Page 299 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

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Fracture Mechanisms in Nonmetals 279

FIGURE 6.23 Effect of hole size on local stress dis-
tribution.

Some researchers [37] have applied fracture mechanics concepts to the failure of composites
panels that contain holes and notches. They assume failure at a critical K, which is usually modified
with a plastic zone correction to account for subcritical damage. Some of these models are capable
of fitting experimental data such as that in Figure 6.24, because the plastic zone correction is an
adjustable parameter. The physical basis of these models is dubious, however. Fracture mechanics
formalism gives these models the illusion of rigor, but they have no more theoretical basis than the
simple strength-of-materials approaches such as the Whitney-Nuismer criterion.
That linear elastic fracture mechanics is invalid for circular holes and blunt notches in com-
posites should be self evident, since the LEFM theory assumes sharp cracks. If, however, a sharp
slit is introduced into a composite panel (Figure 6.25), the validity (or lack of validity) of fracture
mechanics is less obvious. This issue is explored below.
Recall Chapter 2, which introduced the concept of a singularity zone, where the stress and
strain vary as 1/ π from the crack tip. Outside of the singularity zone, higher-order terms, which

FIGURE 6.24 Strength of center-notched composite laminates, relative to the unnotched strength. Taken from
Awerbuch, J. and Madhukar, M.S., “Notched Strength of Composite Laminates: Predictions and Experi-
ments—A Review.” Journal of Reinforced Plastics and Composites, Vol. 4, 1985, pp. 3–159.