Page 107 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

86 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials

|]2.44. You are given the K and n values of two different 2.52. A metal has a strength coefficient K = 700 MPa and n
metals. Is this information sufficient to determine which metal = 0.2. Assuming that a tensile-test specimen made from this
is tougher? If not, what additional information do you need? metal begins to neck at a true strain of 0.2, show that the ul-
|]2.45. A cable is made of two strands of different materi- timate tensile strength is 415 MPa.
als, A and B, and cross sections as follows: 2.53. Plot the true stress-true strain curves for the materials
listed in Table 2.3.
For material A, K = 415 MPa, n = 0.5, AO = 380 mmf. u2.54. The design specification for a metal requires a min-
For material B, K = 210 MPa, n = 0.5, AO = 190 mmz. imum hardness of 80 HRA. If a Rockwell test is performed
and the depth of penetration is 60 /tm, is the material ac-
Calculate the maximum tensile force that this cable can with- ceptable?
stand prior to necking.
|]2.55. Calculate the major and minor pyramid angles for
|l2.46. On the basis of the information given in Fig. 2.6, a Knoop indenter, and compare your results with those
calculate the ultimate tensile strength (engineering) of 304 obtained from Vickers and Rockwell A indenters.
stainless steel.
|]2.56. If a material has a target hardness of 300 HB, what
2.47. In a disk test performed on a specimen 38 mm in is the expected indentation diameter?
diameter and 12 mm thick, the specimen fractures at a stress
|]2.57. A Rockwell A test was conducted on a material
of 275 Mpa. What was the load on it?
and a penetration depth of 0.15 mm was recorded. What is
|]2.48. A piece of steel has a hardness of 330 HB. the hardness of the material? What material would typically
Calculate its tensile strength, in MPa and in psi. have such a hardness value? If a Brinell hardness test were to
2.49. A metal has the following properties: UTS = 480 MPa be conducted on this material, give an estimate of the inden-
and 11 = 0.25. Calculate its strength coefficient, K. tation diameter if the load used was 1500 kg.
2.50. Using only Fig. 2.6, calculate the maximum load in 2.58. A material is tested in tension. Over a 25-mm gage
tension testing of an annealed copper specimen with an origi- length, the engineering strain measurements are 0.01, 0.02,
nal diameter of 5 mm. 0.03, 0.04, 0.05, 0.1, 0.15, 0.2, 0.5, and 1.0. Plot the true
|]2.5 I. Estimate the modulus of resilience for a highly cold strain versus engineering strain for these readings.
worked piece of steel having a hardness of 300 HB; for a piece
of highly cold worked copper with a hardness of 100 HRB.

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2.59. List and explain the desirable mechanical properties you make on these machines to include the effects of temper-
of (a) an elevator cable, (b) a paper clip, (c) a leaf spring for a ature on material properties?
truck, (d) a bracket for a bookshelf, (e) piano wire, (f) a wire 2.63. In tension testing of specimens, mechanical and elec-
coat hanger, (g) the clip for a pen, and (h) a staple. tronic instruments are typically used to measure elongation.
2.60. When making a hamburger, you may have observed Make sketches of instruments that would be suitable for this
the type of cracks shown in Fig. 2.20d. What would you do purpose, commenting on their accuracy. What modifications
to avoid such cracks? [Note: Test hamburger patties by com- would you make to these instruments to include the use of
pressing them at different temperatures, and observe the specimens at elevated temperatures?
crack path (i.e., the path through the fat particles, the meat 2.64. Obtain small pieces of different metallic and non-
particles, or their interface).] metallic materials, including stones. Rub them against each
other, observe the scratches made, and order them in a man-
2.61. An inexpensive claylike material called Silly Putty® is
ner similar to the Mohs hardness numbering system.
generally available in stores that sell toys and games. Obtain a
sample and perform the following experiments: (a) Shape it 2.65. Demonstrate the stress-relaxation phenomenon by
into a ball, and drop it onto a flat surface. (b) Reround the ball tightly stretching thin plastic strings between two nails placed
and place a heavy book on it for 1 minute. (c) Shape the putty at the ends of a long piece of wood. Pluck the strings fre-
into a long rod, and pull on it-first slowly, then very quickly. quently, to test the tension as a function of time. Repeat the
Describe your observations, referring to the specific sections in test at a higher temperature by placing the fixture in an oven
this chapter where each particular observation is relevant. set on low.
2.62. Make individual sketches of the mechanisms of test- 2.66. Demonstrate the impact toughness of a piece of round
ing machines that, in your opinion, would be appropriate for chalk by first using a triangular file to produce a V-notch on
tension, for torsion, and for compression testing of specimens the cylindrical surface (as shown in Fig. 2.19a) and then
at different rates of deformation. What modifications would bending the chalk to break it.