Sliding behavior of metallic glass: Part II. Computer simulations

Molecular dynamics (MD) calculations were used to simulate the sliding of a two-component 2D amorphous system interacting via Lennard–Jones potentials. The friction coefficient showed a transient before reaching an average steady state value. The steady state friction coefficient was observed to decrease with an increasing sliding velocity. Mixing was observed at the sliding interface. The mixed layer grew at a rate that scaled with the square root of time. A density decrease was recorded in the region adjacent to the sliding interface. This spatially corresponded to the softer layer detected experimentally near the worn surface in a Zr_41.2Ti_13.8Cu_12.5Ni_10.0Be_22.5 bulk metallic glass alloy after sliding. Subsurface displacement profiles produced in these simulations were similar to those observed in other material systems. The Navier–Stokes equation was used to analyze the material flow pattern, with results in agreement with data obtained from simulations. This suggests that the observed subsurface displacement profile may be a generic material flow pattern under combined compression and shear.     

Slip-line field solutions of three-point bend specimens with deep notches

Slip-line field solutions of three-point bend specimens with different notch depths, notch angles and notch root radii are presented. For notch angles less than the critical value the plastic region is localized at the ligament but for larger notch angles the plastic region spreads to the flanks of the notch. It is shown that the critical angle decreases with increasing notch depth and decreasing root radius. Solutions for the constraint factor and maximum hydrostatic stress beneath the notch tip are also obtained as a function of both notch depth and root radius for notch angles above and below the critical value.     

Simple apparatus for measuring the dynamic shear modulus of cylindrical specimens

An apparatus is decribed which has been used to determine the shear modulus of cylindrical specimens vibrating in the torsional mode. The specimens are maintained in self-excited vibration by an eddy-current driver, an eddy-current pickup, and a feed back circuit consisting of a phase-locked-loop and an amplitude control module. The system has a frequency stability of 1 part in 10(6) for high-Q materials (a few thousand) in the audio range. In addition, it can automatically track frequency changes of a few percent with negligible error in phase or frequency.     

浅谈Y-74-11№28D型风机三点动平衡做法的经验步骤及"定律"

介绍了Y-74-11№28D型锅炉引风机三点动平衡做法的经验步骤及"定律".     

Experimental investigation of dynamic effects in a two-bar/three-point bend fracture test

A Hopkinson pressure bar has been modified to measure the dynamic fracture properties of materials at loading rates greater than approximately 10(6) MPa ms. Some fundamental dynamic effects associated with the incident stress pulse, such as stress wave propagation characteristics along the Hopkinson bar and within the cracked specimen, the specimen's dynamic response excited by the stress pulse, and the specimen contact situations with the impactor and supports, need to be understood. To better comprehend these fundamental issues, an experimental investigation of these dynamic effects with the emphasis on "loss of contact" was first performed on a two-bar/three-point dynamic bend fracture test setup using a voltage measurement circuit across the specimen/loading-pin interfaces and high-speed photographs. It was demonstrated here that the three-point bend specimen employed with the current two-bar/three-point bend test setup remains in contact with the impactor and supports throughout the first loading duration and that "loss of contact" does not occur. A further improvement using a pulse-shaping technique was employed for achieving a tailored incident pulse. The effect of pulse shaper on the rise time and duration of the incident pulse as well as the dynamic stress equilibrium in the cracked three-point bend has been investigated, for the first time here, with profound implications for significantly improved dynamic three-point bend fracture testing.     

Aggregation behavior analysis of hydrate particles in the bend pipe based on the population balance model

Journal of Mechanical Science and Technology - The hydrate particle aggregation is crucial for the flow safety of oil and gas pipelines. In this paper, coupled with the Eulerian-Eulerian model,...     

