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OCR for page 253
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Appendix C
FIBER-QUALITY PARAMETERS OF SELECTED ASBESTOS,
WHISKER, AND GLASS FIBERS
The strength-diameter effect of fine wires and fibers has been
discovered and rediscovered several times during the lent 250 years.
Each t ime it excited great interest within the sc lent if ic community but
was soon doubted and finally ignored after a few years. It is difficult
to explain the reasons for this cyclical interest in the
strength-diameter effect. One reason could be that no satisfactory
theory has been developed and experimentally proven. Today, it is
possible to observe surface defects and other submicroscopic features,
such as the dissolution pattern of fibers, but the surface structure of
fine fibrous substances still cannot be determined.
In the early 1800s, Karmarsch (1834) completed an extensive and
systematic study on the strength-tiameter effect of 18 different metal
and alloy wires. In 1859, he derived an equation expressing the
relationship between the increasing strength (F.) and the decreasing
diameter (D) of small-diameter wires. His equation (rephrased by
Griffith, 1921) is:
F = A + B/D,
where A and B are constants.
(C-1)
The constant A in the Karmarsch equation (C-1) was interpreted by
Orowan ( 1933) 88 the strength of the internal structure and B as the
strength of the surface structure. However, the Karmarsch equation did
not satisfy all the more than 100 available strength-diameter
measurements. This problem was recently resolved (Zoltai, 1981) by
changing the surface area-to-volume ratio (BID in the Karmarsch equation)
to incorporate other features of the surface structure, e.g., the depth
of the surface layer, the presence of growth steps (Marsh, 1962), and the
effect of longitudinal cleavages (Cook ant Gordon, 1964~. Thus,
of ~ hi (1 + 4R/D)1 + 4k/D
(C-2)
where of is the strength of the fiber, Al is its internal strength, and R
and k are factors expressing the increased strength of the surface layer over
that of the internal structure.
253
OCR for page 254
254
Table C-1 shows the fiber characterist iC8 (internal strength, K and k
constants, etc.) of a selected group of natural and synthetic fibers.
The values shown in the table illustrate the general characteristics of
the examples and the relative magnitudes of the parameters given.
Experimental data in reports by various researchers are difficult to
compare because they often use different expressions of strength,
different methods, and different unite of measurement. Further
limitations result from inaccurate readings of strength and diameter
values from published small-scale graphs.
SIGNIFICANCE OF THE FIBER-QUALITY PARAMETERS
From the measurement of the tensile strength of sets of fibers and
from subsequent calculations (using the above equations), one can
calculate: (1) the internal strength of the fibers (of) and (2) two
constants (K and k), which express the relative increase in the strength
of the surface structure.- There parameters reflect the differences in
the mechanical properties of fibers that grew under different conditions
or that were modified by treatment and wear.
The following conclusions about fiber-quality parameters may be
relevant to the potential health effects of fibers:
( 1) The mechanical properties of fibers are directly related to the
common properties of asbestifonm fibers. Consequently, the three
parameters can be used an numerical indicators of the degree of
asbestifo`= development of fibers.
(2) - The two constants, K and k, must be positive for asbestiform
fibers in order to account for their enhanced strength and flexibility.
- The K and k constants must be equal to zero for crystals that
have no enhanced strength despite a defect-free surface structure.
- The R and k constants must be negative for cleavage fragments
and other fragments whose surfaces are weaker than their internal
structure because of the physical damage introduced by fracturing and
subsequent processes.
(3) Because of the interdependent nature of the common asbestiform
fiber properties, these three parameters may include a direct or indirect
measure of the critical physicochemical property or properties that may
be primarily responsible for the adverse health effects of asbestifonm
fibers. However, the nature of relationship between the fiber quality
and carcinogenic potential of asbestiform fiber" is still unknown.
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259
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OCR for page 260
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Representative terms from entire chapter:
mechanical properties
