Page 300 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

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280 Fracture Mechanics: Fundamentals and Applications

are geometry dependent, become significant. For K to define uniquely the crack-tip conditions and
be a valid failure criterion, all nonlinear material behavior must be confined to a small region inside
the singularity zone. This theory is based entirely on continuum mechanics. While metals, plastics,
and ceramics are often heterogeneous, the scale of microstructural constituents is normally small
compared to the size of the singularity zone; thus the continuum assumption is approximately valid.
For LEFM to be valid for a sharp crack in a composite panel, the following conditions must
be met:

1. The fiber spacing must be small compared to the size of the singularity zone. Otherwise,
the continuum assumption is invalid.
2. Nonlinear damage must be confined to a small region within the singularity zone.

Harris and Morris [38] showed that K characterizes the onset of damage in cracked specimens,
but not ultimate failure, because the damage spreads throughout the specimen before failure, and
K no longer has any meaning. Figure 6.25 illustrates a typical damage zone in a specimen with a
sharp macroscopic notch. The damage, which includes fiber/matrix debonding and matrix cracking,
actually propagates perpendicular to the macrocrack. Thus the crack does not grow in a self-similar
fashion.
One of the most significant shortcomings of tests on composite specimens with narrow slits is
that defects of this type do not occur naturally in fiber-reinforced composites; therefore, the geometry
in Figure 6.25 is of limited practical concern. Holes and blunt notches may be unavoidable in a design,
but a competent design engineer would not be foolish enough to include a sharp notch in a load-
bearing member of a structure.

6.1.3.5 Fatigue Damage

Cyclic loading of composite panels produces essentially the same type of damage as monotonic
loading. Fiber rupture, matrix cracking, fiber/matrix debonding, and delamination all occur in
response to fatigue loading. Fatigue damage reduces the strength and modulus of a composite
laminate, and eventually leads to total failure.
Figure 6.26 and Figure 6.27 show the effect of cyclic stresses on the residual strength and modulus
of graphite/epoxy laminates [39]. Both strength and modulus decrease rapidly after relatively few
cycles, but remain approximately constant up to around 80% of the fatigue life. Near the end of the
fatigue life, strength and modulus decrease further.

FIGURE 6.25 Sharp notch artificially introduced into
a composite panel.