Page 255 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

1656_C005.fm Page 235 Monday, May 23, 2005 5:47 PM

Fracture Mechanisms in Metals 235

FIGURE 5.18 SEM fractographs of cleavage in an A 508 Class 3 alloy. Photographs courtesy of Mr. Sun
Yongqi.

twist angle from the current cleavage plane. Initially, the crack accommodates the twist mismatch
by forming on several parallel planes. As the multiple cracks propagate, they are joined by tearing
between planes. Since this process consumes more energy than the crack propagation on a single
plane, there is a tendency for the multiple cracks to converge into a single crack. Thus, the direction
of crack propagation can be inferred from the river patterns. Figure 5.20 shows a fractograph of
river patterns in a low-alloy steel, where tearing between parallel cleavage planes is evident.

5.2.2 MECHANISMS OF CLEAVAGE INITIATION

Since cleavage involves breaking bonds, the local stress must be sufficient to overcome the cohesive
strength of the material. In Chapter 2, we learned that the theoretical fracture strength of a crystalline