**Numerical Evaluation on Mechanical Properties and Deformation Behaviors of the LSEVI Processed Csf/Mg Composites**

Author:Tian Wen Long

Supervisor:qi le hua

Database:Doctor

Degree Year:2017

Download:73

Pages:187Size:5973K

Keyword:Csf/Mg composites，Liquid-solid extrusion following vacuum pressure infiltration，Mechnical properties，Numerical homogenization，Two-step mean-field homogenization

Short carbon fibers reinforced magnesium alloy matrix composites(Csf/Mg composites)possess the high specific strength,high specific stiffness,low coefficient of thermal expansion and the excellent thermal conductivity,wear resistance,vibration resistance and high tempera-ture performance,which consequently have gained the broad application prospects in the avia-tion,aerospace,automotive,national defense and other fields,etc.,.The liquid-solid extrusion following vacuum pressure infiltration(LSEVI)technique is a new type forming technique for the metal matrix composite proposed by our research group in recent years,and is especially suitable for the preparation of Csf/Mg composites.The essence of this method is to apply a certain gas pressure to the molten magnesium alloy matrix in the protective gas environment,so that the molten magnesium alloy matrix is uniformly infiltrated into the fiber preform and then solidificated by introducing the mechanical pressure.Meanwhile,Csf/Mg composites under the quasi-solid state are extruded and compacted to improve their properties,because of the low deformation resistance and easy flowing of magnesium alloy matrix.The investigations of the mechanical properties and deformation behaviors of the LSEVI processed Csf/Mg composites,not only provide the theoretical reference to the engineering applications of Csf/Mg compos-ites,but can guide the design and fabrication of Csf/Mg composites with high performance.Therefore,there is no doubt that it is very important to predict the mechanical properties and deformation behaviors of the LSEVI processed Csf/Mg composites.Because that experimental tests require expensive experimental equipments and long exper-imental cycle,numerical simulation methods appear to become the excellent choices to predict the mechanical properties and deformation behavior of the LSEVI processed Csf/Mg compos-ites.The mechanical properties and deformation behaviors of the Csf/Mg composites are not only closely related to the mechanical properties of magnesium alloy matrix and short carbon fiber,but also depends on their microstructures.Regarding the complex microstructures,it is difficult to accurately predict the mechanical properties and deformation behavior of the Cs-f/Mg composites by using the traditional numerical simulation methods.Thus,in this study,the numerical homogenization method and the two-step mean-field homogenization method are proposed to predict the mechanical properties and deformation behavior of the Csf/Mg compos-ites.The main contents of this paper are given as follows,(1).The experimental tests of the micro-structures and mechanical properties of the LSEVI pro-cessed Csf/Mg composites.Through the introduction of the LSEVI technique,Csf/Mg compos-ites are fabricated,whose micro-structures,tensile fracture morphology and mechanical proper-ties are then experimentally tested.The results of the experimental tests illustrate that the short carbon fibers are uniformly distributed and there is no defect in the LSEVI processed Csf/Mg composites,and that the bonded strength of the interfaces between the fibers and matrix in the LSEVI processed Csf/Mg composites are moderate.Because of the addition of short carbon fibers into the magnesium alloy matrix,the tensile strengthes and elastic moduli of the Csf/Mg composites are significantly increased,but the fracture elongation and coefficients of thermal expansion of the Csf/Mg composites are significantly decreased.The experimental tests of the micro-structures and mechanical properties of the LSEVI processed Csf/Mg composites can provide experimental support for the assumptions of both numerical homogenization and two-step mean-field homogenization methods,and can also verify the accuracy of both numerical homogenization and two-step mean-field homogenization methods.(2).The numerical evaluation of the mechanical properties of the Csf/Mg composites.Based on the micro-structures,the RVEs of the Csf/Mg composites are generated by implementing the modified RSA algorithm.The periodic boundary condition and numerical homogeniza-tion method are adopted to predict the mechanical properties of the Csf/Mg composites.The predicted mechanical properties of the Csf/Mg composites by the numerical homogenization method are compared against those obtained by the experimental tests and Halpin-Tsai equa-tion such that the numerical homogenization method to predicted mechanical properties of the Csf/Mg composites is validated.The mechanical properties predicted by the numerical ho-mogenization method can be considered as the standard mechanical properties of the Csf/Mg composites.Thus,the numerical homogenization method can be used to validate the two-step mean-field homogenization method.(3).Two-step mean-field homogenization evaluation for thermo-elastic properties of the Cs-f/Mg composites.According to the fiber orientation distribution,the Csf/Mg composites are decomposed into a series of peuso-grains.The mean-field homogenization models are used to predict the thermal-elastic properties of the each decomposed peuso-grain and to average the thermal-elastic properties of all peuso-grains.Consequently,The M-T/Voigt,M-T/Reuss,D-I/Voigt,D-I/Reuss and Q-I/S-I two-step mean-field homogenization models are established and their numerical implementation algorithms are given to predict the thermo-elastic properties of the Csf/Mg composites.Compared with the numerical homogenization method and experimen-tal tests,the M-T/Voigt、M-T/Reuss、D-I/Voigt、D-I/Reuss and Q-I/S-I models are verified to predicted the thermo-elastic properties of the Csf/Mg composites,among of which Q-I/S-I model provides the most accurated predictions.(4).The two-step mean-field homogenization prediction for elasto-plastic deformation behav-iors of the Csf/Mg composites.Combining the secant and tangent formulation constitutive mod-els,the two-step mean-field homogenization method is extended to predict the elasto-plastic deformation behaviors of the Csf/Mg composites and M-T/Voigt,M-T/Reuss,D-I/Voigt and D-I/Reuss two-step mean-field homogenization models are established.Compared with the experimental tests and numerical homongenization method,M-T/Voigt and D-I/Voigt models based on the secant and tangent formulations are validated to predict the elasto-plastic defor-mation behaviors of the Csf/Mg composites.The two-step mean-field homogenization model with secant formulation can only predict the uniaxial tension and shear deformation behaviors of the Csf/Mg composites,while the two-step mean-field homogenization model with tangent formulation can predict the uniaxial tensile,shear and cyclic loading deformation behaviors of the Csf/Mg composites.(5).The two-step mean-field homogenization prediction for elastic properties of the Csf/Mg composites at an elevated temperature and containing a small fraction of liquid phase.Based on the material constitutive method,the liquid magnesium alloy matrix phase is assumed to be equivalently isotropic elastic,and then the elastic properties of the Csf/Mg composites at an elevated temperature and containing a small fraction of liquid phase are predicted by the two-step mean-field homogenization method.By comparing to the numerical homogenization method,the Q-I/S-I model is validated to accurately evaluate the elastic properties of Csf/Mg composites at an elevated temperature and containing a small fraction of liquid phase.(6).The research on the factors affecting mechanical properties and deformation behaviors of the Csf/Mg composites.Using the Q-I/S-I two-step mean-field homogenization model,the effects of fiber volume fraction,fiber aspect ratio,fiber orientation and fiber material properties on the thermo-elastic properties of the Csf/Mg composites are investigated.Based on the M-T/Voigt two-step mean-field homogenization model with the second-order secant and tangent formulations,the effects of fiber volume fraction,fiber aspect ratio,fiber orientation and matrix material properties on the elasto-plastic deformation behaviors of the Csf/Mg composites are investigated.Accroding to the Q-I/S-I two-step mean-field homogenization model,the effects of fiber phase and liquid magnesium phase on the elastic properties of the Csf/Mg composites at an elevated temperature and containing a small fraction of liquid phase are investigated.