Nanoindentation of silica nanoparticles attached to a silicon substrate

The nanomechanical properties of individual silica nanoparticles attached to a single crystal silicon (100) substrate were investigated using nanoindentation technique. The sample used in this study was fabricated by spin coating colloidal silica nanoparticle solution on the silicon substrate and then annealing the sample in a nitrogen environment. Environmental scanning electron microscopy (ESEM), scanning probe microscopy (SPM), and nanoindentation techniques were used to characterize the morphology and mechanical properties of the silica nanoparticles. The elastic modulus and hardness of the silica nanoparticles were found to be 68.9 ± 9.6 GPa and 2.8 ± 0.4 GPa, respectively. Several interesting phenomena were observed at the nanometer scale, including strain hardening, reverse plastic deformation, both ductile and brittle behaviors of the silica nanoparticles, and the increasing deformation resistance of the silica nanoparticles when subjected to repeated indentations.     

Numerical simulation of the freezing process of a water drop attached to a cold plate

This paper presents a direct numerical work on the freezing process of a water drop that is either sessile on or pendant from a cold plate. The numerical technique used is an axisymmetric front-tracking method to represent interfaces that separate different phases. The sessile drop corresponds to positive Bond numbers Bo (i.e., Bo > 0), and the pendant drop represents the other values of Bo. Numerical results show that pendant drops break up into liquid drops when gravity dominates the force induced by surface tension at Bo < 0. That is, a decrease in Bo enhances the breakup of the freezing drop. The breakup also depends significantly on the initial shape of the drop in terms of the contact angle at the plate ?₀, that is, increasing ?₀ induces breakup. In addition, the drop rapidly completes freezing due to breakup. In the case of non-breakup, the increase in Bo reduces the solidified drop height and decreases the time to complete solidification. The freezing process also consumes minimal time with small ?₀. The solidified drop after solidification has a cone near the axis of symmetry due to volume expansion of water upon solidification. This shape of the solidified drop is in accordance with the experimental observation.     

Numerical simulation of vortex-induced vibration of a square cylinder

Vortex-induced vibration (VIV) of a square cylinder in a cross flow is examined numerically. Both the rigid and elastic cases are simulated at a low Reynolds number of 100. The approach solves the unsteady flow field using a finite element method with a deforming grid to accommodate the moving cylinders. As for the cylinder motions, a two-degree-of-freedom structural dynamics model is invoked. Fluid-structure interactions are resolved through iteration at the same time step. The calculated results for the case of rigid cylinder indicated that the non-dimensional vortex shedding frequency (or the Strouhal frequency) of a square cylinder at rest is 0.13, which is in good agreement with the published results. For the elastic case, with the change of the cylinder’s natural frequency, “lock-in” and “beat” phenomena were successfully captured. The phenomena of resonance and galloping can also be indicated.     

Modeling of magnetoelastic resonator using h-parameter analysis

In this paper, an equivalent circuit model for magnetoelastic resonator is introduced. Elements of the model consist of coil inductance, magnetization of the resonator, a parallel RLC resonator representing the resonator resonance and a transformer indicating conversion ratio. This model suggests an approach to describe electrical response and characteristics of the resonator subject to geometries and excitation conditions. Moreover, corresponding techniques for extraction of parameters of the system are developed. Experimental results show that the model gives reasonable approximation of the system and accurately predicts behavior of the system.

     

Laser beam quality and pointing measurement with an optical resonator

We present a compact diagnostic breadboard that is based on an optical ring resonator for measuring beam quality and pointing of single-frequency continuous wave lasers at a wavelength of 1064 nm. To determine the beam quality of the coherent test beam, this optical resonator is used to perform a mode decomposition into Hermite-Gaussian modes. For our laser system, a power fraction in the fundamental Gaussian mode of 97.2%+/-0.2% was measured. Residual misalignment and mis-mode-matching to the resonator as well as the astigmatism and/or ellipticity of the test beam have been determined. Numerical simulations showed that measurements of the M(2) factor and transversal intensity distribution are not suitable for determining this power fraction. To measure the beam pointing, the fundamental mode of the optical resonator was used as a stable reference. The pointing of the test beam was measured with the differential wave front sensing technique up to Fourier frequencies of 1 kHz with a sensitivity to relative pointing of /epsilon/=1x10(-6)/sqrt[Hz]. Pointing measurements with an alternative method were performed and showed good agreement.     

