A split Hopkinson pressure bar(SHPB)device in two-by-two form,including the bar bundle form and the single cylindrical bar form,was designed in response to the demand for the dynamic mechanical experiments for brittle materials such as concrete,rock,etc.The stress waveforms generated through a projectile impacting two different types of incident bars have been studied based on the one-dimensional stress wave theory and numerical simulation method.At last,based on the established two types of mesoscale concrete models with random convex polyhedral aggregates,we performed comparison analysis of SHPB numerical simulations for concrete materials with bar bundle and single cylindrical bar separately,so as to provide technical support for the manufacture and development of this experimental device.The results showed that the introduced two-by-two form SHPB device expanded the scope of practical application,and the wave dispersion effect existing in a large-diameter bar can be evidently reduced when we employed the bar bundle form,indicating its applicability to the dynamic mechanical experiments of concrete-like heterogeneous materials.
Ruiqi GuoHuiqi RenLei ZhangZhilin LongXiangyun WuHailu Wang
This paper uses the discrete element method to model the size and cushion effects during single-particle crushing tests.We propose simplified numerical modeling to examine the effects of particle size and coordination number on particle breakage behavior.We validate the proposed modeling by comparing the numerical results with the experimental data reported in the literature,in terms of the variability of particle tensile strength and axial force-displacement responses.Based on the numerical results,it is clear that a larger particle size entails a higher tensile strength with a larger discreteness.In addition,the characteristic tensile strength increases linearly with an increasing coordination number.Moreover,smaller particles are more susceptible to the cushion effect than larger particles.The numerical results also indicate that an increasing coordination number induces a more ductile mode of failure.Based on these results,we propose an empirical equation for calculating tensile strength,incorporating both the cushion effect and the size effect.
Du-min KuangZhi-lin LongRui-qi GuoPiao-yi YuXu-tong ZhouJie Wang
随着晶体管尺寸按比例缩小,越来越薄的氧化层厚度导致栅上的隧穿电流显著地增大,严重地影响器件和电路的静态特性,为此,基于可靠性理论和仿真,对小尺寸MOSFET(metal-oxide-semiconductor field effect transistor)的直接隧穿栅电流进行研究,并通过对二输入或非门静态栅泄漏电流的研究,揭示直接隧穿栅电流对CMOS(complementary metal oxide semiconductor)逻辑电路的影响。仿真工具为HSPICE软件,MOS器件模型参数采用的是BSIM4和LEVEL 54,栅氧化层厚度为1.4 nm。研究结果表明:边缘直接隧穿电流是小尺寸MOS器件栅直接隧穿电流的重要组成成分;漏端偏置和衬底偏置通过改变表面势影响栅电流密度;CMOS逻辑电路中MOS器件有4种工作状态,即线性区、饱和区、亚阈区和截止区;CMOS逻辑电路中MOS器件的栅泄漏电流与其工作状态有关。仿真结果与理论分析结果较符合,这些理论和仿真结果有助于以后的集成电路设计。
