Research On Curing Process And Impact Damage Monitoring Of Composites Based On FBG Sensors

The composite material has excellent specific strength and specific stiffness, fatigue resistance and durability, has been widely used in load-bearing structure of aircrafts. However, along the thickness direction, there is no reinforcement existed in composite materials, which are very sensitive to injury. Efficient nondestructive testing technology is very important for composite structure damage inspection and continuous monitoring. The optical fiber grating sensor (FBGs) is embedded into the composite material structure, can achieve real-time health monitoring of composite structure, FBGs shows the advantages of low cost, no electromagnetic interference and monitor the internal structure, which is an important trend of a composite structure nondestructive testing technology.The study of FBG sensor embedded in the composite material structures is very important for health monitoring in aircraft structures.Several different composite molding technologies such as hand lay-up, vacuum infusion molding (VIMP), RTM (Resin transfer molding) molding, autoclave molding were used to study the output and protection of optical fiber in this paper.The problem of fiber extraction in the non-parting surface of RTM molding was solved by using optical fiber positioning device and mold loose piece, mold sealing problems was also studied during this system was under positive and negative pressure respectively.In view of autoclave moulding technology, we studied two kinds of eduction mode,end extraction and surface enduction, solved the problem of fiber protection.on After optical fibers were embedded in composites, the cross-section were observed through optical microscope and scanning electron microscope, static mechanics performance of vinyl/glass fiber, epoxy/carbon fiber composite materials were tested respectively which were both embedded by optical fibers.We found that a single optical fiber was embedded in the two composite, their static mechanical properties hadn't been effected a lot. FBG sensor was successfully embedded in this two composite materials -glass fiber/vinyl and carbon fiber/epoxy resin.After FBG sensor was embedded in carbon fiber/epoxy composite material, tensile and bending properties were tested, results showed that the FBG sensor could accurately reflect the changing trend of material internal stress.The use of embedded FBG sensors for vinyl resin curing, composite materials VIMP curing, autoclave curing, molding process was monitored. According to vinyl resin cure monitoring results, the gel point when the vinyl resin curing and when the exothermic peak occurs were determined, and basically corresponding to the results of Roberts Law. The calibration for temperature sensitivity and strain sensitivity coefficient of vinyl resin/glass fiber composite materials with FBG sensor embedded in, by monitoring FBG wavelength and light intensity changes during molding composite materials VIMP, the gel point of the material was determinded, and the influence of temperature was excluded, composite curing process changes within the course of the strain was achieved as well. FBG was embedded in the carbon fiber/epoxy composite aircraft spoiler subscale pieces, under the VIMP molding process at high temperatures, wavelength drift curve in the process of heating up, plastic injection, heat could be obtained, applied the load after forming the parts, the results showed that the survival response of the FBG can be sensitive to stress changes of the composite. FBG sensors and thermocouples were embedded in the carbon fiber/epoxy composites, using autoclave molding process, the wavelength shift curve of the curing process was obtained, the effects of temperature on the FBG signal was edcluded, and the material changes in the course of the internal strain during the whole curing process were obtained.Low-speed compression drop hammer impact was replaced by Quasi-static compression methods, and using FBG sensors monitoring signal, it was verified that the laying of fiber angle, the distance from the impact point on the FBG signals, presented a buried fiber optic sensors to monitor the impact of the FBG sensor network signal layout. According to the monitoring signal is derived from the different angles in the same distance, the same point in different angels from the wavelength shift, verified and modified the formula to establish a theoretical basis for the follow-up study.The FBG sensor is embedded in the glass fiber and carbon fiber composite materials, using the self-made drop test machine for the low speed impact test, using a low frequency and high frequency fiber Bragg grating demodulation instrument for signals were monitored, the results showed that the impact signal could be effectively monitored by a high frequency fiber Bragg grating demodulation instrument,while monitoring datas were great chance by a low frequency fiber Bragg grating demodulation instrument, for reference only. We monitored composite material within the FBG signals of different energy, the same energy of different distance, the same energy between different layers during cumulative impact, wavelength changes and the signal of the vibration during the impact, tchange of the wavelength before and after the impact was studied. Carbon fibre composite material stratification status was detected by ultrasonic C scanning device. The test results showed that the composite material impact signal could be monitored effectively through the FBG optical fiber sensor, internal damage types and damage area of composite which caused by the impact could also be conjectured according to the signal.Finally, composites under quasi-static compression and low-velocity impact process were simulated through ANSYS and LS-DYNA finite element software. The simulation results were compared with the FBG optical fiber sensor monitoring test results, the material of the inner strain variation and the test results were essentially consistent.