| Carbon fiber reinforced composites have the advantages of high specific strength and stiffness, good anti-fatigue performance and durability, etc., which has been widely used in the fields of automobile, ship, aircraft. The composite materials will inevitably generate damages or cracks after a long time exposure to the extreme environment. Therefore, the high efficiency of nondestructive testing technology for composite materials structure for long time continuous real-time monitoring is of a great significance. Optical fiber sensing technology has the advantages of small dimension, light weight and high sensitivity, which will become a popular nondestructive testing method in the future. Fiber Prague grating(FBG) sensor is a kind of optical fiber sensor, which has a strong ability to resist electromagnetic interference, but also is not subject to the influence of light intensity fluctuation, polarization state change, optical fiber connection and coupling loss. FBG sensor embedded into composite materials structure can realize the real-time health monitoring of composite materials.This paper studies the internal thermal strain measurement under the temperature range of20~100℃ by FBG sensors which are temperature-compensated by a free FBG sensor as a reference embedded into the center of cured epoxy resin, two bundles of carbon fiber, carbon fiber/epoxy unidirectional laminates and carbon fiber/epoxy woven fabric laminated composite with the SM125 Optical Sensing Interrogator. The internal thermal strain properties of composites only under a thermal load were analyzed.The conclusions of this investigation are as follows:(1) The temperature calibration of the FBG sensor was carried out. The central wavelength of the fiber Bragg grating had a good linear fitting with the temperature, and fine repeatability.(2) Dynamic Mechanical Analysis(DMA) was used to carry out dynamic thermodynamictest of cured epoxy resin The results showed that when the temperatures was below the glass transition temperature of cured epoxy resin, the mechanical properties of cured epoxy resin was maintained at a high level, while at the temperatures above the Tg, the mechanical properties decreased drastically. In addition, the heating in a short period of time will not affect the test results of composite materials.(3) Performance test of cured epoxy resin embedded with FBG sensor. The results showed that the FBG sensor could not only be aware of the influence of external temperature changes on the thermodynamic properties of cured epoxy resin, but also could accurately sense the thermal stress variation of cured epoxy resin.(4) The tests showed that the central wavelength of the referenced FBG sensor and FBG sensors embedded into the cured epoxy resin, two bundles of carbon fiber composite materials,carbon fiber / epoxy unidirectional laminates and carbon fiber / epoxy plain woven laminate composite were influenced by the only thermal load. The central wavelength was linearly with the increase of temperature and the fitting degree was fine in line with expectations.(5) The tests also indicated that the central wavelength drifts of the four FBG sensors before and the temperature compensation were also affected by temperature change. The central wavelength drifts of the four FBG sensors before and the temperature compensation had a certain error, so it is necessary to compensate the temperature.(6) The internal thermal strain the four kinds of material was analyzed in detail. The internal thermal strain were increased with the temperature rising. And at the same temperature, two bundles of carbon fiber was largest; the internal thermal strain of cured epoxy resin in the second,followed by woven fabric laminated composite materials, and finally carbon fiber / epoxy unidirectional laminates. |
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