三峡地下电站上弯段双曲钢模台车有限元分析

三峡地下电站引水隧洞上弯段属高速高压水流区,空间结构特殊,对衬砌混凝土质量要求高,因此本项目首次提出采用新型隧洞双曲钢模台车进行衬砌施工的方案。按照台车的浇筑顺序,对不同工况进行了有限元分析,完善了台车的结构设计,提高了双曲钢模台车的可靠性和使用寿命,为工程实践提供了较强的理论支撑。     

高速带式输送机的计算机辅助动态设计

运用计算机仿真技术分析了带式输送机在高速运行状态下的动态特性以及输送机配置对提速的影响。介绍了“带式输送机辅助动态设计系统”的功能与结构，该系统根据设计者的输入配置输送机参数，并对输送机在各种速度下的运行状态进行动态仿真计算，记录各种配置下输送机的动态特性，作为高速输送机动态性能设计的辅助手段。     

Analysis of variations of the dynamic characteristics of acoustic-emission sources under static loading of metal specimens

Changes in the energy of acoustic-emission (AE) signals and their cross-correlation coefficient for sources of different types have been experimentally studied for statically loaded specimens. The results allow separation of AE signals appearing during irreversible changes in the region of stress concentrators and signals of other types.     

压力管道动态断裂行为研究

这里研究了压力管道在振动状况下的动态断裂特性。计算了在流体激励下管路系统的各阶模态及位移响应,并利用1/4点位移法对管道中各个位置裂纹的动态应力强度因子进行了有限元计算。经与理论值及静态值结果比较表明,动态下的应力强度因子大约增加10%左右。这里工作对于压力管道的可靠性分析及动态断裂安全评定具有重要的参考价值。     

Computer simulation of dynamic characteristics of tandem cold rolling process

A computer simulation program that can analyze the dynamic behaviors of tandem cold rolling process without laborious experiments in actual mill was developed. By using this simulation program, the stability and accuracy of strip thickness control system were evaluated for various disturbance such as hot band gage, roll eccentricity, and deformation resistance of hot rolled strip. Herein the simulation program was described, and the results of simulation on feedback and feed-forward Automatic Gage Control were quoted as examples showing the effects of analysis on dynamic characteristics.     

Influence of manufacturing errors on the dynamic behavior of planetary gears

Planetary gears are widely used in the transmissions of helicopters, automobiles, aircraft engines, etc. They have substantial advantages such as compactness and a large torque-to-weight ratio. In this work, a plane model of a planetary gear was investigated. The energetic Lagrange formulation was used to recover the equations of motion of the system. A modal analysis was performed, and the influence of gyroscopic effect in particular was scrutinized. The dynamic response was computed by an iterative spectral method. The excitation is induced by time-varying the gearmesh stiffness. The cases of a healthy planetary gear and one with the presence of eccentricity and profile error were compared. The influence on the transmission ratio was also studied.     

Study of dynamic crack propagation in 3PB steel specimens loaded by impact

Crack propagation in ductile steel was investigated using impact-loaded three-point bending (3PB) specimens. Results from experiments and numerical simulations were compared. The specimens were 320 × 75 mm by 10 mm thick. A new 3PB specimen design with reduced width at the ends was developed to avoid the influence of uncertain boundary conditions at the impact heads. One static and two dynamic tests with impact velocities of 30.2 and 45.2 m/s were performed. High-speed photography was used to obtain crack growth and crack tip opening displacement data. Moiré interference patterns were used to directly measure the relative rotation of the two specimen halves. Shear lip fracture surfaces were obtained for all three loading conditions. The experiments indicated no, or only a slight, loading rate influence on the crack propagation.     

Prediction and control of pillow defect in single point incremental forming using numerical simulations

Pillows formed at the center of sheets in Single point incremental forming (SPIF) are fabrication defects which adversely affect the geometrical accuracy and formability of manufactured parts. This study is focused on using FEA as a tool to predict and control pillowing in SPIF by varying tool size and shape. 3D Finite element analysis (FEA) and experiments are carried out using annealed Aluminum 1050. From FEA, it is found out that the stress/strain state in the immediate vicinity of the forming tool in the transverse direction plays a determinant role on sheet pillowing. Furthermore, pillow height increases as compression in the sheet-plane increases. The nature of in-plane stresses in the transverse direction varies from compressive to tensile as the tool-end geometry is changed from spherical to flat. Additionally, the magnitude of corresponding in-plane stresses decreases as the tool radius increases. According to measurements from the FEA model, flat end tools and large radii both retard pillow formation. However, the influence of changing tool end shape from hemispherical to flat is observed to be more important than the effect of varying tool radius, because the deformation zone remains in tension in the transverse direction while forming with flat end tools. These findings are verified by conducting a set of experiments. A fair agreement between the FEM and empirical results show that FEM can be employed as a tool to predict and control the pillow defect in SPIF.

