FAILURE MECHANISMS
4.7 Fatigue
Reported accounts of fatigue mechanisms in textile composites are few and
incomplete. For 3D textile composites, even basic fatigue data, without any observations of
mechanisms, are unavailable for some important loading conditions, especially non-aligned
or multi-axial loads. This survey is correspondingly imperfect. Nevertheless, the available
data do reveal some consistent mechanisms in many classes of textiles, inviting
generalizations that may turn out to apply to all or at least most cases.
1. For cyclic loads aligned with one primary fiber orientation, the local failure events
are much the same as they are in monotonic loading, viz. microcracking; delaminations,
including interply and intraply delaminations in 2D composites; kink band formation; and
tow rupture.
2. In high cycle fatigue, microcracking tends to be less abundant than in monotonic
loading, because damage accumulates inside aligned tows at a few favourable locations at
stresses below the threshold for damage elsewhere. The favourable locations are
determined by the textile architecture and by geometrical imperfections (especially tow
misalignments).
3. At least in 3D interlock weaves, fatigue damage accumulates more rapidly on the
compressive phase of the loading cycle.
4. In parallel with their damage tolerance in monotonic loading, 3D textiles gradually
accumulate spatially distributed fatigue damage. In strain control, they may have extensive
life beyond the first tow failures or the first significant loss of stiffness.
4.7.1 2D Weaves and Braids
Observations for triaxial braids are available for tension-tension fatigue only [4.15].
For loading aligned with the axial tows, fatigue damage commences with tensile
microcracking in resin pockets and microcracking within bias tows that may be driven by
either tension or shear, depending on the bias angle. The composite stiffness declines
rapidly during this phase. After further cycles, the decline in stiffness slows, but axial tows
become disbonded from the bias tows, eventually splitting and rupturing.