某榴弹发射器虚拟样机的建模与仿真

应用机械系统动力学和运动学分析软件ADAMS以某榴弹发射器为例,建立与该榴弹发射器物理样机的简化虚拟样机模型,分析其自动机运动学和动力学特点。根据榴弹发射器的受力关系,在模型的各运动部件施加约束力,设置相关的参数,对虚拟样机模型进行动力学仿真,探讨该榴弹发射器仿真过程中出现的问题并进行优化设计。最后对比以往对同型号的榴弹发射器的实验结果,验证仿真的正确性。     

A resonator microwave sensor for measuring the parameters of Solar-quality silicon

Nondestructive microwave and optical methods for contactless local measurements of resistivity, the lifetimes of minority charge carriers in single-crystal and polycrystalline semiconductors, and lifetimes in photoelectric transducers are investigated.     

隔膜真空泵传动机构平衡计算及性能仿真

隔膜真空泵是工业中用于控制容器压强和实现真空的关键设备之一。在工作过程中真空泵因传动机构不平衡而导致机器振动和噪声过大,从而降低机器的性能和寿命,因此需要对传动机构进行平衡。将真空泵传动机构简化为曲柄滑块机构,运用质量代换方法,计算出使机构残余惯性力最小时的平衡配重,实现了机构的部分平衡。最后利用ADAMS软件,建立了真空泵的虚拟样机,对样机的不平衡惯性力进行了性能仿真,对平衡效果进行了验证,发现不平衡惯性力显著减小。     

一种基于高斯光束的平凹激光腔对准方法

给出了一种基于高斯光束的平凹激光腔对准方法。在高斯准直光束后加一透镜系统,恰当地调整准直高斯光束到某一种汇聚发散的状态。在这种状态下,可使由平凹腔凹面镜和平面镜反射回来的光斑直径大小相仿,解决了不加透镜系统时,两反射回来光斑直径相差很大,难于对准的问题,提高两光斑的对准精度。实验使用束腰为0.6 mm的氦氖光,其后加一优化好的透镜系统,在889 mm的距离下,对凹面镜曲率半径为50 mm的平凹腔进行对准。得到由凹面镜和平面镜反射回来的光斑直径分别为4.8 mm和5.1 mm,平凹腔的角度对准精度达到了3.18'。对准好的腔体在点亮LD后,均能出基模光斑。实验结果与理论分析相符,证明了该对准方法结构简单,执行方便。     

某型自行火炮行驶平顺性仿真研究

平顺性是自行火炮的一项重要性能指标。良好的平顺性能保证武器系统的可靠性,而且能降低驾驶员的劳动强度。以某型自行火炮为研究对象,建立了某型自行火炮的动力学模型,并采用脉冲输入平顺性试验进行了模型验证。仿真分析结果与试验数据吻合度较好,验证了模型的可靠性及有效性。对该自行火炮进行了随机路面行驶平顺性仿真分析,并进行了评价。仿真及评价结果表明,该火炮系统的平顺性较好。     

汽车前纵梁薄壁结构碰撞仿真及优化设计研究

针对前纵梁现有结构吸能特性的不足,对其结构进行了改进,使前纵梁在正面碰撞过程中吸收更多能量。根据前纵梁板厚、焊点的间距及焊点大小等设计参数,建立了前纵梁正面碰撞的有限元模型。采用HyperMesh和LS-DYNA软件,对前纵梁碰撞过程进行了模拟仿真。通过加诱导槽及加加强板的方式,对前纵梁结构进行了优化。研究结果表明,优化后的前纵梁单位质量吸收的能量提高了63.3%。通过该方法使前纵梁提高了吸能特性,降低了车辆碰撞对乘客造成的损害。     

Modeling and analysis of the vibration behavior of a shape memory alloy beam

Shape memory alloys have a high potential for passive isolation of vibrations as well as capability of being used as an active vibration isolation system for their shape memory behavior, i.e. having hysteresis along with superelastic behavior. Study of this behavior necessitates recognition of distinct specifications of this alloy and also presentation of a suitable and simple mathematical model. In this paper, a mathematical model based on Auricchio model, considering asymmetry in tension and compression and also temperature effects on hysteresis at superelastic conditions has been presented. Finally, dynamical behavior of a NiTi beam under free vibration as well as application of sinusoidal and impulse loads upon free–clamped and also simply supported conditions have been analyzed.     

