The Influence of Normal Load and Sliding Speed on Frictional Properties of Skin

The study of frictional properties of human skin is important for medical research, skin care products and textile exploi- tation. In order to investigate the influence of normal load and sliding speed on the frictional properties of skin and its possible mechanism, tests were carded out on a multi-specimen friction tester. When the normal load increases from 0.1 N to 0.9 N, normal displacement and the friction coefficient of skin increase. The friction coefficient is dependent on the load, indicating that both adhesion and deformation contribute to the friction behaviour. The deformation friction was interpreted using the plough model of friction. When sliding speed increases from 0.5 mm·s^-1 to 4 mm·s^-1, the friction coefficient increases and ＂stick-slip＂ phenomena increase, indicating that hysteretic friction contributes to the friction. The hysteretic friction was in- terpreted using schematic of energy translation during the rigid spherical probe sliding on the soft skin surface, which provides an explanation for the influence of the sliding speed on the frictional characteristics of the skin.

Quasi-in-situ investigation into the microstructure and texture evolution of pure magnesium during friction stir welding

The quasi-in-situ microstructure and texture evolution along the real flow path of pure magnesium during friction stir welding were investigated.Five representative stages were involved from the base metal to the formation of the final stir zone.The material experienced compression,preheating,acceleration,deceleration,and subsequent annealing over the course of the welding process.A highly concentrated(0001)texture,denoted as''orientation convergence^,was initiated at the beginning of the acceleration stage(shearing deformation zone)in front of the tool.Both continuous and discontinuous dynamic recrystallization occurred simultaneously in the stir zone,and continuous dynamic recrystallization was determined to be the primary recrystallization mechanism.The marker material morphology and EBSD data were used to elucidate the in-situ evolution of the shear direction and shear plane along with the real flow path.

Molecular dynamic simulation of lubricant spreading： effect from the substrate and endbead

Molecular dynamic simulations based on a coarse-gralned, bead-spring model are adopted to investigate the spreading of both nonfunctional and functional perfluoropolyether （PFPE） on solid substrates. For nonfunctional PFPE, the spreading generally exhibits a smooth profile with a precursor film. The spreading profiles on different substrates are compared, which indicate that the bead-substrate interaction has a significant effect on the spreading behaviour, especially on the formation of the precursor film. For functional PFPE, the spreading generally exhibits a complicated terraced profile. The spreading profiles with different endbeads are compared, which indicate that the endbead-substrate interaction and the endbead-endbead interaction, especially the latter, have a significant effect on the spreading behaviour.

Assessment of Ti-6Al-4V particles as a reinforcement for AZ31 magnesium alloy-based composites to boost ductility incorporated through friction stir processing

Poor ductility is the primary concern of magnesium matrix composites(MMCs)inflicted by non-deformable ceramic particle reinforcements.Metal particles which melt at elevated temperature can be used as reinforcement to improve the deformation characteristics.Ti-6Al-4V particles reinforced AZ31 MMCs were produced through friction stir processing(FSP)which was carried out in a traditional vertical milling machine.The microstructural features as well as the response to external tensile load were explored.A homogenous distribution of Ti-6Al-4V was achieved at every part of the stir zone.There was no chemical decomposition of Ti-6Al-4V.Further,Ti-6Al-4V did not react with Al and Zn present in AZ31 alloy to form new compounds.A continuous strong interface was obtained around Ti-6Al-4V particle with the matrix.Ti-6Al-4V particles underwent breakage during processing due to severe plastic strain.There was a remarkable refinement of grains in the composite caused by dynamic recrystallization in addition to the pinning of smaller size broken particles.Dense dislocations were observed in the matrix because of plastic deformation and the associated strain misfit.Ti-6Al-4V particles improved the tensile behavior and assisted to obtain appreciable deformation before fracture.Brittle mode of failure was avoided.

Modelling of spreading process: effect from hydrogen bonds

Lubricant spreading on solid substrates has drawn considerable attention not only for the microscopic wetting theory but also for the dramatic application in head-disk interface of magnetic storage drive systems. Molecular dynamic simulation based on a coarse-grained bead-spring model has been used to study such a spreading process. The spreading profiles indicate that the hydrogen bonds among lubricant molecules and the hydrogen bonds between lubricant molecules and polar atoms of solid substrates will complicate the spreading process in a tremendous degree. The hydrogen bonds among lubricant molecules will strengthen the lubricant combination intensity, which may hinder most molecules from flowing down to the substrates and diffusing along the substrates. And the hydrogen bonds between lubricant molecules and polar atoms of solid substrates will confine the lubricant molecules around polar atoms, which may hinder the molecules from diffusing along the substrates and cause precursor film to vanish.

