Page 259 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

1656_C005.fm Page 239 Monday, May 23, 2005 5:47 PM

Fracture Mechanisms in Metals 239

(a)

(b)

FIGURE 5.22 SEM fractographs of cleavage initiation in an A 508 Class 3 alloy: (a) initiation at a grain
boundary carbide and (b) initiation at an inclusion near the center of a grain. Photographs courtesy of M.T.
Miglin.

Ritchie, Knott, and Rice (RKR) [30] introduced a simple model to relate fracture stress to
fracture toughness, and to explain why steels did not spontaneously fracture upon the application
of a minimal load. They postulated that cleavage failure occurs when the stress ahead of the crack
tip exceeds σ over a characteristic distance, as illustrated in Figure 5.25. They inferred σ in a mild
f
f
steel from blunt-notched four-point bend specimens and measured K with conventional fracture
Ic
toughness specimens. They inferred the crack-tip stress field from a finite element solution published
by Rice and Tracey [31]. They found that the characteristic distance was equal to two grain diameters