Strain and vibration measurements by FBG sensors for engineering applications

Fiber optic sensors based on the fiber Bragg grating (FBG) technology [1] is widely used to produce quasi-distributed monitoring systems to measure mechanical parameters for Structural Health Monitoring (SHM) of civil engineering structures [2]. FBG sensors are produced in the core of the optical fiber, as a short segment of fiber where a diffraction grating is produced (5-10 mm). The principle of operation of the FBGs is based on the diffraction occurring at the grating: if broadband light propagates, a quasi-monochromatic counter propagating light originates. The wavelength of the diffracted light depends on the value of the refraction index of the core along the grating which in turn is affected by both temperature and strain. Thus, by measuring the wavelength change of the counter propagating light, the change of strain and the temperature can be worked out. The PREFOS Project [3] aims to develop a novel procedure to apply the use of FBGs to monitor the prestressed strands of civil engineering prefabricated components. In this paper, we report results of an experimental campaign intended to measure the sensitivity of saddle-like sensors to induced vibration and mechanical strain. Measurements were done performing static and dynamic tests on a steel strand equipped with 3 sensors. The tests have been performed applying different tensioning to the steel strand. Static measurements were worked out applying a stepwise tensioning increase. Dynamic tests were worked out at each stepped tension level, inducing vibration by both sharp hammer impact and release of hanged weight. The paper is organized as follows, in section 1 a general description of the testbed structure used for the experiments will be provided. In section 2 the results of static analysis and dynamic tests will be presented with the dynamic behavior of the strand evaluated by performing a simple Fast Fourier Transform (FFT). Finally, the conclusions will be drawn.