Page 256 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

1656_C005.fm Page 236 Monday, May 23, 2005 5:47 PM

236 Fracture Mechanics: Fundamentals and Applications

FIGURE 5.19 Formation of river patterns, as a result of a cleavage crack crossing a twist boundary between
grains.

solid is approximately E/π. Figure 5.14, however, indicates that the maximum stress achieved ahead
of the crack tip is three to four times the yield strength. For a steel with σ = 400 MPa and E =
YS
210,000 MPa, the cohesive strength would be ~50 times higher than the maximum stress achieved
ahead of the crack tip. Thus, a macroscopic crack provides insufficient stress concentration to
exceed the bond strength.
In order for cleavage to initiate, there must be a local discontinuity ahead of the macroscopic
crack that is sufficient to exceed the bond strength. A sharp microcrack is one way to provide
sufficient local stress concentration. Cottrell [24] postulated that microcracks form at intersecting

FIGURE 5.20 River patterns in an A 508 Class 3 steel. Note the tearing (light areas) between parallel cleavage
planes. Photograph courtesy of Mr. Sun Yongqi.