ENMA Exams - University of Maryland Honor Pledge: ...

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Unformatted text preview: University of Maryland Honor Pledge: “I pledge on my honor that I have not given or received any unauthorized assistance on this exam.” My name and signature on page 1 attest that I have read and agree with the above statement. Printed Name: ENMA 300 Signature: Final Exam Monday, December 13, 2010 Instructions. Please: 1. All work must be done on the exam itself, do NOT do calculations on scrap paper. 2. Read each question carefully and answer the question asked! 3. If it is appropriate, make assumptions to solve the problems. Be sure to state the assumptions you make. 4. Be consistent within a given problem. 5. Remember you only have 120 minutes to complete the exam. Look at the point values of each question to determine how to spend your time. Point values will not necessarily be split evenly between parts. 6. A person with normal size handwriting should be able to answer each question in the space allotted. If you need more space, continue your answer on the back of the page and indicate which question you are answering. 7. If I cannot understand your answer (due to the legibility of your writing or the "English") I will mark the answer wrong. You do not have to write perfect English sentences, "bullet format" is acceptable as long as I can understand what you are saying. 8. Be sure to define terms in any equations you use! Question #1: Atomic Scale Structures (10 credits) Refer to figure 1 below: FIGURE 1: The crystal structures of diamond and graphite. (a) What is the coordination number of carbon (i) in graphite and (ii) in diamond? (b) What is the expected coordination number of an atom in a close ­packed mono ­atomic crystal? University of Maryland Honor Pledge: “I pledge on my honor that I have not given or received any unauthorized assistance on this exam.” My name and signature on page 1 attest that I have read and agree with the above statement. (c) Explain why the bonding in graphite and in diamond does not lead to a close ­packed crystal. Question # 2: Mechanical Properties. (a) A cylindrical specimen of a hypothetical metal alloy is stressed in compression. If its original and final diameters are 30.00 and 30.04 mm, respectively, and its final length is 105.20 mm, compute its original length if the deformation is totally elastic. The elastic and shear moduli for this alloy are 65.5 and 25.4 GPa, respectively. (6 credits) (b) Some aircraft component is fabricated from an aluminum alloy that has a plane strain fracture toughness of 40 MPa √m. It has been determined that fracture results at a stress of 300 MPa (43,500 psi) when the maximum (or critical) internal crack length is 4.0 mm (0.16 in.). For this same component and alloy, will fracture occur at a stress level of 260 MPa (38,000 psi) when the maximum internal crack length is 6.0 mm (0.24 in.)? Why or why not? (6 credits) University of Maryland Honor Pledge: “I pledge on my honor that I have not given or received any unauthorized assistance on this exam.” My name and signature on page 1 attest that I have read and agree with the above statement. Question #3: Mechanical Properties. For each of the situations given below, identify two important mechanical and/or environmental property considerations, and explain your choice for each: (8 credits) a. A heavy ­duty bulldozer’s blade. [HINT: Think about everything a bulldozer does and might reasonably encounter.] b. The internal structure (e.g., root ­to ­tip, span ­wise spars (beams) and fore ­to ­aft ribs) in the wing of a commercial airliner. c. The solder used in the electronics of a cell ­phone. [HINT: Think of what your cellphone experiences.] d. An artificial hip replacement in a relatively young, 50 ­year ­old. [HINT: Think about the types and durations of loading and the environment.] University of Maryland Honor Pledge: “I pledge on my honor that I have not given or received any unauthorized assistance on this exam.” My name and signature on page 1 attest that I have read and agree with the above statement. Question # 4: Corrosion. (a) Write the possible oxidation and reduction half ­reactions that occur when magnesium is immersed in each