Page 298 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

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278 Fracture Mechanics: Fundamentals and Applications

FIGURE 6.22 Fiber waviness in a graphite-epoxy composite. Photograph provided by A.L. Highsmith. Taken
from Highsmith, A.L. and Davis, J., “The Effects of Fiber Waviness on the Compressive Response of Fiber-
Reinforced Composite Materials.” Progress Report for NASA Research Grant NAG-1-659, NASA Langley
Research Center, Hampton, VA, 1990.

fracture mechanics methodology [33]. A compression after impact test is a common screening
criterion for assessing the ability of a material to withstand impact loading without sustaining
significant damage.

6.1.3.4 Notch Strength

The strength of a composite laminate that contains a hole or a notch is less than the unnotched strength
because of the local stress concentration effect. A circular hole in an isotropic plate has a stress
concentration factor (SCF) of 3.0, and the SCF can be much higher for an elliptical notch (Section 2.2).
If a composite panel with a circular hole fails when the maximum stress reaches a critical value, the
strength should be independent of the hole size, since the SCF does not depend on radius. Actual
strength measurements, however, indicate a hole size effect, where strength decreases with increasing
hole size [34].
Figure 6.23 illustrates the elastic stress distributions ahead of a large hole and a small hole.
Although the peak stress is the same for both holes, the stress concentration effects of the large
hole act over a wider distance. Thus the volume over which the stress acts appears to be important.
Whitney and Nuismer [35] proposed a simple model for notch strength, where failure is assumed
7
to occur when the stress exceeds the unnotched strength over a critical distance. This distance is
a fitting parameter that must be obtained by experiment. Subsequent modifications to this model,
including the work of Pipes et. al. [36], yielded additional fitting parameters, but did not result in
a better understanding of the failure mechanisms.
Figure 6.24 shows the effect of notch length on the strength panels that contain elliptical center
notches [34]. These experimental data actually apply to a boron-aluminum composite, but polymer
composites exhibit a similar trend. The simple Whitney and Nuismer criterion gives a reasonably
good fit of the data in this case.

7 Note the similarity to the Ritchie-Knott-Rice model for cleavage fracture (Chapter 5).