Shear Deformation And Failure Of Metallic Glasses And Their Composites

Bulk metallic glasses(BMGs) and their composites have many excellent mechanical,physical and chemical performances, especially, in the condition of low temperature(below the glass transition temperature) as well as high stress or high strain rate, the materials are easily to behave as localized shear banding manner and display an intense shear sensitivity.Thus, these kinds of materials may be employed in the kinetic enrgy penetrator(KEP), and they are becoming as an active research focus in the military fields of high-speed penetration and armor protection. In the present thesis the shear banding characteristics in the metallic glasses and their composites, includeing the nucleation and propagation of shear bands, and the shear banding-induced failure, are studied based on the finite element method(FEM) simulations, and the shear deformation and failure performances as well as the “self-sharpening” behavior of composite long rods are analyzed.Firstly, based on the coupled thermo-mechanical model, a constitutive model for BMGs,which considers the effects of free volume, temperature and hydrostatic stress, has been further deduced and generalized to the multi-axial stress status. Besides, a failure criterion of critical free volume concentration is introduced from the coalescence mechanism of free volume. The constitutive model and the failure criterion are implemented into LS-DYNA commercial software through the user material subroutine(UMAT), and then FEM simulations on the deformation and failure of BMGs under different scenarios are conducted. The evolutions of different material parameters and the corresponding macroscopic mechanical behaviors are analyzed. It is found that various factors, e.g., the initial free volume concentration, strain rate, initial temperature and the hydrostatic stress,have significant effects. The deformation is dominated by the microstructure of material(i.e., free volume), and the temperature plays an assistant role. Both the strain rate and the initial temperature affect the mechanical behavior of BMGs, especially the influence of strain rate is very significant. Besides, the hydrostatic stress will induce a deformation asymmetry between tension and compression.With considering the inhomogeneity of the inner structure of BMGs, the FEM geometrical model of the randomly distributed shear-band zones in the material is established. FEM simulations for different deformation scenarios, including quasi-statictension, compression and bending as well as instrumented anvil-on-rod impact, are conducted, and the corresponding deformation and failure mechanism of BMGs is analyzed.Relative analysis shows that BMGs exhibit a significant inhomogeneous deformation at the room temperature, i.e., the deformation and failure usually localize into several shear bands.When under the quasi-static condition, the nucleation and propagation of shear band depend on both the stress status and the distribution of shear-band zones. Comparatively, under the impact condition, the effect of shear-band zones decreases.The geometrical models of the fiber / particle reinforced metallic glass(MG) matrix composites are established based on their inner structures, and the modified coupled thermo-mechanical constitutive model is employed to describe the high strength and high shear sensitivity of MG matrix. FEM simulations on the deformation and failure of composites are conducted systemically by integrating with the related quasi-static, dynamic and high-speed impact tests. It shows that the compressive plasticity of composite could be significantly improved compared with the monolithic MG material. However, its tensile plasticity is relatively hard to be improved. When under compression, the deformation mode of fiber reinforced composite will shift with the variations of the internal and external conditions, e.g., the volume fraction of fiber and the strain rate, etc. Comparatively, the particle reinforced composite mainly behaves as shear manner. In the tensile condition, the both kinds of composites behave as brittle fracture, and the fracture surface is almost perpendicular to the loading direction. The corresponding deformation and failure characteristics are firmly related to the shear banding behavior within the material.Finally, integrated with related penetrating tests, FEM simulations on the penetrations of tungsten fiber / metallic glass matrix(WF/MG) and tungsten particle / metallic glass matrix(WP/MG) composite long rods into the targets are conducted. The “self-sharpening”behaviors of composite long rods are analyzed, and the effects of impact velocity, target strength and initial shape of rod nose on the “self-sharpening” property of rod and the consequent penetrating performance are discussed in detail. Besides, the similarity and the difference between these two kinds of composite long rods are analyzed.