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Last class More mechanical testing Fracture toughness - pre-cracked specimen

Last class More mechanical testing Fracture toughness - pre-cracked specimen Creep - constant load, elevated temperature Fatigue - constant load, S/N curves, endurance limit. Fracture surfaces Brittle vs. ductile Crack origins, chevron markings Beachmarks. Today

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Last class More mechanical testing Fracture toughness - pre-cracked specimen

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  1. Last class • More mechanical testing • Fracture toughness - pre-cracked specimen • Creep - constant load, elevated temperature • Fatigue - constant load, S/N curves, endurance limit Fracture surfaces Brittle vs. ductile Crack origins, chevron markings Beachmarks

  2. Today • Atom arrangements in metals • Dislocations

  3. Crystal structures of materials - • where are the atoms? • Polymers - mostly not crystalline • structure extremely complicated • Ceramic materials - complicated • many different types of atom arrangements Metals - really simple - most have one of three types

  4. Crystal structures of pure metals Most pure metals exhibit one of three types 1. cubic close packing (ccp or A1) 2. hexagonal close packing (hcp or A3) 3. A2 (almost universally referred to by the confusing notation 'bcc')

  5. Crystal structures of pure metals cubic close-packed (ccp) close-packed plane of atoms

  6. C B Crystal structures of pure metals cubic close-packed (ccp) ABCABC layer sequence close-packed plane of atoms A C A A B

  7. close-packed plane of atoms hexagon Crystal structures of pure metals hexagonal close-packed (hcp) Zn, Cd, Co, Ti, Zr……

  8. B Crystal structures of pure metals hexagonal close-packed (hcp) ABAB layer sequence close-packed plane of atoms B A B A A

  9. Fe, V, Cr, Mo, W, Ta…… some empty space Crystal structures of pure metals A2 structure - so-called "bcc" metal structure almost close-packed atom planes

  10. Mechanism of plastic deformation connected with existence of defects in atom arrangement known as dislocations Dislocations And now…the rest of the story (on plastic deformation)! Ideally, atom arrangement within a crystal repeats perfectly Mistakes (defects) in repetition occur in reality

  11. Think of edge dislocation as extra plane of atoms partially inserted into crystal Dislocations Situation is this: strength of a material w/ no dislocations is • 20-100 times greater than ordinary materials

  12. Dislocation line 0.00001 cm Dislocations These things are real! We can see them! (in an electron microscope)

  13. Dislocations Move under a shear stress

  14. Dislocations Move under a shear stress

  15. Dislocations Move under a shear stress

  16. Dislocations Move under a shear stress

  17. Dislocations Move under a shear stress

  18. Dislocations Move under a shear stress

  19. Dislocations move along a slip plane Dislocations Permanent change in shape results Dislocation has disappeared

  20. Dislocations Watch a real one move!

  21. Dislocations If hundreds of thousands of dislocations move through material, microscopic steps produced in the surface as below

  22. But it cannot be deformed plastically Dislocations - Initial overview Material w/ NO dislocations is very strong Dislocations weaken a material But dislocations make plastic deformation possible

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