Role of atomic transverse migration in growth of diamond-like carbon films

The growth of diamond-like carbon （DLC） films is studied using molecular dynamics simulations. The effect of impact angle on film structure is carefully studied, which shows that the transverse migration of the incident atoms is the main channel of film relaxation. A transverse-migration-induced film relaxation model is presented to elucidate the process of film relaxation which advances the original model of subplantation. The process of DLC film growth on a rough surface is also investigated, as well as the evolution of microstructure and surface morphology of the film. A preferential-to-homogeneous growth mode and a smoothing of the film are observed, which are due to the transverse migration of the incident atoms.

A New Differential Operator Method to Study the Mechanical Vibration

In this paper, we propose a unified differential operator method to study mechanical vibrations, solving inhomogeneous linear ordinary differential equations with constant coefficients. The main advantage of this new method is that the differential operator D in the numerator of the fraction has no effect on input functions (i.e., the derivative operation is removed) because we take the fraction as a whole part in the partial fraction expansion. The method in various variants is widely implemented in related fields in mechanics and engineering. We also point out that the same mistakes in the differential operator method are found in the related references [1-4].

Analysis of the kinematic characteristics of a high-speed parallel robot with Sch6nflies motion： Mobility, kinematics, and singularity

This study introduces a high-speed parallel robot with Schonflies motion. This robot exhibits a promising prospect in realizing high-speed pick-and- place manipulation for packaging production lines. The robot has four identical limbs and a single platform. Its compact structure and single-platform concept provides this robot with good dynamic response potential. A line graph method based on Grassmann line geometry is used to investigate the mobility characteristics of the proposed robot. A generalized Blanding rule is also introduced into this procedure to realize mutual conversion between the line graphs for motions and constraints. Subsequently, the inverse kinematics is derived, and the singularity issue of the robot is investigated using both qualitative and quantitative approaches. Input and output transmission singularity indices are defined based on the reciprocal product in screw theory and the virtual coefficient by considering motion/force transmission performance. Thereafter, the singular loci of the proposed robot with specific geometric parameters are derived. The mobility analysis, inverse kinematics modeling, and singularity analysis conducted in this study are helpful in developing the robot.

Evaluation of the power consumption of a high-speed parallel robot

An inverse dynamic model of a high-speed parallel robot is established based on the virtual work principle. With this dynamic model, a new evaluation method is proposed to measure the power consumption of the robot during pick-and-place tasks. The power vector is extended in this method and used to represent the collinear velocity and acceleration of the moving platform. After- ward, several dynamic performance indices, which are homogenous and possess obvious physical meanings, are proposed. These indices can evaluate the power input and output transmissibility of the robot in a workspace. The distributions of the power input and output transmissibility of the high-speed parallel robot are derived with these indices and clearly illustrated in atlases. Furtherly, a low- power-consumption workspace is selected for the robot.

Hybrid Navigation System Based Autonomous Positioning and Path Planning for Mobile Robots

Positioning and navigation technology is a new trend of research in mobile robot area.Existing researches focus on the indoor industrial problems,while many application fields are in the outdoor environment,which put forward higher requirements for sensor selection and navigation scheme.In this paper,a complete hybrid navigation system for a class of mobile robots with load tasks and docking tasks is presented.The work can realize large-range autonomous positioning and path planning for mobile robots in unstructured scenarios.The autonomous positioning is achieved by adopting suitable guidance methods to meet different application requirements and accuracy requirements in conditions of different distances.Based on the Bezier curve,a path planning scheme is proposed and a motion controller is designed to make the mobile robot follow the target path.The Kalman filter is established to process the guidance signals and control outputs of the motion controller.Finally,the autonomous positioning and docking experiment are carried out.The results of the research verify the effectiveness of the hybrid navigation,which can be used in autonomous warehousing logistics and multi-mobile robot system.