Modeling and detection of delamination in a composite beam: A polyspectral approach

This work considers the difficult problem of detecting delamination in a composite beam structure based on a polyspectral analysis of the structure's vibrational response. First, a low-dimensional model of the structure is presented that captures the delamination-induced nonlinearity and shows how it influences the beam's dynamic response. A Volterra series solution is then used to approximate the nonlinear beam response. The associated Volterra kernels are derived and then substituted into previously obtained expressions for the power spectral, auto-bispectral and auto-trispectral density functions for multi-degree-of-freedom systems subject to Gaussian distributed, random inputs. The influence of the delamination on the polyspectra is then explored with respect to both delamination size and depth. Specifically, it is shown how the dominant peaks of the polyspectra relate directly to these delamination parameters. The limits of polyspectral approaches to delamination detection are presented.     

Modeling and distributed simulation of supply chain with a multi-agent platform

A supply chain is a complex adaptive system that is very difficult to analyze, in particular, with respect to its dynamics, stochastic, and uncertainty. The traditional analytical model is unable to cope with these characteristics of supply chain. In addition, the analytical model may not be solved due to its complexity and nonlinearity. Simulation, especially the multi-agent-based distributed simulation, is one of the most effective tools to model and analyze supply chain. In this paper, a multi-agent platform framework with multi-layers for modeling and distributed simulation of the complex supply chain is presented. In this platform, generic agents for modeling entities in supply chain are described and a novel simulation modeling approach enabled by the platform is proposed. The platform provides a flexible environment for defining, deploying, simulating, and displaying agent-based simulation models and enables users to instantiate generic agents to model their required simulation models rapidly. A three-echelon supply chain model is developed and simulated to illustrate the use of the simulation platform.     

Direct and transmission milling of suspended silicon nitride membranes with a focused helium ion beam

Helium ion milling of suspended silicon nitride thin films is explored. Milled squares patterned by scanning helium ion microscope are subsequently investigated by atomic force microscopy and the relation between ion dose and milling depth is measured for both the direct (side of ion incidence) and transmission (side opposite to ion incidence) regimes. We find that direct-milling depth varies linearly with beam dose while transmission-milling depth varies with the square of the beam dose, resulting in a straightforward method of controlling local film thickness.     

Modeling and simulation of wear in a pin on disc tribometer

A very efficient, incremental implementation of Archard’s wear model on the global scale for pin wear and disc wear in a pin-on-disc tribometer is presented. The results from the model are in good agreement with experimental results. The identified wear model is implemented in a finite element based tool (Wear-Processor) for 3D wear simulations and the results compare favorably with that from the global wear modeling scheme.     

Vibration of nonuniform carbon nanotube with attached mass via nonlocal Timoshenko beam theory

This paper studies the vibrational behavior of nonuniform single-walled carbon nanotube (SWCNT) carrying a nanoparticle. A nonuniform cantilever beam with a concentrated mass at the free end is analyzed according to the nonlocal Timoshenko beam theory. A governing equation of a nonuniform SWCNT with attached mass is established. The transfer function method incorporating with the perturbation method is utilized to obtain the resonant frequencies of a vibrating nonlocal cantilever-mass system. The effects of the nonlocal parameter, taper ratio and attached mass on the natural frequencies and frequency shifts are discussed. Obtained results indicate that the sensitivity of the frequency shifts on the attached mass increases when the length-to-diameter ratio decreases. Tapered SWCNT possesses higher fundamental frequencies if the taper ratio becomes larger.     

数字式伺服系统的建模与仿真

该动圈式伺服阀控缸系统利用数字控制器控制伺服阀的控制信号,对该系统进行数学建模,并利用仿真了解系统的动态特性。     

压电悬臂梁发电装置的建模与仿真分析

为提高一定尺度压电复合悬臂梁（简称压电梁）的发电能力,建立了单、双晶压电梁发电能力的仿真分析模型,研究了结构尺寸、激励方式及材料性能等对其发电能力的影响规律。研究表明,在基板材料及激励条件相同时,存在不同的最佳厚度比使单／双晶压电梁发电能力最大,双晶梁的最大发电量约为单晶梁的2倍。基板材料不同时,最佳厚度比随（基板与压电材料的）杨氏模量比的增加而减小,铝、钼基板构成的单／双晶压电梁的最佳厚度比分别为（0.7,0.32）和（0.45,0.2）。在相同的厚度比（0.5）及外界激励条件下,杨氏模量比对两种电梁发电能力的影响不同,杨氏模量低于3.3时,双晶梁的发电量均大于单晶梁。