Page 143 - Fiber Fracture

128 P.K. Gupta

Fatigue in Pristine Fibers ............................ 146
Concluding Remarks ................................ 150
Acknowledgements ................................. 15 1
References ...................................... 151

Abstract

Present understanding of strength of bare glass fibers is reviewed. Key experimental
results on the strengths of E-glass and silica fibers are examined to identify factors
which control the strength and fatigue in glass fibers. The strength of pristine fibers
can be classified (a) as intrinsic or extrinsic, and (b) as inert or fatigue. For improved
fiber reliability and production efficiencies, one is primarily interested in extrinsic
fatigue strength. On the other hand, for basic understanding of strength in terms of
the structure of glass, one is interested in the intrinsic inert strength and its variation
with composition. While much work has been done in the past, fundamental questions
remain unanswered about both the extrinsic and the intrinsic strengths. For the extrinsic
strengths, the important questions pertain to the identity of the flaws and the role of
crack nucleation around inclusions. The difficulty in studying large extrinsic flaws lies
in the fact that they occur very infrequently (one flaw in hundreds of kilometers of
fiber!). For the intrinsic strengths, the key questions are (a) what determines the intrinsic
strength of a fiber? and (b) why do pristine flawless fibers exhibit fatigue which is
qualitatively (and to a significant extent quantitatively) similar to that in non-pristine
fibers?.

Keywords

Intrinsic strength; Extrinsic strength; Inert strength; Fatigue strength; Flaws; Slow
crack growth; Weibull distribution; Fractography; E-glass; Silica glass