LECTURE 30 3 11 MECHANICS OF MATERIALS F03 INSTRUCTOR Professor Christine Ortiz OFFICE 13 4022 PHONE 452 3084 WWW http web mit edu cortiz www PLASTICITY OF POLYMERS II SUMMARY LAST LECTURE I Molecular Origins of Plasticity in Metals edge and screw dislocations 2 D and 3 D movement of dislocations slip systems planes directions polycrystalline plastic deformation II Stress versus Strain Curves for Polymers in Uniaxial Tension structure of semicrystalline polymers amorphous cystalline regions chain folding lamellar spherulites macroscopically isotropic highly oriented polymer fiber VIII max Y II VII III IV V amorphous glassy polymer T Tg semicrystalline polymer T Tg VI elastomer T Tg I Lo f Semicrystalline polymers T Tg II linear elastic deformation homogeneous plastic deformation strain hardening III NECK forms and grows unstably i e inhomogeneous plastic deformation IV neck stabilizes V COLD DRAWING neck increases in length by extracting polymer from unnecked region of sample VI entire sample is drawn VII begin stretching of completely drawn sample strain hardening VIII fracture Polyethylene used in milk jugs toys pens ice trays Nylon polyamide MACROSCOPIC MECHANICAL BEHAVIOR Plastic Deformation of Semicrystalline Polymers crystalline lamellae amorphous region Stages and Mechanisms of Plastic Deformation 1 elongation of amorphous chains uncoiling along stress axis 2 rotation tilting of lamellae crystallites toward tensile axis 3 separation of crystallites into block segments partial melting 4 further stretching of crystallites and orientation of amorphous regions along tensile axis void formation stress whitening 5 2nd strain hardening regim recrystallization to a fiber like oriented structure high stiffness Comparison of Amorphous and Semicrystalline Polymers Some Applications of Amorphous Polymers Plastic Deformation of Amorphous Glassy Polymers or Networks T Tg Crazing in Amorphous Glassy Polymers T Tg Crazing in Amorphous Glassy Polymers T Tg polycarbonate craze bulk bulk craze bulk interface main fibril cross tie fibril CRAZE NANO STRUCTURE fibril oriented polymer chains entanglement Crazing in Amorphous Glassy Polymers T Tg Shear DZ s in Amorphous Glassy Polymers T Tg Macroscopic sample necking Microscopic structure CRAZING OR SHEAR DZ s CONSEQUENCES FOR IMPACT RESISTANCE bisphenol A polycarbonate crazing Plastic Deformation and Fracture of Amorphous Components Plastic Deformation and Fracture of Amorphous Components TOUGHENING OF AMORPHOUS POLYMERS TOUGHENING OF AMORPHOUS POLYMERS Typical uniaxial tensile stress strain behavior of polystyrene PS medium impact PS MIPS highimpact PS HIPS and poly acrylonitrile co styrenegraft butadiene ABS
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