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Driven by the constantly rising need for greater bandwidth and faster connection speeds, fiber optic transmission is increasingly standard in modern society.
While it may be understandable to think of fiber optic cable as fragile, this material is relatively sturdy and flexible.
The strength of this material comes from the way it is produced. Fiber optic cable starts off as a pure silica rod or tube. Other materials are added at high temperature using ultra-pure gasses to generate a preform. The preform is hardened and put into a drawing tower where the fiber is pulled into long strings. Precise control of the production sequence results in glass that is very pure, clear and strong.
The fiber that is produced has a theoretical maximum (tensile) strength of around 2 million pounds per square inch. However, the actual maximum is about 10 to 20 percent of that. The cross-sectional area is so tiny, about 20 millionths of an inch, the actual maximum tensile strength ends up being about five to 10 pounds.
The mechanical performance of optical fiber is often classified by factors like strength distribution and fatigue. Strength distribution is written as a probability function known as a Weibull function, which has a slope and intercept parameter. Fatigue describes how fast a flaw grows when subjected to both water and stress.
Types of Strength Testing
Tensile strength tests stretch the fiber and determine the point at which the fiber fails. In dynamic testing, a constant force rate is applied using a stationary capstan fiber tester (SCFT) or a Rotating capstan fiber tester (RCFT). An SCFT stresses the fiber with a capstan connected framework that moves away at a set rate. An RCFT pulls the fiber by revolving one of two connected capstans at a set rate.
A load cell documents the amount of stress at the point of failure, with the strain rate expressed as percent change in length per minute, as it relates to gauge length. The conventional technique for determining dynamic tensile strength involves 30 samples assessed at a strain rate of 3 to 5 percent per minute to ascertain the tensile strength.
The dynamic fatigue parameter (nd) is a measure of fatigue determined by gauging the strength at multiple strain rates, from 0.003 to 30 percent per minute. The median failure stress will deviate based on the demand rate, and the fatigue parameter can be determined based on the slope of the line. At least 15 samples are needed per strain rate to find out the dynamic fatigue parameter.
The testing conditions can have an impact on the outcome. To standardize testing, most assessments are conducted at 25 degrees Celsius and 50 percent relative humidity, although reliable evaluation can happen in a range of conditions. Furthermore, the effects of aging are modeled by artificially aging the fiber at extreme conditions ahead of assessment. A standard 'accelerated aging' conditions used are 85 degrees Celsius and 85 percent relative humidity for 30 days.
The Impact of Gauge Length
Random imperfections introduced during processing usually defines the strength of long stretches of fiber; short lengths typically approach the inert strength, and a lengthy gauge usually is typically weaker.
Most assessment is carried out on 0.5- or 20-meter gauge lengths. The 0.5-meter lengths are used to determine the intrinsic strength of the fiber, while 20-meter samples are used to define the extrinsic strength of the fiber.
A continuous rotating capstan fiber tester (CRCFT) is typically used to assess the low strength (< 350 kpsi) distribution on lengthy fibers, such as those hundreds of kilometers long. This is accomplished by continually testing 20-meter lengths of fiber up to 350 kpsi. If the fiber fails, the load at failure is documented. If the fiber does not fail, then another 20-meter section is examined.
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