Page 292 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

1656_C006.fm Page 272 Monday, May 23, 2005 5:50 PM

272 Fracture Mechanics: Fundamentals and Applications

FIGURE 6.14 Schematic structure of fiber-reinforced composites.

6.1.3.2 Delamination

Out-of-plane tensile stresses can cause failure between plies, as Figure 6.15(b) illustrates. Stresses
that lead to delamination could result from the structural geometry, such as if two composite panels
are joined in a “T” configuration. Out-of-plane stresses, however, also arise from an unexpected
source. Mismatch in Poisson’s ratios between plies results in shear stresses in the x-y plane near the
ply interface. These shear stresses produce a bending moment that is balanced by a stress in the z
direction. For some lay-up sequences, substantial out-of-plane tensile stresses occur at the edge of
the panel, which can lead to the formation of a delamination crack. Figure 6.16 shows a computed
σ distribution for a particular lay-up [22].
z
Although the assumption of a self-similar growth of a dominant crack often does not apply to
the failure of composite materials, such an assumption is appropriate in the case of delamination.
Consequently, fracture mechanics has been very successful in characterizing this failure mechanism.