Page 106 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Quantitative Problems 85

QUALITATIVE PROBLEMS

2.I9. On the same scale for stress, the tensile true 2.32. Wire rope consists of many wires that bend and
stress-true strain curve is higher than the engineering unbend as the rope is run over a sheave. A wire-rope failure is
stress-engineering strain curve. Explain whether this condi- investigated, and it is found that some of the wires, when
tion also holds for a compression test. examined under a scanning electron microscope, display dim-
2.20. With a simple sketch, explain whether it is necessary ples, while others display transgranular fracture surfaces.
Explain these observations.
to use the offset method to determine the yield stress, Y, of a
material that has been highly cold worked. 2.33. A statistical sampling of Rockwell C hardness tests
2.2 I. Explain why the difference between engineering strain are conducted on a material, and it is determined that the
and true strain becomes larger as strain increases. Does this dif- material is defective because of insufficient hardness. The
ference occur for both tensile and compressive strains? Explain. supplier claims that the tests are flawed because the diamond-
cone indenter was probably dull. Is this a valid claim?
2.22. If a material (such as aluminum) does not have an
Explain.
endurance limit, how then would you estimate its fatigue life?
2.34. In a Brinell hardness test, the resulting impression is
2.23. Which hardness tests and scales would you use for
found to be elliptical. Give possible explanations for this
very thin strips of metal, such as aluminum foil? Explain.
result.
2.24. Which of the two tests, tension or compression,
2.35. Some coatings are extremely thin-some as thin as a
would require a higher capacity of testing machine, and why?
few nanometers. Explain why even the Knoop test is not able
2.25. List and explain briefly the conditions that induce to obtain reliable results for such coatings. Recent investiga-
brittle fracture in an otherwise ductile metal.
tions have attempted to use highly polished diamonds (with a
2.26. List the factors that you would consider in selecting a tip radius around 5 nanometers) to indent such coatings in
hardness test. Explain why. atomic force microscopes. What concerns would you have
2.27. On the basis of Fig. 2.6, can you calculate the percent regarding the appropriateness of the results?
elongation of the materials listed? Explain. 2.36. Select an appropriate hardness test for each of the
2.28. If a metal tension-test specimen is pulled and broken following materials, and justify your answer:
rapidly, where would the temperature be highest, and why? a. Cubic boron nitride
2.29. Comment on your observations regarding the con- b. Lead
tents of Table 2.2. C. Cold-drawn 0.5%C steel
2.30. Will the disk test be applicable to a ductile material? d. Diamond
Why or why not? e. Caramel candy
2.3|. What hardness test is suitable for determining the f. Granite
hardness of a thin ceramic coating on a piece of metal?

QUANTITATIVE PROBLEMS

2.37. A paper clip is made of wire 0.7 mm in diameter. If the 2.4l. If you remove the layer of material ad from the part
original material from which the wire is made is a rod 25 mm shown in Fig. 2.30d-for instance, by machining or grinding-
in diameter, calculate the longitudinal engineering and true which way will the specimen curve? (Hint: Assume that the
strains that the wire has undergone during processing. part shown in sketch d in the figure is composed of four
2.38. A 200-mm-long strip of metal is stretched in two horizontal springs held at the ends. Thus, from the top down,
steps, first to 300 mm and then to 400 mm. Show that the you have compression, tension, compression, and tension
springs.)
total true strain is the sum of the true strains in each step; in
other words, the true strains are additive. Show that, in the 2.42. Percent elongation is always defined in terms of the
case of engineering strains, the strains cannot be added to original gage length, such as 50 mm. Explain how percent
obtain the total strain. elongation would vary as the gage length of the tensile-test
2.39. Identify the two materials in Fig. 2.6 that have the specimen increases. (Hint: Recall that necking is a local phe-
lowest and the highest uniform elongations. Calculate these nomenon.)
quantities as percentages of the original gage lengths. 2.43. Make a sketch showing the nature and distribution of
2.40. Plot the ultimate strength vs. stiffness for the materials residual stresses in Fig. 2.31a and b, prior to the material’s
listed in Table 2.2, and prepare a three-dimensional plot for being cut. (Hint: Assume that the split parts are free from any
these materials where the third axis is their maximum elonga- stresses; then force these parts back to the shape they origi-
tion in 50 mm. nally had.)