Page 82 - Fiber Fracture

FORMS OF FIBRE FRACTURE 67

Fig. 9. Tensile fatigue breaks. (a-d) Nylon. (e-g) Polyester. For further explanation, see Fig. 1.

surface wear dominates, so that other methods of flex testing, such as repeated buckling
without pin contact, must be used. In nylon and polyester fibres, surface wear dominates
at high tensions, but it is easy to find conditions in which the broken fibre shows only
minor wear along the line of contact. These are true flex failures, but their form depends
on whether the bending forces over the zone of maximum curvature or the shear forces
along the zone of variable curvature off the pin cause the breakage.

Kink-Band Failure

Most polymer fibres yield at a much lower stress in compression than in tension.
This is due to the formation of kink bands at an angle of about 45" to the fibre axis,
which can be seen internally in polarised light microscopy or projecting on the surface
in scanning electron microscopy, Fig. 10a,b. Due to the difficulty of applying uniform
axial compression, because of whole-fibre buckling, kink bands are usually observed on
the inside of bends. After a single bend, the kink bands can be pulled out and there is no
appreciable loss of strength. Repeated cycling leads to progressively increased damage
at the kink bands, similar to crazing, and eventual rupture across about half the fibre,
Fig. IOc,d. Further cycling leads to complete failure. It is common for a central axial
split to appear, but it is not known whether this is present before the break, due to the
maximum shear stress at the centre of the fibre, or whether it comes later due to the
shear stress at the tip of the break.
Kink-band breaks are easily recognised by their sharp angular form, Fig. 10e.