     

三支承车床主轴的刚度及参数优化

对三支承车床主轴从辅助支承发生作用的条件出发,假设3个支承在载荷作用下的变形呈线性关系,由此对辅助支承约束力求解,从而将超静定问题转化为静定问题,得到了主轴端部柔度(刚度的倒数)计算公式,然后根据主轴部件设计的需要,采用遗传算法对主支承跨距、辅助支承跨距和传动力作用位置等参数进行综合优化,对三支承结构的机床主轴设计有一定的参考意义。     

柔性四杆机构动力学行为研究

针对柔性四杆机构中的典型机构——曲柄摇杆机构,运用有限元方法建立了机构的瞬时模型,进而采用KED方法对变化参数动力学问题进行了动力学行为分析,给出了机构固有频率随机构位置的不同而变化情况及刚、柔性情况下一些关键点的运动量的时间历程及轨迹的对比情况。     

Effects of three parameters on forming force of the single point incremental forming process

This research has examined the effects of three parameter groups on the forming force of single point incremental forming (SPIF) process. The parameters under study include the material types (sheet aluminum, brass and copper), the forming angles (30°, 40° and 50°), and the tool revolution speeds (200, 400 and 600 rpm). The metal forming was carried out using a spherical edge tool which was pressed onto the metal surface to form work pieces of truncated pyramid shape. In the experiment, the forming forces were measured and analyzed to determine an optimal parameter combination, with regard to the material type, forming angle and revolution speed, for the SPIF process. The experimental results showed that all three parameter groups exerted varying influences over the forming force of the SPIF process. The findings indicated that the sheet brass exhibited the highest force value and that the smaller forming angle contributed to the greater forming force. In addition, the higher tool revolution speed resulted in the lower forming force.

     

多视数据拼合的三基准点精确选取技术研究

在逆向工程中,往往需要将从各个不同视角测得的点云数据进行多视定位,统一到一个全局坐标系下,并经过数据融合使之能够便于后续的模型重建。在已提出的多种拼合方法中,基于三个基准点的定位方法是其中的一种简单实用的对齐方法。通过研究三点定位算法,模型数据的对齐精度取决于所选取的三个基准点的测量精度。采用求取参考点顶点质心来代替原有基准点的方法,有效减小了拼合误差并提出了优化后的算法。     

空间长条形反射镜背部三支撑点的设置

采用背部三点支撑方式的空间光学遥感器长条形反射镜目前尚无普遍适用的支撑点设置方法,只能对某一确定尺寸和长宽比的反射镜进行专门设计。本文提出了基于规则模型的、以参数化模型为核心的集成参数化设计方法,并建立了长条形实体反射镜背部三点支撑集成参数化模型。在集成环境中通过试验设计、响应面分析等方法对设计参数进行了分析和优化,结果表明在轴向重力工况,镜面面形峰谷(PV)值优于λ/10(λ=632.5nm)的设计要求下,长条形碳化硅反射镜背部三点支撑的最大适用尺寸为1m。文中给出了最优支撑点布置,并确定了厚径比最优为1/10。最后,对集成参数化分析方法进行了精度分析,结果显示该方法整体误差为6.13%。提出的方法确定了空间长条镜背部三点支撑的适用范围,提供了支撑点最佳布置,为空间相机不同尺寸要求的长条镜设计打下了基础。     

Fabrication of a quantum point contact by the dynamic plowing technique and wet-chemical etching

We have fabricated extremely confined ballistic constrictions using a nanolithography technique based on an atomic force microscope. Vector-scan controlled dynamic plowing with the vibrating tip enables to plastically indent a thin resist layer along a prearranged path. Transfer of the resist pattern into the semiconductor substrate is achieved by a strongly diluted aqueous etchant. In this way approximately 30 nm deep gooves were etched in the channel area of a modulation-doped GaAs/GaA1As field-effect transistor. The quantum point contacts were defined by a broken line whose 60 nm width represents the length and the sub-100 nm gap determines the width of the constriction. At liquid-helium temperature the conductance as a function of gate voltage shows a stepwise increase in units of 2e2/h. Signatures of the conductance quantization persist up to 50 K, which indicates a large subband spacing.