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PE335: Polymer Science and Engineering I Mechanical Behavior of Polymers

PE335: Polymer Science and Engineering I Mechanical Behavior of Polymers. Lesson Outline. States of polymers (MW/ Crystallinity Map) Mechanical Characterization Overview Failure Analysis Tensile Testing Deformation Mechanisms Viscoelasticity Impact Strength.

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PE335: Polymer Science and Engineering I Mechanical Behavior of Polymers

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  1. PE335: Polymer Science and Engineering IMechanical Behavior of Polymers

  2. Lesson Outline • States of polymers (MW/ Crystallinity Map) • Mechanical Characterization Overview • Failure Analysis • Tensile Testing • Deformation Mechanisms • Viscoelasticity • Impact Strength

  3. Molecular Weight- Crystallinity Map of Polymers

  4. Mechanical Characterization Overview • Stress-strain curve (tensile) • Impact resistance • Notched • Un-notched • Product • Creep • Creep Rapture • Creep Deformation • Creep Modulus • Fatigue • Crack propagation resistance • Fracture Toughness • Growth • Dynamic Mechanical Behavior • Hardness • Abrasion Resistance • Temperature • Environment • Fatigue • Constant Load

  5. In-service Failure Analysis Material Selection • Mechanical • Chemical • (environmental) • Thermal Manufacturing Defect Design Defect

  6. Mechanical Stress and Strain

  7. STRESS • Tensile stress, s: • Shear stress, t: Stress unit: N/m2

  8. STRAIN • Tensile strain: • Lateral strain: • Shear strain: Strain is always dimensionless.

  9. Definition of normal stress(axial stress)

  10. Definition of normal strain

  11. Stress and strain components

  12. Tensile Testing The test to measure tensile properties is regulated by international standards such as ISO 527 and ASTM D638 which do not vary significantly and either test will provide good results early in the material selection process.

  13. Testing Machine

  14. STRESS-STRAIN TESTING • Typical tensile specimen • Typical tensile test machine gauge (portion of sample with = length reduced cross section)

  15. Elastic  Plastic (Yield)  Fracture

  16. Flexural Testing Standard specimen thickness is 4 mm (0.16 in), width is 13 mm (0.5 in) and length is 20% longer than the support span.

  17. Elasticity

  18. Hook’s Law • An often quoted material property is the tensile (or Young’s) modulus, E • Hook’s Law: E (Pa) • Copper 1.2*1011 • Polystyrene 3.0*109 • Soft Rubber 2.0*106 MPa GPa

  19. When an element is stretched in one direction, it tends to get thinner in the other two directions. The change in longitudinal and lateral strains are opposite in nature (generally). Poisson’s ratio ν, named after Simeon Poisson, is defined as the ratio of the contraction strain normal to the applied load divided by the extension strain in the direction of the applied load. Poisson’s Effect

  20. Young’s Modulus of Materials

  21. Plastic Deformation

  22. 0.002 Offset Yield Stress

  23. Classification of Polymers According to Mechanical Behavior A: Brittle B: Ductile C: Elastomer

  24. Temperature Effect PMMA

  25. Temperature Transitions

  26. Deformation Behavior

  27. Deformation of Crystalline Lamellae

  28. Elastomers Amorphous

  29. Crazing Shear Bands

  30. Tensile strength of selected plastics Plastic Tensile Strength, MPa Elongation to Break,% A. Amorphous plastics Polystyrene 50 2.5 Poly(methyl methacrylate) 65 10 Poly(vinyl chloride) 50 30 Poly(bisphenol carbonate) 60 125 B. Semicrystalline plastics Polyethylene (HD) 30 600 Polypropylene 33 400 Polytetrafluoroethylene 25 200 Polyamide 66 80 200 Poly(ethylene terephthalate) 54 275 C. Thermosets Phenol-formaldehyde resin 55 1 Epoxy resin 90 2.4 Unsaturated polyester resin 60 3 Note: MPa \ 145 = PSI.

  31. Impact Strength Impact strength is defined in terms of the energy required to fracture a sample when struck with a sharp blow. Charpy Test: the specimen is supported on both ends and struck in the middle. The notch is on the side away from the striker. Izod Test: The sample is supported at one end only, cantilever style. The notch is placed on the same side as the striker. Impact strength is usually reported in units of J/m (Joules per meter of notch)

  32. (Izod) Impact Strength of Plastics Polymer Strength Polystyrene 20–30 High-impact polystyrene, HIPS 80–150 ABS plastics 200–400 Epoxy resin (no filler) 75 Poly(methyl methacrylate) 40–60 Phenol-formaldehyde plastics 20 Poly(vinyl chloride) 40–70 Polycarbonate 600–800

  33. Viscoelasticity Elastic Viscoelastic The viscoelastic behavior of polymeric materials is dependent on both timeand temperature.

  34. Relaxation Modulus • The stress necessary to maintain a constant strain is measured as a function of time (T = const.) • Stress is found to decrease with time • This is a result of molecular relaxation process that take place within the polymer. Stress  time The Relaxation modulus E(t) =  (t)/

  35. (b) Stress Relaxation • During stress relaxation: • Strain is constant • Stress decreased slowly with time. • For elastic solid  the stress remains constant

  36. Creep Stress Strain Time

  37. Creep Constant Load

  38. Creep Compliance

  39. End

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