Health monitoring of composite materials using dual sensing fiber Bragg grating systems

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Smart structures in which structural conditions are monitored with built-in sensors have drawn attention in these days. In smart structures including detection of both impact loading and damage using conventional technique, strain gauges and piezoelectric sensors would be installed. However, these electric sensors have serious drawbacks that they suffer from electromagnetic interference and cannot be used in explosive environment. Moreover, they need individual cables. Therefore, the monitoring system applied to large-scale structures where multi-point sensing is performed would inevitably include complex cable network. In order to overcome these technical difficulties, fiber Bragg gratings (FBGs) featuring multifunction, multiplexibility as well as immunity to electromagnetic interference have been expected to be promising sensors for structural health monitoring.The objective of the present study is to develop a simple FBG sensing system which can measure strain and detect ultrasound … More for structural health monitoring. A fiber-optic system featuring strain measurement and ultrasonic detection was constructed with fiber Bragg gratings based on wavelength-light intensity conversion technique. This fiber Bragg grating sensing system consists of a broadband light source, a broadband optical filter for strain measurement and a narrowband tunable filter for ultrasonic detection. The system was applied to strain measurement in impact loading to carbon fiber-reinforced plastics and the subsequent impact damage detection.Experimental results proved that the minimum detectable strain for FBG and conventional strain gauge are estimated to be 1.2με and 8.1.με, and signal-to-noise ratios are evaluated to be 40dB and 8dB, respectively. FBG sensors would be highly reliable in long-term strain measurement because of its self-referencing and immunity to electromagnetic interference. Furthermore, ultrasonic inspection in which ultrasonic sensitivity was maximized using the tunable filter could detect a 6.3X9mm^2 impact damage. These experimental results have confirmed that the FBG sensing system can be useful tools to monitor strain and damage in composite materials. Less