C. Fibre transducer for damage detection in adhesive layers of wind turbine blades : Annex C - Fibre transducer for damage detection in adhesive layers of wind turbine blades

This report (annex to the summary report "Grundlag for fjernovervågning af vindmøllevingers tilstand (Fase I: Forprojekt)", of a project partly supported by PSO-funding through Elkraft System, contract no. Bro-91.055, FU nr. 1102) describes the work carried out to design and test a fibre optic displacement transducer for detection of damage in adhesive layers of wind turbine blades.

It was chosen to base the transducer on the fibre optic micro-bend principle. The report contains the result of measurements and optical simulations of light transmittance through optical fibres with micro-bends and a suggestion for a micro-bend transducer design specifically suitable for detection of damage in adhesive layers between larger composite structures, as the shells in a wind turbine blade.

Such a damage will cause the joined parts to move slightly relative to each other, and the transducer is designed to change its optical transmittance in accordance to the displacement. Four transducers were manufactured on basis of a specific suggested design. The optical simulation showed that a sensitivity about 1.6 %/um could be expected.

Preliminary experimental investigations showed, that the sensitivity of a fibre optic micro-bend transducer would be in the range between 0.5%/um to 1 %/um depending on the number of bends on the fibre. A measurement on the final transducer showed that the sensitivity was 1.2 %/um. A large 50 % change in transmittance, that is easy to measure, is then obtained for displacements of 40 um.

The sensitivity is anticipated to be sufficient the application, because fully evolved cracks in the adhesive layer typically has a opening of 100 um – 200 um. The tests of the transducer ability to detect damage in adhesive layers both in the lab-test and in the ’full scale test’ were also fully satisfying, although the design of the transducer can be further improved.