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Unformatted text preview: MechEng 382 Winter 2009 Homework #11 Solutions 1. [ A&J-2 , Ex. 21.1] What are the four main generic classes of polymers? For each generic class: (a) give one example of a specific component made from that class; (b) indicate why that class was selected for the component. Solution The four classes are: • Thermoplastics (a) Bottles, cups, packaging. (b) Thermoplastics are light and cheap for these applications. As their inter-chain sec- ondary bonds melt away at far lower temperatures than the chain backbone, they are also easily recycled by heating and re-forming. • Thermosets (a) Matrix material for fiberglass, adhesives, laminates. (b) Thermosets insulate well, are resistant to heat, are hard due to extensive cross- linking, and can be relatively cheap (polyester). • Elastomers (a) Automobile tires, seals. (b) Elastomers are (obviously) highly elastic, allowing large deformations with little to no energy loss or permanent deformation. • Natural polymers (a) Cell walls, musculoskeletal and connective tissues, paper, wood, wool, silk. (b) Products derived from natural polymers are (generally) cheaply available in vast quantities and require little processing. Remark Natural polymers do not have unifying generic mechanical properties allowing them to be classified into the above. Nor is it required in this assignment to speculate as to the evolutionary pressures giving rise to natural polymers. Any example of a biological structure or component derived from one constitutes a suitable response. 1 MechEng 382 Winter 2009 Homework #11 Solutions 2. [ A&J-2 , Ex. 21.3] The photograph (Fig. 1 ) shows a flanged connector in an internally pressur- ized piping system. The material is a polymethylmethacrylate (PMMA). The connector failed during a hydrostatic pressure test, at a pressure of only 70% of the working pressure. At this point, the maximum hoop stress had reached 10MPa. Table 21.5 gives a tensile strength of 80MPa minimum for PMMA. Why do you think the connector failed at only 1/8 of this value?...
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This note was uploaded on 09/28/2010 for the course MECHENG 382 taught by Professor Thouless during the Winter '08 term at University of Michigan.
- Winter '08