FAILURE MECHANISMS
In Section 2, textile composites were categorized as quasi-laminar and nonlaminar,
depending on whether or not they can be considered as laminates modified by the inclusion
of relatively few through-thickness fibers. This distinction is particularly useful in
discussing failure mechanisms. Through-thickness reinforcement was in most cases
introduced to combat undesirable failure mechanisms in 2D laminates. The suppression of
these mechanisms should often still be described in the language of laminate mechanics,
with the effects of the through-thickness reinforcement treated as a perturbation. The reader
is therefore encouraged to review the categorization in Section 2 before proceeding here.
However, the terms quasi-laminar and nonlaminar should be used with care. Some
textiles may appear geometrically and behave elastically as obvious quasi-laminates, yet
show stress-strain characteristics and mechanisms of stress redistribution and damage
accumulation that have no parallel in 2D laminates. A prime example is 3D interlock
weaves, which show extraordinary damage tolerance and other properties related to the
work of fracture by mechanisms that are intimately associated with their 3D nature, as will
be seen in the following.
4.1 Shear
Axial shear failure in bundles of fibers in a polymer matrix begins with arrays of
ogive microcracks aligned between pairs of fibers (Fig. 4-1 and [4.1,4.2]). These cracks
grow and sometimes coalesce amidst considerable microplasticity, which probably involves
crazing and fibril tearing. At higher strains, the damaged matrix divides into pieces of
rubble. The corresponding macroscopic constitutive behaviour can be measured
conveniently and fairly representatively by loading
45° laminates in uniaxial tension. Since
the deviatoric stresses within such a specimen are pure shear within all plies, the specimen
stress/strain response is proportional to that of a single ply under axial shear. Viewed over
the range of strains relevant to ultimate failure, the measured response is often
approximately linear/perfectly plastic, with plasticity occurring above a threshold stress,
τ
c
(Fig. 4-2(a) and [4.1,4.2]).
For composites in which the fibers have much higher modulus than the matrix, the
shear behaviour represented by Fig. 4-2(a) is controlled entirely by the properties of the
matrix deforming under the geometrical constraints imposed by the fibers [4.3,4.4]. Thus
τ
c
should be regarded primarily as a matrix property, with some dependence on parameters
