Elastic–plastic contact analysis of an ellipsoid and a rigid flat

The work presents a finite element model (FEM) of the equivalent von-Mises stress and displacements that are formed for the different ellipticity contact of an ellipsoid with a rigid flat. The material is modeled as elastic perfectly plastic and follows the von-Mises yield criterion. The smaller the ellipticity of the ellipsoid is, the larger the depth of the first yield point from the ellipsoid tip happens. The FEM produces contours for the normalized normal and radial displacement as functions of the different interference depths. The evolution of plastic region in the asperity tip for a sphere (k_e=1) and an ellipsoid with different ellipticities $(k_e=\frac{1}{2}\mathrm{and}\frac{1}{5})$ is shown with increasing interferences. It is interesting to note the behavior of the evolution of the plastic region in the ellipsoid tip for different ellipticities, k_e, is different. The developments of the plastic region on the contact surface are shown in more details in Fig. 7. When the dimensionless contact pressure is up to 2.5, the uniform contact pressure distribution is almost prevailing in the entire contact area. It can be observed clearly that the normalized contact pressure ascends slowly from the center to the edge of the contact area for a sphere (k_e=1), almost has uniform distribution prevailing the entire contact area for an ellipsoid $(k_e=\frac{1}{2})$, and descends slowly from the center to the edge of the contact area for an ellipsoid $(k_e=\frac{1}{5})$.     

An elastic–plastic asperity interaction model for sliding friction

A finite-element model of the interaction of an elastic–plastic asperity junction based on cylindrical or spherical asperities is used to predict sliding friction coefficients. The modelling differs from previous work by permitting greater asperity overlaps, enforcing an interface adhesional shear strength, and allowing material failure. The results of the modelling were also used to predict friction coefficients for a stochastic rough surface. The asperities were based on the titanium alloy Ti-6Al-4V, and the magnitudes of the predicted friction coefficients were generally representative of experimental measurements of sliding friction. The results suggest that friction arises from both plasticity and tangential interface adhesion.     

Elastic–plastic spherical contact under combined normal and tangential loading in full stick

The behavior of an elastic–plastic contact between a deformable sphere and a rigid flat under combined normal and tangential loading with full stick contact condition is investigated theoretically. Sliding inception is treated as a plastic yield failure mechanism, which allows static friction modeling under highly adhesive conditions. Several contact parameters such as: junction tangential stiffness, static friction force and static friction coefficient are extensively investigated. The phenomenon of junction growth and the evolution of the plastic zone in the contact region are briefly described. It is found that at low normal dimensionless loads the static friction coefficient decreases sharply with increasing normal load, in breach with the classical laws of friction. As the normal load further increases the static friction coefficient approaches a constant value that is about 0.3 for many material properties combinations.     

Plastic–elastic Model for Water-based Lubrication Considering Surface Force

Water-based lubrication is an effective method to achieve superlubricity, which implies a friction coefficient in the order of 10^-3 or lower. Recent numerical, analytical, and experimental studies confirm that the surface force effect is crucial for realizing water-based superlubricity. To enhance the contribution of the surface force, soft and plastic materials can be utilized as friction pair materials because of their effect in increasing the contact area. A new numerical model of ...     

Thermomechanical modeling and transient analysis of sliding contacts between an elastic–plastic asperity and a rigid isothermal flat

To investigate thermomechanical contacts between an elastic–plastic sphere and a rigid flat, simulations with slip rates ranging from 0.1 m/s to 10 m/s were performed. As interfaces with strong interfacial bonding but weak substrate were specifically targeted, slip initiation was treated as shear failure of the softer material in numerical simulations. The simulations show that both sliding friction coefficient and friction stress are significantly dependent on slip rate while the maximum static friction coefficient is independent of that. Moreover, the energy release during the transition from full stick to full slip is comparable to the shear fracture energy of the material.     

Effect of as-ground surface and the BALINIT® C and Nb–S coatings on contact fatigue damage in gears

The influence of as-ground surface and the BALINIT^® C and Nb–S coatings on contact fatigue damage have been investigated using testing of case-carburised S156 steel helical gears. The micro-surface features on the as-ground gear flank tend to initiate micro-pitting. In BALINIT^® C coated gears, the surface irregularities are removed by polishing effect resulting in small scale initiation of micro-pitting at the interface between polished and unpolished regions. In Nb–S coated gears, the coating tends to penetrate and fill-up the surface micro-valleys and hence showed only scattered micro-pitting. The Nb–S coated gears did not produce any tooth profile loss whilst the BALINIT^® C coated gears produced profile loss. However, both BALINIT^® C and Nb–S coated gears show enhanced contact fatigue performance.     

