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TRIBOLOGY TEXTBOOK PDF

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WEAR ELSEVIER Wear () Book Review Engineering Tribology by custom-speeches.comms Dr Williams has written a comprehensive book dealing with the eng. FRICTION, WEAR, LUBRICATION A TEXTBOOK IN TRIBOLOGY K.C Ludema Professor of Mechanical Engineering The University of Michigan Ann Arbor. Industrial Significance of Tribology. 3. Origins and Significance of Micro/ Nanotribology. 4. Organization of the Book. 6. References.


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Fundamentals of Physics Textbook Friction, Lubrication, and Wear Technology. pdf Friction, wear, lubrication: a textbook in tribology / by K.C Ludema. Fundamentals of Tribology: • Surfaces in Contact. • Friction. • Lubrication. • Wear. • Concluding Words. • Upcoming Topics in Series. 1/29/ 2. LEC #, TOPICS. 1, Introduction to Tribology (PDF - MB). 2, Introduction to Tribology (cont.) (PDF - MB). 3, Chemical and Physical State of the Solid.

Hydrodynamic Instability. Externally Pressurized Oil Bearings. Gaslubricated Bearings. Elastohydrodynamic Lubrication. Ball Bearings. Surface Roughness Effect.

Friction of Metals. Wear of Metals. The book discusses the basic principles and equations governing Hydrodynamic, Hydrostatic Elastohydrodynamic and Gas Lubrication. The author has made an effort to explain the theory and present an exposition of the fundamentals of fluid film bearings rolling element bearings friction and wear of metals.

Wear of aircraft engines and the barrels of large guns are obvious examples. A less obvious problem is the noise emitted by worn bearings and gears in ships, which is easily detectable by enemy listening equipment. Finally, it is a matter of history that the development of high-speed cutting tool steel in the s aided considerably in our winning World War II.

Friction and wear can affect quality of life. Tooth fillings, artificial teeth, artificial skeletal joints, and artificial heart valves improve the quality of life when natural parts wear out. Wear causes accidents. Traffic accidents are sometimes caused by worn brakes or other worn parts. Worn electrical wiring and switches expose people to electrical shock; worn cables snap; and worn drill bits cause excesses which often result in injury.

The early cars polluted the streets with oil and grease that leaked though the seals. The engine burned a quart of oil in less than miles when in good condition and was sometimes not serviced until an embarrassing cloud of smoke followed the car.

Fortunately there were not many of them!

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Private garages of that day had dirt floors, and between the wheel tracks the floor was built up several inches by dirt soaked with leaking oil and grease. We have come a long way. Progress since the Maxwell has come about through efforts in many disciplines: 1. Lubricants are more uniform in viscosity, with harmful chemical constituents removed and beneficial ones added 2. Fuels are now carefully formulated to prevent pre-ignition, clogging of orifices in the fuel system, and excessive evaporation 3.

Bearing materials can better withstand momentary loss of lubricant and overload 4. Manufacturing tolerances are much better controlled to produce much more uniform products, with good surface finish 5. The processing of all materials has improved to produce homogeneous products and a wider range of materials, metals, polymers, and ceramics 6. Shaft seals have improved considerably Progress has been made on all fronts, but not simultaneously.

The consumer product industry tends to respond primarily to the urgent problems of the day, leaving others to arise as they will. However, even when problems in tribology arise they are more often seen as vexations rather than challenges. The effects include strains, heating, and alteration of chemical reactivity, each of which can act separately but each also alters the rate of change of the others during continued contact between two bodies. The focus in this chapter is upon the strains, but expressed mostly in terms of the stresses that produce the strains.

Those stresses, when of sufficient magnitude and when imposed often enough upon small regions of a solid surface, will cause fracture and eventual loss of material. It might be expected therefore that equations and models for wear rate should include variables that relate to imposed stress and variables that relate to the resistance of the materials to the imposed stress. Though many wear equations have been published which incorporate material properties, none is widely applicable.

The reason is that: The stress states in tests for each of the material properties are very different from each other, and different again from the tribological stress states. In these tests the materials behave elastically when small stresses are applied.

Materials do not actually behave in a linear manner in the elastic range, but linearly enough to base a vast superstructure of elastic deflection equations on that assumption. Deviations from linearity produce a hysteresis, damping loss, or energy loss loop in the stress—strain data such that a few percent of the input energy is lost in each cycle of strain.

The most obvious manifestation of this energy loss is heating of the strained material, but also with each cycle of strain some damage is occurring within the material on an atomic scale.

