Static fatigue life of silica optical fibers and the significance of fiber coating and handling

Abstract

Structural health monitoring with optical fiber sensors requires the embedded or surface bonded optical fibers and sensors to remain functional for the lifetime of the structure being monitored, as repairs are generally impossible. Thus, the feasibility of an embedded optical fiber monitoring concept depends heavily on the durability of the optical fiber. Processes that degrade the mechanical properties of these fibers are therefore of great concern. It is well known that silica optical fibers are vulnerable to moisture degradation, which in combination with applied loads will make flaws grow and eventually lead to fiber failure. Fabrication of optical fiber sensors often involve removal of the protective coating locally, thus exposing the glass surface to moisture and handling loads. Static fatigue experiments lasting up to one year were conducted on coated and uncoated optical fibers to determine the durability of the fibers in terms of time-to-failure as a function of applied loading, environment, and handling. Both an acrylate-coated and a polyimide-coated fiber were studied. Uncoated fiber, representing the worst-case condition, was obtained by chemical removal of the protective coating. A mechanism-based model was introduced to model the data. Results show that the static fatigue effect is very significant with the time-to-failure decreasing with applied load. For the uncoated fiber, it was found that the static fatigue life depended appreciably on how the fiber was handled, with the durability decreasing significantly by the slightest mechanical contact. The results indicate that strain levels of 0.3% should be survivable for years even in the worst case tested, but that above this level the static fatigue durability may become a critical issue.