ENGRI_1110_hw5_2008_AnsKEY

ENGRI_1110_hw5_2008_AnsKEY - ENGRI 1110 Homework 5 Due...

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ENGRI 1110 Homework # 5. Due Friday, Nov. 14, 2008 Note: It is always good with mechanical properties to keep in mind, that when we refer to metals,  ceramics, polymers, etc; that the properties are  generally  true for most materials in that subset.  There are, of course, always exceptions. 1. Compare and contrast the STM with the AFM. Include a) operating principle, b) type of materials appropriate for analysis, and c) ambient conditions required for analysis. 0 a) AFM – measures atomic interaction forces (van der Waals) using a cantilever probe and a laser to measure deflections 1 http://nanoindentation.cornell.edu/Machine/Nanoindentation-Machine.htm STM – measures the tunneling current between a conducting tip and sample http://www.sljus.lu.se/stm/NonTech.html b) AFM – insulators and conductors, including metals, polymers, ceramics and biological samples STM – conductors c) AFM – normal room atmosphere (although vacuum or dry environment is necessary for true atomic resolution due to interference from the natural water layer that forms on surfaces in an uncontrolled atmosphere). Measurements where the sample and the tip are completely immersed in liquid are also possible. STM – vacuum is necessary to prevent impurities or water from disrupting the flow of tunneling current
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2. Draw a stress/strain (engineering) curve for a typical metal and a typical ceramic. For the metal also draw the true stress/strain curve. The green line is a typical stress strain curve for a ceramic. The red line is a typical engineering stress strain curve for a metal, and the blue line is the corresponding true stress strain curve. Strain Stress
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3. Answer a - j with true or false and explain your answer with a short sentence. (a) Grain boundaries are created when crystals of different orientations are joined together. True, assuming you are dealing with identical crystals. (b) In a ceramic the tensile stress is 50 % greater than the yield strength. False. In ceramics there almost isn’t yield strength per say, since ceramics do not generally plastically deform. (c) Metals are not as brittle as ceramics because there are no cracks present. False. There can be cracks present, but with an applied stress the cracks do not propagate to failure like in ceramics; instead, the stress causes the dislocations to move and the material plastically deforms.
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