Page 74 - Fiber Fracture

FORMS OF FIBRE FRACTURE 59

INTRODUCTION

There is great diversity in the way in which fibres fracture, depending on the fibre
type and the mode of application of stress. The subject can be studied at three levels:
the macroscopic applied forces; the microscopic level, which shows the path of fibre
breakage; and the response of the molecules. Attempts at theoretical interpretations of
the gross experimental observations in terms of molecular effects can be misleading,
because abstract statements about the forces involved are related to abstract views
of imperfectly understood structures. The intermediate level, for which the scanning
electron microscope (SEM) provides concrete evidence, must be taken into account in
order to understand the mechanisms of failure.
This paper is intended to set the scene for more detailed discussion by reviewing the
pattern of rupture in different fibres under different conditions. Fig. 1 shows a collection
of different appearances of fibre ends. The first group are tensile breaks, as found in
typical load-elongation tests. The second group are various forms of ‘fatigue’ failure
after repeated loading. The last few include natural ends and other effects, such as
melting, which are not relevant to these proceedings.
The review will necessarily be brief. More examples of the forms of break as seen in
the SEM or occasionally in optical microscopy are included in our book on fibre fracture
(Hearle et al., 1998), which also gives many examples of failure in use.

TENSILE BREAKS

Brittle Fracture

Glass fibres show a classical Griffiths brittle fracture. A smooth crack may run across
the whole fibre, Fig. 2a, but usually the mirror region, which progresses from a flaw,
turns into multiple cracks, the hackle region, Fig. 2b. Breaks of this type occur in
three-dimensionally bonded materials with no yield mechanisms. This group includes
ceramic fibres, Fig. 2c,d, and some carbon fibres, Fig. 2e. There can be deviations
from the simpler forms of Fig. 1. For example, rupture may occur on the plane of
maximum shear stress. All these fibres break at small tensile strains, mostly less than
2%. Surprisingly, the elastomeric fibre, Lycra, Fig. 2f, also shows this type of break
at over 500% extension. However, although this starts extension as a low modulus,
extensible material, it becomes very stiff near the break point.

DuctiZe Failure

The melt-spun thermoplastic fibres, nylon, polyester, polypropylene, show a quite
different form of breakage. In undrawn fibres, which are unoriented or partially oriented,
rupture occurs at the end of a long period of plastic extension at slowly increasing tension.
In oriented fibres, which have been drawn, the stress-strain curve terminates in a short
yield region, the residual plastic extension, before rupture occurs. Break starts as a crack,
usually from a flaw but otherwise self-generated by coalescence of voids, Fig. 3a. The