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Failure of Materials

Understanding of dual nature of steels came after WWII. Of 4700 Liberty Ships and T2 Tankers:250 suffered severe brittle fractures (to break into parts)19 had backs broken, some ships breaking completely in two1200 suffered less severe cracks up to 10 feet in length. Liberty ships and T2 tankers

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Failure of Materials

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    1. Failure of Materials Mechanical properties

    2. Understanding of dual nature of steels came after WWII Of 4700 Liberty Ships and T2 Tankers: 250 suffered severe brittle fractures (to break into parts) 19 had backs broken, some ships breaking completely in two 1200 suffered less severe cracks up to 10 feet in length

    3. Liberty ships and T2 tankers of WWII First time such large ships were welded. Constructed in a hurry by inexperienced welders - Poor welds acted as stress raisers. Failed at relatively low temperatures, typically in North Atlantic during winter. Many failed in calm seas.

    4. Classes of Structural Materials Metals (paper clip) Ceramics and Glasses (peppermint) Polymers (Plastics & Rubbers) (gummy worm/rubber band) Composites (skittles) 4

    5. Principal ways to apply load Science speak Tension Compression Shear Torsion

    6. Tensile testing Show videos of stress strain tests: http://www.youtube.com/watch?v=I28m4FZzqro http://www.youtube.com/watch?v=YdqvGGFIbfc

    7. Load vs elongation

    8. Stress vs Strain

    9. Elastic / Plastic Change

    10. Plastic deformation Plastic deformation Permanent deformation The mechanism for plastic deformation is slip - planes of atoms sliding past one another. Lack of ductility - brittleness

    11. Different types of Strength STRENGTH - resistance to mechanical stress Yield strength –(YS) ability to support a load without significant plastic deformation Tensile strength – (TS) maximum load-carrying ability Breaking (or fracture) strength – (FS) stress at fracture

    12. Properties from tensile test DUCTILITY - permanent, plastic, deformation before fracture TOUGHNESS - ability to absorb energy Energy absorbed during tensile test = area under stress-strain curve

    13. Brittle, Tough, and Soft Materials

    14. FRACTURES - ductile or brittle 14

    15. Ductile vs Brittle

    16. As Temperature decreases things get more Brittle

    17. How Processing affects Properties Demonstrate effect of heat treating with bobby pin Heat and allow to cool Heat and quench

    18. Key Terms Stress Strain Elastic deformation Plastic deformation Fracture Strength Ductility Toughness

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