Simultaneous Measurement of Deformation and Fracture of Composite Structures Using Fiber Bragg Grating Sensors

Because of high specific strength and many other benefits, the use of composites for the large lightweight structures such as modern aircrafts and wind turbines are increasing. However, one of the serious drawbacks of composites is that the structural failure occurs in complex patterns without yielding. Therefore, structural health monitoring has been intensively investigated for the early detection of any problems in structural integrity. One of the promising sensors for this purpose is fiber Bragg grating (FBG) sensor. They can be easily inserted into the layered-structure of the composite materials due to their small size. The excellent multiplexing capability enables measurement to be taken at multiple points along a single sensor line. As well as damage detection, the structural shape measurement also draws attention. Particularly for structures experiencing aerodynamic forces such as wind turbines or helicopter blades, the structural shape itself is important because the applied aerodynamic forces are affected by structural shape deflections. Therefore, the authors have conducted a series of studies on the structural shape estimation of various structures. We have also developed a wavelength division multiplexing (WDM) Bragg grating sensing system for high speed strain sensing as well as low frequency dynamic strains. In the case of high-speed sensing, the interrogator allows a sampling ratio of over 40 kHz for six linearly arrayed FBG sensors per channel. Utilizing the developed interrogator, this paper presents some experimental results for simultaneous measurement of deformation and fracture signals of composite structures. An array of FBG sensors were installed onto composite beam specimens and the acoustic emission (AE) signals due to structural failure was continuously monitored while the overall structural deflection shape was monitored in real time. The reconstructed shapes of the specimens were in good agreement with the shapes captured from photographs taken with a high-speed camera. In summary, it was demonstrated that both fracture signals and the overall deformation shape of composite structures could be simultaneously monitored.