of the following solutions: (i) HCl, (ii) an HCl solution containing dissolved oxygen, (iii) an HCl solution containing dissolved oxygen and, in addition, Fe2+ ions. (7 credits) (b) In which of these solutions would you expect the magnesium to oxidize most rapidly? Why? (2 credits) (c) For each situation below, cite which form of corrosion might occur and why (3 credits): i. When riveted steel gusset plates are used to join and strengthen steel I ­beams. ii. When zinc galvanization is used on carbon steel in the underbody parts of automobiles. iii. When cold ­formed sheets of plain carbon steel for marine hardware are exposed to seawater. University of Maryland Honor Pledge: “I pledge on my honor that I have not given or received any unauthorized assistance on this exam.” My name and signature on page 1 attest that I have read and agree with the above statement. Question #5: Diffusion (6 credits) (a) For substitutional diffusion of zinc in copper, Do = 2.4 x 10 ­5 cm2/s and Ea = 1.96 eV. Calculate the diffusivity (diffusion coefficient) of zinc in copper at 600°C. Express temperature in Kelvin (K) NOTE: k = 8.62 x 10 ­5 eV/K. (b) If the diffusivity (diffusion coefficient) for zinc in copper at 450°C is 1 x 10 ­18 cm2/s, how long will it take at 600°C [using the information in 6a] to achieve the same diffusion profile (penetration depth) as is achieved in 1000 hours at 450°C. Question #6: Phase diagrams and transformations (4 credits) Examine the MgO ­Al2O3 phase diagram shown in figure 2 below. Using the stochiometry, find the exact composition (in wt% Al2O3) of the vertical phase boundary at the bottom of the area marked “MgAl2O4 (ss)”. Mw(Al) = 27 g/mol, Mw(Mg) = 24.3 g/mol, Mw(O) = 16 g/mol, Mw(Al2O3) = 102 g/mol, Mw(MgO) = 40.3 g/mol. University of Maryland Honor Pledge: “I pledge on my honor that I have not given or received any unauthorized assistance on this exam.” My name and signature on page 1 attest that I have read and agree with the above statement. FIGURE 2: The magnesium oxide ­aluminum oxide phase diagram; ss denotes solid solution. Question #7: Phase transformations (15 credits) All the following statements refer to plain carbon steel (no other impurities) a) Explain why is Martensite the strongest of the phases found in Fe ­C steels. You must use the following terms in your answer: diffusionless, interstitial, dislocation, deformation, strain field. (3 credits) b) Circle which of the following microstructures can form under continuous cooling of austenite: Martensite, Spheroidite, Coarse Pearlite, Fine Pearlite, Bainite (2 credits) University of Maryland Honor Pledge: “I pledge on my honor that I have not given or received any unauthorized assistance on this exam.” My name and signature on page 1 attest that I have read and agree with the above statement. (c) Using the isothermal transformation diagram for an iron–carbon alloy of eutectoid composition, specify the nature of the final microstructure (in terms of microconstituents present and approximate percentages of each) of a small specimen that has been subjected to the following time–temperature treatments. In each case assume that the specimen begins at 760°C (1400°F) and that it has been held at this temperature long enough to have achieved a complete and homogeneous austenitic structure (10 credits). (i) Rapidly cool to 400°C (750°F), hold for 500 s, then quench to room temperature. (ii) Reheat the specimen in part (a) to 700°C (1290°F) for 20 (iii)Cool rapidly to 665°C (1230°F), hold for 103 s, then quench to room temperature. (iv) Rapidly cool to 350°C (660°F), hold for 150 s, then quench to room temperature. (v)Rapidly cool to 100C, hold for 104seconds then reheat to 500C for 1000 s. University of Maryland Honor Pledge: “I pledge on my honor that I have not given or received any unauthorized assistance on this exam.” My name and signature on page 1 attest that I have read and agree with the above statement. Question # 8: Crystal Structure (8 credits). Pure iron goes through a polymorphic phase change (if a metal can exist in more than one crystalline form under different conditions of temperature and pressure, it is said to be polymorphic) from BCC to FCC upon heating through 912°C. Calculate the percent volume change associated with the change in crystal structure BCC to FCC if at 912°C, the BCC unit cell has a lattice constant a= 0.293 nm and the FCC unit cell has a lattice constant a= 0.363 nm. University of Maryland Honor Pledge: “I pledge on my honor that I have not given or received any unauthorized assistance on this exam.” My name and signature on page 1 attest that I have read and agree with the above statement. Question #9 : Phase Diagrams.