The temperature response of fiber Bragg gratings (FBGs) embedded in carbon fiber reinforce polymers (CFRP) is investigated. To provide strain-free temperature measurements, two pieces of anti-sticking materials are placed at both sides of the embedded optical fiber and between carbon fiber prepregs, thus providing slippery surfaces that minimize the strain transfer to the FBG sensor. In particular, the impact of different anti-stick materials on the temperature and residual bending strain response of the embedded FBGs is experimentally investigated. Results demonstrate that although some materials can allow for minimum residual strain being transferred to the FBG, their thermal conductivity does not always fulfil the requirements for reliable temperature sensing. It is found out that, among the tested materials, aluminum and copper foils can provide both reliable temperature response (with negligible delay and bias) and minimum residual strain. Using such anti-stick materials, the error induced by the residual strain on FBG temperature measurements is also experimentally evaluated by applying temperature and bending loads (strain) simultaneously to the CFRP packaging. Whilst the study is here performed for FBG-based point sensors, most of results and conclusions are also expected to be valid for applications of embedded distributed optical fiber sensors being affected by strain-temperature cross-sensitivity issues.

Conventional silica optical fibers can be embedded into composite structures or packaging to provide structural monitoring capabilities. In this paper, the microbending optical losses induced by the packaging of a sensing optical fiber into a sandwiched glass-fiber reinforced structure are investigated experimentally and by simulations. Results show that the positioning of the optical fiber within a plain-weave glass fiber-reinforced structure has a critical effect on microbending optical losses due to potentially induced microdeformations of the optical fiber's cylindrical shape. Similar analysis is also carried out with the use of a 1D glass-fiber layer packaging, demonstrating lower residual microbending losses after manufacturing. By the proper positioning of an optical fiber containing a series of wavelength-multiplexed fiber Bragg grating (FBG) sensors over a 2D plain-weave glass fabric, a 10-fold improvement in the induced optical losses is demonstrated, increasing the optical power that reaches the interrogating unit and allowing the monitoring of hundreds of meters over glass-fiber structures. Although all the analysis is focused and verified using a series of FBG sensors, the results and conclusions related to the induced microbending losses are also valid and useful for distributed optical fiber sensing approaches.

The large development of fibre Bragg gratings (FBGs) over decades has made this kind of structures one of the most mature optical fibre sensing technologies existing today, demonstrating key features for a very wide range of applications. FBG sensors are fragile and must be normally protected for real-field applications, although challenging packaging designs are required to mitigate temperature-strain cross-sensitivity issues. Here, a polydimethylsiloxane (PDMS) packaging with a microarray structure that provides gecko-inspired dry adhesion is proposed for strain-free FBG-based temperature sensing. Besides offering protection, the PDMS packaging with an embedded polyamide capillary damps the mechanical strain transferred to the optical fibre, providing FBG-based temperature sensing with a negligible impact of strain. In addition, the microarray structure imprinted on one surface of the packaging provides gecko-inspired dry adhesion based on van der Waals forces. This feature enables the packaged optical fibre sensor to be attached and detached dynamically to nearly any kind of smooth surface, leaving no residuals in the monitored structure. Experimental results verify a fast and accurate temperature response of the sensor with highly mitigated impact of residual strain. The proposed packaged sensor can be used in application where glue is not allowed nor recommendable to be used.

The spatio-temporal evolution of the temperature induced over carbon fiber reinforced polymer (CFRP) laminates during drilling is monitored in real-time using a distributed optical fiber sensor based on optical frequency-domain reflectometry (OFDR). The proposed distributed measurement technique enables the simultaneous monitoring of thousands independent points on a CFRP plate during machining, being of special interest to measure the internal temperature of a workpiece. Experimental results validate the use of distributed OFDR-based sensing for this novel application, demonstrating a precise reconstruction of the two-dimensional (2D) temperature profile around the drilled hole, with a 2 mm spatial resolution and a sampling rate of 23.8 Hz (corresponding to a measurement interval of 42 ms). The high spatial and temporal resolutions provided by OFDR sensing offer unique features for this application, allowing for an accurate identification of the temporal evolution and spatial distribution of the 2D temperature profile originated during drilling. By embedding the sensing optical fiber in the interface between CFRP laminates and metals, a full map of the internal (interlayer) temperature of the CFRP plate can be obtained, demonstrating a feature that cannot be obtained by any other sensing technology. The proposed method can constitute a relevant tool for the identification of potential high temperatures occurring during machining, which could affect the quality of the borehole, induce material defects and compromise the safety of an entire composite structure in service.

With the application of fiber bragg grating (FBG) sensors in the condition monitoring of wind turbine blades, the layout management of FBG sensors with different central wavelengths and manufacturing processes has attracted much attention. A management system of FBG sensor based on RFID technology is proposed. The management system includes three parts: packaging process of FBG sensor with RFID chip, RFID chip stored information and the FBG sensors status monitoring system, the FBG sensor installation on full scale field wind turbine blade and its maintenance. The proposed management system introduces RFID technology into the blade monitoring using FBG sensor. It provides effective digital management method for the manufacturing, the layout, the installation and the maintenance process of the FBG sensor on the wind turbine blade.