Spin–charge separation and electron pairing instabilities in Hubbard nanoclusters

Electron charge and spin pairing instabilities in various cluster geometries for attractive and repulsive electrons are studied exactly under variation of interaction strength, electron doping and temperature. The exact diagonalization, level crossing degeneracies, spin–charge separation and separate condensation of paired electron charge and opposite spins yield intriguing insights into the origin of magnetism, ferroelectricity and superconductivity seen in inhomogeneous bulk nanomaterials and various phenomena in cold fermionic atoms in optical lattices. Phase diagrams resemble a number of inhomogeneous, coherent and incoherent nanoscale phases found recently in high-_Tc$T_c$ cuprates, manganites and multiferroic nanomaterials probed by scanning tunneling microscopy. Separate condensation of electron charge and spin degrees at various crossover temperatures offers a new route for superconductivity, different from the BCS scenario. The calculated phase diagrams resemble a number of inhomogeneous paired phases, superconductivity, ferromagnetism and ferroelectricity found in Nb and Co nanoparticles. The phase separation and electron pairing, monitored by electron doping and magnetic field surprisingly resemble incoherent electron pairing in the family of doped high-_Tc$T_c$ cuprates, ruthenocuprates, iron pnictides and spontaneous ferroelectricity in multiferroic materials.     

Combined rough and finish machining of Ti–6Al–4V alloy by electrochemical mill-grinding

Abstract

This study proposes a combined method for the electrochemical mill-grinding of Ti–6Al–4V alloy to achieve a high material removal rate, high machining accuracy and good surface quality based on rough and finish machining. In the rough machining stage, a maximum feed rate of 2.7?mm min^?1 and a material removal rate of 248.3?mm³ min^?1 were achieved experimentally at a 10?mm cut depth using an abrasive tool with five rows of tool-sidewall outlet holes. In the finish machining stage, there were almost no overcuts or stray corrosions produced. The sidewall surface roughness and sidewall flatness were Ra = 1.06 and 76.8?μm after the finishing stage, which represent a 68% and 79.2% improvement compared with the rough machining stage, respectively. Finally, we fabricated a 1-mm-thick thin-walled structure using the combined machining operations, in which approximately 96% of the total material removal volume was performed at the rough machining stage.     

Multi-objective optimization design of a fixture layout considering locator displacement and force–deformation

The fixture determines the workpiece position in a machining process; therefore, an increasing amount of attention has been given to fixture layout design. While machining, the workpiece position is affected by two major sources: (a) the locator displacement and (b) the force–deformation of the workpiece–fixture system. In the beginning of this paper, a geometric model considering the shape of a locator is developed to analyze the location performance, followed by the presentation of a simplified solving method and a location layout performance index. Second, to complete the force–deformation analysis, a finite element method-based force–deformation model is built and accelerated by a new method with a lower computer memory cost. Based on these two models, multiple objects of fixture layout optimization problems are proposed, and a multi-objective genetic algorithm-based optimization method is constructed. Finally, testing examples are approved to examine the validity of the method represented in this paper. These methods can provide a more accurate prediction of the locating performance in more widely used cases, and they have faster calculating speeds with lower computer memory costs.     

Design and application of a borehole–surface microseismic monitoring system

Borehole–surface microseismic monitoring is a new approach for monitoring artificially induced hydraulic fracturing in an oil or gas field. However, ineffective communication links and incompatible data formats between current borehole- and surface-based monitoring systems mean that borehole–surface monitoring cannot be reported in real time. To solve this problem, the borehole–surface microseismic monitoring system reported here allows fracturing points through inversion and the development of fractures to be viewed in real time. Private cloud technology is used to control the instruments, manage the borehole and the surface database, and process the data. This system ensures high performance and availability of the system. The data acquisition modules and the geophones used in the borehole and surface instruments were calibrated in the laboratory to ensure the consistency of the acquired microseismic signals. The monitoring system located 82 microseismic events with a fracture azimuth of N84°E during fracture production in the Daqing Oilfield. Subsequent analysis showed that the locations of the fractures and their strike directions were consistent with the theory of hydraulic fracture propagation and the local crustal stress field. The results demonstrate that the monitoring system effectively and promptly processed data, thus enabling real-time borehole–surface microseismic monitoring.     

