FAILURE MECHANISMS
failure model (both from [4.5]). The model attributed nonlinearity in shear to degradation
of the matrix stiffness. Once the matrix had yielded (critical maximum principal stress), the
shear stiffness in the model was reduced to 20% of its original value. This approach does
not specify the physical nature of the resulting cracks.
In assessing failure mechanisms in textile composites beyond the proportional limit,
the possibility of shear failure can usually be predicted quite well by comparing the axial
shear stress within individual plies or tows with
τ
c
. In computing the stress-strain
response, reasonable results can usually be obtained by regarding individual plies or tows
as elastic/perfectly plastic in axial shear.
4.2 Monotonic Compression
Quasi-laminar textile composites almost always fail under aligned, monotonic
compression by one of two mechanisms: kink band formation; and delamination, which is
often followed by Euler buckling.
Kind band formation is illustrated by Fig. 4-3. It is a local shear instability in which
a bundle of fibers rotates and ruptures, causing almost total loss of axial strength for the
bundle. The bulk of data confirms that Argon's Law [4.6,4.7]
σ
c
=
τ
c
/
φ
(4.1)
is a serviceable approximation for kink band formation in polymer composites, where
σ
k
is
the critical axial stress for kinking,
τ
c
is the critical shear flow stress of Fig. 4-2(a), and
φ
is
the misalignment angle of the fibers with respect to the applied load (measured in radians).
As Eq. (4.1) shows, the crucial manufacturing issue in optimizing compressive strength is
minimizing the misalignment angle; or, in other words, minimizing fiber or tow waviness.
Fiber defects have a marginal effect on the dynamics and kinetics of shear flow and
fiber rotation; and therefore a negligible effect on the critical stress for kink band formation
in compression. In contrast, the tensile strength of tows is strongly influenced by fiber
defects and therefore handling damage during processing (see Sect. 4.3).
