JEC COMPOSITES MAGAZINE - Issue #44 - October 2008 - Special Asia - 80

KNOW-HOW Control …/… Potential of in-situ measurements by optical fibres sensor is calibrated, the applied stress can be measured by making the difference between the two peak wavelengths. On the interrogating side, there are a few techniques available for reading a FBGS. One of them consists in using a wide bandwidth infrared light source (typically 50 nanometers wide) and an optical spectrum analyzer. However, some commercial devices are on shelf for industrial applications. For example, FBGS interrogators have been developed by CEA LIST, HBM, INSENSYS, FOS&S. The performances of such devices can reach 0.1 picometer of resolution and a sampling frequency up to 500 kHz. Moreover, the bandwidth of the FBGS reflected peak is narrow enough to allow interrogating several sensors simultaneously with the same light source. In practice, up to 10 sensors can be inserted in the same optical fiber for characterizing: • Temperature with a sensitivity of 12 pm/°C in shift of the Bragg wavelength. The resolution of measurement can reach 0.1 pm and the temperature range extends from 0 K to 1,000 K. • Pressure with a sensitivity from 5 to 100 pm/MPa in shift of the Bragg wavelength (depending on the grating). The measurement resolution can reach 0.1 pm and the pressure range could reach 100 MPa. • Strains induced by the surrounding material (polymeric matrix or composite ply) when the grating has been embedded, with a sensitivity of 1 pm/(µm/m), a resolution of 0.1 pm and a measuring strain range from 0.1 µm/m to 20000 µm/m. • Refractive index of the material in which the sensor is embedded. It requires a specific design to match the appropriate range and resolution. Such technique is available only where the applied constraint remains constant along the length of the sensor (from a few millimeters to centimeters). Indeed (Figure 2b), when the applied constraint is no more homogeneous along the grating, it is no longer possible to identify the Bragg wavelength. To make up for this drawback, one may package the grating in order to average the effect of the constraint. This recipe may affect the sensitivity of the sensor for temperature and strain measurements. Another kind of measurement technique combine the Optical Low Coherence Reflectometry (OLCR) and an inverse mathematical method (called layer peeling) to identify the index profile all along the FBGS, so that the full strain profile and its evolution can be measured. Contrary to the previous interrogation technique, a non-homogeneous strain profile can be measured accurately. The spatial resolution of this technique is 20 micrometers, which would amount to place a single strain gage every 20 microns! Spread sensors The second sensors family called « spread » relies on the light propagation properties along the optical fiber. The fiber represents itself a continuously sensitive environment and external constraints are characterized through the properties of the wave propagation along a long length of the fiber. The analysis of the results is based on an echometric principle already widely used in telecom and called « Optical Time Domain Reflectometry ». An optical pulse at a lambda wavelength is injected in the fiber, and despite its transparency, the fiber sends a low part of the incident light back. This is called the Rayleigh backscattering and refers to an optical pulse centered on a chosen Lambda wavelength. In addition, Raman and Brilloin diffusions (non-linear effects) are due to the reflected light at different wavelengths from the incident light. When a constraint is applied to an optical fiber, shifts in the wavelength of Raman and Brilloin diffusions are observed. The Raman component depends on the applied temperature (1 K of resolution), while the Brilloin component depends on both strain and temperature (20 µm/m of resolution). The power of the Rayleigh backscattered light allows to come back to stresses applied to the optical fiber. Analyzing the three spectral components associated to the three kinds of diffusions lead to identify a set of physical quantities. In order to run such analysis, some specifically designed optical fibers are sold (SMARTEC, CEA-LIST) and can be embedded in the structure to be monitored. For example, Omnisens SA proposes an industrial solution for structural health monitoring based on the Brilloin backscattering effect with a spatial resolution of 2 meters and an interrogating duration of approximately 1 minute for a 30 km long optical fiber. Luna-Technologies uses the Rayleigh backscattering principle in an industrial device Fig. 2: Spectral response of an FBGS: a) before and during application of a homogeneous strain; b) typically when the strain state varies along the FBGS JEC Composites Magazine / No44 October 2008

JEC COMPOSITES MAGAZINE - Issue #44 - October 2008 - Special Asia

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #44 - October 2008 - Special Asia