Numerical modelling of rough particle contacts subject to normal and tangential loading

Granular Matter - While there are plenty of experimental studies pertaining to the dust generation from and dustiness of powders, few of them aim at reaching a theoretical understanding of the...     

加载路径对不锈钢球形件内高压成形过程影响

为了研究加载路径对不锈钢球形件内高压成形过程的影响,采用实验方法分析了加载路径对成形过程中缺陷形式的影响,获得了80%膨胀率成形管件的壁厚分布规律.结果表明:当初始内压与屈服强度比值小于0.21时,管坯形成两个皱纹,在整形阶段发生开裂;当初始内压与屈服强度比值大于0.25时,管坯在轴向进给阶段即发生开裂.在初始内压与屈服强度比值为0.21～0.25时,可以成形出合格管件,合格管件最大减薄点位于球形件的最大截面处,最大减薄率为24.5%.本文所成形不锈钢球形件内高压成形区间,合理初始内压与屈服强度比值范围为0.21～0.25.     

Meniscus and viscous forces during normal separation of liquid-mediated contacts

Menisci form between two solid surfaces with the presence of an ultra-thin liquid film. Meniscus and viscous forces contribute to an adhesive force when two surfaces are separated. The adhesive force can be very large and can result in high friction, stiction and possibly high wear. The situation may become more pronounced when the contacting surfaces are ultra-smooth and the normal load is small, as is common for micro-/nanodevices. In this study, equations for meniscus and viscous forces during separation of two flat surfaces, and a sphere and a flat surface, are developed, and the corresponding adhesive forces contributed by these two types of forces are examined. The geometric meniscus curvatures and break point are theoretically determined, and the role of meniscus and viscous forces is evaluated during separation. The influence of separation distance, liquid thickness, meniscus area, separation time, liquid properties and contact angles are analyzed. Critical meniscus areas at which transition in the dominance of meniscus to viscous forces occurs for different given conditions, i.e.?various initial liquid thicknesses, contact angles and designated separation time, are identified. The analysis provides a fundamental understanding of the physics of separation process, and insights into the relationships between meniscus and viscous forces. It is also valuable for the design of the interface for various devices.     

A new view of the onset of plasticity during the nanoindentation of aluminium

In nanoscale contact experiments, it is generally believed that the shear stress at the onset of plasticity can approach the theoretical shear strength of an ideal, defect-free lattice, a trend also observed in idealized molecular dynamics simulations. Here we report direct evidence that plasticity in a dislocation-free volume of polycrystalline aluminium can begin at very small forces, remarkably, even before the first sustained rise in repulsive force. However, the shear stresses associated with these very small forces do approach the theoretical shear strength of aluminium (approximately 2.2 GPa). Our observations entail correlating quantitative load-displacement measurements with individual video frames acquired during in situ nanoindentation experiments in a transmission electron microscope. We also report direct evidence that a submicrometre grain of aluminium plastically deformed by nanoindentation to a dislocation density of approximately 10(14) m(-2) is also capable of supporting shear stresses close to the theoretical shear strength. This result is contrary to earlier assumptions that a dislocation-free volume is necessary to achieve shear stresses near the theoretical shear strength of the material. Moreover, our results in entirety are at odds with the prevalent notion that the first obvious displacement excursion in a nanoindentation test is indicative of the onset of plastic deformation.     

Influence of cyclic loading on the onset of failure in a Zr-based bulk metallic glass

Samples of Zr_62.5Cu_22.5Fe₅Al₁₀ bulk metallic glass are subjected to uniaxial compression. Comparison of tests in monotonic loading and cyclic loading (repeated loading to the onset of plasticity, then unloading) shows that the compressive plasticity of the glass is drastically reduced under cyclic loading. It is argued that this effect arises from stress reversal accelerating the concentration of shear on a dominant shear band. The link with compression–compression fatigue results, and the consequences for low-cycle fatigue of metallic glasses are considered.     

Crystal plasticity investigation of friction effect on texture evolution of Al single crystal during ECAP

A crystal plasticity finite element model has been developed to study the effect of friction between the die wall and the billet on texture evolution during equal channel angular pressing of an aluminum single crystal. Four cases with different coefficients of friction μ = 0, 0.05, 0.1, and 0.15 have been simulated. It has been found that the friction of μ = 0.05 and 0.1 can capture the major texture features shown in the experimental results, and μ = 0.05 predicts a slightly better texture than μ = 0.1. The frictional condition significantly affects texture evolution in the region between 1/2 and 3/4 of the billet thickness from the top surface. It can be attributed to the effect of friction on the corner gap and the distribution of stresses in the die corner.     

