ME108-PracticeExam1

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University of California, Berkeley *

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108

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Mechanical Engineering

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Apr 3, 2024

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ME108 Practice Exam 1 50 points total The exam is scheduled for 50 minutes. Please note any assumptions that you make and show all of your work . 1. (4 points) Type of Materials: a. (1 point) The Columbia Space Shuttle used carbon fibers on the external surface. What is the material type? b. (2 points) Describe two advantages to using this type of material in the design for the Space Shuttle. c. (1 point) Explain one disadvantage to NASA’s design of the Columbia Space Shuttle which used carbon fibers on the external surface. 2. (5 points) Bonds a. (3 points) List and describe the three primary bonds. b. (2 points) Explain how the type of bond can alter material properties without altering material composition.
c. (2 points) For a low alloy steel with signs of large plastic deformation, is the structure more likely to be pearlite or martensite. Why? 3. (7 points) Crystal Structures a. (2 points) Draw a BCC and an HCP unit cell as a reduced-sphere unit cell. b. (2 points) How many full cells are in a BCC and HCP unit cell? BCC ______________ HCP _______________ c. (3 points) Imagine that you work for an alloying firm that would like to convert an element that naturally exists in BCC to an HCP crystal structure. What predicted impact would this have on density and material properties?
4. (16 points+) A Honda Odyssey was driving too fast while going over a speed hump resulting in a dent in the steel frame, which was designed with A36 steel treated to create a tempered martensite microstructure. To repair the frame, the auto body shop heated the steel to 825 o C with a blowtorch for 30 minutes to ensure the entire thickness of the frame was heated to be able to deform the material and repair the dent. The dent was repaired, and the customer was happy. a. (1 point) What type of heat treatment is described here? b. (3 points) What expected changes, if any, might occur to the microstructure of the frame? Explain why. c. (4 points) Draw the expected stress strain curves for the A36 steel before and after the repair. Remember to label your axes. d. (8 points) Label the following properties on the graph above and explain how the repair would impact the material property. i. Young’s modulus: ii. Yield Strength: iii. Percent elongation: iv. Fracture strength:
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v. Toughness: e. ( Bonus point ) Describe a potential risk of such a repair and why the customer may not be happy in the long-run. 5. (6 points) After a long day of Avenging in New York City (NYC), the Hulk decides to knock over the Alamo Statue in the East Village in NYC to sit and rest. The statue is a 3m cube with a Young’s modulus of 100 GPa and a Poisson’s ratio of 0.3. If the cube experienced 0.1% strain in both transverse directions, how much does The Hulk weigh?
6. (2 points each) True/False – Circle one. If False, explain why it is false a. True / False – True stress and strain are defined using the initial length and area of the piece. Explain: b. True / False – Forgetting to zero the displacement and load data at the beginning of a mechanical test, can result in compliance in the data. Explain: c. True / False – van der Waals bonds are the strongest of the three primary bond types between atoms. Explain: d. True/False- All metals, including cast irons, strengthen after yielding. Explain: e. We always start heat treatments with a BCC phase because the interstitial spaces are larger. Explain:
Constants in class: Avogadro’s Number, N A = 6.023X10 23 Possible ME108 Equations (Exam 1): Atomic and Crystal Structure, Heat Treatment: 𝑎 ??? = 4 3 𝑅 𝑎 𝐹?? = 4𝑅/ 2 ? 𝑎 = 1. 633 Mechanical Testing: ? =− ε ??𝑎??????? ε 𝑙??𝑔𝑖???𝑖?𝑎𝑙
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Phase Diagram for Steel