The Friction and Wear Properties of Ti–Al–Nb Intermetallics by Plasma Surface Alloying

Both plasma chromizing and carburization following plasma chromizing (duplex treatment) for Ti–Al–Nb alloy were performed, respectively, and the microstructure, dynamic ultra-microhardness, and elastic modulus of the alloying layer were determined. Using silicon nitride (Si₃N₄) balls as the counterface materials, dry sliding friction tests on the substrate, the chromized layer, and the duplex-treated layer were completed by ball-on-disk tribometer at room temperature. The results indicated that the duplex-treated layer was mainly composed of Cr₂₃C₆, Cr₂Nb, pure chromium, and carbon phases, while the chromized layer consisted of Al₈Cr₅ and Cr₂Nb phases. The ultra-microhardness of the duplex-treated layer was higher than that of the chromized layer, whereas the elastic modulus of the duplex-treated layer was lower than that of the chromized layer. The friction coefficient of the duplex-treated layer was about three times lower than that of the chromized layer, while the wear rate was one order of magnitude lower than that of the chromized layer.     

Surface roughness modeling and optimization of tungsten–copper alloys in micro-milling processes

Nowadays, the micrometric and nanometric dimensional precision of industrial components is a common feature of micro-milling manufacturing processes. Hence, great importance is given to such aspects as online metrology and real-time monitoring systems for accurate control of surface roughness and dimensional quality. A real-time monitoring system is proposed here to predict surface roughness with an estimation error of 9.5%, by using the vibration signal that is emitted during the milling process. In the experimental setup, the z-axis component vibration is measured using two different diameters under several cutting conditions. Then, an adaptive neuro-fuzzy inference system model is implemented for modeling surface roughness, yielding a high goodness of fit indices and a good generalization capability. Finally, the optimization process is carried out by considering two contradictory objectives: unit machining time and surface roughness. A multi-objective genetic algorithm is used to solve the optimization problem, obtaining a set of non-dominated solutions. Pareto front representation is a useful decision-making tool for operators and technicians in the micro-milling process. An example of the Pareto front utility-based approach that selects two points close to both extreme ends of the frontier is described in the paper. In the first case (point 1), machine time is of greater importance, and in the second case (point 2), importance is attached to surface roughness. In general terms, users can select different combinations, at all times moving along the Pareto front.     

Influence cutting parameters on the surface quality and corrosion behavior of Ti–6Al–4V alloy in synthetic body environment (SBF) using Response Surface Method

Most of the global manufacturing of titanium alloys is related to produce biphasic structures with grains alpha and beta. The development of modern applications of titanium alloy is a great challenge due to the chemical composition of Ti–6Al–4V alloy and the complexity of the manufacturing technology. This study proposed an optimal investigation of the variation of cutting speed, feed rate, and depth of cut in the turning of Ti–6Al–4V alloy on surface roughness, cutting efforts, and corrosion resistance. Response Surface Method has been established to optimize and model the responses mathematically. The adequacy of the models and a significant contribution of parameters were determined by analysis of variance (ANOVA). The biocompatibility of the machined surface for different cutting parameters was evaluated by the electrochemical polarization in simulated body fluids (SBF). Furthermore, desirability function analysis was used to determine the optimal values for surface quality, the turning force, and the passivation rate. It was clearly noticed that the multi-responses of the desirability function improved the machine process. The feed rate and depth of cut were the most relevant factors to minimize surface roughness and the turning forces. Moreover, the experimental results showed that the corrosion behavior was strongly related to minimal surface roughness. Finally, the optimization reduced the surface roughness Ra and Rz in 5.5% and 11.9%, respectively and increased the corrosion resistance in 18.8%.     

Frictional performance and near-surface evolution of nanocrystalline Ni–Fe as governed by contact stress and sliding velocity

While early reports on the wear performance of nanocrystalline alloys have suggested enhanced behavior consistent with their higher hardness compared to conventional microcrystalline alloys, there is still limited understanding of the mechanisms and limits of this enhanced behavior. In the present study, we examine the frictional response of a nanocrystalline Ni–20Fe alloy with 34-nm average grain size compared to the same film annealed to an average grain size of 500-nm. We examine the sliding friction performance of these films in contact with a 3.125 mm diameter Si₃N₄ spherical counterface under a range of normal forces (0.1–1.0 N) and sliding speeds (0.25–3.75 mm/s) in a non-oxidizing dry nitrogen environment. Under all conditions, the initial break-in coefficient of friction (COF) starts high, μ≈0.5–0.8, typical of uncoated metallic friction. However, there is an evolution in the COF which depends on normal force and sliding speed. At low sliding speeds (or normal forces), the steady-state COF decreases to μ≈0.2 whereas at higher sliding speeds and normal forces, the steady-state COF remains high at μ≈0.8. Focused ion beam cross-sectioning and TEM imaging reveal that in all cases, a multilayer substructure is formed in the deforming film: a refined ultrananocrystalline layer at the top surface, over a region of coarsened grains, atop the parent nanocrystalline alloy. The key distinction between the high-friction and low-friction conditions appears to lie in the triggering of a delamination process: high-friction conditions are associated with a thickening of the UNC layer through repeated delamination, whereas low-friction conditions are associated with a thin UNC layer that does not delaminate. Finite element analysis is used to aid in the understanding of how the magnitude and location of stresses drive these two distinct regimes.     

