热力耦合下微接触点塑性应变沿深度变化分析

在考虑粗糙实体弹塑性变形、热力耦合、微凸体间相互作用和摩擦热流耦合等影响下,运用有限元法数值模拟具有三维分形特性的粗糙面与刚性平面间滑动摩擦过程,分析了粗糙实体接触凸点塑性变形随深度变化情况。发现：在速度的突变和闪点温度形成时,摩擦接触表层等效塑性应变增大明显;在这一摩擦表层,过不同接触点的纵向剖面塑性应变沿深度分布不同：有的是接触表面塑性变形最大,有的是在接触微凸体表面下某一深度塑性变形最严重,而接触凸点表面的塑性应变稍小些。这与相关文献用SEM研究干摩擦后金属摩擦表层变形照片后发现的结果一致。滑动摩擦过程中,金属粗糙摩擦接触表层塑性变形的不断累积,将会导致材料表层中的夹杂或微观缺陷周围萌生微孔和裂纹源。     

滑动摩擦中不同润滑条件对应力层的影响研究

真实工程表面接触时,微凸体将产生弹塑性变形。建立了在不同润滑条件下二维双粗糙弹塑性分形模型,分析了润滑对滑动摩擦中应力层的分布影响情况。在完全润滑和边界润滑时材料表层下方出现易断裂的循环应力区域,干摩擦时循环应力层消失。     

基于真应力-应变关系的粗糙表面法向接触模型

提出一种粗糙表面的法向弹塑性接触分析的建模方法。基于微凸体的弹塑性有限元接触模型,分别研究了40Cr、45和Q235三种钢材料的微凸体与刚性平面的法向接触特性。有限元模型中采用三种材料的真应力-应变关系,考察了不同强化特性对微凸体接触性质的影响。建立了微凸体在弹性、弹塑性、塑性变形阶段统一的接触变量变化规律的表达式。在此基础上应用概率统计理论建立粗糙表面法向弹塑性接触模型。所建立的接触模型中微凸体接触变量的变化规律完全基于弹塑性有限元模型的计算结果,无需将微凸体的变形过程区分为不同的变形阶段,避免了接触变量在各阶段采用不同函数表达式带来的连续性和光滑性问题,以及在弹塑性阶段采用插值函数的随意性问题。通过与其他接触模型的计算结果相比较,证明了所提出接触模型的合理性。     

微悬臂梁长度对梁末端与底座粗糙面间接触的影响

将磁致伸缩效应等效为材料的热膨胀效应,考虑微梁末端与底座粗糙面微凸体间的相互作用、材料弹塑性变形及微梁弹性恢复力等影响,利用ABAQUS动态模拟了磁致伸缩薄膜悬臂梁末端与底座粗糙面间的接触,分析了不同长度悬臂梁对接触特性的影响。结果表明,重复加卸载过程中,接触面积、法向接触力随着微梁长度的增加呈近似线性增长,而最大等效塑性应变值随微梁长度的增加呈非线性关系,说明微梁长度对梁与底座粗糙面间接触特性的影响十分重要,进而影响到系统工作的可靠性。     

过盈配合接触界面的研究

通过W-M函数建立具有分形特征的等效粗糙表面与刚性平面的接触模型,模拟压缩机叶轮与轴过盈配合下界面的接触。将接触模型导入到有限元软件Ansys中,分析接触模型接触面积、接触压力、滑移面积、粘着面积与变载荷的关系和表面路径上的应力与变形。结果表明,接触时只有个别较高的微凸体发生接触,微凸体肩部区域的应力较大,微凸体中心位置变形量最大,接触界面在一定的法向载荷下接触面积减小,粘着面积增大,配合件发生了宏观的变形。     

法向加-卸载过程中弹塑性微凸体侧向接触能耗研究

针对法向加-卸载作用下双粗糙表面上微凸体接触阻尼能耗问题,提出弹性、弹塑性、塑性微凸体侧向接触能耗计算方法。基于微凸体接触球形假设,根据微凸体侧向接触受力分析,将其分解为垂直于微凸体接触点公切面的法向分力和沿该面的切向分力。采用HERTZ,ETSION理论,分别建立了加-卸载过程中微凸体发生弹性、弹塑性、塑性变形时,法向分力与变形之间关系;依据CATTANEO-MINDLIN黏着-滑移理论,BKE模型,ERITEN模型理论,建立了加-卸载过程中三个变形阶段的切向分力与位移之间关系。利用法向分力-变形和切向分力-位移之间的关系,求得微凸体在法向、切向分力共同作用下产生的应变能耗以及摩擦能耗,进而求得微凸体侧向微观接触在三个阶段下的能耗。研究表明,微凸体侧向接触时耗能包括应变耗能和摩擦耗能,且法向变形量越大,应变耗能、摩擦耗能越大;接触角度越大,应变耗能越大,摩擦耗能越小。     

