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MSE2034 HW#EC B (Optional Extra Credit)

Due Wednesday, March 2 at beginning of class

1 pt each, 8 pts extra credit maximum

1)

An undeformed sample of some metal alloy with a recrystallization temperature of 400Cis measured to have an average grain size of 0.050 mm. The required grain size to obtainsufficient strength in application is 0.020 mm. Explain how it would be possible to alter

the sample to obtain this new structure, or explain why it is not possible to do so.

2) Refer to Fig 8.19. A cylindrical rod of brass with original diameter of 10.2 mm is to becold-worked by a drawing process, maintaining the cylindrical cross-section during the

process. Final properties required are a yield strength of at least 380 MPa and a ductilityof at least 15%EL, with a final diameter of 7.6 mm. Describe a process that could

achieve these properties, giving specific conditions for each required step. NOTE:Multiple sequential steps may be required to prevent failure during processing.

3) Consider a typical sample of window glass with elastic modulus of 69 GPa. If the mostsevere flaws in this sample are internal cracks with a totallength of 0.2 mm and tip radiusof curvature of 0.001 mm, would a 250 MPa tensile stress be sufficient to shatter the

sample? HINT: Use elastic modulus to estimate the ideal strength of the material.

4) Consider a large thick plate (Y=1.1) of polycarbonate with yield strength of 62.1 MPa, plain-strain fracture toughness of 2.2 MPam, and surface cracks 0.05 mm in length(these are very small in comparison to the dimensions of the plate). The plate is stressedto a maximum of 20 MPa in tension. Determine what will occur (will it yield, fracture in

ductile mode, or in brittle mode, or remain intact?).

5) See the data below for Charpy impact tests on a sample of low-carbon steel at varioustemperatures. Plot this data as impact energy vs. T, and estimate the ductile-to-brittle

impact temperature by the following methods: (a) as the average of the impact energylevels at high and low T, (b) as the point of maximum change in impact energy with T

(i.e., the inflection point of the curve), and (c) as the temperature where impact energyreaches 20 J. Briefly comment on the relative merits of these three estimation methods

(which would be most useful in what situations, and why?).

Temperature [C] Impact Energy [J]

50 7640 7530 7120 5810 380 23

-10 14

-20 9

-30 5

-40 1.5

-50 1.5

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6) Given below are some data for fatigue in a typical steel. Use this data to generate an S-N

plot for the steel and estimate the following: (a) the steel fatigue limit; (b) the fatiguelifetime at 400 MPa; and (c) the fatigue strength at 5x10

4cycles. Also, (d) if the given

data represent bending-rotating tests for an axle rotating at 600 RPM [revolutions per

minute], how much actual use time would be allowable for this axle at a stress of 400MPa?

Stress Amplitude [MPa] Cycles to Failure470 104

440 3 x 104

390 105

350 3 x 105

310 106290 3 x 10

6

290 107290

10

8

7) Refer to Figures 9.37 and 9.38 showing creep information for a particular low carbon-nickel alloy. Three samples of this alloy are subjected to a 60 MPa stress for an extended period, one at each of the given temperature levels [427C, 538C, and 649C]. If

possible, estimate the expected rupture lifetime for each sample and the expected percentelongation due to steady-state creep in one year (Note that the data may not permit a

reasonable estimate in certain cases). Comment on what you expect to happenqualitatively to each sample after one year under these conditions.

8) Refer to Figure 9.39, showing data for allowable stress vs. the Larson-Miller parameterfor a S-590 iron alloy. A component made from this alloy must withstand steady stress atT=650C with a rupture lifetime of at least 10 years. Estimate the allowable stress level,

employing a safety factor of 2 in stress to account for substandard samples. Note that theL-M parameter defined in Eqn 9.22 uses a common (base10) logarithm.