(5 credits) The following question references the phase diagram in figure 3 below. For an alloy that has an overall composition of 30 weight percent tin in lead, please determine the following at 150°C: Figure 3 – Tin ­Lead Phase Diagram (a) Draw the expected microstructure from cooling a liquid of the same composition. Label carefully the phases. (b) What is the composition of the α phase in weight percent tin? (c) What is the composition of the β  ­Sn phase in weight percent tin? University of Maryland Honor Pledge: “I pledge on my honor that I have not given or received any unauthorized assistance on this exam.” My name and signature on page 1 attest that I have read and agree with the above statement. Question 10: Phase Diagrams (4 credits) Name the element (letter) in the list that would create a phase diagram like the one shown below (i.e. isomorphous) with copper? What principle(s) did you use to determine your answer? Element Atomic Radius (nm) Crystal Structure Electronegativity Valence Copper (Cu) 0.1278 FCC 1.9 +2 A 0.1241 BCC 1.8 +2 B 0.1249 BCC 1.6 +3 C 0.1250 FCC 1.9 +2 D 0.1332 HCP 1.6 +2 L α A 25% 50% 75% Composition (wt. %B) B University of Maryland Honor Pledge: “I pledge on my honor that I have not given or received any unauthorized assistance on this exam.” My name and signature on page 1 attest that I have read and agree with the above statement. Question 11: Phase Diagrams/Transformations (10 credits). Answer the questions below considering you start with 3.5 kg of austenite containing 0.95 wt% C, cooled to below 727°C (1341°F). (a) What is the proeutectoid phase? (b) How many kilograms each of total ferrite and cementite form? c) How many kilograms each of pearlite and the proeutectoid phase form? (d) Schematically sketch and label the resulting microstructure. University of Maryland Honor Pledge: “I pledge on my honor that I have not given or received any unauthorized assistance on this exam.” My name and signature on page 1 attest that I have read and agree with the above statement. Question #12: Laboratory Questions (6 credits). Select the correct answer from the choices given. (a) How did Copper behave in the Charpy lab? a. Brittle at low temperatures, somewhat ductile at room temperature, very ductile at high temperature b. Ductile at all temperatures c. Classic straight edge fracture propagating from the Charpy “V notch” d. Ductile at low temperatures, somewhat ductile at room temperature, very brittle at high temperature e. None of the above (b) How did Nylon behave in the fatigue lab? a. At 90 degrees it cracked faster, at 45 degrees it took more cycles, at 22.5 degrees it took a lot longer to crack. b. When the Nylon was bent back and forth fast enough it broke with a ductile crack growth. c. It did not break at all in fatigue. d. The fatigue life of Nylon followed the regular S ­N curve. e. None of the above (c) How did Acrylic behave in bending? a. b. c. d. Behaved like a ceramic It showed a low modulus with plastic yielding and some recovery. It showed a high modulus with low rupture stress The force vs. displacement curve showed a classic polymer behavior with a yield area rather than a point and excellent recovery upon unloading e. None of the above (d) What happened when an electric voltage was added to the Copper (+) and Steel ( ­) in the corrosion lab? a. b. c. d. e. The Steel and Copper created a voltage between them to charge the battery. The copper turned dark and tarnished The Copper sulfate changed color The electroplating happened faster None of the above ...
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This note was uploaded on 08/02/2011 for the course ENMA 300 taught by Professor Alsheikhly during the Spring '11 term at University of Maryland Baltimore.

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