Page 254 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

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234 Fracture Mechanics: Fundamentals and Applications

FIGURE 5.17 Optical micrograph (unetched) of ductile crack growth in an A 710 high-strength low-alloy
steel. Photograph courtesy of J.P. Gudas. Taken from McMeeking, R.M. and Parks, D.M., ‘‘On Criteria for
J-Dominance of Crack-Tip Fields in Large-Scale Yielding.” ASTM STP 668, American Society for Testing
and Materials, Philadelphia, PA, 1979, pp. 175–194.
5.2 CLEAVAGE

Cleavage fracture can be defined as the rapid propagation of a crack along a particular crystallo-
graphic plane. Cleavage may be brittle, but it can be preceded by large-scale plastic flow and ductile
crack growth (see Section 5.3). The preferred cleavage planes are those with the lowest packing
density, since fewer bonds must be broken and the spacing between planes is greater. In the case
of body-centered cubic (BCC) materials, cleavage occurs on {100} planes. The fracture path is
transgranular in polycrystalline materials, as Figure 5.1(b) illustrates. The propagating crack
changes direction each time it crosses a grain boundary; the crack seeks the most favorably oriented
cleavage plane in each grain. The nominal orientation of the cleavage crack is perpendicular to the
maximum principal stress.
Cleavage is most likely when the plastic flow is restricted. Face-centered cubic (FCC) metals
are usually not susceptible to cleavage because there are ample slip systems for ductile behavior
at all temperatures. At low temperatures, BCC metals fail by cleavage because there are a limited
number of active slip systems. Polycrystalline hexagonal close-packed (HCP) metals, which have
only three slip systems per grain, are also susceptible to cleavage fracture.
This section and Section 5.3 focus on ferritic steel, because it is the most technologically important
(and the most extensively studied) material that is subject to cleavage fracture. This class of materials
has a BCC crystal structure, which undergoes a ductile-brittle transition with decreasing temperature.
Many of the mechanisms described below also operate in other material systems that fail by cleavage.

5.2.1 FRACTOGRAPHY

Figure 5.18 shows SEM fractographs of cleavage fracture in a low-alloy steel. The multifaceted
surface is typical of cleavage in a polycrystalline material; each facet corresponds to a single grain.
The ‘‘river patterns’’ on each facet are also typical of cleavage fracture. These markings are so
named because multiple lines converge to a single line, much like tributaries to a river.
Figure 5.19 illustrates how river patterns are formed. A propagating cleavage crack encounters
a grain boundary, where the nearest cleavage plane in the adjoining grain is oriented at a finite