LECTURE+1+COE-3001-A

Region 2 proporionality limit 3 elasic

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Unformatted text preview: th SI units, 19 1978 Stress- strain curves for Glass and Cast iron Results show that BRITTLE Materials are generally stronger in compression than in tension! 1/7/13 M. Mello/Georgia Tech Aerospace Reference: Crandall, Dahl, Lardner, IntroducIon to the Mechanics of Solids, McGraw- Hill, 2nd ediIon with SI units, 1978 20 Stress- strain curves for commercial poly- methyl methacrylate (PMMA) Rate dependent behavior (increased strain rate leads to higher flow stress Reference: Crandall, Dahl, Lardner, IntroducIon to the Mechanics of Solids, McGraw- Hill, 2nd ediIon with SI units, 1978 1/7/13 M. Mello/Georgia Tech Aerospace 21 ELASTICITY VS. PLASTICITY ²་  MATERIAL SPECIMEN LOADED UP TO POINT A AND THEN UNLOADED ²་  IF STRESS- STRAIN CURVE IS TRACED IN REVERSE THE MATERIAL IS ELASTIC ²་  POINT E REPRESENTS THE TRANSITION POINT (ELASTIC LIMIT) BEYOND WHICH THE CURVE IS NO LONGER RETRACED AS THE SPECIMEN IS UNLOADED. ²་  UNLOADING CURVE BC IS PARALLEL TO THE INITIAL PORTION OF THE LOADING CURVE (PARALLEL TO TANGENT TO THE LOADING CURVE AT POINT O) ²་  CHARACTERISITC BY WHICH MATERIAL UNDERGOES INELASTIC STRAINS BEYOND THE ELASTIC LIMIT IS KNOWN AS PLASTICITY 1/7/13 M. Mello/Georgia Tech Aerospace 22 CREEP AND STRESS RELAXATION ²་  LOAD IS HELD CONSTANT ²་  DISTANCE BETWEEN SUPPORTS HELD CONSTANT ²་  MATERIAL ELONGATES OVER TIME (t>t0) ²་  WIRE UNDER INITIAL TENSILE STRESS σ0 (at t=t0) ²་  USUSALLY SIGNIFICANT AT HIGH TEMPERATURES ²་  STRESS GRADUALLY DIMINISHES WITH PASSAGE OF TIME (t>t0) ²་  POLYMERS TYPICALLY EXHIBIT STRESS RELAXATION ²་  STEEL, CONCRETE, WOOD AND GLASS EXHIBIT CREEP EVEN AT ATMOSPHERIC TEMPERATURES 1/7/13 M. Mello/Georgia Tech Aerospace 23...
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