Page 268 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

1656_C005.fm Page 248 Monday, May 23, 2005 5:47 PM

248 Fracture Mechanics: Fundamentals and Applications

FIGURE 5.29 Relationship between cleavage fracture toughness and the distance between the crack tip and
the cleavage trigger. Taken from Heerens, J. and Read, D.T., ‘‘Fracture Behavior of a Pressure Vessel Steel
in the Ductile-to-Brittle Transition Region.” NISTIR 88-3099, National Institute for Standards and Technology,
Boulder, CO, 1988.

the site of cleavage initiation. The measured distance from the initiation site to the original crack
tip correlated very well with the measured fracture toughness. In specimens that exhibited
low toughness, this distance was small; a critical nucleus was available near the crack tip. In
specimens that exhibited high toughness, there were no critical particles near the crack tip; the
crack had to grow and sample additional material before a critical cleavage nucleus was found.
Figure 5.29 is a plot of fracture toughness vs. the critical distance r , which Heerens and Read
c
measured from the fracture surface; r is defined as the distance from the fatigue crack tip to the
c
cleavage initiation site. The resistance curve for ductile crack growth is also shown in this plot.
In every case, cleavage initiated near the location of the maximum tensile stress (c.f. Figure
5.14). Similar fractographic studies by Watanabe et al. [33] and Rosenfield and Shetty [48] also
revealed a correlation between J , ∆a, and r .
c
c
Cleavage propagation in the upper transition region often displays isolated islands of ductile
fracture [23, 49]. When specimens with arrested macroscopic cleavage cracks are studied metallo-
graphically, unbroken ligaments are sometimes discovered behind the arrested crack tip. These two
observations imply that a propagating cleavage crack in the upper transition region encounters
barriers, such as highly misoriented grains or particles, through which the crack cannot propagate.
The crack is diverted around these obstacles, leaving isolated unbroken ligaments in its wake. As
the crack propagation continues, and the crack faces open, the ligaments that are well-behind the
crack tip rupture. Figure 5.30 schematically illustrates this postulated mechanism. The energy
required to rupture the ductile ligaments may provide the majority of the propagation resistance a
cleavage crack experiences. The concentration of ductile ligaments on a fracture surface increases
with temperature [49], which may explain why crack-arrest toughness (K ) exhibits a steep brittle-
Ia
ductile transition, much like K and J .
Ic
c