As load and stress are increased, the elastic range may end in one of two ways, either by immediate fracture or by various amounts of plastic flow before fracture.

In the first case, the material is considered to be brittle, although careful observation shows that no material is perfectly brittle. Figure 2. When plastic deformation begins, the shape of the stress—strain curve changes considerably.

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Su is also referred to as the tensile strength TS of the material, but should rather be called the maximum load-carrying capacity of the tensile specimen. The representation of tensile data as given in Figure 2.

The major problem with the representation of Figure 2. For most metals, the yield point may be as low as two thirds the intersection, whereas for steel it is often above. Tensile data are instructive and among the easiest material property data to obtain with reasonable accuracy. However, few materials are used in a state of pure uni-axial tension.

Usually, materials have multiple stresses on them, both normal stresses and shear stresses. These stresses are represented in the three orthogonal coordinate directions as, x, y, and z, or 1, 2, and 3. It is useful to know what combination of three-dimensional stresses, normal and shear stresses, cause yielding or brittle failure. These criteria are not very realistic. From these data Griffith developed a fracture envelope, called a fracture criterion, for brittle material with two-dimensional normal applied stresses, which may be plotted as shown in Figure 2.

Note that the signs on the shear stresses have no influence upon the results. The above two criteria, the Griffith criterion and the von Mises criterion, refer to different end results. The Griffith criterion states that brittle fracture results from tensile normal stresses predominantly, although compressive stresses impose shear stresses which also produce brittle failure.

It is instructive to show the relationship between imposed stresses and the two modes of departure from elasticity, i. This begins with an exercise in transformation of axes of stress.

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Otto Mohr developed a way to visualize the stresses on all possible planes i. Two states of stress will now be shown on the Mohr axes, namely for a tensile test and for a torsion test. In Figure 2. The state of stress on planes chosen at any desired angle relative to the applied load in a tensile test constitutes a circle on the Mohr axes as shown in Figure 2.

Orientation of test specimen with respect to a coordinate axis and positive direction of applied load torque. The stress state on any other plane can as easily be determined. For example, the stress state on a plane oriented The stress state upon an element in the surface of a bar in torsion is shown in Figure 2. A set of balancing shear stresses comprises a plus shear stress and a minus shear stress. These stresses are shown on Mohr axes in Figure 2. The directions of these stresses relative to the applied shear stresses are also shown in Figure 2.

See Problem Set questions 2 a, b, and c.

These two are plastic ductile shearing and tensile brittle failure, two very different and independent properties of solid matter and worthy of some emphasis. These properties are not related, and are not connected with the common assumption that the shear strength of a material is half the tensile strength. We will use a simple, straight-line representation of these properties, bypassing other and perhaps more accurate concepts under discussion in mechanics research.

Our first example will be cast iron, which is generally taken to be a brittle material when tensile stresses are applied.

The critical point is reached when the circle touches the brittle fracture strength line, and the material fails in a brittle manner. This is observed in practice, and there can be few explanations other than that the shear strength of the cast iron is greater than half the brittle fracture strength, i.

In this case the first critical point occurs when the third circle touches the initial shear strength line. The material plastically deforms as is observed in practice. With further strain the material work hardens, which may be shown by an increasing shear limit. Finally, the circle expands to touch the cleavage or brittle fracture strength of the material, and the bar fractures.

Cast iron is thus seen to be a fairly ductile material in torsion. This same type of exercise may be carried out with two other classes of material, namely, ductile metals and common ceramic materials. Ductile metals partly by definition , always plastically deform before they fracture in either tension or torsion.

See Problem Set questions 2 d and e. In the more practical world the stress state on an element cube includes some shear stresses. If one face of a cube can be found with relatively little shear stress imposed, this shear stress can be taken as zero and a Mohr circle can be drawn.

If all three coordinate directions have significant shear stresses imposed, it is necessary to use a cubic equation for the general state of stress at a point to solve the problem: these equations can be found in textbooks on solid mechanics.

If one face of a cube e. The Mohr circle can be constructed by looking into the z face first to visualize the stresses upon the other faces.

The other stresses can be plotted as shown in Figure 2. Again, the stresses on all possible planes perpendicular to the z face are shown by rotation around the origin of the circle. Circle 2 is drawn in the same way. Recall that in Figures 2.

