辐板型式和轮轨接触点位置对车轮声辐射特性的影响

为了分析不同车轮辐板型式和轮轨接触点位置对车轮声辐射特性的影响,建立了车轮有限元-边界元混合振动声辐射模型。首先,根据车轮实际拓扑结构建立三维实体有限元模型,采用分块Lanzos法求解结构的特征值问题,然后采用模态叠加法计算车轮结构在法向单位力激励下的动态响应,将车轮外表面的速度处理成声学边界元的输入,计算车轮的辐射噪声。数值计算中,考虑了S型、直型和波浪型三种辐板型式和轮缘、名义滚动圆处和车轮外侧三个轮轨接触点位置。结果表明,辐板型式和轮轨接触点位置对车轮声辐射具有较明显的影响。而且,不同辐板型式车轮在不同轮轨接触点位置下的声辐射特性也不尽相同。数值分析可以为低噪声车轮的选型提供一定的参考。     

A comparative analysis of contact algorithms in contact shape optimization problems

This paper presents a comparative analysis of contact algorithms used for solving contact shape optimization problems. Specifically, a nonlinear, feasible direction interior point method (FDIPM) for the frictional contact analysis of hyperelastic materials has been implemented in which the friction is introduced using the return mapping approach. This comparative investigation aims to find the cause of instability in sensitivity of the contact pressure nonuniformity (CPN). The results obtained by the FDIPM are found to be comparable with those by the penalty methods (PM) and the augmented Lagrange multiplier methods (ALMM); however, the FDIPM possesses advantages, including good convergence and convenience in modeling. Furthermore, the basic cause of the unstable sensitivity is revealed to be the discretization of the finite element method, which causes the discontinuous increase of contact area with respect to the continuous increase of contact load. To improve the stability of the CPN, an adaptive post-processing technique is proposed.     

Droplet on a fiber: geometrical shape and contact angle

Summary This paper is concerned with the geometrical shape, wetting length, and contact angle of a microdroplet on a fiber by using the method free energy variation. The governing equation and relevant boundary conditions of the microdroplet were re-derived based on the free energy variation of the droplet/fiber system. The geometrical shape of the droplet was determined as the combination of Legendre's elliptical functions of the first and second kinds, corresponding to the previous results in literature [6]. For contact angle θ >15°, a novel efficient semi-analytic approach was proposed to extract the contact angle from experimental data. The given approach can be used as theoretical basis of determining surface tension of fluids based on a sessile drop on a fiber.     

Landmark-free geometric methods in biological shape analysis

In this paper, we propose a new approach for computing a distance between two shapes embedded in three-dimensional space. We take as input a pair of triangulated genus zero surfaces that are topologically equivalent to spheres with no holes or handles, and construct a discrete conformal map f between the surfaces. The conformal map is chosen to minimize a symmetric deformation energy E_sd(f) which we introduce. This measures the distance of f from an isometry, i.e. a non-distorting correspondence. We show that the energy of the minimizing map gives a well-behaved metric on the space of genus zero surfaces. In contrast to most methods in this field, our approach does not rely on any assignment of landmarks on the two surfaces. We illustrate applications of our approach to geometric morphometrics using three datasets representing the bones and teeth of primates. Experiments on these datasets show that our approach performs remarkably well both in shape recognition and in identifying evolutionary patterns, with success rates similar to, and in some cases better than, those obtained by expert observers.     

Nano-scale sliding contact deformation behaviour of enamel under wet and dry conditions

The abrasion response of cross sectional areas of enamel was studied by sliding a rounded diamond conical nano-indenter tip across the surface. The nano-indenter tip (radius ~1,200 nm) was scanned over a specific squared area with a load of 400 μN. Two different environments were chosen: Hank’s balanced salt solution (HBSS) and atmospheric laboratory condition. SEM (Scanning Electron Microscopy) and AFM (Atomic Force Microscopy) were used to characterize the final abraded areas. In addition, single scratches with linear incremented load were performed. The normal load and displacement data were utilized in a complementary manner to support the proposed deformation mechanisms. Greater orientation dependence for the case of the single scratches in relation to the abrasion tests was found. The latter results are discussed in terms of plastic deformation effects. The abrasion mechanisms were found to be the same for both wet and dry measurements and similar to that described in a previous study (Guidoni et al., Wear 266:60–68, 2009; Guidoni, Nano-scale mechanical and tribological properties of mineralized tissues. PhD. Montan University Leoben, Leoben, Austria, 2008). However, scratch deformation under fluid measurements shows greater recovery effects and abrasion resistance.     

