FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

2.2.3 Fiber Strength
The operational and shelf-life of fiberoptic
sensors will depend to a large extent on the mechanical
strength of the glass fibers used in them. In a certain
way, glass fiber is much atronger than steel.
Short, pristine silica fibers, immediately after drawing,
have elastic limits, and ultimate and breaking
tensile strengths, greatly exceeding that of steel
wires. Stress-strain curves for priatine silica fiber
and steel wire are shown in Fig. 2.35. The elaatic
1 I I 1 ! , ( ! , , ,
99 - *# .:
80 -
60 -
./”
./”””
~ 40 - /
u
m
3
~ 20 GAGE LENGTH= 60cm
if
8 -
4
10’0
- 109
“E
~
$ 108
w
u
G
107
106
WIRE
FIBER
10–4 10–3 10–2 10–’ 10°
STRAIN
Fig. 2.35 Stresa versus strain curves for steel wire
and pristine silica (Si02) fiber under ideal
conditions.
limit of steel typically uaed in wire is about 0.2 x
10-9 Newtons/m2 at a atress of about 0.1 percent. Steel
wires tend to break at strains of the order 0.5 percent
and a stress of about 1.5x109 N/m2. On the other hand,
unscratched fibers remain elastic to strains in excess
of 10 percent, corresponding to stresses of about 5 x
109 Newtona per square meter. However, unlike steel,
which may be made malleable and capable of flow-healing
small surface cracks, glass is brittle. Thus, very fine
cracks in glass fibers tend to become stress concentration
centers that propagate transversely across the fiber.
They lead to exceasive strain and ultimately to
complete rupture.
A length of fiber is only as strong as its
weakest section. Under constant tension a length of
fiber will tend to break at its weakest point. The
break will most likely occur where there is a scratch
on the outer surface of the fiber. A long length of
fiber ia more likely to have a weakest point that is
weaker than the weakest point of a short length of
fiber. Thus, fiber strength determination is a procesa
of collecting statistics of failure. This is illustrated
by the reaults of a seriea of tests performed on
a group of test samples, each 60 cm long, cut from sections
distributed uniformly along a 1 km length of fiber.
Each sample was streased to rupture in a tension
test machine. The percentages of the total number of
specimens that failed below a given stress level are
shown as a function of the breaking stress in Fig. 2.36.
Fig.
1 , I I , [
0.5 0.8 10 2.0 3.0 4.0
TENSILE STRENGTH (GN/m2)
2.36 The percentage of optical fiber specimena
that failed as a function of breaking tenaile
strength.
From the graph it may be seen that the first apecimen
broke at approximately 0.5 x 109 N/m2, approximate 10
percent of the specimens broke at 0.8 x 109 N/m J or
leas another 10% broke at atresses between 0.8 and 1.0
x 10 4N/m2, and so forth. A few of the s eclmens, however,
withstood stresses of 4.0 x 109 N/m % before breaking.
From a practical, application-oriented viewpoint,
it is the weakest point in a length of fiber
that determines ita overall strength. Thus, in specifying
fibers for a given application, the maximum
streas or strain to be encountered ahould be determined
and the entire length of fiber to be employed should
be pre-tested at some acceptable safety margin above
this level. Such testing is usually done by reeling
the fiber from one spool to another at a fixed rate,
while maintaining the interreel section under the fixed
specified stress (tension).
There is evidence that preform preparation
and treatment, as well aa the manner in which fibers
are handled after they are drawn, contribute substantially
to their overall atrength. Data on the breaking
or tensile strength of a particular type of fiber drawn
from identically produced preforms is shown in the bargraph
in Fig. 2.37. In the upper portion of Fig. 2.37,
the number of specimens versus breaking strength ia
plotted for 40 specimens taken from a length of fiber
drawn from an ordinarily prepared preform that was
heated as usual in an RF induction furnace. As indicated,
the breaking strengths ranged from approximately
0.50 x 109 N/m2 to 5.5 N/m2 with a maximumof 12 specimena
that broke at 3.0 x 10 4N/m2. Results are presented
in the center aection of Fig. 2.37 on 46 specimens from
a length of fiber drawn, using the RF induction furnace,
from a preform that had been fire-polished prior
to drawing in an effort to eliminate any fine cracks
(microcracks) and other imperfections that might have
existed in its outer surface. In this caae, the first
specimen to break withstood stresses up to 1.8 x 109
N/m2 and 36 of the specimens broke at a stress exceeding
3.8 x 102 N/m2. In the third case, as ahown in the
lower portion of Fig. 2.37, 42 specimens were tested
from a fiber that waa drawn using an infrared laser to
heat the preform, that also had been fire polished
prior to drawing. All of the specimens withstood tensile
atresses up to 4.0 x 109 N/m2 before breaking.
However, instead of being distributed over a very wide
range of breaking tensile strengths, the breaking points
were all in the range from 4.0 to 5.25 x 109 N/m2.
2-16