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Unformatted text preview: EMSE 201 Introduction to Materials Science & Engineering 7 November 2007 Name: S O L U T I O N Test #2 75 minutes; 150 points; 6 questions; 8 pages; 15% of course grade Partial credit will be given for correct set-ups and reasoning. Give units on numerical answers where ap- propriate. Please write all answers on these pages; use the backs if needed. 1) (24 points) Engineering stress-strain curves for three materials, tested in tension, are shown below. Identify each material as either a ceramic, a metal, or a polymer. Justify your answers with three charac- teristics (at least two of these quantitative ) from the stress-strain curves. a) 2 pts for each criterion, up to 6 pts total: Very high strain to failure (400%) Low modulus (~35 MPa/(~0.10) = 0.35 GPa, in the range of LDPE) Low tensile strength (40 MPa) Yield point, followed by rise in strength Polymer ( 2 pts ) b) 2 pts for each criterion, up to 6 pts total: Very low strain to failure (< 0.2%) High modulus (~700 MPa/0.0014 = 500 GPa) High fracture strength (700 MPa) Linear to fracture; no yielding Ceramic ( 2 pts ) c) 2 pts for each criterion, up to 6 pts total: Moderate strain to failure (80%) Moderately high modulus (~1000 MPa/0.005 = 200 GPa) High yield strength (~1.5 GPa) High tensile strength (~2 GPa) Metal ( 2 pts ) Department of Materials Science and Engineering 1 of 8 Case Western Reserve University 40 MPa 400% EMSE 201 Introduction to Materials Science & Engineering 7 November 2007 2) The figure at right shows the engineering stress-strain curves for a low-carbon steel at three different degrees of cold work. a) (10 points) Give approximate numerical values of the modulus of toughness for this alloy at each of these three degrees of cold work. Which property has the dominant ef- fect on the trend in toughness versus cold work in this material? Use the rougher of the two formulas for estimating modulus of toughness ( 2 pts ): U t 2245 f ut . Take num- bers from the graph: %CW ut ,MPa f U t ,MPa 0 46010 241% 110 4 49510 191% 94 24 59510 141% 83 ( 2 pts for each answer). The more precise formula gives a similar trend, with less overestimation. Though strength increases with increasing %CW, the strain to failure decreases more strongly and therefore has the dominant effect on the trend in toughness ( 2 pts ). b) (10 points) Describe how the modulus of resilience would vary with degree of cold work for this material. Justify your answer....
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This document was uploaded on 11/08/2011.
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