Three different sensors for hydrogen detection have been built and tested within a research project for the European Space Agency. One type is a FBG coated with a palladium layer, detecting the hydrogen by metal hindrance, the strains transmitted to the grating by shear. It works only as a detector and can not quantify the H2 percentage in a gas mixture. A main drawback, common with all palladium based sensors, was a strong temperature dependence, which makes its response time too large at low temperatures. The other two types were intensity based sensors; one of them was a micromirror, with a palladium thin layer at the cleaved end, detecting changes in the backreflected light. The other one as a tapered fibre coated also with palladium; hydrogen will change the refractive index of the palladium, and consequently the amount of losses in the evanescent wave.
A trade-off analysis of sensor performances was done, comparing reproducibility, repetitiveness, robustness, multiplexability, response time and cost. FBG sensor was found to be the most reliable sensor among the optical fibres sensors considered, and the preferred one for space applications.
Embedded fiber Bragg gratings (FBGs) are sensitive to changes of near strain fields in a composite host monolithic structure, typical of aircraft airframes. FBGs have been embedded in different configurations (a typical position is the skin -- stiffener interface in a monolithic structure) for detecting events associated to damage occurrence. Thus, it is possible to think in FBGs not only as strain sensors, in a classical load monitoring configuration, but as a part of a structural health monitoring (SHM) system in composite structures dimensioned following damage tolerance criteria.
Airbus Espana, INTA and UPM are collaborating in a joint research directed to increase their experience and the knowledge in the use of fiber Bragg gratings as strain and damage sensors in advanced composite structures for aerospace applications. In certain conditions, fiber Bragg gratings are able to detect damage by measuring sudden changes in the strain distribution of composite monolithic structures due to delaminations or debonding of stiffening elements. However, strain measurement of FBGs can be compromised by spurious phenomena like birefringence promoted by manufacturing induced residual stresses, with the subsequent spectral peak splitting, and spectral chirping induced by local non uniform strain fields. These effects add a new issue to the difficult task of demodulating the spectral response of fiber Bragg gratings when employed as strain sensors in composite structures. However, photoelastic models already developed allow obtaining considerable information about this phenomena, and relate them with local changes in the stress field promoted by structural damage. Thus, it is possible to think in FBGs not only as load monitoring sensors, but as a part of SHMS for composite structures designed under damage tolerance criteria.
There are currently available many software tools for modeling the processing of composite materials, that help designers to evaluate the process constraints and the feasibility of different concepts. Nevertheless, several manufacturing tests are still required for adjustment of the control parameters before production may start. Real time monitoring is the only way to validate the numerical results and to get a deeper knowledge on the process evolution. Final objective would be a closed loop known as 'Intelligent Material Processing'.: process model - in situ sensors - predictive control, able to react on real time to small disturbances, adapting the process parameters for optimal results. This paper concentrates on the sensor development for two aerospace processes, autoclave curing and RTM, and it present the results obtained on a real aircraft structural part, a five meter diameter frame for the fuselage of Airbus A380 . An optical fiber system has been implemented to monitor the movement of the resin flow front during the injection and the internal residual strains. The procedure has the advantage of being very robust, and it may be used for complex geometry of the part. It has been demonstrated the feasibility of the procedure to work at an industrial environment; the results are being used to refine the data on the material properties, as the preform permeability, and to improve the process control.
The single peak of the spectrum of a fiber Bragg grating written in a standard low birefringence optical fiber splits in two peaks when a transverse strain field is applied due to the promotion of a strain induced birefringence. In a composite laminate, the theory predicts a plane stress state, which allows using a single embedded Bragg grating to obtain simultaneously both components of the strain field. After a brief review of the photoelastic theory, and the experimental verification of the suitability of the plane stress state hypothesis, some experiments are done with thick graphite fiber composite laminates with different lay-up sequences. Strong residual stresses are detected inside the laminate, in accordance with the theory of composite laminates. The response of embedded Bragg gratings must take into account these internal stresses in order to adequately interpret the experimental results.
Windmills are large composite structures, usually located at difficult-to-access sites, bearing strong dynamic loads in a harsh environment. In-service inspection involves dismounting the blades, a very costly process. Instead of inspecting the windmills regularly, they are usually overdesigned. Furthermore, the design of a windmill is based on estimated wind conditions, while the real wind is seldom measured. The main inconvenience for it are the characteristics of the conventional equipment needed to carry out the measurements. The substitution of traditional sensors for Bragg gratings and piezoelectronics to measure the strain field and vibrational frequencies adds the advantages of a smaller size, no drift, no EMI and the possibility of embedding them while manufacturing, so the windmill is fully equipped when installed. Long-term measurements are possible to check both the in-service conditions and the degradation of the structure. Two possibilities were tested: embedding of the sensors while manufacturing with low disturbance of the process and surface-bonding of the sensors. Windmill qualification test were carried out to check the survivability of the Bragg gratings and the piezoelectric sensors under extreme environment are presently running. Results are hopeful. The next step could be a permanent connection via modem to a remote controller that can use the acquired data to map the wind conditions and/or the structural health.
Intracore fiber Bragg gratings have been extensively used as longitudinal strain sensors both bonded and embedded in numerous applications, fulfilling the same task as conventional resistive strain gages. Comparative results obtained in composite laminates with both types of sensors show an excellent correlation in those places where the strain distributions are smooth. In this case, optical sensors offer additional advantages: unnecessary calibration, multiplexing capability, small size, embedding ability, etc. But optical sensors reveal all their potential in those locations where the anisotropy of the composite structural element promotes strong strain distributions, and complex stress fields. In these cases, the analysis of the distorted spectrum of a grating submitted to an intense strain gradient offers a big amount of information, even without using spectrum integration methods. Furthermore, the knowledge of the optical behavior of fiber Bragg gratings submitted to transverse loads allows having additional information about the residual stress field promoted during the manufacturing process, and the stress release due to the machining of the part. This paper demonstrates theoretically and experimentally that fiber Bragg gratings can be valuable tools not only to monitor complete composite structures in service, but to analyze the stress and strain state of those particular configurations in which any kind of information, due to their complexity and high requirements, is essential.
Due to the small dimensions of the adhesive layer, the high non-uniformity of the strain field and the non linear elastic behavior of the adhesive material, the strain distribution at an adhesive joint can be predicted by FEM, but can not be experimentally obtained with classical approaches; only non standard procedures like Moire interferometry, or special artifacts like KGR extensometers may afford some insights on the behavior of the adhesive. Due to their small size, ensuring low perturbation of the strain field, and their innate ability to measure strain and strain gradient along the sensor, fiber Bragg gratings offer a good opportunity to solve this problem, and it is a good example of situations that may benefit from these new sensors. Fiber Bragg gratings may be placed or at the interface, within the adhesive layer, or embedded at the adherents, if these were made of composite material. Tests may be run at different temperatures, changing the adhesive characteristics from brittle to pseudoplastic without additional difficulties. When loading the joint, the strain field is obtained by analyzing the distorted spectrum of the reflected light pulse; the algorithm for doing it has already been published. A comparison with theoretical results is done, and the validity and utility of these sensors for this and similar applications is demonstrated.
The effect of embedded optical fibers on the static and fatigue interlaminar shear strength of a glass reinforced polyester laminate is evaluated. Seventy identical specimens with and without embedded optical fibers were tested researching more than 500,000 cycles in some cases. It was found that the optical fiber does not have a negative influence on the laminate, neither static nor in fatigue. These tests were included into a project to develop smart wind turbine blades.