pa7_Oct27_09 - a) What is the relaxation time? (Ans. about...

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AERO3700 PA 7 Questions + Quiz in last 30 minutes Tuesday October 27 Purpose: Familiarization and review of polymers and ceramics using notes and text. Callister: 12.41 (Ans. a. 101 MPa, b. 9.34 x 10 -3 mm), 12.45, 12.46 (Ans. a. 265 GPa, b. 195 GPa), 13.4 (Fig 12.25, a. about 2220 o C, b. about 2070 o C), 13.15, 13.16 A . An aerospace engineer is required to design an elastomeric link which is 140 mm long and which pulls less at the attachment points as time passes, until the stress approaches 0 MPa after 1 year. The engineer having taken AERO3700 immediately realizes that stress relaxation can be exploited to achieve this design requirement. A few tests show the following: a stress of 11 MPa is needed to stretch a 100 mm piece of the elastomer to the required length; and after 42 days at 20 o C the elastomer exerts only 5.5 MPa.
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Unformatted text preview: a) What is the relaxation time? (Ans. about 60.5 days) b) What stress will be exerted by the elastomer in the same stretched position after 90 days? (Ans. 2.5 MPa) c) Will the stress approach the design stress of 0 MPa after 1 year? (Engineering judgement is required to solve this problem. (Ans. Yes it will.) Explain your answer. d) Tests at 25 o C show that the relaxation time is 50 days. What is the activation energy for stress relaxation for this elastomer. Express your answer in cal/mole. (Ans. Q is about 6613 cal/mole) e) The engineer is informed that the actual service temperature will be 30 o C. What is the relaxation time at this temperature? (Ans. About 42 days.) f) Will the stress approach 0 MPa in 1 year at this temperature? (Ans. It is not likely.) M. McDill AERO3700...
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