Page 294 - Fiber Fracture

276 J.W.S. Hearle

were observed, such as splits that terminated at both ends without breaks and peeling of
a layer from the surface.

Surface Abrasion

The shear stresses associated with surface rubbing cause severe abrasion of the
surface of aramid fibres. In yams that have been carelessly handled, fibrillation is clearly
apparent. In laboratory tests with rubbing on a pin, the surface becomes worn away until
break occurs.

Axial Compression, Bending and Flex Fatigue

At the molecular level, HM-HT polymer fibres can be regarded as lightly bonded
assemblies of slender rods, which will buckle under axial compression. It is not easy to
put whole fibres into uniform axial compression, because of buckling at the fibre level,
and most studies are from compression on the inside of a bend. Compressive yield stresses
of an HMPE fibre, estimated from recoil studies, have been reported by Allen (1987) to
be only 0.07 GPa, 2.5% of the tensile strength; for Kevlar, he reports 0.37 GPa, 11 %
of tensile strength. Using estimates from bending tests, van der Zwaag and Kampschoer
(1987) report values of 0.5 and 0.9 GPa for aramid (Twaron) fibres. Sikkema (2001) gives
compressive strengths of 0.4 GPa for PBO, 0.58 GPa for Twaron, but 1.6 GPa for M5
fibre. This order of values from HMPE to M5 reflects the increase in transverse bonding.
The effects of axial compression are seen internally in fibres viewed in polarised
light microscopy and coming out of the surface in SEM pictures (e.g. Fig. loa). The
yielding on the compressive side allows the neutral plane to move outwards and so
prevents the tensile strain on the outside reaching its break value. Schoppee and Skelton
(1974) found that Kevlar, as well as other lower-modulus fibres, could be bent back on
itself without breaking. An interesting exception to this rule for polymer fibres was an
experimental X500 Monsanto fibre shown in Fig. lob. This has broken on the tension
side, but with axial splitting as distinct from the classical brittle fracture that would
occur in glass and similar fibres. The additional hydrogen bonding shown in Fig. 1 may
be the cause of this difference. Weakness in shear is commonly seen as a disadvantage,
but it does eliminate brittleness in bending.

Fig. 10. (a) Bent Kevlar 49 fibre. (b) Bent X-500 fibre. From Schoppee and Skelton (1974).