FAILURE MECHANISMS
fabrication. With this proviso, delamination is not the strength limiting mechanism even
after impact [4.19].
4.2.4 3D Braids
Detailed observations of the mechanisms of failure in 3D braids have not been
reported. Experiments have been conducted in load control rather than displacement
control, which frustrates attempts to identify kink bands post mortem. However, if the
braid architecture contains significant proportions of nominally straight, aligned tows, it is
very likely that their kinking will be the primary mechanism of composite failure. By
delocalizing mechanisms similar to those operating in 3D interlock weaves, one might again
expect very high strains to ultimate failure in favourable cases.
4.3 Monotonic Tension
Under aligned tension, almost all load is borne by the aligned fibers. Their rupture
is the primary failure mechanism and determines ultimate strength. However, depending on
the fiber architecture, tensile or shear failures of the matrix can cause nonlinearity at much
lower loads.
Tow rupture strengths in weaves or braids are usually ~30-50% lower than would
be expected from the properties of pristine fibers or comparable unidirectional tape
laminates (Sect. 3). For example, AS4 carbon tows with an internal volume fraction of
70% apparently have rupture strengths near 1.5 Gpa in 3D interlock weaves [4.1]. The
manufacturer quotes 4 GPa for the strength of AS4 fibers
1
while data for equivalent tape
laminates [4.17] imply tow ply strengths exceeding 2 GPa.
Unidirectional composites have lower strengths than implied by bare fiber strength
data because flaws on neighbouring fibers in a composite are coupled by the matrix. When
one fiber fails in a bundle of bare fibers, the load it sheds is picked up equally by all
surviving fibers, without any small group of them bearing excessive loads. This favours
higher bundle strength. When one fiber fails in a polymer composite, in contrast, there is a
significant stress concentration acting on its neighbours [4.20]. Nearby flaws in separate
fibers are thus grouped into larger flaws and composite strength tends to be lower.
Various mechanisms can be conjectured to explain why tow strengths in textiles are
even lower still. 1) The rigours of textile processing are always likely to damage tows. 2)
1
Data sheets of Hercules Inc., Wilmington, Delaware.
