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For many applications in the aerospace industry, monitoring of structural performance is becoming increasingly important in order to reduce maintenance and inspection costs and enhance efficiency. One of the most attractive sensors to build large area sensor networks has become the fiber-optic Bragg grating. Despite the fact that Bragg grating sensors have been of considerable interest to the structural health monitoring community for the last few years, many challenges associated with applying Bragg grating sensors in real-world applications have not yet been solved. One of the major issues remaining is the method of attaching the sensors to the structure to be monitored. It is essential to understand the optical signal of the sensor and its relation to the actual strain fields in or on the structure. DaimlerChrysler and EADS have studied several solutions for the fiber attachment over the past few years. This paper discusses three of these methods, which are currently under investigation in both lab and field tests.

The goal of this paper is to discuss different data interpretation concepts for structural health monitoring based on Lamb wave propagation. Surface-mounted piezoelectric transducers are used as wave emitters and receivers. The waves emitted interact with discontinuities and experience a change in their propagation characteristics when damage is generated. By comparing sensor signals collected before and after damage has been generated, the condition of the structure can be determined. Two approaches are proposed for characterizing the damage. The initial approach analyzes only the first wave packet reflected at the damage and is based on a simple time-of-flight analysis in conjunction with a geometric method. The size of the damage can be estimated with this technique by using several pairs of piezoelectric sensors. The second technique considers additional modes and is based in particular on the presence of modeconversion processes. This technique offers the potential for enhancing measurement accuracy. An estimation of the size and location of the damage becomes possible with just three piezoelectric sensors. It has been found that, in practice, the superposition of wave packets represents one of the main problems in realizing this approach. The decomposition of superimposed wave packets is investigated using wavelet transform and the pulse compression technique. It is shown that noise and dispersion represent a major drawback, hence rendering signal analysis distinctly more difficult.

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In future aircraft, integrated sensors for monitoring structural performance and structural integrity will become increasingly important. For integration into carbon41berreinforced composite sflctures, fiberoptic Bragg grating sensors represent a very promising approach. In order to characterize the behavior of integrated sensors for different mechanical load cases theoretical and experimental investigations were conducted. In the theoretical part of the work, an analytical approach was first used to determine the strain sensitivity of the Bragg sensor and the interface sfresses at the coating. Using this model the effect oflongitudinal and transverse tension has been studied for different stacking sequences of the composite material.We then compared the results of the analytical model with a finite element analysis. A good correlation between the analytical and the numerical approach was found. For the experimental investigations the CFRP was subjected to the load cases of transverse and longitudinal tension and longitudinal compression. For the strain sensitivity of the integrated Bragg sensors a good consistency of the theoretical and experimental work was found. As a result of the investigations, conclusions can be drawn as to the design of sensor coatings with regard to the minimization of transverse strain sensitivity effects and interface stresses. For commercially available coatings, high interface stresses were identified for strain levels relevant to practical applications (0.5%). Furthermore, transverse strain sensitivity effects have to be taken into account for interpretation ofthe sensor signals. However, linear behavior ofthe integrated sensor can be observed.