铸型尼龙端面扭动摩擦接触模型及实验验证

基于刚性平冲头压在弹性半无限空间的理论研究,建立了玻璃纤维增强铸型尼龙(MC尼龙)复合材料与45#钢的端面扭动摩擦模型,并开展了铸型尼龙端面扭动摩擦学试验.结果表明:摩擦扭矩-角位移(T-θ)曲线随角位移幅值的增加由直线形、椭圆形最终转变为准平行四边形;纯MC尼龙及其复合材料(3%GF、5%GF)在扭动角分别为0.5°、1°和2°时,扭动接触界面处于完全滑移状态,其不同的摩擦系数直接影响黏着半径与扭动角的关系,即导致了不同的扭动界面接触机制;MC尼龙复合材料的扭矩随扭动角的增加急剧减小至稳定状态,由扭矩与扭动角的关系还可知部分滑移状态时的扭矩值高于完全滑移状态的扭矩.不同角位移下的实验扭矩与计算曲线相似,揭示了该扭动摩擦模型可近似预测MC尼龙复合材料的扭矩.     

纳米TiO2和SiO2填充尼龙的摩擦磨损行为

制备了纳米SiO2和纳米TiO2填充PA1010尼龙复合材料,测定了复合材料的力学性能,并采用MM-200型摩擦磨损试验机考察了尼龙复合材料在干摩擦条件下同45#钢配副时的摩擦磨损行为.结果表明:填充纳米颗粒可以提高尼龙复合材料的力学性能;纳米SiO2和纳米TiO2作为填料可以提高PA1010的耐磨性,降低摩擦系数,其中纳米颗粒的最佳质量分数为10%;纳米颗粒填充尼龙1010复合材料同45#钢配副时主要呈现粘着和疲劳磨损特征.     

纳米Al2O3和Fe2O3填充尼龙PA1010的摩擦磨损行为

采用模具挤压成型方法制备了纳米Al2O3和Fe2O3填充PA1010尼龙复合材料，采用MM-200型摩擦磨损试验机考察了所制备的尼龙复合材料在干摩擦条件下同45#钢对摩时的摩擦磨损行为。研究结果表明，填充纳米Al2O3使得PA1010尼龙复合材料的摩擦系数增大，而填充纳米Fe2O3使得摩擦系数降低；纳米Al203和Fe2O3填充尼龙复合材料的耐磨性能优于尼龙；当纳米填料的质量分数从10％提高到20％时，纳米Fe2O3填充尼龙的磨损量增大，纳米Al2O3填充尼龙的磨损量无明显变化，2种填料填充尼龙复合材料的摩擦系数变化不大．纳米Fe2O3填充尼龙复合材料同45#钢对摩时主要呈现粘着磨损和轻微疲劳磨损特征，而纳米Al2O3填充尼龙复合材料呈现脆性疲劳开裂特征。纳米Fe2O3填充尼龙复合材料在偶件磨损表面形成的转移膜更加均匀和连续，故其减摩抗磨性能优于纳米Fe2O3填充尼龙复合材料。     

纤维增强铸型尼龙在水润滑条件下的摩擦磨损性能研究

考察了玻璃纤维和碳纤维增强MC尼龙在水润滑条件下的摩擦磨损特性,并借助扫描电子显微镜和表面形貌仪分析了磨损机理。结果表明：在水润滑条件下,纤维增强MC尼龙的摩擦系数比干摩擦下的低,耐磨性优于未增强的基体材料;其中碳纤维增强MC尼龙比玻璃纤维增强MC尼龙具有更低的摩擦系数和更高的耐磨性能;碳纤维增强MC尼龙的磨损机理主要是粘着转移,同时伴有犁削作用,而玻璃纤维增强MC尼龙的磨损机理主要是犁削作用。     

表面纳米化中碳钢在干摩擦条件下的摩擦磨损性能研究

采用高能喷丸方法对45#钢进行表面纳米化处理,在环-盘摩擦磨损试验机上评价表面纳米化处理45#钢与T10钢摩擦副在干摩擦条件下的摩擦磨损试验,通过分析其磨损表面形貌探讨表面纳米化对中碳钢磨损行为的影响.结果表明:在载荷为10～60 N条件下,表面纳米化中碳钢的摩擦系数较处理前有所降低;在载荷为10～40 N时,其磨损质量损失较处理前有所降低;表面纳米化能够减弱中碳钢的疲劳磨损效应,提高中碳钢的摩擦磨损性能.     

