Page 264 - Shigley's Mechanical Engineering Design

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Failures Resulting from Static Loading 239
Figure 5–21
Brittle behavior Ductile behavior
Failure theory selection
flowchart.
< 0.05 ≥ 0.05
f

No Yes No Yes
·
Conservative? S = S ?
yt
yc

Mod. Mohr Brittle Coulomb-Mohr Ductile Coulomb-Mohr
(MM) (BCM) (DCM)
Eq. (5–32) Eq. (5–31) Eq. (5–26) No Yes
Conservative?

Distortion-energy Maximum shear stress
(DE) (MSS)
Eqs. (5–15) Eq. (5–3)
and (5–19)

namely, Coulomb Mohr, or modified Mohr. Figure 5–21 provides a summary flowchart for
the selection of an effective procedure for analyzing or predicting failures from static
loading for brittle or ductile behavior. Note that the maximum-normal-stress theory is
excluded from Fig. 5–21 as the other theories better represent the experimental data.

5–12 Introduction to Fracture Mechanics

The idea that cracks exist in parts even before service begins, and that cracks can grow
during service, has led to the descriptive phrase “damage-tolerant design.” The focus of
this philosophy is on crack growth until it becomes critical, and the part is removed
from service. The analysis tool is linear elastic fracture mechanics (LEFM). Inspection
and maintenance are essential in the decision to retire parts before cracks reach cata-
strophic size. Where human safety is concerned, periodic inspections for cracks are
mandated by codes and government ordinance.
We shall now briefly examine some of the basic ideas and vocabulary needed for
the potential of the approach to be appreciated. The intent here is to make the reader
aware of the dangers associated with the sudden brittle fracture of so-called ductile
materials. The topic is much too extensive to include in detail here and the reader is
urged to read further on this complex subject. 9

9 References on brittle fracture include:
H. Tada, P. C. Paris, and G. R. Irwin, The Stress Analysis of Cracks Handbook, 3rd ed., ASME Press,
New York, 2000.
D. Broek, Elementary Engineering Fracture Mechanics, 4th ed., Martinus Nijhoff, London, 1985.
D. Broek, The Practical Use of Fracture Mechanics, Kluwar Academic Pub., London, 1988.
David K. Felbeck and Anthony G. Atkins, Strength and Fracture of Engineering Solids, 2nd ed.,
Prentice-Hall, Englewood Cliffs, N.J., 1995.
Kåre Hellan, Introduction to Fracture Mechanics, McGraw-Hill, New York, 1984.