gm14_18notes2

gm14_18notes2 - Block 3 Materials/Structural Integrity...

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Unformatted text preview: Block 3 Materials/Structural Integrity Three purposes to this block of lectures: 1. To allow you to understand limits on stresses that a structure can carry: 1-D and multiaxial in order to design 2. To allow you to understand the origins (and limits) of the strength in the different classes of material identifying unobtainium, origins and limitations on models for strength 3. To allow you to select materials for specific functions, understand design process. Note: Failure occurs when a structure cannot meet its design requirements. Does not necessarily correspond to material failure e.g buckling. We will focus on material aspects here. Materials can behave in a variety of ways according to the loading. Examples: 1. a piece of metal wire (paper clip): Can twang it elastic deformation Bend it - leading to permanent deformation Cyclic loading rupture into two pieces (fatigue) 2. A piece of aluminum foil: Load it up and it necks down before fracture Put a notch in it and it tears 3. A balloon (pressurized aircraft fuselage) Stick a pin in it and it explodes catastrophic failure 4. e.g piece of solder wire with a weight hanging from it under a bright light - creep We will examine four types of failure in Unified; Yield (ductile failure), Fracture (brittle failure), Fatigue (cyclic loading) and creep (time/temperature dependent failure) necessarily brief. Read Ashby and Jones for more details. M15 Yielding and Plasticity Reading: Ashby and Jones ch. 8, 11 Characteristic stress-strain curve and material response for a ductile metal under uniaxial loading Unloading and reloading is elastic, permanent strain: u [ Load-extension curve for a bar of ductile metal (e.g. annealed copper) pulled in tension. F F F A F = 0 F = 0 F = 0 F = 0 A o I F u F F F Image adapted from: Ashby, M. F., and D. R. H. Jones. Engineering Materials 1 . Pergamon Press, 1980. Slope E Tension F u Reversible elastic deformation Permanent plastic deformation Image adapted from: Ashby, M. F., and D. R. H. Jones. Engineering Materials 1 . Pergamon Press, 1980. Most engineering alloys have an initial linear elastic region followed by yield and non-linear plasticity: Key features of stress-strain response: Yield stress, s y Tensile strength s ts Permanent strain after unloading Linear elastic unloading-reloading Work hardening (increasing yield stress with increasing plastic strain ). Image adapted from: Ashby, M. F., and D. R. H. Jones. Engineering Materials 1 . Pergamon Press, 1980. s n s TS s y s n f Final fracture Onset of necking s TS Tensile strength ( Plastic strain after fracture, 0.1% Strain 0.1% Proof stress 0.1% Yield strength ) Slope is Young's modulus, E Up to now we have not distinguished between compressive and tensile response of materials. Need to be careful with large plastic strains: Apparent difference between compression and tension due to increase in cross-sectional area in compression vs. reduction in tension. compression vs....
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This note was uploaded on 01/28/2012 for the course AERO 16.01 taught by Professor Markdrela during the Fall '05 term at MIT.

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gm14_18notes2 - Block 3 Materials/Structural Integrity...

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