Role of contact electrification and electrostatic interactions in gecko adhesion

Geckos, which are capable of walking on walls and hanging from ceilings with the help of micro-/nano-scale hierarchical fibrils (setae) on their toe pads, have become the main prototype in the design and fabrication of fibrillar dry adhesives. As the unique fibrillar feature of the toe pads of geckos allows them to develop an intimate contact with the substrate the animal is walking on or clinging to, it is expected that the toe setae exchange significant numbers of electric charges with the contacted substrate via the contact electrification (CE) phenomenon. Even so, the possibility of the occurrence of CE and the contribution of the resulting electrostatic interactions to the dry adhesion of geckos have been overlooked for several decades. In this study, by measuring the magnitude of the electric charges, together with the adhesion forces, that gecko foot pads develop in contact with different materials, we have clarified for the first time that CE does contribute effectively to gecko adhesion. More importantly, we have demonstrated that it is the CE-driven electrostatic interactions which dictate the strength of gecko adhesion, and not the van der Waals or capillary forces which are conventionally considered as the main source of gecko adhesion.     

Measurement of contact angle and work of adhesion at high temperature

A critical review is given of the present state of knowledge and future perspectives in high-temperature contact angle measurement. Experimental results obtained by the different versions of the sessile drop method and by various procedures are given in order to illustrate the two main sources of scatter in wettability data, the first being related to the quality of the substrates and the second to control of the furnace atmosphere.     

Unilateral contact with adhesion and friction between two hyperelastic bodies

In the present paper, we consider a thermodynamic model using the contact kinematics developed by A. Curnier, Q.C. He and J.J. Téléga [C. R. Acad. Sci. Paris Sér. II 314 (1992) 1] involving unilateral contact, adhesion and Coulomb friction between two homogeneous, isotropic and hyperelastic bodies. Adhesion is described by an internal state variable β introduced by M. Frémond [C. R. Acad. Sci. Paris Sér. II 295 (1982) 913; J. Theor. Appl. Mech. 6 (1987) 383]. Taking the case of contact between a hyperelastic solid and a plane support, we formulate the associated boundary value problem as a minimization problem when no friction is involved. When the intensity of the adhesion obeys a ‘static' law, we obtain an existence result for this problem.     

Providing adhesion strength for a substrate-coating system under contact loading

The authors of the article discuss some feasible means of determining geometrical parameters of a discrete-type coating with a view to ensure adhesion strength of the substrate-coating composite. The stress-strain calculations are performed for an element of a carbide tool with a vacuum-plasma-deposited coating, under contact loading, allowing for friction forces.     

Selective synthesis of metallic nickel particles with control of shape via wet chemical process

Metallic nickel particles with shapes varying from sea urchin-like to spherical have been selectively synthesized via aqueous chemical reduction. The phase structure and morphology of particles have been investigated by means of X-ray diffraction and scanning electron microscopy. It was found that sodium carbonate greatly accelerated the reduction process to form needle-like dendrites under atmospheric pressure. The probable formation mechanism of the sea urchin-like particles is also discussed. In the mild reduction process, the particle size of nickel nanospheres could be easily controlled by adjusting the experimental parameters.     

DEM simulations: mixing of dry and wet granular material with different contact angles

In solid mixing the raw materials typically differ at least in one material property, such as particle size, solid density and wetting properties, which in turn influence particle mobility. For example, smaller particles can percolate through the voids of larger ones under the influence of strain and gravity. This may produce fine particle accumulation at the bottom of the mixing vessel which results in undesired, inhomogeneous final products. When wet particles with different wetting properties need to be mixed, heteroagglomeration may occur as another segregation mechanism. We present a new capillary bridge force model to study segregation in moist cohesive mixing processes using DEM. New analytical equations of best fit are derived by solving the Young–Laplace equation and performing a regression analysis, in order to investigate discontinuous mixing processes of dry and moist materials with different particle sizes and different contact angles. Compared to a dry mixing process, mixing efficiency is improved by the addition of a small amount of liquid. While percolating segregation is reduced, heteroagglomerates occur in the wet mixing process.     

