Interface Frictional Properties

44 and 4.45 show the increase in the debond length, f, and displacement, as a result of the reduction of p (from Po = 0.22 to p = 0.07) under cyclic loading. It is interesting to note that both I and 5 remain constant until the coefficient of friction, p, is reduced to a critical value p. (= 0.144 and 0.166, respectively for fiber pull-out and fiber push-out). The implication is that the debond crack does not grow 

Variation of debond length, i, as a function of coefficient of friction, p. After Zhou et al. (1993). 

From comparison of the plots between the two loading geometry in Figs. 4.44 and 4.45, it is generally noted that a substantially larger N is required for debond crack 

Aboudi, J. (1983). The effective moduli of short fiber composites. Int. J. Solids Struct. 19, 693-707. 

Ananth, C.R. and Chandra, N. (1995). Numerical modelling of fiber push-out test in metallic and intermetallic matrix composites mechanics of failure process. J. Composite Mater. 29, 1488-1514. 

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WUliams, N. D. and HoulUian, M. E (1987), Evaluation of interface friction properties between geosynthetics and soils. Proceedings of the Geosynthetics 87 Conference, Industrial Fabrics Assodation International, St Paul, Minnesota, pp. 616-627. 

The frictional properties of TPs, specifically the reinforced and filled types, vary in a way that is unique from metals. In contrast to metals, even the highly reinforced plastics have low modulus values and thus do not behave according to the classic laws of friction. Metal-to-thermoplastic friction is characterized by adhesion and deformation resulting in frictional forces that are not proportional to load, because friction decreases as load increases, but are proportional to speed. The wear rate is generally defined as the volumetric loss of material over a given unit of time. Several mechanisms operate simultaneously to remove material from the wear interface. However, the primary mechanism is adhesive wear, which is characterized by having fine particles of plastic removed from the surface. 

Hydropolymer gel has been considered as a possible candidate for an artificial articular cartilage in artificial joints because it exhibits very low friction when it is in contact with a solid. The origin of such low friction is considered to be associated with the water absorbed in the gel [83-86], some of which is squeezed out from the gel under the load and serves as a lubricant layer between the gel and solid surface, resulting in hydrodynamic lubrication [87, 88]. Although the structural information about the interfacial water is important to understand the role of water for the low frictional properties of hydrogel in contact with a solid and the molecular structure of lubricants other than water at solid/solid interfaces have been investigated theoretically [89-91] and experimentally [92-98], no experimental investigations on water structure at gel/solid interfaces have been carried out due to the lack of an effective experimental technique. 

The analytical solutions derived in Sections 4.3 and 4.4 for the stress distributions in the monotonic fiber pull-out and fiber push-out loadings are further extended to cyclic loading (Zhou et al., 1993) and the progressive damage processes of the interface are characterized. It is assumed that the cyclic fatigue of uniform stress amplitude causes the frictional properties at the debonded interface to degrade... 

Nowadays attention is turned also to the supermolecular level, that is, to the morphologic aspects, to the nature of interfaces, to the formation of new phases, or of particular aggregates (liquid crystals, gels, etc.). Interest has also been directed to the study of chain mobility for its influence on frictional properties of polymers. In recent years there have been many successful approaches to a microscopic theory (in contrast to a phenomenological approach) of the physi-comechanical behavior of macromolecular materials. 

Differences in the frictional properties of most plastics can be explained in terms of the ratio of shear strenghth to hardness. Shooter and Tabor observed that the coefficients of friction for polytetrafluoroethylene are 2—3 times lower than anticipated by this calculation. It is believed that this discrepancy is caused by the inherently low cohesive forces between adjacent polymer chains and is responsible for the absence of stick-slip. The large fluorine atoms effectively screen the large carbon-fluorine dipole, reducing molecular cohesion so that the shear force at the interface is low. The shear strength of the bulk material is higher because of interlocking molecular chains. 

The heat evolved by friction at the interface of two rubbing bodies passes by conduction into the material of both. The resulting interfacial temperature at equilibrium is a function of specific parameters such as the coefficient of friction, the loading force, the velocity of sliding, the dimensions of the interface, the properties of the materials, etc. The classical theory of heat conduction has been applied to the interfacial temperature problem with good to moderate success. The calculations are often so intricate that the physical picture is lost in the complexity therefore our introductory consideration of interfacial temperature will be the simplified descriptive approach immediately following. 

The interfacial rheology of polymers at a solid interface dominates the friction properties and strongly depends on the degree of slip of polymers on a surface. 

As mentioned in the Introduction, the recent interest for reducing friction — promoting slip — at solid-liquid interfaces was initially motivated by the ever growing field of microfluidic devices where the role of channel surfaces is considerably enhanced compared with the macroscale. It is in this particular context that super-hydrophobic surfaces have been introduced, and we have presented in Section 2 a review of the different theoretical and experimental works showing their remarkable frictional properties in laminar (low Reynolds numbers) flows. 

The reinforcement spacing of Model M-1, P-1 and I-l were all 6 cm. They were reinforced with medical bandage gauze, plastic window screen and iron window screen, respectively. They were all in an open netlike pattern, with similar frictional properties of interface with the soil. The tensile moduh of the three materials were different. They can be used to compare the influence of reinforcement modulus on the slope. The reinforcement spacing of Model M-2 and P-2 were both 3 cm, which can also be used to compare the influence of reinforcement modulus. 

The coefficient of friction of the bulk material is another very important property. One can distinguish both internal and external coefficient of friction. The internal coefficient of friction is a measure of the resistance present when one layer of particles slides over another layer of particles of the same material. The external coefH-cient of friction is a measure of the resistance present at an interface between the polymeric particles and a wall of a different material of construction. The coefficient of friction is simply the ratio of the shear stress at the interface to the normal stress at the interface. Friction itself is the tangential resistance offered to the sliding of one solid over another. 

Influence of the Polymer Architecture. The time dependence of the friction reduction with PLL- -PEG adsorption suggests that the frictional properties of these interfaces are closely related to the areal density of the PEG chains immobilized near the surface. The full effect of this areal density is revealed through an analysis of the coupled contribution of the PEG chain length and grafting ratio to the... 

The primary reason to use lubricants is to reduce friction and wear between two interacting surfaces. Hydrocarbon oils have the proper friction properties to meet these requirements but their low viscosity may cause them to be forced out of the contact region between interfaces. Powders of low MW PIEE may be added to liquid lubricants to provide reserve lubrication in case the liquid phase is forced out. Low-MW PTFE that is used this way is sometimes called an extreme pressure or boundary additive. The type of PTFE used in lubricants may be from either suspension or dispersion polymerization but the small particle size of dispersion-type PTFE is usually preferred to help maintain dispersion in the oil. Many Journal articles and patents have been published that report the performance of lubricating oils with and without the addition of PTFE. For example, Rico et al. [44] provided the results of an extreme pressure wear study of steel balls (Shell four-ball test) with several mineral oils containing four different percentages (1-10%) of PTFE. 

The filler and the matrix form a whole by the interface. Stress can be transferred evenly through the interface, and the area shows excellent performance. In addition, as a result of the existence of the interface, composite materials produce discontinuous physical properties, such as interface friction and electrical resistance, heat resistance, dimensional stability, impact resistance, and so forth, and can prevent the expansion of the crack and reduce stress concentration. Therefore,... 

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