Performance and surface alloying characteristics of Cu–Cr and Cu–Mo powder metal tool electrodes in electrical discharge machining

The main objective of this study is to investigate the effect of Cu–Cr and Cu–Mo powder metal (PM) tool electrodes on electrical discharge machining (EDM) performance outputs. The EDM performance measures used in the study are material removal rate (MRR), tool electrode wear rate (EWR), average workpiece surface roughness (R_a), machined workpiece surface hardness, abrasive wear resistance, corrosion resistance, and workpiece alloyed layer depth and composition. The EDM performance of Cu–Cr and Cu–Mo PM electrodes produced at three different mixing ratios (15, 25, and 35 wt% Cr or Mo), compacting pressures (P_c = 600, 700, and 800 MPa), and sintering temperatures (T_s = 800, 850, and 900 °C) are compared with those machined with electrolytic Cu and Cu PM electrodes when machining SAE 1040 steel workpiece. Analyses revealed that tool materials were deposited as a layer over the work surface yielding high surface hardness, strong abrasion, and corrosion resistance. Moreover, the mixing ratio, P_c, and T_s affect the MRR, EWR, and R_a values.     

Surface Characterization and Erosion–Corrosion Behavior of Q235 Steel in Dynamic Flow

The study aims at investigating the surface evolution and erosion–corrosion behavior of Q235 steel during erosion–corrosion process in various dynamic flows. For the purpose, true flow fields with the average flow velocities of 0.4 and 0.8 m/s and impact angles of 0°, 30° and 90° to the sample surface were successfully measured by particle image velocimetry. The topography of erosion–corrosion surface was observed by laser scanning confocal microscopy. The evolution of localized corrosion pattern is found to be determined by impact angle, i.e., round or elliptical corrosion pit corresponds to impact angle of 90° and ribbon-like corrosion pit corresponds to 0°. The deeper corrosion pits were observed at impact angle of 30° than those at the other two impact angles owing to combined effects of shear and normal stresses. Electrochemical impedance spectroscopy of samples shows smaller radiuses of capacitive loops at velocity of 0.8 m/s than those at 0.4 m/s. Equivalent circuit analysis implies unstable surface state of sample in dynamic flow. Above results indicate that the flow velocity and impact angle play the key role in the erosion–corrosion behavior of Q235 steel.     

Engagement behavior of lubricated porous annular disks. Part I: Squeeze film phase — surface roughness and elastic deformation effects

The lubricated clutch engagement behavior of two annular disks, one of which is covered with a layer of porous material, was analyzed. The engagement model dealt with was postulated to consist of three phases: squeeze film phase, mixed asperity contact phase and consolidating contact phase. In the squeeze film phase, the porous material is considered to be elastically deformable and to have a rough surface. Solutions of pressure distribution, load capacity and film thickness versus time have been obtained for various geometric, material and operating parameters. In the paper following (Part II) the consolidating phase behavior was analyzed based on the theory of poroelasticity, and comparison was made between analysis and experiment at squeeze film and consolidating contact phases indicating good qualitative agreement. The investigation of mixed asperity contact phase is not yet completed.     

Optimization of surface treatment to enhance fiber–matrix interface and performance of composites

Oxidation treatment with concentrated HNO₃ was employed to the carbon fabric (CF) for various time intervals (30–180 min) to observe the effect of treatment on two simultaneous processes involved viz. improvement in its adhesion with the matrix and reduction of fiber strength which in turn is responsible for change in the performance properties of composites. Seven composites with untreated and acid treated CF were developed based on the polyetherimide (PEI) matrix and evaluated for adhesive wear properties under various loads (200–600 N) against mild steel disc. 90 min treated CF composite indicated the best tribological properties and showed 30% reduction in specific wear rate (K₀) and 23% in coefficient of friction (μ) respectively at 600 N load. Treatment beyond this time proved detrimental for improvement in properties. Field emission scanning electron microscopy (FE-SEM) showed increase in roughness with treatment time, while atomic force microscopy (AFM) studies indicated substantial increase in roughness value. Scanning electron microscopy (SEM) of worn surfaces supported the wear mechanisms and improvement in adhesion between fiber and matrix.