碳纤维织物/环氧复合材料油润滑下摩擦学性能

采用半干法制备碳纤维织物增强环氧树脂基自润滑复合材料,研究钢背衬复合材料与45钢在环-环端面浸油润滑状态下的摩擦学特性,考查载荷、速度和碳织物类型对复合材料摩擦磨损性能的影响,并采用扫描电子显微镜对复合材料及偶件磨损表面进行观察与分析.结果表明:轻载高速启动可显著提高单向碳织物/环氧复合材料的摩擦磨损性能,边界润滑状态下的碳织物/环氧复合材料主要表现出黏着磨损特性,对偶钢环上出现的网状转移膜大大改善了材料的摩擦学性能;平纹碳织物/环氧复合材料因表面织物纹理使得润滑油能深入到摩擦表面各区域,在重载下表现出较低的摩擦系数.     

润滑条件下钢表面氧化膜结构的SAM研究

作者在球-盘式摩擦磨损试验机上采用阶梯式加载和20~#机油滴注式润滑进行了GCr15钢球对45~#钢盘的摩擦磨损试验。通过摩擦系数的变化记录了在不同滑动速度下摩擦磨损状态发生转化时的临界载荷,从而获得了反映摩擦副摩擦学特性的P-V图,并用扫描俄歇微探针仪(SAM)研究了该摩擦副处于不同状态时表面氧化膜的结构,提出了钢表面氧化膜的结构层次模型,论证了钢的摩擦磨损特性与氧化膜结构之间的关系。     

炭纤维织物/环氧树脂基复合材料的摩擦磨损性能研究

研究了炭纤维织物/环氧(CF/EP)复合材料与45#钢在环-环端面干摩擦状态下的摩擦磨损特性,考查了制备工艺和MoS2与石墨不同配比等对CF/EP复合材料干摩擦性能的影响,采用扫描电子显微镜观察复合材料及其偶件磨损表面形貌.结果表明:半干法制备的环氧树脂体积分数为40%的CF/EP复合材料的摩擦系数稳定;纯CF/EP复合材料主要表现为粘着磨损特性;MoS2与石墨改性后复合材料摩擦磨损性能明显改善,质量比为1.5∶1的MoS2与石墨改性CF/EP复合材料具有最佳的摩擦磨损性能,其稳态摩擦系数为0.14～0.15,磨损量3.15×10-5 mg/r.     

CF/PTFE纤维混编织物增强环氧复合材料干摩擦特性

采用碳纤维与聚四氟乙烯纤维(CF/PTFE)混编织物增强,制备了环氧树脂基自润滑复合材料,研究了钢背衬复合材料与45钢在环-环端面干摩擦状态下的摩擦学特性,考查了纤维织物、摩擦热、载荷、速度对材料摩擦磨损性能的影响,用红外热像仪、热电偶及风冷方式对摩擦副温度进行监控,用激光共聚焦显微镜和扫描电子显微镜对复合材料及偶件磨损面进行了观察与能谱分析.结果表明:与碳织物相比,混编纤维织物大大改善了复合材料的摩擦学性能,改善效果极大依赖于摩擦温度、载荷和速度参数.PTFE纤维磨损后在树脂基体及偶件表面形成减摩型转移膜层,材料表现为疲劳磨损特征.摩擦高温使复合材料摩擦学特性改变,黏结磨损加剧,偶件钢环表面出现氧化磨损,树脂基体塑性流动,摩擦力增大.混编纤维的排布方式影响复合材料的摩擦磨损性能,摩擦面上大量破碎的碳纤维易使偶件表面转移膜受到破坏,复合材料转变为以磨粒磨损为主,减摩主要源于磨屑中的润滑组分.     

聚酰亚胺复合材料与不同对偶材料滑动摩擦磨损性能

本文考察了碳纤维（CF）和固体润滑剂（LW）填充热塑性聚酰亚胺（TPI）复合材料与45^#钢、镍铬合金以及铜对摩时的摩擦磨损性能,利用SEM和光学显微镜观察磨损表面形貌.研究表明：与硬度高的钢和镍铬合金摩擦时,由于CF的增强和承载作用降低了材料的摩擦系数与磨损率,LW的加入增强了转移膜与对偶面间的结合力,进一步降低TPI的摩擦系数;与较软的铜摩擦时,由于铜偶件存在较大磨损,破坏了转移膜的稳定性,TPI及其复合材料的摩擦性能较差;TPI与钢和镍铬合金摩擦时以黏着磨损为主,与铜摩擦时以磨粒磨损为主.薄而均匀的转移膜有利于摩擦系数与磨损率的降低.     

一种基于直接计算高阶奇异积分的断裂力学双边界积分方程分析法

相对于有限元法,边界单元法在求解断裂问题上有着独特的优势,现有的边界单元法中主要有子区域法和双边界积分方程法.采用一种改进的双边界积分方程法求解二维、三维断裂问题的应力强度因子,对非裂纹边界采用传统的位移边界积分方程,只需对裂纹面中的一面采用面力边界积分方程,并以裂纹间断位移为未知量直接用于计算应力强度因子.采用一种高阶奇异积分的直接法计算面力边界积分方程中的超强奇异积分;对于裂纹尖端单元,提供了三种不同形式的间断位移插值函数,采用两点公式计算应力强度因子.给出了多个具体的算例,与现存的精确解或参考解对比,可得到高精度的计算结果.      