Effects of dry friction on the dynamics of a spherical pendulum

The effects of dry friction on a spherical pendulum are considered. Equations are derived for the angular velocity of the rotation for the plane of oscillation. The behavior of a pendulum in the initial stage is examined in the linear approximation. Translated from Izmeritel'naya Tekhnika, No. 6, pp. 3–5, June, 1998.     

Cooperative adhesion and friction of compliant nanohairs

The adhesion and friction behavior of soft materials, including compliant brushes and hairs, depends on the temporal and spatial evolution of the interfaces in contact. For compliant nanofibrous materials, the actual contact area individual fibers make with surfaces depends on the preload applied upon contact. Using in situ microscopy observations of preloaded nanotube hairs, we show how nanotubes make cooperative contact with a surface by buckling and conforming to the surface topography. The overall adhesion of compliant nanohairs increases with increasing preload as nanotubes deform and continuously add new side-wall contacts with the surface. Electrical resistance measurements indicate significant hysteresis in the relative contact area. Contact area increases with preload (or stress) and decreases suddenly during unloading, consistent with strong adhesion observed for these complaint nanohairs.     

晶体塑性模型描述多晶体循环加载中的Bauschinger效应

为了研究循环加载过程中织构对多晶材料Baushinger效应的影响,利用经典晶体塑性模型及含随动硬化的晶体塑性模型模拟AA6104铝合金循环加载力学行为.研究了多晶体中晶粒取向差异对材料宏观塑性行为的影响.详细分析了经典晶体塑性模型可描述多晶体循环加载Bauschinger效应机理,定量分析了多晶有限元模型中晶体取向差异对模拟结果的影响.结果表明多晶体中由于晶粒取向差异而造成的晶粒间相互作用力使得多晶体模型宏观卸载时晶粒内的残余应力是产生Bauschinger效应的主要原因,采用含随动硬化的晶体塑性模型能够较好地模拟具有织构的AA6014铝合金的循环加载过程.     

Influence of LDH conversion coatings on the adhesion and corrosion protection of friction spot-joined AA2024-T3/CF-PPS

Layered double hydroxide(LDH)conversion coatings loaded with corrosion inhibitors were suggested for the surface treatment of the aluminum alloy 2024-T3,prior to friction spot joining with carbon-fiber reinforced polyphenylene sulfide(AA2024-T3/CF-PPS).Vanadate was used as a model corrosion inhibitor.Lap shear testing method revealed an increase of approx.20%of the joint’s adhesion performance when treated with LDH and before exposure to salt spray.The evaluation of the joints after exposure to salt spray demonstrated a significant difference in the corrosion behavior of the joints when the AA2024-T3 is treated with LDH loaded with nitrate and vanadate species.The LDH intercalated with nitrate revealed a clear improvement in the mechanical and corrosion resistance performance of the joints,even after 6 weeks of salt spray.However,the LDH intercalated with vanadate failed in providing protection against corrosion as well as preserving the mechanical properties of the joints.The effect of the galvanic corrosion was further investigated by zero resistance ammeter(ZRA)measurements as well as localized scanning vibrating electrode technique(SVET).     

Structure-dependent adhesion and friction on highly ordered metallic nanopore membranes

Highly ordered metallic nanopore membranes are fabricated by direct deposition of nickel on typical porous anodic alumina (PAA) templates. The large-area uniform nanopore arrays of the PAA templates are accurately transferred to the metallic nanopore replicas, depending on the thickness of the deposited metal and the pore size of the base template. We demonstrate the ready tunability of the pore size and reproducibility of the metallic nanopore structure in a wide range of pore sizes. The adhesion and friction characteristics of the metallic replicas are studied according to the pore size using atomic force microscopy (AFM). As the pore diameter increases, the friction coefficients increase nonlinearly, and the adhesive forces scarcely change. These characteristics are understood in terms of the structural properties of the replicas, specifically the surface morphology and the real contact area. Initial pore formation from a flat thin film reduces the adhesive force by up to four times.     

