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Name____________________ ME 328 Exam #6 Winter 2005-06 Closed book, closed notes, no help from other students. Show all work for full credit. All justifications should use materials engineering principles. 1. A higher Weibull modulus means a) the material has a higher thermal conductivity b) the material has a lower coefficient of thermal expansion c) the material has a higher average strength d) the material has a lower relative scatter in strength 2. Your company defines the design stress as the stress at which no more than one half of one percent of the parts fail. Material property data tables show Ceramic A with nominal strength of 800 MPa and Weibull modulus of 18 and Ceramic B with nominal strength of 850 MPa and Weibull modulus of 10. Which is likely to be preferred for your application? Explain. 3. Describe (in words and/or pictures) a method for manufacturing tempered glass. 4. We need a ceramic to go quickly from low temperature to high temperature without fracture. We would like: i. Young’s Modulus a) high b) low c) doesn’t matter ii. Coef of thermal exp. a) high b) low c) doesn’t matter iii. Tensile strength a) high b) low c) doesn’t matter 5. Ceramics typically have ionic/covalent bonding while metals have metallic bonding. Therefore, we expect that, on average, compared to metals, ceramic’s i. Young modulus is a) higher b) lower c) can’t predict ii. Coef of thermal exp. is a) higher b) lower c) can’t predict iii. Melting temperature is a) higher b) lower c) can’t predict iv. Thermal conductivity is a) higher b) lower c) can’t predict v. Ductility is a) higher b) lower c) can’t predict 6. Describe the difference between “Pressed” and “Isostatically Pressed” in the manufacture of ceramics. 7. In a glass-ceram cooktop, a glass part is heated to convert the microstructure to ceramic. T F The heating process causes the part to become amorphous T F The heating process can be described as “vitrification” 8. Several statements regarding the brittle nature of ceramics are given. Only one is completely true and in line with the principles discussed in this course. Which is it? a) Ceramics are brittle because all high strength materials are brittle. b) Ceramics are brittle because of their high elastic moduli, E. c) Ceramics are brittle because of the absence of slip systems in complex crystals. d) Ceramics are only brittle at low temperatures. At half the melting point. they become ductile. e) They are brittle because dislocations are blocked by either cations or anions. 9. Exercise 1 at the end of the Stienstra notes concerns the potential use of Silicon Nitride as a replacement for metal in making the balls in a machine tool ball bearing. What did the exercise conclude? Choose all correct answers. a) Ceramics are too brittle to ever be used. b) The higher density of the ceramic balls are in their favor. c) The stiffer ceramic bearings can reduce the life of the steel race due to higher contact stresses at maximum loads. d) The ceramic bearings will be cheaper and easier to manufacture than those of steel. 10. Often in the manufacturing of ceramics a powder with binder added is formed into a shape and sintered. During the sintering process a) the part shrinks as the pores decrease in size b) the powders melt into a liquid and flow into the shape of the mold c) the part expands due to heat and a phase change d) the part age hardens as a second phase precipitates to block dislocation motion Match the ceramic to the application. One ceramic will clearly be better than others. A = Alumina Z = Zirconia C = Silicon carbide N = Silicon nitride ____________11. Insulator for spark plugs ____________12. Gas turbine component, requiring high thermal shock resistance ____________13. Body armor side plates ____________14. Used in heat exchangers in chemical industry operated at high temp. ____________15. A cutting blade for low temperature use ____________16. Low electrical conductivity and high thermal conductivity are needed.
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