Page 153 - Fiber Fracture

138 P.K. Gupta

Intrinsic
Region
mH

,

Ln L+
Fig. 2. Length dependence of the extrinsic inert strength of fibers with bimodal distribution.

this reason the fracture initiating flaws in long lengths of fibers are generally different
than those in small lengths of fibers. Decrease of strength with increase in length
or diameter is generally considered a strong indicator of extrinsic strengths. Intrinsic
strengths, on the other hand, are constant with respect to length and diameter. Fig. 2
shows schematically the length dependence of the strength of fibers having a bimodal
Weibull distribution of strengths. Eq. 7, which gives the length and diameter dependence
of the inert extrinsic strength of fibers, is valid only when a single mode is dominant.
Much of the past work on fiber strengths reported in the literature has two intertwined
themes for improvement in the average fiber strength: (1) by adjustments of glass com-
position, and (2) by adjustments in the fiber manufacture process. The first approach,
in principle, requires an understanding of the intrinsic strength as a function of com-
position. However, frequently, composition changes lead to changes in the nature and
density of extrinsic flaws and consequently influence the extrinsic strength. Adjustments
in the process are generally designed to eliminate the sources of low strengths which
are caused by (relatively large) flaws. In addition, large cracks grow rapidly in fatigue
and tend to dominate the fatigue behavior of the long fibers. Thus the strength and
fatigue behavior of large cracks is of considerable technological interest in applications
requiring long lengths (such as glass fibers for telecommunication).