汶川地震诱发文家沟巨型滑坡 碎屑流基本特征及成因机制初步分析

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2008年5月12日,汶川M80地震在四川省绵竹市清平乡文家沟内诱发一巨型滑坡。通过现场调查得知,滑坡前后缘高差455m,厚度20～30m,滑面为基岩层面,初始方量2750×107m3。滑体在运动中转化为碎屑流。滑坡-碎屑流总的水平运动距离为4022m,垂直运动距离为1443m,遗留的堆积物体积达5×107m3。滑坡距映秀—北川断裂仅36km,位于其下盘,地震烈度达XI度。滑坡导致文家沟中48人遇害,并形成一条完整的地震次生地质灾害链。初步分析表明滑坡启动速度快,滑坡向碎屑流转化过程明显、地点明确。碎屑流运动过程复杂,伴有强烈的“气垫效应”和“前缘气浪冲击效应”。作者认为,文家沟滑坡的高启动速度是长持时强烈地震动作用的结果,与山体的猛烈碰撞是导致滑体解体并转化为碎屑流的原因。     

Preface: Orbits around asteroid

One of the core issues in modern celestial mechanics is the orbital dynamics in the near-regime gravitational field of as- teroids, which provides deep insights into the mathematical nature of a class of nonlinear systems, and plays as a critical basis for in situ explorations of different science goals. Lots of efforts have been made to reveal the characteristics of orbital motion in the vicinity of asteroids, and to improve the skills of asteroid research in methodology.     

Effet de l'anisotropie élastique cristalline sur la distribution des facteurs de Schmid à la surface des polycristauxInfluence of crystalline elasticity anisotropy on Schmid factor distribution at the free surface of polycrystals

Experimental studies of the plasticity mechanisms of polycrystals are usually based on the Schmid factor distribution supposing crystalline elasticity isotropy. A numerical evaluation of the effect of crystalline elasticity anisotropy on the apparent Schmid factor distribution at the free surface of polycrystals is presented. Cubic elasticity is considered. Order II stresses (averaged on all grains with the same crystallographic orientation) as well as variations between averages computed on grains with the same crystallographic orientation but with different neighbour grains are computed. The Finite Element Method is used. Commonly studied metals presenting an increasing anisotropy degree are considered (aluminium, nickel, austenite, copper). Concerning order II stresses in strongly anisotropic metals, the apparent Schmid factor distribution is drifted towards small Schmid factor values (the maximum Schmid factor is equal to 0.43 instead of 0.5) and the slip activation order between characteristic orientations of the crystallographic standard triangle is modified. The computed square deviations of the stresses averaged on grains with the same crystallographic orientation but with different neighbour grains are a bit higher than the second order ones (inter-orientation scatter). Our numerical evaluations agree quantitatively with several observations and measures of the literature concerning stress and strain distribution in copper and austenite polycrystals submitted to low amplitude loadings. Hopefully, the given apparent Schmid factor distributions could help to better understand the observations of the plasticity mechanisms taking place at the free surface of polycrystals. To cite this article: M. Sauzay, C. R. Mecanique 334 (2006).     

Nonlinear behavior of matrix-inclusion composites under high confining pressure: application to concrete and mortar

This paper is devoted to a micromechanics-based simulation of the response of concrete to hydrostatic and oedometric compressions. Concrete is described as a composite made up of a cement matrix in which rigid inclusions are embedded. The focus is put on the role of the interface between matrix and inclusion which represent the interfacial transition zone (ITZ). A plastic behavior is considered for both the matrix and the interfaces. The effective response of the composite is derived from the modified secant method adapted to the situation of imperfect interfaces. To cite this article: T.H. Le et al., C. R. Mecanique 336 (2008).     

Dynamics and Breakup of Viscoelastic Liquids (A Review)

The phenomena of hydrodynamic breakup of liquid jets, drops, films, bridges, and filaments are reviewed for liquids with viscoelastic properties. The reasons for breakup are capillary instabilities, collisions with rigid obstacles, and other forms of dynamic action. The relationship between the properties of the liquids and the features of the breakup process is discussed.     

Flow stability analysis and excitation using pulsating jets

Classical flow stability applied to transition from laminar to turbulent flow may also describe the behavior of vorticity fluctuations created by a pulsating jet placed along a solid boundary. A numerical laminar flow experiment involving a pulsating jet placed along the surface of a duct with flow separation downstream, resulted in eliminating most part of the separated flow region. Applying the same approach to a turbulent flow, it was possible to develop a turbulent stability flow formulation and apply successfully turbulent pulsating jet flow separation control. To cite this article: D. Skamnakis, K. Papailiou, C. R. Mecanique 333 (2005).