考虑蠕变特性的橡胶微凸体与金属表面接触特性分析

金属-橡胶接触广泛存在于密封结构中,密封接触表面上微凸体间的相互作用会直接影响整个密封界面的接触特性,进而影响其密封性能。基于粗糙密封界面的单个微凸体,考虑橡胶的蠕变特性,采用理论分析和仿真研究相结合的方式研究橡胶微凸体与金属表面的接触特性。通过橡胶蠕变特性的实验结果,构建橡胶蠕变计算模型;构建半球微凸体与金属平板间的有限元模型,进行考虑蠕变特性的仿真,分析其接触特性,并与Hertz接触理论的计算值进行对比。结果表明：在蠕变阶段,接触半径、法向变形量和最大等效蠕变应变均随蠕变时间的增加而增大,最大接触压力随蠕变时间增大而减小,这均可能导致密封性能的下降;随压力载荷的增大,接触半径、法向变形量、最大接触压力和最大等效蠕变应变均增大,但增大的趋势逐渐减小;橡胶微凸体与金属表面间的等效模量随蠕变时间的增加而减小,随压力载荷增大而增大。     

粗糙表面微凸体对液黏传动的影响

为研究液黏传动过程中粗糙表面的承载特性,将分形理论引入到两粗糙表面摩擦过程之中,分析传动过程中混合摩擦和边界摩擦两阶段的微凸体承载过程,考虑微凸体弹塑性变形,对M-B模型进行修正,建立修正的微凸体承载模型。建立基于修正M-B模型的微凸体承载模型。通过数值仿真得到有效面积系数、分形参数对液黏调速离合器传动过程的影响规律;对修正的微凸体承载模型的计算结果与M-B模型的计算结果进行对比分析。结果表明:微凸体接触载荷和传递转矩随着面积比的增大而增大,当有效面积系数与尺度系数增大时,接触载荷与传递转矩均有所增大;分形维数为1.5时,微凸体接触载荷与传递转矩最小且随面积比的变化最为缓慢;在整个接触区域内,弹性变形区域的面积、接触载荷以及传递转矩最大,其次是弹塑性变形区域,塑性变形区域最小;考虑弹塑性变形时,微凸体接触载荷与传递转矩均有所下降;修正M-B模型和M-B模型间的修正系数范围在25%以内,修正系数随着有效面积系数、尺度系数的增大而增大,随着分形维数的增大而减小。     

新的粗糙表面弹塑性接触模型

提出一种新型的粗糙表面弹塑性微观接触模型.该模型的建立基于接触力学理论和接触微凸体由弹性变形向弹塑性变形及最终向完全塑性变形的转化皆是连续和光滑的假设.研究单个微凸体在载荷逐渐增加时的变形规律,并重点推出弹塑性变形区间的接触方程.在此基础上应用概率统计理论导出了粗糙表面的接触载荷、平均分离和实际接触面积之间的数学关系式.在不同的塑性指数和载荷条件下,该模型与GW弹性模型和CEB弹塑性模型就实际接触面积和法向距离的预测结果进行了对比.结果表明,在同样塑性指数和载荷条件下比GW模型预测的实际接触面积大但法向距离小,且两者的差距随塑性指数和载荷的增加而增大.因此该模型的预测结果更加符合人们的试验观察和直觉,能够更加科学和合理地描述两个粗糙表面的微观和宏观接触状态.     

双粗糙平面法向接触建模方法的有限元对比分析

基于统计学方法,Greenwood和Tripp(GT模型)提出双粗糙平面法向接触(双粗模型)可以等效为单粗糙平面与刚性平板之间的接触(单粗模型),但这种等效处理缺乏对接触应力场分析以及材料硬化的考虑。为进一步研究GT模型,基于高斯粗糙表面数字化表征,通过控制自相关长度和滤波方法,得到双粗糙平面及其等效单粗糙平面;借助有限元软件ABAQUS对2组模型法向接触进行建模及分析。结果表明:2组模型预测的接触刚度和接触面积符合较好,但接触压力与接触面积关系存在差异;2组模型预测的等效应力和接触压强的应力幅值以及云图的分布区域大致相近,但双粗模型由于存在大量微凸体侧接触,弱化接触状态以及材料硬化,因而应力幅值偏低。     

Thickness and grain size dependence of limit strains in sheet metal stretching

A theoretical analysis is presented of thickness and grain size dependence of the limit strains in isotropic sheet metals under biaxial tension. Sheet surfaces are assumed to consist of asperities which increase proportionally with the equivalent strain and the grain diameter of the material. The limit strains are given as the sum of the strain up to Swift instability modified by taking into consideration the effect of surface asperities and the subsequent strain up to a localized neck which is based on the Marciniak analysis. The thickness ratio of the material in the Marciniak model is determined making use of the magnitude of the surface roughness at the modified Swift instability.Numerical results show that the limit strains decrease in all cases as thickness to grain size ratio decreases. The increase in surface roughness with plastic strain varies from material to material and this is taken into account in predicting limit strains.     