The inner cube in Figure 2. The von Mises equation, Equation 1, suggests otherwise. The Mohr circle embodies the Tresca yield criterion, incidentally. Experiments in yield criteria often show data lying between the Tresca and von Mises yield criteria. This behavior is sometimes modeled by arrays of springs and dashpots, though no one has ever seen them in real polymers. Two simple tests show visco-elastic behavior, and a particular mechanical model is usually associated with each test, as shown in Figure 2.

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The decrease in E of polymers over time of loading is very different from the behavior of metals. That would be equivalent to a stress relaxation test, though very little relaxation occurs in the metal in a short time a few hours. For polymers which relax with time, one must choose a time after quick loading and stopping, at which the measurements will be taken. Typically these times are 10 seconds or 30 seconds. The second values for E for four polymers are given in Table 2.

Table 2. Both are strain rate frequency, f, for a constant amplitude and temperature T dependent, as shown in Figure 2. The location of the curves on the temperature axis varies with strain rate, and vice versa as shown in Figure 2.

The temperature—strain rate interdependence, i. Adapted from Ferry, J. It is most accurately determined while measuring the coefficient of thermal expansion upon heating and cooling very slowly.

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The value of the coefficient of thermal expansion is greater above Tg than below. Polymers do not become transparent at Tg; rather they become brittle like glassy solids, which have short range order. Crystalline solids have long range order; whereas super-cooled liquids have no order, i. An approximate value of Tg may also be marked on curves of damping loss energy loss during strain cycling versus temperature. The damping loss peaks are caused by morphologic transitions in the polymer.

For example, PVC shows three peaks over a range of temperature. This transition is thought to be the point at which the free volume within the polymer becomes greater than 2.

These take place at lower temperature and therefore at smaller free volume since the side chains require less free volume to move. The glass transition temperature for polymers roughly correlates with the melting point of the crystalline phase of the polymer. The laboratory data for rubber have their counterpart in practice.

For a rubber sphere the coefficient of restitution was found to vary with temperature, as shown in Figure 2. The sphere is a golf ball. An example of visco-elastic transforms of friction data by the WLF equation can be shown with friction data from Grosch see Chapter 6 on polymer friction.

See Problem Set question 2 f. In the polymers this behavior is attributed to dashpot-like behavior. In metals the reason is related to the motion of dislocations even at very low strains, i.

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Thus there is some energy lost with each cycle of straining. Some typical numbers for materials are given in Table 2. Hardness indenters should be at least three times harder than the surfaces being indented in order to retain the shape of the indenter.

Indenters for the harder materials are made of diamonds of various configurations, such as cones, pyramids, and other sharp shapes. Indenters for softer materials are often hardened steel spheres. Loads are applied to the indenters such that there is considerable plastic strain in ductile metals and significant amounts of plastic strain in ceramic materials.

The size of indenter and load applied to an indenter are adjusted to achieve a compromise between measuring properties in small homogeneous regions e. For ceramic materials and metals, most hardness tests are static tests, though tests have also been developed to measure hardness at high strain rates referred to as dynamic hardness. Hardness testing of these materials is done with a spring-loaded indenter the Shore systems, for example.

Note that each system offers several combinations of indenter shapes and applied loads. This value changes with time so that it is necessary to report the time after first contact at which a hardness reading is taken. Typical times are 10 seconds, 30 seconds, etc.

Automobile tire rubbers have hardness of about 68 Shore D 10 s. Notice the stress states applied in a hardness test. With the sphere the substrate is mostly in compression, but the surface layer of the flat test specimen is stretched and has tension in it. Thus one sees ring cracks around circular indentations in brittle material.

The substrate of that brittle material, however, usually plastically deforms, often more than would be expected in brittle materials.

In the case of the prismatic shape indenters, the faces of the indenters push materials apart as the indenter penetrates. Brittle material will crack at the apex of the polygonal indentation. This crack length is taken by some to indicate the brittleness, i.

See the section on Fracture Toughness later in this chapter.It might be expected therefore that equations and models for wear rate should include variables that relate to imposed stress and variables that relate to the resistance of the materials to the imposed stress.

Petersburg formerly Leningrad , said friction was due to hypothetical surface ratchets. Crystalline solids have long range order; whereas super-cooled liquids have no order, i.

The pressure on those points is therefore very high. Now since: Thus, the local resistance to sliding varies and some asperities will slip when low values of friction force are applied. Materials do not actually behave in a linear manner in the elastic range, but linearly enough to base a vast superstructure of elastic deflection equations on that assumption. It is not a science by itself although research is done in several different sciences to understand the fundamental aspects of tribology.

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