Solid mechanical contacts

Contact mechanics, in the classical sense, describes the behavior of solids in contact under the action of an external load. The first studies in the area of contact mechanics date back to the seminal publication "On the contact of elastic solids of Heinrich Hertz in 1882 [ 1 ]. The original Hertz theory was applied to frictionless non-adhering surfaces of perfectly elastic solids. Lee and Radok [2], Graham [3], and Yang [4] developed the theories of contact mechanics of viscoelastic solids. None of these treatments, however, accounted for the role of interfacial adhesive interactions. 

Contact mechanics deals with the deformation of solids in contact. Consider two elastic bodies, shown schematically in Fig. 3, of radii of curvature R[ and Rt, Young s moduli E and E2, and Poisson s ratios and V2. Define... 

Triboelectric Series. Prediction of triboelectric behavior in granular solids is hampered by difficult-to-control factors such as particle shape, prior mechanical contacts, material purity, and particle moisture content (which is usually related to airborne humidity). In the absence of any reliable predictive model for powder electrification, the practical requirements of industry necessitate an empirical approach (Taylor and Seeker,... 

Equipment for reactive absorption is of the same types as for physical absorption, towers of various kinds and stirred tanks. Packed or tray towers are the most common, but spray or bubble towers are used for their mechanical simplicity and when there is a likelihood of clogging. Thus S02 is scrubbed from air with a spray of lime slurry in a tower. Fluorine waste gases form solids on contact with water, so they are scrubbed by bubbling the gas through water in an empty tower. 

A traditional explanation of solid friction, which is mainly employed in engineering sciences, is based on plastic deformation.12 Typical surfaces are rough on microscopic length scales, as indicated in Figure 3. As a result, intimate mechanical contact between macroscopic solids occurs only at isolated points, typically at a small fraction of the apparent area of contact. 

Leaching of chemicals from complex materials or matrices is a complicated phenomenon in which many factors may influence the release of the specific organic compounds and inorganic ions. Important factors include major element chemistry, pH, redox, complexation, liquid to solid ratio, contact time, and biological activity. To describe fully the leaching of SWMs/COMs under field conditions, a battery of leaching tests was specifically designed to simulate various physical and chemical release mechanisms. 

Plastic deformation (strain). When two surfaces of ductile materials are placed in contact and the load exceeds the elastic limit of one of the two materials, plastic deformation or strain occurs. The plastic deformation of one surface when two surfaces are in solid-state contact can occur in the presence or absence of lubricants. In fact, in some instances, the presence of lubricants can increase the deformability of the solid surfaces by a mechanism such as the Rehbinder effect. Plastic deformation of the solid surface is, therefore, observed in the presence of lubricants. Plastic deformation is accommodated by the generation of slip lines for dislocation flow in the solid surface. Dislocations are line defects in the solid and they are site of higher energy state on the surface. Thus, they interact or react more rapidly with certain chemical agents than do the bulk surfaces (Buckley, 1981 Lunarska and Samatowicz, 2000). 

Film conductances are also often defined for the impedance to thermal conduction when two solid conductors are placed in mechanical contact. A significant contact resistance is often observed when, on a microscopic scale, heat transfer involves an air-gap between the materials. Under such conditions, phonon propagation must be replaced by the kinetic interaction amongst gaseous atoms and then back to phonon heat transfer in the next solid. Fibrous and foam insulation axe effective thermal insulators because of the numerous contact resistances involved in the transfer of heat. 

Goryacheva I. Contact Mechanics in Tribology, Solid Mechanics and Its Applications. Kluwer Academic Publishers 1998. 

The PEVD system used in this investigation is schematically shown in Eigure 36. A Na -p/ P -alumina disc, 16 mm in diameter and 5 mm in thickness, was used as the solid electrolyte with a working electrode on one side and both counter and reference electrodes on the other. To simplify data interpretation, the same electrode material, a Pt thick film, was used for all three electrodes, so the measured potential difference could be directly related to the average inner potential difference between the working and reference electrode. In order to make good electrical and mechanical contact, Pt meshes, with spot-welded Pt wires, were sintered on the Pt thick films as electron collectors and suppliers. 

Consider a 3-dimensional, corrugated solid placed on a smooth substrate as a simplified model for a mechanical contact, which is a subtle case (Section III. C.5). The macroscopic contact will then consist of individual junctions where asperities from the corrugated solid touch the substrate. A microscopic point of contact p then carries a normal load Ip, and a shear force fp will be exerted from the substrate to the asperity and vice versa. These random forces fp will try to deform both solids. For the sake of simplicity, let us only consider elastic deformations in the top solid. Asperities in intimate contact with the substrate will be subject to a competition between the (elastic) coupling to the top solid and the interaction with the substrate. If the elastic stress exceeds the local critical shear stress c,p of junction p, the contact will break and asperity p will find a new mechanical equilibrium position. In order for (5 to exceed Uep, the area A = tiL- over which the random forces accumulate must be sufficiently laige. The value of L where this condition is satisfied is called the elastic... 

Abstract In this chapter we discuss the results of theoretical and experimental studies of the structure and dynamics at solid-liquid interfaces employing the quartz crystal microbalance (QCM). Various models for the mechanical contact between the oscillating quartz crystal and the liquid are described, and theoretical predictions are compared with the experimental results. Special attention is paid to consideration of the influence of slippage and surface roughness on the QCM response at the solid-liquid interface. The main question, which we would like to answer in this chapter, is what information on... 

In order to analyze the influence of the different loading mechanisms on the QCM response one has to model a dependence of the mechanical impedance Zl or the complex resonance frequency shift on the chemical and physical properties of the contacting mediiun. Various models for the mechanical contact between the oscillating quartz crystal and the outer medium are discussed below. The QCM is now so widely and extensively used that, in the framework of this chapter, it is not possible to review all the available literature. Hence we limited ourselves here to a review of the experimental data and theoretical ideas concerning the studies of structure and interaction at solid-hquid interface. Furthermore, we did not present here studies on... 

Evidently if S > 0 then k>+1. Were S>0 so that > o-, + a, this would imply that the solid-gas interface would immediately coat itself with a layer of the liquid phase and replace the supposedly higher free energy per unit area of direct solid-gas contact, cr g, by the supposedly lower sum of the free energies per unit area of solid-liquid and liquid-gas contacts, cr i + cr, thereby lowering the free energy of the system. However, in thermodynamic equilibrium this cannot be realized (Gibbs 1906, Rowlinson Widom 1982). Therefore, for a spreading film in thermodynamic equilibrium k = +1 S = 0), and locally there is a state of mechanical equilibrium at the contact line between the three phases. 

When any two surfaces are brought into solid state contact and subsequently separated the nature of one or both surface frequently has changed as a result of the contact. It is even more likely to occur when mechanical forces are imposed on the contact. Polymeric materials are not different than other solids in this respect. The surface and near surface changes that have occurred in polymers may however be more difficult to characterize, in part because of the difficulty in identifying these materials with analytical tools. 

Among these models, one usually distinguishes rather arbitrarily between mechanical and specific adhesion, the latter being based on the various types of bonds (electrostatic, secondary, chemical) that can develop between two solids. Actually, each of these theories is valid to some extent, depending on the nature of the solids in contact and the conditions of formation of the bonded system. Therefore, they do not negate each other and their respective importance depends largely on the system chosen. 

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