Material properties friction

In order to develop the proper dow pattern, knowledge of a material s dow properties is essential. Standard test equipment and procedures for evaluating sohds dow properties are available (6). Direct shear tests, mn to measure a material s friction and cohesive properties, allow determination of hopper wall angles for mass dow and the opening size required to prevent arching. Other devices available to evaluate sohds dowabiUty include biaxial and rotary shear testers. 

The main characteristic properties of asbestos fibers that can be exploited in industrial appHcations (8) are their thermal, electrical, and sound insulation nonflammabiUty matrix reinforcement (cement, plastic, and resins) adsorption capacity (filtration, Hquid sterilization) wear and friction properties (friction materials) and chemical inertia (except in acids). These properties have led to several main classes of industrial products or appHcations... 

For many centuries the application of materials for low friction and wear in sliding and rolling contacts primarily involved wood, stone, leather, iron, and copper. Almost all engineering materials have since been employed at one time or another in the continuing search for the best bearing material. Final selection is commonly a judgment based on the most essential material properties, ease of appHcation, and cost. 

The friction and wear of plastics are extremely complex subjects which depend markedly on the nature of the application and the properties of the material. The frictional properties of plastics differ considerably from those of metals. Even reinforced plastics have modulus values which are much lower than metals. Hence metal/thermoplastic friction is characterised by adhesion and deformation which results in frictional forces that are not proportional to load but rather to speed. Table 1.7 gives some typical coefficients of friction for plastics. 

In the transition zone, EHL is still important, but as more water is removed, EHL at the microscale (MEHL) becomes more important, and when the water layer is reduced to molecular levels, another mechanism, BL takes over. Since BL is the main mechanism by which friction is generated in the overall skidding process, any material properties which increase the proportion of BL in the transition zone relative to EHL, i.e., accelerate the transition from EHL to BL, will have an impact on overall skid performance. As discussed above, modulus is an important factor in determining the rate of water removal in EHL. Eor MEHL, it is the modulus on the microscale at the worn surface of the tread that is critical. There is evidence that after a certain amount of normal wear, a significant part of the surface of silica-filled compounds is bare silica, whereas in black-filled compounds, the surface is fully covered by rubber.The difference in modulus between rubber and silica is very large, so even if only part of the worn surface is bare silica, it would make a significant impact on the... 

A 2D soft-sphere approach was first applied to gas-fluidized beds by Tsuji et al. (1993), where the linear spring-dashpot model—similar to the one presented by Cundall and Strack (1979) was employed. Xu and Yu (1997) independently developed a 2D model of a gas-fluidized bed. However in their simulations, a collision detection algorithm that is normally found in hard-sphere simulations was used to determine the first instant of contact precisely. Based on the model developed by Tsuji et al. (1993), Iwadate and Horio (1998) incorporated van der Waals forces to simulate fluidization of cohesive particles. Kafui et al. (2002) developed a DPM based on the theory of contact mechanics, thereby enabling the collision of the particles to be directly specified in terms of material properties such as friction, elasticity, elasto-plasticity, and auto-adhesion. 

The contact force between two particles is now determined by only five parameters normal and tangential spring stiffness kn and kt, the coefficient of normal and tangential restitution e and et, and the friction coefficient /if. In principle, kn and k, are related to the Young modulus and Poisson ratio of the solid material however, in practice their value must be chosen much smaller, otherwise the time step of the integration needs to become unpractically small. The values for kn and k, are thus mainly determined by computational efficiency and not by the material properties. More on this point is given in the Section III.B.7 on efficiency issues. So, finally we are left with three collision parameters e, et, and which are typical for the type of particle to be modeled. 

In spite of the apparent sensitivity to the material properties, the direct assignment of the phase contrast to variation in the chemical composition or a specific property of the surface is hardly possible. Considerable difficulties for theoretical examination of the tapping mode result from several factors (i) the abrupt transition from an attractive force regime to strong repulsion which acts for a short moment of the oscillation period, (ii) localisation of the tip-sample interaction in a nanoscopic contact area, (iii) the non-linear variation of both attractive forces and mechanical compliance in the repulsive regime, and (iv) the interdependence of the material properties (viscoelasticity, adhesion, friction) and scanning parameters (amplitude, frequency, cantilever position). The interpretation of the phase and amplitude images becomes especially intricate for viscoelastic polymers. 

Molding compounds were among the earliest applications for solid phenolic resins. The molded articles exhibit high-temperature, flame, and chemical resistance retention of modulus at elevated temperatures and hardness. Systems with good electrical properties can be formulated at low costs. Adhesive materials and friction materials (brakes) are made from molding compounds. 

The crucial point is to simulate the previously identified basic stress modes impact and friction under well defined stress conditions. This way, material properties can be related to attrition caused by these stress modes and the respective attrition mechanisms in effect. For this purpose it was chosen to perform single particle experiments in simple experimental setups to realize the defined stress conditions. Details on these setups are given in the next section. 

The results presented show that three levels have to be distinguished when investigating attrition processes. The first one is the stress mode as derived from the process function which is essential to know if the attrition process is to be simulated successfully in a simple experimental setup. The second point is the material reaction to this stress mode, i.e. the material function which varies depending on material properties like storage and loss modulus as measured by DMA. Finally, the microscopic attrition mechanisms (see [18] for impact and [19,20] for sliding friction) describing the formation of attrition on a microscopic scale constitute the bottom level. 

During a collision, the colliding solids undergo both elastic and inelastic (or plastic) deformations. These deformations are caused by the changes of stresses and strains, which depend on the material properties of the solids and the applied external forces. Theories on the elastic deformations of two elastic bodies in contact are introduced in the literature utilizing Hertzian theory for frictionless contact and Mindlin s approach for frictional contact. As for inelastic deformations, few theories have been developed and the available ones are usually based on elastic contact theories. Hence, an introduction to the theories on elastic contact of solids is essential. 

Coefficient of friction A material property of the contacting surfaces and/or of the contaminants and other films at their interface. 

Equation 4.1-4 suggests that the static coefficient of friction is a material property characteristic of the pair of solid surfaces and, specifically, of the softer solid... 

The perception that friction and wear of materials is not a material property but a system property of the elements involved in the process in conjunction with the combined stresses is widely accepted among system developers. 

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