Drag Reducing and Increasing Mechanism on Triangular Riblet Surface

Drag reducing and increasing mechanism on riblet surface has been studied through computational fluid dynamics(CFD).Drag reduction is achieved through the optimization of riblet geometry which would affect flow structure inside riblet grooves.Force and flow structure on riblet surface are analyzed and compared with those of smooth surface based on the k-εturbulence model.Drag reducing and increasing mechanism is proved to be related to microvortexes induced inside riblets which lead to Reynolds shear stress reduction significantly and is considered to be the dominant factor resulting in wall friction reduction.Simulation results also show that the pressure drag generating from the deviation of static pressure on the front and rear ends of riblets occurs and grows exponentially with Mach number,which can cause drag increasing.Furthermore,near-wall vortical structures,Reynolds shear stress and static pressure on riblet surfaces are also analyzed in detail.

Dynamic characteristics of large permanent magnet direct‐drive generators considering electromagnetic–structural coupling effects

Dynamic characteristics of large permanent magnet direct‐drive generators(PMDGs)considering electromagnetic–structural coupling effects are analyzed in this study.Using the conformal mapping method,the scalar magnetic potential of the air gap magnetic field considering the slot effect is calculated.On the basis of the discrete current element and magnetic equivalent circuit model,the local magnetic saturation effect of the stator and rotor is quantitatively simulated and the air gap magnetic field intensity distribution is obtained via numerical simulation.A series of uniformly distributed equivalent electromagnetic springs are introduced to develop an electromagnetic–structural coupling finite element PMDG model.The proposed air gap field analysis method is verified by the finite element analysis results.On the basis of the test platform for the Goldwind 1.5MW PMDG,both modal and dynamic response tests for the stator/rotor coupling system are conducted,and the results are compared with the natural frequencies,mode shapes,and vibration responses obtained using the numerical model.The effects of the air gap length and rotor speed on the natural frequencies of the coupling system are analyzed.The proposed model has the potential to accurately evaluate the PMDG vibration energy,avoiding resonance points,and maintaining stable operations of the unit.

Hydrophobicity mechanism of non-smooth pattern on surface of butterfly wing

Twenty-nine species (24 genera, 6 families) of butterflies typical and common in northeast China were selected to make qualitative and quantitative studies on the pattern, hydrophobicity and hydrophobicity mechanism by means of scanning electron microscopy and contact angle measuring system. The scale surface is composed of submicro-class vertical gibbosities and horizontal links. The distance of scale is 48—91 μm, length 65—150 μm, and width 35—70 μm. The distance of submicro-class vertical gib-bosities on scale is 1.06—2.74 μm, height 200—900 nm, and width 200—840 nm. The better hydropho-bicity on the surface of butterfly wing (static contact angle 136.3°—156.6°) is contributed to the co-effects of micro-class scale and submicro-class vertical gibbosities on the wing surface. The Cassie equation was revised, and new mathematical models and equations were established.

Laser additive manufacturing of Zn porous scaffolds: Shielding gas flow, surface quality and densification

Zn based metals have exhibited promising prospects as a structural material for biodegradable applications. Pure Zn porous scaffolds were produced by laser powder bed fusion(LPBF) based on data files of designing and CT scanning. Massive Zn evaporation during laser melting largely influenced the formation quality during LPBF of Zn metal. The metal vapor in processing chamber was blown off and suctioned out efficiently by an optimized gas circulation system. Numerical analysis was used to design and testify the performance of gas flow. The surface of scaffolds was covered with numerous particles in different sizes. Processing pores occurred near the outline contour of struts. The average grain size in width was8.5m, and the hardness was 43.8 HV. Chemical plus electrochemical polishing obtained uniform and smooth surface without processing pores, but the diameter of struts reduced to 250 αm from the design value 300 m. The poor surface quality and processing pores were resulted by the splashing particles included spatters and powders due to the recoil force of evaporation, and the horizontal movement of liquid metal due to overheating and wetting. The insufficient melting at the outline contour combined with good wetting of Zn liquid metal further increased the surface roughness and processing pores.

