Owing to their excellent mechanical properties, particularly high strength and stiffness, composite components are increasingly used in aerospace structures. Modern planes (e. g. Boeing B787) include many subassemblies made from composite materials, especially carbon fibre reinforced composites (CFRC). Their (volumetric) percentage in the total aircraft parts is as high as 80%, which amounts to 50% of the ready aircraft weight. However, the development of defects in composite materials is much more complicated than in metallic materials and it is still not fully explored (Diamanti et al., 2002). For this reason, in recent years much research has been devoted to structural health monitoring (SHM), also of aerospace structural components, by means of fibre-optic measuring techniques. The research projects are aimed at developing systems for monitoring aerospace structures not only during their everyday operation, but also during the processes of production (e. g. pressure and vacuum forming, resin impregnation and curing, machining, assembly, etc.) of the particular composite components.
In (Takahashi et al., 2010) the authors describe a method of developing a system for the continuous monitoring of the composite parts of the plane’s fuselage (a partition with reinforcements) through a system controlling the state (distribution) of strain on its surface during the entire production stage and service life. The developed system was based on Bragg gratings integrated with the structure of a CFRC reinforced composite. Through the measurement of local strains the system can detect and locate defects and other damage arising in the structure’s material. The authors also showed that it was possible to measure the strain of composite samples in the course of the whole process of pressure forming and described its effect on the tested object. In order to determine the optimum location of point FBG sensors on the reinforced composite partition a numerical FE model was created and loaded on its outer surface with a pressure corresponding to the maximum design pressure. Two types of defects: reinforcement separation and local impact into the partition resulting in the delamination of the composite were implemented in the virtual model. Thanks to this it became possible to determine the optimum distance between FBG sensors, ensuring detection of defects in any area of the investigated object. It was estimated that the intersensor distance for sensors with a sensitivity of 10 pe, installed on the reinforcements should not exceed 100 mm. In the case of the sensors controlling the surface of the partition, one FBG grating is capable of detecting a defect within an area 300 mm in diameter.
A major problem which designers of systems for continuous structural health monitoring encounter is the location of measuring transducers and their optimization with regard to the number of measuring points. The constraints put on the number of sensors are aimed at reducing the measuring system costs (e. g. costs of sensors, instrumentation, measuring units, system installation). They are also dictated by difficulties with the on-line processing of large quantities of measurement information. A large number of sensors does not always guarantee system reliability. Proper software with implemented data processing algorithms, capable of keeping the user informed about potential risks, is also needed.
Besides research on optimizing the arrangement and number of sensors (e. g. through numerical modelling and the hybrid (model + experiment) approach), purely experimental research on implementing the very wide networks of sensors (comprising a few thousand and more sensors) is being conducted. The aim is to create SHM systems which, similarly as the human nervous system (nerves/sensors, the brain/the information processing unit), enable the monitoring of entire engineering structures (Balageas et al., 2001).
Exemplary research on implementing an SHM system comprising a large number of sensors is presented in the paper (Childers et al., 2001). The authors describe a measuring system consisting of 3000 measuring points based on fibre Bragg gratings, for monitoring the strain of a transport plane wing. Static tests on 12 m long composite wing made of carbon-epoxy composite with Kevlar inserts were carried out. A system consisting of 466 foil strain gauges was used as the reference. A novel system for handling many FBG sensors in a single fibre had been specially designed for the tests. The system is based on the optical frequency domain technique (OFDR) and is capable of simultaneously reading signals from 4 measuring heads. Each of the heads is 8 m long and contains 800 sensors spaced at every 10 mm.
The results obtained from the two different measuring systems were very similar. Moreover, it was found that thanks to the use of the fibre-optic system the total weight of the measuring system could be considerably reduced and at the same the number of measuring points could be increased. As a result, the behaviour of the structure under real loads could be more accurately represented. In addition, owing to the small number of cables the system based on Bragg gratings turned out to be much easier to install than the strain gauge system (Childers et al., 2001).