Page 324 - Materials Science and Engineering An Introduction

296 • Chapter 8 / Failure

7
Fatigue failure is not to occur for at least 10 cy- 10 T(20 + log t r )(°R–h)
3
cles when the maximum load is 250 N. Possible
materials for this application are the seven alloys 25 30 35 40 45 50 100
having S–N behaviors displayed in Figure 8.20.
Rank these alloys from least to most costly for
this application. Assume a factor of safety of 2.0
and that the distance between load-bearing points
is 80.0 mm (0.0800 m). Use cost data found in
Appendix C for the following alloys: 100

Alloy designation Alloy designation Stress (MPa) 10 Stress (10 3 psi)
(Figure 8.20) (Cost data to use—Appendix C)
EQ21A-T6 Mg AZ31B (extruded) Mg
70Cu-30Zn brass Alloy C26000
2014-T6 Al Alloy 2024-T3
10
Ductile cast iron Ductile irons (all grades)
1
1045 Steel Steel alloy 1040 Plate, 12 16 20 24 28
cold rolled 10 T(20 + log t r )(K–h)
3
4340 Steel Steel alloy 4340 Bar, Figure 8.35 Logarithm stress versus the Larson–
normalized Miller parameter for an 18-8 Mo stainless steel.
Ti-5Al-2.5Sn titanium Alloy Ti-5Al-2.5Sn (From F. R. Larson and J. Miller, Trans. ASME, 74, 765, 1952.
Reprinted by permission of ASME.)
You may also find useful data that appears in
Appendix B. 8.2FE Which type of fracture is associated with inter-
granular crack propagation?
Data Extrapolation Methods (A) Ductile
8.D4 An S-590 iron component (Figure 8.33) must
have a creep rupture lifetime of at least 20 days at (B) Brittle
650 C (923 K). Compute the maximum allowable (C) Either ductile or brittle
stress level.
(D) Neither ductile nor brittle
8.D5 Consider an S-590 iron component (Figure 8.33) Estimate the theoretical fracture strength (in
that is subjected to a stress of 55 MPa (8000 psi). At 8.3FE
MPa) of a brittle material if it is known that frac-
what temperature will the rupture lifetime be 200 h?
ture occurs by the propagation of an elliptically
8.D6 For an 18-8 Mo stainless steel (Figure 8.35), predict shaped surface crack of length 0.25 mm that has a
the time to rupture for a component that is subjected tip radius of curvature of 0.004 mm when a stress
to a stress of 100 MPa (14,500 psi) at 600 C (873 K). of 1060 MPa is applied.
8.D7 Consider an 18-8 Mo stainless steel component (A) 16,760 MPa
(Figure 8.35) that is exposed to a temperature of (B) 8,380 MPa
650 C (923 K). What is the maximum allowable
stress level for a rupture lifetime of 1 year? 15 years? (C) 132,500 MPa
(D) 364 MPa
FUNDAMENTALS OF ENGINEERING 8.4FE A cylindrical 1045 steel bar (Figure 8.20) is
QUESTIONS AND PROBLEMS subjected to repeated compression–tension stress
8.1FE The following metal specimen was tensile cycling along its axis. If the load amplitude is
tested until failure. 23,000 N, calculate the minimum allowable bar
diameter (in mm) to ensure that fatigue failure
will not occur. Assume a factor of safety of 2.0.

Which type of metal would experience this type of (A) 19.4 mm
failure? (B) 9.72 mm
(A) Very ductile (C) Brittle (C) 17.4 mm
(B) Indeterminate (D) Moderately ductile (D) 13.7 mm