# hw8f10 - increase decrease or stay the same a in the...

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Homework #8 Due Tuesday 11/9/10 1. For the three parameter model shown above, provide relationships between the total stress and the stress in each element, and the total strain and the strain in each element. 2. For a Voigt-Kelvin model, describe its behavior under stress relaxation. 3. For the four parameter model shown above draw a plot showing its response under creep. Be sure to show what happens when the stress is removed. Do not derive the equations; the plot should just show the general trends. Also indicate on the plots how each of the elements contributes to the response (see class notes for example). 4. A particular polymer is represented by a Maxwell element. When a tensile stress of 1.0 kPa is applied for 10.0 seconds, the maximum length attained by the sample is 1.15 times the original length. After this 10.0 seconds the stress is removed, and the length of the sample becomes 1.10 times the original length. What is the relaxation time of the Maxwell element? 5. For each of the following, indicate whether the stress relaxation modulus will

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Unformatted text preview: increase, decrease, or stay the same. a. in the rubbery region, increase the number of crosslinks b. in the rubbery region, increase the crystallinity c. in the glassy region, decrease the crystallinity d. in the leathery region, increase temperature e. in the leathery region, decrease the time at which you measure the stress relaxation modulus Continued on next page. 6. Using the figure below, estimate values for the stress relaxation modulus of PMMA under the following conditions: a. 120˚ C and 1000 hours b. 110˚ C and 1000 hours c. 110˚ C and 0.0001 hours d. 92˚ C and 10 hours e. 40˚ C and 10,000 hours 7. A polymer with a Tg of 50˚ C has a 2 hour creep compliance of 10-5 cm 2 /dyne at 80˚ C. At what time will it have the same creep compliance at 150˚ C? 8. The tensile strength of polystyrene is much less sensitive to strain rate than high density polyethylene. Explain why this is so....
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hw8f10 - increase decrease or stay the same a in the...

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