Reliability Assessment of Optical Fibers under Tension and Bending Loads
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The mechanical failure of optical fibers must be avoided to ensure reliability of fiber-based systems. The first stress event in a fiber's lifetime is the proof test. The proof test will alter the fiber's strength distribution for all subsequent processing and applications. Thus it is critical to know the fiber strength distribution after proof test (post-proof strength distribution). It is generally assumed that the proof test truncates the strength distribution at the proof test stress level. But, many users are concerned because they know that theoretically it has been shown that after proof test the strength of fiber may be much less than the proof test stress level, and that the minimum post-proof strength is determined only by the unloading rate during the proof test. But this theoretical result is not consistent with historical field data. Historically no one has documented failures stresses below the proof stress level. This dissertation resolves this apparent contradiction by reviewing the theory and conducting a probabilistic assessment. As optical fibers are used more and more in computer and switching gear backplanes, a new potential mechanical reliability problem arises due to the necessary bends introduced in optical fibers. Previous researchers were concerned with the uniform stress optical fibers saw in long haul underground applications, but bending places a non-uniform stress along the fiber surface. So it is inaccurate to borrow fiber usage mechanical guidelines from long-haul application. This dissertation reviews existing theories and then develops a new analytic approach to assess the mechanical reliability of optical fibers under bending loads and static fatigue conditions. This new analytic approach is verified through a simple static two-point bend experiment. Finally the newly developed reliability assessment method is used to develop new guidelines for bending application and examples are presented to show how the approach can be used to attack some very common mechanical reliability problems with optical fibers.