Page 287 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

1656_C006.fm Page 267 Monday, May 23, 2005 5:50 PM

Fracture Mechanisms in Nonmetals 267

FIGURE 6.8 Craze zone in polypropylene. Photograph provided by M. Cayard.

Fracture occurs in a craze zone when individual fibrils rupture. This process can be unstable
if, when a fibril fails, the redistributed stress is sufficient to rupture one or more neighboring fibrils.
Fracture in a craze zone usually initiates from inorganic dust particles that are entrapped in the
polymer [16]. There are a number of ways to neutralize the detrimental effects of these impurities,
including the addition of soft second-phase particles.
Crazing and shear yielding are competing mechanisms; the dominant yielding behavior depends
on the molecular structure, stress state, and temperature. A large hydrostatic tensile component in the
stress tensor is conducive to crazing, while shear yielding favors a large deviatoric stress component.
Each yielding mechanism displays a different temperature dependence; thus the dominant mechanism
may change with temperature.

6.1.2.3 Crack-Tip Behavior
As with metals, a yield zone typically forms at the tip of a crack in polymers. In the case of shear
yielding, the damage zone resembles the plastic zone in metals, because slip in metals and shear in
polymers are governed by similar yield criteria. Craze yielding, however, produces a Dugdale-type
strip-yield zone ahead of the crack tip. Of the two yielding mechanisms in polymers, crazing is
somewhat more likely ahead of a crack tip, because of the triaxial tensile stress state. Shear yielding,
however, can occur at crack tips in some materials, depending on the temperature and specimen
geometry [17].
Figure 6.9 illustrates a craze zone ahead of a crack tip. If the craze zone is small compared to
4
the specimen dimensions, we can estimate its length ρ from the Dugdale-Barenblatt [18, 19]
strip-yield model:

π  K  2
ρ =  I  (6.17)
8  σ 
c

4 Another implicit assumption of Equation (6.17) is that the global material behavior is linear elastic or linear viscoelastic.
Chapter 8 discusses the requirements for the validity of the stress-intensity factor in polymers.