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MEE 312 – Engineering Materials
Due Wednesday 15 in class
1) Briefly (in a single phrase or short sentence) describe/define the following terms in your own words and, if relevant, include an example:
- Strain hardening f) Residual stresses
- Cold working g) Annealing
- Springback h) Polygonized subgrain structure
- Frank-Read source i) Recrystallization temperature
- Texture strengthening j) Hot working
2) Some of the key mechanical properties of 2024 aluminum alloy (AA), obtained from tensile testing (MatWeb), are shown below:
|Modulus of Elasticity||10,600 ksi|
|Yield strength||11 ksi|
- Draw the engineering stress-strain curve for 2024 AA (qualitative). Annotate the stress-strain diagram with the mechanical properties above and their respective values.
- On the same stress-strain diagram, using a dashed line, construct the stress-strain curve (qualitative) for a 2024 AA that has been subjected to 50% cold work. Explain the following:
- Would you expect the modulus of elasticity (stiffness) to differ between the two curves? Why or why not?
- Would you expect the yield strength and UTS to be higher or lower for the cold-worked material and why?
- Would you expect the ductility to be higher or lower for the cold-worked material and why?
3) Metals with what type of crystal structure are least responsive/receptive to cold working? Most responsive/receptive?
4) Data from a tensile test conducted on a metallic specimen is provided in an MS Excel spreadsheet (tensile_raw.xlsx).
- a) Convert these “semi-raw” measurements to an engineering stress-strain curve and a true stress-strain curve. Plot both curves on the same set of axes.
- b) Briefly describe (in words) how you would determine the stiffness, yield strength, UTS, ductility, and toughness of the material (no need to go through the actual calculations).
- c) Determine the strain-hardening exponent n for the plastic portion of the stress-strain curve. What type of crystal structure do you think the metal has? (Extra Credit)
5) Are polycrystalline ceramics responsive to cold working? Amorphous ceramics? Thermoset polymers? Thermoplastic polymers? Why or why not? If yes, is the strain hardening mechanism the same as in metals?
6) Fill in the table below as it relates to the different stages of annealing:
|Change in microstructure||New grains nucleate||Grains enlarge|
|Change in mechanical properties||Further reduction in strength, further increase in ductility|
|Temperature||Below T_rc||Well above T_rc|
7) Reproduce Example 8-5 on pp. 289-290 of the textbook (7th ed.) in your own words. (Don’t just copy things down without thoroughly understanding each step.) What did you learn about the interplay between cold working and annealing in deformation processing?
8) Consider the following material property data obtained experimentally as a material was progressing through the three different stages of annealing:
|Annealing Temperature (C)||Electrical Conductivity (ohm-1 cm-1)||Yield Strength (MPa)||Grain Size (µm)|
- Estimate the recrystallization temperature? How do you know?
- Estimate the grain growth temperature? How do you know?
- Estimate a suitable temperature for a stress-relief anneal.
- Estimate a suitable temperature for hot working.
9) For each of the qualities/characteristics below, list whether it corresponds to cold working (CW), hot working (HW), or both.
Reduces ductility Suited to form & shape large parts
Provides good surface finish Reduces porosity
Deforms material above T_rc Produces anisotropic behavior
Does not increase strength Increases strength
Deforms material below T_rc Provides poor surface finish
Easier to control dimensional Harder to control dimensional
Material continuously recrystallizes Decreases stiffness (modulus of
during deformation elasticity)
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