Effects of shot-peening on surface contact angles of biomaterials

It was shown previously that (i) if the surface of a biomaterial is covered with TiO₂ (tetragonal structure oxide), it shows a high initial contact angle and a high change rate in contact angle (i.e. a higher spreading process); while (ii) cubic structure oxides show relatively lower spreading rates in 1% NaCl solution at 25°C. Shot-peening has been applied to biomaterials (especially titanium and its alloys) to improve their fatigue strength. It is well known that shot-peening causes surface roughening. The effects of surface roughness on wettability are not well documented. Therefore, in the present study, the effects of shot peening on the initial contact angle and changes in it as a function of time, were investigated. In addition, the spontaneous half-cell potential of all tested biomaterials were measured to correlate the wettability phenomenon to initial surface chemistry. Pure titanium and its alloys, including Ti-6AI-4V and NiTi alloys, AISI Type 316L stainless, Co-Cr alloy, and pure nickel, were mechanically polished, shot-peened and pre-oxidized at 300°C for 30 min in pure oxygen. It was found that (i) shot-peening homogenized the surface conditions in terms of initial contact angles, (ii) TiO₂ oxide shows a higher spreading coefficient, while cubic structure oxides show a lower value, and (iii) the spreading coefficient was correlated to the magnitude of the spontaneous half-cell potential.     

Frictionless elastic contact problem for a curved rigid punch of arbitrary shape

Summary A new method is proposed for the analysis of elastic contact problems for a curved punch of non-elliptic planform under the action of a normal force. The punch base is assumed to be a quadratic surface. The method is based on an integral representation for the reciprocal distance between two points obtained by the author earlier. Some general relationships are established between the applied force and the punch settlement. Specific formulae are derived for a punch whose planform has a shape of a polygon, a rectangle, a rhombus and a cross. An example of a finite rigid cylinder lying on its generator and pressed against an elastic half-space is considered in detail. The method allows to have singular stresses at the cylinder edges and zero stresses at the rest of the boundary of the contact domain. The last condition serves for defining the width of the domain of contact. All the formulae are checked against the solutions known in the literature, and a good accuracy is confirmed in a sufficiently wide range of the aspect ratio.With 3 Figures     

Self-organized meso-patterning of soft solids by controlled adhesion: elastic contact lithography

The surface of a soft elastic film becomes unstable and deforms when a rigid flat plate is brought into its contact proximity, without application of any external pressure. These isotropic undulations have a characteristic wavelength, lambda approximately 3H, where H is the film thickness. The wavelength is independent of the adhesive interactions and the mechanical properties of the film. We present here a mini-review of our recent work on techniques of aligning, modulating, and ordering the instability structures by the use of simple 1-D patterned stamps, by changing the stamp-surface separation, by slow shearing of a flat stamp and by confining the instability in soft narrow channels. The generality of the technique for different soft materials is illustrated by patterning cross-linked polydimethylsiloxane (PDMS), aluminum coated PDMS and hydrogels films. Use of a flexible stamp such as a metal foil provides enhanced conformal contact by adhesive forces, which aids large area patterning without critically maintaining a parallel configuration and uniform pressure between the stamp and the film. The technique has the potential to develop into a new soft lithography tool--"Elastic Contact Lithography" suitable for rapid, large area micron and sub-micron self-organized patterning of a variety of soft materials without any special equipments.     

Effects of short-fiber shape on tensile properties of reinforced rubber

The tensile properties under various conditions were investigated to ascertain the optimum conditions to yield the best tensile properties. Fiber aspect ratio (AR: length of fiber/diameter of fiber), diameter ratio (DR: sphere diameter of dumbbell/diameter of fiber), interphase condition and fiber content were all considered as variables which impact the tensile strength, tensile moduli, pull-out force. In general, under good interphase conditions the tensile strength increased when the fiber aspect ratio was more than 20. The short-fiber reinforced SBR with a big end (DR = 3) did not show the dilution effect under interphase conditions when the fiber aspect ratio was more than 20. In case of short-fiber reinforced NR, when the specimen had DR = 3 and AR≥20, the dilution effect only showed up in the no-coated one. The tensile moduli were significantly improved due to the fiber aspect ratio, fiber content and good interphase at the same diameter ratio. The pull-out force increased with the diameter ratio, and keeping the diameter ratio the same, better interphase conditions also resulted in a higher pull-out force. Overall, it was found that the fiber aspect ratio, fiber diameter ratio, and interphase condition all have an important effect on tensile properties.