Investigating Size Effects In Microcosmic Elasto-Plastic Deformations Under Complex Loadings

Micro-scaled metal parts are extensively used in micro electromechanical system(MEMS),which is the driving force of product intellectualization.mechanical properties at the micro-scale are obviously different from the macro-scale counterpart,which is mainly manifested as a series of micro-scale effects.Micro-scale mechanical properties such as yield strength are closely related to the specimen size.The macroscopic classical plasticity theory does not include intrinsic parameters,so it is not able to capture the micro-scale effects.Based on the strain-gradient elastoplastic theory,in this paper we pay attention to the contributions of both elastic and plastic deformations during elastoplastic deformation processes.The specific research contents are as follows:1.A simplified version of strain gradient elasto-plasticity is proposed.We have neglected both the higher-order stresses conjugated with strain gradients and the elastic characteristic lengths which is far smaller than plastic ones.As a result,we have achieved a model with one single characteristic length.Correspondingly,the expression is updated for the total elasto-plastic effective strain.Numerical examples show that the simplified theory can still work well in capturing main features on the micro-scale.2.Tension-torsion tests on micro-scaled wires are investigated.We study the torsional mechanical response of the wire with different degrees of pre-stretching deformation.Studies have shown that the wire still has relatively obvious micro-scale effects in the cyclic torsion stage after pre-stretching,but the pre-tension significantly reduces the yield strength during torsion.According to the Taylor-based flow rule,pre-tension decreases the elastic deforming zone of the torsion stage.When pre-tension is large enough,the torsion process begins directly with plastic deformation.In the process of cyclic torsion,the pre-tension deformation will weaken the anomalous Bauschinger effect,plastic softening as well as the extra gradient hardening.In a word,this study shows that the torsional yield strength of wires depends on its fabrication and machining histories,which serves as a possible reasonable explanation for differences among experimental results reported in the literature.3.Dynamic growth of micro-scaled voids are solved by accounting for influences of inertia,strain gradients and temperature softening.We analyze the effects of the loading rate of the far-field hydrostatic tensile load and the different hydrostatic tensile loads.Studies have shown that in the early stage of void growth,the inertia effect will hinder the growth of the void.The larger void has a higher degree of hindrance.When the growth reaches a certain stage,the inertial effect will promote the continuous growth.The gradient effect greatly increases the yield strength near the void surface,because the yield strength in the neighborhood of voids is greater than the yield strength at the macroscopic scale.The gradient effect has a great hardening effect on the vicinity of the void surface,improves the threshold stress of the void dynamic growth and delays the void dynamic growth.Under fast loadings,most of the plastic work will be converted into heat energy,which will cause the local temperature of the void surface to rise,and even reach the melting point.This temperature rise will soften the metal material near the void surface,reduce the threshold stress for continuous void growth,and somehow contradict the gradient hardening effect.A larger loading rate will promote the rapid growth of the void.A higher hydrostatic tensile load will cause a bigger growth rate.