Friction Characteristics of Impregnated and Non-Impregnated Graphite against Cemented Carbide under Water Lubrication

The friction behavior of resin-impregnated and non-impregnated graphite sliding against a cemented carbide in dry, oil, and water environments using a ring–ring configuration was investigated. Friction coefficients were recorded at various speeds. The results showed that the impregnated graphite exhibited much better friction properties under water or oil lubrication than non-impregnated graphite, and the impregnated graphite could remain in the stable friction regime under high pressure × velocity（pv）.Based on scanning electron microscopy and Raman spectroscopy analyses, the different characteristics between impregnated and non-impregnated graphite were able to be attributed to the structure of the graphite and wettability of the lubricants.

Conceptual design and kinematic analysis of a novel parallel robot for high-speed pick-and-place operations

This paper deals with the conceptual design, kinematic analysis and workspace identification of a novel four degrees-of-freedom （DOFs） high-speed spatial parallel robot for pick-and-place operations. The proposed spatial parallel robot consists of a base, four arms and a 11/2 mobile platform. The mobile platform is a major innova- tion that avoids output singularity and offers the advantages of both single and double platforms. To investigate the characteristics of the robot＇s DOFs, a line graph method based on Grassmann line geometry is adopted in mobility analysis. In addition, the inverse kinematics is derived, and the constraint conditions to identify the correct solution are also provided. On the basis of the proposed concept, the workspace of the robot is identified using a set of presupposed parameters by taking input and output transmission index as the performance evaluation criteria.

An energy efficiency evaluation method for parallel robots based on the kinetic energy change rate

Benefit from the high payload-to-weight ratio, parallel robots are expected to have a high potential for energy savings. However,it is a challenging issue to evaluate the energy efficiency of parallel robots with a quantitative method. Quantitative energy efficiency evaluation methods include energy efficiency evaluation models and indices which mathematically describe the relationship between energy consumers in models and design variables of robots, such as geometry, mass and inertia parameters.Considering the structural features of parallel robots, the chains and the end effectors are identified as two separated energy consumers. Besides, the chains in parallel robots are identified as a transmission system which transfers energy from drives to the end effectors. On this basis, an energy efficiency evaluation model considering the change rate of kinetic energy stored in chains is built. The kinetic energy change rate of chains is influenced by design variables of robots as well as motion of the end effector.In order to give a quantitative description of energy efficiency performance of parallel robots, indices considering arbitrary velocity vector of the end effector are proposed. The evaluation method is suitable for all kinds of parallel robots with various motion conditions. Furthermore, the method can be used to optimize machining parameters and guide the design of energyefficient machines.

ADHESION STRENGTH AND MORPHOLOGIES OF r BMSCs DURING INITIAL ADHESION AND SPREADING

Cell adhesion plays an important role in cell physiology. A better understanding of this process could facilitate many clinical therapies. In this study, Rat bone marrow-derived mesenchymal stem cells （rBMSCs） were cultured on glass substrate, and the morphology and adhesion strength were characterized. The cell morphology was defined as spherical, adhesive, and spreading. The adhesion strengths of the different morphologies exhibited different distribu- tion patterns. The spherical cells exhibited low adhesion strength; the adhesive cells exhibited rapidly increasing adhesion strength while their diameters remained relatively constant. The ad- hesion strength increased with the cell diameter in the spreading cells. These findings suggest that adhesion strength can be quickly assessed by examining the cell morphology.

Scaling analysis of current influence on Hastelloy surface roughness in electro-polishing process

In this study, a series of Hastelloy tapes were electro-polished, and the dividing method was used to carry out a detailed investigation on the influence of polishing current(I) on root mean square(R_q) at various image scales(L). The electro-polishing is found to be effective mainly at L smaller than 10μm, where the R_q–I relationship could be fitted by an exponential decay function with a residual roughness value. An approximate model of electro-polishing process was established to interpret the exponential decay function. This study provides a quantified insight into the electro-polishing process, which could help to obtain more understanding of its mechanism.

Laser Texturing of Rolled Surfaces

To reduce friction and improve clarity of steel surfaces, laser texturing was used to produce known surface roughness profile on rolled surfaces using a Nd:YAG laser. Laser texturing process was analyzed to select the laser parameters. The surface roughness hardness and abrasion resistance were then measured and analyzed. The results show that the surface roughness is harder than the matrix, which fits for the requirements of laser texturing. The surface roughness also has good abrasion resistance. The intensity and distribution of the single pulse are the key points affecting the surface roughness profile.