Page 288 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

1656_C006.fm Page 268 Monday, May 23, 2005 5:50 PM

268 Fracture Mechanics: Fundamentals and Applications

FIGURE 6.9 Schematic crack-tip craze zone.

which is a restatement of Equation (2.74), except that we have replaced the yield strength with σ ,
c
the crazing stress. Figure 6.10 is a photograph of a crack-tip craze zone [17], which exhibits a
typical stress-whitening appearance.
The crack advances when the fibrils at the trailing edge of the craze rupture. In other words,
cavities in the craze zone coalesce with the crack tip. Figure 6.11 is an SEM fractograph of the
surface of a polypropylene fracture toughness specimen that has experienced craze-crack growth.
Note the similarity to fracture surfaces for microvoid coalescence in metals (Figure 5.3 and
Figure 5.8).
Craze-crack growth can either be stable or unstable, depending on the relative toughness of the
material. Some polymers with intermediate toughness exhibit sporadic, so-called stick/slip crack
growth: at a critical crack-tip-opening displacement, the entire craze zone ruptures, the crack arrests,
and the craze zone reforms at the new crack tip [3]. Stick/slip crack growth can also occur in materials
that exhibit shear yield zones.

6.1.2.4 Rubber Toughening
As stated earlier, the rupture of fibrils in a craze zone can lead to unstable crack propagation.
Fracture initiates at inorganic dust particles in the polymer when the stress exceeds a critical value.
It is possible to increase the toughness of a polymer by lowering the crazing stress to well below
the critical fracture stress.

FIGURE 6.10 Stress-whitened zone ahead of a crack tip, which indicates crazing. Photograph provided by
M. Cayard.