Page 32 - Fiber Fracture

FIBER FRACTURE: AN OVERVIEW 17

10

2

0 5 IO 15 20 30
Time (h)
Fig. 11. The superior creep resistance of the doped tungsten filament compared to that of an undoped one
(after Horascek, 1989).

The objective is to have a high density of such tiny bubbles so that they pin the grain
boundaries effectively against sliding, thus resulting in a microstructure that is stable at
the operating temperature. These gas bubbles retard the recrystallization of the wire and
give it a very superior creep resistance at the high temperatures prevailing in a glowing
lamp and thus a longer life than that of the undoped filament. Fig. 11 shows the superior
creep resistance of the doped tungsten filament compared to that of an undoped one
(Horascek, 1989).

GLASS AND CERAMIC FIBERS

Ceramic fibers have been researched extensively in the last quarter of the twentieth
century past decade. One of the important applications has been the use of ceramic fibers
(10-125 p.m in diameter) as reinforcement of ceramic matrices to make ceramic matrix
composites for high-temperature applications. Improvements in fracture resistance,
strength, and creep resistance have been shown due to the incorporation of ceramic
fibers into ceramic matrix composites. Another important application has been in the
area of optical glass fibers. Specifically, in the nineteen nineties the demand for optical
glass fibers increased dramatically due to the advances in telecommunications industry
and the availability of the Internet. Optical glass fiber cables are complex structural
products. They may contain metal and composite parts for strengthening, water sealants,
plastic jackets, etc. When such cables are used in water, corrosion of the metal parts
results in the production of hydrogen. If this hydrogen is trapped inside the cable, it
can result in an increase in the attenuation of the optical signal via the optical glass
fiber. Stresses may also result in the optical glass fiber because of installation. In the
case of the optical fibers, the flaws can form during the processing when dust particles
and other particles adhere to the surface of the doped silica fiber. Furthermore, there is
an increasing demand for optical glass fiber with a large bandwidth and one that can