(Old)
Viscosity is a fundamental characteristic property of all liquids. When a liquid flows, it has an internal resistance to flow. Viscosity is a measure of this resistance to flow or shear. Viscosity can also be termed as a drag force and is a measure of the frictional properties of the liquid. Viscosity is a function of temperature and pressure. Although the viscosities of both liquids and gases change with temperature and pressure, they effect the viscosity in a different manner.
(Viscosity of liquids: theory, estimation, experiment and data, Dabir S. Viswanath, pg. 1)
(New and better)
Viscosity is the measure of the internal friction of a fluid. This friction becomes apparent when a layer of fluid is made to move in relation to another layer. The greater the friction, the greater the amount of force required to cause this movement, which is called shear.Shearing occurs whenever the fluid is physically moved or distributed, as in pouring, spreading, spraying, mixing, and so on. Highly viscous fluids, therefore, require more force to move than less viscous materials.
Consider two parallel planes of fluid of equal area separated by unit distance, and moving in the same direction at different velocities. The force required to maintain this difference in velocity is proportional to the difference in velocity though the liquid, called velocity gradient.
Velocity gradient is a measure of the change in velocity at which the intermediate layers move with respect to each other. It describes the shearing the liquid experience and is thus called shear rate. Its unit of measure is called the reciprocal second (s^-1). The force per unit are required to produce the shearing action is called shear stress.
The defining equation for viscosity is called Newton's equation, which is
Absolute (dynamic) viscosity is a measure of how resistive the flow of a fluid is between two layers of fluid in motion.
Viscosity affects the magnitude of energy loss in a flowing fluid.
High viscosity fluids require greater shearing forces than low-viscosity fluids at a given shear rate.
(Fluid Mechanics, Heat Transfer, and Mass Transfer: Chemical Engineering Practice By K. S. Raju, p 1.2)
2 - Tribology: (Engineering Mechanics, Material Science)
(Old)
Tribology has been defined as the science practice of an interacting surfaces in relative motion and the practices related there to.
Elements of Tribology:
(a) Friction
(b) Wear
(c) Lubrication
The subject "Tribology" generally deals with the technology lubrication, friction control and wear prevention of surface having relative motion under load. However, the surface interaction phenomena are so closely related to all above disciplines. This makes the study of tribology a multidisciplinary concept. Thus, tribology is truly an interdisciplinary science and is very useful for its practitioners.
(Tribology, H.G. Phakatkar, R.R. Ghorpade, pg. 1.1)
(New and better)
Tribology, which focuses on friction, wear and lubrication of interacting surfaces in relative motion is a new field of science defined in 1967 by a committee of the Organization for Economic Cooperation and Development. "Tribology" is derive from the Greek word "tribos" meaning rubbing or sliding. After an initial period of skepticism, as is inevitable for any newly introduced word or concept, the word tribology has gained gradual acceptance. As the word tribology is relatively new, its meaning is still unclear to the wider community and humorous comparisons with tribes or tribolites tend to persist as soon as the word tribology mentioned.
Wear is the major cause of material wastage and loss of mechanical performance and any reduction in wear can result in considerable savings, Friction is a principal cause of wear and energy dissipation. Considerable savings can be made by improved friction control. It is estimated that one-third of the world's energy resources in present use us needed to overcome friction in one form or another. Lubrication is an effective means of controlling wear and reducing friction. Tribology is a field of science which applies an operational analysis to problems of great economic significance such as reliability, maintenance and wear of technical equipment ranging from household appliances to spacecraft.
(Engineering Tribology By Gwidon W. Stachowiak, Andrew W. Batchelor, p 2)
3 - Wafer Testing: (Electronics)
(No older version)
Once the circuits have been built on the wafers, they are tested to determine the resultant yield of operational circuits (good or bad, speed, power consumption, etc.) and tagged for reference. The wafers are then diced (cut) into individual units and sorted of binned based on the prior testing creating what is referred to as the device or die blank. This process is generally referred to as binning (Fig 14.8). Observe it is the device (not the wafer) that is packaged and placed into video games, cell phones, laptops etc. A single wafer generates anywhere from 30 to 900 devices, and sometimes, the exact type of device is not determines until it is tested. that is, the testing not only determines whether the device is usable or not, but also determines its final part identity. Differences between devices from the same wafer typically occur as result of speed and power consumption. Wafer testing has a short cycle time (3-10 days) and involves a purely sequential process through test operations except for rework.
(Planning Production and Inventories in the Extended Enterprise, By Karl G. Kempf, p.327)
4 - ASHBY PLOT: (Materials)
(Old)
Of more importance to us in materials selection processes is the availability of data upon which to base our decisions. The nature of data needed in early stages differs greatly in its level of precision and bread from that needed in later stages. At the conceptual stage, the designer requires approximate property values for the widest possible range of materials. It is an this stage that the materials selection charts of Ashby are particularly useful. Such a plot assists the designer in selecting an appropriate class of material for consideration. The specific plot to use depends upon the design criteria; for example modulus versus density may be important for one application, whereas thermal conductivity versus tensile strength is important for another.
(An introduction to materials engineering and science for chemical and material engineers. Brian S. Mitchell)
(New and better)
Ashby plots provide a convenient method of comparing the properties of materials. these plots were originally devised to establish a materials performance index based on specific design objectives. For example, a meaningful material performance index (M) of a stiff beam for a lightweight load bearing application would be M= EIp, which is defined as the specific modulus. Then all materials represented on an Ashby plot of elastic modulus E, versus density p, which fall on a line of slope equal to unity, would have an equal value of M, and therefore equally satisfy the design criterion of equivalent performance.
(Principles Of Brazing By David M. Jacobson, Giles Humpston, p 222-223)
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