Page 145 - Fiber Fracture

130 P.K. Gupta

(2) How is intrinsic strength related to other intrinsic properties like Young’s modulus
or surface tension of a glass?
(3) Is intrinsic strength controlled by crack nucleation in a flaw-free glass?
(4) Is there any role of structural or stress relaxation in determining the intrinsic strength
of a glass?
(2) Pristine fibers exhibit fatigue much like fibers containing microcracks. However,
the mechanisms of fatigue in the absence of cracks are not well understood at present.
Strength of a glass fiber depends on the way it is measured. In other words, the
measured strength is a function of testing variables such as the type of test (pure
tension or bend test), the strain rate used, and the relative humidity and the temperature
of the testing environment. The testing parameters influence the measured value of
the strength because of a stress-enhanced environment-induced phenomenon known as
fatigue. Strength measured in the absence of fatigue is called inert strength. Fatigue
causes the strength of a glass sample to degrade with time in the presence of stress.
Because of fatigue, failure can occur after a sufficiently long time at values of stress
much less than the inert strength. The mechanism of fatigue in glasses containing cracks
is reasonably well understood. It occurs as a result of a chemical reaction between
adsorbed water and the stressed siloxane bond at the crack tip leading to slow growth of
the crack length (Wiederhorn et al., 1980):
(tensile stress)
H20 (adsorbed) + =Si-0-Sir + 2Si-OH
(Siloxane bond) (broken bonds)

Interestingly, pristine fibers also exhibit fatigue which is qualitatively similar 10
fatigue in fibers with cracks. However, the mechanism of fatigue in the absence of
cracks is not clear at present. Such an understanding is required for the estimation of the
life times of fibers under low levels of stress (Gupta et al., 2000).
The purpose of this paper is to provide a review of strength of bare glass fibers.
The emphasis is on fundamental aspects. Methods to protect the high strength of fibers,
for example by application of coatings (Kurkjian et al., 1993; Dwight, 2000), are not
included. Much of the data discussed throughout this paper are for two compositions:
silica and E-glass (see Table 1). This is because these two compositions have been
studied most extensively and reliable data under a variety of test conditions are available
from several independent sources.
The basic concepts of glass fiber strength are summarized first along with an
introduction of the relevant terminology. The subject of glass fiber strength naturally
partitions into four separate categories: (a) extrinsic, inert strength; (b) extrinsic, fatigue
Strength; (c) intrinsic, inert strength; and (d) intrinsic, fatigue strength. Extrinsic strength
is discussed next as it is better understood than intrinsic strength. This provides a sound
basis for subsequent discussion of intrinsic strength which is followed by concluding
remarks.