Page 148 - Fiber Fracture

STRENGTH OF GLASS FIBERS 133

smaller-diameter E-glass fibers (5-20 pm), testing in pure tension is the only method
available at present for measuring intrinsic strengths.
It should be pointed out that the intrinsic strength of a glass is not a constant.
The intrinsic strength values for a given composition vary with testing conditions
(environmental humidity, and temperature).

Extrinsic Strength, S

Flaw-controlled strengths are called extrinsic. The strengths of non-pristine fibers are
extrinsic by definition. Extrinsic strengths show larger COV than can be accounted for
from fiber diameter variations alone. The additional variation in strength arises from the
variation in the severity of the most severe flaws in different samples. Gupta (I 987) has
carried out a detailed analysis of the measured strength distributions by combining the
variations in fiber diameter and in the flaw-severity statistics.

Inert Strength, SO

Strength measured in the absence of fatigue is called the inert strength. Inert strength
should not be confused with intrinsic strength. Inert refers to absence of fatigue while
intrinsic refers to absence of flaws. In this paper, the inert strength is denoted by SO.
In principle, inert strength can be measured by using testing conditions which
minimize fatigue (such as dry environment or vacuum and sufficiently large strain rates
so that the fatigue reaction does not have time to progress). All these approaches have
been attempted with varying degrees of success. However, inert strengths are measured
most conveniently by testing at the liquid Nz temperature (77 K) where the rate of the
fatigue reaction is sufficiently small to be considered negligible. Several experiments
have demonstrated that the measured strength at room temperature under conditions of
high vacuum approaches the measured strength at the liquid nitrogen (LN) temperatures
(France et al., 1980; Roach, 1986; Smith and Michalske, 1989). In other words, there is
no significant temperature dependence of the inert strength. The temperature dependence
from other intrinsic properties such as E is negligible. It follows, therefore, that

So = SLN (4)
where SLN is the strength measured at the liquid Nz temperature.

Theoretical Strength, sfh

The strengths of pristine fibers at the liquid nitrogen temperature, StN, are the highest
strength values measured for a given composition. One might expect StN in the absence
of flaws to be equal to the theoretical strength, S,, of a glass:
s&I = sa (5)
Unfortunately, at present, theoretical estimates of S,h do not exist for multicomponent
compositions (such as E-glass) and are not reliable even for simple one-component
glasses. For example, estimates of the theoretical strength of silica based on cohesive