Soldering fiber Bragg grating sensors for strain measurement

When measuring strain with a FBG sensor fixed to a structure by an adhesive, one will notice, that only axial strain is monitored. This is due to the low Young's modulus of both standard coating materials and adhesives. Metal coated fibers with high Young's modulus have become available recently, which may be surface bonded by soldering. Motivated by this, finite element simulations with different coating materials and adhesives were carried out. Increasing the Young's modulus of coating and adhesive in the simulation shows a difference in transverse strain of around 20% for the two cross sectional fiber axes. On this basis a process to substitute standard fiber coatings with a copper coating is described. Satisfying results were obtained using a copper electroplating process. Especially its simple experimental access is noticeable. A sensor modified in the electroplating process is soldered to a cantilever beam and a quasi-static strain measurement is performed. Soldering the modified fiber sensors changes the spectral response of the FBG. Towards shorter wavelength which is a result of the strains induced by cooling.