Page 289 - Fiber Fracture

272 J.W.S. Hearle

EXPERIMENTAL OBSERVATIONS

Tensile Failures

An early view of fracture of para-aramid fibres was given by Yang (1993, p. 97). who
refers to three basic forms. The caption to his fig. 3.28 describes fracture morphology
of Kevlar aramid fibre in tensile breaks as: ‘Type (a), pointed break: type (b) fibrillated
break: type (c) kink-band break,” The kink-band breaks, which extend over a length
approximately equal to a fibre diameter can be attributed to fibres that have been
weakened by axial compression and will be discussed in a later section.
The other two types will occur in relatively undamaged fibres. An example of type (a)
was shown in fig. 6c,d and of type (b) in fig. 6b (see paper by Hearle, 3rd paper in this
volume). The axial splitting, whether single or multiple, commonly extends over about
100 fibre diameters. Type (a) shows a gradual tapering towards the tip. Yang (1993, p.
97) points out that the diameter at the final break point is about 2-4 pm compared to 12
pm for the whole fibre: “Thus the true fibre strength based on the fibre cross-sectional
area at break is very high.” If the final break is due to axial tensile failure, when the
reduction in aspect ratio means that tensile rupture is easier than shear cracking, this
implies an ultimate molecular tensile strength of 30 to 100 GPa. There are alternative
explanations of why two forms of break, namely the single split of type (a) and the
multiple splitting of type (b), are observed.
Yang attributes the difference to differences in fibre type and test conditions: “Pointed
fibre breaks are often observed on Kevlar 49 aramid fibres [post-treated to increase initial
modulus] at slow strain rate. It reflects a highly ordered lateral fibre structure and is
generally associated with high fibre strength.”
In contrast to Yang’s view, our SEM studies (Hearle et al., 1998, chapter 7) showed
that the same fibre break could have one end of type (a) and one of type (b). We
attributed this to break starting at a surface flaw and proceeding by a crack which split
into multiple cracks, as shown in Fig. 6a. Necessarily, as shown in Fig. 6b, the upper
bifurcation in this diagram reaches the opposite side of the fibre first, thus naturally
leading to one single-split end of type (a) and one fibrillated end of type (b). The
only way of avoiding this geometrical consequence is if, as shown in Fig. 6c, another
bifurcation moves faster than the uppermost one. However, the splits on the left end then
point in the wrong direction.
If breaks started from internal flaws, both ends would show multiple splitting. It
is also possible that the snap-back after break, which, as shown in Fig. 7, causes
complicated modes of deformation, might lead to multiple splitting of an initial end of
type (a). Breaks of type (a) would occur on both ends if the crack does not bifurcate.
There are probably elements of truth in both explanations. The geometrical expla-
nation for a combination of pointed and fibrillated ends certainly seems valid for the
example quoted, but other scenarios could lead to two pointed ends or two fibrillated
ends. There may be bias towards different combinations with different forms of Kevlar
and Twaron and different test conditions. Examination of a large number of breaks would
be needed to clarify the position. Most SEM studies have been limited to the few studies
needed to show different, and not necessarily statistically common, forms of break.