Study On Hygrothermal Residual Stress Of 3D Braided Carbon Fiber/epoxy Resin Composites

Because the three-dimensional braided carbon fiber reinforced epoxy resin matrix composites is formed by the reinforcement which have the complicated geometric structure and matrix, so it could overcome the problems of conventional laminated composites, such as low strength in the thickness direction,low interlaminar shear strength and are prone to interlaminar delamination, so, greatly improve it strength and stiffness. As a result, it will certainly be widely applied in many fields, including aviation and spaceflight. However, Composites are susceptible to heat and moisture during the storage and long-term service. due to the significant mismatch in the moisture induced volumetric expansion between the matrix and the fibers, and thus leads to the evolution of localized stress fields in the composite, thereby forcing the mechanical properties of composite to change. In this paper, the stress value of C3D/EP composite was measured by using Micro-Raman Spectroscopy technique. The whole model of composite was established and ABAQUS was used to simulate and analyze the stress field induced by moisture.Firstly,Moisture stress field of different fiber directions in the composite were characterized by Microscopic Raman Spectrometer. The result shows that during the long-term moisture absorption process,the axial residual stresses within the fibers transform from thermal resiual compressive stresses before absorption into hygrothermal tensile stresses after absorption. This fact suggests that tensile residual stress was generated during moisture absorption process. And residual stress caused by long-term moisture can reach a higher level . Moreover, the evolution process of stress fields for carbon fiber measured in different directions are different to some extent. The temperature has apparent impact on stress fields due to moisture induced expansions, which was illuminated by the fact that the stress level of the specimens at the exposure duration was higher for T=80℃than that observed for T=37℃.Furthermore, A new approach-Kalman filter was adopted to determine critical moisture diffusion parameters such as: maximum moisture content, and the diffusivity for C3D/EP composite exposed to the humidity of RH=100%, temperature at 37℃. The conclusive result are: maximum moisture content C*=0.93%, diffusivity D= mm2/s. Using these best estimates obtained from the kalman filter 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 kalman filter.At last, ABAQUS was used to analyze the stress field of carbon fiber epoxy composite materials during moisture absorption process with the three-dimensional model. The analyzed results show that the calculated results and experimental results of moisture absorption curve are consistent. The results of three-dimensional model show that the value of stress was increased with the moisture absorption time, so as to the axial stress of carbon fiber which was about 45.5 MPa when moisture saturation.