Study On The Residual Stress Of 3D-C_f/Mg Composites

Three-dimensional carbon fiber reinforced magnesium composites (3D-C_f/Mg) are candidate materials for aerospace structures and advanced weapon systems due to their high specific strength, high specific stiffness and low coefficient of thermal expansion (CTE). In addition, compared to unidirectional composites and laminated composites, 3D-C_f/Mg is more competitive because of its higher stiffness and strength in three orthogonal directions, especially in thickness direction. However, there is a serious mismatch of volume shrinkage and coefficient of thermal expansion (CTE) between the carbon fiber frame and magnesium alloy, resulting in a large residual stress in 3D-C_f/Mg, which will affect the mechanical properties of materials, and even lead to material cracking.In order to improve the mechanical properties, this thesis will investigate the residual stress of 3D-C_f/Mg composites after different treatments. Firstly, a nonlinear elastoplastic model is proposed and a finite element method is used to study the thermal residual stress in 3D-C_f/Mg study and its modification after different treatments. Then, an experimental investigation was realized by SEM and XRD, which indicates the influences of thermal residual stress on the microstructure of the composites. Finally, the mechanical properties of 3D-C_f/Mg composites are studied in order to analysis the influence of residual stress on the strength, modulus and hardness.The result of numerical simulation shows that high-temperature prepared 3D-C_f/Mg exhibit a large thermal residual stress, which is 147.32MPa on the average in the matrix, after cooling to room temperature. After heat-treatment at 120℃and 240℃, thermal residual stress in the matrix does not change significantly, which is 138.38MPa and 138.25MPa respectively. However, after cold-treatment, the residual stress in the matrix significantly decreases. After cold-treatments at -78℃and -196℃, the average residual stress in the matrix respectively decreases to 99.40MPa and 34.16MPa.Analyze the microstructure of the materials by SEM and XRD after heat-treatment at 120℃and cold-treatment at -196℃. The results show that high-temperature prepared 3D-Cf/Mg exhibit a large thermal residual stress, causing the deformation of cross-section of the carbon fiber and an increase of interplanar spacing of the matrix. After heat-treatment at 120℃, the residual stress changes a little, therefore, the cross-section of the carbon fiber is still oral and the interplanar spacing of the matrix is slightly larger after treatment. However, after cold-treatment at -196℃, the residual stress obviously decreases, so the cross-section of carbon fiber changed from oval to round and the interplanar spacing of the matrix return to the value of that of AZ91D.Finally, study the influence of the residual stress on the strength, modulus, and hardness the composites. The results show that after heat-treatment, the residual stress decreases slightly. And the flexural modulus and hardness decreased slightly, respectively by 5.5% and by 4.2%. However, as the plastic property of the matrix has been improved, the flexural strength increased by 7.0%. In the other hand, after cold-treatment, the residual stress in the composite significantly reduced and the flexural modulus increased by 9.2%. However, the flexural strength and hardness decreased by 19.9% and 17.2% respectively.