On the theory of plasticity with smooth surfaces of loading

The well-known theories of plastic yield with smooth surfaces of loading are developed and generalized on the basis of a model of a nonlinear anisotropically hardened medium based on the concept of slip. Unlike the commonly used procedure of application ofa priori known laws of hardening (variation of the surface of loading in the process of plastic deformation), we suggest a method for the experimental evaluation of a universal function of the material appearing in the constitutive equations for arbitrary complex loading processes including elastic unloading and plastic deformation in the direction opposite to the initial one. The constitutive equations are relatively simple and, hence, can easily be used for the statement and solution of boundary-value problems in the theory of plastic yield. Dnepropetrovsk State University, Dnepropetrovsk. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 6, pp. 63–70, November–December, 1997.     

Influence of the loading path on fatigue crack growth under mixed-mode loading

Fatigue crack growth tests were performed under various mixed-mode loading paths, on maraging steel. The effective loading paths were computed by finite element simulations, in which asperity-induced crack closure and friction were modelled. Application of fatigue criteria for tension or shear-dominated failure after elastic–plastic computations of stresses and strains, ahead of the crack tip, yielded predictions of the crack paths, assuming that the crack would propagate in the direction which maximises its growth rate. This approach appears successful in most cases considered herein.     

Temperature and stresses in the vicinity of friction contacts of microasperities in the process of braking

We develop an approach to the solution of heat problems of friction simulating the operation of brake systems. The thermal and stressed state of an element of the friction pair is estimated by using the criterion of octahedral tangential stresses. Franko L'viv State University, L'viv State Agricultural University, State University “L'vivs'ka Politekhnika,” L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 6, pp. 72–80, November–December, 1996.     

ALE方法模拟爆炸载荷作用下球壳反直观行为

本文利用任意拉格朗日与欧拉（ALE）方法,模拟了球壳在爆炸载荷作用下的动力反直观行为,得到了球壳中点位移时程曲线以及最终变形模式等动力响应。在研究中,考虑了材料的应变率效应,并对球壳的矢高和厚度对其反直观行为的影响进行了参数分析。结果表明,只有炸药距球壳距离在特定范围内,球壳才能发生反直观行为;当炸药当量一定时,球壳发生反直观行为的爆心距区域段随球壳矢高增加而减少,矢跨比大于一定值后,将不会发生反直观行为     

粗糙结合面法向接触的能量耗散与阻尼特性研究

为了能够预测粗糙结合面法向接触的能量耗散和阻尼的变化规律,在Hertz和Abbott-Firestone经典接触模型的基础上,提出了一种加载-卸载混合弹塑性接触模型。首先,研究单个微凸体在完全弹性和塑性接触状态下的受力特性,推导出混合弹塑性状态下接触面积和力的数学表达式,进而研究能量变化规律和接触阻尼特性。然后,通过文献中的实验数据,并且和完全弹性、塑性状态下的接触模型进行对比,验证了该模型的有效性。在此基础上,分析能量耗散和法向接触阻尼与法向变形量、硬度以及硬度指数的关系。结果表明,结合面能量耗散随着法向变形量的增加而变大,而法向接触阻尼随之减小;在法向变形量一定时,接触阻尼随着材料硬度的增加而变大,而硬度指数的影响很小。     

The effect of temperature on the nanoscale adhesion and friction behaviors of thermoplastic polymer films

Adhesion and friction tests were carried out in order to investigate the effect of temperature on the tribological characteristics of poly(methylmethacrylate) (PMMA) film using AFM. The pull-off and friction forces on the PMMA film were measured under a high vacuum condition (below 1 × 10(-4)?Pa) as the temperature of the PMMA film was increased from 300 to 420?K (heating) and decreased to 300?K (cooling). Friction tests were also conducted in both high vacuum and air conditions at room temperature. When the temperature was 420?K, which is 25?K higher than the glass-transition temperature (T(g)) of PMMA, the PMMA film surface became deformable. Subsequently, the pull-off force increased in proportion to the maximum applied load during the pull-off force measurement. In contrast, when the temperature was under 395?K, the pull-off force did not depend on the maximum applied load. The friction force began to increase when the temperature rose above 370?K, which is 25?K lower than the T(g) of PMMA, and rapidly increased at 420?K. Decrease of the PMMA film stiffness and plastic deformation of the PMMA film were observed at 420?K in force-displacement curves. After the heating to 420?K, the coefficient of friction was measured under air condition at room temperature and was found to be lower than that measured before the heating. Additionally, the RMS roughness increased as a result of heating.