Study On Dynamic Compressive And Tensile Properties Of Mg Alloy Based Hybrid Nanocomposites

Advanced magnesium alloy based nanocomposites reinforced with carbon nanotubes(CNTs) and nanoparticles have excellent dynamic mechannical performance and they would be widely used in automobile and aeronautical fields. The research on dynamic mechanical properties of magnesium alloy based composites has an important guiding meaning for their optimal design and manufacture.The dynamic behavior of the nano-composites was studied by experiments and finite element method (FEM). Several kinds of composites have been made in which the total mass fraction of surface modified multiwalled carbon nanotubes(MWCNTs) and SiC nanoparticles always is1%. The microstructure was characterized by metalloscope and tranmission electron microscope(TEM). Quasi-static compressive and tensile mechanical properties of magnesium matrix nanocomposites with different hybrid ratios were tested. It is found that both the tensile and compression performance were improved compared with magnesium alloy matrix, the tensile mechanical property is better when the hybrid ratio of CNTs and nano-sized SiC particles is7:3, while the compression performance is better when the hybrid is3:7, and it is also found that the tensile and compressive stress-strain curves were not symmetrical. Then the dynamic compressive and tensile mechanical properties for magnesium alloy matrix and its composites were tested by SHPB technique and simulated by FEM respectively, and the parameters of the magnesium alloy matrix constitutive relation were identified. The numerical predictions of dynamic compressive and tensile mechanical properties for the composites were found to represent the observed behaviour in the experimental tests fairly well, and this provides the reliable constitutive parameters for the dynamic constitutive relation at high temperature of the heterogeneous anisotropic composites. Finally, Two-dimensional micromechanics models for magnesium nanocomposites were set by using finite element software LS-DYNA, to simulate the local stress field near the reinforcements. In addition, the stress-strain curves were ploted based on the simulative results, and the influences of the hybrid ratio, volume fraction, temperature and strain rate on the dynamic compressive and tensile mechanical properties of the composites were discussed. FEM calculation results show that magnesium based nanocomposites exhibit significantly higher yield stresses and compressive/tensile strengths, much better ductility, and thus a higher energy absorption capacity under dynamic compressive and tension. Among of them, the dynamic compression properties is best when the hybrid mass ratio is5:5for CNTs and SiC nanoparticles. The Johnson-Cook constitutive models of the magnesium composites under dynamic compressive were fitted based on the calculated stress-strain data. It is found that the flow stress increased with the increase of strain rate and decreased with the increase of temperature, but the influence of the strain hardening was greater. When the dynamic tensile behaviour was compared with results of dynamic compression behavior, and significant tension-compression asymmetry in the response for the magnesium based nanocomposites is observed. A possible reason for this asymmetry is the occurrence of twinning in compression and its absence in tension.