Page 323 - Materials Science and Engineering An Introduction

Questions and Problems • 295

8.38 A cylindrical component constructed from an 8.44 (a) Using Figure 8.31, compute the rupture life-
S-590 alloy (Figure 8.31) is to be exposed to a time for an S-590 alloy that is exposed to a tensile
tensile load of 20,000 N. What minimum diameter stress of 400 MPa at 815 C.
is required for it to have a rupture lifetime of at Compare this value to the one determined
least 100 h at 925 C? (b)
from the Larson–Miller plot of Figure 8.33, which
#
8.39 From Equation 8.24, if the logarithm of P s is is for this same S-590 alloy.
plotted versus the logarithm of s, then a straight
line should result, the slope of which is the stress Alloys for High-Temperature Use
exponent n. Using Figure 8.32, determine the 8.45 Cite three metallurgical/processing techniques
value of n for the S-590 alloy at 925 C, and for the that are employed to enhance the creep resistance
initial (lower-temperature) straight line segments of metal alloys.
at each of 650 C, 730 C, and 815 C.
Spreadsheet Problems
8.40 (a) Estimate the activation energy for creep 8.1SS Given a set of fatigue stress amplitude and
(i.e., Q c in Equation 8.25) for the S-590 alloy cycles-to-failure data, develop a spreadsheet that
having the steady-state creep behavior shown in allows the user to generate an S-versus-log N plot.
Figure 8.32. Use data taken at a stress level of 300
MPa (43,500 psi) and temperatures of 650 C and 8.2SS Given a set of creep strain and time data,
730 C. Assume that the stress exponent n is inde- develop a spreadsheet that allows the user to gen-
pendent of temperature. erate a strain-versus-time plot and then compute
the steady-state creep rate.
#
(b) Estimate P s at 600 C (873 K) and 300 MPa.
8.41 Steady-state creep rate data are given in the fol- DESIGN PROBLEMS
lowing table for a nickel alloy at 538 C (811 K): 8.D1 Each student (or group of students) is to obtain
# an object/structure/component that has failed. It
-1
` s (h ) S(MPa) may come from the home, an automobile repair
10 7 22.0 shop, a machine shop, and so on. Conduct an inves-
10 6 36.1 tigation to determine the cause and type of failure
(i.e., simple fracture, fatigue, creep). In addition,
propose measures that can be taken to prevent fu-
Compute the stress at which the steady-state
h (also at 538 C).
creep is 10 5 1 ture incidents of this type of failure. Finally, submit
a report that addresses these issues.
8.42 Steady-state creep rate data are given in the
following table for some alloy taken at 200 C (473 K): Principles of Fracture Mechanics
# 8.D2 (a) For the thin-walled spherical tank discussed
-1
` s (h ) S[MPa (psi)] in Design Example 8.1, on the basis of the critical-
2.5 10 –3 55 (8000) crack size-criterion [as addressed in part (a)], rank
2.4 10 –2 69 (10,000) the following polymers from longest to shortest criti-
cal crack length: nylon 6,6 (50% relative humidity),
If it is known that the activation energy for creep polycarbonate, poly(ethylene terephthalate), and
is 140,000 J/mol, compute the steady-state creep poly(methyl methacrylate). Comment on the magni-
rate at a temperature of 250 C (523 K) and a stress tude range of the computed values used in the rank-
level of 48 MPa (7000 psi). ing relative to those tabulated for metal alloys as
provided in Table 8.3. For these computations, use
8.43 Steady-state creep data taken for an iron at a data contained in Tables B.4 and B.5 in Appendix B.
stress level of 140 MPa (20,000 psi) are given here:
(b) Now rank these same four polymers relative
# to maximum allowable pressure according to the
-1
` s (h ) T(K) leak-before-break criterion, as described in part
6.6 10 –4 1090 (b) of Design Example 8.1. As before, comment
8.8 10 –2 1200 on these values in relation to those for the metal
alloys tabulated in Table 8.4.
If it is known that the value of the stress exponent
n for this alloy is 8.5, compute the steady-state The Fatigue S–N Curve
creep rate at 1300 K and a stress level of 83 MPa A cylindrical metal bar is to be subjected
(12,000 psi). 8.D3 to reversed and rotating–bending stress cycling.