Page 164 - Fiber Fracture

STRENGTH OF GLASS FIBERS 149

Table 2. Intrinsic strengths and related properties of silica and E-glass fibers
E-glass Silica
S; (GPa) 6.0 14
C,; (nm) 7.2 1 .O
&; (%) 8.3 20
S* (RT) (GPa) 3.6 6.0
C* (RT) (nm) 20 5.6
SA IS' (RT) 1.7 2.3
E (GPa) 72 70
K, (MPa m'/*) 0.9 0.8

in Table 2 along with the strength values and the values of pertinent fracture mechanical
parameters for two glass compositions (E-glass and silica). The crack size corresponding
to inert intrinsic strengths are about 1 nm in silica and about 7 nm in the case of E-glass
fibers.
Before speculating on the nature of the nm size flaws, we should emphasize that
because the strength is known to be affected by the environmental humidity, the strength
controlling flaws must be on the fiber surface. Could these flaws be related to the
nanoscale intrinsic roughness which has been observed on the surface of pristine fibers
(Gupta et al., 2000)? The measured RMS roughness on pristine silica fiber surface is
about 0.2 nm and the peak-to-valley roughness is about 1.5 nm. Thus it seems possible
that because of its random nature, the roughness may act like a classic Griffith elliptical
notch at some locations on the pristine surface, and may lead to stress concentrations
which ultimately control the pristine inert strength of fibers. This is also supported by
the work of Yuce et al. (1992) who studied the increase in surface roughness upon aging
in silica fibers and found that the strength decreases with increase in surface roughness
(see Fig. 13 and Kurkjian et al., 1993), approximately linearly on a log-log plot with
a slope of approximately -0.22. Clearly, more work is needed to examine in detail the
role of intrinsic surface roughness as a source of intrinsic flaw controlling the strength
of pristine fibers.
What other nm size features, present in the structure of a glass, can control the
intrinsic strength of glass fibers? The structure of E-glass is complex and not well
studied. But the structure of silica glass has been very well studied (Grimley et al.,
1990). A 1 nm size scale in silica structure corresponds approximately to the size of
two planar six-membered rings in the silica structure. If a siloxane bond is broken,
the elongated opening created by the two neighboring rings is about 1 nm long. This
feature may control the intrinsic strength of silica fibers. There is evidence of broken
bonds in silica fibers. For example, it has been shown (Hibino and Hanafusa, 1985)
that application of tensile stress leads to generation of point defects (Le., broken bonds)
such as E' and NBOHC centers in silica fibers. These can be detected using either
ESR or photo-luminescence techniques. Kokura et al. (1989) have demonstrated by
ESR the generation of point defects in silica glass during mechanical crushing. Thus, it
appears that nm size cavities may exist in pristine fibers. Whether they are the strength
controlling flaws remains to be studied.