Plastic deformation analysis in machining of Inconel-718 nickel-base superalloy using both experimental and numerical methods

Surface region plastic deformation of Inconel-718 nickel-base superalloy workpieces was evaluated when machined under orthogonal cutting conditions at various cutting speeds. Plastic deformation analysis was accomplished by determining the residual stress and plastic strain distributions in the surface region. The residual stresses were tensile and maximum near the surface and decreased in magnitude with an increase in depth beneath the machined surface. Similarly, the plastic strains were maximum near the surface and decreased with an increase in depth beneath the machined surface. In addition, a finite element simulation of orthogonal machining was carried out for predicting the residual stress and plastic strain distribution. In general, the trend of the curves predicted by the finite element model was similar to those found experimentally.     

The variation of real contact area between surfaces with contact pressure and material hardness

The real area of contact between solids under high contact pressure is examined, and values obtained experimentally are compared with theoretical predictions. The theory considers the compression of an array of uniform prismatic wedges, and predicts a rise in the strength of the asperities when adjacent plastic deformation fields interact. It is assumed that the material on which the wedge-shaped asperities are supported is in a state of hydrostatic stress; this is replicated in the experiment by constraining the bulk material of the specimens. One specimen was single point machined to produce a uniform array of wedges and the other specimen abraded to produce a more random surface texture. Good agreement was found between the compression of the machined surface and the theory. The behaviour of the abraded surface was found to be in good agreement with predictions based on an equivalent uniform array; however, the rigorous theory gave too much strength to the smaller asperities, as their substrate material was found to deform both elastically and plastically.It is demonstrated that material hardness is an important variable; knowledge of initial hardness is not adequate since final contact area depends on final hardness.     

2.25Cr—1Mo复杂应力状态下低周疲劳裂纹扩展规律研究

通过对2.25Cr-1Mo材料进行不同温度下的低周疲劳试验,并应用NHRDS有限元程序进行裂纹尖端各应力应变分量,各当量弹、塑性应变范围的计算,利用当量J积分范围,研究了该材料在复杂应力状态下的低周疲劳裂纹扩展规律,结果表明,在复杂应力状态下2.25Cr-1Mo材料的低周疲劳裂纹扩展速率可采用当量J积分范围进行表征。     

Role of surface texture of harder surface on subsurface deformation

Plastic deformation and damage accumulation below the contact surfaces play an important role in sliding wear of ductile materials. In this study, metallographic techniques have been used to determine the extent of plastic deformation and strain localization events that occur at various depths beneath the worn surface in the subsurface zones of the ductile pins slid against harder plate with various surface textures. Results showed that the magnitude of plastic strain gradient and the depth of highly deformed zone correlate with coefficient of friction, which in turn depends on the surface texture of harder counterface, under both dry and lubricated conditions. It was also observed that the gradient of equivalent strain, as it approaches the worn surface, was higher under dry conditions when compared to that under lubricated conditions.     

Real area of contact and boundary friction in metal forming

Upper-bound models for asperity flattening on a workpiece surface undergoing bulk plastic deformation are developed. It is found that the effective hardness of the surface can be greatly reduced by the presence of underlying plastic flow. Theoretical predictions of the variation of real area of contact with strain show excellent agreement with experiments using model asperities in rolling. Friction models which allow for the reduction in effective hardness are developed for cases in which roughness is concentrated on either the workpiece or tooling.     

Contact mechanics and the wear of metals

It is commonly observed that metallic wear debris takes the form of thin platelets, leading to the term ‘delamination wear’. Modelling this phenomenon has proved a stiff challenge in Contact Mechanics since the fractures which give rise to wear particles lie parallel, or nearly so, to the surface; i.e. on planes of maximum compressive stress. Sectioning the surface layer beneath a wear track has revealed it to have acquired severe plastic strains, which suggests that the cracks are ductile fractures, driven by plastic strain rather than elastic stress intensity. The paper reviews recent research into the progressive plastic deformation of surfaces in repeated sliding: the process known as ‘ratchetting’. Included is an analysis of ‘running-in’ of rough surfaces by repeated sliding and a discussion of the criterion of rupture under cyclic plastic strain.     

FINITE ELEMENT ANALYSIS OF SUBSTRATE LOCAL PLASTIC DEFORMATION INDUCED BY CRACKED THIN HARD FILM

It has been postulated that, with tensile loading conditions, micro-cracks on thin hard film act as stress concentrators enhancing plastic deformation of the substrate material in their vicinity. Under favorable conditions the localized plastic flow near the cracks may turn into macroscopic plastic strain thus affects the plasticity behaviors of the substrate. This phenomenon is analyzed quantitatively with finite element method with special attention focused on the analysis and discussion of the effects of plastic work hardening rate, film thickness and crack depth on maximum plastic strain, critical loading stress and the size of the local plastic deformation zone. Results show that micro-cracks on thin hard film have unnegligible effects on the plasticity behaviors of the substrate material under tensile loading.