Mechanical Properties of Fibers


. The mechanical properties of fibers are equally important as that of transmission properties. The fibers must be able to sustains exerted during the cabling process.
Two basic mechanical properties of glass fibers are identified.
1. Strength
2. Static Fatigue
1. Strength
. The strength of the fiber is limited due to stress at surfaces or micro cracks. A hypothetical model of micro crack is shown in Fig. 1. This is popularly known as Griffith micro crack. The micro crack is elliptical shaped.

. The strength of fiber crack is expressed as, 

. A fiber contains many randomly distributed micro cracks of different sizes. Therefore fiber strength should be expressed statistically. The commutative probability of failure of a fiber is given as,

       where,
              L      = Fiber length
              σ      = Stress level
              N(σ) = Total cracks per unit length
. The expression for N(σ) is given by Weibull

       Where L0, σ0 and m are constant relating to initial inert strength distribution. The Weibull expression is given by

 2. Static Fatigue
. The static fatigue is the process of slowly growing micro cracks (flaws) due to humid conditions and tensile stress. There is possibility of fiber failure due to growing micro cracks. Also because of chemical erosion at the flaw tip due to water molecules, the flaw increases. To protect fiber from environmental erosion, coating are applied immediately after the manufacturing of fiber.
. Proof testing is the method for high assurance of fiber reliability. In proof testing the fiber is subjected to a tensile load greater than the load at the time of manufacturing and installation. The fibers are rejected if it is does not pass the test. The failure probability Fs for a fiber after it has been proof tested is given as,


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