me_350_-_lect_3_-_ch_3_4___5.20090831.4a9c15e71c7367.44862480

Me_350_-_lect_3_-_ch_3_4___5.20090831.4a9c15e71c7367.44862480

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Ch 3 - Mechanical Properties of Materials Hardness Effect of Temperature on Properties Fluid Properties Viscoelastic Behavior of Polymers
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Hardness Resistance to permanent indentation High hardness generally means more resistance to scratching and wear (desirable for tooling, etc.) Common test methods: Metals and general purpose - and Ceramics - Vickers & Knoop Polymers -
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Widely used for testing metals and nonmetals of low to medium hardness A hard ball is pressed into specimen surface with a load of 500, 1500, or 3000 kg Load divided into indentation area = Brinell Hardness Number (BHN), F = indentation load, kg; D b = diameter of ball, mm, D i = diameter of indentation, mm Brinell Hardness Test ) ( 2 2 2 i b b b D D D D F HB - - = π
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Effect of Temperature on Properties
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Recrystallization in Metals Most metals strain harden at room temperature according to the flow curve ( n > 0) But if heated to sufficiently high temperature (above the recrystallization temperature) and deformed, strain hardening does not occur Instead, new grains are formed that are free of strain The metal behaves as a material; that is, n = T r T m (temp in Kelvin)
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Recrystallization and Manufacturing Recrystallization can be exploited in manufacturing Heating a metal above its recrystallization temperature prior to deformation allows a greater amount of straining, and lower forces and power are required to perform the process Forming metals at temperatures above recrystallization temperature is called
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Fluid Properties and Manufacturing Fluids flow - They take the shape of the container that holds them Knowing fluidic properties are important Examples: Metals are cast in molten state Glass is formed in a heated and fluid state Polymers are almost always shaped as fluids
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Viscosity in Fluids Viscosity is the resistance to flow that is characteristic of a given fluid Viscosity is a measure of the internal friction when velocity gradients are present in the fluid
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Me_350_-_lect_3_-_ch_3_4___5.20090831.4a9c15e71c7367.44862480

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