5th International Workshop on Structural Health Monitoring, Stanford University, 2005 PERFORMANCE EVALUATION OF FIBER BRAGG GRATINGS OVER A LARGE TEMPERATURE RANGE Authors: M. Frövel (1), Ignacio Baraibar (1), J.M. Pintado (1), Daniel Stelzl (1), A. Güemes (2) 1 ABSTRACT This paper reports on tensile test results of the performance of fiber Bragg grating sensors, FBGSs, at temperatures from –150ºC to 160ºC embedded in carbon/epoxy composite material. Two test samples, with in total 4 acrylic coated and 4 uncoated FBGSs, were subjected to identical test conditions to allow for statistical analysis of the data. The strain response of the FBGSs changed in the measured temperature range between –100 and 50ºC on average by about 0,02 pm/με 100ºC -1 and on average by about 0,04 pm/με 100ºC -1 between 50 and 160ºC. Only insignificant differences have been found in the average strain sensitivity of the coated and the uncoated sensors although a significant higher standard deviation of the measured values have been observed in the uncoated sensors. INTRODUCTION The application of embedded fiber optic sensors for loads monitoring in flight structures requires that the sensors are able to perform over a large range of temperatures. In order to use fiber optic sensors effectively, and their results may be accepted by certification authorities, one must have a thorough understanding of the sensor’s limits and how it responds under various environmental conditions. Many papers have demonstrated an almost linear peak wavelength drifting with strains, expressing it by a coefficient, called the strain sensitivity or the conversion factor, K, of the FBGS. This conversion factor is considered as a constant, obtained experimentally over a narrow operating range. How this constant varies with temperature in acrylic coated and uncoated FBGSs being embedded in carbon/epoxy composite material is the main purpose for this paper. THEORY For a plane strain state, the Bragg sensor output in terms of the laminate strains ‘far’ from the sensor, that is embedded in parallel to unidirectional 0º layers, and for a given temperature can be described as: (1) INTA , Dpt Materials and Structures. Crta. de Ajalvir p.k. 4; 28850 Torrejón de Ardoz, MADRID – SPAIN, Frovelm@Inta.es (2) Univ. Politécnica Madrid, Dpt Aeronautics. Pza.Cardenal Cisneros, 3; 28040 MADRID – SPAIN, aguemes@dmpa.upm.es