Study On Raman Microscopy Of Hygrothermal Residual Stress In 3D Braided Carbon Fiber/Epoxy Resin Composites And Finite Element Analysis

Three-dimensional braided carbon fibers reinforced epoxy resin composites (C_3D/EP) are formed by the four-step braided reinforcement and epoxy matrix. The overall strength and stiffness of the composites have been significantly improved. It will certainly be widely applied in many fields, including aviation, aerospace, civil infrastructure and chemical industry. The composite will inevitably serve in hot and humid environment and other harsh environment. Because of the huge difference of the coefficient of thermal expansion (CTE) and wet expansion (CWE) between the fibers and resin, residual stresses usually occur in hot and humid environment. In this paper, Raman spectroscopy was used to analyze the residual stress in the composite, and the results are as follows.Firstly, the moisture absorption curves of C_3D/EP composites with different braided angles were obtained in the enviorment of T=37℃and R_H=100%. Moisture stress field of different fiber directions in the composites of different braided angles were characterized by Microscopic Raman Spectrometer at 0 day, 196 days and 335 days, respectively. The result showed that during the long-term moisture absorption process, the axial residual stresses in the fibers all decreased and tended to be stable in a range.The thermal residual stress test of unidirectional fiber reinforced composite (CL/EP) that has the same 45% volume fraction with C_3D/EP indicated that, the thermal residual stress of C_3D/EP in the preparation process was 4 to 5 times higher than that of CL/EP, the reason for which was the fibers in the C_3D/EP seriously squeezed and twisted.This paper also applied the Raman mapping to test the microarea in the C_3D/EP and obtained the residual stress in the surface of the composite. The result was conventional with the traditional point Raman spectroscopy, which can be used for guiding future work.The modeling of C_3D/EP composite was established by UG in this paper. The whole model of the composite was established and ABAQUS was used to simulate and analyze the stress field induced by moisture. This can provide a reference to evaluate the strength and life of composite in service process.Furthermore, A new Adaptive Kalman filter (AKF) was adopted to determine two critical moisture diffusion parameters, that is, maximum moisture content and the diffusivity, for C_3D/EP composite exposed to the humidity of R_H=100% and T= 37℃. The conclusive result was, maximum moisture content C=0.74% and diffusivity D= 6 .7×10^-4 mm²/s. With these best estimates obtained from the AKF as inputs, the moisture diffusion process was simulated by finite element program ABAQUS and the close match between the simulated results and the experimental observations supports the accuracy of the estimated diffusion parameters determined from the AKF.