ANALYTICAL METHODS FOR TEXTILE COMPOSITES
4.2.2 Stitched and Stitched-Knitted Laminates
Stitching, either alone or in combination with weaving (usually uniweave plies) or
knitting, is very effective in suppressing delamination. In the absence of delamination,
failure in monotonic compression commonly occurs via kink band formation [4.18]. The
critical stress for kinking is generally much higher than in 2D weaves and braids, because
the in-plane fibers in stitched uniweave or stitched-knitted laminates tend to be better
aligned. Stitched uniweave and stitched/knitted laminates are good candidates for
applications involving compression.
When a kink band forms in a stitched laminate, it propagates unstably through the
thickness of the specimen [4.18]. The load-bearing capacity of the material drops
immediately to zero; the material exhibits brittle failure.
4.2.3 3D Weaves
Under aligned loads, 3D interlock weaves also fail by kink band formation
[4.1,4.18]. However, the failure is generally not nearly as brittle as in a stitched laminate.
Each kink band that forms is confined to a single aligned tow and does not spread
catastrophically into neighboring tows. Ultimate failure is only achieved by the cumulative
effect of many distinct kinking events, which may be spread over a substantial volume of
the composite. Strains to ultimate failure of 3-15% have been measured, depending on the
details of the architecture and the test configuration [4.1,4.18].
Misaligned tow segments constitute geometrical flaws, with the strength or critical
local stress for each flaw depending on the misalignment angle via Eq. (4.1). The statistics
of kink formation depend on how these flaws are distributed in strength and space. Broader
distributions favour delocalized kinking sequences and high strain to failure [4.18].
The peak compressive stress also depends on the statistics of the misalignment
fluctuations in the aligned tows. The highest values of misalignment tend to be somewhat
higher than in tape laminates, at least in current 3D interlock weaves [4.1]. The peak load is
commensurately lower than in equivalent tape laminates.
Local delaminations are frequently observed in 3D weaves under compression. But
delamination crack growth is limited and Euler buckling of delaminated tows is not a failure
path provided the through-thickness reinforcement is not heavily crimped during
