Download - Day 30: Mechanical Behavior
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DAY 30: MECHANICAL BEHAVIOR Temperature dependence of Moduli Mechanism of plastic deformation. Cold work and annealing mean different
things for polymers.
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TEMPERATURE DEPENDENCE OF MODULUS Here is the definition of relaxation modulus for a
polymer. The strain 0 is imposed in the creep test.
0
ttER
Modulus is a function of temperature.
As we expect, the moduli are higher for higher temperatures.
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REGIMES OF BEHAVIOR – DEPEND ON TEMPERATURE We have 1. Glassy, E nearly
const.2. Leathery Big
change in E3. Rubbery, E
nearly constant4. Rubber Flow, E
falling5. Viscous Flow, E
drops greatly, it’s a liquid. Glass temp. middle of leathery
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NOTE THE EFFECTS OF CRYSTALLINITY / TACTICITY Three forms of PS behave a lot differently.
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DEFORMATION IN SEMI-CRYSTALLINE THERMOPLASTIC
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STRESS STRAIN CURVE
Neck starts at yield
Neck propagates
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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.
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TENSILE RESPONSE: BRITTLE & PLASTIC
10
brittle failure
plastic failure
(MPa)
x
x
crystalline regions
slide
fibrillar structure
near failure
crystalline regions align
onset of necking
Initial
Near Failure
semi-crystalline
case
aligned,cross-linkedcase
networkedcase
amorphousregions
elongate
unload/reload
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.)
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TENSILE RESPONSE: ELASTOMER CASE
• Compare to responses of other polymers: -- brittle response (aligned, crosslinked & networked polymer) -- plastic response (semi-crystalline polymers)
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.)
(MPa)
initial: amorphous chains are kinked, cross-linked.
x
final: chainsare straight,
stillcross-linked
elastomer
Deformation is reversible!
brittle failure
plastic failurex
x