Study Of Surface Passivation Effect Of Smallscale Materials Based On Higher-order Strain Gradient Plasticity Theories

With the rapid development of micro-nano technology,a large number of micron or submicron metal devices are applied in micro-electromechanical systems and integrated circuits.In miniaturized equipment,the microscale metal structure almost always appears in the form of a combination of one or more external media,the deformation generated in the process of processing and use affects each other,and the metal structure generates a dense oxide layer on the surface due to exposure to air.These physical constraints on the plastic deformation formed on the surface or interface will lead to size effect and passivation effect.Since the traditional plasticity theory does not include length scales,it is impossible to explain such mechanical behavior.The prediction and simulation based on strain gradient plasticity theory is the main method to study the size effect and passivation effect.It is of great significance to better understand the effect of surface passivation on the mechanical behavior of materials in both basic science and related technical applications.In this thesis,the surface passivation effect in microscale field is taken as the breakthrough point,and two kinds of higher-order strain gradient plasticity theory are numerically implemented by using the finite element method,respectively,and the influence of surface passivation and size of material on the mechanical behavior of microscale materials are studied.The main contents include:(1)A numerical formulation of the variational constitutive updates for the FleckHutchinson higher-order strain gradient plasticity theory has been constructed to solve the initial boundary value problems.In this formulation,the potential energy functional is discretized into a set of linear equations by using the variational extremum principle.This method is used to solve the boundary value problem in the field of uniform displacement,and the effects of dissipative strain gradient and surface passivation are revealed.Compared with the differential equation method,this method shows better computational stability and accuracy.(2)Based on Gudmundson higher-order strain gradient plasticity theory,a planar 8-node element containing both displacement and plastic strain degrees of freedom is established and the passivation effect can be modeled by constraining the degrees of freedom of plastic strain.The thin metal slab under shear,the thin film with surface passivation under tension and the thin film under bending are modeled by using the element.Closed form solution of higher-order equilibrium equation for wire torsion with different boundary conditions is obtained by ignoring the higher-order dissipative stress,and the effect of surface passivation on the materials with purely energetic gradient is analyzed.