Composite materials combine the advantages of lightweight and high strength. This is in particular of interest for the development of large constructions e.g. aircraft, space applications, wind turbines etc. One of the shortcomings of using composite materials is the complex nature of the failure mechanisms which makes it difficult to predict the remaining lifetime. Therefore, condition and health monitoring are important for using composite materials for the critical parts of a construction. Different types of sensors are used to monitor composite structures. These include ultrasonic, thermography, shearography and fiber optics. Optical fiber sensors can be surface mounted or embedded in the composite construction to provide the unique advantage of in-operation measurement of mechanical strain and other parameters of interest. This is identified as a promising technology for Structural Health Monitoring (SHM) or Prognostic Health Monitoring (PHM) of composite constructions. Among the different fiber optic sensing technologies, Fiber Bragg Grating (FBG) sensors are the most mature and widely used ones. FBG sensors can be realized in an array configuration with many FBGs in a single optical fiber. In the current project, different aspects of using embedded FBG for composite wind turbine monitoring are investigated. The demonstration of using FBG sensor array for temperature and strain sensing and monitoring of a 5.8 m long scale model of a glass fiber monopile is investigated. The results indicate that the embedded FBGs successfully show the temperature increase during curing of the resin. During the static and dynamic tests, the embedded FBG sensors provided accurate monitoring.@en

Elastic guided waves are carriers of information of the (change in) condition of plate-like structures like wind-turbine blades, airplane wings and road surfaces on bridges. To measure these guided waves we do not have to use piezos. Other sensors offer interesting benefits like contactless sensing, embedding, or measuring without electricity. However, quantitively comparing them is not trivial: the sensors all have different geometries, operating principles and are sensitive to different mode shapes of guided waves. We designed and performed an experiment to quantitatively compare the performance of five state of the art sensors (piezo, in-fiber interferometer, FBG, free-space interferometer, and ring resonator sensors) to measure So and Ao guided elastic waves. The measurements were performed on guided waves in an 8 mm steel plate, in the 60-150 kHz range. The dimensions of the plate and the positioning of the sources and sensors was chosen such that the So and Ao waves arrived in separate time windows. The in-fiber interferometer was the sensor that came closest to the piezo, that was used as reference sensor (-11 dB difference in SNR), the other optical based sensors have SNR values below -30 dB compared to the piezo. The measurements and simulations show that it is important to have two quantitative SNR measures for the performance to measure guided waves: one for the So and one for the Ao wave. For one sensor we found a difference of 22 dB between these two SNR measures.

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Background: The mechanical properties of small minimally invasive instruments are limited and thus must be treated as flexible instruments. Proper functional behavior of these instruments can be significantly enhanced when the instrument is equipped with a shape sensor to track the path of the flexible instrument. MRI compatible instruments, and thus the corresponding paths, are long in particular. Therefore, the accuracy of the tip position is stringent. Approach: We have developed and realized a thin Fiber Bragg Grating (FBG) based fiber optical shape sensor. The main advantages of this fiber optical sensor are its minimum dimensions, the intrinsic MRI compatibility, and the ability of sensing deformation with submicro-strain accuracy. The shape sensor consists of three fibers, each equipped with multiple FBG's, which are integrated physically by gluing and can be positioned inside an flexible instrument. In this study a critical component analysis and numerical error analysis were performed. To improve performance, a calibration procedure was developed for the shape sensor. Results and Conclusion: With current state of the art interrogators it is possible to measure a local deformation with a triplet of FBG sensor very accurately. At high radii of curvature, the accuracy is dominated by the interrogator, whereas at low radii of curvature, the position of the fibers is leading. The results show that position error of a single segment of the shape sensor (outer diameter of 220 μm, a segment length of 23.5 mm and a minimum bending radius of 30 mm) could be measured with accuracies (3σ) of 100 μm for low radius of curvature upto 8 μm for high radii of curvature.@en