10-09-Lecture

10-09-Lecture - 1 Defo rm a tion and Dis locations Part A...

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Unformatted text preview: 1 Defo rm a tion and Dis locations Part A ENGR 213 DEFORMATION TYPES Materials respond to stress by deforming elastic deformation Recoverable Reversible e.g., spring plastic deformation (PD) permanent, irreversible Breaking and reforming bonds Movement of defects PD more effective use of matl PD Saves $$ - Example Plain carbon steel Bridge support 60,000 lbs load Elastic defo only ( < 40kpsi) Since = F/A, A = F/ A = 60klbs/40kpsi = 1.5 in 2 PD allowed ( < 80kpsi) A = 0.75 in 2 Material cost reduced by 50% Weight reduced by 50% Early Attempt at Strength Prediction assumed perfect crystals theoretical yield strength ( theor ) Assumed planes of atoms slide across each other Must break ALL bonds approximately G/2 experimental yield strength ( exp ) orders of magnitude smaller smaller due to more favorable mechanism of dislocation motion (only break FEW bonds) PD INVOLVES DISLOCATIONS edge dislocation shear stress perpendicular to dislocation line movement parallel to direction of stress screw dislocation shear stress parallel to dislocation line movement perpendicular to direction of stress mixed DISLOCATION MOVEMENT Unsatisfied bonds Higher energy Disloc moves along slip plane Wave-like movement See video puredun DISLOCATION CHARACTERISTICS - 1 dislocation density Length/vol Metals 10 3 /mm 2 to 10 10 /mm 2 Ceramics 10-1 /mm 2 to 10 4 /mm 2 lattice strains around dislocation...
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10-09-Lecture - 1 Defo rm a tion and Dis locations Part A...

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