Exam I Sol
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University of California, Los Angeles *
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183A
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Aerospace Engineering
Date
Feb 20, 2024
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4
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1 MAE 183A: Introduction to Manufacturing Processes University of California, Los Angeles (UCLA) Winter 2024 Instructor: CJ Kim Examination I January 29: 8:35–9:25 AM (50 minutes) Write within the given spaces. More than enough spaces are given. Name: _____
Solutions
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, __________________________
Last First SCORE:
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55
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55
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Problem 1 (15 pts) Multiple choice questions. Circle the correct answer(s). (1) (1 pt) Which one of the following engineering materials is defined as a compound containing metallic and nonmetallic elements: (a) ceramic, (b) composite, (c) metal, or (d) polymer? (2) (1 pt) Which of the following is a surface defect in a crystal lattice structure: (a) edge dislocation, (b) grain boundaries, (c) interstitialcy, (d) Schottky defect, (e) screw dislocation, or (f) vacancy? (3) (1 pt) If engineering stress and true stress were measured simultaneously during a "compression" test, true stress would be (a) higher than, (b) lower than, or (c) same as engineering stress (4) (1 pt) Which one of the following types of stress strain relationship best describes the behavior of most metals at room temperature: (a) elastic and perfectly plastic, (b) elastic and strain hardening, (c) perfectly elastic, or (d) none of the above? (5) (1 pt) The strength and hardness of steel increase as carbon content (a) increases or (b) decreases? (6) (2 pts) Ferrous metals include which of the following (two correct answers): (a) aluminum, (b) cast iron, (c) copper, (d) gold, and (e) steel? (7) (2 pts) Which of the following alloying elements are most commonly associated with stainless steel (two best answers): (a) chromium, (b) manganese, (c) molybdenum, (d) nickel, and (e) tungsten? (8) (1 pt) Which one of the following is the correct definition of ultimate tensile strength (UTS), as derived from the results of a tensile test (remember the dogbone specimen?) on a metal specimen: (a) the stress encountered when the stress-strain curve transforms from elastic to plastic behavior, (b) the maximum load divided by the final area of the specimen, (c) the maximum load divided by the original area of the specimen, or (d) the stress observed when the specimen finally fails?
2 (9) (1 pt) A ceramic material can be defined as a microscopic physical mixture consisting of a metal or semimetal and one or more nonmetals: (a) true or (b) false? (10) (1 pt) Which of the following best characterizes the structure of glass-ceramics: (a) polycrystalline, (b) vitreous, (c) single crystal, or (d) polymeric? (11) (1 pt) A linear molecular structure (
) is softer in the solid state and less viscous in the molten state than a branched structure (
) for the same polymer: (a) true or (b) false? (12) (1 pt) The thermal expansion properties of metals are generally (a) greater than, (b) less than, or (c) the same as those of polymers? (13) (1 pt) Which one or two of the three polymer types involve(s) cross-linking: (a) thermoplastics, (b) thermosets, or (c) elastomers? Probem 2 (20 pts) Short answer questions. Write within the given space. (1) (2 pts) Define steel. Answer. An iron–carbon alloy containing 0.02% to 2.11% carbon. Accepted: ~0% to ~2% (2) (2 pts) What is the difference between a live center and a dead center, when these terms are used in the context of workholding in a lathe? Answer. A center holds the workpiece at the tailstock end of the lathe. A live center is mounted in bearings and rotates with the work. A dead center does not rotate - the work rotates about it. (3) (2 pts) How do grain boundaries contribute to the strain hardening phenomenon in metals? Answer. Grain boundaries block the continued movement of dislocations in the metal during straining. As more dislocations become blocked, the metal becomes more difficult to deform; in effect it becomes stronger. (4) (2 pts) Draw a typical true stress vs. true strain curve.
(5) (2 pts) What is sintering? Answer. Sintering is the process of compacting solid particles into a solid object at an elevated temperature, which is above 2/3 of and below the melting temperature in absolute unit
).
3 (6) (2 pts) Is copper considered easy or difficult to weld? Why? Answer. Copper is considered difficult to weld because its high thermal conductivity makes the heat flow away from the joint rather than being concentrated to permit melting of the metal. (7) (2 pts) What is so special about the superalloys? What distinguishes them from other alloys. Answer. The superalloys are generally distinguished by their strength and resistance to corrosion and oxidation at elevated temperatures. (8) (2 pts) What is the primary mineral in glass products? Answer. Silica, or silicon dioxide (SiO
2
). (9) (2 pts) What is graphene? Answer. Graphene is a one-atom-thick planar sheet of carbon. (10) (2 pts) Cross-linking (curing) of the various thermosetting plastics is accomplished by one of several ways. Name two. Answer. Two of (1) temperature-activated systems; (2) catalyst-activated systems; (3) mixing-activated systems; (4) ultraviolet light (UV) activated systems. Problem 3 (10 points) From the textbook and the lecture note, you have learned three-point test. The lecture note, in addition, asked you to study “four-point bending test”. (a) (5 pts) Draw the four-point bending test in a fashion similar to the drawing of the three-point bending test in the textbook and lecture note. Solution
: (b) (5 pts) What problem or shortcoming of the 3-point test does the 4-point test overcome? Solution
: The two points of forcing down creates a uniform maximum stress “section” between the two, making sure there are defects in the section. In comparison, the three-point test has a maximum stress “point”. This is not a problem with most materials, which have numerous defects, but is a problem with a specimen with a low defect density. By expanding the maximum stress region from a point to a section, you let the scarcity of defects not affect the test result.
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4 Problem 4 (10 points)
The phase diagram for the Nb-W alloy is shown on the right. Ignore the dotted line. For an alloy of 20% Nb and 80% W, we are solidifying the alloy from a molten state gradually (i.e., always in an equilibrium state). (a) (1) What is the liquidus temperature? Ans: 3090°C (b) (1) What is the solidus temperature? Ans: 2920°C (c) (1) What is the composition of the first solid to form during solidification? Ans: 91% W (d) (1) What is the composition of the last liquid to solidify during solidification? Ans: 63% W (e) (1) What is the composition of the liquid phase at 3000°C? Ans: 71% W (f) (1) What is the composition of the solid phase at 3000°C? Ans: 85% W (g) (2) What is the amount of liquid phase at 3000°C? Show the work. Ans: Liquid ~ (85-80)/(85-71) = 36% (h) (2) What is the amount of solid phase at 3000°C? Show the work. Ans: Liquid ~ (80-71)/(85-71) = 64%