Page 21 - Fiber Fracture

6 K.K. Chawla

As spun (dried)

Strain - (wet)
As coagulated

i kat-treated

Fig. I. Schematic stress-strain curves of rigid-rod polymeric fibers in tension and compression. Such fibers
show very high strength under axial tension but have poor properties under axial compression, torsion, and
in the transverse direction.

organic fibers fail in compression at strains el%. Microbuckling or shear banding is
responsible for easy failure in comprcssion. The spider dragline silk fiber seems to
be an exception to this. In general, highly oriented fibers such as aramid fail in a
fibrillar fashion. The term fibrillar fracture here signifies that the fracture surface is not
transverse to the axis but runs along a number of planes of weakness parallel to the
fiber axis. As the orientation of chains in a fiber becomes more parallel to its axis, its
axial tensile modules (E) increases but the shear modulus (G) decreases, i.e. the ratio
E / G increases tremendously. During failure involving compressive stresses, fibrillation
occurs, which results in a large degree of new surface area. This fibrillation process
results in high-energy absorption during the process of failure, which makes these fibers
useful for resistance against ballistic penetration.
Various models have been proposed to explain this behavior of high-performance
fibers. Fig. 3 shows two compressive failure models: (a) elastic microbuckling of poly-
meric chains; and @) misorientation. The microbuckling model involves cooperative in-
phase buckling of closely spaced chains in a small region of fiber. The misorientation
model takes into account structural imperfections or misorientations that are invariably
present in a fiber. In the composites literature it has been reported that regions of