Page 144 - Fiber Fracture

STRENGTH OF GLASS FIBERS 129

INTRODUCTION

Fibers constitute a major component of the glass industry. The traditional fiber-
glass companies manufacture either reinforcement fibers (E-glass being the principal
composition, see Table 1) or fibers for thermal and acoustic insulations (Gupta, 1988;
Dwight, 2000). During the last two decades, a new glass fiber industry, namely the
fiber optics industry, has seen explosive growth based on the use of silica glass fibers
as optical waveguides (Izawa and Sudo, 1987; Hecht, 1999). These applications require
high tensile strengths over long periods of time (as much as 20 years in the fiber-optic
telecommunication applications). As a consequence, there has been a great interest in
the past in studying the strength of glass fibers and much has been learned and reviewed
in the literature (Kurkjian, 1985; Kurkjian et al., 1993).
Presently, glass fibers are produced routinely with strengths which are more than
adequate for their intended use. For example, silica fibers are manufactured with a
proof-tested strength of 700 MPa. With the achievement of such high strengths, the tech-
nological interest in furthering the understanding of strength of glass fibers has subsided
in recent years and has shifted, instead, towards improving the production efficiencies
by elimination of the sources of low strength fibers (Gulati, 1992). Nonetheless, there
remains a strong fundamental interest in the strength of glass fibers for two reasons.
(1) The strength of pristine (flaw free) fibers is the ‘intrinsic’ strength of a glass
composition.
Intrinsic strength refers to strength of a glass containing no flaws either in the bulk
or on the surface. The intrinsic strength is determined by the composition and structure
of a glass. Strength controlled by flaws is called extrinsic. Unlike intrinsic strength,
extrinsic strength is not a unique function of glass composition. While much has been
learned about extrinsic strength, the understanding of intrinsic strength remains poor
and unsatisfactory. For example, answers to simple questions such as the ones listed
below are not available at present.
(1) How does intrinsic strength vary with composition of a glass?

Table I. Compositions of technologically important glass fibers
Composition E-glass S-glass Silica
(wt%)
Si02 52-56 65 100
A1203 12-16 25
B203 5-10
CaO 16-25
MgO 0-6 10
Na20 + K20 &2
Fiber information
Diameter (Km) 5-30 125
Composition profile homogeneous core/clad
Method of making melt preform