Numerical Analysis And Fiber Bragg Grating Monitoring Of Thermocuring Processes Of Carbon Fiber/epoxy Composites

Due to light weight,high specific strength and stiffness,fatigue and corrosion resistance,carbon fiber reinforced polymer(CFRP)composites have been widely used in numerous areas such as aerospace,vehicles,wind power,ships,sports equipment,etc.However,during the fabrication of CFRP composites,the residual stress is inevitably induced,which can deteriorate mechanical performance,dimensional stability and durability of composite structures.Therefore,it's of great scientific and technical significance to analyze the in-mold hot curing process and internal stress evolution of CFRP composite structures,and then regulate their demolding deformation to achieve high-accuracy,low-defect,low-cost manufacture,and solve the bottleneck problems when composite structures are applied on a large scale in the fields of aerospace,wind power,ships,etc.The curing process of CFRP composites involves heat transfer,curing heat generation,chemical shrinkage,thermal expansion,stress relaxation,etc.In order to characterize the process mentioned above,the thermo-chemical model and three-dimensional viscoelastic constitutive model were established.Then the corresponding finite element model was accomplished in ABAQUS combining the subroutines USDFLD?HETVAL?UMAT and UEXPAN,and the model was validated by comparison with the results from literatures.During the application of CFRP composites,in order to meet distinct load carrying conditions,various structures with different shapes are designed,of which L-shaped and T-shaped structures are two typical components and have attracted much attention.This work analyzed the deformation evolution of the L-shaped composite structure during cure using the validated model,and investigated the impact of mold constraint on the deformation.For the T-stiffened skin assembly,the stiffener plays a significant role in the overall performance.This paper studied the influence of the stiffener on stress concentration,stress redistribution and deformation of the composite assembly during cure.To analyze the dependence of residual stress and deformation on the stiffener's size,a series of assemblies with different fillet radii,flange lengths and web heights were established.Finally,the sensitivity of residual stress and deformation on the stiffener's size was concluded.In this work,the fiber Bragg grating(FBG)temperature sensor and strain sensor were embedded in the composite laminate in order to in situ monitor the temperature and strain evolution.The reliability of the monitored strain can be evaluated by comparison with the simulated result.The results reveal that for the L-shaped structure,the spring-in phenomenon can be observed after cure.When the mold constraint is taken into account and free sliding is allowed between mold and part,the deformation decreases after demolding compared with the case without mold constraint;if no sliding is allowed between mold and part,the deformation increases significantly after demolding compared with the case without mold constraint,and the deformation increases as the coefficient of thermal expansion of mold increases.For the T-stiffened skin assembly,the overall residual stress is mainly sensitive to fillet radius,while the residual stress of the skin component is almost not affected by the stiffener's size.The maximum deformation of the assembly is located in the skin component,and is mainly sensitive to the flange length.For the FBG monitoring,the monitored temperature by FBG temperature sensor can accurately reflect the actual temperature in the laminate.The monitored strain by FBG strain sensor cannot precisely characterize the actual strain evolution at the early stage of cure.Nevertheless,when the interfacial strength between resin and FBG increases enough to transfer the resin strain effectively,the monitored strain change matches well with the actual strain change.