Optical fiber Bragg grating sensors embedded in CFRP compos- ites for structural health monitoring of aerospace structures Authors: Malte Frövel, INTA, Composite Materials Department, Torrejón de Ardoz, Spain Encarna del Olmo, EADS CASA ESPACIO SL, Madrid, Spain José María Pintado, INTA, Composite Materials Department This paper describes the investigation work to validate the performance of optical fiber Bragg gratings, FBGS, as strain and temperature sensors in a wide temperature range and different load and ambient conditions. Test campaigns have been performed in tensile and compression load in dry and humid condition with acrylic and polyimide coated FBGSs be- ing embedded in both, unidirectional and quasi isotropic carbon/epoxy or car- bon/bismaleimide composite material and in a temperature range from -55ºC to 120ºC for aeronautical applications and -110 to 200ºC for space applications. The tests try to simulate the in service ambient conditions of aircraft and spacecraft structures. The FBGS performed satisfactorily in all the conditions but showed among others an influence of the testing tem- peratures, the dry or wet specimen condition and the load direction on the sensor strain sensitivity K. Introduction Fiber optic sensing systems are considered for structural health monitoring of aeronauti- cal and space applications due to many ad- vantages, like multiplexability, very small dimensions, embeddable in composite mate- rials and immunity to electro magnetic inter- ferences that can lead to solutions with the potential to out perform their conventional counterparts. Most advantageous fiber optic sensors for that purpose are Fiber Bragg Grating Sensors, FBGS. Monitoring in flight structures requires that the sensors are able to perform over a large temperature range, in different humidity conditions and under ten- sile and compression load and their response under all these operative environmental con- ditions of an aircraft and space vehicle must be well understood for the flight certification of these sensors. A FBGS respond on a mechanical load and on a thermal load or a combination of both with a wavelength shift, WLS, of its charac- teristic optical wavelength, WL, that is nor- mally a well defined peak of about 0,2 nm of spectral width at -3dB. This cross sensitivity permits to use a FBGS for both, strain and temperature measurement, but requires that the temperature, T, of each FBG-strain sen- sor must be known to subtract the thermal induced sensor response. The thermal in- duced WLS λ a of the FBGS in a temperature range of a space structure as for instance between -110ºC and 200ºC can reach about 3000 microstrain. Temperature induced dif- ferences in the strain sensitivity K of an em- bedded FBGS that are insignificant in a nar- row temperature range around room tem- perature, RT, can increase in the same tem- perature range of the mentioned space struc- ture to 8 or 10% and must be taken into ac- count /1,2/. When subtracting the tempera- ture induced WLS, λa and adapting the strain sensitivity K on the actual temperature of the FBG-strain sensor, the mechanical strain [με] at temperature T can be calculated according to the following equation (1). K a ⋅ Δ − Δ = 0 ) ( λ λ λ ε (1) With: λ 0 = Initial WL at zero load and reference T [pm] Δλ= measured WL λ - λ 0 [pm] Δλa= T induced WLS λa - λ 0 [pm] K = normalized strain sensitivity at test T [pm/pm/με] The temperature of the FBG-strain sensors can be assessed with conventional electrical T sensors or with FBG temperature sensors, FBG-T sensors. It is advantageous using FBG-T sensors because in that case no addi- tional lecture equipment is needed besides the FBGS lecture equipment used for the FBG-strain sensors. FBG-T sensors can be introduced in the same optical fiber where the FBG-strain sensors are located. The quantity of temperature sensors that are needed to assess all the thermal induced sensor responses depend on the tempera- ture gradients of the host structure and the required accuracy of the measurements. As a consequence depending on the thermal load cases in one case one T sensor can Adaptronic Congress 2008, 20-21 May, Berlin 1