Page 285 - Materials Science and Engineering An Introduction

8.5 Principles of Fracture Mechanics • 257

SEM Micrograph

Grains Path of crack propagation

(a) (b)
Figure 8.6 (a) Schematic cross-section profile showing crack propagation through the interior of grains for trans-
granular fracture. (b) Scanning electron fractograph of ductile cast iron showing a transgranular fracture surface.
Magnification unknown.
[Figure (b) from V. J. Colangelo and F. A. Heiser, Analysis of Metallurgical Failures, 2nd edition. Copyright © 1987 by John Wiley
& Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.]

In some alloys, crack propagation is along grain boundaries (Figure 8.7a); this
intergranular fracture is termed intergranular. Figure 8.7b is a scanning electron micrograph showing
fracture a typical intergranular fracture, in which the three-dimensional nature of the grains may
be seen. This type of fracture normally results subsequent to the occurrence of processes
that weaken or embrittle grain boundary regions.

8.5 PRINCIPLES OF FRACTURE MECHANICS 1
Brittle fracture of normally ductile materials, such as that shown in the chapter-opening
Figure b (of the oil barge), has demonstrated the need for a better understanding of the
mechanisms of fracture. Extensive research endeavors over the past century have led to
fracture mechanics the evolution of the field of fracture mechanics. This subject allows quantification of the
relationships among material properties, stress level, the presence of crack-producing
flaws, and crack propagation mechanisms. Design engineers are now better equipped to
anticipate, and thus prevent, structural failures. The present discussion centers on some
of the fundamental principles of the mechanics of fracture.

1 A more detailed discussion of the principles of fracture mechanics may be found in Section M.4 of the Mechanical
Engineering (ME) Online Module.