Page 165 - Fiber Fracture

150 P.K. Gupta

700

H 6oo

300
1 10 100
MAXIMUM (MEOIAN) ROUGHNESS, nm
Fig. 13. Strength of silica fibers as a function of the RMS surface roughness (Yuce et al., 1992)

CONCLUDING REMARKS

The strength of pristine fibers can be classified (a) as intrinsic or extrinsic, and (b)
as inert or fatigue. For most applications and for improved production efficiencies, one
is primarily interested in improving 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.
While the fibers are being produced routinely with strengths adequate for their
respective technological applications, fundamental questions about both the extrinsic
and the intrinsic strengths remain unanswered. For the extrinsic strengths, the important
questions pertain to the identity of the flaws and the role of crack nucleation and
residual stresses around inclusions. The difficulty is partly because these large flaws
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 fibers exhibit fatigue which is qualitatively (and to a large extent
quantitatively) similar to that in non-pristine fibers?
Atomistic modeling of fracture in topologically disordered solids such as silicate
glasses remains at a primitive stage primarily because of a lack of knowledge of the
anharmonic aspects of the interatomic potentials.