ANALYTICAL METHODS FOR TEXTILE COMPOSITES
4.7.2 3D Interlock Weaves
Just as under monotonic loading, the principal failure mechanism for interlock
weaves subject to aligned cyclic compression is kink band formation [4.37]. The kinks
form in segments of nominally straight tows that have unusually high misalignment (e.g.,
Fig. 4-11). Fatigue damage accumulation apparently consists of damage to the resin within
individual tows. This may either allow rotation of fibers and an increase of the
misalignment angle,
φ
, of the affected segment; or a gradual lowering of the critical shear
flow stress,
σ
c
[4.37,4.38]. Some evidence indicates that the latter is more likely in
interlock weaves [4.37]. In either case, the criterion for kink band formation, Eq. (4.1),
will eventually be satisfied and kink failure will occur in the tow.
Figure 4-11. A kink band formed in fatigue in a misaligned segment of a stuffer in
an AS4/1895 layer-to-layer interlock weave (from [4.37]).
Just as in monotonic loading, a kink failure in one tow does not usually propagate
unstably into neighbouring tows. The failed tow debonds from the surrounding composite
via a peripheral matrix crack, minimizing local stress concentrations. Since misaligned
segments, which act as geometrical flaws, tend to be broadly distributed in space in 3D
interlock weaves, subsequent kinking tends to occur elsewhere in the composite [4.18].
Ultimate failure under strain control might involve a widely distributed accumulation of
separate kink events.
References
4.1 B. N. Cox, M. S. Dadkhah, and W. L. Morris, "Failure Mechanisms for 3D Woven
Composites in Tension, Compression, and Bending," Acta Metall. Mater. 42 (1994)
3967-84.
