Failure stress distributions can be used to predict fiber reliability at a variety of alternative conditions. TSB-61 shows mathematically how this can be done. To complete a given reliability projection, the tests used to characterize a distribution must be controlled for the following:
• Population of fiber, e.g., coating, manufacturing period, diameter
• Gage length, i.e., length of section that is tested
• Stress rates
• Testing environment
• Preconditioning or aging treatments
• Sample size
This method measures the strength and the stress corrosion parameters of optical fiber at specified constant strain rates. It is a destructive test, and is not a substitute for prooftesting.
This method is used for those typical optical fibers for which the median fracture stress is greater than 3.1 GPa (450 kpsi) in 0.5 m gage lengths at the highest specified strain rate of 25 %/min. For fibers with lower median fracture stress, the conditions herein have not demonstrated sufficient precision.
This method tests the fatigue behavior of fibers by varying the strain rate. The test is applicable to fibers and strain rates for which the relationship of log of failure stress vs. log of stress rate is essentially linear. Other approaches are feasible for non-linear results.
Typical testing is conducted on 0.5-m gage lengths with sample numbers ranging from 15 to 30. The realm of probability that is characterized with a typical test does not approach the level needed for installed cable when failure rates as low as 10-5 break/km are required. To assess probabilities at this low level, use ITM-1. This FOTP is useful, however, in comparing the effects of different environmental treatments, or to measure either strength or the stress corrosion parameter.
The test environment and any preconditioning or aging is critical to the outcome of this test. There is no agreed upon model for extrapolating the results for one environment to another environment. For failure stress at a given stress rate, however, as the relative humidity increases, failure stress decreases. Both increases and decreases in the measured stress corrosion parameter and strength distribution parameters have been observed as the result of preconditioning at elevated temperature and humidity for even a day or two.
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