Research On Curing Residual Stress Of Carbon Fiber Composite Material Based On FBG Sensors

Since the advanced composite materials have excellent specific strength, specific stiffness, anti-corrosion, anti-abrasion and fatigue resistance, they have taken the first place in the four structure materials of aircraft. However, the curing residual stresses, which is always, during the curing process of composite material, induced by the mismatch of thermal expansion coefficients between reinforced fiber and resin, the shrinkage in the resin polymerization reaction or other reasons, can reduce the performance of composite materials and increase their assemble difficulty. So it is very meaningful to monitor the curing process of composite materials with efficient non-destructive testing technology. To monitor the health condition of composite structure using built-in sensors method is also an important development trend of composite structure non-destructive testing technology. Among numerous sensors, the optical fiber grating (FBG) sensor, which is a superior sensor with magnetic and electrical resistance, can be embedded in the composite materials with little interference to monitor the whole autoclave curing process of prepreg.Firstly, the stress sensitive coefficient, the temperature sensitive coefficient and the thermal stability performances at high and low temperature of FBG sensors had been thoroughly studied in this paper. The results showed that the stress sensitive coefficient, the temperature sensitive coefficient and the thermal stability performances at high and low temperature of FBG sensors with different central wavelength were very similar, and they were all consistent with the theoretical analysis value. When the FBG sensor encapsulated by metal casing and the naked one were put into the epoxy resin, the gel point and glass transition temperature of resin curing reaction could be easily and immediately identified from two signals of FBG sensors, and the curing reaction heat with the curing shrinkage after cooling could be also calculated. Besides, it was found that the heat release of curing process was very slow.Secondly, in the unidirectional laminate autoclave curing process, a Taguchi orthogonal test was designed according to five different curing conditions (including the second heating time, the second holding time, the cooling mode, the curing pressure and the temperature of pressure releasing), which could affect the curing residual stresses. And the FBG sensors were embedded into the middle of prepreg in order to monitor the whole process of curing. After the results of orthogonal test were analyzed by Taguchi soft, the important factor in the curing test and interaction between them could be obtained. Subsequently, in terms of the optimal test of important factor, the optimum curing condition fitting to this test and the minimal residual stresses (-6.49MPa) had also been found. In addition, different curing conditions had great influence on curing residual stresses. Even the two FBG sensors at same layer of different direction showed different signal changes.Thirdly, this paper had researched on curing behavior and curing residual stresses of different ply sequences and different thickness. In the beginning, the curing behaviors of orthogonal and quasi isotropic layers at cooling stage were mainly studied by means of analyzing the curing signals which were monitored by FBG sensors. Subsequently, the paper compared the curing behaviors of symmetric and asymmetric laminate, and found out the general rule that how symmetric and asymmetric laminate affected the FBG sensors.Finally, X ray diffraction method was used to verify the residual stresses measured by FBG sensors. Since the surface of specimens in X ray diffraction method required to be polished, it would release part of curing residual stresses. The value measured by X ray diffraction method was less than that measured by FBG sensors, but the general trend was still the same.Meanwhile, interlaminar shear strength of the unidirectional composite laminate specimens with different curing residual stress had been tested, and the test data was processed according by certain steps subsequently. The results revealed that the relationship between B benchmark shear strength value under four curing conditions and curing residual stresses monitored by FBG sensors was an approximate curve. That was a great impact on the shear strength when the curing residual stress monitored by FBG sensors was more than14MPa. However, when the curing residual stresses monitored by FBG sensors were less than8MPa, the impact was very little.Finally, the heat capacity of stainless steel mold was relatively large, which led to a certain temperature gradient between the upper and lower surface of the laminate during the cooling stage of curing process. The curing process of symmetric and asymmetric layers were simulated with the convection boundary condition by ANSYS finite element software. The simulation results obtained were compared with the test results monitored by FBG sensors, which showed that changing trend of cured prepreg with different stacking sequence and different layer thickness was consistent with test results.