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Day 30: Mechanical Behavior

Day 30: Mechanical Behavior. Temperature dependence of Moduli Mechanism of plastic deformation. Cold work and annealing mean different things for polymers. Temperature Dependence of Modulus.

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Day 30: Mechanical Behavior

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  1. Day 30: Mechanical Behavior • Temperature dependence of Moduli • Mechanism of plastic deformation. • Cold work and annealing mean different things for polymers.

  2. Temperature Dependence of Modulus • Here is the definition of relaxation modulus for a polymer. The strain e0 is imposed in the creep test. • Modulus is a function of temperature. • As we expect, the moduli are higher for higher temperatures.

  3. Regimes of Behavior – Depend on Temperature • We have • Glassy, E nearly const. • Leathery Big change in E • Rubbery, E nearly constant • Rubber Flow, E falling • Viscous Flow, E drops greatly, it’s a liquid. Glass temp. middle of leathery

  4. Note the effects of crystallinity / tacticity • Three forms of PS behave a lot differently.

  5. Deformation in Semi-Crystalline Thermoplastic

  6. Stress Strain curve Neck propagates Neck starts at yield

  7. Drawing and Annealing • Drawing, or Cold Work. Take advantage of the increased strength and stiffness caused by the orientation of the chains. This can actually be used as a final step in manufacturing polymers as it is in metals. Note: drawing just imparts strength / stiffness in one direction! How is this different from CW in metals? • Annealing. (1) If the material is already drawn, it has much the same effect of softening as in metals. BUT (2) If the material is not drawn it can impart strength and stiffness (at least in some polymers) by enhancing crystallinity.

  8. fibrillar structure Near Failure near failure Initial aligned, networked crystalline cross- case regions linked slide case semi- amorphous crystalline crystalline regions case regions align elongate Tensile Response: Brittle & Plastic s (MPa) brittle failure x onset of necking plastic failure x unload/reload e Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from Figs. 15.12 & 15.13, Callister 7e. (Figs. 15.12 & 15.13 are from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.)

  9. Tensile Response: Elastomer Case final: chains are straight, still cross-linked Deformation initial: amorphous chains are is reversible! kinked, cross-linked. s (MPa) brittle failure x Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along elastomer curve (green) adapted from Fig. 15.15, Callister 7e. (Fig. 15.15 is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.) plastic failure x x elastomer e • Compare to responses of other polymers: -- brittle response (aligned, crosslinked & networked polymer) -- plastic response (semi-crystalline polymers)

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