Conventional aircraft repair techniques employ bolted or riveted metallic reinforcements, which frequently introduce additional stress concentrations leading to further cracking and creating areas difficult or impossible to inspect. Bonded composite repairs (“patches”) result in the elimination of stress concentrations caused by additional fastener holes, improved strength to weight ratio and present a sealed interface. This reduces even further the danger of corrosion and fretting under the repair, gives greater flexibility in design and lessens application time while lengthening fatigue life.
Embedding optical fibres and sensors into the patch, and combining this with advanced data collection and processing systems, creating a so-called “smart patch”, will enable the real-time assessment of aircraft structural integrity resulting in reliable prediction of maintenance requirements for repaired structures. This paper describes the current state of the art in smart patch technology, and includes a detailed description of the measurement problem and of the work being undertaken to solve it, at both the component and system level. An analysis of typical crack behaviour, based on FE modelling is presented and this demonstrates the need for optical strain sensors having a very short gauge length. The paper discusses the advantages and limitations of very short Fibre Bragg Gratings (FBGs) in this context and also provides early experimental data from 1mm and 2mm gratings which have been fabricated for this purpose. The paper also describes the impact of the measurement and environmental constraints on the design of the FBG interrogation system and presents the results of initial trials. The work is being undertaken in the framework of a collaborative project (ACIDS) which is co-funded by the European Commission.
Ice accretion on flying surfaces affects the aerodynamic performance and handling qualities of aircraft, and may require different pilot corrective action, dependent upon the surface that ice is accreting onto. The current methodology for ice detection usually relies on an indirect method, normally based on ambient air temperature, and liquid water content. When a pre-set threshold level is reached, the ice protection system is activated, whether or not ice is accreting on
critical surfaces. This method is not cost effective or efficient for an ice protection system. Air Conformal Ice Detection System (ACIDS) obviates these problems by using a 'direct’ method of detection and measurement the presence and thickness of ice. This paper outlines some of the preliminary experimental work done on the optical properties of ice grown in an icing tunnel on the leading edge of an aerofoil leading to the development of a Fibre Optic Direct Ice
Detector sensor (DID) with emphasis. The result of this studies have shown that with suitable processing it is possible to use fibre optic sensors to determine the thickness of ice and texture of the ice accreted in the vicinity of the sensor. In the latter part of this paper basic fibre optic architecture is discussed and together with some preliminary results for representative icing runs.
We observed and investigated self-starting quasi-periodic pulsation in Er-doped fiber laser at 30 - 100 mW pump power. Pulse with duration of 10 - 50 ns and peak power of 50 - 200 W are generated at a quite stable repetition rate in the range of 300 - 500 microsecond(s) . In contrast with previous experiments the pump level in our experiment is significantly lower. At this low pump power we found no nonlinear effect except SBS influencing on the laser dynamics. The experimental results were explained by a theoretical model based on cooperative dynamics of Rayleigh backscattering and Stimulated Brillouin Scattering (SBS). Using digital oscilloscope, we traced in details different stages of Q-switching pulse formation process: growth of the spontaneous radiation, lasing due to Rayleigh backscattering, appearance and growth of the first order SBS Stokes radiation and the second order Stokes radiation, lasing suppression due to saturation of the population inversion in Er-doped fiber by the SBS Stokes radiation. Good agreement between theory and experiment have been demonstrated.
Recently a new mechanism for passive Q-switching in fiber lasers based on cooperative dynamics of linear Rayleigh backscattering (RS) and Stimulated Brillouin Scattering (SBS) has been reported in Yb- and Er-doped fiber lasers with high pump powers (greater than 2 W). At such high pump levels, the intensity of the light generated inside the fiber laser cavity exceeds considerably the SBS threshold, so that the conditions for passive Q-switching due to nonlinear backscattering are easily achieved and, for this reason, no special optimization of the laser configuration is required. Here, we report on experiments in Er-doped fiber lasers with low pump power levels. The Q-switching operational mode of Er-doped fiber lasers was observed at pump power levels in the range of 30 - 100 mW. Throughout the experiments, we traced in details different stages of Q-switching pulse formation process: growth of the spontaneous radiation, lasing due to Rayleigh backscattering, appearance and growth of the first order SBS Stokes radiation and the second order Stokes radiation, lasing suppression due to saturation of the population inversion in Er-doped fiber by the SBS Stokes radiation. In general, the process was slower in comparison with previous experiments at high pump power level. The output pulse duration was in the range of 10 - 20 ns and the peak power of the pulses was less than approximately 100 - 200 W. For this reason, all nonlinear processes except SBS did not influence the pulse formation process. The experimental results are well explained by a theoretical model based on RS-SBS dynamics.
Optical fiber sensors for monitoring of structures (OSMOS) is a European collaborative research project which has, over the past three years, embraced a number of technological issues related to the problem of structural monitoring in the civil engineering and aerospace industries. A key technical objective of the program was the measurement of temperature and strain using a single sensor length. A laboratory prototype using the differential sensitivities of polarimeters based on the fundamental, LP01 mode and the first higher order LP11 mode of polarization maintaining fiber demonstrated parameter recovery to within 2 C and 5 (mu) (epsilon) . A receiver enabling quasi-distributed measurements to be made with a linear spatial resolution of 70 cm using white light polarimetry was assembled. White light polarimetry was also used in conjunction with pressure sensitive fiber to detect impact damage on a composite radome structure. Impacts of 5 Joules in magnitude were detected with a spatial resolution of around 1 cm. Microwave radio frequency subcarrier measurement techniques were used to develop the engineering processes necessary to integrate optical sensors into civil engineering structures for simulated applications trials. This enabled issues such as stress transfer, mechanical bonding and sensor protection to be addressed. For the aerospace industry, embedding of optical fiber sensors remains an important issue. Here we developed techniques for embedding connectorized fibers such that the component could be machine finished after curing, an important feature of the manufacturing process.