8-9 material-selection-all

8-9 material-selection-all - ME250 Design and Manufacturing...

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Materials selection ME250 Design and Manufacturing I
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2 What? Why? Material properties Major classes of materials Material selection Summary Outline
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3 Given: Basic shape and function of parts Find: Best materials for manufacturing parts considering properties Weight Static load resistance Impact resistance Service life Temperature resistance Corrosion resistance Cost Compatible manufacturing processes etc. . What?
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4 Various materials and processes used in tractor What?
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5 Why?
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6 There are many materials with varying properties, cost, and compatible mfg processes Why?
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7 Material-property compatibility (more later) Why? material strength /weight forming corrosion resistance coloring recycle joining conductor? Metals ++ +++ + + +++ +++ either Ceramics +++ + +++ + + + no Glasses +++ +++ +++ +++ +++ + no Polymers + +++ + +++ ++ ++ no Composites +++ + depends + + + either
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8 Process-material compatibility (more later) Why?
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9 Shape-process compatibility (more later) Why?
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10 Similar shape can be made with different mfg processes (more later) Why? casting forging extrusion machining assembling
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11 Design, material selection, process selection are dependent to each other Typically proceed with iteration process Why? Design Needs Material selection Process selection Function
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12 Mechanical properties Stiffness [Force/area] Strength [Force/area] Fatigue [force/area that survives 10 6 cycles] Fracture toughness [Force/area * length^0.5] Material properties
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13 Physical properties Density [mass/volume] Thermal expansion [length/length/deg] Melting temperature [deg] Specific heat [energy/mass/deg] Thermal conductivity [energy/time/length/deg] Electrical resistance [voltage/current] Corrosion resistance Material properties
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14 Stiffness and strength (informal definitions) Materials deforms and eventually breaks under force Stress = force/area applied to material piece Strain = displacement/length under stress Yield strength = stress at which material stops to spring back Ultimate strength = stress at which material breaks Stiffness and strength are defined in stress-strain curve obtained by tensile test Material properties
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15 Material properties Tensile test machine
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16 Material properties Stress-strain relationship (tensile test) yield strength: S y Fracture strength: S f (ultimate) tensile strength: S ut
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17 Material properties Tensile test results Brittle material Ductile material
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18 Material properties Engineering stress A 0 = cross sectional area before loading Why “engineering”? True stress s = P / A > eng A is hard to measure during tests Typically given in MPa (Mega = 10 6 ) or psi 0 eng P A
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19 Material properties Engineering tensile strain l 0 = length before loading Unitless or given in % 0 0 t ll e l
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20 Material properties Young’s modules s = true (not engineering) stress during elastic deformation (will return to original length if unloaded) Measure of stiffness; aka modulus of elasticity Typically given in GPa (giga = 10 9 ) or psi t E e
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21 Material properties Yield strength P 0.002 = load at 0.2% permanent deformation (will not return to original length if unloaded) Obtained by drawing a line parallel to elastic loading line at e t = 0.002
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This note was uploaded on 03/21/2010 for the course MECHENG 250 taught by Professor Stevis during the Fall '09 term at University of Michigan.

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8-9 material-selection-all - ME250 Design and Manufacturing...

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